WO2016124645A1

WO2016124645A1 - Method for ferroelectric polarization switching using polarized light

Info

Publication number: WO2016124645A1
Application number: PCT/EP2016/052288
Authority: WO
Inventors: José Francisco FERNÁNDEZ LOZANO; Adolfo DEL CAMPO GARCIA; Fernando Rubio Marcos
Original assignee: Consejo Superior De Investigaciones Científicas
Priority date: 2015-02-04
Filing date: 2016-02-03
Publication date: 2016-08-11

Abstract

The present invention refers to a method for switching the polarization of a ferroelectric material by applying a polarized coherent light at a light polarization angle θ, and while the polarized coherent light is being applied, varying said light polarization angle θ. Additionally, the invention is directed to a method for writing data in a data storage device comprising switching the polarization of a ferroelectric material according to the method of the invention and to a data storage device comprising a ferroelectric material wherein the stored data is written by switching the polarization of the ferroelectric material according to the method of the invention.

Description

METHOD FOR FERROELECTRIC POLARIZATION SWITCHING USING

POLARIZED LIGHT

FIELD OF THE INVENTION

The present invention relates to the field of polarization switching and, more particularly, to the field of polarization switching in ferroelectric materials.

BACKGROUND

Ferroelectrics materials are characterized by exhibiting spontaneous and stable polarization, which can be usually reoriented by an applied external electric field. On the electrically switchable nature of this polarization underlie various ferroelectric devices, such as nonvolatile ferroelectric random access memory (FeRAM) . In such memory devices, the storage of data bits is achieved by motion of domain walls that separate regions with different polarization directions. Therefore, an external voltage pulse can switch the polarization between two stable directions, representing "0" and "1". This behavior is responsible for a read/write process that can be completed within nanoseconds. The major drawback of FeRAM is that requires a circuitry access, limiting their practical implementations for commercial use due to their difficult integration into devices as compared to their conventional magnetic random access memory counterparts. Consequently, there is a need for a method for switching the polarization of ferroelectric materials without the need of a circuitry access.

C.L. Sones and co-workers [Central Laser Facility Annual Report 2004/2005, 185-187; Central Laser Facility Annual Report 2005/2006, 182-184] developed a method for switching the polarization of litium niobate (LN) using an intense light beam (pulsed UV laser light) . This method, however, produces irreversible changes at the surface of the ferroelectric material since it is based on the lithography technique.

Multiferroic oxides (oxides that exhibit more than one type of ferroic order parameter simultaneously) combine the advantages of both ferroelectricity and ferromagnetism for new FeRAM that can be written electrically, but read magnetically. Recently it was demonstrated significant room temperature multiferroicity and magnetoelectric coupling by monitoring changes in ferroelectric domain patterns induced by magnetic fields in PZTFT and epitaxial thin films of BiFe0₃ [Evans, D.M. et al. Nat. Comm. 4, 1534 (2013); Wang, J. et al . Science 299, 1719-1722 (2003)]. This effect is attractive for photovoltaic devices and renewable solar energy use because large photovoltages can be generated by domain walls [Yang, S. Y. et al. Nat. Nanotechnol. 5, 143-147 (2010)]. A drawback for using multiferroic oxides in memory devices is the presence of decreased electronic conductivity in ferroelectric domain walls resulting in uncompensated accumulated charge which are known as "strongly" charged domain walls (sCDW) [Seidel, J. et al . Nat. Mater. 8, 229-234 (2009); Farokhipoor, S. et al . Phys . Rev. Lett. 107, 127601 (2011); Maksymovych, P. et al . Nano Lett. 12, 209-213 (2012); Meier, D. et al . Nat. Mater. 11, 284-288 (2012)]. These sCDW contain high local mechanical stress due to the accumulated charge and therefore, the polarization of multiferroic oxides is expected to be less effectively switched by means of an electrical field or pressure [Warren, W. L. et al. Appl. Phys. Lett. 67, 866-868 (1995); Warren, W. L. et al . J. Appl. Phys. 77, 6695-6702 (1995)]. US 5,239,504 Al describes a method to switch the polarization of a multiferroic oxide material (PZT) by means of an external electric field. Since the applied field is insufficient to cause a switching of the polarization of the material, an electrostrictive film is put in contact with said material to reduce the mechanical stress at the sCDW and therefore enabling the polarization to be switched in accordance with the applied electric field. US 6,108,111 A describes a data storage device comprising two electrodes and a ferroelectric crystalline thin film layer. Such device may be written for non-volatile storage and read out using optical processes by setting the polarization state of each individual ferroelectric device by applying a linearly polarized light beam .

Recently, in a pioneering study, T. Sluka and co-workers [Nat. Commun . 4, 1808 (2013)] demonstrated the correlation between the existence of sCDW in ferroelectric BaTi0₃, BTO, and their enhanced electromechanical properties (electron-gas like conductivity while individual domains remained excellent insulators) . Therefore, the discovery of this stimulant behavior from sCDW described for BTO-based material and the potential technological applications from these enhanced functionalities raises a need for an efficient and non-invasive method to switch ferroelectric domains without the need of electrical connections or physical contact, for example, by means of an electrostrictive film.

SUMMARY OF THE INVENTION

The authors of the present invention have found an unexpected coupling between polarized light and ferroelectric polarization, which modifies the stress induced in ferroelectric materials at the domain walls by varying the light polarization angle Θ of a coherent light. Therefore, by the method of the present invention light energy is directly converted in ferroelectric domain wall motion which causes a switching in the polarization resulting in a method for switching the polarization of the ferroelectric material without the need of electrical connections or physical contact. In addition, the method of the present invention leads to a non-invasive (no damages are produced at the surface of the ferroelectric material) and reversible process.

Accordingly, in a first aspect, the invention is directed to a method for switching the polarization of a ferroelectric material comprising applying a polarized coherent light at the surface of the ferroelectric material with a light polarization angle Θ, which is the angle at the plane perpendicular to the propagation direction of the light and parallel to the plane of the surface of the ferroelectric material, and, while said polarized coherent light is being applied, varying the light polarization angle Θ in the plane parallel to the surface of the ferroelectric material .

In a second aspect, the invention is directed to a method for writing data in a data storage device comprising switching the polarization of a ferroelectric material according to the method as defined above.

Finally, the invention refers to a data storage device comprising a ferroelectric material, and characterized in that the stored data is written by switching the polarization of the ferroelectric material according to the method as defined above.

FIGURES

Figure 1: Ferroelectric domain walls moving by polarized light. Figure 2: Structural characterization of BaTi0₃ (BTO) single crystal by XRD, wherein:

(a) is the unit cell of BaTi0₃ in the tetragonal structure. The direction of ferroelectric polarization is indicated by an arrow .

(b) is the XRD pattern of BTO single crystal. The inset shows a detail of the XRD diffraction pattern in the 2Θ range from 44° to 46°, corresponding to (002) and (200) peaks of the tetragonal symmetry.

Figure 3: Characterization of BTO single crystal through Atomic Force Microscopy (AFM), wherein:

(a) is an optical image of BTO single crystal, which shows the domain structure.

(b) is an AFM image of BTO single crystal, which shows the domain topography inside the marked white box of (a) .

(c) is an AFM topography data along the arrow inside the marked white box of (a) (d) is a detail of the domain boundary topography, which correspond to LB-1 and LB-2 regions of (c) .

Figure 4 : Mapping of the ferroelectric domains structure of BTO single crystal by Confocal Raman spectroscopy (CRM), wherein:

(a) is an optical image of the c-plane of BTO single crystal. The region marked as a white rectangle shows the positions where the XY Raman image and XZ Raman depth scan image has been performed and correspond with the area marked as a white rectangle of previous AFM analysis.

The Raman image shows the domain distribution by regions marked as 7, 8 and 9 and shown in grey scale intensity code at the surface (b) and in the depth scan (c) . The Raman image resulted from mapping the different single Raman spectra collected in each pixel of the marked rectangle area in (a) . Raman spectra having same spectral shift for the Raman modes are identified under same intensity of grey.

(d) are Raman spectra of BTO for the regions marked as 7, 8 and 9 in (b) and (c) . The numbers 1-6 next to the vibrational peaks represent the main Raman modes. The inserts show magnified Raman spectra, ascribed to the 4 and 5 Raman modes, respectively .

Figure 5: Schematic illustration of the BTO domain structure built by combining the AFM and Raman mapping information, for the regions marked as 7, 8 and 9 in Figs. 3 and 4 respectively. Scheme shows a domain structure composed of a-domain and c- domains with a head-to-head configuration of the polarization vectors. The head-to-head configuration maximizes the internal stress close to the domain wall. As consequence of these internal stresses the a-c-domains are hindered by b-domains. The b-domain structure minimizes the internal stress of the domain wall by a bundle of alternate a-domain and c-domain.

Figure 6: Motion of the ferroelectric domain boundaries under polarized light, wherein: (a-g) are sequence of Raman depth scans images showing the switching of the c-domains and b-domains in the BTO cross section for different light polarization angles between 0° and 90°. The scheme localized at the top of (a) represents the coordinates where the light polarization angle Θ at the surface of the crystal varies in the plane XY . Additionally, the light polarization angle value is indicated on the left of each Raman image. The Raman spectra taken in two representative points selected along the dotted line denotes as 10-11, and 12-13 in the set (a-g) are shown in Fig. 9.

(h-n) Sequence of Raman shift depth scans images showing the Raman shift at three main points in the complex structure representing a-domain, a-c-domain wall and b-domain which are inscribed as 14— >15 , 16— >11 and 18— >1 9 , respectively.

(o) represents the Raman shift evolution for the three main points inscribed in (h-n) as a function of the light polarization angle (Θ) ; and the relative motion of the domain as a function of the light polarization angle (Θ) .

Figure 7: Effect of the light polarization angle Θ in the Raman modes of BTO, wherein:

(a) are Raman spectra of a-domain (point variations 14— >15 in Fig. 6h) .

(b) are Raman spectra of b-domain (point variations 16→17 in Fig . 6h) , and

(c) are Raman spectra of a-c-domain wall (point variations

18→19 in Fig. 6h) as a function of the light polarization angle Θ collected. The inserts show magnified Raman spectra, ascribed to the 4 and 5 Raman modes, respectively.

(d) Comparison of Raman spectra at low wave number region for main domain structures at θ=0° (left) and 9=90° (right), each Raman image indicates on the top of the panel (d) the light polarization angle Θ value. The numbers next to the Raman peaks represent assignments. The rhombohedral Raman modes associated to the BTO phase are signaled as R in the Raman spectra of the panel (d) .

Figure 8: Mapping of the domain structure of the (KNa) Nb0₃-based ceramics (KNN) by Confocal Raman spectroscopy (CRM) , wherein:

(a) is an optical image of the surface of (KNa) Nb0₃-based ceramics. The region marked as a grey rectangle, and indicated as 21, shows the positions of the XY Raman image.

(b) is the surface Raman image resulted from mapping the different single Raman spectra collected in each pixel of the marked area 20 in (a) . Raman spectra having same spectral shift for the Raman modes are identified under same grey intensity.

Figure 9: Ferroelectric domain wall motion induced by polarized light on (KNa) Nb0₃-based ceramics (KNN) , wherein:

(a-g) is a sequence of Raman images showing the switching of the domain structure of the KNN surface section (21), for light polarization angles between -90° → 0° —> 90°. The scheme localized at the top of the Fig. 9(a) represents the coordinates where the light polarization angle Θ at the surface of the crystal varies in the plane XY . Additionally, the value of the light polarization angle Θ is indicated on the left side of each Raman image .

(h-n) is sequence of Raman shift images showing the Raman shift at the two main point which are inscribed as 22—>23. The relative motion of domain is illustrated along the line marked as 22 —> 23.

(o) represents the Raman shift evolution for two main adjacent domains of the complex structure, representing the relative motion of the domain with the light polarization angle Θ and the corresponding Raman Shift in Fig. 9 (a-g) .

Figure 10: Mapping of the domain structure of the Bi₄Ti₃0i₂ (BIT) ceramic by Confocal Raman spectroscopy (CRM), wherein: (a) is an optical image of the surface of Bi₄Ti₃0i₂ (BIT) ceramic. The region marked as a grey rectangle, and indicated as

(24), shows the positions of the XY Raman image. The region marked as a black rectangle, and indicated as (25), that is rotated around 45° respect to the grey rectangle (24) and represents the area where the ferroelectric domain moves accordingly with the variation of light polarization angle Θ. Inside the black rectangle (25) it is located a pore (26) that served as a reference point.

(b) is the surface Raman image resulted from mapping the different single Raman spectra collected in each pixel of the marked area 24 in (a) . Raman spectra having same spectral shift for the Raman modes are identified under same grey intensity. Pore (26) is represented in black-grey due to the absence of Raman spectra.

(c-i) is sequence of Raman shift images showing the Raman shift at a ferroelectric domain wall that separated two adjacent ferroelectric domains with distinct ferroelectric orientation, which are inscribed as 27 → 28. The relative motion of the ferroelectric domain is illustrated by comparison of the position of domain wall that varies with light polarization angle from 27 → 28 and the fix point of the pore (2) resulting in a relative domain motion of around 5 μιτι.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs .

The present invention refers to a method for switching the polarization of a ferroelectric material, comprising applying a polarized coherent light at the surface of the ferroelectric material at a light polarization angle Θ, which is the angle at the plane perpendicular to the propagation direction of the light and parallel to the plane of the surface of the ferroelectric material, and while said polarized coherent light is being applied, varying the light polarization angle Θ in the plane parallel to the surface of the ferroelectric material.

In said method the switching of the polarization of the ferroelectric material is reversible and occurs without the need of electrical connections or physical contact.

In the context of the present invention the term "ferroelectric material" relates to a material that exhibits, over a range of temperature, a spontaneous electric polarization that can be reversed or reoriented (known as "switching"), usually, by application of an electric field. Regions with different orientations of the polarization vector coexist within a ferroelectric sample, and are called "polarization domains" or "ferroelectric domains" or just "domains". The regions that separate two adjacent domains are called "domain walls".

The method of the present invention can be applied to a ferroelectric material as a single-crystal material or as a poly-crystalline material.

In a preferred embodiment, the ferroelectric material as defined above is a thin-film layer. In the context of the present invention, "thin-film layer" refers to a layer of ferroelectric material ranging from fractions of a nanometer to a film having a thickness up to 20 μιτι in thickness.

In a particular embodiment, the ferroelectric material is a ceramic .

In the context of the present invention the term "ceramic" relates to any inorganic crystalline material, compounded of a metal and a non-metal.

In a preferred embodiment, the ferroelectric material has a structure selected from perovskite-type structure, ilmenite-type structure, tungsten-bronze-type structure, Aurivillius-type structure, potassium dihydrogen phosphate-type structure, potassium titanyl phosphate system-type structure, and oxygen tetrahedral-type structure.

In the context of the present invention the term "perovskite-type structure" relates to a family of crystalline materials of formula AB0₃ wherein A is atom charged +2 and B is atom charged +4. Examples of ceramic perovskites suitable in the method of the present invention are BaTi0₃-based materials, Pb (Ti, Zr) 0₃-based materials, (K, a) b0₃-based materials,

(Bi , a) Ti0₃-based materials. The perovskite structure is shown to be the most common one in ferroelectric ceramics.

In the context of the present invention the term "-based material" refers to a materials that belong to the same parent structure, as example perovskite BaTi0₃ parent structure, and content different cations in solid solution or as a dopant in order to modify the ferroelectric properties of the parent structure .

In the context of the present invention the term "ilmenite- type structure" refers to a family of crystalline materials which support ferroelectricity and is related to the perovskite structure in that it is exhibited by materials with the general formula A'B'0₃, but where the A' cation is too small to fill the coordinated site of the perovskite structure. The structure is made up of hexagonal close-packed layers of oxygen ions, with the A and B ions occupying the octahedrally coordinated sites between the layers. This structure can also be considered to be related to the perovskite structure in that both are based on oxygen octahedra. Examples of ceramic ilmenites suitable in the method of the present invention are LiNb0₃ and LiTa0₃.

In the context of the present invention the term "tungsten- bronze-type structure" relates to a very large family of oxygen octahedral crystalline materials with the general formula [A1₂A2₄C₄] [B1₂B2₈] O₃₀. The Bl and B2 sites are octahedrally coordinated by oxygens and have similar sizes and valences to the B sites in the perovskites. The Al and A2 sites are surrounded by four and five columns of B0₆ octahedra respectively. The three-fold coordinated C sites in the structure are frequently empty, but can be occupied by small uni- or divalent cations (e.g. Li⁺ or Mg²⁺) . There are a wide range of ferroelectric tungsten-bronzes, which frequently show non-stoichiometry . Examples of ceramic tungsten bronzes suitable in the method of the present invention are PbNb₂0₆ and PbTa₂0₆. Substitution of Pb²⁺ by Ba²⁺ can also be used to produce a useful ferroelectric ceramic material Pbi/₂Bai/₂ b₂0₆. Other examples of tungsten bronze ferroelectrics are Sr _x Bai-_X Nb₂0₆, Ba₂NaNb₅0i₅ and

In the context of the present invention the term "Aurivillius-type structure" relates to a class of ferroelectrics based on an oxygen octahedral crystalline structure. They have in common with the perovskites in that they consist of layers or slabs of perovskite blocks with the general formula (A'' _m-iB' ' _m 0_3m+i) ^2~ separated by (M₂0₂) ²⁺ layers in which the M cation is in pyramidal coordination with four oxygens, the M being at the apex of the pyramid. In the ferroelectric phases, M is usually Bi and m is usually between one and five. The A and B cations follow the usual ionic radius and valence criteria of the perovskites, and we see ferroelectric compounds in which A''=Bi, La, Sr, Ba, Na, K etc. and B''=Fe, Ti, Nb, Ta, etc. Examples of Aurivillius ferroelectric suitable in the method of the present invention are Bi₄Ti₃0i₂ and SrBi₂Ta₂0₉

Other crystalline structures which support ferroelectricity as oxygen octahedral ferroelectrics are for example groups of phosphates (in which the phosphorous ions are tetrahedrally coordinated by oxygens) which have found applications in optical systems as the potassium dihydrogen phosphate and the potassium titanyl phosphate system.

There are other examples of oxygen tetrahedral ferroelectrics as lead germanate (Pb₅Ge₃0n) or gadolinium molybdate, Gd₂(Mo0₄)₃ that possess a structure consisting of corner-linked Ge0₄ tetrahedra or M0O₄ tetrahedra respectively.

In a preferred embodiment, the ferroelectric material is selected from BaTi0₃, (K, a) Nb0₃-based materials and Bi₄Ti₃0i₂.

In the context of the present invention, the term "coherent light" refers to light in which the phases of all electromagnetic waves at each point on a line normal to the direction of the incident beam are identical, i.e. waves are in phase. The most common coherent light for practical uses is laser light since its spatial coherence allows it to be focused to a tight spot, to stay narrow over long distances, and having high temporal coherence which allows it to have a very narrow spectrum, i.e., it only emits a single color of light (monochromatic) .

Therefore, in a preferred embodiment, the coherent light of the method as defined above is a laser light.

Examples of laser lights suitable in the method of the present invention are those emitted by gas lasers, chemical lasers, metal-vapor lasers, dye lasers, solid-state lasers, semiconductor lasers or the like.

In a preferred embodiment, the laser light of the method as defined above is emitted by a Nd:YAG laser operating at 532 nm .

In another particular embodiment, the coherent light of the method as defined above is a laser light selected from those emitted by continuous wave lasers, pulsed laser and ultrafast lasers. More preferably, the coherent light of the method as defined above is a light emitted by a pulsed laser. Even more preferably, the coherent light of the method as defined above is a light emitted by a pulsed laser having pulses from every Ιμεβο to every 10 nsec.

In the context of the present invention, the term "polarized coherent light" refers to a coherent light as defined above wherein the emitted un-polarized light is transformed into polarized light. Polarized light waves are light waves in which the vibrations occur in a single plane. The polarization of a light beam identifies the direction, which is always at right angles to the direction of propagation, in which the electric field is vibrating. This vibration can be simple having only one direction along the beam path (linear polarization) or it can be complex. In the latter case, there are many possibilities, but the most commonly encountered behaviors are those of circular and elliptical polarization. With circular polarization, the electric field changes its orientation by 360° within one wavelength; with elliptical polarization the rate of change is the same, but this time the magnitude of the field varies as well .

Therefore, in a preferred embodiment, the polarized coherent light of the method as defined above is a linearly- polarized coherent light, more preferably a linearly-polarized laser .

Furthermore, light can be polarized by transmission, by reflection, by refraction or by scattering.

In the method of the invention, the polarized coherent light is preferably polarized by reflection. Even more preferably, the polarized coherent light is polarized using an optical filter which allows passing the light waves of a specific polarization and blocks waves of other polarizations. Examples of polarizer are half-wave plate polarizer, wire-grid polarizer, absorptive polarizers, beam-splitting polarizers, birefringent polarizers and thin-film polarizers.

Moreover, prior methods for switching the polarization of ferroelectric materials used un-polarized intense light beams such as pulsed UV laser light. These methods, however, produce irreversible structural changes in the ferroelectric material due to the high temperatures reached at the surface of material. However, the method of the present invention leads to a non- invasive since no damages are produced at the surface of the ferroelectric material .

Therefore, in another preferred embodiment, the polarized coherent light of the method as defined above is absorbed by the surface of the ferroelectric material in a dose equal or less than 1 Jem^-2.

In the context of the present invention, the term "dose" refers to the energy density, i.e. energy power absorbed by surface area of the ferroelectric. An absorption dose higher than the dose 1 Jem^-2 generate a heating of the ferroelectric that produce a symmetry change from the ferroelectric phase to a high temperature centro-symmetric phase and the ferroelectric property vanished. The temperature limit that corresponds to the above dose is defined as the Curie temperature of the ferroelectric material. As an example, for BaTi0₃, the Curie temperature is 125°C.

In another preferred embodiment, the polarized coherent light of the method as defined above emits at a wavelength from around 213 nm to around 2900 nm.

In the context of the present invention, the term "surface" refers to the external layer of the ferroelectric material where the polarized coherent light is not absorbed and therefore Raman spectra are allowable to be collected. The surface region depends on the extinction coefficient of the ferroelectric material and on the appearance of defect as grain boundaries or dislocations .

In a preferred embodiment, the surface of the method as defined above is a layer with a thickness equal or less than 20 ]i .

In a particular embodiment, the surface of the ferroelectric material is polished, thermally etched, chemically etched or combinations thereof. These surface treatments are used to improve sample surface finishing avoiding topographic artefacts due to micro- or nano- roughness at the surface of the ferroelectric material.

In the context of the present invention, the term "polished" refers to a surface treatment usually performed in two steps. The first step consists in a "hard polished" or grinding process normally using silicon carbide abrasive papers, to obtain parallel surfaces. In a second step, the surface of material is "softly polished" normally with diamond paste to obtain mirror finish surfaces. The polished surface is characterized because the median roughness Ra is less or equal to 1 μιι.

In the context of the present invention, the "thermal etching" refers to thermal treatment of the polished surface at a temperature <90% of the sintering temperature of the ferroelectric material . An example of thermal etching in a (KNa) Nb0₃-based ferroelectric ceramic consists in the thermal treatment of the polished surface at 1025°C during 10 minutes.

In the context of the present invention, "chemical etching" refers to an immersion of the polished surface into a chemical bath, and the action of the chemical to reveal the domain structure at the surface. An example of chemical etching in a ferroelectric ceramic consists in the immersion of the polished surface in aqueous solution of 5% HC1 during 10 second.

In a preferred embodiment, the surface of the ferroelectric material of the method of the present invention is at least polished.

Therefore, the method of the present invention comprises applying the polarized coherent light at the surface of the ferroelectric material at a light polarization angle Θ.

In the context of the present invention, the term "light polarization angle Θ" refers to the angle at the plane perpendicular to the propagation direction of the light (light propagates in the direction of the z axis) and parallel to the plane of the surface of the ferroelectric material (plane XY) (see Figure 1) . Therefore, axes X and Y are defined respect to the material surface and with the Z-axis always normal to the surface and its positive direction going into the surface material .

The method of the present invention further comprises that while the polarized coherent light is applied, the light polarization angle Θ is varied. The light polarization angle Θ varies in the plane parallel to the surface of the ferroelectric material (plane XY) as a result of different direction of the polarized coherent light.

Without being bounded to any theory, it is believed that the energy intensity provided by the light polarization angle Θ variation while said polarized coherent light is applied is related to a coupling between the polarized light and the ferroelectric material crystal structure. As a result, either charge or stress redistribution occurs when exposed to light and, therefore, a domain walls motion occur, given as a result a switch in the direction of the polarization domain.

Therefore, the light polarization angle Θ of the method as defined above varies in such a way that the rate of relative motion of a ferroelectric domain wall (R_RM) is between around 4 nm per unit of degree and around 50 nm per unit of degree.

In the present invention, the term "rate of relative motion of a ferroelectric domain wall" or "R_RM" is defined as follows:

wherein :

AR_M is the spatial displacement of the position of the domain wall between two given angles defined by Δ Θ ; and

Δ Θ is the variation of the light polarization angle Θ between two given angles Θ having any value between 0° to 90°. In a particular embodiment, ΔΘ is the varation of the light polarization angle Θ between 0^° and 15^° degrees, between 15^° and 30^°, between 30^° and 45^°, between 45^° and 60^°, between 60^° and 75^°, or between 75^° and 90^° . In another particular embodiment, ΔΘ is the variation of the ligth polarization angle teta between 0^° and 90^° .

Since the direction of polarization of light interacts non- linearly with the crystallographic direction of the ferroelectric domain, it is observed that there are two regimes of R_RM wherein a slow variation regime starts at around 4 nm per unit of degree and a fast variation regime goes up to around 50 nm per unit of degree.

In addition, as explained above, the method of the present invention leads to a reversible process. Since each direction of polarization has an associated light polarization angle Θ, in the present invention, "reversible process" refers to the fact that, by the method of the invention, it is always possible to going back and forward to a given polarization direction by going back and forward to the corresponding associated light polarization angle Θ.

Another aspect of the invention refers to a method for writing data in a data storage device comprising switching the polarization of a ferroelectric material according to the method as defined above.

Specifically, writing data bits in such devices is achieved by switch or motion of domain walls that separate regions with different polarization directions on a ferroelectric material. Therefore, by the method of the invention, the application of a polarized coherent light can switch the polarization between two stable directions, representing "0" and "1".

Therefore, the method of present invention allows writing data bits in a data storage device without the need of a circuit access or physical contact. Moreover, the present invention refers also to a data storage device comprising a ferroelectric material, and characterized in that the stored data is written by switching the polarization of the ferroelectric material according to the method as defined above.

EXAMPLES

The present invention will now be described by way of examples which serve to illustrate the construction and testing of illustrative embodiments. However, it is understood that the present invention is not limited in any way to the examples below .

Example 1: BaTi0₃ (BTO)

Sample Preparation

BTO single crystals used in this example were grown with an orientation in the <100> plane, with 5mm x 5 mm x 1 mm in dimension, and produced by top-seeded solution growth (TSSG) provided by PIKEM Ltd (UK) . The single crystals were only polished and no further thermal and/or chemical etching was used. They were maintained at >25°C keeping the material warm by a resistance and a temperature controller.

Identification of the crystalline symmetry and crystallographic orientation of single crystals of BTO

A basic identification of the structure (tetragonal symmetry) and of the crystallographic orientations (c-plane and a-plane) of a BTO single crystal was made according to X-Ray Diffraction analysis (XRD, X'Pert PRO Theta/2theta of Panalytical, Cu Ka radiation) .

Fig. 2a shows the P4mm unit cell of BTO. The direction of ferroelectric polarization is indicated by the arrow in Fig. 2a. Elongation of the unit cell along the c-axis and the deviation of the c/a ratio from unity (see Table 1) are used as an indication of the presence of the ferroelectricity . The XRD pattern of the BTO single crystal is shown in Fig. 2b. The absence of most of the diffraction peaks observed for polycrystalline samples, especially the most intense one (101)/ (110) double peak at 31.5-31.6° confirm the single-crystal nature of the sample. Only two families of peaks ((001) and (h00)) are detected. This indicates that the sample is not single-domain and present two different orientations. In addition, the splitting of the (001) pc peaks into (001) and (hOO) confirms the tetragonal symmetry of the sample (see insert of the Fig. 2b for the (002) and (200) peaks) . The refinement gave lattices parameters a = 3.9899 A and c = 4.0340 A (see Table 1), which are very close to reported values for tetragonal phase [Ghosh, D. et al. Adv. Funct. Mater. 24, 885-896 (2014)] . Considering the orientation degree of the crystal, the relative intensity of (hOO) and (001) peaks are reversed compared to powder data. Thus, the (001) orientation, or c-plane, is the dominant one .

Table 1. Unit-cell parameters of BaTi0₃ single crystal, in the tetragonal P4mm structure, at room temperature.

Characterization of the domain structure of the BTO single crystal by Atomic Force Microscope

A sample of a BTO single crystal was imaged by Atomic Force Microscopy (AFM) in AC Mode using Arrow FM cantilevers (Nanoworld, Germany) with a resonance frequency in the range of 70-90 kHz and damping of r = 50%, recording both topography and phase images simultaneously.

Fig. 3a depicts an optical microscopy image of the BTO single crystal sample aligned perpendicular to the AFM cantilevers. The area of 150 x 30 μιτι delimits the selected area where the purely topographic information is collected through the AFM. Fig. 3b shows a detailed AFM topographic image of the domain structure as a consequence of the differences in crystallographic orientations. The AFM line scans along the arrow marked in Fig 3a is illustrated in Fig. 3c. From AFM scan (horizontal arrow in Fig. 3c), it can be determinate the different regions separated by domain walls. The domain structure is mainly composed of domains width ranged from 40 to 50 μιτι in thickness, and showed height differences of -120 nm in adjacent domains having different contrast (bright and dark regions) in the AFM image. Another feature derived from the AFM analysis is the domain boundary differences which are soft transitions. The details of the domain boundary topography are magnified in Fig. 3d, which are signaled as A and B in Fig. 3c.

Mapping of the domain structure of the BTO single crystal by Confocal Raman Microcopy

The sample (BTO single crystal) was cleaned with acetone and ethanol before characterization by Confocal Raman Microscopy (CRM) . Raman spectral resolution of the system was down to 0.02 cm^-1 and obtained using a frequency-doubled NdYAG laser operating at 532 nm and a lOOx objective lens (NA = 0.9) . The incident laser power was 40 mW and the integration time is 0.2 seconds. The optical diffraction resolution of the CRM was limited to about -250 nm laterally and -500 nm vertically. The microscopy sample was mounted on a piezo-driven scan platform having 4 nm lateral and 0.5 mm vertical positional accuracy. The piezoelectric scanning table allows steps of 3 nanometers (0.3 nm in the vertical direction) , giving a very high spatial resolution. The microscope base was also equipped with an active vibration isolation system, active 0.7-1000 Hz. Collected spectra were analyzed by using Witec Control Plus Software.

Figure 4a shows an optical micrograph of the polished surface of the BTO single crystal aligned perpendicular to the Raman laser. The area of 150 x 30 μιτι denotes the selected area where the Raman spectra are collected at a plane located just below the surface of the sample where the Raman intensity is maximized. The acquisition time for a single Raman spectrum was 1 second, thus the acquisition of a Raman image consisting of 150 x 30 pixels (4500 spectra) required 75 minutes for the planar-section. Features such as Raman peak intensity, peak width or Raman shift from the recorded Raman spectra were fitted with algorithms to compare information and to represent the derived Raman image.

Raman active phonons of the tetragonal P4mm crystal symmetry are represented by 3A1 + Bl + 4E. Long-range electrostatic forces induce the splitting of transverse and longitudinal phonons, which results in split Raman active phonons represented by 3 [Al (TO) + Al (LO) ] + Bl + 4 [E (TO) + E (LO) ] . The assignments, both symmetry and nature (first and second order) , of the observed Raman modes on the BTO single crystal are summarized as (1) for Raman modes of Ei (TO) , Ai (TO) , E(T0₂) , E(LO) and A₁ (LO) ; (2) for Raman modes of A₁ (T0₂) ; (3) for Raman modes of E (T0₃ + L0₂) and Bl; (4) for Raman modes of Ei(L0₃), Ai(L0₂) and E (T0₄) ; (5) for Raman modes of E (T0₅) and Ai(T0₃) ; and (6) for Raman modes of E (L0₄) and A₁ (L0₃) .

Fig. 4b shows Raman spectra of the BTO surface. Fig. 4c shows Raman depth scan image of the BTO cross-section. Both, Fig. 4b and Fig. 4c, reveal the presence of ferroelectric domain structures. Fig. 4d shows the average Raman spectra obtained in two adjacent domains. (7), (8) and (9) points of Fig. 4b correspond to the a-domain and c-domains, respectively, which are separated by a 90° domain wall (a-c-domain wall) . The details of the Raman active phonons of the BTO single crystal are magnified in inserts in Fig. 4d, which were assigned to E (T0₄) and Al (TO₃) Raman modes, respectively. These two bands are located at around to 480 and 520 cm^-1, and they are signaled as (4) and (5) in Fig. 4d.

There is a clear correlation between c-domain (out-of-plane polarizations) assignation by CRM and the protrusion evidenced by AFM, as well as between a-domain (in-plane polarization) assignations by CRM with the depletion determined by AFM. The dimensional variation is in good agreement with the distortion of the tetragonal structure in which c/a >1.18 However, the most unexpected result obtained from the Raman image of the surface was the appearance of a new type of domains which are represented in the Fig. 4b. This unusual continuous domain boundary can be unambiguously identified by CRM imaging at a-c- domain wall. It is located at the step detected from the topographical analysis by AFM, such as evidenced in Fig. 3d. Here we use the term "b-domain" (8) to refer thereafter to the domain boundaries for the sake of simplicity.

In order to clarify the complex nature of the b-domain, a depth scan Raman image of the cross section of BTO was carried out. As a relevant result, Fig. 4c, more than one b-domain was observed that hindered only the a-c-domain wall. There is a set of b-domains parallel between them that growth in the c-domain region (see the Fig. 6 for a detailed configuration of domain boundaries at the cross-section) . These b-domains are absent in the c-a-domain walls. The Raman spectrum of the b-domain (Fig 4d) is a clear combination between a-domain and c-domain Raman spectra .

The phenomenology model based on the formation of the b- domain structure in BTO described here is schematically represented as shown in Fig. 5. Said scheme is built by combining the AFM and Raman mapping information. The complex domain structure is composed of a-domain, c-domains and b-domain that appear in the a-c-domain wall within the plane ({101}pc) that slopes 20° to the top surface. The a-c-domain wall it associated with a head-to-head 90° domain wall where the mechanical stress is enhanced and it is envisaged that the head- to-head configuration maximizes the bound charge at close to the domain wall. The b-domains lead to minimize the internal stress by increasing the domain wall density, as illustrated in the insert of the Fig. 5.

In situ ferroelectric domain switching under applied light source

In situ observations of polarization switching was carried out by application of polarized light parallel to the BTO crystal surface along the depth scan direction, denoted as θ=0° and marked as X axis, and perpendicular to depth scan direction, denoted as 9=90° and inscribed as Y axis. In all cases the light incidence was normal to the sample surface.

In the above conditions the polarized light was parallel to the polarization direction of the a-domain for 9=0° and it was perpendicular such polarization direction for 9=90°, but remained in the in-plane polarization of the a-domain. In the case of c-domain the polarized light was always perpendicular to the out-of-plane polarization of the material. For the b-domain, it is expected a combination of both behaviors as a result of the bundle of a-c-subdomains .

Figs. 6 (a-g) shows a sequential depth scan Raman images in which ferroelectric domains move along the X axis for both the b-domain and the c-domains. The 10→11 and 12→13 lines serve a reference guide to follows the domain moving. When the polarized light reaches 9=90°, the Raman spectrum of the c-domain (9) evolves toward b-domain (8) (Figs. 6 (a-g) ) that account for a structural change. The Raman shift is an indicator of the crystal stress and correlated with tetragonality and polarization .

Fig. 6 (h-n) displays depth scans Raman shift images related shift of the Al (T0₂) Raman mode for the different light polarization angle. The Al (T0₂) Raman active phonon is a symmetrical mode, which is detected as relatively strong scattering signals in BTO because of a near-perfect equilateral octahedral symmetry. Raman shifts as large of 15 cm^-1 (from 205 cm^-1 to 220 cm^-1) were observed in the complex domain structure. The higher Raman shift observed corresponds to c-domain near the a-c-domain wall. This fact implies a higher tetragonality in this region and as a consequence a higher off centering of the Ti-cation that it is traduced in higher polarizability . The surprising increase of polarizability could be explained by the presence of local charge accumulation at the 90° domain wall. The higher Raman shift is thus an evidence of the enhancement of the local electric field provided by the head-to-head a-c-domain wall. Besides, the local electrical field also competes with the compressive stress. By the contrary the b-domain (point inscribed as (18) in Fig. 6h) represents the redshift regions as a consequence of the subdomain structure in which higher domain density reduced the stress and also the tetragonality.

The Raman shift evolved with the light polarization angle with a translation of the domain structure as it is stated above. The a-c-domain wall moves and the region with local charge accumulation moves accordingly, this region undergo redshift with the light angle. The redshift seems to be more relevant for θ>45°. The most relevant redshift occurs in a- domain that indicates also a stress relief or in some extent a structural modification of the crystal lattice. As a whole the domain structure promotes a polarization reduction by the effect of the polarized light that originated the domain moving. It must be taken into account that the light is always in-plane polarized. The light polarization is parallel at θ=0° but perpendicular at 9=90° to the a-domain polarization. The redshift variations denote a coupling between the domain orientation and the coherent light polarization.

Fig. 6o summarizes the extent to which the domain position is affected by the light polarization angle. This relative motion is illustrated by using as reference the lineS marked as 10 → 11 and 12 —> 13 in Fig. 6 (a-g) . This analysis shows that the relative motion of the domains is approximately 2.16 ±0.09 μιτι when the light polarization changes from 0 to 90°. Accordingly, with the light polarization angle the relative domain displacement experience two different regimes. From 45- 90° the relative motion approximately triplicate the one observed between 0-45°, Fig. 6 (d-g) . In addition, there is a progressive change of the c-domain nature that reaches the top of the single crystal surface that become b-domain in nature. The previous experiments are attempted in two ways: i) the light polarization angle increase in steps from 0°—>90° and thereafter from 90°—>0° , and ii) the light polarization angle randomly switched from the selected steps. In all the cases the observation of domains is reversible and the relative motion remains under the error bars represented in the Fig. 6o .

Fig. 6o also represents the Raman shift for three main points in the complex structure representing a-domain, c-domain and b-domain, which are inscribed as (14), (18) and (16) in Figs. 6h and o. The three representative Raman points experiment a redshift for the Al (T0₂) Raman mode. When the polarization light rotates 45-90°, the redshift is three time higher than in 0°-45° range. This fact clearly shows a correlation between the domain wall relative motion and the structural changes. The point (16) (c-domain near of a-c-domain wall) presents a higher stress and higher tetragonality region that it is less sensitive to the light polarization angle. The presence of a local electric field or stress concentration effects seem to be more relevant than the out-of-plane polarization in the c-domain region near of the a-c-domain wall.

The occurrence of the domain motion with polarization light angle is related to the energy provides by the polarized light coupled with the crystal structure. To analyze such coupling an evolution of surface a-domain, b-domain and c-domain as a function of the light polarization angle is attempted, Figure 7. The Raman spectra modified with the increasing of light polarization angle in two main aspects:

i) higher intensity in the low wave number region (< 200 cm^-1) as well appearance of new Raman modes (Fig. 7d) ;

ii) larger asymmetry for the a-domains spectra, as shown in the insert of the Fig. 7a.

Example 2: K₀.₅Na₀.₅Nb0₃ (KN )

Sample Preparation

The K₀.₅ a₀.₅ bO₃ (KNN) ceramic was prepared by microwave- hydrothermal synthesis. The raw materials used were potassium hydroxide (KOH, 99.0%), sodium hydroxide (NaOH, 99.5%), niobium oxide (Nb205, 99.99%) and cetyltrimethylammonium bromide (CTAB) . First, the mixture solutions of KOH, and NaOH with a ratio K⁺/(K⁺+Na⁺)= 0.6 were adjusted to concentration 8N. Then, 100 ml of solution was poured in a Teflon lined autoclave joint lg of Nb205, joint the additive (CTAB = 0.04 g) . This solution was heated and mixed to the desired temperature. Hydrothermal treatments were carried out at 200°C for 3h. The precipitate was washed three times with deionized water and dried at 120°C for two hours. With the aim of preparing a sintered sample, the dried powders were uniaxially pressed under 300 MPa in a disk- shaped sample. This sample was sintered at 1125°C for 2 h in air, obtaining a density of -4.40 g/cm3.

KNN samples were polished and thermal etching was used to reveal the domain structure. With this aim, the surface of the pellets was carefully polished in two steps. The first step consisted in a "hard polished" or grinding process using Silicon carbide abrasive papers (MetaServ® 250 Grinder-Polisher, BUEHLER, An ITW Company) . This first step was carried out for each pellet to obtain parallel surfaces. Finally in a second step, the surface of the pellet was softly polished with diamond paste to obtain mirror finish surfaces using a VibroMet® 2 Vibratory Polisher (BUEHLER, An ITW Company) . In consequence, the micro-roughness is inhibited resulting in an improvement of the sample surface finish. Finally, the KNN ceramic sample was thermal etching at 950 °C for 5 min.

Mapping of the domain structure by CRM

The characterization experiments of the ferroelectric domains of KNN by CRM were performed using a Confocal Raman Microscope (Witec alpha-300R) . Raman spectra were obtained using a frequency-doubled NdYAG laser operating at 532 nm and a 100X objective lens (NA = 0.9) . The incident laser power was 20 mW and the integration time 0.2 seconds. The optical diffraction resolution of the Confocal Microscope was limited to about -250 nm laterally and -500 nm vertically. Raman spectral resolution of the system was down to 0.02 cm^-1. The microscopy sample was mounted on a piezo-driven scan platform having 4 nm lateral and 0.5 mm vertical positional accuracy. The microscope base was also fitted with an active vibration isolation system, active 0.7-1000 Hz. Collected spectra were analysed by using Witec Control Plus Software.

Figure 8a shows an optical micrograph of the polished surface of the ceramic aligned perpendicular to the Raman laser. The area of 25 χ 25 μιτι denotes the selected area (marked black box) where the Raman spectra were collected at a plane located just below the surface of the sample where the Raman intensity was maximized. The acquisition time for a single Raman spectrum was 200 milliseconds, thus the acquisition of a Raman image consisting of 100 x 100 pixels (10000 spectra) required 33 minutes. Features such as Raman peak intensity, peak width or Raman shift from the recorded Raman spectra were fitted with algorithms to compare information and to represent the derived Raman image, Fig. 8b. The vibrations of the B06 octahedron consist of lAlg (vl) + lEg (v2) +2Flu (v2, v4) + F2g (v5) + F2u (v6) modes. Of these vibrations, lAlg (vl) + lEg (v2) +1F1U (v3) are stretching modes and the rest, bending modes. In particular, Alg (vl) symmetrical mode and F2g (v5) antisymmetric mode are detected as relatively strong scattering signals in KNN based materials because of a near-perfect equilateral octahedral symmetry .

Raman spectra having same Raman shift are classified by correlating the grey intensity with the Raman intensity. The combination of grey scale results in an image of the ceramic microstructure which reveals the presence of a ceramic grain with striped ferroelectric domains. This Raman image provides a scenario to study the domain structure in polycrystalline samples synthesized by hydrothermal method.

Fig. 8c shows magnified Raman image of the domain distribution at the surface scan, the area marked on the white box. This measurement shows that there are relevant differences related to the polarization orientation in the domains of each grain corresponding to alternate in plane and out of plane domains, and as a consequence the existence of 90° domain walls between adjacent striped or lamella regions. In principle the adjacent domains should be separated by a 90° domain wall.

The ferroelectricity is a cooperative phenomenon and oriented larger domains are required to get a net higher polarization. The formation of domains walls reduces the total polarization. Piezoelectric ceramics required to be consolidated at high temperatures by means of the sintering process. The randomly oriented polycrystals of the paraelectric phase suffer a crystal lattice distortion when cooling below Tc. The grain boundaries impose a difficulty to accommodate the stress resulted from this lattice distortion and as a consequence the domain structure is formed. The smaller the grain size, the smaller the domain is and as a consequence the domain density increases as a way to compensate the higher stress originated in the ceramics.

In situ ferroelectric domain switching using polarized light source .

In situ observations of polarization switching were carried out by application of polarized light. In all cases the light incidence is normal to the sample surface and the polarization direction of light parallel to the surface. Fig. 9 (a-g) shows a sequential Raman image on the surface of the KNN ceramics in which ferroelectric domains move along the X axis. The 22→23 line serves a reference guide to follows the domain moving. From Fig. 9 (a-g) , one can deduce that the light polarization angle alters the Raman modes, the Raman spectrum of the red-domain evolves toward blue-domain (Figs. 9 (a-g) ) that account for a structural change.

In order to further determine the stress degree associated with the domain moving, the Raman shift image of the A_lg Raman mode for each light polarization angle is also displayed in Fig. 9 (h-n) . So, the Raman shift evolved with the light polarization angle with a translation of the domain structure such as it is evidenced from Fig. 9 (h-n) . Considering the results of Raman shift image, it is deduced that this fact clearly shows a correlation between the domain wall relative motion and the structural changes, as shown Fig. 9 (a-g) .

Finally, Fig. 9o summarizes the extent to which the domain position is affected by the light polarization angle. This relative motion is illustrated by using as reference the line marked as 22 —> 23 in Fig. 9 (a-g) and Fig. 9 (h-n) , respectively. This analysis shows that the relative motion of the domains is approximately 780 ± 90 nm when the light polarization changes from 0 to 90° .

Example 3 : Bi₄Ti₃0i₂

Sample Preparation

The Bi₄Ti₃0i₂ ceramic having Aurivillius structure was prepared by solid state reaction. Rutile Ti0₂ (Alfa Aesar, mean particle size 1.04 μιτι) and a-Bi₂0₃ (Aldrich, particle size 11.89 μιτι) with 99.9% purity were used as starting materials. The appropriate stoichiometric amounts of the starting materials were mixed in an attrition mill with 1.2 mm Zr0₂ balls in water for 3 hours. A 0.6% wt . of T5003 Rohm&Haas dispersant was added to improve homogenization . The powders were overnight dryed at 75 °C and sieved through a 0.1 mm mesh and calcined at different temperatures, 600 °C and 800 °C, for 2 hours with constant heating and cooling rates of 3 °C/min. The calcined powders were again attrition milled on water adding a 0.6%wt. of dispersant T5003 Rohm&Haas and dispersed by a high shear Ika Ultraturraz T50 for 10 minutes at 4000 rpm. The powders have been dryed again and shieved through a 0.1 mm mesh. The resultant powder were pressed at 200 MPa and sinterized at 1100 °C during 2 h.

BIT samples were polished. To this aim, the surface of the pellets was carefully polished in two steps. The first step consisted in a "hard polished" or grinding process using Silicon carbide abrasive papers (MetaServ® 250 Grinder-Polisher, BUEHLER, An ITW Company) . This first step was carried out for each pellet to obtain parallel surfaces. Finally in a second step, the surface of the pellet was softly polished with diamond paste to obtain mirror finish surfaces using a VibroMet® 2 Vibratory Polisher (BUEHLER, An ITW Company) . As a result, the micro-roughness is inhibited resulting in an improvement of the sample surface finish.

Mapping of the domain structure by CRM

The observation of the domains was carried out by a confocal Raman microscope (CRM) coupled to an AFM (Witec alpha- 300R) . Raman spectra were obtained using a frequency-doubled NdYAG laser operating at 532 nm and a 100X objective lens (NA = 0.9) . The incident laser power was 1.5 mW and the integration time 0.2 seconds. The optical diffraction resolution of the confocal microscope was limited to about ~200 nm laterally and ~500 nm vertically. Raman spectral resolution of the system was down to 0.02 cm^-1. The microscopy sample was mounted in a piezo- driven scan platform having 4 nm lateral and 0.5 nm vertical positioning accuracy. The piezoelectric scanning table allows steps of 3 nanometers (0.3 nm in vertical direction), giving a very high spatial resolution for the confocal Raman microscopy. The microscope base was also equipped with an active vibration isolation system, active 0.7-1000 Hz. The system allows studying the same area of the sample by selecting the adequate objective of the microscope. Collected spectra were analyzed by using Witec Control Plus software. The domains and stresses can be study in detail by Confocal Raman Microscopy (CRM) . When polarized laser light is used, this technique allows to distinguish different grain orientations in a ceramic and to relate them with the domains. This is an advantage of the use of CRM for the study of ferroelectric domains with respect to other techniques, such as SEM.

Figure 10a shows an optical micrograph of the polished surface of the Bi₄Ti₃0i₂ ceramic aligned perpendicular to the Raman laser. The area of 20 ^χ 20 μιτι denotes the selected area (square remark on the panel a, which is marked as 24) where the Raman spectra were collected at a plane located just below the surface of the sample where the Raman intensity was maximized. The acquisition time for a single Raman spectrum was 500 milliseconds, thus the acquisition of a Raman image consisting of 100 x 100 pixels (10000 spectra) required 83 minutes. Features such as Raman peak intensity, peak width or Raman shift from the recorded Raman spectra were fitted with algorithms to compare information and to represent the derived Raman image, Fig. 10b.

Raman spectra having same Raman shift are classified by correlating the color intensity with the Raman intensity. The combination of color results in an image of the ceramic microstructure which reveals the presence of ceramic grains with striped ferroelectric domains. This Raman image provides different scenarios to study the domain structure in polycrystalline samples synthesized by solid state reaction. This measurement shows that there are relevant differences related to the polarization orientation in the domains inside a Bi₄Ti₃0i₂ grain. Accordingly, the grain showed a domain structure that corresponds to a sequence of alternate in plane and out of plane domains, and as a consequence the existence of 90° domain walls between adjacent striped or lamella regions. In principle the adjacent domains should be separated by a 90° domain wall.

In situ Β±₄Τ±₃0₁₂ -ferroelectric domain switching using polarized light source .

The in situ Bi₄Ti₃0i₂ domain switching was monitored taking subsequent surface-scans at different light polarization angles of the same area, using as position reference a significant point of the sample surface visible with the optical microscope. Surface scan Raman images had 15 μιτι of length, 3 μτη of width, 60 x 12 spectra of 1500 milliseconds of integration time at 1.5 mW of laser power. Thus, the acquisition of a Raman image required 18 minutes.

In situ observations of polarization switching were carried out by application of polarized light on the area marked as number 25 of the Fig. 10b. In all cases the light incidence is normal to the sample surface and the polarization direction of light parallel to the surface. Fig. lOc-i shows a sequential Raman image on the surface of the Bi₄Ti₃0i₂ ceramics in which ferroelectric domains move along the XY plane. The pore signaled as (26) and 27→28 line serve as reference guide to follows the domain moving.

To sum up, the relative motion is illustrated by using as reference the lines marked as 27 —> 28 in Fig. lOc-i. This analysis reveals again that the relative motion of the domains is approximately 4.5 ± 0.5 μιτι when the light polarization changes from 0 to 90°.

Claims

1. - A method for switching the polarization of a ferroelectric material comprising applying a polarized coherent light at the surface of the ferroelectric material at a light polarization angle Θ, which is the angle at the plane perpendicular to the propagation direction of the light and parallel to the plane of the surface of the ferroelectric material, and, while said polarized coherent light is being applied, varying the light polarization angle Θ in the plane parallel to the surface of the ferroelectric material.

2. - The method according to claim 1, wherein the ferroelectric material is a single-crystal material or a poly-crystalline material .

3. - The method according to any of claims 1 to 2, wherein the ferroelectric material is a thin-film layer.

4. - The method according to any of claims 1 to 3, wherein the ferroelectric material is a ceramic.

5. - The method according to claim 2 to 4, wherein the ferroelectric material has a structure selected from perovskite- type structure, ilmenite-type structure, tungsten-bronze-type structure, Aurivillius-type structure, potassium dihydrogen phosphate-type structure, potassium titanyl phosphate system- type structure, and oxygen tetrahedral-type structure

6. - The method according to claim 5, wherein the ferroelectric material is selected from BaTi0₃, (K, Na) b0₃-based materials and

7. - The method according to any of claims 1 to 6, wherein the coherent light is a laser light.

8. - The method according to any of claims 1 to 7, wherein the polarized coherent light is absorbed by the surface of the ferroelectric material in a dose equal or less than 1 Jem^-2 .

9. - The method according to any of claims 1 to 8, wherein the polarized coherent light emits at a wavelength from around 213 nm to around 2900 nm.

10. - The method according to any of claims 1 to 9, wherein the surface of the ferroelectric material is a layer with a thickness equal or less than 20 μιτι.

11. - The method according to any of claims 1 to 10, wherein the surface of the ferroelectric material is polished, thermally etched, chemically etched or combinations thereof.

12. - The method according to claim 11, wherein the surface of the ferroelectric material is at least polished.

13. - The method according to any of claims 1 to 12, wherein the light polarization angle Θ varies in such a way that the rate of relative motion of a ferroelectric domain wall (R_RM) is between around 4 nm per unit of degree and around 50 nm per unit of degree.

14. - A method for writing data in a data storage device comprising switching the polarization of a ferroelectric material according to the method of any of claims 1 to 13.

15. - A data storage device comprising a ferroelectric material, and characterized in that the stored data is written by switching the polarization of the ferroelectric material according to the method of any of claims 1 to 13.