CN112420709A

CN112420709A - Conversion of PbTiO3/SrTiO3Method of vortex domain of superlattice material

Info

Publication number: CN112420709A
Application number: CN201910785451.3A
Authority: CN
Inventors: 陈潘; 白雪冬
Original assignee: Institute of Physics of CAS
Current assignee: Institute of Physics of CAS
Priority date: 2019-08-23
Filing date: 2019-08-23
Publication date: 2021-02-26
Anticipated expiration: 2039-08-23
Also published as: CN112420709B

Abstract

Transition PbTiO₃/SrTiO₃A method of providing a vortex domain of a superlattice material, comprising the steps of: for PbTiO with vortex domain₃/SrTiO₃The superlattice material exerts a force with a pressure of 0.4-1.0GPa, and the PbTiO is₃/SrTiO₃The vortex domain of the superlattice material becomes an in-plane domain; removing the force, the PbTiO₃/SrTiO₃The in-plane domains of the superlattice material revert back to vortex domains. The invention realizes the mechanical transformation of the vortex domain which is difficult to observe by common experimental means for the first time, and determines that the domain structure after the transformation of the vortex domain is the in-plane domain. PbTiO 2₃/SrTiO₃The super lattice material has small size, high density and regular arrangement of vortex domain structure and can be converted into in-plane domain exclusively under the action of external force,PbTiO₃/SrTiO₃superlattice materials are ideal memory cells.

Description

Conversion of PbTiO3/SrTiO3Method of vortex domain of superlattice material

Technical Field

The invention belongs to the field of ferroelectric storage. In particular, the invention relates to converting PbTiO₃/SrTiO₃A method of swirling domains of a superlattice material.

Background

As the demand for memory capacity is higher and higher due to the development of human society and the progress of technology, the conventional field effect transistor or magnetic memory capacity has been approaching the limit, and new high capacity memory materials are urgently required. The ferroelectric memory has the characteristics of high speed and low power consumption, and therefore, the ferroelectric memory is also paid more and more attention and researched.

The ferroelectric vortex domain is a structure in which electric polarization vectors continuously rotate to form vortex, is different from the traditional ferroelectric domain, has the size of only about 4 nanometers, and can improve the capacity of the existing ferroelectric storage by about 5 orders of magnitude when used for storage, so that the capacity reaches 48 multiplied by 10¹²Bits per square inch.

For ferroelectric vortex domains, the response of the vortex domain under an external field must be known. Computer storage involves two processes, first logging data, which is related to the binary of the computer, thus requiring two states, corresponding to the '0' and '1' of the computer. The vortex domain can be used as one state, and the vortex domain is converted into an in-plane domain under the action of a force field and can be used as another state, so that the basic condition of storing data is realized. Another process is reading data, and since the vortex domain and the transformed in-plane domain respond differently to the optical signal, for example, the second harmonic signal of the vortex domain is lower than that of the in-plane domain, the stored state can be read out as '0' or '1' by the optical signal, and the stored data state can be known.

Because the size of the vortex domain is too small, the general experimental means such as X-ray are difficult to accurately study the overturning process, and the existing research only stays at the theoretical stage. The dynamics of the vortex domain are still very deficient experimentally, such as whether the vortex domain can change under an external field, how it changes, what the changed structure is, and these information are very important for realizing storage. The invention adopts the means of an in-situ electron microscope to realize and detect the process of converting the vortex domain into the in-plane domain.

Disclosure of Invention

The invention aims to provide a transition PbTiO₃/SrTiO₃A method of swirling domains of a superlattice material.

The above object of the present invention is achieved by the following means.

In the context of the present invention, the term "PbTiO₃/SrTiO₃The "cycle period of the superlattice material" means that a layer of PbTiO is used₃And a layer of SrTiO₃Cycling is performed for one period.

The invention provides a transition PbTiO₃/SrTiO₃A method of providing a vortex domain of a superlattice material, comprising the steps of: for PbTiO with vortex domain₃/SrTiO₃The superlattice material exerts a force with a pressure of 0.4-1.0GPa, and the PbTiO is₃/SrTiO₃The vortex domain of the superlattice material becomes an in-plane domain; removing the force, the PbTiO₃/SrTiO₃The in-plane domains of the superlattice material revert back to vortex domains.

Preferably, in the method of the invention, the pressure is between 0.4 and 0.6 GPa.

In the method, if the pressure is less than 0.4GPa, the vortex domain can not be transformed; if the pressure is greater than 1.0GPa, the film is damaged and cannot be recovered.

Preferably, in the method of the present invention, the PbTiO is₃/SrTiO₃PbTiO in superlattice material₃The thickness of (A) is 10-16 PbTiO₃The thickness of the unit cell; the PbTiO is₃/SrTiO₃SrTiO in superlattice materials₃Has a thickness of 10-16 SrTiO₃Thickness of the unit cell.

Preferably, in the method of the present invention, the PbTiO is₃/SrTiO₃The cycle period of the superlattice material is 10-16.

Preferably, in the method of the present invention, the PbTiO is₃/SrTiO₃The superlattice material is prepared by a method comprising the following steps of: depositing on DyScO by using pulsed laser₃Growing SrRuO on the substrate₃As an electrode, sequentially growing PbTiO at intervals₃And SrTiO₃。

Preferably, in the method of the present invention, the SrRuO₃Has a thickness of 30-50 SrRuO₃Thickness of the unit cell.

Preferably, in the method according to the invention, the application of a force at a pressure of 0.4-1.0GPa is performed by a method comprising the steps of:

(1) adding PbTiO₃/SrTiO₃Preparing a section electron microscope sample from the superlattice material, and mounting the sample on one end of a sample rod of a transmission electron microscope;

(2) and a metal tungsten needle point is arranged on a slide block head at the other end of the sample rod as a mechanical pressure head, and the tungsten needle point is moved in the transmission electron microscope to apply a force of 0.4-1.0GPa to the sample.

Preferably, in the method of the present invention, the tungsten tip is pretreated by electrochemical etching.

Preferably, in the method of the present invention, the step (1) of preparing the cross-sectional electron microscope sample is performed by a method including the following steps:

adding the PbTiO₃/SrTiO₃Bonding the superlattice material with a silicon wafer, heating and curing, and cutting into pieces with thickness of 0.75-1.2 mm; and grinding the silicon wafer to a thickness of 30-50 μm, fixing the silicon wafer on a molybdenum ring, moving the silicon wafer up to an ion thinning instrument, and thinning the silicon wafer.

The principle of the invention is as follows:

PbTiO₃/SrTiO₃the formation of vortex domains in the superlattice is a result of mutual competition among elastic performance, electrostatic energy and gradient energy in the system to achieve balance. Under the action of an external force field, the balance relation of the three energies is necessarily broken, and the vortex domain cannot stably exist under the condition and can be converted. In the method of the invention, the force field is applied by a needle tip, the needle tip is precisely controlled by a control system, the system controls the distance the needle tip moves so as to control the magnitude of the applied force, and the transition of the vortex domain is characterized by a transmission electron microscope. The invention obtains the transformation process of the vortex domain under the action of the force field by means of the transmission electron microscope, and determines the transformed domain structure by utilizing the atomic resolution of the transmission electron microscope.

The invention has the beneficial effects that:

the invention realizes the mechanical transformation of the vortex domain which is difficult to observe by common experimental means for the first time, and determines that the domain structure after the transformation of the vortex domain is the in-plane domain. PbTiO 2₃/SrTiO₃The vortex domain structure of the superlattice material has small size, large density and regular arrangement, and can be uniquely converted into an in-plane domain under the action of external force,thus, PbTiO₃/SrTiO₃Superlattice materials are ideal memory cells. Therefore, the method has great application prospect in the industry and can be used as a new high-density storage device.

Drawings

Embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 shows PbTiO of the present invention₃/SrTiO₃A vortex domain mechanical transformation diagram of the superlattice material;

FIG. 2(a) in FIG. 2 is PbTiO prepared in example 1 of the present invention₃/SrTiO₃A low power high angle annular dark field image of the superlattice material; FIG. 2(b) is a high power high angle annular dark field image; FIG. 2(c) is a dark field image and FIG. 2(d) is an electron diffraction image;

FIG. 3 shows PbTiO prepared in example 1 of the present invention₃/SrTiO₃A timing diagram of a vortex domain mechanical transition of the superlattice material;

fig. 4(a) in fig. 4 is a graph of the magnitude of the applied force in embodiment 1 of the present invention; FIG. 4(b) shows PbTiO prepared in example 1 of the present invention₃/SrTiO₃A timing diagram of a vortex domain mechanical transition of the superlattice material;

FIG. 5(a) in FIG. 5 shows PbTiO prepared in example 1 of the present invention₃/SrTiO₃Electron diffraction patterns of superlattice materials with increasing force; FIG. 5(b) is a lattice parameter plot;

FIG. 6(a) in FIG. 6 is a PbTiO prepared according to example 1 of the present invention₃/SrTiO₃High resolution images of superlattice materials with increasing forces; FIG. 6(b) is an inverse Fourier transform of FIG. 6 (a);

FIG. 7(a) in FIG. 7 is PbTiO prepared in example 1 of the present invention₃/SrTiO₃Macroscopic atomic images of superlattice materials; FIG. 7(b) is a corresponding stress profile of FIG. 7 (a); fig. 7(c) shows a high-magnification atomic image of a region converted into an in-plane domain.

Detailed Description

The present invention is described in further detail below with reference to specific embodiments, which are given for the purpose of illustration only and are not intended to limit the scope of the invention.

The process of the present invention is schematically illustrated in FIG. 1. In FIG. 1, the black and gray interlayers represent PbTiO, respectively₃And SrTiO₃And (3) a layer. In PbTiO₃There are vortex domains in the layer that are aligned between clockwise and counterclockwise rotation. When the tungsten needle tip is used for applying pressure to the film, the stress is strongest at the position close to the needle tip due to the stress gradient distribution, and the stress far away from the needle tip is weakened, so that the vortex domain is converted after the stress reaches a certain strength.

Example 1

1) PbTiO growth by pulsed laser deposition₃/SrTiO₃A superlattice thin film. The substrate used is DyScO₃First growing SrRuO₃As a metal electrode, the energy of the laser at this time was 400mJ, the frequency was 10Hz, the growth temperature was 690 degrees, and the oxygen pressure was 80 mTorr. Then sequentially growing 11 single-cell PbTiO at intervals₃And SrTiO₃At the moment, the energy of the laser is 360mJ, the frequency is 10Hz, the growth temperature is 600 ℃, the oxygen pressure is 200mTorr, and the growth is stopped after 15 periods;

2) adhering the obtained sample and a silicon wafer by using M-bond 610 glue, cutting into a thickness of 1mm after heating and curing for half an hour, grinding the silicon wafer on sand paper by using a sample preparation tool, and grinding the silicon wafer to a thickness of 30 mu M, wherein the silicon wafer transmits red light; then fixing the sample on a molybdenum ring, moving the sample to an ion thinning instrument for thinning, wherein the thinning voltage is 3KeV, the angle of a gun is 6 degrees, and after thinning and punching, the angle of the gun is changed into 4 degrees. Obtaining a cross-section sample for a transmission electron microscope after successful thinning;

3) placing the obtained cross section sample at one end of an in-situ sample rod, and placing a tungsten needle point subjected to electrochemical corrosion on a sliding block head at the other end of the sample rod to serve as a mechanical probe;

4) in a transmission electron microscope, adjusting to a dark field mode, and acquiring a dark field image by a camera; by (200)_pcImaging diffraction spots, namely clearly seeing the contrast of a vortex domain, moving a tungsten needle point to apply pressure to a sample, wherein the vortex domain transformation area is larger and larger along with the increase of the pressure; wherein, the lower corner mark pc is a pseudo cubic structure mark;

5) repeating the step 4) by using a sample rod capable of quantitatively measuring the force to obtain the quantitative force;

6) adjusting the transmission electron microscope to a diffraction mode, moving the tungsten needle point to apply pressure, obtaining an electron diffraction image by a camera, and recording the process of applying force;

7) adjusting the transmission electron microscope to a high-resolution mode, clearly seeing the lattice image, moving the tungsten needle point to apply pressure, obtaining a high-resolution image by a camera, and recording the vortex domain conversion process;

8) the transmission electron microscope is adjusted to a scanning mode, the atomic image can be clearly seen, the tungsten needle point is moved to apply pressure, and the camera obtains the atomic image.

Characterization and testing

FIG. 2(a) in FIG. 2 is PbTiO prepared in this example₃/SrTiO₃A low power high angle annular dark field image of the superlattice material. The white arrows in fig. 2(a) indicate the atom displacements, which are calculated according to the Matlab program written. The white arrows in fig. 2(a) show the clockwise and counterclockwise aligned vortex structures. Fig. 2(b) is a high power high angle annular dark field image. FIG. 2(c) shows a dark field image, and (002) in FIG. 2(d) is selected as the diffraction pattern_pcThe vectors are arranged regularly with light and dark in the figure and contrast of vortex domains. Fig. 2(d) shows an electron diffraction image. Wherein PbTiO grown in the step (1)₃/SrTiO₃The superlattice thin film is shown in fig. 2 (a). FIG. 2(a) shows that the contrast is relatively bright PbTiO₃Layer, relatively dark, of SrTiO₃And (3) a layer. The structure of the vortex domains is clearly shown in fig. 2(b) of fig. 2, the white arrows indicate the direction of the electric polarisation, and the arrows constitute a continuously rotating vortex structure and are arranged clockwise and counter-clockwise.

FIG. 3 shows PbTiO prepared in this example₃/SrTiO₃Timing diagrams of the vortex domain mechanical transitions of superlattice materials. Fig. 3 shows that the contrast of the particles is a vortex domain, and the contrast of the particles is transformed into a uniform contrast after pressure is applied. This means that the vortex domains are transformed. As the pressure increases, the transition area becomes larger and larger, and the vortex domain recovers upon removal of the pressure.

Fig. 4 is a graph showing the magnitude of the force applied in the present embodiment. Fig. 4(a) shows the magnitude of the applied pressure.

FIG. 5(a) in FIG. 5 shows PbTiO prepared in this example₃/SrTiO₃Electron diffraction patterns of superlattice materials are obtained with increasing force. FIG. 5 is a photograph taken during the application of force (002)_pcA diffraction image.

Fig. 5(b) is a lattice parameter diagram. The diffraction spots of the superlattice and the diffraction spots of the vortex domains on both sides of the superlattice are clearly visible in the initial state. The intensity of the diffraction spots belonging to the vortex domains becomes weaker as the force field increases, and the weakening of the intensity indicates that the vortex domains are transformed by the force field, since the intensity of the diffraction spots is proportional to the number of the vortex domains. As the force field continues to increase, the vortex domain diffraction spots disappear, indicating that a majority of the vortex domains in the region have transitioned. From the diffraction spots, the changes in-plane and out-of-plane lattice constants were calculated simultaneously, and it was found that the out-of-plane lattice constant decreased, eventually leading to a c/a ratio less than 1, indicating that the vortex domain was finally transformed into an in-plane domain.

FIG. 6(a) in FIG. 6 PbTiO prepared in this example₃/SrTiO₃The superlattice material yields high resolution images with increasing force. As shown in fig. 6(a), the contrast in the high resolution image becomes more and more uniform with increasing pressure, indicating that the vortex domains are transformed into a uniform structure. Fig. 6(b) is an inverse fourier transform diagram of fig. 6 (a). Fig. 6(b) shows a clear grain contrast, which is one-to-one corresponding to the number of vortex domains, and as the pressure increases, the number of grains decreases until disappearing, meaning a change in the number of vortex domains.

FIG. 7(a) in FIG. 7 is PbTiO prepared in this example₃/SrTiO₃Macroscopic atomic images of the superlattice material. Between the four images in FIG. 7(a) are scanned images obtained by moving the needle tip forward by the same length. Fig. 7(b) is a corresponding stress distribution diagram of fig. 7 (a). Fig. 7(b) shows an initial state, and the stress distribution exhibits a sine wave shape, which corresponds to the vortex domain. As stress is applied, the region under the tip transforms into uniform stress, meaning that the vortex domain transforms, the stress increases, and the area of the transformed region increases. FIG. 7(c) is a transition to a surfaceHigh power atomic image of the region of the inner domain. FIG. 7(c) is a high-resolution atomic image taken in the transition region, and displacement polarization vectors calculated by the program are white arrows which are uniformly directed in the plane, illustrating PbTiO₃/SrTiO₃The vortex domains of the superlattice material are converted into in-plane domains.

Claims

1. Transition PbTiO₃/SrTiO₃A method of providing a vortex domain of a superlattice material, comprising the steps of: for PbTiO with vortex domain₃/SrTiO₃The superlattice material exerts a force with a pressure of 0.4-1.0GPa, and the PbTiO is₃/SrTiO₃The vortex domain of the superlattice material becomes an in-plane domain; removing the force, the PbTiO₃/SrTiO₃The in-plane domains of the superlattice material revert back to vortex domains.

2. The method of claim 1, wherein the pressure is 0.4-0.6 GPa.

3. The method of claim 1, wherein the PbTiO is₃/SrTiO₃PbTiO in superlattice material₃The thickness of (A) is 10-16 PbTiO₃The thickness of the unit cell; the PbTiO is₃/SrTiO₃SrTiO in superlattice materials₃Has a thickness of 10-16 SrTiO₃Thickness of the unit cell.

4. The method of claim 1, wherein the PbTiO is₃/SrTiO₃The cycle period of the superlattice material is 10-15.

5. The method of claim 1, wherein the PbTiO is₃/SrTiO₃The superlattice material is prepared by a method comprising the following steps of:

depositing on DyScO by using pulsed laser₃Growing SrRuO on the substrate₃As an electrode, sequentially growing PbTiO at intervals₃And SrTiO₃。

6. The method of claim 5, wherein the SrRuO₃Has a thickness of 30-50 SrRuO₃Thickness of the unit cell.

7. A method according to claim 1, wherein applying a force at a pressure of 0.4-1.0GPa is performed by a method comprising:

8. The method of claim 7, wherein the tungsten tip is pre-treated with electrochemical etching.

9. The method of claim 7, wherein the step (1) of producing a cross-sectional electron microscope sample is performed by a method comprising: