FR2846459A1 - Method of realization of magnetic nanostructure with improved dynamic performance for high speed read heads, uses irradiation or ion implantation to create region in nanostructure with different magnetic properties to its surroundings - Google Patents

Abstract

The magnetic nanostructure (6) induces different magnetic properties in adjacent parts (7,8) of the structure. A zone with an easy axis of magnetization, and/or a different magnetization fluctuation dissipation parameter, is created in the central zone (7) surrounded by a peripheral zone (8). The different magnetic properties are created by irradiation or ion implantation.

Description

The invention relates to a method for producing a nanostructure

  magnet having improved dynamic performance of magnetization reversal, and the nanostructure obtained by the method.

  Magnetic nanostructures produced by etching, by ion irradiation (patent FR 2 773 632), metal deposition on a resin and selective dissolution of the resin, deposition on a modulated texture substrate ["Magnetic Domain Confinement by Anisotropy Modulation", are known. SP Li, WS Lew, JAC Bland, L. Lopez-Diaz, CAF Vaz, M. Natali and Y. Chen, Phys. Rev. Lett. 88, 087202 (2002)], or electrochemical deposition in holes, formed in structured magnetic layers deposited on a support and having thicknesses of nanometric dimensions. Such nanostructures can be used for microwave applications of magnetic materials, for magnetic sensors and for the magnetic storage of information, for example on hard disks, magneto-optical disks and M-RAM memories (Magnetic Random

Access Memories).

  In the case of magnetic information storage applications,

  the nanostructures are made in such a way as to have separate magnetic zones arranged in a regular structure or network and capable of constituting memory points.

  The evolution of information processing techniques requires that the storage surfaces such as hard disks have a very high storage density, the requirements for this areal density being more and more large and requiring a growth of 25%. order of 100% per year. In order to be able to use such very high surface density information storage surfaces, it is nevertheless necessary to be able to efficiently process the data streams, which supposes that it is possible to use read and write frequencies on the areas of the data. These discs are very high and constantly increase as a function of the surface densities of the discs. These frequencies are, at present, of the order of 400 Mbit / s. It is therefore necessary to have systems (reading sensors, and magnetic storage medium of

  information) having a very fast switching of the magnetization, that is to say, able to perform in a very short time significant variations in their magnetization. The necessary frequencies are now approaching the physical limits related to the shape of the structures used and their type of excitation or actuation.

  In addition, to cope with the challenges of obtaining high surface densities of information storage, it is envisaged to change hard drive technology, and move to a recording medium whose magnetization is perpendicular to the plane of the disc. For such a disk, it is necessary to realize a new type of read head, and assist the reversal of the magnetization of the bit by the presence of a soft magnetic sub-layer placed between the disk substrate and the nanostructures. The speed of switching of such a strongly coupled bit + underlayer system under the impetus of a novel writing head is controversial to this day [see for example "Simulation of the off-track capability of "Jin Zhen, N. Bertram, B. Wilson, R. Wood., IEEE Transactions on Magnetics. vol.38, no.2, pt.2; March 2002; p.1429-35.]. In addition, the manufacturers of magnetic recording equipment 20 seek in the medium term to manufacture memories M-RAM supplementing or

  directly competing semiconductor RAMs.

  The increase of the working frequencies of the M-RAM memories, with good conditions of reliability, is therefore an objective of the memory manufacturers to which improvements in the realization and the excitation of the magnetic nanostructures can compete, to obtain dynamic properties. , in particular reversal of the magnetization of these

  structures, which are significantly improved.

  In a magnetic system such as a nanostructure, according to the LANDAU-LIFSHITZ-GILBERT equation, the reversal speed of the mantle M depends on an external magnetic field, Hex, which is the magnetic field applied to the nanostructure , from a Haniso anisotropy field that depends on the angle between the crystalline axes of the sample and its local magnetization, a Hex exchange field that depends on the local gradients of the magnetization and a field demagnetizing demagnetizing which mainly depends on the disorientation between the magnetization and the direction

  in the largest dimension of the magnetic nanostructure.

  The reversal of the magnetization also depends on a parameter 5 AC, called damping parameter, which represents the dissipation of the fluctuations

  of magnetization in the nanostructure.

  In current devices using nanostructures whose properties are uniform within each nanostructure, the torque exerted to cause the reversal of the magnetization results from a triggering external magnetic field which is possibly amplified by the fields.

  of anisotropy and demagnetizing which accelerate the process as soon as the magnetization is out of the zone close to the axis of easy magnetization of the structure.

  In current devices using nanostructures, reasons for stability and reproducibility of the magnetization reversal process dictate that the magnetization of the structure should be kept as uniform as possible during rollover. Under these conditions, the contribution of the exchange field to the turning speed remains very low. Due to the flat shape of the nanostructure commonly used, the turnaround time is then dominated by the contribution of the demagnetizing field which appears as soon as the magnetization leaves the plane of the nanostructure. With the known physical parameters, it seems difficult to go below 100 ps of turnaround time. To approach this time limit, it is necessary to use a material having a low dissipation parameter (ca <0.1). Moreover, with the current memory sizes, it is difficult to obtain a consistent reversal of the magnetization without post-reversal fluctuations, so that the return to equilibrium of the fluctuations can not occur in a relatively short finite time. only if the dissipation is effective (optimal value equal to 1). Because of these conflicting conditions, as regards the optimization of the dissipation parameter ax, the time required between two successive writes, in the case of a structure according to the prior art, is at best a few hundred picoseconds. .

  The reversal of the magnetization in an area having a uniform magnetization therefore requires a time which may be excessive in certain applications envisaged, in particular in the case of the magnetic storage of information. This is even more critical in the case of application to sensors, which must especially be fast enough to ensure real-time monitoring of any variation in magnetization.

  It has therefore been sought to optimize the reversal time of the magnetic nanostructures used in sensors and in the information storage memories, by acting on the external field applied to the nostructure for its reversal. Work has been carried out on the generation of ultrashort magnetic field pulses by fast electronic methods or electro-optical switches. In particular, these studies have shown that in Permalloy alloy nanostructures, a magnetic field that is antiparallel to magnetization, if it is too weak or too strong, does not allow the magnetization to be reversed. If the antiparallel field to

  the magnetization is too strong, the magnetization can "bounce", too much energy having been injected compared to the instantaneous dissipation capabilities of the magnetic structure.

  It has therefore been proposed to apply carefully chosen magnetic field sequences. By implementing, in addition to a magnetic field along an axis of easy magnetization of the nanostructure, a transverse field, the reversal time can be lowered to a nanosecond, as is disclosed in US-6,163,477.

  For Permalloy alloy nanostructures, it has also been shown [see for example "Ultrafast precessional magnetization reversal by picosecond magnetic field pulse shaping", Th. Gerrits, H.A.M. Van Den Berg, J.

  Hohlfeld, L. Bâr & Th. Rasing, Nature, Vol 418 p. 6897, (2002)] that if the external magnetic field is applied perpendicularly to the axis of easy magnetization (transverse field only) and is sufficiently strong, the minimum reversal time is the half precession period, that is to say, this time decreases almost inversely with the demagnetizing field, and thus reversals can be performed in typically 140 ps.

  It has been shown, on the other hand, that an adequate choice of the duration of application of the magnetic field makes it possible to significantly minimize the time of return to equilibrium after excitation. This method is, however, sensitive to the angle between the applied field and the proper axes of the demagnetizing voltage of the nanostructure, as well as the temporal dependence of the applied field. This technique is therefore difficult to apply, both as regards the production of nanostructures and the application of

field of excitation.

  On the other hand, in the case of hard disks, we use magnetic layers composed of grains whose axes tensors

  magneto-crystalline anisotropy energy and demagnetizing energy are disoriented from one grain to another. The methods presented above therefore lose their relevance for hard disk type applications.

  It has also been proposed, for example in US Pat. No. 5,695,864, to use strong spin polarized electric currents, either to switch the magnetization alone or as a means of assistance. The latest models show that this technique probably does not allow

  switch the magnetization in less than a nanosecond.

  Injection of strong electric currents has also been suggested as a means to decrease the equilibrium return time of the nanostructures.

  However, these techniques must implement magneto-resistive nanopiles to bring the current to the nanostructure, the realization and dimensional control of these magneto-resistive pillars posing problems difficult to solve technically.

  None of the techniques currently proposed and summarized above therefore make it possible to solve in an optimal way the problem of the reversal of the magnetization of nanostructures and to pass under

  Magnetic nanostructure reversal time less than 100 ps.

  Magnetic structuring methods according to FR30 2.773 are known. 632, or ["Magnetic Domain Confinement by Anisotropy Modulation", S. P. Li, W. S. Lew, J. A. C. Bland, L. Lopez-Diaz, C. A. F. Vaz, M. Natali and Y. Chen, Phys. Rev. Lett. 88, 087202 (2002)] which make it possible to modify locally the magnetic properties of a material, in zones of a

width of the order of one micrometer.

  However, such methods have never been implemented to produce nanostructures having at least one zone having greatly improved dynamic magnetization reversal properties. The object of the invention is to propose a method for producing a generic type of magnetic nanostructure having improved dynamic performance of magnetization reversal, consisting in inducing different magnetic properties in adjacent parts of the nanostructure, such that so that the susceptibility of the magnetization of a zone of the nanostructure can be great also at the high frequencies and that the reversal of this magnetization can be obtained in a very short time, substantially less than the magnetization reversal time of the

  magnetic structures used until now in the storage of information.

  For this purpose, different magnetic properties are induced in

  a central zone and in a peripheral zone of the nanostructure in contact with the central zone over a substantial part of the periphery of the central zone, with the aim of obtaining improved dynamic magnetization reversal performance.

  Preferably, a magnetic anisotropy of the nanostructure is created such that the axis of easy magnetization in the central zone of the nanostructure has a substantially different orientation from the axis of easy magnetization in the peripheral zone. In this way, a magnetization gradient is created in the vicinity of the boundary between the central zone and the peripheral zone which is used to obtain an exchange magnetic field.

  important favoring the reversal of magnetization.

  Preferably, the parameter a which reflects the dissipation of the magnetic field, in the LANDAU-LIFSHITZ-GILBERT equation, is substantially greater in the peripheral zone than in the central zone of the nanostructure.

  Preferably, at least one of an ion implantation and irradiation is carried out in the peripheral zone of the nanostructure, so that a parameter reflecting the dissipation of the magnetization fluctuations after an excitation for the reversal of magnetization is superior in

  the peripheral zone, with respect to the central zone of the nanostructure.

  The invention also relates to a magnetic nanostructure having different magnetic properties in adjacent zones, characterized in that it comprises a central zone and a peripheral zone in contact with the central zone over a substantial part of the periphery of the zone. central zone, in which the magnetic properties of the nanostructure are different, and it has improved dynamic performance of magnetization reversal. Preferably: the nanostructure according to the invention is such that at least one of the

  magnetic properties: axis of easy magnetization, magneto-crystalline anisotropy, field of exchange, parameter reflecting the dissipation of magnetization waves in the nanostructure after excitation is different in the central zone and in the peripheral zone of the nanostructure .

  the nanostructure is such that the peripheral zone exhibits a magneto-crystalline anisotropy, or an elongated shape, or any form giving it an anisotropy of shape, and is in contact with the central zone, over a substantial part of the periphery of the zone; Central.

  The invention also relates to an excitation method for producing the reversal of the magnetization of at least one zone of the nanostructure, characterized by the fact that an external magnetic field of direction perpendicular to the magnetization of a magnetic field is exerted. central zone of the nanostructure, 25 in the direction of the magnetization of a peripheral zone surrounding the

  central zone and having magnetic properties different from those of the central zone, so as to obtain a processional-type magnetization reversal, from the torque produced by the external magnetic field on the central zone of the amplified nanostructure by a field 30 exchange at the boundary between the central zone and the peripheral zone which have easy magnetization axes of different directions.

  Preferably, the central zone of the nanostructure has an axis of easy magnetization perpendicular to the axis of easy magnetization of the zone

  peripheral and we exercise a magnetic field along the axis of easy magnetization of the peripheral zone.

  Preferably, an electric current is circulated from the peripheral zone to the central zone of a nanostructure comprising a central zone surrounded by a peripheral zone having magnetic properties different from those of the peripheral zone, in a direction perpendicular to a wall. separating the central zone and the peripheral zone and simultaneously applying a magnetic field outside the nanostructure, so as to produce a torque associated with the electric current which adds or retracts to the torque produced by the magnetic field outside and produce, facilitate or inhibit a reversal of magnetization in the direction of the electric current, by approximation or removal

  magnetization directions of the central zone and the peripheral zone.

  The invention also relates to the application of the nanostructures according to the invention to the production of information carriers such as

  M-RAM type memories, magneto-optical disks and hard disks whose recording medium is composed of nanostructures.

  The invention also relates to the application of the following nanostructures

  the invention to the realization of magnetic sensors fast and sensitive.

  The invention also relates, in particular, to a magnetic memory in the form of a hard disk comprising a memory magnetic layer consisting of a plurality of nanostructures according to the invention deposited directly on a support of the hard disk without underlayer soft magnetic, associated with a writing head to ensure recording on the hard disk.

  In order to clearly understand the invention, reference will be made by way of examples, with reference to the appended figures, of several embodiments of nanostructures according to the invention and various methods of obtaining

  a magnetic reversal in the central zone of the nanostructure.

  FIG. 1 is a schematic perspective view of an assembly

  for the fabrication of controlled gradient nanostructures of magnetic properties, by irradiation.

  FIG. 2 is a conventional sectional view of a magnetic material comprising nanostructures with a controlled gradient of properties.

  magnetic resins obtained by irradiation or any other equivalent process.

  Figures 3A to 3F and 4A to 4F show, in a conventional manner, easy magnetization directions of adjacent areas of nanostructures as obtainable by an irradiation method.

  FIGS. 5A, 5B, 5C and 5D are conventional views of a nanostructure according to the invention and according to a first embodiment during four successive reversal steps of the magnetization of a central zone of the nanostructure, under the action of a magnetic field antiparallel to the magnetization of the central area Figure 6 is a diagram giving the components of the moment

  Magnetic applied to a nanostructure according to the invention and according to a second embodiment, as a function of time, during the reversal of the magnetization under the effect of a transverse field.

  FIGS. 7A to 71 are conventional views of a nanostructure according to the invention during different phases of the reversal of the magnetization under the effect of a transverse field perpendicular to the axis of easy magnetization of the central zone of the nanostructure .

  Figure 8 is a schematic view showing the action of a current

  transverse electric on the reversal of the magnetization of a nanostructure according to the invention.

  FIG. 1 diagrammatically shows part of a magnetic support consisting of a substrate 1, a magnetic layer 2 whose thickness is of nanometric dimension and, of two non-magnetic layers 3a and 3b between which is placed the layer in

magnetic material 2.

  The layer of magnetic material 2 may be a multilayer or a single layer. In any case, it is continuous, and the process of nanostructuring, for example by irradiation and ion implantation as it

  will be described later allows to achieve, in this layer of magnetic material 2, controlled gradient nanostructures of magnetic properties.

  In a first step, the upper surface of the layer 3b disposed above the layer of magnetic material 2 is covered by a mask 4 made of an ion-absorbing material used to

  modify the magnetic properties of layer 2.

  The mask 4 makes it possible to produce nanostructures in the layer

  Magnetic 2, with a lateral resolution better than 30 nm.

  For the implementation of the invention, irradiations are made in particular to obtain different magnetic properties in a central zone whose dimensions are smaller than 100 nm and in a peripheral zone of nanostructures.

  As represented by the arrow 5, the upper surface of the magnetic structure partially covered by the mask 4 is irradiated or implanted with an ion beam 5 whose energy and the nature of the ions are chosen so as to ensure irradiation and ion implantation in certain areas of the magnetic material which make it possible to obtain the magnetic property gradients (magnetocrystalline anisotropy and dissipation parameter ax) desired. Subsequently, we remove the mask so

  recovering a flat surface (3b).

  In FIG. 2, the magnetic layer (or multi-layer) 2 between the layers 3a and 3b is shown in section at the end of the irradiation carried out by the ion beam 5.

  Zones 2a which have undergone irradiation and zones 2b covered by the mask which have not undergone irradiation, in layer 2, have

  different magnetic properties.

  For example, as shown in FIGS. 3A to 3F, the axes of easy magnetization in the different adjacent zones 2a and 2b of the layer of irradiated magnetic material 2 may have directions perpendicular to each other which are themselves either perpendicular or

  parallel to the faces of the magnetic layer of nanometric thickness.

  The directions of easy magnetization in the plane of section are represented by arrows and the directions of magnetization perpendicular to the plane of section are represented by circles in which is represented a point or a cross, according to the direction of the magnetization perpendicular to the plan he

  cutting. The central and peripheral zones may have different dissipation parameters.

  The conventions are the same in the case of Figures 4A to 4F which

  show the different configurations in which the easy manageability directions are antiparallel to each other in the adjacent zones 2a and 2b of the nanostructures.

  FIGS. 3A to 3F and 4A to 4F show that numerous configurations are possible with easy magnetization axes either perpendicular to each other in the adjacent zones, or parallel to each other but with magnetic anisotropies of different values.

  In addition, for the implementation of the invention, it is possible to envisage orientations resulting from the magnetic anisotropy between the adjacent zones of the nanostructures according to the invention, forming between them any angles, with respect to the axis of easy magnetization of the adjoining zones and consequently a gradient of any magnetization direction at the boundary between the central zones and the peripheral zones

nanostructures.

  In addition, the changes in the magnetic properties of the adjacent zones of the magnetic layer in which the nanostructures 20 are produced according to the invention are carried out in such a way that the parameter aE representative of the dissipation of the magnetization fluctuations in the nanostructure is substantially different in the central zones and in the peripheral zones of the nanostructures.

  Generally, an ion implantation will be carried out using ions whose nature (depending on the chemical composition of the magnetic layer) makes it possible to obtain a much greater dissipation in the

  peripheral area only in the central zone of nanostructures.

  To increase the dissipation parameter a, in the peripheral zone of a nanostructure, ions of elements having a strong spin-orbit coupling, such as gold (Au), must be implanted in this zone.

  antimony (Sb), platinum (Pt) or rare earth elements such as erbium (Er) or dysprosium (Dy), this list given by way of example is not limiting. Ion implantation should be done in the

  or in any non-magnetic metal layers between which the magnetic layers are sandwiched.

  FIGS. 5A, 5B, 5C and 5D show, in four successive phases of the reversal of the magnetization of a central zone, a nanostructure produced according to the invention comprising a peripheral zone having, for this example, the shape of a cross whose arms are perpendicular to each other and a central zone of square shape which is contained entirely in a part of the nanostructure located at the intersection

arms of the cross.

  The nanostructure as a whole is designated by reference numeral 6, the

  central zone 7 and the peripheral zone 8. The central zone 7 of the nanostructure 6 is a square whose side measures approximately 85 nm and the magnetization of this central zone 7 is directed along an axis Z perpendicular to the Fig. 5A to 5D and facing the front of the planes of Figs. 5A to 5D.

  In every point, the local magnetization vector is symbolized by an arrow giving its projection in the plane (xy) of the nanostructure. The following Mz component z of the local magnetization is symbolized by a circle or a cross depending on whether it is oriented towards the rear or the front of the planes of FIGS. 5 and 7, the size of the cross or circle being proportional to Mz.

  The magnetization of the central zone 7 is oriented along Z. The peripheral zone 8, in its two arms in cross, has magnetization directions (represented by arrows) in the plane of the figure substantially in the direction of the arms of the cross, with the exception of the boundary zone between the peripheral zone 8 and the central zone 7 in which a gradient of the magnetization direction is observed between the bisector X + Y in the plane of the nanostructure of a on the one hand, and on the other

  Z direction perpendicular to the plane of the nanostructure.

  We can achieve the reversal of the magnetization of the central zone 7 of the nanostructure 6 in two different ways: a first way by applying an antiparallel field to the magnetization along the axis (Oz), and a second way by applying a field parallel to the magnetization of the peripheral zone in the vicinity of the central zone, as will be described on the

figure 7.

  In the first case, the magnetization of the central zone 7 of the nanostructure 6 is carried out by means of a field of 1 kOe directed

  5 along the Z axis, opposite the direction of easy magnetization of the central zone.

  FIG. 5A shows the equilibrium state of the nanostructure

6 in the absence of applied field.

  FIG. 5B shows the state of the central zone 7 and of the peripheral zone 8 (magnetization represented by the arrows) after a

  duration of application of the field in the direction of the Z axis, in the central zone, of 152 ps. In the corners of the central zone 7 there is a beginning of the magnetization reversal in the form of the zones 9 in which the magnetization is turned to lie in the direction of the applied field.

  The central portion 7a of the central zone 7 has remained magnetized in

  the initial direction, this central zone having a substantially circular shape.

  FIG. 5C shows the state of the nanostructure after a

  field application time in the Z direction of 206 ps.

  In a large part of the central zone, on the periphery of the zone, the magnetization has turned in the direction of the Z axis and in the

  opposite direction to the original direction of the magnetization.

  There remains in the center of the central zone, an area 7'a in which

  the magnetization has not rocked yet.

  FIG. 5D shows the state of the nanostructure 6 after

  an application of the field with a duration of 270 ps.

  The central zone is magnetized uniformly in the direction of the applied field. The excess of energy in the form of a magnetization wave 10 is evacuated towards the peripheral zone where it is all the more rapidly attenuated because a high value of the parameter a, obtained by ion implantation, has been induced. the peripheral area. This type of reversal o the peripheral zone is used as dissipative zone allows

  a quick return to the equilibrium of the magnetization in the nanostructure.

  It should be noted that, in the case of a nanostructure according to the invention as described above, the central zone is totally inserted into the peripheral zone, these zones being made in the same continuous layer of magnetic material.

  In the case of points made in a magnetic type information storage memory according to the known techniques, the zones constituting the memory points are isolated zones or inserted in a non-magnetic zone, and the reversal of the magnetization of the nanostructure is realized by an external magnetic field Hext. The outer field that is applied to isolated areas of memory

  constituting the memory points, for example 1 kOe can be applied along the axis OZ isolated areas constituting the equivalent of the central zones of the nanostructure according to the invention.

  In the case of an isolated zone according to the known techniques, the initial pitch of the magnetization Hext x M is zero and the reversal is slow to start (duration greater than 1 ns). The reversal of the magnetization is in this case initiated by a nucleation. For nanostructure sizes substantially greater than the exchange lengths and the width of a Bloch wall, one can observe topological singularities (360-degree walls) which result from the upturn and which then disturb

  the reproducibility of this reversal of magnetization.

  During the reversal, magnetization fluctuations propagate in the nanostructure and bounce on its edges; their attenuation time is of the order of 1 ns, so that this additional time must be waited for to carry out a subsequent write operation on the memory point.

  In the case of a nanostructure according to the invention such as the nanostructure 6 which has been described, the central zone is not isolated but inserted in the peripheral zone 7 which has undergone a different irradiation so as to present a easy magnetization different from the axis of easy magnetization of the central area. The mode of reversal of the magnetization in this configuration of a nanostructure having a central zone and a peripheral zone which are not isolated from each other has advantages which

will be exposed below.

  First, the initial pair Hex x M is important at the boundary between

  the peripheral zone and the central zone of the nanostructure where an easy magnetization axis gradient is observed. The magnetization therefore reacts immediately upon application of the external field.

  The inversion is initiated in all cases at the periphery of the nanostructure (see Figure 5B) without topological singularities can be generated. At the end of the reversal (FIG. 5D), the magnetization wave is evacuated towards

  the peripheral zone where the dissipation characterized by the parameter a can be artificially increased by ion implantation, to obtain a faster return to equilibrium.

  The nanostructures according to the invention as described with reference to FIGS. 5A to 5D which are characterized in particular by a

  gradient of axis of easy magnetization and by a modulation of the local damping parameter ax make it possible to significantly increase the initial efficiency of the magnetic field applied for the reversal of the magnetization, to improve the reproducibility of the reversal and optimize the return time to equilibrium.

  However, by using configurations as described and shown in FIGS. 5A to 5D, the time required for the reversal of the magnetization of the central zone of the nanostructures can not be reduced below a limit imposed by the wall dynamics of the structure. This limit being typically 200 ps.

  In order to significantly reduce the turnaround time of

  the magnetization of nanostructures according to the invention, it has been proposed a mode of excitation of the nanostructures corresponding to FIGS. 3A to 3F making it possible to obtain a reversal of the "processional" type, that is to say a reverberation using the torque produced by an external field perpendicular to the axis of easy magnetization of the central zone.

  In the case of a nanostructure according to the invention as shown, for example in FIG. 7A, the central zone 7 is entirely

  surrounded by a peripheral zone 8, the magnetic properties of the central zone and the peripheral zone being different, in particular as regards the axis of easy magnetization. The central zone 7 and the peripheral zone 8 are made continuously, so that the central zone 7 is not isolated, as in the case of memory points of following structures

the prior art.

  In the case of an insulated central zone having a magnetization along the axis OZ, if it is desired to return the magnetization by precession around an applied field of 1 kOe perpendicular to OZ, it is necessary to apply a field greater than the corrected anisotropy field of the demagnetizing field, this field being for example of the order of 5 kOe, for the materials as used in the examples which will be described later. This necessary field can be even larger (10 kOe) in the case of applications

hard drive type.

  An object of the invention is therefore to obtain a precession reversal for weaker fields making it possible to obtain a gain in efficiency of the magnetic memory, by using a central zone 7 coupled by exchange interaction with a peripheral zone 8 of axis of easy different magnetization. As will be explained later, a field applied in the peripheral zone 8 is chosen parallel to the axis of easy magnetization of the peripheral zone, so that only the central zone 7 is affected by the external magnetic field and the magnetization. of the peripheral zone 8 remains constant. The nanostructure according to the invention shown in FIG. 7A, in the absence of an applied external field, comprises a substantially rectangular central zone 7 whose side measures 64 nm and a peripheral zone 8 completely surrounding the central zone 7 of rectangular shape whose

  size is 128 x 256 nm2 in our example.

  The nanostructure was obtained by ion irradiation, the axis of easy magnetization of the central zone 7 being directed along the axis OZ perpendicular to the plane of the figure and the axis of easy magnetization of the peripheral zone, substantially in the direction of the axis OY, according to which the zone

  Rectangular device 8 has its largest dimension. A transverse field of 1.5 kOe is applied along the axis OY.

  The various stages of the reversal of the magnetization are shown in FIGS. 7B to 7G.

  Moreover, in FIG. 6, the curves Mx, My and Mz of the mean magnetization of the nanostructure are represented in the form of the indexed curves a, b and c under the effect of a transverse field initiated at t = 0. The curves 11 represent the magnetization if the external magnetic field is never removed. The magnetic moment components Mx, My, Mz, respectively, are represented as curves 12a, 12b, 12c.

  during the relaxation if the magnetic field pulse is stopped after t = 131 ps and in the form of the curves 13a, 13b, 13c, respectively, the components Mx, My, Mz of the magnetic moment, during its relaxation if the Magnetic field pulse is stopped after a pulse of 72 ps, the applied field of 1.5 kOe, in the direction OY of the major axis of the rectangle constituting the peripheral zone 8.

  FIG. 7B shows the state of the nanostructure after a duration of 12 μs of application of the transverse excitation magnetic field, the FIG.

  7C, after 42 μs, and Figure 7D, after 60 μs.

  Figures 7E, 7F and 7G show, respectively, the nanostructure after durations of 89 ps, 131 ps and lns following the initial moment of application of a transverse magnetic field pulse of 1.5 kOe.

  FIG. 7H shows the nanostructure after a duration of application of the field in the direction OY of the nanostructure of a duration of 131 ps followed by a relaxation of 174 ps, sufficient to reach equilibrium.

  Figure 71 shows the state of the structure after an application of the transversal field in the OY direction of 72 ps followed by a relaxation

  300 ps, enough to reach balance.

  As it appears in FIGS. 7B, 7C and 7D, and in FIG. 6, the initial component of the magnetization of the central zone 7 of the nanostructure along the OZ axis completely disappears after a duration of the order of 72. ps and, as it is visible in FIGS. 7E, 7F and 7G, the total reversal of the magnetization continues to reach a state of complete equilibrium after

typically 300 ps.

  As can be seen in FIG. 7H and FIG. 71, an applied field pulse with a duration of about 72 μs is sufficient to obtain, after relaxation, an equilibrium structure whose magnetization is completely reversed. application of a magnetic field of the same amplitude but

  longer to bring no benefit.

  To obtain the reversal of the magnetization of the nanostructure according to the invention, by using a transverse field directed along the plane of the nanostructure, in a direction substantially perpendicular to the easy axis of magnetization of the central zone 7, It is necessary to apply the transverse field for a duration of only 72 ps (instead of 200 ps in

  the case of a field applied along the direction - OZ of the central zone).

  Processional type reversal is possible for weaker fields than in the case of isolated nanostructures manufactured according to the state

  of the art, because the central zone is coupled by exchange interaction with a peripheral zone whose axis of easy magnetization is different. Preferably, the applied field of transverse direction is parallel to the axis of easy magnetization of the peripheral zone, so that the peripheral zone 20 is not affected by this field and keeps a constant magnetization.

  Only the magnetization of the central zone is affected and undergoes a reversal under the effect of a motor torque produced by the exchange field which is added to the applied field. Moreover, as can be seen in FIGS. 7H and 71, for a judiciously chosen applied field (here 1.5 kOe), regardless of when the applied field is cut after at least 72 ps, the state

  to which relaxes the central zone 7 has an antiparallel magnetization axis with respect to the initial magnetization axis. The method is therefore virtually insensitive to the fluctuations of the various parameters involved to trigger the reversal. These properties have been shown to be maintained within an applied field gap defined according to the nanostructure.

  Another advantage of the processional reversal applied to a nanostructure according to the invention is that the time of return to equilibrium is very low. There is only 300 ps between the start of the applied field pulse and the time when the memory point can be reused.

  sre, for a next read or write operation.

  In addition, as indicated above, globally, the magnetization of the peripheral zone hardly changes during the magnetization reversal under the effect of a transverse field. The boundary conditions of the central zone are always identical and this results in an excellent reproducibility of the mode of reversal of the magnetization in the central zone.

  Because the flipping requires only a short period of application of the external field (for example 72 ps), the system is very energy efficient.

  The invention of locally modulating the axis of easy magnetization in a microstructure comprising a central zone and a peripheral zone of a magnetic information storage device makes it possible to obtain a rapid processional reversal of the magnetization. this speed resulting from a strong exchange interaction between the central zone and the

peripheral area.

  The precessional reversal is, on the other hand, efficient because it consumes little energy and is intrinsically reproducible.

  A processional type reversal therefore makes it possible to use at

  better the capacities of a nanostructure according to the invention.

  In the case of a hard disk composed of such nanostructures, it is possible to record on such a hard disk with a writing head generating a magnetic field not perpendicular to the surface of the disk, such as for example a read head identical to those used for current hard disks, that is to say disks for which the magnetization is orthoradial in the plane of the disk. It is however necessary for said read head to generate a magnetic field whose rise time is substantially lower than the reversal time of the nanostructure. Our

  This concept also reduces the constraints on the production of perpendicular magnetization recording media, since it is no longer necessary to manufacture a soft magnetic underlayer under the magnetic layer.

  memory gnetics. When applied to the manufacture of random access magnetic memories, the nanostructures according to the invention are associated so that their central zones constitute a set of memory points on which reading and writing operations are carried out. selectively, through addressing.

  Addressing the memory points formed by the central zones of

  Nanostructures according to the invention may be assisted by injecting electric current through the peripheral zone, towards the central zone of the nanostructure, as shown in FIG. 8.

  FIG. 8 shows, in a conventional manner, the central zone 7 of the nanostructure 1 whose easy magnetization axis is perpendicular to the plane of FIG. 8 and directed towards the front. There is shown in the form of rectangles a conductive line 14 serving to produce the magnetic field parallel to the magnetization of the zone 16 '. In the form of rectangles 15a and 15b, the conductive zones surrounding the zone are shown in the same way. central 7 of the nanostructure, via which an electric current is injected perpendicular to the boundary separating the central zone from the peripheral zone. The conductive line 14 is electrically isolated from

zones 7, 15 and 16.

  In the form of an arrow 16, in a peripheral zone 16 'adjacent to the central zone 7 of the nanostructure 1, an electric current propagating in the direction of the arrow 16, in the peripheral zone 16' and in a direction substantially perpendicular to a partition wall of the central zone and the peripheral zone of the nanos25 tructure. The arrow 16 is also the magnetization of the peripheral zone. During its transport, the electric current acquires a spin polarization. The torque generated by the electric current 16, according to whether the current is directed in a positive direction or in a negative direction, promotes the approximation or removal of the directions of the magnetizations of the central zone and the peripheral zone in a common plane. to these two magnetizations. As a result, the torque produced by the electric current can be added to or withdraw from the effect of the exchange torque produced by an external magnetic field pulse generated by a current in the conductive line 14, so that it is possible to select, using the electric current 16 and the magnetic field generated by the current in the conductive line 14, the memory element constituted by a central zone 7 of a nanostructure which is selectively turned over, in a network of memory points. Whatever the initial state of the magnetization of the nanostructure, the magnetic field can always be applied in the same direction. Each line 14 can therefore be connected to a unipolar power supply, which is simpler than the bipolar power supplies currently required.

  in MRAM applications according to the state of the art.

  The method according to the invention for producing nanostructures whose magnetic properties and in particular the axis of easy magnetization and the damping parameter ac are modified locally, for example by irradiation and ion implantation, makes it possible to obtain a magnetization reversal. accelerated, either by applying a magnetic field along the axis of easy magnetization of the central zone of the nanostructure, or by applying a

  transversal field, that is to say along the axis of easy magnetization of the periphery to obtain a processional type reversal.

  The turning characteristics do not depend on the exact shape of the central zone of the nanostructure but only on the size of the central zone. In addition, the magnetization reversal depends little on the precise duration of the applied field. The realization and excitation of the nanostructures can therefore be obtained in a simpler way. In addition, when a transverse excitation field is used to obtain a precession reversal, the reversal can be obtained in a time of the order of 72 ps, that is to say a time three times less than the time required for

  reversal by a field applied anti-parallel to the axis of easy magnetization of the central zone of the nanostructure.

  Since the exact shape of the central zone of the nanostructure does not affect the turning conditions, the realization of an array of memory points consisting of central zones of nanostructures according to the invention, for example by irradiation with using a mask or any

  Another process of the "lithographic" type is considerably simplified.

  The nanostructures according to the invention also make it possible to accelerate the return to equilibrium after a magnetization reversal, because a peripheral zone can be produced, in particular by ion implantation whose damping parameter cc is substantially greater. to that of the central zone. The return to equilibrium is also accelerated by exchange coupling between the peripheral zone and the central zone. Each zone has specific functions thanks to a judicious choice of its magnetic properties. To apply strong electric currents spin polarized, either 10 to switch alone the magnetization, or as means of assistance to

  the switching of the magnetization in memory point arrays, magneto-resistive nanopillars have been used until now to inject current into the memory points. Such nanopiliers are generally made by "lithography", so that their section has an area at least equal to 12 o I is the resolution of the lithographic method for producing nanopiliers.

  In the case of the nanostructures according to the invention, the conducting section for bringing the current to the central zone of the nanostructure is defined, for one of its dimensions, from the resolution of a "lithographic" method, by example for making arms of a cross of width I (in the case of a nanostructure as shown in Figures 5A to 5D) and secondly, for its second dimension, by the thickness of the magnetic deposit (thickness E total layers deposited on a substrate), in which the nanostructures are produced by irradiation. The resolution obtained on a magnetic layer deposition thickness is much better than the resolution of a lithographic method (E "1) so that the current densities that can be obtained in the case of the nanostructures according to the invention are much greater than in the case where magneto-resistive nanopiliers are used, a substantial gain in energy is obtained Furthermore, the production of peripheral zones around the memory points does not really reduce the surface density of the memory because the peripheral zones are used to transport the electrical addressing currents in the case of the MRAMs, and because zones separating the nanostructures are necessary in the case of a disk-type application

hard based nanostructures.

  The invention is not limited strictly to the embodiments which

have been described.

  It is thus possible to envisage the manufacture of nanostructures in which the central zone and the peripheral zone have different shapes

of those that have been described.

  In particular, a peripheral zone surrounding

  completely a central zone having an elongated shape and more generally any geometric shape giving it an anisotropy of form.

  Preferably, the largest dimension of the central zone and less than 100 nm, so as to fully benefit from the exchange field.

  In the case of the examples described above, the peripheral zone and the central zone have a common boundary which extends substantially around the periphery of the central zone. However, for the implementation of the invention, it is possible to design nanostructures in which the peripheral zone and the central zone are in contact only along part of the periphery of the central zone; in this case, however, the contact between the peripheral zone and the central zone must be carried out over a substantial part of the periphery of the central zone, that is to say from 50% to 100% of the length

from this circumference.

  According to the use of the magnetic structures including the nanostructures according to the invention, the materials used to produce the magnetic layers in which the nanostructures are made can have various compositions.

  These magnetic layers may be constituted for example by alloys or thin layers with strong magnetocrystalline anisotropy such as layers of iron-platinum, iron-palladium or cobaltplatin alloys, or more isotropic alloys deposited on antiferromagnetic regions such as iron-manganese, manganeseiridium or manganese-platinum alloys inducing, by interlayer exchange, easily different axes of easy magnetization, or else materials deposited on substrates whose surface state has been artificially modulated so as to induce

  different growth conditions in the central zone and the peripheral zone, and thus induce different easy directions of magnetization.

  The nanostructures according to the invention having a central zone and

  a peripheral zone with different magnetic properties can be obtained by a different method of ion irradiation and implantation.

  For example, a variable magnetic anisotropy can be obtained by depositing magnetic layers on a substrate having juxtaposed areas having different surface properties, so that the magnetic layers deposited on the different areas of the substrate

  exhibit different magnetic anisotropy properties.

  When using a method of irradiation and ion implantation of magnetic layers, the zones irradiated by the ion beam may be the peripheral zones or the central zones of the nanostructures according to the desired properties. It is also possible to perform ion irradiation and implantation different from the central zones and the peripheral zones. The invention applies in particular to the production of supports for

  magnetic storage of information, such as hard disks, magneto-optical disqueques and M-RAM memories.

  The invention makes it possible in particular to obtain hard disks on which information can be recorded with a writing head producing a magnetic field of direction substantially parallel to the plane of the disk. Such a disk consists of a plurality of nanostructures according to the invention. Due to the energy gain obtained, when implementing

  nanostructures according to the invention, the memory layer can be deposited directly on a support of the hard disk, without soft magnetic intermediate sub-layer, to close the field lines generated by the writing head, which is usually necessary in this type of Hard disk magnetization perpendicular to the plane of the hard disk.

Claims (11)

  1.- Method for producing a magnetic nanostructure (6)   comprising inducing at least one magnetocrystalline anisotropy, an easy magnetization axis and a damping parameter having different values in adjacent portions of the nanostructure (6), characterized in that different magnetic properties are induced in a central zone (7) and in a peripheral zone (8) of the nanostructure in contact with the central zone (7) on a substantial part of the periphery of the central zone (7), in the aim of obtaining improved dynamic magnetization reversal performance.   2. A process according to claim 1, characterized in that induces a magnetic anisotropy of the nanostructure, so that the axis of easy magnetization of the central zone (7) has a different orientation of   the axis of easy magnetization of the peripheral zone (8).   3. Process according to claim 2, characterized in that   performs at least one of an ion implantation and irradiation in the peripheral zone (8) of the nanostructure (1), so that a parameter (a) reflecting the dissipation of the magnetization fluctuations after an excitation for the magnetization reversal is greater in the peripheral zone (8), relative to the central zone (7) of the nanostructure (1).   4. Magnetic nanostructure having different magnetic properties in adjacent zones (7, 8), characterized in that it comprises a central zone (7) and a peripheral zone (8) in contact with the central zone, on a substantial part of the periphery of the central zone (7) in which the magnetic properties of the nanostructure (1) are different, and it has improved dynamic performance of magnetization reversal.   5. Nanostructure according to claim 4, characterized in that at least one of the following magnetic properties: easy axis of mantation, magneto-crystalline anisotropy, exchange field, parameter (a) reflecting the dissipation of waves magnetization in the nanostructure (6) after excitation, is different in the central zone (7) and in the zone   peripheral (8) of the nanostructure (1).   6. Nanostructure according to any one of claims 4 and 5,   characterized in that the peripheral zone (8) has a magneto-crystalline anisotropy, or an elongate form, or any form giving it an anisotropy of shape, and is in contact with the central zone (7), on a substantial part of its circumference. 7. A method for exciting a magnetic nanostructure (6) to produce the reversal of the magnetization of at least one zone of the nanostructure (6) characterized by the fact that an external magnetic field of perpendicular direction is exerted at the magnetization of a central zone (7) of the nanostructure (6), in the direction of the magnetization of a peripheral zone (8) in contact with the central zone (7) over a substantial part of its circumference and having magnetic properties different from those of the central zone, so as to obtain a processional-type magnetization reversal, from the torque produced by the external magnetic field on the central zone (7) of the nanostructure (6). ) amplified by an exchange field at the boundary between the central zone (7) and the peripheral zone (8)   which have easy magnetization axes of different directions.   8. The excitation method according to claim 7, characterized by   in that the central zone (7) of the nanostructure (1) has an axis of easy magnetization perpendicular to the axis of easy magnetization of the peripheral zone (8) and that a magnetic field is exerted along the axis of easy magnetization of the peripheral zone (8).   9. An excitation method according to any one of the claims   7 and 8, characterized in that an electric current is circulated from the peripheral zone (8) to the central zone (7) of a nanostructure (6) comprising a central zone (7) surrounded by a peripheral zone (8) having magnetic properties different from those of the peripheral zone (8), in a direction perpendicular to a separation wall of the central zone (7) and the peripheral zone (8) and which is applied, simultaneously, A magnetic field outside the nanostructure (6), so as to produce a torque associated with the electric current which adds or retracts to the torque produced by the external magnetic field and to produce, facilitate or inhibit a magnetization reversal according to the sense of the electric current, by bringing the magnetization directions of the zone closer or further   central (7) and the peripheral zone (8).   10. Magnetic memory comprising a plurality of nanostructures   (1) according to any one of claims 4 to 6 and constituting one of the following 5 information storage devices: hard disk, magnetooptical disk, memory M-RAM.   11. Magnetic memory according to claim 10, in the form   a hard disk having a magnetic memory layer consisting of a plurality of nanostructures according to any one of claims 4 to 6, deposited directly on a hard disk medium without soft magnetic sub-layer, associated with a writing head for ensure the recording on the hard disk.