Magnetic thin wire memory and method of driving magnetic thin wire memory

Abstract

To provide a magnetic thin wire memory having a structure which is easy to process and stabilizes memory operation of a magnetic thin wire.SOLUTION: A magnetic thin wire memory 1 includes, on a substrate 10, a magnetic thin wire 30 having a linearly formed magnetic body 31 and a cap layer 32 made of a non-magnetic heavy metal material and disposed on the magnetic body 31, the cap layer 32 has a first area 3F and a second area 3S having different spin orbit torque due to the cap layer 32, and the second area is periodically formed for each unit bit length L. The second area 3S has a smaller spin orbit torque than that of the first area 3F.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a magnetic nanowire memory, and more particularly to a magnetic nanowire memory utilizing a current-induced domain wall motion phenomenon in which a domain wall moves when a current pulse is applied, and a method for driving the magnetic nanowire memory. [Background technology]

[0002] Research is underway on magnetic nanowire memory, which records and reproduces binary information corresponding to the direction of magnetization, such as upward or downward, in a medium made of magnetic material processed into a nanowire shape. Magnetic nanowire memory employs a nanowire-shaped conductor (hereinafter referred to as a recording element) orthogonally arranged above or below the magnetic nanowire via an interlayer insulating layer. Information is recorded by locally reversing the magnetization in the magnetic nanowire using the current-induced magnetic field generated by applying a current to the recording element (Patent Document 1). Parallel and stacked arrangements of such magnetic nanowire memories enable ultra-high-speed operation through parallel synchronous control. Furthermore, because magnetic nanowire memory does not require mechanically moving parts, high operational reliability is ensured. Patent Document 2 describes a magnetic nanowire with periodic constrictions at regular intervals. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-027802 [Patent Document 2] Patent No. 6093146 Summary of the Invention [Problem to be solved by the invention]

[0004] The operation of magnetic nanowire memory consists of "recording," which forms magnetic domains corresponding to recording bits, "driving," which shifts the formed magnetic domains in the magnetic nanowire, and "reading," which detects the magnetization direction of the magnetic domains. Here, "driving" utilizes the current-induced domain wall motion phenomenon, in which domain walls move when a current pulse is applied. However, there are various types of current-induced domain wall motion that actually occur. For example, there are variations in magnetic properties within a single magnetic nanowire, and as a result, even when the same driving current pulse is applied, the shift length (the distance the magnetic domain moves) and magnetic domain length (the length of the magnetic domain itself after driving) of the magnetic domain after driving may differ depending on the location within the same magnetic nanowire.

[0005] Furthermore, for example, when a large number of magnetic nanowires are formed in parallel on a single substrate, a phenomenon may occur in which, even on the same substrate, the magnetic domain lengths of the magnetic domains recorded by applying a recording current to the recording element and reversing the magnetization in its vicinity are different for each of the multiple magnetic nanowires. Furthermore, even if recording domains of the same magnetic domain length could be formed in such a large number of parallel magnetic nanowires during the "recording" stage, it is currently difficult to stably move each recording domain uniformly in all parallel magnetic nanowires to the position of the reproducing element that performs "reproduction" when these are moved by applying the same number of drive pulses.

[0006] Therefore, as described in Patent Document 2, for example, it is conceivable to define the shift length (the distance the magnetic domain moves) by periodically providing constrictions at regular intervals in the magnetic nanowire so that the length corresponds to each unit bit length (the length of the magnetic domain that defines one bit). However, when creating a constriction for each unit bit length, high-precision microfabrication is required to realize the magnetic nanowire as designed, and it is difficult to adjust the trapping force by changing the shape and size of the constriction. Therefore, further improvement in yield is desired.

[0007] The present invention has been made in view of the above-mentioned circumstances, and has as its object to provide a magnetic nanowire memory that is easy to process and has a structure that stabilizes the memory operation of the magnetic nanowire, and a method for driving the magnetic nanowire memory. [Means for solving the problem]

[0008] In order to solve the above problem, the magnetic nanowire memory of the present invention comprises a magnetic nanowire having a magnetic body formed in a straight line on a substrate and a cap layer made of a non-magnetic heavy metal material and arranged on the magnetic body, the cap layer having a first area and a second area in which the spin-orbit torque caused by the cap layer is different, and the second area is formed periodically for each unit bit length.

[0009] According to this configuration, the magnetic nanowire memory has a first area and a second area, each area having a different spin orbit torque (SOT). The magnetic nanowire memory has the second areas periodically arranged at regular intervals so as to have a length for each unit bit length, allowing the shift length to be specified. By implanting specific ions into a desired area of ​​the cap layer, it is possible to change the spin orbit torque of that area. Alternatively, by forming a heavy metal material different from that of the cap layer in a desired area of ​​the cap layer, it is possible to change the spin orbit torque of that area. The magnetic nanowire memory does not require the high-precision microfabrication required for conventional constrictions, and is easy to process.

[0010] Furthermore, in the magnetic nanowire memory, in the first area of ​​the magnetic nanowire where the spin-orbit torque is large, the spin-orbit torque assists the domain wall motion during magnetic domain driving, lowering the threshold current value of the drive current and facilitating domain wall motion. In the second area of ​​the magnetic nanowire where the spin-orbit torque is small, the threshold current value of the drive current becomes relatively high during magnetic domain driving, making domain wall motion more difficult. This enables stable driving while maintaining a constant magnetic domain length at the entrance of each second area, which is formed periodically for each unit bit length. [Effects of the Invention]

[0011] The present invention provides the following excellent effects. The magnetic nanowire memory according to the present invention has an easily processable structure in which areas with different spin-orbit torques are periodically arranged at regular intervals. The magnetic nanowire memory allows the magnetic domain length to be constant when the magnetic domain is driven, thereby stabilizing memory operation. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a perspective view schematically showing a magnetic nanowire memory according to a first embodiment of the present invention. [Figure 2] 1 is a top view schematically showing a magnetic nanowire memory according to a first embodiment of the present invention. [Figure 3] FIG. 3 is a cross-sectional view taken along the line AA′ in FIG. 2. [Figure 4] FIG. 10 is a perspective view schematically showing a magnetic nanowire memory according to a second embodiment of the present invention. [Figure 5] FIG. 10 is a top view schematically showing a magnetic nanowire memory according to a second embodiment of the present invention. [Figure 6] 6 is a cross-sectional view taken along the line BB' in FIG. 5. [Figure 7] FIG. 10 is a cross-sectional view of a magnetic nanowire memory according to a third embodiment of the present invention. [Figure 8] FIG. 10 is a cross-sectional view of a magnetic nanowire memory according to a fourth embodiment of the present invention. [Figure 9] FIG. 1 is a schematic diagram of a magnetic domain shift in a magnetic nanowire memory. [Figure 10] FIG. 2 is an explanatory diagram of a driving current flowing through a magnetic nanowire. [Figure 11] 10A to 10C are top views showing the manufacturing process of the magnetic nanowire memory according to the third embodiment. [Figure 12] 1A and 1B are magneto-optical microscope images obtained in a verification experiment and explanatory diagrams thereof. DETAILED DESCRIPTION OF THE INVENTION

[0013] A magnetic nanowire memory according to an embodiment of the present invention will be described with reference to the drawings. Note that the size and positional relationship of components shown in each drawing may be exaggerated for clarity. The following will be described in detail in the following sections: 1. Structure of the magnetic nanowire memory, 2. Manufacturing method of the magnetic nanowire memory, 3. Driving method of the magnetic nanowire memory, and 4. Verification experiment data.

[0014] [1. Structure of magnetic nanowire memory] (First embodiment) The structure of the magnetic nanowire memory according to the first embodiment will be described with reference to Figures 1, 2, and 3. In each figure, the longitudinal direction of the magnetic nanowire 30 is the x-axis direction, the longitudinal direction of the recording element 50 is the y-axis direction, and the film thickness direction is the z-axis direction. The magnetic nanowire memory 1 includes, on a substrate 10, a number of magnetic nanowires 30 (two as an example in FIG. 1), an interlayer insulating layer 40, a recording element 50, and a reproducing head 60, arranged in this order in parallel to the substrate 10. In order to insulate the magnetic nanowires 30 and the recording element 50, a SiO x An insulating layer 20 such as the above is formed.

[0015] 3, the magnetic nanowire memory 1 includes a magnetic nanowire 30 having a magnetic body 31 formed linearly on a substrate 10, and a cap layer 32 made of a non-magnetic heavy metal material and disposed on the magnetic body 31. The cap layer 32 has a first area 3F and a second area 3S that have different spin-orbit torques due to the cap layer 32, and the second area 3S is formed periodically for each unit bit length L. Furthermore, as an example, the magnetic nanowire memory 1 has a structure in which the second area 3S has a smaller spin-orbit torque than the first area 3F. Note that a small spin-orbit torque due to the cap layer 32 means that the absolute value of the spin-Hall angle of the second area 3S in the cap layer 32 is small. The spin-Hall angle represents the efficiency of conversion from electric current to spin current. Note that the names of the first area 3F and the second area 3S are relative, and therefore the features of the first area 3F and the second area 3S may be interchanged, although this is not shown in the figures. That is, as another example, the magnetic nanowire memory 1 may have a structure in which the first area 3F has a smaller spin-orbit torque than the second area 3S.

[0016] In the following description, it is assumed that the spin-orbit torque in the second area 3S is smaller than that in the first area 3F. 3, in the first area 3F and the second area 3S, the magnetic body 31 of the magnetic nanowire 30 has a flat shape with a constant thickness. Similarly, the cap layer 32 has a flat shape with a constant thickness. The thickness of the cap layer 32 is 3 nm or more, and although there is no particular upper limit, the thicker the cap layer 32, the higher the power consumption, so the thickness is set to a desired value within the allowable range of power consumption.

[0017] In this embodiment, for example, the cap layer 32 is doped only in the second area 3S with ions that decrease the spin Hall angle of the heavy metal material of the cap layer 32. Alternatively, the cap layer 32 is doped only in the first area 3F with ions that increase the spin Hall angle of the heavy metal material of the cap layer 32.

[0018] Here, the second areas 3S of the cap layer 32 of the magnetic wire 30 specifically refer to the respective positions where the shift of the magnetic domain wall should be stopped. In other words, the second areas 3S are positions where the shift of the magnetic domain is trapped. In this sense, regarding the longitudinal length of the magnetic wire 30, the length of the second areas 3S may be shorter than the length of the first areas 3F. Alternatively, the length of the second areas 3S may be the same as the length of the first areas 3F.

[0019] The recording element 50 is formed linearly in a direction perpendicular to and intersecting with the magnetic nanowire 30. By arranging the recording element 50 perpendicular to the magnetic nanowire 30, the magnetization direction of the magnetic domain formed in the magnetic nanowire can be locally reversed by a current magnetic field generated by a current flowing through the recording element 50, thereby enabling information recording. In particular, by arranging the recording element 50 perpendicular to a large number of magnetic nanowires 30, information can be recorded efficiently. The reproducing head 60 is formed directly above the magnetic nanowire 30 with the interlayer insulating layer 40 interposed therebetween, and at a position spaced a predetermined distance longer than the unit bit length L from the recording element 50 . The reproducing head 60 detects the magnetization direction of the magnetic domains accumulated inside the magnetic nanowire 30, and may be, for example, a reproducing head similar to an existing hard disk. As shown in FIG. 3, a first area 3F and a second area 3S are arranged between the recording element 50 and the reproducing head 60.

[0020] (Second embodiment) The structure of the magnetic nanowire memory according to the second embodiment will be described with reference to Figures 4, 5, and 6. Note that the same components as those in the first embodiment are given the same reference numerals (i.e., numbers in the drawings) and descriptions thereof will be omitted. The magnetic nanowire memory 1B includes, in this order, a recording element 50 and an insulating layer 20, an interlayer insulating layer 40, a magnetic nanowire 30, and a reproducing head 60 on a substrate 10 and parallel to the substrate 10. The magnetic nanowire memory 1B differs from the magnetic nanowire memory 1 in the stacking order of some components, but has the same feature as the magnetic nanowire memory 1, in that it has a first area 3F and a second area 3S with different magnitudes of spin-orbit torque, and the second area 3S is formed periodically for each unit bit length L.

[0021] 6, in the magnetic nanowire memory 1B, the magnetic body 31 and the cap layer 32 of the magnetic nanowire 30 have a flat shape with a constant film thickness in the first area 3F and the second area 3S. These features are similar to those of the magnetic nanowire memory 1 shown in FIG. The recording element 50 is formed linearly in a direction perpendicular to and intersecting the magnetic nanowire 30. However, the recording element 50 differs from the magnetic nanowire memory 1 shown in Fig. 3 in that the recording element 50 is disposed below the magnetic nanowire 30 via an interlayer insulating layer 40. The reproducing head 60 is formed directly above the magnetic nanowire 30 and at a position spaced apart from the recording element 50 by a predetermined length longer than the unit bit length L.

[0022] (Third embodiment) The magnetic nanowire memory according to the third embodiment will be described with reference to FIG. 7. Note that the same components as those in the first embodiment are given the same reference numerals (i.e., numerals in the drawings) and their description will be omitted. The magnetic nanowire memory 1C according to the third embodiment has a pattern of convex portions 32b on the cap layer 32 in each second area 3S. The cap layer 32 is formed of a first heavy metal material. On the other hand, the convex portions 32b are formed of a second heavy metal material different from the first heavy metal material. The sign of the spin Hall angle of the second heavy metal material is different from the sign of the spin Hall angle of the first heavy metal material. Note that the sign of the spin Hall angle is either positive (plus) or negative (minus).

[0023] In the magnetic nanowire memory 1C, materials with different signs of spin-Hall angle are in contact with each other in the second area 3S of the magnetic nanowire 30, so that the spins of the materials cancel each other out at the interface, thereby reducing the intrinsic spin-orbit torque of the first heavy metal material constituting the cap layer 32. An example of a heavy metal material with a positive sign of the spin-Hall angle is platinum (Pt). An example of a heavy metal material with a negative sign of the spin-Hall angle is tantalum (Ta). When the first heavy metal material is platinum (Pt), for example, the second heavy metal material can be tantalum (Ta). The types of the first heavy metal material and the second heavy metal material are not limited to the exemplified elements.

[0024] As shown in Fig. 7, in the first area 3F and the second area 3S, the magnetic body 31 and the cap layer 32 of the magnetic nanowire 30 have a flat shape with a constant film thickness. These features are similar to those of the magnetic nanowire memory 1 shown in Fig. 3. However, the convex-shaped portion 32b on the cap layer 32 is formed for each unit bit length L as the second area 3S where the spin-orbit torque of the cap layer 32 is small. Moreover, the area other than the convex-shaped portion 32b on the cap layer 32 is formed as the first area 3F where the spin-orbit torque of the cap layer 32 is large. The magnetic wire 30 and the pattern of the convex portion 32b are covered with an interlayer insulating layer 40.

[0025] As shown in the cross-sectional view of FIG. 7, the multiple convex portions 32b are arranged at regular intervals (unit bit length L) in the longitudinal direction (x-axis direction) of the magnetic wire 30. In addition, in the xy plan view, as an example, the longitudinal direction of the convex portions 32b is the direction perpendicular to the longitudinal direction of the magnetic wire 30 (they are arranged so that their length in the y-axis direction is longer than their length in the x-axis direction). As an example, the convex portions 32b are formed into an oval shape in a plan view (see FIG. 11(e)). The shape of the convex portions 32b is not particularly limited as long as they are formed on the magnetic wire 30 at regular intervals.

[0026] The recording element 50 is formed linearly in a direction perpendicular to and intersecting with the magnetic nanowire 30. The reproducing head 60 is formed directly above the magnetic nanowire 30 with the interlayer insulating layer 40 interposed therebetween, and at a position spaced a predetermined length longer than the unit bit length L from the recording element 50. As shown in FIG. 7, a first area 3F and a second area 3S are arranged between the recording element 50 and the reproducing head 60. These features are similar to those of the magnetic nanowire memory 1 shown in FIG. 3.

[0027] (Fourth embodiment) The magnetic nanowire memory according to the fourth embodiment will be described with reference to Fig. 8. Note that the same components as those in the second and third embodiments are given the same reference numerals (numbers, i.e., numerals in the drawings) and descriptions thereof will be omitted. The magnetic nanowire memory 1D according to the fourth embodiment has a pattern of convex portions 32b on the cap layer 32 in each second area 3S. The cap layer 32 is made of a first heavy metal material. On the other hand, the convex portions 32b are made of a second heavy metal material different from the first heavy metal material. The sign of the spin Hall angle of the second heavy metal material is different from the sign of the spin Hall angle of the first heavy metal material. These features are similar to those of the magnetic nanowire memory 1C shown in FIG. 7.

[0028] 8, in the magnetic nanowire memory 1D, the magnetic body 31 and the cap layer 32 of the magnetic nanowire 30 have a flat shape with a constant film thickness in the first area 3F and the second area 3S. The convex portion 32b on the cap layer 32 is formed for each unit bit length L as the second area 3S where the spin orbit torque of the cap layer 32 is small. Moreover, the area other than the convex portion 32b on the cap layer 32 is formed as the first area 3F where the spin orbit torque of the cap layer 32 is large. These features are the same as those of the magnetic nanowire memory 1C shown in FIG. 7.

[0029] The recording element 50 is formed linearly in a direction perpendicular to and intersecting with the magnetic nanowire 30, and is disposed below the magnetic nanowire 30 via an interlayer insulating layer 40. The reproducing head 60 is formed directly above the magnetic nanowire 30 and at a position spaced apart from the recording element 50 by a predetermined length longer than the unit bit length L. These features are similar to those of the magnetic nanowire memory 1B shown in FIG.

[0030] [2. Manufacturing method of magnetic nanowire memory] Next, details of each part of the magnetic nanowire memory and a manufacturing method thereof will be described with reference to the drawings as appropriate. (substrate) The substrate 10 is a base for constructing the magnetic nanowire memory 1, and is a substrate in the broad sense for forming the recording element 50 and the magnetic nanowire 30, and known substrate materials can be used. Specifically, the surface is thermally oxidized to form SiO xA Si substrate on which a film is formed is suitable. Alternatively, insulating substrates such as a GGG (gadolinium gallium garnet) substrate, a SiC (silicon carbide) substrate, an MgO (magnesium oxide) substrate, an AlN (aluminum nitride) substrate, or a Ge (germanium) single crystal substrate, which are known transparent substrate materials, can be used. In this way, at least the surface of the substrate 10 serves as an insulator to prevent short-circuiting of metal materials such as the recording element 50 and magnetic nanowire 30 formed thereon.

[0031] (recording element) The recording element 50 is made of common electrode metal materials such as metals such as Cu, Al, Au, Ag, Ta, Cr, and Ti, or alloys thereof, and Au is particularly suitable because it has high conductivity and is chemically stable, and it is more preferable to use a base film that improves film adhesion. The recording element 50 is formed on a prepared substrate using a metal material by forming a film using a known method such as sputtering, and then processing it into the shape of the recording element 50 using photolithography (or electron beam lithography), etching (or lift-off method), etc. Alternatively, the recording element 50 may be formed by forming an opening on a prepared substrate using photolithography (or electron beam lithography) and etching, then plating the substrate with a metal material and filling the opening with the metal material.

[0032] (Interlayer insulating layer) The interlayer insulating layer 40 is for insulating the recording element 50 from the magnetic nanowire 30. The interlayer insulating layer 40 is made of, for example, SiO X and AlO X oxide films such as SiN X Known insulating materials such as MgF2 can be used.

[0033] (magnetic thin wire) The magnetic nanowire 30 is patterned by processing a magnetic material. The magnetic nanowire 30 can be made of a known magnetic material, specifically, a multilayer film such as a Co / Pt multilayer film in which a transition metal such as Fe, Co, Ni and a noble metal such as Pd or Pt are repeatedly stacked, an alloy (RE-TM alloy) or multilayer film of a rare earth metal and a transition metal such as Tb-Fe-Co, Gd-Fe, or the like, or an L 10 Examples of such materials include FePt and FePd, which are ordered alloys of the Fe-based alloy. In this embodiment, the thin wire made of a magnetic material is called a magnetic body 31.

[0034] A cap layer 32 is formed on the outermost surface of the magnetic nanowire 30 to prevent the magnetic properties of the magnetic material constituting the magnetic body 31 from deteriorating due to exposure to the atmosphere or infiltration by chemicals during the manufacturing process. In this embodiment, the cap layer 32 is made of a non-magnetic heavy metal material. Specifically, platinum (Pt), tantalum (Ta), tungsten (W), iridium (Ir), etc. are suitable.

[0035] (Area where the spin-orbit torque is periodically reduced for each bit length) In each embodiment, an area (second area 3S) where the spin-orbit torque is locally reduced is provided for each unit bit length L of the magnetic nanowire memory, specifically only at the position where the domain wall shift should be stopped, and the magnetic domain shift is trapped there, thereby making the shift length and magnetic domain length constant. Below, several examples of methods for forming this area that reduces the spin-orbit torque will be described.

[0036] <Method 1> By implanting specific ions into desired areas of the cap layer 32, it is possible to change the spin-orbit torque in those areas. Basically, implanting any kind of ion into a heavy metal such as platinum (Pt) causes unwanted disturbances in the band gap, which changes the spin-orbit torque in the area into which the ions are implanted. Empirically, in this case, the spin-orbit torque is often reduced. Therefore, when forming the cap layer 32, the second areas 3S with a small spin-orbit torque can be periodically formed for each unit bit length L by a simple process of implanting ions that reduce the spin Hall angle of the heavy metal material of the cap layer 32 into the regions that will become the second areas 3S.

[0037] However, the cap layer 32 only needs to have an area where the magnitude of the spin-orbit torque is relatively large and an area where it is small, and is not limited to the case where the spin-orbit torque is reduced by ion implantation. Therefore, when forming the cap layer 32, ions that increase the spin-Hall angle of the heavy metal material of the cap layer 32 can be implanted into the region that will become the first area 3F to periodically form second areas 3S with a relatively small spin-orbit torque for each unit bit length L. For example, it is known that a material obtained by implanting sulfur (S) ions into platinum has a large spin-Hall angle. Sulfur (S) ions are an example of ions that increase the spin-Hall angle of the heavy metal material of the cap layer 32.

[0038] In the case of the magnetic nanowire memory 1, 1B (see Figures 3 and 6), when fabricating the cap layer 32 of the magnetic nanowire 30, sulfur (S) ions or the like are locally injected into the heavy metal material of the cap layer 32 at regular intervals using a resist template or the like, thereby making it possible to locally change the spin-orbit torque. In this way, when forming the cap layer 32 in the magnetic nanowire memory 1, 1B, the second area 3S can be formed periodically for each unit bit length L by simple processing such as implanting ions into the first area 3F or the second area 3S.

[0039] <Method 2> As another method, in the case of the magnetic nanowire memories 1C and 1D (see FIGS. 7 and 8), the second areas 3S with a small spin-orbit torque can be formed by providing a pattern of convex portions 32b (heavy metal) periodically at each unit bit length L so as to be bonded to the outermost surface of the cap layer 32 in the film thickness direction. In this case, after patterning the magnetic nanowire 30, the areas with a small spin-orbit torque (second areas 3S) are formed by lithography. The material used for the second areas 3S is preferably a heavy metal different from that of the cap layer 32 and having a spin-Hall angle with a different sign. As a result, the spin-Hall angle of the second areas 3S changes, and the spin-orbit torque can be reduced.

[0040] (Method of forming periodic convex portions 32b) <Method 2-1> In the magnetic nanowire memory 1C shown in FIG. 7 and the magnetic nanowire memory 1D shown in FIG. 8, convex portions 32b are formed as second areas 3S in which the spin-orbit torque of the cap layer 32 is small for each unit bit length L. For example, a processing method for the second areas 3S is as follows: first, a magnetic film made of a magnetic material is formed on the substrate 10; then, a metal film is formed using a first heavy metal material and processed into a thin wire. In this way, a flat magnetic nanowire 30 including the cap layer 32 is patterned. Thereafter, a pattern using a second heavy metal material is additionally formed using a lithography method or the like. According to this method, after the cap layer 32 is formed, the second areas 3S can be periodically formed for each unit bit length L by a simple process of periodically adding the second heavy metal material.

[0041] <Method 2-2> Another method for forming the convex portions 32b is to first form a pattern of a layer of a second heavy metal material on the magnetic nanowire 30 including the cap layer 32, and then remove unnecessary portions that will become the first areas 3F (concave pattern). In this case, a resist mask or the like is formed on the magnetic nanowire pattern on which the layer of the second heavy metal material has been formed, and then the layer of the second heavy metal material is partially trimmed by a milling method or the like to thin its film thickness. This allows for the formation of a pattern of periodic convex portions 32b as the second areas 3S in which the spin-orbit torque of the cap layer 32 is small. This allows the second areas 3S to be periodically formed for each unit bit length L by a simple process of removing unnecessary portions on the regions that will become the first areas 3F from the magnetic nanowire pattern on which the layer of the second heavy metal material has been formed.

[0042] Note that the above-mentioned methods 1, 2-1, and 2-2 involve ion implantation into the cap layer 32 to form an area that periodically reduces the spin-orbit torque, the addition of a second heavy metal material that forms the convex portion 32b, or partial removal of the layer formed of the second heavy metal material, and do not require special processing at least for the magnetic body 31 portion, making it possible to manufacture a magnetic nanowire memory that is easier to process than conventional methods.

[0043] [3. Driving method of magnetic nanowire memory] Next, a method for driving the magnetic domains recorded in the magnetic nanowire memory 1 will be described with reference to FIGS. First, the magnetic domain shift in the magnetic nanowire memory 1 will be described with reference to FIG. In order to shift the magnetic domains by current when driving the magnetic nanowire memory 1, it is necessary to apply a current above a certain level, that is, it has been experimentally confirmed that there is a threshold current density for the current domain wall motion phenomenon used for driving. As shown in Figure 9, the magnetic nanowire 30 comprises a magnetic body 31 processed into a nanowire shape and a cap layer 32 for protecting the magnetic body 31 from exposure to the atmosphere and infiltration by chemical solutions during the manufacturing process.

[0044] For the sake of explanation, as an example, in the initial state, when an external magnetic field is applied in the upward direction perpendicular to the film surface of the magnetic nanowire 30, the magnetic body 31 is magnetized upward as shown in Figure 9(a). The open upward arrow 35 indicates that the magnetization direction is upward. After that, when an external magnetic field in the opposite direction is applied near the center of the magnetic nanowire 30, a magnetic domain 37 with reversed magnetization is formed as shown in Figure 9(b).

[0045] Thereafter, when a pulse current is applied to the magnetic nanowire 30, the magnetic domain 37 can be shifted to the right in Fig. 9(c) . Here, the cap layer 32 is formed of a non-magnetic heavy metal material with large spin-orbit coupling, such as Pt, Ta, W, or Ir, and when a drive current is applied to the magnetic nanowire 30, a spin current is generated in the cap layer 32 due to the spin Hall effect.

[0046] As is well known, in the case of materials such as Pt and Ir, which are transition metals in the sixth period of the periodic table, when a current is passed through them, spins pointing toward the page move upward, and spins pointing away from the page move downward, resulting in a spin current. Therefore, materials such as Pt are defined as having a positive spin Hall angle. On the other hand, in the case of materials such as Ta and W, when a current is passed through them, spins pointing away from the page move upward, and spins pointing toward the page move downward, resulting in a spin current. Therefore, materials such as Ta are defined as having a negative spin Hall angle.

[0047] In this embodiment, the cap layer 32 made of a heavy metal and the magnetic body 31 are bonded together, so that a spin current is directly injected from the cap layer 32 into the magnetic body 31 at the interface between the cap layer 32 and the magnetic body 31, generating a spin-orbit torque 39. That is, spins pointing, for example, into the page gather on the surface of the cap layer 32 that is in contact with the magnetic body 31, aligning the electron spin direction in one direction, generating a torque. At this time, a torque is applied from the cap layer 32 made of a heavy metal to the magnetic body 31, and with the assistance of this torque, the magnetic domain 37 can be driven with less power consumption. A material with a larger spin Hall angle, which represents the efficiency of conversion from electric current to spin current, has a larger effective torque injected from the cap layer 32 into the magnetic body 31, which contributes to reducing the power consumption of magnetic domain driving.

[0048] Next, an example of a method for driving the magnetic domains recorded in the magnetic nanowire memory 1 will be described with reference to FIG. 10(a) is a partial cross-sectional view of the magnetic nanowire 30 of the magnetic nanowire memory 1. As shown in Fig. 10(a), in the first area 3F where the spin-orbit torque is large in the cap layer 32 of the magnetic nanowire 30, the spin-orbit torque assists the domain wall motion during magnetic domain driving, so the threshold current value of the drive current is low and domain wall motion becomes easy. On the other hand, in the second area 3S where the spin-orbit torque is small in the cap layer 32 of the magnetic nanowire 30, the threshold current value of the drive current is relatively high during magnetic domain driving, making it difficult for the domain wall motion to occur.

[0049] The threshold current value of the drive current flowing through the magnetic nanowire 30 may be set by converting it from the threshold current value of the current flowing through the magnetic body 31. The cross-sectional structure of the magnetic nanowire shown in FIG. 10(a) can be represented, for example, by the electric circuit shown in FIG. 10(b). Here, the cross-sectional area of ​​the magnetic body 31 is constant in the longitudinal direction, and the electric resistance value corresponding to the cross-sectional area is Rm. The cross-sectional area of ​​the cap layer 32 is constant in the longitudinal direction, and the electric resistance value corresponding to the cross-sectional area is Rc. In this case, when a drive current I is applied to the magnetic nanowire 30, a shunt current I1 flows through the magnetic body 31, while a shunt current I2 flows through the cap layer 32 (I = I1 + I2).

[0050] The method for driving the magnetic nanowire memory according to the embodiment includes a step of passing a driving current having a predetermined pulse interval through the magnetic nanowire 30. The driving current I flowing through the magnetic nanowire 30 has a pulse section that changes with time as shown in Fig. 10(c). The driving current I has a first threshold current value TH1 and a second threshold current value TH2. The first threshold current value TH1 is set as the lower limit of the current value at which the domain wall motion is easy in the first area 3F. The second threshold current value TH2 is set as the upper limit of the current value at which the domain wall motion is difficult in the second area 3S.

[0051] As shown in FIG. 10(c), the pulse section is made up of, for example, a section from time t1 to time t2 and a section from time t2 to time t3. The section from time t1 to time t2 is a section in which a signal is applied to the magnetic wire 30 such that the current value of the current flowing through the magnetic wire 30 in the second area 3S becomes a domain wall-driving current value greater than the second threshold current value TH2. The section from time t2 to time t3 is a section in which a signal is applied to the magnetic wire 30 so that the current value of the current flowing through the magnetic wire 30 in the first area 3F becomes smaller than the first threshold current value TH1. In this section, the current value of the driving current is gradually reduced from the domain wall driving current value, and the current value of the driving current I becomes smaller than the first threshold current value TH1 before time t3, as shown by the imaginary circle in FIG. 10(c).

[0052] As a result, as shown in Figure 10(a), at the time when driving of the unit bit length L corresponding to the consecutive second area 3S and first area 3F is completed, the current value of the current flowing through the magnetic wire 30 becomes below the lower limit of the current value (first threshold current value TH1) at which the domain wall is likely to move, making it difficult for the domain wall to shift. On the other hand, at the entrance of the second area 3S where the spin-orbit torque is small, the current value of the current flowing through the magnetic wire 30 becomes relatively high, exceeding the upper limit of the current value (second threshold current value TH2) at which the domain wall is unlikely to move, making it even more difficult for the domain wall to shift. For this reason, when the value of the current flowing through the magnetic wire 30 becomes smaller than the first threshold current value TH1 at the timing when driving of the unit bit length L ends, the shift of the magnetic domain stops. This enables stable driving while maintaining a constant magnetic domain length at the entrance part of each second area 3S formed periodically for each unit bit length L.

[0053] Furthermore, the magnetic nanowire memory 1B (FIG. 6) according to the second embodiment differs from the magnetic nanowire memory 1 in the stacking order of some components, but can provide similar effects. Furthermore, a similar effect can be obtained in a configuration in which a pattern of convex portions 32b is provided on the cap layer 32 in each second area 3S, as in the magnetic fine wire memory 1C of the third embodiment (Figure 7) and the magnetic fine wire memory 1D of the fourth embodiment (Figure 8).

[0054] Although the magnetic nanowire memory according to each embodiment of the present invention has been described above, the scope of the present invention is not limited to these descriptions and should be broadly interpreted based on the claims. Furthermore, it goes without saying that various changes and modifications based on these descriptions are also included in the scope of the present invention. For example, the magnetic nanowire memory 1 can include a control circuit for controlling the formation and driving of magnetic domains. Such a control circuit generates current magnetic fields in two directions at different timings in the recording element 50, thereby controlling the formation of magnetic domains in the magnetic nanowire 30. The control circuit also controls the on / off of the drive current supplied to the magnetic nanowire 30, thereby controlling the shifting of the magnetic domains formed in the magnetic nanowire 30.

[0055] [4. Verification Experiment Data] In order to confirm the effect of the magnetic nanowire memory of this embodiment, the following verification experiment was carried out. <Magnetic nanowire memory prototype> The magnetic nanowire memory 1C shown in Fig. 7 was fabricated by the following procedure. First, as shown in Fig. 11(a), a substrate 10 on which an insulating layer 20 was formed was prepared. That is, the surface of the substrate 10 was thermally oxidized to form SiO XA Si substrate on which a film was formed was prepared. A thin magnetic nanowire 30 was formed on this Si substrate as shown in FIG. 11(b). Specifically, a Co / Tb multilayer film was formed by repeatedly stacking a 0.95 nm-thick terbium (Tb) layer and a 0.35 nm-thick cobalt (Co) layer four times, and patterning the film to form a magnetic material 31. To prevent oxidation of the magnetic nanowire material, a 3 nm-thick metal film using platinum (Pt) as a conductive material was deposited on the top surface of the Co / Tb multilayer film in the same vacuum to form a cap layer 32. In other words, the film structure of the magnetic nanowire 30 was Pt (3 nm) / [Co (0.35 nm) / Tb (0.95 nm)]4. The processing procedure was lithography, film deposition, and lift-off in that order.

[0056] Next, as shown in FIG. 11(c), convex portions 32b were formed on the surface of the magnetic nanowire 30 at a period of each unit bit length. Specifically, a tantalum (Ta) layer with a thickness of 3 nm was deposited and patterned to form multiple convex portions 32b. The processing procedure was lithography, deposition, and lift-off in that order. The sign of the spin-Hall angle of Ta is different from the sign of the spin-Hall angle of Pt, the material of the cap layer 32. Therefore, the effective spin-orbit torque is smaller in the area where Ta is formed than in other areas.

[0057] Next, as shown in FIG. 11(d), an interlayer insulating layer 40 was formed on the Si substrate from above the magnetic nanowire 30 and the plurality of convex portions 32b. Specifically, a silicon nitride film (SiN X ) layer and a 25 nm thick silicon oxide film (SiO X ) were stacked in this order and patterned to form the interlayer insulating layer 40. That is, the interlayer insulating layer 40 has a film structure of SiO X (25 nm) / SiN X The fabrication process is lithography, film formation, and lift-off in that order.

[0058] Finally, as shown in FIG. 11(e), the recording element 50 and a measurement extraction electrode pad (not shown) were formed in the same manner as described below. Specifically, a 3-nm-thick tantalum (Ta) layer, a 150-nm-thick silver (Ag) layer, a 20-nm-thick gold (Au) layer, and a 5-nm-thick tantalum (Ta) layer were stacked in this order and patterned. In other words, the recording element 50 and electrode pad layer were multilayer films with a film configuration of Ta (5 nm) / Au (20 nm) / Ag (150 nm) / Ta (3 nm). The processing procedure was lithography, film formation, and lift-off in that order.

[0059] <Conditions and procedures for evaluation experiment 1 (recording)> An upward external magnetic field was applied to the magnetic nanowire memory fabricated by the above procedure, and the magnetic nanowire was initialized to a uniform upward magnetization direction. Then, a pulse current was applied to the recording element 50, and the magnetization direction of the region of the magnetic nanowire 30 adjacent to the recording element 50 was reversed to a downward direction by the induced current magnetic field. In other words, a downward magnetization direction was recorded in the region of the magnetic nanowire 30 adjacent to the recording element 50, and a downward magnetic domain was formed. The pulse current conditions at this time were a pulse width of 12 [μs] and a current value of 71 [mA].

[0060] <Results of Evaluation Experiment 1 (Recording)> Figure 12(a) shows the results of observing the magnetic domains recorded in the magnetic nanowire memory using a magneto-optical microscope. Figure 12(b) is a schematic diagram of the magneto-optical microscope image of Figure 12(a), and the area with dots in Figure 12(b) indicates the area of ​​the recorded downward magnetic domain. In the magneto-optical microscope image of Figure 12(a), this area appears whiter than the other areas in the magnetic nanowire.

[0061] <Conditions and procedures for evaluation experiment 2 (shift)> Next, a pulse current was applied to the magnetic nanowire 30 in which the downward magnetic domain was formed as a drive current for shifting the magnetic domain. That is, the recording magnetic domain in which the magnetization was reversed in the magnetic nanowire 30 was driven to the right. The pulse current conditions at this time were a pulse width of 12 μs, a current rise time of 1 μs, a current fall time of 1 μs, and a current value of 2.5 mA.

[0062] <Results of evaluation experiment 2 (shift)> FIG. 12(c) shows the results of observing the driven magnetic domains at this time with a magneto-optical microscope. FIG. 12(d) is a schematic diagram of the magneto-optical microscope image of FIG. 12(c), and the dotted areas in FIG. 12(d) indicate the areas where the magnetic domains have shifted. Specifically, in the magnetic nanowire 30, the magnetic domains recorded from the location corresponding to the right end of the recording element 50 to the left end of the second convex-shaped portion 32b (second area) have shifted to the left end of the third convex-shaped portion 32b (second area). The magneto-optical microscope image of FIG. 12(c) reveals that this region is whiter than other regions in the magnetic nanowire. In other words, the driving of the magnetic domains has stopped at the left end of the convex-shaped portion 32b, i.e., the area where Ta is formed on the surface of the magnetic nanowire 30.

[0063] This is thought to be because the magnetic domain recorded as shown in Figure 12(a) shifted to the right in Figure 12 when a driving current pulse was applied, and because the spin-orbit torque in the adjacent second area 3S was small, the threshold current value was relatively large, making it difficult for the moving magnetic domain to move the domain wall, causing it to stop shifting on the spot. Therefore, this evaluation experiment 2 confirmed that by providing an area with a small spin-orbit torque, magnetic domains are trapped during driving. [Explanation of symbols]

[0064] 1,1B,1C,1D magnetic nanowire memory 3F Area 1 3S Area 2 10 Substrate 20 insulating layer 30 Magnetic thin wire 31 Magnetic material 32 Cap Layer 32b Convex shape part 40 Interlayer insulation layer 50 Recording element 60 Playhead L unit bit length

Claims

1. a magnetic nanowire having a magnetic body formed linearly on a substrate and a cap layer made of a non-magnetic heavy metal material and disposed on the magnetic body; the cap layer has a first area and a second area in which spin-orbit torques due to the cap layer are different; The magnetic nanowire memory is characterized in that the second areas are formed periodically for each unit bit length.

2. 2. The magnetic nanowire memory according to claim 1, wherein the second area has a smaller spin-orbit torque than the first area.

3. 3. The magnetic nanowire memory of claim 2, wherein the cap layer is doped only in the second area with ions that decrease the spin Hall angle of the heavy metal material, or only in the first area with ions that increase the spin Hall angle of the heavy metal material.

4. the cap layer is formed of a first heavy metal material; a convex portion formed on the cap layer in each of the second areas and made of a second heavy metal material different from the first heavy metal material; 3. The magnetic nanowire memory according to claim 2, wherein the sign of the spin Hall angle of the second heavy metal material is different from the sign of the spin Hall angle of the first heavy metal material.

5. 5. The magnetic nanowire memory according to claim 3, wherein the magnetic nanowire, an interlayer insulating layer, and a recording element are provided on the substrate in this order in parallel with the substrate.

6. 5. The magnetic nanowire memory according to claim 3, further comprising a recording element and an insulating layer, an interlayer insulating layer, and the magnetic nanowire, which are provided on the substrate in this order in parallel with the substrate.

7. the recording element is formed linearly in a direction perpendicular to and intersecting the magnetic nanowire, a read head is further provided at a position directly above the magnetic nanowire with the interlayer insulating layer interposed therebetween and spaced a predetermined length longer than a unit bit length from the recording element; 6. The magnetic nanowire memory according to claim 5, wherein the first area and the second area are disposed between the recording element and the reproducing head.

8. the recording element is formed linearly in a direction perpendicular to and intersecting the magnetic nanowire, a read head is further provided on the magnetic nanowire and at a position spaced apart from the recording element by a predetermined length longer than a unit bit length; 7. The magnetic nanowire memory according to claim 6, wherein the first area and the second area are disposed between the recording element and the reproducing head.

9. A method for driving the magnetic nanowire memory according to any one of claims 2 to 4, comprising: a step of passing a drive current having a predetermined pulse section through the magnetic nanowire; a first threshold current value is set for the drive current as a lower limit of the current value at which domain walls are likely to move in the first area, and a second threshold current value greater than the first threshold current value is set as an upper limit of the current value at which domain walls are unlikely to move in the second area; The pulse section is a section in which a signal is applied to the magnetic nanowire such that the current value of the current flowing through the magnetic nanowire in the second area becomes a domain wall-driving current value greater than the second threshold current value; a section in which a signal is applied to the magnetic nanowire so that the current value of the current flowing through the magnetic nanowire in the first area becomes smaller than the first threshold current value.

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