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 conductive material and disposed on the magnetic body 31, and has a first area 3F and a second area 3H in which a ratio of thickness of the magnetic body 31 to total layer thickness from the substrate 10 to the cap layer 32 is different, and the second area is periodically formed for each unit bit length. The total layer thickness from the substrate 10 to the cap layer 32 in the second area 3H is larger than that in the first area 3F, and a ratio of thickness of the magnetic body 31 to the total layer thickness in the second area 3H is smaller than that in 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 conductive material and placed on the magnetic body, and has a first area and a second area in which the proportion of the film thickness of the magnetic body to the total layer thickness from the substrate to 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, and each area has a different ratio of the magnetic body's film thickness to the total layer thickness from the substrate to the cap layer disposed on the magnetic body. The magnetic nanowire memory has the second areas periodically arranged at regular intervals to match the length of each unit bit length, allowing the shift length to be specified. The magnetic nanowire memory does not require the high-precision microfabrication required to create conventional constrictions, making it easy to process.

[0010] Furthermore, in magnetic nanowire memory, the magnetic nanowire includes a cap layer made of a conductive material, so the drive current applied to the magnetic nanowire to drive the domain wall is split between the magnetic material and the cap layer. Because the proportion of magnetic material differs between the first and second areas, the split current flowing through the magnetic material differs in each area. When a drive current is applied, the split current flowing through the magnetic material in the first area, where the magnetic material occupies a large proportion of the film thickness, is smaller in the second area, where the magnetic material occupies a small proportion of the film thickness. When the split current flowing through the magnetic material in the second area, where the magnetic material occupies a small proportion of the film thickness, becomes smaller than the threshold current value that enables domain wall movement, the magnetic domain shift stops. This enables stable drive 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 in which the ratio of the film thickness of the magnetic material to the total thickness from the substrate to the cap layer disposed on the magnetic material is different are periodically provided at regular intervals. The magnetic nanowire memory makes it possible to keep the magnetic domain length 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 first modified example. [Figure 8] FIG. 10 is a cross-sectional view of a magnetic nanowire memory according to a second modification. [Figure 9] FIG. 2 is an explanatory diagram of a driving current flowing through a magnetic nanowire. [Figure 10] 10A and 10B are top views showing a manufacturing process of the magnetic nanowire memory according to the first modified example. [Figure 11] 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. Principle of maintaining a constant magnetic domain length in the magnetic nanowire memory, 4. Method of driving the magnetic nanowire memory, and 5. 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 (first 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 conductive material and disposed on the magnetic body 31. The magnetic nanowire memory 1 has a first area 3F and a second area 3H in which the proportion of the film thickness of the magnetic body 31 to the total layer thickness from the substrate 10 to the cap layer 32 differs, and the second area 3H is formed periodically for each unit bit length L. Furthermore, as an example, the magnetic nanowire memory 1 has a structure in which the total layer thickness from the substrate 10 to the cap layer 32 is greater in the second area 3H than in the first area 3F, and the proportion of the magnetic material film thickness to the total layer thickness is smaller in the second area 3H than in the first area 3F.

[0016] Note that the names of the first area 3F and the second area 3H are relative, and therefore the features of the first area 3F and the second area 3H may be interchanged, although this is not shown in the drawings. That is, as another example, the magnetic nanowire memory 1 may have a structure in which the total layer thickness from the substrate 10 to the cap layer 32 is greater in the first area 3F than in the second area 3H, and the proportion of the magnetic body film thickness to the total layer thickness is smaller in the first area 3F than in the second area 3H.

[0017] In the following description, it is assumed that the total layer thickness from the substrate 10 to the cap layer 32 is greater in the second area 3H than in the first area 3F. In this case, the second area 3H specifically refers to the respective positions where the shift of the domain wall should be stopped. In other words, the second area 3H is a position where the shift of the magnetic domain is trapped. Regarding the longitudinal length of the magnetic nanowire 30, the lengths of the first area 3F and the second area 3H may be equal, or the length of the second area 3H may be shorter than the length of the first area 3F. In the first embodiment, the magnetic nanowire memory 1 has areas (convex patterns) where the cap layer 32 is thick and other areas (concave patterns) formed for each unit bit length L.

[0018] Here, the total layer thickness from the substrate 10 to the cap layer 32 (hereinafter simply referred to as the total layer thickness) is the sum of the film thickness of the magnetic body 31 and the film thickness of the cap layer 32. As shown in Fig. 3, the total layer thickness in the second area 3H is larger (thicker) than the total layer thickness in the first area 3F. Furthermore, the proportion of the film thickness of the magnetic body 31 to the total layer thickness in the second area 3H is smaller than the proportion of the film thickness of the magnetic body 31 to the total layer thickness in the first area 3F.

[0019] As shown in Figure 3, in the first area 3F and the second area 3H, the magnetic body 31 of the magnetic nanowire 30 has a flat shape with a constant thickness. The thickness of the cap layer 32 in the second area 3H is formed to be thicker than the thickness of the cap layer 32 in the first area 3F. This allows the second area 3H to be formed periodically for each unit bit length L by simple processing, such as adding a conductive material to periodically thicken the cap layer when forming the cap layer 32, or removing unnecessary portions from a cap layer that has been previously formed thick. The thickness of the cap layer 32 is 3 nm or more, and there is no particular upper limit, but since the thicker the cap layer, the higher the power consumption. Therefore, the thickness is set to a desired value within the allowable power consumption range.

[0020] 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 3H are arranged between the recording element 50 and the reproducing head 60.

[0021] In the magnetic nanowire memory 1, the magnetic nanowire 30 includes a cap layer 32 made of a conductive material, so that the current applied to the magnetic nanowire 30 to drive the domain wall is divided between the magnetic body 31 and the cap layer 32. In the magnetic nanowire memory 1, the total layer thickness in the second area 3H is greater than the total layer thickness in the first area 3F, and the proportion of the film thickness of the magnetic body 31 to the total layer thickness in the second area 3H is smaller than the proportion of the film thickness of the magnetic body 31 to the total layer thickness in the first area 3F. Therefore, the current diverted to the magnetic body 31 in the second area 3H is smaller than the current diverted to the magnetic body 31 in the first area 3F. Therefore, as will be described in detail later, the current diverted to the magnetic body 31 is smaller at the entrance of the second area 3H than in the first area 3F. For this reason, when the current diverted to the magnetic body 31 in the second area 3H becomes smaller than the threshold current value that enables domain wall motion, the magnetic domain shift stops. This allows stable driving while maintaining a constant magnetic domain length at the entrance of each second area 3H formed periodically for each unit bit length L.

[0022] (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 and description thereof will be omitted. The magnetic nanowire memory 1B includes, on a substrate 10 and parallel to the substrate 10, a recording element 50 and an insulating layer 20, an interlayer insulating layer 40, a conductive magnetic nanowire 30, and a reproducing head 60, in this order. The magnetic nanowire memory 1B differs from the magnetic nanowire memory 1 in the stacking order of some components, but has a first area 3F and a second area 3H in which the proportion of the film thickness of the magnetic body 31 to the total layer thickness is different, and the second area 3H is formed periodically for each unit bit length L. However, in the magnetic nanowire memory 1B, the total layer thickness from the substrate 10 to the cap layer 32 is the sum of the film thickness of the magnetic body 31, the film thickness of the cap layer 32, and the thickness of the interlayer insulating layer 40.

[0023] 6, in the magnetic nanowire memory 1B, the magnetic body 31 of the magnetic nanowire 30 has a flat shape with a constant thickness in the first area 3F and the second area 3H. The thickness of the cap layer 32 in the second area 3H is formed to be thicker than the thickness of the cap layer 32 in the first area 3F. 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. As with the magnetic nanowire memory 1, the magnetic nanowire memory 1B can be stably driven while maintaining a constant magnetic domain length at the entrance of each second area 3H formed periodically for each unit bit length L.

[0024] (First Modification) A modified example (first modified example) of the magnetic nanowire memory 1 according to the first embodiment shown in Fig. 3 will be described with reference to Fig. 7. Note that the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted. The magnetic nanowire memory 1C according to the first modified example differs from the magnetic nanowire memory 1 according to the first embodiment in that it further includes conductive members 70 periodically arranged for each unit bit length L on the substrate 10 and directly below the magnetic nanowire 30 in the second area 3H.

[0025] In the first area 3F of the magnetic nanowire memory 1C, the total layer thickness from the substrate 10 to the cap layer 32 is the sum of the film thickness of the magnetic body 31 and the film thickness of the cap layer 32. On the other hand, in the second area 3H of the magnetic nanowire memory 1C, the total layer thickness from the substrate 10 to the cap layer 32 is the sum of the film thickness of the magnetic body 31, the film thickness of the cap layer 32, and the thickness of the conductive member 70. Note that the film thicknesses of the magnetic body 31 and the cap layer 32 of the magnetic nanowire 30 are kept constant in each area.

[0026] Since the magnetic nanowire memory 1C has conductive members 70 periodically arranged below the magnetic nanowire 30, the current applied to the magnetic nanowire 30 to drive the domain wall is not only diverted between the magnetic body 31 and the cap layer 32, but also diverted to the conductive members 70 in the second area 3H. In the magnetic nanowire memory 1C, the total layer thickness of the second areas 3H is greater than the total layer thickness of the first areas 3F, and the proportion of the film thickness of the magnetic body 31 to the total layer thickness in the second areas 3H is smaller than the proportion of the film thickness of the magnetic body 31 to the total layer thickness in the first areas 3F. Therefore, the magnetic nanowire memory 1C also becomes capable of stable driving while maintaining a constant magnetic domain length at the entrance portions of the second areas 3H periodically formed for each unit bit length.

[0027] The conductive member 70 is made of a conductive material such as platinum (Pt), tantalum (Ta), or ruthenium (Ru). The thickness of the conductive member 70 is set within the allowable range of its power consumption. Furthermore, the inventors of the present application conducted experiments to verify the balance between the ease of driving when shifting magnetic domains and the trapping force that stops the magnetic domains at desired positions when a drive current is passed through the magnetic nanowire according to this modification, and found that the upper limit of the thickness of the conductive member 70 is approximately 70% of the film thickness of the magnetic body 31. For this reason, it is desirable that the thickness of the conductive member 70 be 70% or less of the film thickness of the magnetic body 31.

[0028] As shown in the cross-sectional view of FIG. 7, the plurality of conductive members 70 are arranged at regular intervals (unit bit length L) in the longitudinal direction (x-axis direction) of the magnetic nanowire 30. In addition, in the xy plan view, as an example, the longitudinal direction of the conductive members 70 is the direction perpendicular to the longitudinal direction of the magnetic nanowire 30 (the conductive members 70 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 conductive members 70 are formed into an oval shape in a plan view (see FIG. 10(b)). The shape of the conductive members 70 is not particularly limited as long as they are formed on the magnetic nanowire 30 at regular intervals.

[0029] (Second Modification) A modified example (second modified example) of the magnetic nanowire memory 1B according to the second embodiment shown in Fig. 6 will be described with reference to Fig. 8. Note that the same components as those in the second embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted. The magnetic nanowire memory 1D according to the second modified example differs from the magnetic nanowire memory 1B according to the second embodiment in that it further includes conductive members 70 periodically arranged for each unit bit length L on the interlayer insulating layer 40 and directly below the magnetic nanowire 30 in the second area 3H.

[0030] In the first area 3F of the magnetic nanowire memory 1D, the total layer thickness from the substrate 10 to the cap layer 32 is the sum of the film thickness of the magnetic body 31, the film thickness of the cap layer 32, and the thickness of the interlayer insulating layer 40. On the other hand, in the second area 3H of the magnetic nanowire memory 1D, the total layer thickness from the substrate 10 to the cap layer 32 is the sum of the film thickness of the magnetic body 31, the film thickness of the cap layer 32, the thickness of the interlayer insulating layer 40, and the thickness of the conductive member 70. In each area, the film thicknesses of the magnetic body 31 and the cap layer 32 of the magnetic nanowire 30 and the thickness of the interlayer insulating layer 40 are kept constant.

[0031] For the reasons described above, in the magnetic nanowire memory 1D as well, it is preferable that the thickness of the conductive member 70 be 70% or less of the film thickness of the magnetic material. As with the magnetic nanowire memory 1C, the magnetic nanowire memory 1D also enables stable driving while maintaining a constant magnetic domain length at the entrance portion of each second area 3H that is periodically formed for each unit bit length.

[0032] [2. Manufacturing method of magnetic nanowire memory] Next, details of each part of the magnetic nanowire memory and its manufacturing method will be described. (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.

[0033] (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.

[0034] (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.

[0035] (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.

[0036] 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 formed from a conductive metal material, and a material whose conductive properties are resistant to deterioration due to oxidation, etc., is preferred. Specific examples include platinum (Pt), tantalum (Ta), and ruthenium (Ru).

[0037] (Method for forming a periodic convex pattern on a cap layer) <Method 1> In the magnetic nanowire memory 1 shown in FIG. 3 and the magnetic nanowire memory 1B shown in FIG. 6, an area where the cap layer 32 is thick is formed for each unit bit length L. For example, a processing method for this is to first form a magnetic film made of a magnetic material on the substrate 10, then form a metal film made of a conductive metal material and process it into a thin wire. In this way, a flat magnetic nanowire including the cap layer is patterned. After that, a procedure is taken in which a pattern made of a conductive material is additionally formed by lithography or the like to periodically thicken the thickness of the cap layer.

[0038] <Method 2> Another method is to form a pattern of a magnetic nanowire including a cap layer by forming a thicker cap layer in advance, and then remove the unnecessary portions that will become the recessed pattern. In this case, after forming a resist mask or the like on the magnetic nanowire pattern with a thicker cap layer, the cap layer is partially trimmed by a milling method or the like to thin its film thickness. This allows a periodic convex pattern to be formed as areas where the cap layer 32 is thick. Furthermore, for example, when a driving current for shifting the recording magnetic domain formed in the magnetic nanowire 30 is passed through the magnetic nanowire 30 from a pulse current source, in areas where the cap layer 32 is thick, the current shunted to the magnetic material 31 is smaller than in other areas within the same magnetic nanowire 30.

[0039] <Method 3> 7, an area where the total layer thickness from the substrate 10 to the cap layer 32 is thick is formed for each unit bit length L. This is formed by forming a conductive member 70 for each unit bit length on the substrate 10 (on the insulating layer 20) in advance and then depositing a magnetic nanowire material. Note that in the case of the magnetic nanowire memory 1D shown in FIG. 8, the area is formed by forming a conductive member 70 for each unit bit length on the interlayer insulating layer 40 in advance and then depositing a magnetic nanowire material.

[0040] After the periodic convex pattern of the conductive members 70 is formed in this manner, the magnetic material 31 is deposited on top of it to a constant thickness. Furthermore, the cap layer 32 is deposited on top of it to a constant thickness. By doing this, the shape of the periodic convex pattern of the conductive members 70 is sequentially transferred to the upper surface of the upper metal film, and a convex pattern similar to the periodic convex pattern of the conductive members 70 is formed on the upper surface of the cap layer 32.

[0041] Specifically, the periodic convex pattern of the conductive member 70 is formed by lithography using a conductive material such as platinum (Pt), tantalum (Ta), or ruthenium (Ru) to form a linear (one-dimensional) convex pattern. This allows the formation of the periodic convex steps according to the first or second modification. In these cases, when a driving current is passed through the magnetic nanowire 30, the shunt current of the magnetic body 31 is smaller in the area where the convex pattern is formed than in other areas of the same magnetic nanowire 30.

[0042] Note that the above methods 1 to 3 involve adding or partially removing material from the cap layer 32 to form a periodic convex pattern, or forming a conductive member 70 in advance, and do not require special processing of the magnetic body 31, making it possible to manufacture a magnetic nanowire memory that is easier to process than conventional methods.

[0043] [3. Principle of maintaining constant magnetic domain length in magnetic nanowire memory] Next, the principle of keeping the magnetic domain length constant in the magnetic nanowire memory 1 will be described with reference to FIGS. 9(a) and 9(b) (and also with reference to FIGS. 1 to 3 as appropriate). In order to shift the magnetic domains by current when driving the magnetic nanowire memory 1, it is necessary to apply a current of a certain level or more. In other words, it has been experimentally confirmed that there is a threshold current density for the current-induced domain wall motion phenomenon used for driving. As shown in Figure 9(a), the magnetic nanowire 30 comprises a magnetic body 31 processed into a thin wire shape and a cap layer 32 that protects the magnetic body 31 from exposure to the atmosphere and infiltration by chemical solutions during the manufacturing process. Here, the cap layer 32 is formed of a conductive material, and the driving current applied to the magnetic nanowire 30 is divided between the magnetic body 31 and the cap layer 32.

[0044] If the cross-sectional areas of the magnetic body 31 and the cap layer 32 are constant along the length of the magnetic nanowire 30, the ratio of the shunt currents in the magnetic body 31 and the cap layer 32 during driving will be a constant value based on their respective resistance values. If a driving current is applied to the magnetic nanowire 30 so that the shunt current flowing through the magnetic body 31 is equal to or greater than the threshold current value required for driving, the magnetic domain can be driven. In the following explanation, it is assumed that the cross-sectional area of ​​the magnetic body 31 is constant, and the threshold current density is the threshold current value.

[0045] The magnetic nanowire memory 1 is provided periodically with second areas 3H for each unit bit length L, in which the total layer thickness from the substrate 10 to the cap layer 32 is large and the proportion occupied by the film thickness of the magnetic material is small, so that the shunt current to the magnetic material 31 is small in the second areas 3H. The magnetic nanowire memory 1 is fabricated by processing so as to create second areas 3H in which the film thickness of the magnetic nanowire 30 is thick, and specifically, is processed so that the film thickness of the cap layer 32 periodically becomes thicker (convex pattern) for each unit bit length L. As a result, the resistance value of the cap layer 32 arranged in the second areas 3H (convex pattern) in which the film thickness of the magnetic nanowire 30 is thick differs from the resistance value of the cap layer 32 arranged in the other first areas 3F (concave pattern).

[0046] For this reason, when a driving current flows through the magnetic nanowire 30, the current is diverted differently between the magnetic body 31 and the cap layer 32. The resistance value of the cap layer 32 in the second area 3H, which is relatively thick, is smaller than the resistance value of the cap layer 32 in the other first area 3F. Therefore, in the second area 3H, which is thick, the current diverted to the cap layer 32 is larger than the current diverted to the cap layer 32 in the other first area 3F. Therefore, the current diverted to the magnetic body 31 arranged in the second area 3H, where the cap layer 32 is thick, is relatively smaller than the current diverted to the magnetic body 31 arranged in the first area 3F, where the cap layer 32 is thin. In short, the current diverted to the magnetic body 31 can be made smaller in the second area 3H, where the cap layer 32 is thick, compared to the other first area 3F.

[0047] If the shunt current flowing to the magnetic body 31 in the second area 3H where the cap layer 32 is thick is made smaller than the threshold current value required for driving during magnetic domain driving, it becomes difficult for the magnetic domain wall to move, and the shift movement stops immediately. For this reason, in this embodiment, the second area 3H where the cap layer 32 is thick is used as a structure in which the shunt current to the magnetic body 31 is reduced for each unit bit length L of the magnetic nanowire memory 1.

[0048] The cross-sectional structure of the magnetic nanowire shown in Fig. 9(a) can be represented by the electric circuit shown in Fig. 9(b). Here, the explanation will be given assuming that the following conditions are set. Note that the contribution of the resistance value to the longitudinal length of the magnetic nanowire 30 is ignored. The magnetic body 31 has a constant cross-sectional area in the longitudinal direction for the first area 3F and the second area 3H, and an electrical resistance value Rm corresponding to the cross-sectional area. In the first area 3F where the film thickness (cross-sectional area) of the cap layer 32 is small, the electrical resistance value corresponding to the cross-sectional area is Rc1. In the second area 3H where the film thickness (cross-sectional area) of the cap layer 32 is large, the electrical resistance value corresponding to the cross-sectional area is Rc2 (Rc2 <Rc1)である。 When a driving current I is applied to the magnetic wire 30, for example, in the first area 3F, a shunt current I1 flows through the magnetic body 31, while a shunt current Ia flows through the cap layer 32 (I=I1+Ia). When a driving current I is applied to the magnetic wire 30, for example, in the second area 3H, a shunt current I2 flows through the magnetic body 31, while a shunt current Ib flows through the cap layer 32 (I=I2+Ib).

[0049] Under the above conditions, the shunt current I1 of the magnetic body 31 in the first area 3F is expressed by the following formula (1), while the shunt current I2 of the magnetic body 31 in the second area 3H is expressed by the following formula (2). Furthermore, when the drive current I is a constant value, the shunt current I2 of the magnetic body 31 in the second area 3H, which has a relatively thick film thickness, is smaller than the shunt current I1 of the magnetic body 31 in the first area 3F (formula (3)). The principle of making the magnetic domain length constant in the magnetic nanowire memories 1B, 1C, and 1D described above is the same.

[0050]

number

[0051] [4. Driving method of magnetic nanowire memory] Next, an example of a driving method for driving the magnetic domains recorded in the magnetic nanowire memory 1 will be described with reference to Fig. 9. The driving method for the magnetic nanowire memory according to the embodiment includes a step of passing a driving current having a predetermined pulse section through the magnetic nanowire 30. The drive current flowing through the magnetic nanowire 30, that is, the drive current I in the above-mentioned formulas (1) and (2), has a pulse section that changes with time as shown in FIG. 9(c).

[0052] As shown in FIG. 9(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 period from time t1 to time t2 is a period in which a signal is applied to the magnetic wire 30 from a pulse power supply (not shown) such that the current value of the shunt current I2 flowing through the magnetic body 31 in the second area 3H of the magnetic wire 30 becomes a domain wall driving current value greater than the threshold current value TH that enables domain wall movement. The period from time t2 to time t3 is a period during which a signal is applied to the magnetic wire 30, so that the current value of the shunt current I2 flowing through the magnetic body 31 in the second area 3H becomes smaller than the threshold current value TH, and the current value of the shunt current I1 flowing through the magnetic body 31 in the first area 3F becomes larger than the threshold current value TH. In this period, 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 threshold current value TH before time t3, as shown by the imaginary circle in FIG. 9(c).

[0053] As a result, at the timing when driving of the unit bit length L ends, the shunt current I2 flowing through the magnetic body 31 becomes equal to or less than the threshold current value TH. Therefore, at the entrance portion of the second area 3H where the film thickness is thick, the resistance value Rc2 of the cap layer 32 is smaller than the resistance value Rc1 of the cap layer 32 in the first area 3F, and the shunt current I2 to the magnetic body 31 becomes smaller. Therefore, the shift of the magnetic domain stops, and driving can be performed while maintaining a constant magnetic domain length at the intervals between the protrusions and recesses of the cap layer 32 on the surface of the magnetic nanowire 30.

[0054] Furthermore, a similar effect can be obtained in a configuration in which the thickness of the cap layer 32 is kept constant and a convex pattern made of conductive material 70 is periodically formed below the magnetic nanowire 30, as in the magnetic nanowire memory 1C (Figure 7) according to the first modified example. Furthermore, the same effect can be obtained with the magnetic nanowire memory 1B according to the second embodiment (FIG. 6) and the magnetic nanowire memory 1D according to the second modified example (FIG. 8).

[0055] 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.

[0056] [5. 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. 10(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 X A Si substrate on which a film was formed was prepared. As shown in FIG. 10(b), a plurality of conductive members 70 were formed on this Si substrate. Specifically, a metal film with a thickness of 3 nm was formed on the Si substrate using platinum (Pt) as a conductive material, and convex steps with a width corresponding to the unit bit length L were periodically patterned as the conductive members 70. The processing procedure was lithography, film formation, and lift-off in that order.

[0057] Next, as shown in FIG. 10(c), a thin magnetic nanowire 30 was formed on the pattern of the conductive member 70. 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 the magnetic material 31. To prevent oxidation of the magnetic nanowire material, a 3 nm-thick metal film was formed on the top surface of the Co / Tb multilayer film in the same vacuum using platinum (Pt) as a conductive material, forming a cap layer 32. In other words, the film configuration of the magnetic nanowire 30 was Pt (3 nm) / [Co (0.35 nm) / Tb (0.95 nm)]4. The processing procedure was lithography, film formation, and lift-off in that order.

[0058] Next, as shown in FIG. 10(d), an interlayer insulating layer 40 was formed on the Si substrate from above the magnetic nanowire 30. 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 The fabrication process is lithography, film formation, and lift-off.

[0059] Finally, as shown in FIG. 10(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.

[0060] <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 adjacent to the recording element 50 in the magnetic nanowire 30 was reversed to a downward direction by the induced current magnetic field. In other words, a downward magnetization direction was recorded in the region adjacent to the recording element 50 in the magnetic nanowire 30, and a downward magnetic domain was formed. The pulse current conditions at this time were a pulse width of 30 [μs] and a current value of 72 [mA].

[0061] <Results of Evaluation Experiment 1 (Recording)> Figure 11(a) shows the results of observing the magnetic domains recorded in the magnetic nanowire memory using a magneto-optical microscope. Figure 11(b) is a schematic diagram of the magneto-optical microscope image of Figure 11(a), and the area with dots in Figure 11(b) indicates the area of ​​the recorded downward magnetic domain. In the magneto-optical microscope image of Figure 11(a), this area appears whiter than the other areas in the magnetic nanowire.

[0062] <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 30 μs, a current rise time of 1 μs, a current fall time of 2 μs, and a current value of 3.3 mA.

[0063] <Results of evaluation experiment 2 (shift)> FIG. 11(c) shows the results of observing the driven magnetic domains at this time using a magneto-optical microscope. FIG. 11(d) is a schematic diagram of the magneto-optical microscope image of FIG. 11(c), and the dotted areas in FIG. 11(d) indicate the areas where the magnetic domains have shifted. Specifically, in the magnetic nanowire 30, the magnetic domains recorded from the point corresponding to the right end of the recording element 50 to the first convex portion (conductive member 70) have shifted to the left end of the second convex portion (conductive member 70). In the magneto-optical microscope image of FIG. 11(c), this region appears whiter than other regions in the magnetic nanowire. In other words, the magnetic domains stopped moving at the left end of the conductive member 70, i.e., the platinum (Pt) convex pattern portion.

[0064] 11(a), the magnetic domains recorded as shown in Fig. 11(a) moved toward the thicker second area 3H due to the application of a driving current pulse, and a current smaller than the threshold current value flowed through the magnetic material 31 arranged in that second area 3H, making it difficult for the moving magnetic domains to move their domain walls, causing them to stop shifting. Therefore, this evaluation experiment 2 confirmed that the shift of magnetic domains during driving was stopped when the current flowing through the magnetic material due to the shunting became equal to or less than the threshold current value. [Explanation of symbols]

[0065] 1,1B,1C,1D magnetic nanowire memory 3F Area 1 3H Area 2 10 Substrate 20 insulating layer 30 Magnetic thin wire 31 Magnetic material 32 Cap Layer 40 Interlayer insulation layer 50 Recording element 60 Playhead 70 Conductive material 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 conductive material and disposed on the magnetic body; a first area and a second area in which the ratio of the film thickness of the magnetic material to the total thickness from the substrate to the cap layer is different; The magnetic nanowire memory is characterized in that the second areas are formed periodically for each unit bit length.

2. the second area has a larger total layer thickness from the substrate to the cap layer than the first area; 2. The magnetic nanowire memory according to claim 1, wherein the proportion of the film thickness of the magnetic material in the second area to the total layer thickness is smaller than that in the first area.

3. In the first area and the second area, the magnetic body has a flat shape with a constant film thickness, 3. The magnetic nanowire memory according to claim 2, wherein the thickness of the cap layer in the second area is formed to be thicker than the thickness of the cap layer in the first area.

4. In the second area, conductive members are further provided on the substrate and directly below the magnetic nanowire, and are arranged periodically for each unit bit length; 3. The magnetic nanowire memory according to claim 2, wherein the thickness of the conductive member is 70% or less of the film thickness of the magnetic 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; The pulse section is a section in which a signal is applied to the magnetic nanowire such that the current value of the shunt current flowing through the magnetic body in the second area becomes a domain wall driving current value greater than a threshold current value that enables domain wall motion; a section in which a signal is applied to the magnetic wire such that the current value of the shunt current flowing through the magnetic material in the second area is smaller than the threshold current value, and the current value of the shunt current flowing through the magnetic material in the first area is larger than the threshold current value.

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