Superparamagnetic magnetic tunnel junction element and computing system

Abstract

[Problem] To provide a superparamagnetic tunnel junction element and a computing system using same, the tunnel junction element having excellent operation stability (robustness) against an external magnetic field and being suitable for the computing system based on probabilistic computing. [Solution] This superparamagnetic tunnel junction element 10 comprises a first ferromagnetic layer group 14 including a ferromagnetic material, a second ferromagnetic layer group 12 including a ferromagnetic material, and a barrier layer 13 disposed between the first ferromagnetic layer group 14 and the second ferromagnetic layer group 12, wherein: the first ferromagnetic layer group 14 includes a 1-1st ferromagnetic layer 14A_1, a 1-2nd ferromagnetic layer 14A_2, and a first non-magnetic bonding layer 14B_1; the 1-1st ferromagnetic layer 14A_1 is composed of a ferromagnetic material, and the magnetization direction thereof varies with a first time constant; the first time constant is one second or shorter; and the first non-magnetic bonding layer 14B_1 contains at least one among Ru, Ir, Rh, Cr, and Cu.

Description

[Technical Field]

[0001] This invention relates to a superparamagnetic magnetic tunnel junction element and a computing system using the same. [Background technology]

[0002] Conventional computing systems can efficiently handle large-scale problems involving iterative operations such as arithmetic, but they struggle to handle computationally complex problems such as optimization problems. In recent years, probabilistic computing, alongside quantum computing, has attracted attention as a computational principle that can handle such complex processing that conventional computing systems struggle with relatively easily. Therefore, the development of dedicated computing system hardware specifically for probabilistic computing has become a crucial issue.

[0003] In a computing system dedicated to probabilistic information processing, a random number generation unit is required whose output fluctuates randomly between 0 and 1 over time, and whose ratio of 0 to 1 can be controlled by an external input current (or voltage). Within this random number generation unit, a circuit or solid-state element capable of generating a random output signal is required.

[0004] In recent years, magnetic tunnel junctions have attracted attention as solid-state devices capable of generating random output signals. A magnetic tunnel junction typically consists of a first ferromagnetic layer group made of a ferromagnetic material, a second ferromagnetic layer group also made of a ferromagnetic material, and a barrier layer made of an insulator formed between the first and second ferromagnetic layer groups. For example, if the magnetization direction of the first ferromagnetic layer group is designed to be freely reversible, and the magnetization direction of the second ferromagnetic layer group is designed to be substantially fixed, the direction of magnetization of the first ferromagnetic layer group can be detected by the level of electrical resistance by utilizing the tunnel magnetoresistance effect, and this can then be assigned to and used as information between 0 and 1. In this case, the first ferromagnetic layer group is also called the free layer, and the second ferromagnetic layer group is also called the fixed layer.

[0005] Magnetic tunnel junction elements can be used as memory elements for non-volatile memory by designing them so that the magnetization direction of the free layer does not easily change in response to thermal disturbances. On the other hand, if the magnetization direction is designed to easily change in response to thermal disturbances, they can be used as solid-state elements that generate random output signals and are applied to computing systems that perform probabilistic information processing. The stability of the magnetization direction in response to thermal disturbances is called thermal stability, and the energy barrier E between two states is defined by the thermal disturbance k. B The value obtained by dividing by T (E / k B It is quantitatively expressed using the thermal stability index T). A state in which the magnetization direction fluctuates with a short time constant due to thermal disturbance is called superparamagnetism, and a magnetic tunnel junction element designed so that the free layer exhibits superparamagnetism is called a superparamagnetic magnetic tunnel junction element, etc.

[0006] Conventionally, some applications of magnetic tunnel junction elements to probabilistic information processing have involved performing numerical calculations on a hypothetical magnetic tunnel junction element designed with a free layer thermal stability index of 0, and proposing methods for realizing probabilistic information processing (see, for example, Non-Patent Document 1 or Non-Patent Document 2). In addition, some studies have presented experimental results concerning superparamagnetic magnetic tunnel junction elements (see, for example, Non-Patent Documents 3 to 8), and others have presented experimental results concerning superparamagnetic magnetic tunnel junction elements, along with the results of principle verification of probabilistic information processing using them (see, for example, Non-Patent Document 9).

[0007] Furthermore, there are studies that provide a physical definition of the time constant τ, which represents the frequency of temporal fluctuations in the magnetization direction of a magnetic tunnel junction element (for example, Non-Patent Document 10), and studies that demonstrate that a superparamagnetic magnetic tunnel junction element designed so that both the first and second ferromagnetic layers exhibit superparamagnetism shows characteristics suitable for probabilistic information processing (see, for example, Non-Patent Document 11). [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Kerem Yunus Camsari, Rafatul Faria, Brian M. Sutton, and Supriyo Datta, “Stochastic p-Bits for Invertible Logic”, Phys. Rev. X, 2017, vol. 7, 031014 [Non-Patent Document 2] Kerem Yunus Camsari, Sayeef Salahuddin, Supriyo Datta, “Implementing p-bits With Embedded MTJ”, IEEE Electron Device Letters, 2017, vol. 38, 1767 [Non-Patent Document 3] Yang Lv, Jian-Ping Wang, “A single magnetic-tunnel-junction stochastic computing unit”, 2017 IEEE International Electron Devices Meeting (IEDM), 2017, DOI: 10.1109 / IEDM.2017.8268504 [Non-Patent Document 4] Mukund Bapna and Sara A. Majetich, “Current control of time-averaged magnetization in superparamagnetic tunnel junctions”, Appl. Phys. Lett., 2017, vol. 111, 243107 [Non-Patent Document 5] Alice Mizrahi, Tifenn Hirtzlin, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Julie Grollier & Damien Querlioz, “Neural-like computing with populations of superparamagnetic basis functions”, Nature Communications, 2018, vol. 9, 1533

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[0009] One of the challenges in the social implementation of computing systems using superparamagnetic magnetic tunnel junction elements is their operational stability (robustness) against external magnetic fields. In environments where computing systems are used, magnetic fields ranging from several milliteslas to tens of milliteslas are expected to be applied by various external factors. Superparamagnetic magnetic tunnel junction elements need to operate stably without significantly changing their characteristics even under such external magnetic fields.

[0010] The free layer of a superparamagnetic tunnel junction element is designed with a small anisotropic magnetic field, making it generally sensitive to external magnetic fields, and overcoming this challenge is not easy. For example, the superparamagnetic tunnel junction elements shown in Non-Patent Documents 4 and 8 change from a state where they continuously output 0 regardless of time to a state where they continuously output 1 with only about a 1 millitesla change in the applied magnetic field. Such constraints on disturbing magnetic fields significantly limit the applications for computing systems using superparamagnetic tunnel junction elements, or necessitate large and costly magnetic field shields, which are undesirable for applications. While the dependence of the characteristics on external magnetic fields is not clearly shown in Non-Patent Documents 3, 5 to 7, and 9, they present similar problems to those in Non-Patent Documents 4 and 8.

[0011] This invention addresses these challenges and aims to provide a superparamagnetic magnetic tunnel junction element that exhibits excellent operational stability (robustness) against external magnetic fields and is suitable for computing systems based on probabilistic information processing, as well as a computing system using the same. [Means for solving the problem]

[0012] To achieve the above objective, the superparamagnetic magnetic tunnel junction element according to the present invention comprises a first ferromagnetic layer group containing a ferromagnetic material, a second ferromagnetic layer group containing a ferromagnetic material, and a barrier layer disposed between the first ferromagnetic layer group and the second ferromagnetic layer group, wherein the first ferromagnetic layer group comprises a 1-1 ferromagnetic layer, a first non-magnetic coupling layer, and a 1-2 ferromagnetic layer, the 1-1 ferromagnetic layer is composed of a ferromagnetic material and its magnetization direction changes with a first time constant, the first time constant being 1 second or less, and the first non-magnetic coupling layer contains at least one of Ru, Ir, Rh, Cr, and Cu.

[0013] Alternatively, the superparamagnetic magnetic tunnel junction element according to the present invention comprises a first ferromagnetic layer group containing a ferromagnetic material, a second ferromagnetic layer group containing a ferromagnetic material, and a barrier layer disposed between the first ferromagnetic layer group and the second ferromagnetic layer group, wherein the first ferromagnetic layer group comprises a 1-1 ferromagnetic layer, a first non-magnetic coupling layer, and a 1-2 ferromagnetic layer, the 1-1 ferromagnetic layer being composed of a ferromagnetic material, its magnetization direction changing with a first time constant, the first time constant being 1 second or less, and the magnetization of the 1-1 ferromagnetic layer and the 1-2 ferromagnetic layer being coupled by the first non-magnetic coupling layer such that the direction is stable in a direction that is almost parallel to each other.

[0014] The superparamagnetic tunnel junction element according to the present invention exhibits excellent operational stability (robustness) to external magnetic fields because the first non-magnetic coupling layer contains at least one of Ru, Ir, Rh, Cr, and Cu, or because the magnetizations of the first-1 ferromagnetic layer and the first-2 ferromagnetic layer are coupled by the first non-magnetic coupling layer in a manner that is stable in antiparallel directions relative to each other. For this reason, the superparamagnetic tunnel junction element according to the present invention is suitable for computing systems based on probabilistic information processing.

[0015] In the superparamagnetic magnetic tunnel junction element according to the present invention, the second ferromagnetic layer group has at least a second-first ferromagnetic layer, the second-first ferromagnetic layer is composed of a ferromagnetic material, and its magnetization direction may be substantially fixed.

[0016] Furthermore, in the superparamagnetic magnetic tunnel junction element according to the present invention, the second ferromagnetic layer group has at least a second-first ferromagnetic layer, the second-first ferromagnetic layer is composed of a ferromagnetic material, its magnetization direction changes with a second time constant, the second time constant may be 1 second or less. In this case, the second ferromagnetic layer group further has a second non-magnetic coupling layer and a second-second ferromagnetic layer, the second non-magnetic coupling layer may contain at least one of Ru, Ir, Rh, Cr, and Cu, or the second ferromagnetic layer group further has a second non-magnetic coupling layer and a second-second ferromagnetic layer, the magnetization of the second-first ferromagnetic layer and the second-second ferromagnetic layer may be coupled by the second non-magnetic coupling layer such that the antiparallel directions are stable relative to each other.

[0017] In the superparamagnetic tunnel junction element according to the present invention, the thickness of the first nonmagnetic coupling layer is preferably in the range of 0.5 nanometers or more and 1.1 nanometers or less, or 1.7 nanometers or more and 2.5 nanometers or less. Furthermore, in the superparamagnetic tunnel junction element according to the present invention, the first ferromagnetic layer group is preferably circular or elliptical in shape, with a ratio of its major axis to its minor axis of 1 or more and 3 or less, and its minor axis of 80 nanometers or less.

[0018] The superparamagnetic magnetic tunnel junction element according to the present invention has a value obtained by dividing the average magnetic volume [Tesla cubic meters] of the 1-1 ferromagnetic layer and the 1-2 ferromagnetic layer by the coupling strength [Tesla] of the 1 non-magnetic coupling layer, which is 2.5 × 10⁻¹⁰. -23 [cubic meters] or less, more preferably 1 x 10 -23 [m 3 It is preferable that the coupling strength [Tesla] of the first non-magnetic coupling layer is 1 [Tesla] or less.

[0019] The computing system according to the present invention comprises a weighting circuit, a plurality of random number generation units connected by the weighting circuit, and an output circuit, wherein each random number generation unit has a superparamagnetic magnetic tunnel junction element according to the present invention.

[0020] The computing system according to the present invention has a superparamagnetic magnetic tunnel junction element according to the present invention, and therefore exhibits excellent operational stability (robustness) against external magnetic fields. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a superparamagnetic magnetic tunnel junction element that exhibits excellent operational stability (robustness) against external magnetic fields and is suitable for computing systems based on probabilistic information processing, as well as a computing system using the same. [Brief explanation of the drawing]

[0022] [Figure 1] This is a block diagram showing a computing system according to an embodiment of the present invention. [Figure 2] (a) XZ cross-sectional view and (b) XY plan view of a superparamagnetic magnetic tunnel junction element according to the first embodiment of the present invention. [Figure 3] This is an XZ cross-sectional view schematically showing the stacked structure of the first ferromagnetic layer group of the superparamagnetic tunnel junction element according to the first embodiment of the present invention. [Figure 4]This is an XZ cross-sectional view schematically showing the stable magnetization state of the first ferromagnetic layer group in the superparamagnetic magnetic tunnel junction element of the first embodiment of the present invention, when (a) the first-1 ferromagnetic layer is oriented in the -x direction, and when (b) the first-1 ferromagnetic layer is oriented in the +x direction. [Figure 5] (a) Structure of a conventional superparamagnetic tunnel junction element, and (b) Structure of a superparamagnetic tunnel junction element according to the first embodiment of the present invention. The principle for obtaining the desired characteristics is explained in the (left) schematic diagram of the film structure and magnetization direction, and (right) the time-averaged magnetization when an external magnetic field H is applied. <m>This is a schematic diagram showing the response and the most stable state of magnetization at that time (indicated by the white arrow). [Figure 6] (a) A graph showing the measurement results of the response of a conventional superparamagnetic tunnel junction element as shown in Figure 5(a), and (b) a superparamagnetic tunnel junction element according to the first embodiment of the present invention, when a magnetic field is applied in the direction of the easy magnetization axis. [Figure 7] This graph shows the measurement results of the response of a superparamagnetic tunnel junction element according to the first embodiment of the present invention (filled plot) and a conventional superparamagnetic tunnel junction element (outlined plot) when a magnetic field is applied in the direction of the hard magnetization axis. [Figure 8] (a) XZ cross-sectional view and (b) XY plan view schematically represent the structure of a modified example of the superparamagnetic magnetic tunnel junction element of the first embodiment of the present invention. [Figure 9] This is an XZ cross-sectional view of a superparamagnetic magnetic tunnel junction element according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0023] Hereinafter, with reference to the drawings, a superparamagnetic magnetic tunnel junction element according to an embodiment of the present invention and a computing system using the same will be described.

[0024] [1. Basic Structure of Computing Systems] Figure 1 shows a block diagram of a computing system 1 according to an embodiment of the present invention. The computing system 1 according to an embodiment of the present invention includes a weighted logic circuit 200, a plurality of random number generation units 100 connected by the weighted logic circuit 200, and an output circuit 300. The random number generation unit 100 includes at least one superparamagnetic tunnel junction element 10.

[0025] In probabilistic information processing, an energy (cost function) for solving a given problem is set mathematically, and the magnitude of the input signal to be introduced to each bit for minimizing the energy (cost function) is described as a function of the states of other bits. This function is programmed into the weighting circuit 200.

[0026] An input signal V is input to the random number generation unit 100, IN and an output signal V is output from the random number generation unit 100. The random number generation unit 100 generates an output signal V that varies moment by moment with a first time constant τ1, OUT but its time-average characteristic changes sigmoidally with respect to the input signal V. OUT IN

[0027] The weighting circuit 200 calculates the input signal V to each random number generation unit 100 in real time according to the output signal V from each random number generation unit 100 that varies moment by moment, OUT and feeds it back to each random number generation unit 100. This loop is continued for a certain period of time T. IN COMP

[0028] The output circuit 300 continuously measures the output signal V from each random number generation unit 100 that varies moment by moment, OUT and accumulates the solutions corresponding to the output signal V from each random number generation unit 100 at each time for a certain period of time T. OUT COMP The time T depends on the scale and nature of the problem and the accuracy of the solution to be obtained, but it is preferably 1000 times or more the first time constant τ1. As a result, in the case of an optimization problem, the optimal solution is obtained as the most frequently observed state. Also, in the case of a sampling problem, the distribution of states itself becomes a sample of the solution corresponding to the given problem. COMP

[0029] [2. Basic Structure of Giant Magnetoresistive Magnetic Tunnel Junction (First Embodiment)] Figure 2 schematically shows the structure of the superparamagnetic tunnel junction element 10 of the first embodiment of the present invention. Figure 2(a) is an XZ cross-sectional view, and Figure 2(b) is an XY plan view. The superparamagnetic tunnel junction element 10 shown in Figure 2 has two terminals, one above and one below. The circuit configuration of a random number generation unit 100 for probabilistic information processing using such a two-terminal superparamagnetic tunnel junction element 10 is disclosed, for example, in Non-Patent Document 2, and is therefore omitted here.

[0030] The superparamagnetic magnetic tunnel junction element 10 includes a lower electrode 11, a second ferromagnetic layer group 12 provided adjacent to the upper surface of the lower electrode 11, a barrier layer 13 provided adjacent to the upper surface of the second ferromagnetic layer group 12, a first ferromagnetic layer group 14 provided adjacent to the upper surface of the barrier layer 13, and an upper electrode 15 provided adjacent to the upper surface of the first ferromagnetic layer group 14. The order of the first ferromagnetic layer group 14 and the second ferromagnetic layer group 12 may be reversed.

[0031] In the first embodiment, the second ferromagnetic layer group 12 includes a ferromagnetic material, and its magnetization direction is substantially fixed. The barrier layer 13 is made of an insulator. The first ferromagnetic layer group 14 is composed of a laminated structure including a ferromagnetic material, and at least a portion of its magnetization direction changes freely with a first time constant τ1 due to thermal disturbance. The specific laminated structure will be described in detail later. Preferably, both the second ferromagnetic layer group 12 and the first ferromagnetic layer group 14 have an easy magnetization axis (in-plane easy magnetization axis) in the direction in the film plane. However, both may also have an easy magnetization axis (perpendicular easy magnetization axis) in the direction perpendicular to the film plane.

[0032] The lower electrode 11 and the upper electrode 15 are made of a metallic material. The lower electrode 11 and the upper electrode 15 are electrically connected to the wiring shown in the diagram.

[0033] The first time constant τ1 is given by a constant time T COMP It needs to be sufficiently shorter than τ1. The ratio depends on the scale of the problem and the precision of the solution to be obtained, but τ1 is T COMP It should be less than 1 / 1000 of that, and preferably less than 1 second.

[0034] The superparamagnetic magnetic tunnel junction element 10 has an elliptical shape within the film plane, with its minor axis being L1 and its major axis being L2. Alternatively, at least the first ferromagnetic layer group 14 has an elliptical shape within the film plane, with its minor axis being L1 and its major axis being L2. The preferred design range for the minor axis L1 and major axis L2 will be described later.

[0035] Figure 3 is an XZ cross-sectional view showing the stacked structure of the first ferromagnetic layer group 14. The first ferromagnetic layer group 14 of the superparamagnetic tunnel junction element 10 of the first embodiment of the present invention has a structure in which the first-1 ferromagnetic layer 14A_1, the first non-magnetic coupling layer 14B_1, and the first-2 ferromagnetic layer 14A_2 are stacked in this order or the reverse order. The first-1 ferromagnetic layer 14A_1 and the first-2 ferromagnetic layer 14A_2 are made of ferromagnetic material, and the first non-magnetic coupling layer 14B_1 is made of non-magnetic material. The first-1 ferromagnetic layer 14A_1 is adjacent to the barrier layer 13, and at least the magnetization direction of the first-1 ferromagnetic layer 14A_1 needs to change freely by thermal disturbance with a first time constant τ1.

[0036] Figure 4 schematically shows typical examples of the possible directions of magnetization for the first-first ferromagnetic layer 14A_1 and the first-second ferromagnetic layer 14A_2. In the present invention, the magnetizations of the first-first ferromagnetic layer 14A_1 and the first-second ferromagnetic layer 14A_2 are coupled in an almost antiparallel direction via the first non-magnetic coupling layer 14B_1. As a principle of magnetic coupling, the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction can be used. As shown in Figure 4, when the first-first ferromagnetic layer 14A_1 is oriented in the -x direction, the first-second ferromagnetic layer 14A_2 is stable in the +x direction, and when the first-first ferromagnetic layer 14A_1 is oriented in the +x direction, the first-second ferromagnetic layer 14A_2 is stable in the -x direction. Note that the magnetization states shown here schematically represent the configuration in a thermally stable state. During operation, the magnetizations of the two layers may temporarily be shifted from the ±x direction, and there may also be moments when they are shifted from an antiparallel state. Note that the saturation magnetization (M) of the first-first ferromagnetic layer 14A_1 and the first-second ferromagnetic layer 14A_2 S The magnetic volume per unit area (M) is given by the product of () and the effective film thickness (t). S The ratio of t) is preferably designed to be 20% or less, and more preferably 10% or less. This makes it easier to obtain the effects of the present invention. The principle will be described later.

[0037] In Figure 2, the second ferromagnetic layer group 12 is depicted as being composed of a single ferromagnetic layer. However, in reality, it is desirable for the second ferromagnetic layer group 12 to also have a multilayer structure consisting of multiple layers, and in particular, it is desirable for it to consist of two ferromagnetic layers antiferromagnetically coupled by a pinning layer and a non-magnetic layer made of an antiferromagnetic material. Since these are the same structures used in general non-volatile magnetic memory, a detailed explanation will be omitted.

[0038] Furthermore, in Figures 3 and 4, the first ferromagnetic layer group 14 is shown to have a structure in which the 1-1 ferromagnetic layer 14A_1 and the 1-2 ferromagnetic layer 14A_2 are antiferromagnetically coupled via the first non-magnetic coupling layer 14B_1. However, for example, the number of layers may be further increased, and a structure may be adopted in which layers are stacked in the order of 1-1 ferromagnetic layer 14A_1, 1-1 non-magnetic coupling layer 14B_1, 1-2 ferromagnetic layer 14A_2, 1-2 non-magnetic coupling layer 14B_2, 1-3 ferromagnetic layer 14A_3, ..., 1-N non-magnetic coupling layer 14B_N, 1-N+1 ferromagnetic layer 14A_N+1, and the magnetization of opposing ferromagnetic layers is formed to be coupled in an antiparallel direction via each non-magnetic coupling layer.

[0039] [3. Principle] Next, using Figure 5, the principle by which the present invention realizes a superparamagnetic magnetic tunnel junction element 10 with excellent resistance to external magnetic fields will be explained. For the purpose of explanation, a conventional structure in which the first ferromagnetic layer group 14 is composed of a single layer of ferromagnetic layers (Figure 5(a)) and the structure of the present invention in which the first ferromagnetic layer group 14 is composed of antiferromagnetically coupled 1-1 ferromagnetic layer 14A_1 and 1-2 ferromagnetic layer 14A_2, as described above, are shown (Figure 5(b)). The left half of the figure is a schematic diagram of the film structure and magnetization direction, and the right half is the time-averaged magnetization when an external magnetic field H is applied. <m>The response and the most stable state of magnetization at that time (white arrow) are schematically shown. For simplicity, the magnetic volume (M) of the first-first ferromagnetic layer 14A_1 and the first-second ferromagnetic layer 14A_2 are shown here. S Assume that t) are equal and completely cancel each other out.

[0040] In the case of a single-layer ferromagnetic material (Figure 5(a)), for example, when a magnetic field is applied in the +x direction, the magnetization tends to orient in the +x direction because the Zeeman energy decreases as the magnetization points in the +x direction. Conversely, when a magnetic field is applied in the -x direction, the magnetization tends to orient in the -x direction because the Zeeman energy decreases as the magnetization points in the -x direction. Therefore, even if it fluctuates superparamagnetically in time resolution, when the time average is taken, as shown in the figure on the right, the time-averaged magnetization <m>The resistance changes in a sigmoid function manner with respect to the magnetic field. In magnetic tunnel junction elements, the resistance is low or high depending on the magnetization direction, so the time-averaged magnetization <m>The magnetic field dependence directly corresponds to the resistance of the superparamagnetic magnetic tunnel junction element 10, and the output signal V from the random number generation unit 100. OUT This will be reflected.

[0041] On the other hand, in the first embodiment of the present invention, a superparamagnetic magnetic tunnel junction element using artificial antiferromagnetic coupling (Figure 5(b)), whether the magnetic field is applied in the +x direction or the -x direction, the magnetization of the first-1 ferromagnetic layer 14A_1 and the first-2 ferromagnetic layer 14A_2 are coupled in an antiparallel direction. Therefore, if one ferromagnetic layer tries to face the direction of the magnetic field, the other ferromagnetic layer opposes that change. Consequently, within a certain magnetic field range, the time-averaged magnetization <m>This does not change significantly with respect to the magnetic field. Above a certain magnetic field (flop magnetic field), the magnetization of the 1-1 ferromagnetic layer 14A_1 and the 1-2 ferromagnetic layer 14A_2 stabilizes in a state where the angle is intermediate between 0 and 180 degrees, like scissors, and as the magnetic field increases, the scissors close, resulting in the average magnetization. <m>This changes in response to the magnetic field. Thus, as shown in the right figure of Figure 5(b), the average magnetization over a wide magnetic field range is <m>A state is achieved in which the element does not respond significantly to a magnetic field, that is, a superparamagnetic magnetic tunnel junction element that is robust to an external magnetic field is realized.

[0042] Next, let's consider the case where a magnetic field is applied in the y-direction or z-direction. In this case, if the material consists of a single layer of ferromagnetic material (Figure 5(a)), the energy barrier for inversion is reduced or pushed to a higher energy state, thus shortening the time constant τ1 of the magnetization fluctuation. On the other hand, if an artificial antiferromagnetic coupling is used (Figure 5(b)), even if the magnetization of one ferromagnetic layer tries to align with the direction of the magnetic field, the other layer opposes it, so the effect of the applied external magnetic field is suppressed. Therefore, as with the case of a magnetic field in the x-direction, robust behavior against an external magnetic field is achieved.

[0043] [4. Materials and Shape] Next, we will describe the materials that can be used in each layer of the superparamagnetic magnetic tunnel junction element 10 of the first embodiment of the present invention. Non-magnetic and conductive metals can be used for the lower electrode 11 and the upper electrode 15. Specifically, examples include Ta, W, Ti, Ru, Cu, Cu-N, Ti-N, and Ta-N. The film thickness is designed to be in the range of approximately 5 nanometers to 80 nanometers.

[0044] The second ferromagnetic layer group 12 is preferably constructed by laminating a pinning layer typically composed of an antiferromagnetic material, a second-first ferromagnetic stationary layer composed of a ferromagnetic material, a stationary layer coupling layer composed of a non-magnetic material, and a second-first ferromagnetic stationary layer composed of a ferromagnetic material, in this order from the substrate side. Here, the second-first ferromagnetic stationary layer and the second-second ferromagnetic stationary layer are coupled in an antiparallel direction via the stationary layer coupling layer.

[0045] For the pinning layer, Pt-Mn alloy, Ir-Mn alloy, Pd-Mn alloy, etc., can be used. Its film thickness is designed to be in the range of approximately 5 nanometers to 30 nanometers. For the 2-1 ferromagnetic stationary layer, Co, Fe, Co-Fe alloy, etc., can be used. Its film thickness is designed to be in the range of approximately 1 nanometer to 4 nanometers. For the stationary layer bonding layer, Ru, etc., can be used. Its film thickness is designed to be in the range of approximately 0.6 nanometers to 2.5 nanometers. For the 2-2 ferromagnetic stationary layer, Co-Fe-B alloy, Fe-B alloy, etc., can be used. Its film thickness is designed to be in the range of approximately 1 nanometer to 4 nanometers.

[0046] An insulating, non-magnetic material can be used for the barrier layer 13. In particular, MgO is preferred. Its film thickness is designed to be approximately 0.8 nanometers to 2.0 nanometers.

[0047] The first ferromagnetic layer 14A_1 and the first ferromagnetic layer 14A_2, which constitute the first ferromagnetic layer group 14, can be made of materials such as Co-Fe-B alloy, Fe-B alloy, or Co-Fe alloy. In all cases, the film thickness is designed to be in the range of approximately 1.2 nanometers to 3.5 nanometers.

[0048] The first non-magnetic coupling layer 14B_1, which constitutes the first ferromagnetic layer group 14, is composed of Ru, Ir, Rh, Cr, Cu, and alloys containing these materials, with Ru or Ir being particularly preferred. Its film thickness is designed so that the magnetizations of the first-first ferromagnetic layer 14A_1 and the first-second ferromagnetic layer 14A_2 are coupled in an antiparallel direction. When using the RKKY interaction, the film thickness for Ru or Ir is in the range of 0.5 nanometers to 1.1 nanometers, or 1.7 nanometers to 2.5 nanometers. By composing the first non-magnetic coupling layer 14B_1 with the materials described above, the magnetization of the first-first ferromagnetic layer 14A_1 and the magnetization of the first-second ferromagnetic layer 14A_2 can be coupled in an antiparallel direction. This makes it possible to realize a superparamagnetic magnetic tunnel junction element 10 with excellent operational stability (robustness) against an external magnetic field, according to the principle described above.

[0049] The ratio of the minor axis L1 to the major axis L2 (aspect ratio: L2 / L1) of the first ferromagnetic layer group 14 is preferably around 1 to 3. This results in low magnetic anisotropy and properties suitable for realizing a superparamagnetic state. Furthermore, the minor axis L1 is preferably 100 nanometers or less, and more preferably 80 nanometers or less. There is no particular lower limit for L1, but from the viewpoint of microfabrication accuracy, the lower limit with current technology is around 10 nanometers. Note that the shape of the first ferromagnetic layer group 14 may be a rectangle, a rhombus, or other shape other than the ellipse shown in Figure 2. In this case, the minor axis L1 and major axis L2 are defined as the lengths in the short and long directions.

[0050] The shape of the barrier layer 13 and the second ferromagnetic layer group 12 may be the same as or different from that of the first ferromagnetic layer group 14. Forming them with the same shape simplifies the manufacturing process. On the other hand, if the barrier layer 13 and the second ferromagnetic layer group 12 are formed larger than the first ferromagnetic layer group 14, the number of manufacturing steps increases, but there is an advantage in that the influence of the leakage magnetic field from the second ferromagnetic layer group 12 on the first ferromagnetic layer group 14 can be reduced. Here, a structure in which the second ferromagnetic layer group 12 is formed on the substrate side relative to the first ferromagnetic layer group 14 (Figure 2) is assumed.

[0051] [5. Examples] Next, the experimental results of the superparamagnetic tunnel junction element 10 of the first embodiment of the present invention, as performed by the inventors, are shown. The inventors fabricated a superparamagnetic tunnel junction element 10 having the following film configuration. Si substrate with thermal oxide film / Ta(5) / PtMn(20) / Co(2.4) / Ru(0.85) / CoFeB(2.4) / MgO(1.0) / CoFeB(1.8) / Ru(0.74) / CoFeB(2.1) / Ta(5) / Ru(5) / Ta(50) Here, the numbers in parentheses represent the film thickness, and the unit is nanometers.

[0052] The composition of PtMn is Pt 38 -Mn 62 The composition of CoFeB is (Co 75 -Fe 25 ) 75 -B 25 (The numbers represent atomic%). Ta(5) on the substrate side corresponds to the lower electrode 11, PtMn / Co / Ru / CoFeB to the second ferromagnetic layer group 12, MgO(1.0) to the barrier layer 13, CoFeB(1.8) to the first-1 ferromagnetic layer 14A_1, Ru(0.74) to the first non-magnetic coupling layer 14B_1, CoFeB(2.1) to the first-2 ferromagnetic layer 14A_2, and Ta(5) / Ru(5) / Ta(50) to the upper electrode 15. The difference in film thickness between the first-1 ferromagnetic layer 14A_1 and the first-2 ferromagnetic layer 14A_2 is due to consideration of the magnetic dead layer associated with the formation of the Ta layer on the upper electrode 15. Layers other than MgO were deposited by DC magnetron sputtering, and the MgO layer was deposited by RF magnetron sputtering.

[0053] After the deposition of the multilayer films, microfabrication was performed using lithography technology. The second ferromagnetic layer group 12 to the first ferromagnetic layer group 14 were patterned together with substantially the same shape. Typical experimental results are shown below, but similar properties were confirmed within the range of 40 to 70 nanometers for the short axis L1, 60 to 110 nanometers for the long axis L2, and 1.1 to 1.7 for the aspect ratio.

[0054] After microfabrication, a heat treatment was performed at 300 degrees Celsius for 2 hours while applying an in-plane magnetic field of 1.2 Tesla. The purpose of this heat treatment is to induce an exchange bias at the interface between PtMn and Co in the second ferromagnetic layer group 12.

[0055] Figure 6 shows the typical time (time constant) τ for which the magnetization of the first ferromagnetic layer group 14 and the second ferromagnetic layer group 12 resides in parallel directions when a magnetic field is applied in the easy magnetization axis direction (major axis direction of the ellipse / x direction) of the fabricated superparamagnetic magnetic tunnel junction element 10. P (Filled plot), and typical values ​​of the time spent in the antiparallel direction (time constant) τ AP The results of measuring the changes (indicated by white plots) are shown. Figure 6(a) shows a conventional structure in which the first ferromagnetic layer group 14 is composed of a single ferromagnetic layer, and Figure 6(b) shows the measurement results for the structure disclosed in the present invention, in which the first ferromagnetic layer group 14 is composed of two antiferromagnetically coupled ferromagnetic layers. As is clear from the figures, in the conventional structure, the residence times of the parallel and antiparallel states change in the range of 10 nanoseconds to 150 nanoseconds in response to a magnetic field of about 1 millitesla, whereas in the structure of the present invention, the residence time changes only in the range of a few nanoseconds to 20 nanoseconds even when a magnetic field of about 5 millitesla is applied, confirming the effect of the present invention.

[0056] Next, Figure 7 shows the measurement results of the change in the time constant of magnetization reversal of the superparamagnetic magnetic tunnel junction element 10 when a magnetic field is applied in the direction of the hard magnetization axis (minor axis direction of the ellipse / y direction) within the film surface. Here, the time constant τ is on the vertical axis. ave is τ ave =( τ P ×τ AP ) 1 / 2 This is defined as and corresponds to the first time constant τ1 of the fluctuations of the fabricated superparamagnetic magnetic tunnel junction element 10. In the figure, the conventional structure is shown as a white plot, and the structure of the present invention is shown as a lightly filled plot. In the conventional structure, the residence time changes by about eight orders of magnitude for a magnetic field of several millitesla, whereas in the structure of the present invention, it changes by only one to two orders of magnitude even when a magnetic field of about 10 millitesla is applied, and the effect of the present invention can be similarly confirmed.

[0057] Next, based on the experimental results shown in Figures 6 and 7, a theoretical model was constructed that well describes the behavior of the first ferromagnetic layer group 14, and based on this theoretical model, design guidelines for the first ferromagnetic layer group 14 to obtain better characteristics were considered. As a result, the magnetic volume (saturation magnetization M) of the first-1 ferromagnetic layer 14A_1 and the first-2 ferromagnetic layer 14A_2 was determined. S The ratio of the product of the magnetic volume and volume V to the coupling strength of the two layers by the first non-magnetic coupling layer 14B_1 determines the robustness against an external magnetic field, and it was found that the smaller the magnetic volume and the greater the coupling strength, the better the robustness.

[0058] Specifically, the magnetic volume is Tm 3 When expressed in units of [Tesla cubic meter], and the bonding strength is expressed in units of T [Tesla], the magnetic volume [Tm 3 ] / bond strength[T] is 2.5 × 10 -23 [m 3 When the following conditions are met, sufficient robustness is obtained against an external magnetic field of about 2 mT, and 1 × 10 -23 [m 3 It was found that sufficient robustness against an external magnetic field of about 5 mT can be obtained when the following conditions are met. Here, the coupling strength of the two ferromagnetic layers due to the first nonmagnetic coupling layer 14B_1 is given by the magnetic field (saturation field) in which the magnetization of the two layers is directed in one direction in the magnetization curve.

[0059] Considering current shielding technologies, robustness against magnetic field disturbances of approximately 2 mT would enable use in consumer applications, and robustness against magnetic field disturbances of approximately 5 mT would enable use in more demanding environments such as automotive applications. Therefore, in the first ferromagnetic layer group 14, the magnetic volume [Tm 3 The average of [ ] divided by the coupling strength [T] due to the first non-magnetic coupling layer 14B_1 is 2.5 × 10 -23 [m 3 ] More preferably 1 × 10 -23 [m 3 It is preferable that the following conditions apply.

[0060] In addition, if the coupling strength [T] due to the first nonmagnetic coupling layer 14B_1 is too large, the first time constant τ1 becomes longer, and the calculation time T COMP It was found that this process takes a long time. Detailed calculations revealed that this problem becomes more pronounced when the coupling strength is 1[T] or higher. In other words, it is preferable that the coupling strength [T] of the first nonmagnetic coupling layer 14B_1 be 1 Tesla or less.

[0061] [6. Modified example (3-terminal structure)] Figure 8 shows the structure of a modified example of the superparamagnetic tunnel junction element 10 of the first embodiment of the present invention. Figure 8(a) is an XZ cross-sectional view, and Figure 8(b) is an XY plan view. Unlike the superparamagnetic tunnel junction element 10 shown in Figure 2, the superparamagnetic tunnel junction element 10 shown in Figure 8 has three terminals, two of which are connected to the lower electrode 11, and the remaining one is connected to the upper electrode 15. In addition, a first ferromagnetic layer group 14 is formed on the upper surface of the lower electrode 11, and a second ferromagnetic layer group 12 is formed on the lower surface of the upper electrode 15.

[0062] In the modified superparamagnetic magnetic tunnel junction element 10, the first ferromagnetic layer group 14 utilizes the spin orbit torque (SOT) generated by the in-plane current introduced into the lower electrode 11. The origin of the spin orbit torque can be the spin Hall effect, spin anomalous Hall effect, topological Hall effect, Rashba-Edelstein effect, magnetic spin Hall effect, etc. The circuit configuration of the random number generation unit 100 using the three-terminal superparamagnetic magnetic tunnel junction element 10 is disclosed in Non-Patent Literature 1 and is therefore omitted here.

[0063] In this modified example, the first ferromagnetic layer group 14 has a structure in which the 1-1 ferromagnetic layer 14A_1 and the 1-2 ferromagnetic layer 14A_2 face each other via the first non-magnetic coupling layer 14B_1. In this case as well, the output signal V from the random number generation unit 100 is affected by the spin-orbit torque. OUT Input signal V IN It can be varied accordingly, enabling the realization of a computing system for probabilistic information processing, and at the same time, robust properties against external magnetic fields are achieved through artificial antiferromagnetic coupling.

[0064] In this case, it is desirable to use a material capable of generating a large spin-orbit torque for the lower electrode 11. Specifically, examples include 5d transition metals such as Ta, W, Hf, Pt, and Bi, and alloys containing them.

[0065] [7. Basic structure of a superparamagnetic magnetic tunnel junction (second embodiment)] Next, a superparamagnetic tunnel junction element 10 according to a second embodiment of the present invention will be described. The difference between the second embodiment and the first embodiment is that, in the first embodiment, the magnetization direction of the ferromagnetic layers constituting the second ferromagnetic layer group 12 was substantially fixed, whereas in the second embodiment, the magnetization direction of the ferromagnetic layers constituting the second ferromagnetic layer group 12 is not fixed, and at least a portion of its magnetization is designed to fluctuate with a second time constant τ2. Furthermore, it is desirable that this second time constant τ2 be 1 second or less. In addition, it is desirable that there be a small time correlation between the magnetization fluctuations of the first ferromagnetic layer group 14 and the magnetization fluctuations of the second ferromagnetic layer group 12.

[0066] Figure 9 is an XZ cross-sectional view schematically showing a typical structure of a superparamagnetic magnetic tunnel junction element 10 according to a second embodiment of the present invention. In the second embodiment, the second ferromagnetic layer group 12 has a structure in which a second-first ferromagnetic layer 12A_1, a second non-magnetic coupling layer 12B_1, and a second-second ferromagnetic layer group 12A_2 are stacked. The second-first ferromagnetic layer 12A_1 is adjacent to the barrier layer 13, and its magnetization direction needs to fluctuate with a second time constant τ2. Similar to the case of the first-first ferromagnetic layer 14A_1, the first non-magnetic coupling layer 14B_1, and the first-second ferromagnetic layer group 14A_2 described in the first embodiment, the second-first ferromagnetic layer 12A_1 and the second-second ferromagnetic layer group 12A_2 are antiferromagnetically coupled via the second non-magnetic coupling layer 12B_1. The materials and structures that can be used for the second-first ferromagnetic layer 12A_1, the second non-magnetic coupling layer 12B_1, and the second-second ferromagnetic layer group 12A_2 are the same as those for the first-first ferromagnetic layer 14A_1, the first non-magnetic coupling layer 14B_1, and the first-second ferromagnetic layer group 14A_2, so a detailed explanation is omitted.

[0067] In the second embodiment, the state of the second ferromagnetic layer group 12 also constantly fluctuates due to heat with a second time constant τ1. However, even in this case, because the correlation of fluctuations with the first ferromagnetic layer group 11 is small, the tunnel magnetoresistance of the superparamagnetic tunnel junction element 10 changes with a time constant roughly equivalent to that of the first time constant τ1 and the second time constant τ2. The superparamagnetic tunnel junction element 10 of the second embodiment of the present invention can also be incorporated into the random number generation unit 100.

[0068] In the second embodiment, the second ferromagnetic layer group 12 also achieves robust operation against an external magnetic field by the same principle as described for the first ferromagnetic layer group 14 in the first embodiment.

[0069] The usefulness of a superparamagnetic magnetic tunnel junction element in which both ferromagnetic layers on either side of the barrier layer 13 exhibit superparamagnetism for probabilistic information processing is described in Non-Patent Literature 11. Furthermore, in the stacked structure assumed in Non-Patent Literature 11, since artificial antiferromagnetic coupling is not used, the effect of static magnetostatic coupling (dipole interaction) between the two ferromagnetic layer groups adversely affects operation. However, in the superparamagnetic magnetic tunnel junction element 10 of the second embodiment of the present invention shown in Figure 9, the magnetization of both the first ferromagnetic layer group 14 and the second ferromagnetic layer group 12 is substantially canceled out by multiple ferromagnetic layers, so the static magnetic field generated externally is small. Therefore, the effect of dipole interaction is greatly reduced, and faster random number generation and more stable operation can be expected compared to the structure assumed in Non-Patent Literature 11.

[0070] In practice, the present invention can be implemented even if the second ferromagnetic layer group 12 consists only of the second-first ferromagnetic layer 12A_1. [Industrial applicability]

[0071] The superparamagnetic tunnel junction element and random number generation unit according to the present invention can also be used for applications other than computing systems specialized for probabilistic information processing. For example, they may be used as random number generators for cryptography. [Explanation of Symbols]

[0072] 1: Computing Systems 10: Superparamagnetic magnetic tunnel junction element 11: Lower electrode 12: Second ferromagnetic layer group 12A_1: 2nd-1st ferromagnetic layer 12A_2: 2nd-2nd ferromagnetic layer 12B_1: Second nonmagnetic coupling layer 13: Barrier layer 14: First ferromagnetic layer group 14A_1: 1st-1st ferromagnetic layer 14A_2: 1st-2nd ferromagnetic layer 14B_1: First nonmagnetic coupling layer 15: Upper electrode 100: Random number generation unit 200: Weighting Circuit 300: Output Circuit< / m> < / m> < / m> < / m> < / m> < / m> < / m>

Claims

1. A first ferromagnetic layer group containing a ferromagnetic material, A second ferromagnetic layer group containing a ferromagnetic material, It has a barrier layer disposed between the first ferromagnetic layer group and the second ferromagnetic layer group, The first ferromagnetic layer group comprises a first-1 ferromagnetic layer, a first non-magnetic coupling layer, and a first-2 ferromagnetic layer, wherein the first-1 ferromagnetic layer is composed of a ferromagnetic material, its magnetization direction changes with a first time constant, the first time constant is 1 second or less, and the first non-magnetic coupling layer contains at least one of Ru, Ir, Rh, Cr, and Cu. A key feature is the superparamagnetic magnetic tunnel junction element.

2. A first ferromagnetic layer group containing a ferromagnetic material, A second ferromagnetic layer group containing a ferromagnetic material, It has a barrier layer disposed between the first ferromagnetic layer group and the second ferromagnetic layer group, The first ferromagnetic layer group comprises a first-1 ferromagnetic layer, a first non-magnetic coupling layer, and a first-2 ferromagnetic layer, wherein the first-1 ferromagnetic layer is composed of a ferromagnetic material, its magnetization direction changes with a first time constant, the first time constant being 1 second or less, and the magnetizations of the first-1 ferromagnetic layer and the first-2 ferromagnetic layer are coupled by the first non-magnetic coupling layer such that they are stable in a nearly antiparallel direction. A key feature is the superparamagnetic magnetic tunnel junction element.

3. The superparamagnetic magnetic tunnel junction element according to claim 1 or 2, characterized in that the second ferromagnetic layer group has at least a second-first ferromagnetic layer, the second-first ferromagnetic layer is composed of a ferromagnetic material, and its magnetization direction is substantially fixed.

4. The superparamagnetic magnetic tunnel junction element according to claim 1 or 2, characterized in that the second ferromagnetic layer group has at least a second-first ferromagnetic layer, the second-first ferromagnetic layer is composed of a ferromagnetic material, its magnetization direction changes with a second time constant, and the second time constant is 1 second or less.

5. The superparamagnetic magnetic tunnel junction element according to claim 4, wherein the second ferromagnetic layer group further comprises a second nonmagnetic coupling layer and a second-second ferromagnetic layer, and the second nonmagnetic coupling layer contains at least one of Ru, Ir, Rh, Cr, and Cu.

6. The superparamagnetic magnetic tunnel junction element according to claim 4, wherein the second ferromagnetic layer group further comprises a second nonmagnetic coupling layer and a second-second ferromagnetic layer, and the magnetizations of the second-first ferromagnetic layer and the second-second ferromagnetic layer are coupled by the second nonmagnetic coupling layer such that they are stable in a direction that is almost parallel to each other.

7. The superparamagnetic magnetic tunnel junction element according to any one of claims 1 to 6, characterized in that the thickness of the first nonmagnetic coupling layer is in the range of 0.5 nanometers or more and 1.1 nanometers or less, or 1.7 nanometers or more and 2.5 nanometers or less.

8. The superparamagnetic magnetic tunnel junction element according to any one of claims 1 to 7, characterized in that the first ferromagnetic layer group has a circular or elliptical shape, the ratio of its major axis to its minor axis is 1 or more and 3 or less, and its minor axis is 80 nanometers or less.

9. The average of the magnetic volumes [Tesla cubic meters] of the 1-1 ferromagnetic layer and the 1-2 ferromagnetic layer, divided by the coupling strength [Tesla] of the 1 non-magnetic coupling layer, is 2.5 × 10⁻¹⁰. －２３ The superparamagnetic magnetic tunnel junction element according to any one of claims 1 to 8, characterized in that the volume is less than or equal to [cubic meters] and the coupling strength [tesla] of the first nonmagnetic coupling layer is 1 [tesla] or less.

10. Weighting circuit and Multiple random number generation units connected by the aforementioned weighting circuit, It has an output circuit, Each random number generation unit has a superparamagnetic magnetic tunnel junction element as described in any one of claims 1 to 9. A computing system that is characterized by its features.

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