DATE2024.04.26 #Press Releases
Success in Controlling Magnetic States of Antiferromagnets Using Exchange Bias
--A major step toward ultra-fast and ultra-low-power memory--
Mihiro Asakura (Doctoral student, Department of Physics)
Tomoya Higo (Project Associate Professor, Department of Physics
Research Fellow, Institute for Solid State Physics, The University of Tokyo)
Takumi Matsuo (Doctoral student, Department of Physics
Doctoral student, Johns Hopkins University)
Ryota Uesugi (Project Researcher, Department of Physics
During the research: Doctoral student, Institute for Solid State Physics, The University of Tokyo)
Satoru Nakatsuji (Professor, Department of Physics
Project Professor, Institute for Solid State Physics, The University of Tokyo
Director, Trans-scale Quantum Science Institute, The University of Tokyo
Research Professor, Johns Hopkins University)
Daisuke Nishio-Hamane (Technical Specialist Staff, Institute for Solid State Physics)
Key Points
- Researchers have discovered that the magnetic order in the Weyl antiferromagnet Mn3Sn, a promising material for nonvolatile memory, can be controlled at room temperature using exchange bias.
- Exchange bias is crucial for stabilizing data retention and operations in magnetoresistive memory (MRAM), marking a significant advancement toward next-generation antiferromagnetic memory with unparalleled speed and minimal power consumption.
- This technology leverages the inherent non-self-demagnetizing property of antiferromagnetic materials, allowing for the induction of magnetic anisotropy in any direction by exchange bias, which could be applied to innovative operations like brain-type computing and multi-level recording.
Control of Magnetic Order in a Weyl Antiferromagnet by Exchange Bias
Summary
A research group led by Graduate Student Mihiro Asakura, Project Associate Professor Tomoya Higo, and Professor Satoru Nakatsuji at the Graduate School of Science, The University of Tokyo, has discovered that the magnetic order in Weyl antiferromagnet Mn3Sn(Note 1), which has attracted attention as a next-generation nonvolatile memory material, can be controlled by exchange bias(Note 2).
Amidst increasing data demands due to advances in information, communication, AI, and IoT technologies, there is a pressing need for faster, more power-efficient information processing technologies beyond traditional silicon-based semiconductors. Magnetoresistive Random Access Memory (MRAM(Note 3)), a nonvolatile memory that retains data without power consumption, addresses this need. By substituting ferromagnetic materials in MTJ(Note 4) devices by antiferromagnetic materials, it is possible to dramatically increase the operating frequency from the GHz band to the THz band. Therefore, development of antiferromagnetic materials for MRAM is currently underway to realize memory that is both nonvolatile (power-saving) and ultra-fast.
The research group has developed the Weyl antiferromagnet Mn3Sn, in which the topological electronic state plays an important role, and has demonstrated that Mn3Sn is a suitable antiferromagnetic material for memory application as an alternative to ferromagnetic materials through the realization of the electrical writing (published in Nature in 2020 and 2022, press release (1), (2)) and reading (published in Nature in 2023, press release (3)) of nonvolatile data (magnetic state), an essential function for MRAM applications (Figure 1).
Figure 1: MTJ device configuration
(a) Configuration of MTJ device using ferromagnetic materials. From the bottom, there are a recording layer to store information as a magnetic state, a reference layer for comparing the magnetic state with the recording layer when reading out, and a pinning layer for fixing the magnetic state of the reference layer by exchange bias. (b) All-antiferromagnetic device, in which the ferromagnetic layers are replaced by the Weyl antiferromagnet. The writing and reading phenomena have already been demonstrated, and in this study, the effect of fixing by exchange bias is demonstrated.
In this study, the research group has demonstrated that (i) exchange bias appears at the interface between Mn3Sn and a different magnetic material, and (ii) the magnetic state of Mn3Sn can be controlled using the exchange bias (Figure 2). Exchange bias is widely used in conventional MRAM as a means to stabilize the magnetic state of certain ferromagnetic layers in MTJ devices to reduce the error rate during data read/write operations and to improve thermal stability. This result, which shows that the magnetic state of antiferromagnetic memory materials can be stabilized by exchange bias, combined with the already demonstrated writing and reading techniques, is expected to play a central role in the realization of ultra-fast and ultra-power-efficient memory. In addition, this result was obtained at the interface between polycrystalline materials fabricated on a thermally oxidized silicon substrate using the sputtering method, which enables the use of a fabrication process that is highly compatible with existing semiconductors and MRAM.
Figure 2: Observation of exchange bias in the Weyl antiferromagnet Mn3Sn at room temperature.
Perpendicular magnetic field dependence of the Hall resistivity of the Weyl antiferromagnet Mn3Sn/pinning layer MnN at room temperature after cooling in a magnetic field. Field cooling(Note 2) is performed from 400 K by applying cooling fields of ±9 T in the perpendicular direction. The black curve shows the Hall loop without field cooling. A shift toward the negative field direction was observed when a positive cooling field was applied (red curve). By changing the sign of the cooling field, a reversal of the shift direction was also observed (green curve).
The research group has also succeeded in the observation of a characteristic property of the Weyl antiferromagnet that reflects its "giant responses comparable to those of ferromagnets with negligibly small magnetization”. The chiral antiferromagnetic order characterized by the cluster magnetic octupole polarization(Note 1) has an "Omnidirectional" property that allows the direction of polarization to be fixed in any direction without being affected by demagnetizing fields(Note 5) (published in Advanced Functional Materials in 2021: press release (4)). In this study, it has been shown that the magnetic anisotropy induced to the chiral antiferromagnetic order of Mn3Sn by exchange bias, can be applied in any direction without the influence of the sample geometry or shape anisotropy(Note 5), depending on the direction of the cooling field. This demonstration of the omnidirectional property of the exchange bias shows that the magnetic state of Mn3Sn can be stabilized in any direction. This unique property observed in the system would lead to the development of new computation methods, such as brain-type calculation and multi-level recording(Note 6).
In addition to the importance in application, the interfacial magnetic coupling effect(Note 2) is a very interesting research topic in the physics of magnetism because various physical phenomena can be observed depending on the combination of magnetic materials at the interface. In this study, we have observed a new type of exchange bias at the interface of a magnetic multilayer, in which magnetic anisotropy can be applied in any direction by using the Weyl antiferromagnet. This achievement is expected to contribute not only to the development of new computation methods, but also to the further development of research on interfacial magnetic coupling.
Publication Information
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Journal name Advanced Materials Title of paper Observation of Omnidirectional Exchange Bias at All-Antiferromagnetic Polycrystalline HeterointerfaceAuthor(s) M. Asakura, T. Higo*, T. Matsuo, R. Uesugi, D. Nishio-Hamane, and S. Nakatsuji* (*: responsible author)
(*: Responsible author)
DOI No.
Research Grant
This research is supported by the Japan Science and Technology Agency (JST) under “Innovation of Photoelectric Technologies using Spintronics” (Program Manager: Satoru Nakatsuji) in the technology theme of "Innovative device technologies to achieve ultra-high level information processing in the age of trillion sensors" (General Manager: Yoshihiro Oishi), JST-MIRAI (Grant number; JPMJMI20A1), "Functional Magnets and Device based on Topological Electronic Structure" (Research Director: Satoru Nakatsuji) in the research area of "Creation of Core Technology based on the Topological Materials Science for Innovative Devices" (Research Supervisor: Masahito Ueda), JST-CREST (Grant number: JPMJCR18T3), and “Innovation of quantum electronics based on topological materials” (Japan-side PI: Satoru Nakatsuji, Partner PI: Collin Broholm) in the research field of “Research related to quantum computers and quantum technology in general, including on quantum materials with innovative functionality, which contributes to the realization of a productivity revolution” (Program Officer: Norio Kawakami), JST-ASPIRE (Program Director: Kenjiro Miyano) (Grant number: JPMJAP2317), Murata Research Foundation research project (Principal Investigator: Tomoya Higo), and others.
Terminology
Note1 Weyl antiferromagnet Mn3Sn, cluster magnetic octupole polarization
Electrons have a degree of freedom called spin, which is one of the origins of magnetism in matter. Depending on the arrangement of the spins in the atoms that make up the material, magnetic materials can be classified into several categories. In the ferromagnetic material, which is generally known as magnet, the spins are aligned in the same direction and exhibit a large magnetization, while in the antiferromagnetic material, the spins are oriented so that they cancel out each other and the overall magnetization is almost zero. In general antiferromagnets, as in the pinning layer used in this research, the spins cancel out each other by facing antiparallel direction, and are called collinear antiferromagnets. On the other hand, the Weyl antiferromagnet Mn3Sn has a spin structure as shown in Figure 3. Its antiferromagnetic magnetic order (chiral antiferromagnetic order) is described by the cluster magnetic octupole represented by the red arrow in the figure, which is known to have a symmetry similar to that of ferromagnets. Due to the symmetry of the magnetic structure, giant responses according to the magnetic order are observed in Mn3Sn. ↑up
Figure 3:Crystal and magnetic structure of the Weyl antiferromagnet Mn3Sn
Mn3Sn has a lattice plane (kagome plane) composed of manganese (Mn) and tin (Sn) stacked in alternating positions in the [0001] direction The arrows on the Mn atoms indicate the magnetic spin direction. The red arrow in the right figure indicates the cluster magnetic octupole corresponding to the magnetization in ferromagnets, which changes the direction according to the magnetic order of the Mn moments.
Figure 4: Exchange bias between a ferromagnet and antiferromagnet
(a) Magnetic state of the pinning(antiferromagnetic) layer/ferromagnetic layer above the magnetic transition temperature (Néel temperature, TN) of the antiferromagnetic layer. The curve showing the response of the ferromagnet is usually centered with respect to the magnetic field. (b) Magnetic state after cooled to below the magnetic transition temperature of the antiferromagnet with a magnetic field. The magnetic state of the pinning layer is stabilized depending on the orientation of the applied magnetic field (cooling field). (c) The exchange bias from the pinning layer, which shows almost no response to an external magnetic field, retains the reversal of the magnetization of the ferromagnetic layer. This effect is observed as a shift in the response of the ferromagnetic layer.
Note 2 Exchange bias, interfacial magnetic coupling effect, field cooling
As shown in Figure 4, when a multilayer of ferromagnetic layer/antiferromagnetic layer (pinning layer) is cooled from above the magnetic transition temperature (Néel temperature, TN) of the antiferromagnet (Figure 4(a)) to below the transition temperature (Figure 4(b)) with a magnetic field (field cooling), the antiferromagnetic order is ordered depending on the orientation of the applied magnetic field. This antiferromagnetic order shows almost no response to external disturbances such as magnetic fields, and is extremely stable. If magnetic coupling occurs between the magnetic elements near the interface between the ferromagnetic and antiferromagnetic layers, the antiferromagnetic layer acts to stabilize the magnetic order of the ferromagnetic layer during cooling (Figure 4(c)), and the ferromagnetic layer is also stabilized against external disturbances (exchange bias). This phenomenon can be viewed as an emergence of a magnetic anisotropy, preferred orientation of the magnetic order, in the ferromagnetic layer. This effect is observed as a shift in the hysteresis curve of the ferromagnetic layer. ↑up
Note 3 Magnetoresistive Random Access Memory (MRAM)
In Magnetoresistive Random Access Memory (MRAM), the magnetic order direction is used as 1-bit information. Since magnetic order is maintained without an external power supply, MRAM can be used as a power-saving nonvolatile memory that does not lose information even when the power is turned off.↑up
Note 4 Magnetic tunnel junction (MTJ)
In a structure called a magnetic tunnel junction (MTJ), in which an insulator tunnel barrier layer is sandwiched between two magnetic layers, the resistance between the magnetic layers changes depending on the direction of magnetic order in the two magnetic layers, known as the tunnel magnetoresistance (TMR). It is used in memory technology because the magnetic state can be read out as high or low resistance.↑up
Note 5: Self-demagnetization, shape anisotropy
In ferromagnetic materials with large magnetization, a magnetic field is generated inside the material due to its own magnetization. Since this magnetic field works in a direction that cancels its own magnetization, it is difficult to stabilize the magnetization in an arbitrary direction, which is called self-demagnetization. This effect is highly dependent on the shape of the magnetic material and is the cause of magnetic anisotropy called shape anisotropy. ↑up
Note 6 Brain-type calculation, multi-level recording
Normal memory elements record information in binary values of "0" and "1," but when they have the ability to retain more information, it is called multi-level recording. When analog-like information can be recorded by this method, it is expected to be utilized for brain-type calculation, a computation method that mimics cranial nerves. ↑up