Muhammad Ikhlas (Project Researcher, Department of Physics)

Tomoya Higo (Project Associate Professor, Department of Physics / Research Fellow, Institute for Solid State Physics)

Sayak Dasgupta (Project Researcher, University of British Columbia)
Project Researcher, Institute for Solid State Physics)

Oleg Chernishov, Professor, Johns Hopkins University

Satoru Nakatsuji, Professor, Department of Physics/ Project Professor, Institute for Solid State Physics
Director, Trans-scale Quantum Science Collaborative Research Organizations)

Key points of the presentation

• Giant piezomagnetic effect in antiferromagnets has been realized at room temperature.
• First success in controlling the sign of the anomalous Hall effect, which is normally controlled by the magnetic field, by crystal distortion.
• The technology to control the magnetic state of antiferromagnets by distortion may be applied to various devices such as MRAM.

Summary of Presentation

The University of Tokyo Graduate School of Science, Professor Satoru Nakatsuji, Project Associate Professor Muhammad Ikhlas, Associate Professor Tomoya Higo, Project Researcher Sayak Dasgupta, University of British Columbia, Canada, Assistant Professor Brad Ramshau, Cornell University, USA, Graduate Student Graduate Student Florian Seys, Ph, In collaboration with an international joint research group consisting of Associate Professor Shunichiro Tachibanataka at Chuo University, Professor Oleg Chernishov at Johns Hopkins University, USA, and Group Leader Clifford Hicks at the University of Birmingham, UK, an antiferromagnet that exhibits a giant anomalous Hall effect ^{(Note 1)} at room temperature despite having almost no magnetization (Note ²⁾ We have demonstrated that the sign of the anomalous Hall effect can be controlled by uniaxial strain (strain) in _Mn3Sn.

Antiferromagnets not only have a response speed of spin ^{(Note 2} ) that is 100~1000 times faster than that of ferromagnets ^{(Note 2)}, but also have the property of being unaffected by leakage fields when magnetized as a device because their magnetization is very small. Therefore, when applied to magnetoresistive memory (MRAM) ^{(Note 3} ), which is a strong candidate for nonvolatile memory ^{(Note 3)}, it is expected to realize ultra-high density MRAM that is driven at ultra-high speed in the terahertz band (picosecond range). On the other hand, the above advantage of extremely small magnetization has been an issue because it is difficult to write information (control signals) in antiferromagnetic materials. The highly efficient signal control method using uniaxial strain discovered in this research complements conventional signal control methods for magnetic materials, such as magnetic fields and currents, and opens up a wider range of applications of antiferromagnets in spintronics technology.

The research results were published online in the British scientific journal Nature Physics at midnight on Friday, August 19, 2022 (Japan Standard Time).

Publication details

Background of the research
Currently, information processing in computers and smartphones uses volatile semiconductor memory, which loses information when power is not supplied. In recent years, the Internet has made our society highly informatized, and in order to process the explosive increase in information with low power consumption, nonvolatile memory that does not lose information even when the power is turned off is being developed. MRAM using ferromagnetic materials known as magnets has been studied as a nonvolatile memory that can satisfy both low power consumption and high rewrite endurance, and its practical application has been progressing in recent years. This is because antiferromagnetic materials are not suitable for MRAM. This is because antiferromagnetic materials are expected to achieve information storage speeds of picoseconds (100 to 1000 times faster than conventional ferromagnetic materials), and because they have no net magnetization due to the arrangement of spins that cancel each other's magnetization, making operations 100 times faster and enabling, in principle, higher density MRAM. This is because the magnetization cancels each other out and there is no net magnetization. On the other hand, in order to use antiferromagnetic materials, it was necessary to develop a technology to detect and control electrical signals corresponding to "0" and "1" information.

This research group has been studying an antiferromagnet _Mn3Sn, which is composed of manganese (Mn) and tin (Sn), and has revealed that readout signals ^{such as the} anomalous Hall effect, anomalous Nernst effect ^{(Note 3), and} magnetooptic Kerr effect ^{(Note 5)}, which have been believed to be difficult to detect in antiferromagnets, can be detected at room temperature The reason why these signals can be obtained is because of the NONCO effect. These signals are obtained because _{Mn3Sn, which} exhibits a noncollinear (non-collinear) antiferromagnetic spin structure, has an extended magnetic octupolar polarization ^{(Note 6)} similar to a magnetic pole (Figure 1a).

Figure 1: (a) Magnetic structure of _{Mn3Sn, which} has an alternating stacked Kagome lattice composed of magnetic atoms of manganese (Mn, pink and blue spheres) in the c-axis direction; below 430 K (about 150 °C), Mn spins exhibit a non-colinear antiferromagnetic order called the inverse 120° structure. The spins on the two-layer Kagome lattice (pink and blue arrows) show that the six units of spins, called extended magnetic octupoles, shown as hexagons, are aligned in the same direction. The black arrow (K) indicates the direction of magnetic octupole polarization. (b) Piezoelectric strain stage for resistance measurement, capable of applying uniaxial strain in tensile and compressive directions with high accuracy and over a wide range. _Mn3Sn processed into a bar shape is fixed on two Ti plates, which are operated laterally in the figure by a piezoelectric actuator, to measure resistivity and anomalous Hall effect.

Although magnetic fields and electric currents are commonly used to control the above signals, we focused on strain in this study. We developed a piezoelectric strain measurement stage for resistivity measurement (Fig. 1b) that can apply uniaxial strain to a pure _Mn3Sn single-crystal sample in the tensile and compressive directions with high accuracy and over a wide range, and measured the change in anomalous Hall signal due to strain (Fig. 2a). As a result, we found that _Mn3Sn exhibits a piezomagnetic effect ^{(Note 7)} at room temperature. Normally, a distortion of about 1% is required to produce observable changes in electrical transport properties such as the anomalous Hall effect, but in this study, we succeeded in changing the Hall signal produced by the anomalous Hall effect with a very small distortion of about 0.1% (Figure 2b). The Hall signal not only changes in magnitude but also reverses its sign, indicating that the signal can be controlled very efficiently by distortion in _the non-colinear antiferromagnet _Mn3Sn. In previous studies of ferromagnetic materials, the Hall signal changed in response to the direction of magnetization. On the other hand, both experimental and theoretical studies have shown that in _Mn3Sn, the minute magnetization produced by the piezomagnetic effect and the Hall signal can be controlled separately by distortion (Figure 3).

Figure 2: (a) Schematic diagram of the measurement configuration of the anomalous Hall effect under strain. Here, the Hall voltage VH generated in the z-axis direction of the sample is measured under tensile strain in the x direction, where Hy is the external magnetic field, I is the current, and εxx is the strain in the x direction. (b) Magnetic field dependence of the Hall resistivity of _Mn3Sn under various strains εxx.

Figure 3: The uniaxial strain signature in _Mn3Sn allows independent control of the orientation of the extended magnetic octupole polarization K, which is the origin of the micro-magnetization _{MS and} the anomalous Hall effect in _Mn3Sn caused by the piezomagnetic effect.

Future Prospects
Thin films are necessary for the development of antiferromagnetic MRAM and other electronic devices. Many research groups around the world are currently working on the development of thin films of _Mn3Sn and related materials, and research in the field of antiferromagnetic spintronics focusing on their film interface properties is gaining momentum. In thin films, lattice mismatch with the substrate and piezoelectric materials make it possible to control readout signals including anomalous Hall effects due to distortion effects. Indeed, in a device that can invert the Hall signal with an electric current using _Mn3Sn thin films (spin-orbit torque magnetic reversal device), it becomes clear that the Hall signal can be electrically controlled with high efficiency by introducing perpendicular magnetic anisotropy due to strain (Nature 607, 474- 479 (2022). This has led to the development of new techniques for controlling signals by distortion, such as the following. The advanced technology for controlling the magnetic state of antiferromagnets by distortion developed in this research is an important guideline for realizing faster and lower power consumption electrical control of the Hall signal described above. It is expected that this technology will be applied in future research on improving the functionality of various magnetic devices, including MRAM.

The anomalous Hall effect used as a readout signal in this study originates from the Weyl semimetallic state ^{(Note 8)}, which is a topological electronic state of the non-colinear antiferromagnet _Mn3Sn. The properties originating from the topology of materials have attracted much attention in recent years in solid state physics, and the electrical control of the Weil semimetallic state is of great academic interest. The newly developed method for controlling the anomalous Hall effect, including sign inversion due to uniaxial distortion, is expected to open up new phenomena in the Weyl semimetallic state that have not been observed before.

This research is being conducted as part of the JST-MIRAI (Japan Science and Technology Agency) "Innovative Device Technologies for Ultra-High Information Processing in the Trillion-Sensor Era" research project, entitled "Creation of Fundamental Technologies for Spintronics Photoelectric Interfaces," under the research theme of "Innovative Device Technologies for Realizing Ultra-High Information Processing in the Trillion-Sensor Era," project number: JPMJJ. (Principal Investigator: Satoshi Nakatsuji), and "Development of Functional Magnetic Materials and Devices Based on Topological Electronic Structures and Their Functionality" (Research Director: Masahito Ueda) in the research area of "Creation of Materials and Devices with Innovative Functions Based on Topological Materials Science" (Research Director: Yoshihiro Oishi) under the Strategic Research Initiative Team Research Program (JST-CREST). The research was conducted as part of the research project "Development of Functional Magnetic Materials and Creation of Devices Based on Topological Materials Science" (Research Director: Masahito Ueda).

Members of this research team:

Muhammad Ikhlas	Project Researcher, Department of Physics, Graduate School of Science, The University of Tokyo
Sayak Dasgupta	Project Researcher, Department of Physics and Astronomy, University of British Columbia
Florian Seis	D., Department of Physics, Cornell University
Tomoya Higo	Project Associate Professor, Department of Physics, Graduate School of Advanced Materials Science, The University of Tokyo / Research Fellow, Quantum Matter Research Group, Institute for Solid State Physics, The University of Tokyo
Shunichiro Tachibanataka	Associate Professor, Department of Physics, Faculty of Science and Engineering, Chuo University
Brad RAMSHAU	Assistant Professor, Department of Physics, Cornell University
Oleg Chernishov	Professor, Department of Physics and Department of Astronomy, Johns Hopkins University
Clifford Hicks	Group Leader, Department of Physics and Department of Astronomy, University of Birmingham
Satoru Nakatsuji	Professor, Department of Physics, Graduate School of Science, The University of Tokyo / Specially Appointed Professor, Quantum Materials Group, Institute for Solid State Physics, The University of Tokyo / Director, Trans-scale Quantum Science Collaborative Research Organizations, The University of Tokyo

Journals

Journal name	Nature Physics
Title of paper	Piezomagnetic switching of anomalous Hall effect in an antiferromagnet at room temperature
Author(s)	M. Ikhlas+, S. Dasgupta+, F. Theuss, T. Higo, S. Kittaka, B. J. Ramshaw, O. Tchernyshyov, C. W. Hicks and S. Nakatsuji* (+ : equal contribution, * : corresponding author)
DOI number	10.1038/s41567-022-01645-5

Terminology

Note 1: Anomalous Hall effect

The Hall effect is a phenomenon in which an electromotive force is generated in a material capable of conducting electricity in a direction perpendicular to the magnetic field and electric current. This is caused by the direction of motion of electrons flowing as an electric current being bent by the magnetic field when a magnetic field and electric current are applied perpendicular to each other. In ferromagnetic materials, the Hall effect is generated by controlling the direction of the magnetic poles without an external magnetic field. This effect is called the anomalous Hall effect. Recently, it has been found that the anomalous Hall effect also appears in special antiferromagnets and spin liquids with virtual magnetic fields (an effective magnetic field existing in wavenumber space, a new physical concept that stems from the topology of the electronic structure). ↑up

Note 2 Antiferromagnetic, spin, and ferromagnetic bodies

A magnetic body is a material that possesses a microscopic magnet caused by the spinning motion of electrons called "spin. These magnetic bodies exhibit magnetic ordering in which macroscopic numbers of spins are aligned in some pattern, and are classified into ferromagnetic bodies, in which the spins are aligned in a uniform direction and have magnetic poles like a magnet, and antiferromagnetic bodies, in which adjacent spins are aligned in an antiparallel or mutually canceling manner and have no magnetic poles (no magnetic properties). The two categories are antiferromagnetic (no magnetic properties) and ferromagnetic (no magnetic poles). ↑up

Note 3 Nonvolatile memory, magnetoresistive memory (MRAM )

Unlike existing semiconductor memory, this type of memory does not lose recorded information even when the power is turned off. Several types of memory with different data storage methods have been developed, including magnetoresistive memory (MRAM), resistive memory (ReRAM), and phase-change memory (PRAM). MRAM, the focus of this research, is a nonvolatile memory that uses magnetic states (changes in resistance corresponding to the direction of magnetic poles) to write and read information. When the magnetic poles (in the case of Mn3Sn, the magnetic octupole polarization ^{(Note 6))} are oriented vertically (up and down) with respect to the film surface, a perpendicular binary state, high density, low power consumption, and improved thermal stability can be expected. ↑up

Note 4 Anomalous Nernst effect

The Nernst effect is a phenomenon in which an electromotive force is generated perpendicular to the magnetic field and temperature gradient in a material capable of conducting electricity. This is caused by the fact that the magnetic field bends the flow of electrons from the high-temperature side to the low-temperature side when the magnetic field and temperature gradient are applied perpendicular to each other. Similar to the anomalous Hall effect, the Nernst effect appears even in a zero magnetic field in ferromagnetic materials and special antiferromagnetic materials with a virtual magnetic field, and is called the anomalous Nernst effect. In this case, an electromotive force can be obtained by directing the magnetic poles or virtual magnetic field perpendicular to the temperature gradient instead of the magnetic field. ↑up

Note 5 Magneto-optic Kerr effect

When linearly polarized light is incident on a ferromagnetic material, the polarization plane of the reflected light rotates according to the direction of the magnetic poles, a phenomenon called the magnetooptic Kerr effect. Since the direction of rotation of the plane of polarization of light enables non-contact and nondestructive measurement of the orientation of the magnetic poles, it is widely used as a method to directly observe the magnetic domain of ferromagnetic materials. Although it was thought to be difficult to observe in antiferromagnets, it has been found that this effect can be observed in special antiferromagnets with extended magnetic octupolar polarization, such as the non-colinear antiferromagnet Mn3Sn, as well as the anomalous Hall and Nernst effect, and is used for magnetic domain observation. ↑up

Note 6 Extended magnetic oct upole polarization

Ferromagnetic bodies, known as magnets, have two poles (magnetic poles), the N and S poles, but spins located at each lattice point of a magnetic body also have two poles, which are called magnetic dipoles. The characteristic combination of spins created when considering a unit with spins located at multiple lattice points is called an extended magnetic multipole, and as the number of spins constituting the unit increases to one, two, or three, the names of the combinations change to magnetic dipole, quadrupole, octupole, and so on. In the spin structure of the antiferromagnet _Mn3Sn, a unit of extended magnetic octupoles can be considered with six spins arranged on two Kagome lattices and has extended magnetic octupolar polarization as shown in Figure 1a. The magnetic octupolar polarization of Mn3Sn serves as a parameter that controls the direction of the virtual magnetic field, the source of anomalous Hall effects, etc. anomalous Hall effect, etc. Therefore, even antiferromagnets without magnetic polarization can exhibit giant responses similar to those seen in ferromagnets. ↑up

Note 7 Piezomagnetic effect

This is a phenomenon in which magnetic polarization (magnetization) occurs in a magnetic material due to externally applied stress (strain). Magnetization and strain are linearly coupled. In magnetic materials exhibiting the piezomagnetic effect, not only can magnetization be induced by strain, but the sample itself can also be physically deformed by the application of a magnetic field. ↑up

Note 8 Weil semimetallic state

Substances with particles of zero mass (Weil particles) described according to the Weil equation proposed by Hermann Weil in 1921 are called Weil half-metals. In Weil semimetals, Weil points appear as pairs with different chirality (right-handed and left-handed degrees of freedom) as a topological electronic structure. This pair of Weil points corresponds to the N and S poles of a magnet in momentum space. In ordinary Weil semimetals, the Weil points are created by the crystal structure of the material. On the other hand, magnetic materials with Weil points created by magnetism are called Weil magnets (more broadly, topological magnets). In Weil magnets, it is possible to control the Weil points and the associated virtual magnetic field by controlling the magnetic poles and magnetic octupole polarization, which is an attractive property from the viewpoint of application. The virtual magnetic field generated between Weil points is large enough to correspond to an external magnetic field of 100 ~ 1000 tesla (T), and is the origin of the giant anomalous Hall effect, etc. ↑up