New Physics and Application of Antiferromagnet Uncovered

The research group of Professor Hideo Ohno and Associate Professor Shunsuke Fukami of Tohoku University has studied the control of magnetization using a current applied to heterostructures comprising an antiferromagnet. They found that the current gives rise to a flow of electron spin in the antiferromagnet, which induces magnetization switching in a neighboring ferromagnet.

The obtained results shed light on a new physics of antiferromagnet and also open various pathways toward ultralow-power integrated circuits and other novel applications such as neuromorphic computing.

Spintronics devices that can store information via the magnetization direction under no power supply, are expected to realize ultralow-power integrated circuits. 　A key issue for the application is how to achieve a fast and reliable magnetization switching with low power consumption.

Recently, a switching scheme utilizing the flow of electron spin, the so-called spin current, originating from the spin-orbit interaction, has attracted a great deal of attention as a new method to achieve fast and reliable control of magnetization. This scheme has been observed in heterostructures typically consisting of ferromagnet and nonmagnetic heavy metal layer and is called the spin-orbit torque induced magnetization switching.

fig. 1: Schematics of the antiferromagnet-ferromagnet bilayer system studied in this work. The current applied to the bilayer gives rise to a flow of electron spin in the perpendicular direction to the film plane. The magnetization of ferromagnet in the vicinity of interface is biased in the film plane direction due to an interaction with the antiferromagnet, which allows for the field-free switching.

The research group investigated the spin-orbit torque induced switching in an antiferromagnet-ferromagnet bilayer system. Until now, the motion of electron spin in antiferromagnetic materials has not yet been studied well. They fabricated switching devices from a stack with an antiferromagnetic PtMn and a ferromagnetic Co/Ni multilayer, and electrically evaluated the switching properties at room temperature. They found that the current flowing in the antiferromagnet generates a spin-orbit torque large enough to induce the magnetization switching in the neighboring ferromagnet.

It is notable that whereas the spin-orbit torque switching in nonmagnet-ferromagnet bilayer systems studied previously requires in-plane external field, the present system allows field-free switching owing to a unique property arising at the antiferromagnet-ferromagnet interface.

fig. 2: The Hall resistance versus applied current measured at zero magnetic fields. The Hall resistance represents the perpendicular component of magnetization. The reversed component of magnetization depends on the magnitude of applied current.

Furthermore, they found that in specific stack structures, the reversed portion of magnetization can be controlled in an analogue manner by the magnitude of the applied current, and this feature can also be attributed to the nature of the antiferromagnet.

This work is significant from both the physics and application points of view. In terms of physics, the obtained results allow for deeper understanding of antiferromagnet and spin transport phenomena, such as the topological Hall effect. In terms of application, the external-field-free switching achieved in this work shows promise for the implementation of spin-orbit torque devices for future ultralow-power integrated circuits. In addition, since the analogue-like behavior observed here resembles the operation manner of synapses in the brain, the present antiferromagnet-ferromagnet device could be key to realizing neuromorphic computing, which is known to achieve efficient information processing.

This work is supported by the ImPACT Program of CSTI and R&D Project for ICT Key Technology to Realize Future Society of MEXT.

Publication Details :

Authors: S. Fukami, C. Zhang, S. Dutta Gupta, A. Kurenkov and H. Ohno
Title: Magnetization switching by spin-orbit torque in an antiferromagnet-ferromagnet bilayer system
Journal: Nature Materials
DOI: 10.1038/NMAT4566

Tohoku University

New magnetic behaviour in nanoparticles discovered

Schematic representation of the antiferromagnetic coupling
between a magnetic Fe3O4 soft core and a magnetic
Mn3O4 hard shell. The image of an electronic high-resolution
transmission microscope, superimposed on a map of electronic
energy loss spectroscopy (EELS), reveals the high quality of the
interface with a coherent increase between the two phases.

The phenomenon, observed by researchers from the UAB and the ICN2, could lead to important technological applications, such as smaller sized digital memories.

Electronic devices such as mobile phones and tablets spur on a scientific race to find smaller and smaller information processing and storage elements. One of the challenges in this race is to reproduce certain magnetic effects at nanometre scale.

An international collaboration of scientists led by researchers from the Universitat Autònoma de Barcelona Department of Physics and the Institut Catala de Nanociencia i Nanotecnologia, and with the participation of the Universitat de Barcelona, has been able to reproduce in particles measuring 10 to 20 nanometres a magnetic phenomenon of great importance in magnetic devices: the antiferromagnetic coupling between layers.

This phenomenon appears when coupling layers of materials with different magnetic properties, which allows controlling the magnetic behaviour of the whole device. This property has very important technological applications. For example, it forms an important part of data reading systems found in hard drives and in the MRAM memories of computers and mobile devices.

Researchers have managed for the first time to reproduce this phenomenon in nanoscopic materials, measuring a mere few tens of atoms in diameter. They managed to do this by using iron-oxide particles surrounded by a thin layer of manganese-oxide and vice versa: manganese-oxide particles covered by a layer of iron-oxide. The discovery provides an unprecedented control of the magnetic behaviour of nanoparticles, since it permits controlling and easily adjusting their properties without having to manipulate their shape or composition, solely by controlling the temperature and the magnetic fields surrounding it.

“We've been able to reproduce a magnetic behaviour not previously observed in nanoparticles, and this paves the way for miniaturisation up to limits which seemed impossible for magnetic storage and other more sophisticated applications such as spin filters, magnetic codifiers and multi-level recording”, explain Josep Nogués, ICREA research professor, and Maria Dolors Baró, professor of Applied Physics.

The research, published today in Nature Communications, included the participation of professors Maria Dolors Baró and Santiago Suriñach from the Department of Physics of the UAB; ICREA research professor Josep Nogués, from the Department of Physics of the UAB and ICN2; researchers from the Department of Inorganic Chemistry and from the Department of Electronics at the University of Barcelona (UB); researchers from the Complutense University of Madrid; the Università degli Studi di Firenze, Italy; the St. Petersburg Nuclear Physics Institute, Russia; the Stockholm University, Sweden; the NCSR in Greece; the Oak Ridge National Laboratory, USA; the Miami University, Ohio, USA; and the Argonne National Laboratory, USA.


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