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Successful Functionality Verification of New Spin Device by Controlling Magnetic Field (Press Release)

Release Date
02 Jul, 2012
  • BL25SU (Soft X-ray Spectroscopy of Solid)
- A new path in sight for achieving rare metal-free magnetic devices capable of dual functionality: storage and arithmetic -

Osaka University
Japan Synchrotron Radiation Research Institute (JASRI)
Tohoku University

Researchers at Osaka University, Japan Synchrotron Radiation Research Institute (JASRI), and Tohoku University jointly conducted a study on the inversion of magnetic coupling at the interface connecting ferromagnetic and antiferromagnetic bodies at controlled temperatures (isothermal process), and successfully visualized this dynamic process. The inversion process plays an important role in information processing, such as reading data from a hard disk drive.

The ferromagnetic and antiferromagnetic body*1 are the most common examples of magnetic materials. The ferromagnetic body has a long history of widespread applications - including magnets and magnetic storage devices - owing to its property of spontaneous magnetization. The antiferromagnetic body, on the other hand, has had only scarce practical applications because of its inability to magnetize spontaneously. However, with the emerging knowledge of exchange magnetic anisotropy*2 - a strong magnetic effect generated at the junction interface with a ferromagnetic body - it has found many applications in recent years in the area of spin electronics*3 devices (e.g. magnetic random access memory), as well as the reading head of HDDs.

In processes based on conventional technology, the direction of exchange magnetic anisotropy is factory implanted at elevated temperatures, and, once it is fixed, post-implementation modifications (in terms of intensity and direction) are not available. If the properties associated with exchange magnetic anisotropy - direction and intensity - should be rendered modifiable, a new breed of spin electronics devices is very likely to emerge that combines the two functions - i.e. arithmetic and storage - in a single device. These two functions have traditionally required two distinct devices for implementation. Aiming at verifying the feasibility of controlling the properties of exchange magnetic anisotropy after manufacture, the research group conducted experiments using the soft X-ray solid spectroscopy beam line (BL25SU) available at SPring-8*4 (a large-scale synchronous radiation facility), and successfully demonstrated its potential. The soft X-ray magnetic circular dichroism measurement*5 used in these experiments is capable of ultra-high sensitivity analysis of magnetic properties inherent in a very thin layer (equivalent to several atoms in thickness), which play an important role in exchange magnetic anisotropy. In the experiments reported here, we made use of brute force (extreme magnetic field in excess of 105 Gauss) to forcibly flip the direction of antiferromagnetic spin. Future advancement of the research to reduce the required intensity of magnetic fields is expected to enable high-speed control within the device. In addition, the research used only chromium oxide (Cr2O3), available in abundance at cheap prices, for functional verification - many present-day devices make use of antiferromagnetic material (Mn-Ir) that contains Ir (a rare metal). In this context, this can be a breakthrough achievement that raises the possibility for replacing rare metals with common elements, as well as to promote technical advancement of spin electronics devices.

These results appeared in the online version of Applied Physics Letters (a physics journal published in US) on the 29th of June, 2012 (EST).

"Isothermal switching of perpendicular exchange bias by pulsed high magnetic field"
Y. Shiratsuchi, T. Nakamura, K. Wakatsu, S. Maenou, H. Oikawa, Y. Narumi, K. Tazoe, C. Mitsumata, T. Kinoshita, H. Nojiri and R. Nakatani
Applied Physics Letters, Vol. 100, p. 262413 (2012).


Fig.1. Conceptual representation of exchange magnetic anisotropy inversion under strong magnetic field
Fig.1. Conceptual representation of exchange magnetic anisotropy inversion under strong magnetic field

The research group found that, in the system characterized by exchange magnetic anisotropy, forced spin inversion in the ferromagnetic body is not accompanied by spin inversion in the antiferromagnetic body, but only by small changes in spin orientation.
Based on the knowledge from this finding, the group successfully inverted the direction of exchange magnetic anisotropy forcibly by application of an extremely strong magnetic field, leading to inversion of the spin orientation in ferromagnetic bodies.

Fig.2. Schematic representation of directional change in exchange magnetic anisotropy under the effect of strong magnetic field
Fig.2. Schematic representation of directional change in exchange magnetic anisotropy
under the effect of strong magnetic field

The horizontal axis represents the magnetic field intensity, and the vertical axis represents spin direction. In the two figures located in the upper left and lower right, the displacement of the center of the profile from the position where the magnetic field intensity vanishes (0) represents the intensity of exchange magnetic anisotropy, and the direction of displacement represents that of exchange magnetic anisotropy. The experiment has successfully demonstrated that the direction of exchange magnetic anisotropy can be arbitrarily commuted by applying a strong magnetic field. ((A): displaced in the positive direction of external magnetic field, (C): displaced in the negative direction)

Fig.3. Strong magnetic field can invert the orientation of antiferromagnetic spin (schematic representation)
Fig.3. Strong magnetic field can invert the orientation of antiferromagnetic spin
(schematic representation)

The horizontal axis represents photonic energy obtainable from synchrotron radiation light, and the vertical axis represents spin direction. The figures show that, after application of a strong magnetic field, the direction of antiferromagnetic spin becomes inverted (i.e. the profile changes sign along the vertical axis). The state can be reverted by application of a strong magnetic field in the opposite direction.

Fig.4. Comparison: conventional semiconductor memory, spin memory (spin electronics), and the new spin memory based on the results of this research
Fig.4. Comparison: conventional semiconductor memory, spin memory (spin electronics),
and the new spin memory based on the results of this research

Conventional semiconductor memory needs an uninterrupted flow of current because it maintain information based on the presence (or depletion) of electric charge in a capacitor. Spin electronics devices maintain information even when the power is lost because the direction of ferromagnetic spin (N- and S-pole of a magnet) represents data. The spin electronics devices developed up to the present provide only one channel for information input, i.e. the spin direction of the ferromagnetic body 2 in the figure is fixed. This situation is quite wasteful. Well-devised application of this technology can raise the possibility of controlling both of the spin directions in two ferromagnetic bodies.

*1 Ferromagnetic and antiferromagnetic body

The ferromagnetic body is a class of magnetic bodies characterized by its property of being attracted by a magnet. These also show a tendency to become magnets themselves. Inside the ferromagnetic body, magnetization (electron spin) tends to achieve alignment in the same orientation. In contrast, each electron spin in an antiferromagnetic body tends to align itself in the direction opposite that of the neighboring electron spin. Therefore, an antiferromagnetic body does not generate external magnetic flux, showing no tendency to be attracted by a magnet.

*2 Exchange magnetic anisotropy
The antiferromagnetic body does not become a magnet by itself, but it can exert a huge effect on the magnetic properties of a ferromagnetic body if joined together with a ferromagnetic body. Normally, magnetization (direction of N- and S-pole) of a stand-alone ferromagnetic body follows the direction of the magnetic field in which it is placed. This property is clearly exemplified by the fact that a compass always orients itself to the determined direction. But, the magnetization of a ferromagnetic body, if joined together with an antiferromagnetic body, orients itself to a fixed direction determined by the direction of antiferromagnetic spin, defying to follow the direction of a magnetic field of weak strength. Reading information from magnetic storage devices – HDDs and magnetic random access memory – is performed based on this effect.

*3 Spin electronics
The electronics of the 20th century evolved solely through exploitation of the charge of the electrons in semiconductors. The magnet also has its origin in electrons: electron spin gives rise to variety of magnetic properties. The electron has a spin as well as a charge. Spin electronics aims to exploit an additional degree of freedom (i.e. electron spin), in addition to the electron charge that has been by far the dominant player in semiconductor electronics. Simultaneous exploitation of electron charge and spin enables breakthrough new devices with novel functionalities beyond the capabilities of conventional semiconductor devices.

*4 SPring-8
A RIKEN facility located in Harima Science Garden City (Hyogo prefecture) is capable of producing the world's highest intensity synchronous radiation. The management and promotion of utilization of this facility are undertaken by JASRI. The name “SPring-8” comes from “Super Photon ring-8GeV.” An electron flying at nearly the speed of light, if deflected from its original trajectory through the effect exerted by a magnet, emits an electromagnetic wave in a direction tangential to its trajectory, which is called radiation light (or synchrotron radiation). At present, there are three “3rd Generation” large scale synchronous radiation facilities in the world: SPring-8 (Japan), APS (USA) and ESRF (France). The acceleration energy available at SPring-8 (8 billion electron volts) enables the provision of an extremely wide spectrum of radiation light: from far infrared to visible, vacuum ultraviolet, and soft X-ray up to hard X-ray. SPring-8 provides a theater for collaborative works involving researchers inside and outside Japan, and the research conducted at this facility cover such diverse areas as material science, geoscience, life science, environmental science, and various applications in industrial sectors.

*5 Synchrotron radiation X-ray magnetic circular dichroism
X-ray is a class of electromagnetic wave, just like light and radio waves, that moves through space accompanied by the propagation of electric and magnetic waves along its path. Circular polarized light is an electromagnetic wave in which the electric and magnetic waves change direction in a rotary manner as it progresses. When a circular polarized X-ray is absorbed in a magnetic material, the amount of energy absorbed varies depending on the magnetic state of the electrons in the material. The rotational direction of the electric field – clockwise or counter-clockwise - also affects the amount of energy absorbed. The X-ray analysis method that utilizes this phenomenon for the analysis of magnetic bodies is called X-ray Magnetic Circular Dichroism (XMCD).

For more information, please contact:
  Assoc.Prof. Yu Shiratsuchi (Osaka University)
    E-mail : mail1

  Dr. Tetsuya Nakamura (JASRI)
    E-mail : mail2

  Prof. Hiroyuki Nojiri (Institute for Materials Research, Tohoku University)
    E-mail : mail3

  Assoc.Prof. Yasuo Narumi (Institute for Materials Research, Tohoku University)
    E-mail : mail4