Successful Visualization of “Tilted” Spins at the Junction Interface of Dissimilar Magnetic Bodies (Press Release)
- Release Date
- 07 Aug, 2012
- BL25SU (Soft X-ray Spectroscopy of Solid)
Osaka University
Japan Synchrotron Radiation Research Institute (JASRI)
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The research team, consisting of Dr. Yu Shiratsuchi (Associate Professor, Osaka University), Dr. Tetsuya Nakamura (Senior Scientist, JASRI) and others, tackled a study of microscopic magnetic behavior at the interface connecting a ferromagnetic and antiferromagnetic body, and successfully revealed the origin of strong magnetic coupling that makes the material combination useful for information retrieval in a hard disk. When two magnetic bodies with different characteristics, i.e. a ferromagnetic and antiferromagnetic body,*1 are joined together, the combination gives rise to a set of new magnetic properties at the junction interface that neither of the bodies possess in isolation. This phenomena is called exchange magnetic anisotropy*2 and its properties have been used in a wide range of devices, including hard disk drives (information retrieval), angular velocity (gyration) sensors for automobiles, and spin electronics*3 devices, such as magnetic random-access memory. In spite of such practical uses, discussions on the detailed mechanisms that give rise to exchange magnetic anisotropy are still continuing after the nearly 60 years since its discovery in the middle of the 1950s. The research group obtained a clear answer to the long-standing question regarding exchange magnetic anisotropy: “Does the antiferromagnetic spin flip or not?” The answer is: “Antiferromagnetic spin only tilts into an inclined position, but does not flip.” The study used the soft X-ray magnetic circular dichroism*5 measurement available at SPring-8*4 to detect minute antiferromagnetic spin signals. Elucidation of the twisted arrangement of tilted antiferromagnetic spins and the pursuit of thinner antiferromagnetic layers provide important parameters for achieving smaller devices with larger capacity. Efficient use of the results is expected to have significant implications on the design of future electronics devices that take advantage of “twisted" ferromagnetic spin. These results appeared on the online version of Physical Review Letters (a physics journal published in US) on the 8th of August, 2012. Publication: |
<<Figures>>
antiferromagnetic body (Mn-alloy, e.g. Mn3Ir) and in the target of this research.
A conventional antiferromagnetic body, shown in (a), has multiple possibilities for spin direction (6 or more), making identification of spin direction difficult. The antiferromagnetic body used in this research, shown in (b), has only two possible spin directions (up or down), making clear determination of spin direction possible.
Do the antiferromagnetic spins at the junction interface between a ferromagnetic and antiferromagnetic body flip (a) or remain stationary (b)? This has been the biggest question concerning the system. To solve this question, high accuracy measurement of antiferromagnetic spins is required. The antiferromagnetic body shown in Fig. 1(b) was used for the determination, as it has only limited freedom of spin orientations. The experiment led to the conclusion, as shown in (c), that the antiferromagnetic spins do tilt, but never flip. Tilting of antiferromagnetic spins at the junction interface produces a twisted spin arrangement in the antiferromagnetic body. This twisted spin arrangement is the key feature that determines the peculiar magnetic property (exchange magnetic anisotropy) at the junction interface between a ferromagnetic and antiferromagnetic body. Future development of new magnetic materials that allow effective manipulation of twisted spin arrangement will enable the creation of a much higher level of exchange magnetic anisotropy.
<<Glossary>>
*1 Ferromagnetic and antiferromagnetic bodies
The ferromagnetic body is a class of magnetic bodies characterized by its property of being attracted by a magnet. Such bodies also show a tendency to become magnets themselves. Inside a 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 to the direction opposite to that of the neighboring electron spin. Therefore, antiferromagnetic bodies do not generate external magnetic flux, showing no tendency to be attracted to a magnet.
*2 Exchange magnetic anisotropy
Antiferromagnetic bodies do not become magnets, but they can exert a huge effect on the magnetic properties of a ferromagnetic body if joined to one. Normally, the 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 to an antiferromagnetic body, orients itself to a fixed direction determined by the direction of antiferromagnetic spin, no longer following the direction of a magnetic field of normal strength. Reading information from magnetic storage devices - HD and magnetic random access memory - is performed based on this effect.
*3 Spin electronics
Electronics in the 20th century evolved solely through the exploitation of the charge of the electrons in semiconductors. The magnet also has its origin in electrons: electron spin gives rise to a 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 breakthroughs with new devices that have novel functions beyond the capability 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
The X-ray is a class of electromagnetic waves, like light and radio waves, that move through space accompanied by the propagation of electric and magnetic waves along their path. Circular polarized light is an electromagnetic wave in which the electric and magnetic waves change direction in a rotary manner as they progress. When a circular polarized X-ray is absorbed in a magnetic material, the amount of energy absorbed varies depending on the magnetic state of 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).
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For more information, please contact:
Dr. Tetsuya Nakamura (JASRI) |
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