SPring-8, the large synchrotron radiation facility

The collaborative research group, consisting of researchers from RIKEN (president: Ryoji Noyori), University of Tokyo (president: Junichi Hamada), Kobe University (president: Hideki Fukuda), Hiroshima University (president: Toshimasa Asahara), and JASRI (president: Tetsuhisa Shirakawa), discovered that the osmium (Os) atom in artificial compound Cd₂Os₂O₇^*1 has two spin orientations (inward and outward orientation). The discovery is considered to pave the way to developing novel magnetic recording materials, i.e. those without magnetic properties. The group’s lead researchers include Professor Takahisa Arima (team leader of Spin Order Research Team, RIKEN SPirng-8 Center (director: Tetsuya Ishikawa)) and Jun-ichi Yamaura (research associate, Institute for Solid State Physics, University of Tokyo).

The electron is a negatively charged spinning entity, and, accordingly, has the property of a microscopic magnet. Materials with the electron spins arranged in the same direction exhibit magnetic properties and have found a wide variety of practical applications, a typical example of which is the magnetic storage material used in a hard disk. In magnetic storage materials, the orientation of electron spin, or the direction of the north pole, imparts digital meaning to the stored information (“0” or “1”).

In this study, the collaborative research group focused attention on artificial compound Cd₂Os₂O₇. It has an interesting property in that it undergoes a change of state-from metal to semiconductor-at temperatures below -52°C. The collaborative research group hypothesized that the driving force behind the state change was the development of a regular alignment of electron spins at lower temperatures, and examined the electron spin arrangement of Os atoms using the synchrotron radiation X-ray available at SPring-8.^*2 The result was the discovery of a collective realignment of all electron spins that belonged to the regular tetrahedron made up of four Os atoms: all of them align themselves together, facing the inward or outward direction, in synchronization with the state change from metal to semiconductor, or vise versa. This peculiar alignment endows the substance with two characteristics: cancellation of magnetic properties because of mutually opposing electron spins, and capability of representing binary information, i.e. digital “0” and “1.” Because of the former property, it is affected very little by a strong magnet placed in the direct vicinity, raising the possibility of delivering novel storage materials with unprecedented characteristics.

The expectations are that the discovery will lead to, by synthesizing practical materials with the same class of spin arrangement, the development of a new breed of storage materials in the future that deliver unconventional properties.

The results of the research were published in the online version of “Physical Review Letters” (a US physics journal) on May 28.

Publication:
"Tetrahedral Magnetic Order and the Metal-Insulator Transition in the Pyrochlore Lattice of Cd₂Os₂O₇"
J. Yamaura, K. Ohgushi, H. Ohsumi, T. Hasegawa, I. Yamauchi, K. Sugimoto, S. Takeshita, A. Tokuda, M. Takata, M. Udagawa, M. Takigawa, H. Harima, T. Arima, and Z. Hiroi
Physical Review Letters 108 24 , 247205 (2012), published online13 June 2012