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Promise Found for the Development of “Non-magnetic” Material for Magnetic Recording (Press Release)

Release Date
24 May, 2012
  • BL02B1 (Single Crystal Structure Analysis)
  • BL19LXU (RIKEN SR Physics)
- Discovery of hitherto unknown electron spin arrangement that renders a material non-magnetic -

University of Tokyo
Kobe University
Hiroshima University
Japan Synchrotron Radiation Research Institute (JASRI)

• The electron spin arrangement in Cd2Os2O7 (artificial compound) was determined for the first time using facilities at SPring-8.
• Confirmation of two modes of electron spin arrangement holds promise for representing digital information (“0” and “1”).
• The electron spins of the compound cancel each other, making the compound a non-magnetic entity as a whole.

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.

"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


Fig.1. Crystal structure of Cd2Os2O7
Fig.1. Crystal structure of Cd2Os2O7

(Left): Crystal structure of Cd2Os2O7. Cadmium (Cd), osmium (Os), and oxygen (O) atoms are represented in green, light brown, and blue respectively. This atomic arrangement is called the “pyrochlore structure.” (Right): Os-atom lattice of the Cd2Os2O7 crystal structure (other atoms not shown). The lattice consists of linked tetrahedrons.

Fig.2. Two modes of electron spin alignment
Fig.2. Two modes of electron spin alignment

Schematic view of the Os-lattice extracted from Cd2Os2O7 crystal, and its electron spin alignment at low temperatures (≤ -52°C).
(a) The tetrahedrons in Cd2Os2O7 crystal structure have two possible orientations (tetrahedrons A and B), and are arranged so that each one of the linked tetrahedrons has opposite orientation to the other. For example, if electron spins in tetrahedron-A are outwardly oriented, then, those in neighboring tetrahedron-B are inwardly oriented (see right figure). If this state is to represent digital “0,” the state with reversed spin orientations-inwardly oriented in tetrahedron A, and outward in tetrahedron B-can be considered to represent digital “1.” In this way, the two modes of electron spin arrangements can be used to symbolize binary information (digital “0” and “1”). (b) Tetrahedron A: the four electron spins at the tetrahedron’s apexes cancel each other, resulting collectively in a non-magnetic material. This unique property holds significant promise for developing recording materials with novel functionalities that are beyond the reach of conventional magnetic storage materials.

*1 Cd2Os2O7

An artificial metal oxide derived from pyrochlore,*3 prepared by replacing its calcium (Ca) atoms with cadmium (Cd), and its niobium (Nb) atoms with osmium (Os).

*2 SPring-8
SPring-8, with its synchrotron radiation beams, provides a wide range of research opportunities ranging from basic research (e.g. analysis of tiny particles brought back by the asteroid probe “Hayabusa”) to practical industrial applications (e.g. lithium battery and drug discovery). The research reported here utilized two beam lines (BL19LXU, BL02B1).

*3 Pyrochlore
A naturally occurring ore (raw material used for refining metallic niobium) with an ideal composition formula of Ca2Nb2O7. Actual instances of the ore include compositional variations: some of the constituent atoms may be replaced with dissimilar ones (Ca with Na, Nb with Ta, O with F) and additional H may be included. The crystal structure (called the “pyrochlore structure”) is known to allow easy material design, as it permits relatively free replacements of one atom with types of dissimilar atoms while maintaining its basic structure. Many pyrochlore-type materials have been artificially synthesized.

For more information, please contact:
  Ph.D. Takahisa Arima (RIKEN)

  Ph.D. Hiroyuki Osumi (RIKEN)

  Dr. Junichi Yamaura (The University of Tokyo, The Institute for Solid State Physics)

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