A research group of Dr. Hiroya Sakurai, Senior Researcher, Dr. Taras Kolodiazhnyi, Senior Researcher, and Dr. Yuichi Michiue, Principal Researcher of the Superconducting Properties Unit, National Institute for Materials Science (NIMS, President: Sukekatsu Ushioda), Dr. Eiji Muromachi, Vice President of NIMS, and others, in joint work with Professor Hikomitsu Kikuchi and Mr. Yuichi Tanabe of the University of Fukui, discovered a new material, NaCr2O4, which displays a novel type of colossal magnetoresistance effect. The new material was developed by ultra-high pressure synthesis.

Materials in which electrical resistance changes by an order of magnitude when a magnetic field is applied are called colossal magnetoresistance (CMR) materials. Virtually all known CMR materials are oxides of manganese, and their CMR mechanism also depends on a special ferromagnetic-metallic phase of manganese ions. However, new CMR mechanisms and material search guidelines which do not rely on manganese oxides have been demanded.

In this research, a new material, NaCr2O4, was developed by ultra-high pressure synthesis, focusing on the following two points: (1) Calcium ferrite structures have both a 1-dimensional crystal structure and a structure which displays magnetic frustration, and (2) oxides with tetravalent ions of Cr have a special electronic state.

It was found that a CMR effect occurs in NaCr2O4, which is not a ferromagnetic metal, but rather, is an antiferromagnetic semiconductor. Although the CMR effect appears over a wide temperature range, i.e., the entire temperature range below the magnetic transition temperature, this is a CMR effect with a new mechanism, which has the novel feature of not displaying history effects with respect to temperature or the magnetic field.

This result has important implications for the search for CMR materials, as it is also necessary to consider the antiferromagnetic semiconductors, which had seemed unrelated to the CMR effect until now. The new mechanism proposed as a result of this research has the potential to become a new material search guideline, as the CMR effect can be considered to occur in the diverse structures of various transition metal compounds.

This research was carried out with support from the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) Program, Grant-in-Aid for Basic Research A (22246083), Basic Research C (21560025), of the Japan Science and Technology Agency and Designated Research Regions (19052005) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). This achievement has been published in Angewandte Chemie International Edition vol. 51 (2012) 6653-6656.

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Hiroya Sakurai

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Prof. Hikomitsu Kikuchi

Graduate School of Engineering, University of Fukui

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