Press Release

Uncovering the origin of strain-induced magnetization in strongly correlated insulators

July 10, 2013

University of Tokyo
High Energy Accelerator Research Organization, KEK

A research group at the University of Tokyo, led by Assistant Professor Jun Fujioka and Professor Yoshinori Tokura, together with the team at the Institute of Materials Structure Science, KEK, including Assistant Professor Yuichi Yamasaki, Associate Professor Hironori Nakao, and Professors Reiji Kumai and Yoichi Murakami found that the fabrication of a thin film of perovskite LaCoO₃ and the crystal-lattice strain by substrate results in a new complex spin and orbital ordering of electrons with spontaneous magnetization.

Strongly correlated electron systems, where electrons strongly interact with each other, widely show self-organization of electrons at the nanometer scale. For instance, magnetization is the result of the ordering of electron spins in materials. Such ordering can be obtained by tuning the material control parameter such as the effective one-electron band-filling or band-width.

The research group focused on perovskite LaCoO₃, which is a known non-magnetic insulator, with the aim of controlling the spin state to induce ordering of the spin and orbital degrees of freedom that results in changes in magnetism. The magnetic state of the LaCoO₃ thin film was investigated using magnetization measurements, and the crystal structure and the resonant X-ray scattering were measured using synchrotron X-ray diffraction (XRD) at the Photon Factory, KEK. Anomalous temperature dependence of the magnetism was found to exist at 126 K, which is higher than the onset temperature of spontaneous magnetization, 94 K. Furthermore, the spin state of Co ions in the material was found to change from the non-magnetic low-spin state to the intermediate- and the high-spin state with slight strain in the crystal. The orbital ordering of intermediate- and high-spin states was found to occur at 126 K, resulting in the spin ordering at 94 K with spontaneous magnetization. Fig.1 shows a probable spin and orbital ordering pattern based on the above results.

This research uncovered a mechanism to transform non-magnetic to magnetic materials by applying slight crystal strain. One possible application of this mechanism is the development of highly sensitive strain sensors.

This achievement was published in the online version of the American scientific journal “Physical Review Letters” (June 12, 2013). This work was conducted as part of the funding program for the World-Leading Innovative R&D on Science and Technology (FIRST Program) project “Quantum Science on Strong Correlation” (Principal investigator: Yoshinori Tokura).

Fig.1. One possible spin-orbital ordering pattern. The arrows and lobes indicate electron spins and orbitals, respectively. The solid lines depict the Co crystal lattice.

Publication "Spin-orbital superstructure in strained ferrimagnetic perovskite cobalt oxide" Physical Review Letters Authors：J. Fujioka, Y. Yamasaki, H. Nakao, R. Kumai, Y. Murakami, M. Nakamura, M. Kawasaki, and Y. Tokura