Emergence of ferromagnetism due to charge transfer in compressed ilmenite powder using super-high-energy ball milling

Satoshi Ohara; Takashi Naka; Kousuke Sunakawa; Shiro Kubuki; Mamoru Senna; Takeshi Hashishin

doi:10.1038/s41598-020-62171-z

Emergence of ferromagnetism due to charge transfer in compressed ilmenite powder using super-high-energy ball milling

Sci Rep. 2020 Apr 2;10(1):5293. doi: 10.1038/s41598-020-62171-z.

Authors

Satoshi Ohara¹, Takashi Naka², Kousuke Sunakawa³, Shiro Kubuki³, Mamoru Senna⁴, Takeshi Hashishin⁵

Affiliations

¹ Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan. ohara@jwri.osaka-u.ac.jp.
² National Institute for Materials Science, 2-1-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.
³ Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachi-Oji, Tokyo, 192-0397, Japan.
⁴ Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, 223-8522, Japan.
⁵ Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, Kumamoto, 860-8555, Japan. hashishin@msre.kumamoto-u.ac.jp.

Abstract

Ilmenite, FeTiO₃, is a common mineral in nature, existing as an accessory phase in the most basic igneous and metamorphic rocks, for example, it is derived from the upper mantle. Therefore, an understanding of the high-pressure physics of FeTiO₃ is of fundamental importance in the study of rock magnetization. Here, we provide experimental evidence of lattice compression of FeTiO₃ powder using super-high-energy ball milling, enabling the very high collision energy of 420 times gravitational acceleration. A sample obtained as an ilmenite- hematite 0.5FeTiO₃·0.5Fe₂O₃ solid solution showed a decrease in molar volume of approximately 1.8%. Consequently, the oxidation state in FeTiO₃ powder was changed into almost Fe³⁺Ti³⁺, corresponding to 87% Fe³⁺ of the total Fe for FeTiO₃, resulting in the emergence of ferromagnetism. This new ferromagnetic behaviour is of crucial importance in the study of rock magnetization which is used to interpret historical fluctuations in geomagnetism. In addition, the super-high-energy ball mill can be used to control a range of charge and spin states in transition metal oxides with high pressure.