Invariable Simultaneous Emergence of Antiferromagnetic Order and Tetragonal Deformation in CoO Nanocrystals

Yoichi Ishiwata1+ , Gaku Kawahara1, Keisuke Akase1, Taisuke Tominaga1, Hayato Miyazaki, Hirofumi Ishii, Akira Matsuo, Koichi Kindo, Yuji Inagaki, Kenta Akashi, Tatsuya Kawae, Tetsuya Kida, Satoshi Suehiro, Masashi Nantoh, and Koji Ishibashi

It remains unclear why type-II antiferromagnetic order with a propagation vector along a trigonal axis and a tetragonal
Jahn–Teller deformation coexist in a low-temperature phase of CoO. Nanocrystals generally have a lower magnetic
transition temperature but crystal size reduction does not affect structural transition locally induced by a crystal field. On
this basis, we investigated the magnetic and structural properties of CoO nanocrystals. Both transition temperatures were
equally reduced in nanocrystals, which indicates that magnetic order triggers the tetragonal deformation. This
observation is consistent with a model that the Jahn–Teller deformation is assisted by spin–orbit energy splitting of t2g
electrons in the high-spin Co2+ ion, proposed by Kugel and Khomskii.

J. Phys. Soc. Jpn. 93, 044603(2024)

93

044603