Advanced Electron Microscopy Group, National Institute for Materials Science^* Spin Super Structure Project, ERATO-JST, Japan^** Correlated Electron Research Center (CERC), AIST, Japan^*** HREM Research, Inc., Japan^**** Department of Physics, University of Tokyo, Japan^*****
○Yoshio Matsui^* Toru Asaka^* Xiuzhen Yu^** Yasuhide Tomioka^*** Yoshio Kaneko^** Takuro Nagai^* Koji Kimoto^* Kazuo Ishizuka^**** Yoshinori Tokura^*****

Ordered double perovskite, AE₂FeMoO₆ (AE = Ca, Sr and Ba), has two kinds of ions, i.e. Fe³⁺ and Mo⁵⁺, at the B site in the perovskite-type structure. The Fe³⁺ and Mo⁵⁺ ions are ordering in manner of rock-salt type structure. In the magnetic aspects, the local spins on the Fe³⁺ (S = 5/2) and Mo⁵⁺ (S = 1/2) ions couple antiferromagnetically. This leads to ferrimagnetism. This system contains essentially a crystallographic antiphase-domain structure which is caused by a spatial distribution of the Fe/Mo ordering. It has been reported that the antiphase boundary (APB) plays a role of the magnetic domain wall (MDW) where the spins couple antiferromagnetically [1]. Concerning the interactions between APB and MDW, Yu et al. have demonstrated the direct observation of the pinning effect of MDW at APB in Ba₂FeMoO₆ by combined use of dark-field imaging and Lorentz transmission electron microscopy (LTEM) [2]. Here, we report detail research of the crystallographic antiphase domain and magnetic domain structures for a single crystal of Ba₂FeMoO₆, by semi-quantitative LTEM observation.
We observed bending and branching of the MDW at the APB. In addition, the MDWs perfectly coincide with the APBs in most parts of the observed area. We found that the magnetic domain corresponds one-to-one with antiphase domain at many regions. Besides, it was revealed that the magnetic nanodomains formed at the short-range Fe/Mo ordering region. We consider that the magnetic nanodomains were formed by restriction of coherent magnetic ordered region due to the crystallographic short-range ordering, and fragmentation of magnetic domain and spatial fluctuation of local magnetic moments on the APB.

[1] H. Q. Yin et al. J. Appl. Phys. 87, 6761 (2000).
[2] X. Z. Yu et al. J. Elect. Microsc. 54, 61 (2005).