Journal of Magnetism and Magnetic Materials 310 (2007) 1572–1574 Pinning effect of the antiphase and grain boundaries on magnetic domains in double perovskite A 2 FeMoO 6 X.Z. Yu a,Ã , T. Asaka b , Y. Tomioka c , Y. Kaneko a , M. Uchida a , J.P. He a , T. Nagai b , K. Kimoto b , Y. Matsui b , Y. Tokura a,c,d a Spin Superstructure Project, ERATO, JST, c/o AIST, Tsukuba Central 4, Tsukuba 305-8562, Ibaraki, Japan b Advanced Electron Microscopy Group, NIMS, Tsukuba 305-0044, Ibaraki, Japan c Correlated Electron Research Center, AIST, Tsukuba 305-8562, Ibaraki, Japan d Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan Available online 21 November 2006 Abstract Magnetic domain structures and microstructures of ferrimagnets A 2 FeMoO 6 (A ¼ Ba or Ca) with double perovskite-type structure were studied by Lorentz transmission electron microscopy (LTEM) and dark-field imaging technique. The magnetization distribution was obtained using the transport-of-intensity equation method based on LTEM images. In the Ba 2 FeMoO 6 single crystal, the magnetic domain walls are pinned at antiphase boundaries due to the Fe/Mo anti-sites, while in the Ca 2 FeMoO 6 polycrystalline sample, magnetic domain walls are pinned at grain boundaries. These results indicate that underlying crystal defects such as the antiphase and grain boundaries affect the local magnetization distribution. r 2006 Elsevier B.V. All rights reserved. PACS: 61.16.Bg; 75.30.Kz; 74.62.Dh Keywords: Transmission electron microscopy; Pinning effect; Magnetic domain; Antiphase boundary; Grain boundary Ordered double perovskite A 2 FeMoO 6 (A ¼ Sr, Ba, or Ca) is known as the tunneling magnetoresistance material that behaves as a half metal with intrinsic tunneling junctions [1]. The crystal structure consists of alternating FeO 6 and MoO 6 octahedra, where the Fe 3+ (3d 5 , S ¼ 5/2) and Mo 5+ (4d 1 , S ¼ 1/2) ions couple antiferromagnetically to produce a ferrimagnetic ground state. The ordering of Fe 3+ /Mo 5+ ions is not perfect in the Ba 2 FeMoO 6 single crystal. If a Mo 5+ site is locally displaced by a Fe 3+ ion and vice versa, the local structure results in an ‘‘anti-site defect.’’ It was shown that such anti-site defects affect the magnetic domain structure [2,3]. In the ordered Sr 2 FeMoO 6 polycrystalline sample, it was suggested that the low-field-sensitive magnetoresistance is due to the field suppression of spin-dependent scattering at grain boundaries [4]. To clarify the effect of crystal defects such as antiphase and grain boundaries on magnetic domains, we studied magnetic domain structures and microstructures of A 2 FeMoO 6 (A ¼ Ba or Ca) with the direct observation using Lorentz transmission electron microscopy (LTEM) and the dark-field imaging technique. The magnetization distribution was obtained using the transport-of-intensity equation (TIE) method. Ba 2 FeMoO 6 single-crystalline and Ca 2 FeMoO 6 poly- crystalline samples were prepared by the floating-zone technique [1] and the solid-state reaction [5], respectively. Mechanical polishing and argon-ion thinning using a Gatan-600N machine with an acceleration voltage of 4 kV were used to prepare electron-transparent thin samples. Magnetic domain structures and microstructures of A 2 FeMoO 6 were studied using a Lorentz electron microscope (Hitachi HF-3000L) equipped with a cold-field emission gun [6] at an acceleration voltage of 300 kV. The magnetization distribution was obtained using the commercial software, QPt, for DigitalMicrograph [7], which is based on the TIE [8,9]. ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1054 Ã Corresponding author. Tel.: +81 29 861 2653; fax: +81 29 858 8344. E-mail address: xiuzhen-yu@aist.go.jp (X.Z. Yu).