Advanced Nano Characterization Center, National Institute for Materials Science / Waseda University^* National Institute for Materials Science^** Sophia University^*** Waseda University^****
○Masahiro Nagao^* Toru Asaka^** Yutaka Yamauchi^*** Ryota Hatakeyama^*** Takuro Nagai^** Atsushi Yamazaki^**** Koji Kimoto^** Hideki Kuwahara^*** Yoshio Matsui^**

Perovskite manganites have a wide variety of electronic/magnetic structures depending on the interplay among spin, charge, orbital, and lattice degrees of freedom. In Nd_1-xSr_xMnO₃, various electronic/magnetic ground states such as a ferromagnetic and an antiferromagnetic charge order state appear. Among these series, there have been few studies of the C-type antiferromagnetic phase which has 1D-like orbital chain structure, although the other magnetic structures have been studied extensively to understand the properties of the manganites such as colossal magnetoresistance, charge/orbital order. Here we show the crystal structure and the textures of the C-type antiferromagnetic phase for single crystals of Nd_1-xSr_xMnO₃ (x=2/3, 3/4, 4/5) by means of transmission electron microscopy.
The electron diffraction (ED) patters of x=2/3, 3/4, obtained at room temperature and 82K, indicated tetragonal I4/mcm structure corresponding to the previous neutron diffraction study. In contrast, the ED pattern of x=4/5 demonstrated the emergence of diffuse scattering around 460K at the forbidden reflections points for I4/mcm structure. The diffuse scattering changed into the sharp spot at the vicinity of Neel temperature, ~300K. The observation provides the evidence for the presence of a different structure from I4/mcm.
We carried out dark-field imaging in order to elucidate the detail of the structural phase transition. In the high-temperature paramagnetic phase, the nanometer-scale domains with another structure embedded in I4/mcm matrix were observed. The nanodomains evolved to become large in size with decreasing temperature. Eventually, the other structure was replaced I4/mcm phase at Neel temperature. This behavior would be regarded as the diffuse phase transition.