Observations of Magnetic Domain Structure Change in Nd 2 Fe 14 B Magnets at Elevated Temperature with External Magnetic
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Observations of Magnetic Domain Structure Change in Nd2Fe14B Magnets at Elevated Temperature with External Magnetic Field by Lorentz Microscopy Toshimasa Suzuki1, Koichi Kawahara1, Haruka Tanaka1 and Kimihiro Ozaki2 1 Materials Research and Development Laboratory, Japan Fine Ceramics Center (JFCC), Nagoya, 456-8587, Japan. 2 Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, 463-8560, Japan. ABSTRACT In this study, we conducted the in-situ observations of the magnetic domain structure change in Nd2Fe14B magnets at elevated temperature by transmission electron microscopy (TEM) / Lorentz microscopy. The in-situ observations in Nd2Fe14B magnets revealed that the magnetization reversal easily occurred at the elevated temperature. At more than 180oC, the magnetic domain wall motion could be observed by applying the magnetic field of less than 20 mT. The motion of the magnetic domain wall was discontinuous and the domain wall jumped to one grain boundary to the neighboring grain boundary at 180oC. On the other hand, the continuous domain wall motion within grain interior as well as discontinuous domain wall motion was observed at 225oC, and some grain boundaries showed still strong pinning effect even at 225oC. The temperature dependence of the pinning effect of grain boundaries would not uniform. INTRODUCTION Nd2Fe14B magnets with excellent properties have extended the applications in the field of motors for Hybrid Electric and Electric Vehicles. The permanent magnet motors are often used at elevated temperature as high as 200oC. It is well known that the Nd2Fe14B magnets without dysprosium (Dy) show the decrease of coercivity at elevated temperatures. In order to attain high coercivity of sintered Nd2Fe14B magnets even at high temperature, various methods have been employed [1-3]. However, the coercivity of developed Nd2Fe14B magnets were worse than the coercivity of Nd2Fe14B with Dy [4]. Therefore, in order to reduce deterioration of coercivity of Nd2Fe14B magnets at elevated temperatures, it is important to observe generating sites of reverse magnetic domains and pinning sites against magnetic domain walls motion in Nd2Fe14B magnets. In this study, we observed the magnetic domain structure change in fine-grained Nd2Fe14B magnets at elevated temperatures by transmission electron microscopy (TEM) / Lorentz microscopy. EXPERIMENT Fine-grained sintered Nd2Fe14B magnets without Dy were prepared using Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) processed Nd2Fe14B powder. Details of preparation of sintered magnets were described in ref. 5. The average grain size of sintered Nd2Fe14B magnet was about 380 nm (Fig. 1). A coercivity of the sintered
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Nd2Fe14B magnet is 1.2 T. Prior to preparation of thin foils suitable for TEM observations, the sintered Nd2Fe14B magnet was thermally demagnetized in Ar atmosphere at 350oC. Thin foils with dimensions of ca. 10 μm x 10 μm x 100 nm were prepared by a focused ion beam (FIB
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