Ordered phase (fct) FePt is a promising material for high-density magnetic recording medium since it has a large magnetic anisotropy energy (Ku) to overcome the thermal fluctuation problem[1]. The fct phase is obtained by annealing the as-made fcc phase, and a twin boundary is often formed in crystal grains during this transformation process. At the twin boundary, the c-axis of each fct region crosses at an angle of 88 degrees[2]. Since the magnetic anisotropy is induced along the c-axis, distorted spin microstructure is generated at this twin boundary. This microstructure is expected to cause a reduction in the coercivity (Hc) as reported for the composite media[3].
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