Anomalous positive magnetoresistance inFe0.75Mn1.35As

With increasing temperature, tetragonal Fe0.75Mn1.35As compound exhibits a first-order phase transition at T-s=165 K from a ferrimagnetic (FI) phase with a large unit-cell volume to an antiferromagnetic (AF) phase with a small unit-cell volume. An external magnetic field induces a metamagnetic transition from the AF to the FI state above T-s, stabilizing the phase with the larger cell volume. With increasing the number of warming and cooling cycles, the resistivity curve, rho(T), shifts to higher values, and this shift is ascribed to a pronounced reduction of the mean-free path of electron scattering. After several thermocycles, a large, irreversible lattice expansion of c axis of 0.18% occurs. The positive magnetoresistance ratios as large as 12% and 6% for the first magnetic-field cycles are observed at 170 and 167 K, respectively, at 5 T. The overall magnetotransport behaviors on the subsequently second and third magnetic-field cycles at 170 K are similar to that in the first magnetic-field cycle, but with MR ratios of only 1.6% and 0.6%, respectively, at 5 T.