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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.

The magnetic and electrical transport properties of ferromagnetic semiconductor Mn1�xCrxTe x = 0.04, 0.08, and 0.14 compounds have been investigated. These compounds have ferromagnetic behavior with hysteresis loops showing a coercivity of 300– 985 Oe at 5 K. The hysteresis loop is observed even at room temperature for Mn0.86Cr0.14Te. The substitution of Cr for Mn leads to a change from an antiferromagnetic state of MnTe to a ferromagnetic or ferrimagnetic state of Mn1�xCrxTe. Moreover, the incorporation of Cr into the host antiferromagnetic semiconductor MnTe lattice is confirmed by the structural characterization, which proves further that the ferromagnetic properties are not a result of the secondary phase. The typical feature of the thermally activated conduction processes for semiconductors has been verified by electrical property measurement. DOI: 10.1103/PhysRevB.72.193308 PACS numbers: 75.50.Pp Ferromagnetic semiconductors are key materials for spin injection in electronic and optoelectronic semiconductor devices, 1 such as spin transistors, polarizing light-emitting diodes, and nonvolatile storage devices 2 that can be controlled by weak magnetic fields. Usually, ferromagnetic semiconductors are obtained by doping magnetic impurities into host non-magnetic semiconductors, such as Mn-Ge, MnSi, Cr-Ge, and Mn-CuO. 3–6 Recently, room temperature ferromagnetism was reported in Mn-GaN, Cr-GaN, Co-TiO2, and Co-ZnO. 7–10 Although the ferromagnetism in those materials was attributed to the expected ferromagnetism of the diluted magnetic semiconductors DMSs on the basis of magnetization measurements and crystallographic studies, it is still doubtful whether the ferromagnetism observed comes from the DMSs or the magnetic precipitates. 11 To obtain a real ferromagnetic semiconductor, it is important to ensure that after the doping, the same/similar crystal structure still exists with certain solid solubility between magnetic impurity and host semiconductor. On the other hand, it is interesting to detect whether any other possibilities exist for realizing a ferromagnetic semiconductor: Could the element substitution for an antiferromagnetic semiconductor induce a change from antiferromagnetic to ferromagnetic or ferrimagnetic state while maintaining its semiconductor behavior?