Giant magnetoresistance of magnetic granular phase of FeRh ordered alloy

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Since the ferromagnetic-antiferromagnetic phase transition is first ordere transition, the large change in resistivity caused by the transition is considered to be difficult to use. Howecer, it became clear that the change in resistivity caused by an application of magnetic field during the midway of the phase transition from high temperature ferromagnetic phase to low temperature antiferromagnetic phase has a small and reversible hysteresis. Therefore, the application of this change in resistivity is possible.In ordere to make clear the mechanism of this large change in resistivity, samples were cold rolled and performed various kind of heat treatments. Then, the change in resistivity was discussed in relation to the phase transition and magnetic microstructure. From this experiment, it is considered that the magnetic structure which appears on the midway of magnetic phase transition is the mixture of ferromagnetic and antiferromagnetic phases mixed like the so colled granular state, and a part of magnetoresistance is due to the spindependent diffractions of conduction electrons which occur at the magnetic grain boundary. These results were presented to the journal of the magnetics society of japan. Then, we tried to observe the magnetic granular state by MFM,and obtained results which suport our consideration.As an applicational attempt, we tried to get large change in magnetoresistance by weak magnetic field. A Fe-Rh sample 0.5mm long 2mm wide 1mm thick was sandwiched between two iron plates 5mm long and 2mm wide. Then sample was connected with the iron plates by introducing one pulse of current from a spot welder. This heterostructural sample showed large relative magnetoresistance change of 5.4% by 6.6kOe. This change is 5 times larger than that obtained for a sample without iron plates. Rh conposition dependence of the change in magnetoresistance was examined. The sample with 51 at% Rh showed the largest magnetic field dependence of magnetoresistance.