Compositional dependence of the giant magnetoresistance in FexRh1-x thin films

In this article we report on the magnetic and transport properties of FexRh 1-x thin films, prepared by evaporation in high vacuum, in the composition range 0.41 < x(Fe) < 0.59. Upon annealing (at a temperature of 870 K or hig her) a certain volume fraction transforms to the ordered CsCl-type (alpha') FeRh phase. Close to room temperature this phase shows a hysteretic transi tion between the antiferromagnetic (AF) and the ferromagnetic (F) state for samples with x(Fe) < 0.5, which gives rise to a magnetoresistance (MR) eff ect. Although the magnetic transition was never found to be complete, it is shown that the full MR ratio can be obtained by extrapolation of the measu red MR ratio as a function of the relative change of the magnetization at t he transition. The AF --> F transition is only observed for films with x(Fe ) < 0.505 +/- 0.015, for which the alpha' phase with this (fixed) compositi on is present together with a nonmagnetic Rh-rich fcc-type phase, as is sho wn from a combination of x-ray diffraction, Mossbauer spectroscopy, and mag netization studies. This observation, which was not expected from the phase diagrams available from the literature, can explain our finding that the f ull MR ratio observed for films in this compositional range is independent of the Fe content. The full MR ratio is 85 +/- 6% at room temperature, whic h is very close to the MR ratio observed for bulk FeRh, implying a high str uctural quality of the films grown. X-ray diffraction and transmission elec tron microscopy were used to quantify microstructural aspects such as the g rain diameter and strain. The highest MR ratio actually observed is 58%, in a 4400 kA/m field at 275 K. The full MR increases to approximately 150% at 250 K, and, as estimated from the resistivity of F and predominantly AF sa mples with almost the same composition, to 680 +/- 100% at 4.2 K. (C) 1999 American Institute of Physics. [S0021-8979(99)01802-2].