Biphasic Fe-Rh nanowires synthesized by AC electrodeposition

Abstract

Fe and Fe90Rh10 (nominal) polycrystalline nanowire arrays, 20 nm in diameter and about 1-3 mm in length, are successfully synthesized by AC electrodeposition into the cylindrical pores of anodized aluminum oxide (AAO) nanotemplates. The effects of Rh addition to Fe nanowires on the resulting microstructure, magnetic hysteresis and magnetoresistance properties are explored. As deposited Fe90Rh10 nanowires are biphasic, with large alfa-Fe grains (>100 nm) and clusters of very small (1.7 nm) grains of the ClCs type a´-FeRh phase, with a composition near Fe42Rh58. Hysteresis loops as a function of the sample orientation relative to the external applied field indicate that the easy magnetization axis is parallel to the nanowire long axis at all temperatures. The coercive field decreases with rhodium addition, as expected for an effective uniaxial anisotropy controlled by shape effects; on contrary, the relative remanence (or squareness) slightly increases. The temperature dependence of the coercive field is consistent with a mechanism involving nucleation of inverse domains, with an apparent energy barrier at zero fields between 2.6 and 4.1 eV. Room temperature magnetoresistance in Fe90Rh10 arrays is negative and shows hysteresis at low fields. At low temperature a large hysteresis is observed for fields between the knees in the hysteresis loop, with field ranges of positive and negative magnetoresistance slopes, arising in domain wall nucleation and expansion to complete polarization reversal and in conventional anisotropic magnetoresistance, respectively.