Magnetic viscosity in iron-rhodium nanowires

Abstract

Noble/transition bimetallic nanowires of nominal composition Fe90Rh10 are AC electrodeposited into 20 nm diameter hexagonally self-assembled alumina nanopores. Nanowires about 18 nm in diameter and 1 μm long are polycrystalline and multiphase. Wires contain α-Fe grains and very small grains of the ClCs-type α′-Fe(Rh) phase. The room temperature magnetization mechanism and the thermal stability of nanowire magnetic configurations are further investigated by measuring the dependence of the coercive field on the applied field sweeping rate R. From these data a mean fluctuations field value of μ0HFR = (9.0 ± 0.5) mT is obtained (at coercivity) and an effective activation magnetic moment μac = 5400 μB is estimated, with μB the Bohr magneton. At the coercive field (about 45 mT) the resulting activation lengths become lAC ≈ 6.4 and 6.7 nm for α-Fe and α′-Fe(Rh), respectively. Assuming an effective magnetic anisotropy, considering magnetostatic shape contributions in addition to the magnetocrystalline one, the domain wall thickness δw in α-Fe grains and in the α′-Fe(Rh) become δwFe = 13.4 nm and δwFeRh = 10.9 nm respectively. These values are comparable to the activation lengths estimated at the coercive field in each phase. These facts strongly indicate that irreversible polarization reversal in these nanowires takes place by local curling, involving localized nucleation modes.