L10-FeNi ordered phase in AC electrodeposited iron-nickel biphasic nanowires

Abstract

Nanowire arrays with nominal composition FexNi100-x (x = 0, 18, 53, 93, 100) have been synthesized by AC electrodeposition into the cylindrical pores of alumina templates. Except for the composition Fe53Ni47, nanowires are single-phase, consisting of small grains of the A1-FeNi disordered phase. Near the equiatomic nominal composition nanowires are biphasic, consisting of grains of the A1-FeNi disordered phase (γ-FeNi phase) as well as grains of the L10-FeNi ordered phase (γ″-FeNi phase). Even in nanowires with high Fe content the α-Fe bcc phase is absent. The coercive field and reduced remanence dependences on composition are non monotonic but go through a local maximum near the equiatomic composition. Biphasic nanowires behave as a magnetic single phase, exhibiting a unique switching field for polarization reversal in the entire range between 5 K and 300 K. The linear dependence of the coercive field on temperature and the dependence of the coercive field on the angle between the nanowire long axis and the applied field are consistent with a polarization reversal mechanism controlled by the nucleation by curling and expansion of inverse domains, assisted by thermal fluctuations and the applied field. The effective uniaxial anisotropy resulting from these measurements suggests that the shape anisotropy makes a small contribution to the high coercive field observed (170 mT). The effective uniaxial magnetocrystalline constant measured at room temperature, 130–170 k Jm−3, may be explained by considering nucleation at favorable sites in an exchange-hardened γ-FeNi soft phase.