Giant magnetoresistance, structural and magnetic properties of glass-coated Fe–Ni–Cu microwires

Abstract

Glass-coated Fe–Ni–Cu microwires prepared by Taylor's technique exhibit negative magnetoresistance (MR) of 8.15% at 77 K and 6.35% at 300 K in a magnetic field of 9.0 T. The MR is of the same origin found in the granular giant magnetoresistance (GMR) materials and is distinguishable from the giant magnetoimpedance (MI) commonly seen in soft magnetic microwires. MI displays a peak at zero field for RF currents with frequencies less than 20 MHz and it crosses over to a sharp dip at higher frequencies. This crossover is ascribed to the skin-depth-limited response primarily governed by the field dependence of the permeability. Micro-X-ray absorption near edge structure (micro-XANES) spectroscopy data were collected at the K edges of Cu, Ni and Fe and revealed that the Fe atoms in the as-cast sample are in FCC configuration and they remain in the FCC phase throughout the annealing processes. The MR decreases to ∼2.5% as the annealing temperature increases to 500°C. The loss of GMR upon annealing is attributed to the growth of FCC Fe–Ni-rich magnetic particles. The increase in the Fe–Ni particle size also results in higher room temperature coercivity. When the annealing temperature is increased to 500°C, a wasp-waisted hysteresis loop is observed which arises from the locking-in of the domain walls by the directional order of atoms due to diffusion under the influence of the local magnetic field. A magnetically hard glass-coated microwire with coercivity of 600 Oe is obtained after annealing at 700°C for 1 h.