Effect of grain size on magnetic properties and microstructure of Ni80Fe20 thin films

Abstract

Ni–Fe thin films were deposited from a Ni80Fe20 target on a glass substrate by direct current magnetron sputtering. The film thickness was varied from 300Å to 1000Å and the films are formed at room temperature (RT) and annealed for 1h at 250°C. Plane-view high-resolution transmission electron microscopy was used to examine the microstructure of each NiFe thin film and to determine the grain sizes. Electron diffraction patterns reveal that the as-deposited and annealed NiFe thin film had a face-centered cubic NiFe. It also indicates that the crystallization of the annealed NiFe film is stronger than that of the as-deposited film. The coercivity and magnetic squareness (SQR) and remanent and saturation magnetization ratio (Mr/Ms) ratio of the in-plane magnetic hysteresis loop were measured using a superconducting quantum interference device. The coercitivity as well as the magnetic remanence properties and squareness ratio of the NiFe thin films is related to the distribution of grain sizes. The SQR value of the NiFe thin film increases upon annealing because the ordered spins in the NiFe thin film are randomized by the thermal vibration. Accordingly, Mr is increased and Ms is reduced. Hence, the SQR value decreases as the thickness of thin film increases because the refinement of the grains reduces the effective anisotropy, strengthening the ferromagnetic exchange coupling interaction.