Annealing effects and degradation mechanism of NiFe/Cu GMR multilayers

Abstract

Structural, transport and magnetic properties of sputtered Ni 80Fe 20/Cu multilayers showing giant magnetoresistance (GMR) were studied using X-ray reflectometry and diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect. In particular, mechanisms of the GMR degradation at elevated temperatures were investigated. Multilayers with an individual layer thickness of 2 nm show a sharp drop of the GMR after annealing at about 250°C. Whereas below this temperature grain growth and defect reduction contribute to a partial improvement of the GMR, above ∼250°C interdiffusion between Ni and Cu appears to lead to layer intermixing and to the degradation of transport and magnetic properties. Moreover, the initial 〈1 1 1〉 texture sharpens, and strong tensile stresses arise in the layer stack. We correlated the structural alterations to changes in the magnetic properties such as the strength of the antiferromagnetic coupling (bilinear and biquadratic) and the magnetic anisotropy. Above 250°C an increasing magnetic inhomogeneity of the Permalloy layers can be inferred from the FMR linewidth broadening.