Magnetotransport effects of ultrathin Ni80Fe20 films probed in-situ

TL;DR

This study investigates how ultrathin Ni80Fe20 films grown on different substrates exhibit evolving magnetoresistance properties during growth, transitioning from tunneling to anisotropic magnetoresistance as film structure develops.

Contribution

It provides a detailed analysis of the magnetotransport evolution during film growth, linking structural changes to magnetoresistance behavior across various substrates.

Findings

Transition from tunneling to anisotropic magnetoresistance with increasing film thickness

Substrate-dependent differences in magnetoresistance response during growth

Identification of two ferromagnetic contributions in thicker Al2O3-grown films

Abstract

We have investigated the magnetoresistance of Permalloy (Ni80Fe20) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and SiO2 substrates. Growth and transport measurements were carried out under cryogenic conditions in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties are ascertained during growth. With increasing thickness the films exhibit a gradual transition from tunneling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains as well as the transition from superparamagnetic to ferromagnetic behavior of the film. Using an analysis based on a theoretical model of the island growth, we find that the observed evolution of the magnetoresistance in the tunneling regime originates from the changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunneling magnetoresistance and anisotropic magnetoresistance were found. We attribute this to an increasingly pronounced island growth and slower percolation process of Permalloy when comparing growth on SiO2, MgO and Al2O3 substrates. The different growth characteristics result in a markedly earlier onset of both tunneling magnetoresistance and anisotropic magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in a structure of the film containing two different contributions to the ferromagnetism which lead to two distinct coercive fields in the high thickness regime.