Magnetic anisotropy in giant magnetoresistive CoAg granular films (abstract)

Abstract

A correlation is found between the room temperature giant magnetoresistive (GMR) behavior and the direction of the easy axis of the magnetization in molecular beam epitaxy grown CoAg granular films. Samples containing between 15 and 50 at. % Co were prepared by co-evaporation at temperatures ranging from 0 to 600 °C. GMR curves are recorded with the current in the film plane and the external field either parallel or perpendicular to the film. Out-of-plane torque magnetometry has been used to determine the anisotropy energies and carry out a full directional analysis. Below Co concentrations of 30 at. %, the GMR curve for the field perpendicular to the plane always decreases more rapidly than for the field parallel to the plane. In contrast, for higher Co concentrations, the in-plane GMR decreases more rapidly at low fields and then crosses over the out-of-plane GMR curve. Thus, the maximum GMR is always observed when the field is perpendicular to the plane. Torque measurements show that for all growth temperatures, the direction of the easy axis of magnetization changes from out-of-plane to in-plane with increasing Co concentration. This crossover shifts to lower Co concentrations as the growth temperature increases. The maximum anisotropy energy density is larger in the case where the easy axis lies in the film plane. Unexpectedly, for all growth temperatures two uniaxial anisotropy contributions are found to coexist in films containing 20 at. % of Co. We attribute our findings to the shape anisotropy of the magnetic grains, which is dominated by columnar growth at low and thin film like behavior at high Co concentrations, respectively.