Spin-dependent scattering in nanocrystalline Fe:GMR

Abstract

Granular Fe(1 1 0) thin films have been grown on c-sapphire substrates by triode sputtering. Islands size range from 11 nm in-plane diameter and 2 nm height up to 100 nm in-plane and 23 nm height, depending on the deposition time. Strong changes in the spin-dependent scattering are found in samples with in-plane diameter below 30 nm. This behavior is due to a low-temperature spin freezing of the island boundary magnetic regions, producing a suppression of the exchange coupling between islands. A consequence of the magnetic decoupling is the random arrangement of the individual magnetization, determined by the magnetocrystalline anisotropy of each island, that results in a spin-dependent induced increase of the resistivity below the freezing temperature. By applying a high enough magnetic field, magnetic ordering recovers regardless of the temperature, obtaining a typical metallic behavior for the thermal dependence of the resistivity. The ratio of the low-temperature resistivities with and without magnetic field yields magnetoresistance values up to 5.5% for 16.5 nm in-plane island sizes. The following two conclusions are obtained: (i) nanocrystalline Fe can be considered as a low-temperature GMR-like system and (ii) Fe grain boundaries are not ferromagnetic at low temperature but behave as a reentrant spin-glass system.