Low Temperature Magnetic Properties of Nanocrystalline Iron

Abstract

Nanocrystalline iron has been obtained by sputtering on different substrates. Resistivity measurements point out an anomalous behaviour of the thermal dependence of the resistivity at low temperatures which dissapears when a high magnetic field is applied. Strong changes in the spin dependent scattering in islanded Fe (110) thin films are 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, which results in a spin-dependent induced increase of resistivity below freezing temperature. After application of a magnetic field, magnetic ordering recovers regardless of temperature, obtaining a typical metallic behavior for the temperature dependence of the resistivity. Ratio of low temperature resistivities, measured with and without magnetic field, yields magnetoresistance values of up to 5.5% for 16.5% nm in-plane island sizes. The following two conclusions are obtained, i) nanocrystalline Fe can be considered a low temperature GMR-like system and ii) Fe grain boundaries are not ferromagnetic at low temperature, but behave as reentrant spin-glass.