Magnetoresistance in Composite Thin Films

In this work the magnetic properties of epitaxial co-evaporated Fe:MgO anti-granular films have been investigated. These films consist of MgO nanoparticles in an Fe matrix. Fe and MgO were co-evaporated on MgO(001) substrates in the ratio of Fe:MgO = 4:1,3:1, 2:1 and 1:1 and ex-situ annealed in steps of one hour up to 6 hours enhancing the uniaxial anisotropy and saturation magnetization. An alternating gradient force magnetometer (AGFM) and a vibrating sample magnetometer (VSM) were used to determine the magnetic properties of the films. An enhancement of the saturation magnetization of
up to 20 % was found. The Fe:MgO = 1:1 samples showed magnetisation reversal which was dominated by dipole-dipole interactions, whereas for higher Fe content samples the
reversal was dominated by intergranular exchange interactions. The samples show different indications of thermally activated grains, in particular the Fe:MgO = 1:1 as deposited sample seems to have a large fraction of these grains. A 4-point-probe-system was used for magnetoresistance measurements. Isotropic and anisotropic components of magnetoresistance (MR) were found. The MR probably originates from anisotropic MR (AMR) and granular giant MR (GMR) and/or granular tunnelling MR (TMR) effects. The MR measurements support the conclusions from the magnetization measurements regarding the magnetic reversal mechanisms in the films. The previously existing MR measurement system was modified and most of the existing code of the measurement program was rewritten and extended. By doing so the reproducibility of the measurements improved from ±0.2% for MR [1] down to ±0:0001% for optimal measurement conditions. A program to calibrate the Gauss probe to a secondary standard probe was written.