Thickness dependence of tunneling magneto-resistance effect in granular Fe-Al2O3 films

The magneto-transport properties of the granular Fe-0.5(Al2O3)(0.5) solids with various film thickness have been investigated, The temperature dependence of the resistivity shows that all samples follow a tunneling type equation p(T) = p(0)exp(T-o/T)(1/alpha) with p approximate to 0.1 similar to 1 Omega cm at 300K and alpha = 2. The results of magneto-resistance at 300K indicate that the value is about 4.5% for thick films. If increases slowly with decreasing Trim thickness. At about 150nm, a peak value of TMR is found, which is determined to be 5%. Then TMR drops sharply with further reducing film thickness. It becomes only 3% at t = 20nm. At low temperatures, the TMR effect becomes larger and shows similar thickness dependence. However, the values of T-o that relate to the geometry of the tunneling barrier show behavior of opposite thickness dependence. A small T-o is associated with a large TMR effect. In this report, the correlation of the film microstructure and the TMR effect are discussed in details.