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 ρ(T)=ρ₀exp(T₀/T)¹α/ with ρ≈0.1~1 Ωcm at 300 K and α=2. The results of magneto-resistance at 300 K indicate that the value is about 4.5% for thick films. It increases slowly with decreasing film thickness. At about 150 nm, 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=20 nm. At low temperatures, the TMR effect becomes larger and shows similar thickness dependence. However, the values of T ₀ that relate to the geometry of the tunneling barrier show behavior of opposite thickness dependence. A small T₀ is associated with a large TMR effect. In this report, the correlation of the film microstructure and the TMR effect are discussed in details