Magnetic tunnel junctions (MTJs) attracted significant interest in recent years for magnetoelectronic applications. To make the MTJs usable as low-field(low-power) device application, it is critical to have small switching field and uniform switching field distribution. In micromagnetic simulation, the switching field decreases,as a free layer becomes thinner and has a lower saturation magnetization. In this study, we fabricated MTJs with three kinds of free layer, NiFe, CoFe, and NiFe/CoFe, and investigated effect of thickness of the free layer on magnetic properties.
The MTJs of $Ta(50Å})/NiFe(80Å)/IrMn(200Å)/CoFe(40Å})/Al_2O_3/FM/Ta(150Å) (FM : NiFe, CoFe, CoFe/NiFe) were deposited on thermally oxidized Si substrate by a magnetron sputtering system. The barrier was formed by oxidizing 17Å Al layer using ozone atmosphere. The junctions were patterned by Si shadow masks and the junction area was 150 ㎛ × 50 ㎛.
NiFe free layer showed lower MR ratio than CoFe free layer at as deposited state, although they have almost the same spin polarization. It is because NiFe has the tendency to be contaminated, during vacuum break for the mask change, more than CoFe. The NiFe free layer also showed lower switching field than CoFe. This is in consistent with the simulation results because CoFe has higher saturation magnetization than NiFe.
After annealing at 250 ℃ in vacuum, MR ratio increased and had no dependency on free layer thickness. The MR increase may be associated with the redistribution of excess oxygen at the interface between the barrier and the magnetic free layerby oxygen migration to Al oxide layer. Therefore, the slightly oxidized magnetic layer is reduced and the spin polarization increases. Because the NiFe layer is more easily contaminated, the annealing effect is more effective with NiFe free layer. The fact that MR ratio is independent of the free layer thickness in CoFe and NiFe indicates spin dependent scattering is dominated by interface scattering than bulk scattering. After annealing, the switching field also decreased. It is due to the relief of residual stress and the reduction of slightly oxidized magnetic layer by oxygen redistribution.
At 77K, both switching field and MR ratio increased. The increase of the switching field is due to reduced nucleation of magnetization reversal by thermal fluctuation, and the increase of MR ratio was due to the increase of spin polarization of magnetic layer and the decrease of spin independent tunneling at the lower temperature.
There was a critical thickness for free layers, below which no MR ratio was detected. NiFe 10Å, $CoFe 6Å, and CoFe 3Å /NiFe 6Å showed no MR ratio. To study the reason why there is no MR ratio for the thin free layer, TEM cross section images were taken. In case of NiFe 10Å, it showed island-like structure, and the film was discontinuous. But in case of NiFe 20Å, the film was continuous. If the magnetic film is not continuous, some islands have the magnetization with the opposite directions due to the demagnetization by the adjacent island. So, the total magnetization becomes zero. It is the reason why there is no MR ratio for the thin free layer.
