Characterization of transport processes in magnetic tunnel junctions

Abstract

Magnetic and transport properties of thin film ferromagnet-insulator-ferromagnet tunnel junctions were studied. These magnetic tunnel junctions possess a magnetic field dependent resistance (magnetoresistance). The junction resistance is a maximum when the electrode magnetizations are antiparallel. The insulating barrier layer was formed by deposition of a thin metallic Al layer followed by plasma oxidation. Junctions were fabricated with combinations of Co, Co ₅₀Fe₅₀, and Ni₈₀Fe₂₀ ferromagnetic layers. Magnetization and resistance measurements versus magnetic field indicated the electrodes in Co/AlO_x/Co₅₀Fe ₅₀ and Ni₈₀Fe₂₀/AlO_x/Co junctions did not achieve a stable antiparallel alignment. This was achieved in Ni₈₀Fe₂₀/AlO_x/Co and Co/AlO_x/Co junctions through the use of an antiferromagnetic Ni_0.5Co_0.5O layer to exchange bias one of the electrodes. The magnetoresistance of Co/AlO_x/Co/Ni_0.5 Co_0.5O junctions was measured as a function of Al thickness. The maximum magnetoresistance occurred for junctions with 1.2 nm of Al (before oxidation).

The conduction mechanisms in Co/AlO_x/Co/Ni _0.5Co_0.5O junctions were studied by making conductance measurements on adjacent junctions with the structure Co/AlO_x/Nb/Ni _0.5Co_0.5O. The barrier for both types of junctions was produced in a single deposition and oxidation, thus providing a high degree of uniformity between both types of junctions. A single top electrode was used for both types of junctions. Because they shared the same barrier, conductance measurements on the junctions with a superconducting Nb electrode provided information about the transport processes in the magnetic tunnel junctions. At 1.5 K the junctions with superconducting electrodes had a large zero-bias conductance that varied with barrier thickness, in part due to ballistic channels in parallel with the tunnel barrier. The magnetoresistance in the magnetic tunnel junctions also was a function of barrier thickness. The magnetoresistance was correlated with the zero-bias conductance in the adjacent junctions with a superconducting electrode. As the zero-bias conductance decreased the magnetoresistance increased, implying that the ballistic channels responsible for the zero-bias conductance were not magnetoresistive and that their presence reduced the total magnetoresistance of the magnetic tunnel junctions.