Relation between exchange coupling and enhanced coercivity in the free layer of a patterned magnetic tunnel junction

A magneto-optical Kerr effect system with a spatial resolution of 2 μm was used to measure the local M-H loops for the free layer of a magnetic tunnel junction with a structure of Ta/Cu/Ta/NiFe/Cu/Mn₇₅Ir ₂₅/Co₇₀Fe₃₀/Al₂O₃/Co ₇₀Fe₃₀/Ta to investigate the exchange bias field H _E and the coercivity H_C for the free layer. The H _E and H_C measured along the direction of the free layer varied symmetrically with respect to the junction center. The measurements indicate that the enhanced H_C correlated with H_E, and H_E could be reasonably explained by using an "orange-peel- type" coupling based on variations in the thickness of the pinned layer along the direction of the free layer. The variation in H_E along the pinned-layer's direction could be ascribed to that of the free-layer's thickness, and the increase in H_E at the junction edge along the pinned layer was due to a decrease in the thickness of the free layer near the edge. However, the nearly constant H_C along the pinned layer indicates that the thickness of the free layer can be excluded from the mechanism for enhancing H_C, which is a unique difference in the parameters involved in H_E and H_C, and in the mechanism for enhancing H_C.