Influence of reference layer stability on the switching performance of sub-micron-sized magnetic tunnel junctions

Abstract

Arrays of sub-micron-sized, magnetic tunnel junctions are patterned and the switching field (Hc) of the free layer (FL) and its distribution (σ) are measured by Kerr magnetometry. A correlation of Hc and σ with the magnetic stiffness (stability against external magnetic fields) of the reference system (RS) is observed. For simple pinned RSs the stiffness is varied by different materials (CoFe vs CoFeB) and its thicknesses, while for artificial antiferromagnetic pinned systems the thickness of the exchange coupling layer of Ruthenium is modified. It is shown that less stiffness causes a lower Hc and larger σ. This effect is assumed to be due to magnetic mirror charges induced in RS by the magnetic dipoles of the patterned FL elements. These mirror charges screen the dipole stray field of the FL resulting in lower effective shape anisotropy, which goes along with reduced switching fields. Micromagnetic simulations performed for different pinning strengths of a single layer RS clearly support this observation. From high field measurements perpendicular to the unidirectional exchange anisotropy of the RS, an order parameter λ can be extracted quantifying the stiffness (∼1∕λ). λ is found to be linearly correlated to σ and Hc.