Magneto-optical detection of non-collinear magnetization states in ferromagnetic multilayers

We have experimentally studied the relationship in between non-collinear magnetization states in ferromagnetic (FM) multilayers and their resulting magneto-optical (MO) properties. Hereby, we observe that the phase of the complex-valued MO parameters are especially sensitive towards non-collinear magnetization states and enable their unambiguous detection. For the purpose of our experimental study, we designed, fabricated and characterized a set of epitaxial FM/NM/FM multilayers with in-plane uniaxial anisotropy, in which the non-magnetic (NM) interlayer thickness t was varied, so that tunable FM interlayer exchange coupling strength in between the two FM layers could be achieved. Furthermore, the two FM layers were made from different alloys, so that they exhibit different levels of magnetocrystalline anisotropy, which enables a collinear to non-collinear magnetization state transition upon applying a magnetic field H away from the in-plane easy axis for samples with sufficiently large t . Utilizing generalized MO ellipsometry, we determined the full reflection matrix R as a function of H and we observed that the phases of the complex-valued MO coefficients in R change with H in multilayers that have sufficiently weak interlayer coupling strength, i.e. large t, which can only happen if non-collinear magnetization states of varying non-collinearity occur in those samples. For samples with small t, corresponding to strongly exchange coupled FM layers, this effect is absent, consistent with the existence of collinear magnetization states in those multilayers for all H values.