Domain-wall-assisted asymmetric magnetoimpedance in ferromagnetic nanostripes

Abstract

With micromagnetic simulations, we study the dynamical magnetic response of a ferromagnetic nanostripe deposited on an AC-current-carrying nonmagnetic layer (that induces an AC Oersted field inside the nanostripe), in the presence of a static axial field. Included into a circuit, the structure induces the magnetoimpedance (MI) whose value is proportional to the time derivative of the transverse magnetization of the nanostripe, hence, it is directly related to the dynamical magnetic response of the stripe. Paying especial attention to the regime of low values of the axial field, we study in detail the mechanism of the dynamical response, thus, MI which is based on the oscillations of the positions of 90° (head-to-tail) domain walls (DWs). Such DWs are present in nanostripes of uniaxial-transverse or four-fold (in-plane) magnetic anisotropies of a small (up to 10 nm) thickness, whose combination of the structural and shape anisotropies stabilizes the structure of domains of diagonal (to stripe axes) magnetization, with a non-zero axial component of the overall magnetization of the nanostripe. For the nanostripes of specific materials (uniaxial Co, cubic Fe3O4), that ordering results in a significant asymmetry of the DW-based dynamical response with respect to the reversal of the axial field. For the nanostripes of other materials (cubic: Fe, Fe3Pt), the DW-based magnetic response is almost insensitive to the axial field, thus, the low-field MI is inefficient. We explain different dynamical behaviors of the nanostripes within the analytical (Walker-like) model of the DW motion combined with a phenomenological model of the DW-based MI, including the DW interactions. The maximum relative shift of the DW-based impedance of the nanostripes ranges from about 50% for Fe3O4 to above 200% for Co nanostripes, whereas the (low-)field regime of the DW-based response (the anisotropic MI regime) is as wide as 30 kA m−1.