Current-induced nonlinear magnetoelectric effects in strontium hexaferrite

Abstract

We report on the observation of nonlinear magnetoelectric effects at room temperature due to a dc current in the ferrimagnetic $M$-type strontium hexaferrite platelets. Utilizing microwave measurement techniques and data on the shift in magnetic mode frequencies, it was found that a dc current along the hexagonal $c$ axis resulted in a significant decrease in the saturation magnetization and an increase in the uniaxial magnetocrystalline anisotropy field. These changes in the magnetic order parameters were directly proportional to the square of applied electric field and were found to be much higher than variations due to Joule heating. A phenomenological theory that takes into account the current-induced magnetobielectric (MBE) effects is proposed. Expressions for coupling coefficients for MBE effects have been obtained and have been calculated from the variations in magnetic order parameters. The electric field $E$ (or current) tuning of the magnetic modes in $\mathrm{Sr}M$ reported here is orders of magnitude stronger than strain mediated $E$ tuning of magnetic resonance in hexaferrite-ferroelectric composites. The nonlinear magnetoelectric effects in hexaferrite, therefore, open up an avenue for the realization of $E$-tunable broadband microwave and millimeter wave ferrite signal processing devices such as resonators and filters.