Suppression of Coercivity in Nanoscale Graded Magnetic Materials

Abstract

We investigate temperature-dependent magnetization reversal of $\mathrm{Co}\mathrm{Ru}$ graded films, in which a predefined depth-dependent exchange-coupling strength J follows a V-shaped profile. Magnetometry reveals an extended temperature range below the Curie temperature ${T}_{C}\phantom{\rule{0.25em}{0ex}}$ where the reversal of the magnetization M is not accompanied by the conventionally occurring hysteresis, in stark contrast with homogeneous $\mathrm{Cr}\mathrm{Ru}$ reference films. This is caused by the temperature-driven paramagnetic (PM)-ferromagnetic (FM) phase transition, which does not occur in the entirety of the graded material but only in well-defined nanoscopic regions at any given temperature, enabling the creation of two internal PM/FM interfaces that assist the external magnetic field in reversing the magnetization of the FM graded sample region. Hysteretic reversal is recovered at sufficiently low sample temperatures or by using graded structures with very steep J gradients, so that no PM/FM interfaces form inside the exchange-coupled layer material that could influence the magnetization reversal process. Our findings open interesting material design options, and we envision wide application potential, since we succeed in engineering a temperature gap between the temperature dependent magnetization and hysteresis onsets, a capability of interest for any technology benefitting from field-free magnetization switching.