Broadband magnetotransport in bulk and thin films of self and Sr-doped LaMnO₃

Colossal magnetoresistance (CMR) discovered nearly three decades ago in the Mn-perovskite oxide family (hole-doped LaMnO₃) triggered a flurry of activities in search of a similar phenomenon in other oxides. However, the majority of available reports deal with CMR measured with direct current or low-frequency current (f < 1kHz) and magnetic fields often more than 1 T. While ac impedance in the frequency range 100 Hz to a few MHz range is often used to study dielectric relaxation and magnetocapacitance effect in insulating oxides, ac impedance in metallic oxides is seldom reported. Recent experimental work on the ac magnetoimpedance in the frequency range from ~1kHz to 3 GHz done in our lab has revealed many exciting phenomena: colossal magnetoresistance at low fields (~ 40-90 % for H ≤ 1 kOe at room temperature) and a transition from negative to positive magnetoresistance. As the frequency of the microwave current increases beyond 0.9 GHz, the low-field magnetoresistance exhibits a positive peak and its position shifts up linearly or non-linearly with increasing strength of the applied dc magnetic field, depending on the hole content. We propose that this feature is most likely due to current-driven paramagnetic/ferromagnetic resonance, but detected electrically. I will present some results derived from self-doped LaMnO₃, Sr-doped LaMnO₃ bulk and thin films and in a paramagnetic DPPH molecule[1-4]. This technique can potentially probe spin dynamics in other materials too but is rarely exploited. In the end, I will relate our approach to recent works on the inverse spin Hall effect and spin-orbit torque-induced ferromagnetic resonance observed in non-oxide thin films.