Magnetoresistance of hole-doped ${\mathrm{LaMnO}}_3$ at frequencies above a few MHz has been seldom reported compared to numerous studies on magnetoresistance measured at direct current. Here, we contrast the high-frequency magnetoresistance in the frequency range $f=0.9–3\mathrm{GHz}$ in polycrystalline ${\mathrm{LaMnO}}_3$ (LMO) synthesized by microwave irradiation of oxide precursors in a microwave furnace (MW-LMO) and by conventional heating (CH-LMO) in an electrical furnace. The structure at room temperature changes from orthorhombic in CH-LMO to rhombohedral in MW-LMO. A combination of magnetization and resistivity studies suggests that the CH-LMO sample is a canted antiferromagnetic insulator below 140 K but the MW-LMO is a ferromagnetic metal with $T_C=240\mathrm{K}$. While the high-frequency resistance of the CH-LMO sample at 300 K decreases gradually with increasing strength of the applied dc magnetic field for all $f$, a peak appears at $H=H_r>0$ when $f\ge 1.4\mathrm{GHz}$ in the MW-LMO sample and $H_r$ increases linearly with $f$. We attribute the observed features in the high-frequency magnetoresistance of MW-LMO to current-driven resonant excitation of spins in the paramagnetic state. Its absence in the CH-LMO sample highlights the need for an optimum hole density to observe this effect.

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• Electrical properties
• Magnetotransport