BISTABILITY OF THE MAGNETIC-RESONANCE OF CONDUCTION ELECTRONS IN GALLIUM OXIDE

The electron paramagnetic resonance (EPR) spectrum of conduction electrons in beta-Ga2O3 single crystals exhibits a bistability with a magnetic field memory at temperatures up to room temperature. This effect manifests itself by the occurrence of different line shapes and positions of EPR lines recorded upon forward and backward sweeps of the external magnetic field. The difference between the resonance fields in these two modes can reach 0.1 mT, which is larger than the peak to peak line width of the nonsaturated EPR line in beta-Ga2O3. We show that this effect is the consequence of the dynamic polarization of gallium nuclei upon resonance saturation of conduction electron spins (Overhauser effect). Within a determined range of external magnetic field B0, the effective magnetic field B(eff) seen by the electron spin takes two stable steady states. One stable state is taken during a forward sweep of B0 and the other during a backward sweep. This effect is maximum at about 150 K, but some hysteresis remains at room temperature. The bistability observed in beta-Ga2O3 results from the conjunction of two features in the EPR of conduction electrons: line width extremely narrow (0.047 mT at 150 K) and a strong isotropic hyperfine interaction which results in a large Overhauser shift of the EPR signal, about 0.3 mT in the backward sweep mode. A third feature is that this phenomena is maximum at moderate saturation (s almost-equal-to 0.66) of the electron spin system, which is easy to produce in beta-Ga2O3 because Of the long electron spin-lattice relaxation time (T1 almost-equal-to 4 x 10(-7) s at 150 K). The steady-state EPR intensity at time t > 0 and at a chosen magnetic field B-0(r) was found to depend on the initial (memorized) magnetic field B-0(0) at time t < 0, which demonstrates the existence of a magnetic field memory.