MAGNETIC BISTABILITY AND OVERHAUSER SHIFT OF CONDUCTION ELECTRONS IN GALLIUM OXIDE

We study the intrinsic magnetic bistability of conduction-electron spi ns in gallium oxide beta-Ga2O3 by electron-paramagnetic-resonance (EPR ) spectroscopy. This compound, normally an insulator, becomes an n-typ e semiconductor when synthesized under reducing conditions. The crysta ls studied in this work have a conductivity of 180-200 OMEGA-1 cm-1 at room temperature and 140 OMEGA-1 cm-1 at liquid-helium temperature. I t has been previously shown [J. Phys. Chem. 96, 5513 (1992)] that the hyperfine interaction between conduction-electron spins and nuclear sp ins of gallium is responsible for a strong dynamic nuclear polarizatio n when the EPR of conduction electrons is saturated (Overhauser effect ). The resulting nuclear field acting on the electron spins was found to be bistable, which causes a hysteresis of the resonance line. In th is work we demonstrate that hysteresis can be theoretically produced b y three different control parameters: the external magnetic field B0, the microwave frequency nu, and the microwave field B1 (or the microwa ve power P). A model is presented for the EPR line shape under bistabl e dynamic nuclear polarization, which is in fair agreement with experi mental results for gallium oxide. We verify in this compound the exist ence of hysteresis of the EPR intensity upon positive and negative var iations of the incident microwave power. The effect of sample size on bistability is also investigated. It is shown that this phenomenon can disappear if the sample size is larger than the skin depth of the com pound. Bistability at room temperature in gallium oxide is found to be very sensitive to this size effect. The Overhauser shift of conductio n electrons is also studied as a function of temperature. This paramet er gives details on the hyperfine interaction between charge carriers and magnetic nuclei despite the extreme motional narrowing of the EPR line. The results are interpreted in terms of electronic transport at low temperatures involving an impurity band formed by oxygen vacancies acting as shallow donors.