Effect of an applied magnetic field on the abrupt donor collapse in a ferromagnetic semiconductor

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Abstract
With rising temperature, the spin disorder of a ferromagnetic semiconductor can drive a donor electron to collapse from a large-radius state to a severely localized (small-polaronic) donor state. The application of a magnetic field suppresses the spin deviations and increases the temperature of the donor collapse. With weak magnetic fields, the temperature of the donor collapse is found to increase as the square root of the applied magnetic field. As a result of this sublinear dependence of the temperature of the donor collapse on magnetic field strength, the application of small magnetic fields can induce substantial increases of the temperature of the donor collapse. Furthermore, using the previously determined parameters for a donor in EuO, a magnetic field of 5 T produces an increase of the temperature of the donor collapse of over 10 K. The agreement of the calculated shift of the donor collapse with the observed shift of the temperature of the metal-to-insulator transition in heavily doped EuO is consistent with the proposal that the metal-to-insulator transition in heavily doped EuO is primarily caused by the donor collapse.