Condensed Matter Physics Seminar

2 p.m., Thursday, April 26, 2001
Room 1201, Physics Building

Ferromagnetism in diluted magnetic semiconductors

Ravin Bhatt

(Department of Electrical Engineering and Princeton Center for Complex Materials, Princeton University)

Abstract: We develop a model of impurity bands due to charged dopants in a diluted magnetic semiconductor (DMS), coupled to magnetic ions, appropriate to both II-VI and III-V based DMS. Though the parent Hamiltonian contains only antiferromagnetic interactions, we show that the system undergoes a transition from a paramagnetic to an essentially ferromagnetic state at low temperatures for low carrier densities. Further, we demonstrate, using a combination of analytic, numerical mean-field, and Monte Carlo methods, that for carrier densities both in the insulating as well as the metallic phases, the nature of the ferromagnetic phase is rather unusual. This results in unusual spin excitations, and correspondingly unconventional thermodynamic properties. In particular, neither critical point theories of homogeneous magnets, nor long wavelength spin-wave expansions around the zero temperature ordered state, are found to be useful descriptions of the thermodynamic and magnetic behavior. Implications for spin transport will be discussed.

Host: Sankar Das Sarma

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