Dilute magnetic semiconductors and oxides
Re-entrant ferromagnetism in a class of diluted magnetic semiconductors.
M.J. Calderón, S. Das Sarma
Phys. Rev. B 75, 235203 (2007). cond-mat/0611384 .
Considering a general situation where a semiconductor is doped by magnetic impurities leading to a carrier-induced ferromagnetic exchange coupling between the impurity moments, we show theoretically the possible generic existence of three ferromagnetic transition temperatures, T1 > T2 > T3, with two distinct ferromagnetic regimes existing for T1 > T > T2 and T < T3. Such an intriguing re-entrant ferromagnetism, with a paramagnetic phase (T2 > T > T3) between two ferromagnetic phases, arises from a subtle competition between indirect exchange induced by thermally activated carriers in an otherwise empty conduction band versus the exchange coupling existing in the impurity band due to the bound carriers themselves. We comment on the possibility of observing such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic oxides.
Theory of carrier mediated ferromagnetism in dilute magnetic oxides.
M.J. Calderón, S. Das Sarma.
Annals of Physics 322, 2618 (2007). cond-mat/0603182 .
We analyze the origin of ferromagnetism as a result of carrier mediation in diluted magnetic oxide semiconductors in the light of the experimental evidence reported in the literature. We propose that a combination of percolation of magnetic polarons at lower temperature and RKKY ferromagnetism at higher temperature may be the reason for the very high critical temperatures measured (up to ~700K).
Impurity-Semiconductor band hybridization effects on the critical temperature of diluted magnetic semiconductors.
M.J. Calderón, G. Gomez-Santos and L. Brey,
Phys. Rev. B 66, 075218 (2002). cond-mat/0203404
We have studied the critical temperature of diluted magnetic semiconductors by means of Monte Carlo
simulations and coherent-potential-approximation (CPA) calculations. In our model for this system, the magnetic
ions couple with the carriers through an antiferromagnetic exchange interaction J and an electrostatic
interaction W. The effective impurity potential J-W controls the hybridization between the magnetic impurities
and the hole charge on the dopants. We find that the critical temperature depends substantially on the hole
charge on the magnetic impurities. The CPA critical temperature is always lower than that obtained in the
Monte Carlo simulations, although all trends in the simulation results are reproduced in the CPA calculations.
Finally we predict the existence of pockets of phase segregation instability close to the carrier’s band edges.