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Abstract
We investigate the magnetic and the transport properties of diluted magnetic semiconductors using a spin-fermion Monte-Carlo method on a simple cubic lattice in the intermediate coupling regime. The ferromagnetic transition temperature Tc shows an optimization behavior with respect to the absolute carrier density pabs and the magnetic impurity concentration x as seen in the experiments. Our calculations also show an insulator-metal-insulator transition across the optimum pabs where the Tc is maximum. Remarkably, the optimum pabs values lie in a narrow range around 0.11 (holes/site) for all x values and the ferromagnetic Tc increases with x. We explain our results using the polaron percolation mechanism and outline a new route to enhance the ferromagnetic transition temperature in experiments.
Highlights
Diluted magnetic semiconductors (DMS) are materials of strong interest due to both, their novel ferromagnetism and potentiality for future spintronics[1,2,3,4,5]
In the higher impurity concentration regime the ferromagnetic Tc increases with decrease in the hole density as compared to the un-codoped samples
Magnetic and transport properties in DMS are the consequence of the competition between the carrier mediated ferromagnetic spin-spin interaction and the carrier localization
Summary
∑ μ ci†σciσ, i where ci†σ (ciσ) are the fermion creation (annihilation) operators at site i with spin σ and t is the nearest neighbor (〈ij〉) hopping parameter. Magnetic moment clustering and the direct exchange interaction between the localized spins are neglected. In order to get the same hole density throughout the calculations (hole density checked using Fermi-Dirac distribution) we tune the chemical potential μ during the annealing process at each temperature. We employ exact diagonalization based Monte-Carlo (ED + MC) approach to anneal the system towards the ground state at fixed density and temperature. In this method the classical spin SR is updated at a site and the internal energy is calculated by exact diagonalization of the carriers in the background of the new spin configuration. Monte-Carlo scheme is implemented by diagonalizing a Hamiltonian reconstructed from a cluster around the to-be-updated site rather than diagonalizing the full lattice. All physical quantities are averaged over ten different randomly localized spin configurations
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