Costi TA, Bergqvist L, Weichselbaum A, von Delft J, Micklitz T, Rosch A, Mavropoulos P, Dederichs PH, Mallet F, Saminadayar L, et al. Kondo Decoherence: Finding the Right Spin Model for Iron Impurities in Gold and Silver. PHYSICAL REVIEW LETTERS. 2009;102:056802.

AbstractWe exploit the decoherence of electrons due to magnetic impurities, studied via weak localization, to resolve a long-standing question concerning the classic Kondo systems of Fe impurities in the noble metals gold and silver: which Kondo-type model yields a realistic description of the relevant multiple bands, spin, and orbital degrees of freedom? Previous studies suggest a fully screened spin S Kondo model, but the value of S remained ambiguous. We perform density functional theory calculations that suggest S = 3/2. We also compare previous and new measurements of both the resistivity and decoherence rate in quasi-one-dimensional wires to numerical renormalization group predictions for S = 1/2, 1, and 3/2, finding excellent agreement for S = 3/2.

Mavropoulos P, Lezaic M, Blügel S.

Ferromagnetism in nitrogen-doped MgO: Density-functional calculations. PHYSICAL REVIEW B. 2009;80:184403.

AbstractThe magnetic state of nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be spin polarized with a moment of 1 mu(B) per nitrogen atom and to interact ferromagnetically via the double-exchange mechanism in the full concentration range. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The disorder is described within the coherent-potential approximation, with the exchange interactions harvested by the method of infinitesimal rotations. The Curie temperature T-C, calculated within the random-phase approximation, increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single nitrogen atoms, also interstitial nitrogen atoms, dimers and trimers, and their structural relaxations are discussed with respect to the magnetic state. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase in T-C is difficult to achieve, requiring a particular attention to the choice of chemistry.

Slipukhina I, Arras E, Mavropoulos P, Pochet P.

Simulation of the enhanced Curie temperature in Mn5Ge3Cx compounds. APPLIED PHYSICS LETTERS. 2009;94:192505.

AbstractMn5Ge3Cx films with x >= 0.5 were experimentally shown to exhibit a strongly enhanced Curie temperature T-C compared to Mn5Ge3. In this letter we present the results of our first principles calculations within Green's function approach, focusing on the effect of carbon doping on the electronic and magnetic properties of the Mn5Ge3. The calculated exchange coupling constants revealed an enhancement of the ferromagnetic Mn-Mn interactions mediated by carbon. The essentially increased T-C in Mn5Ge3C is well reproduced in our Monte Carlo simulations and together with the decrease of the total magnetization is found to be predominantly of an electronic nature.