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We study the exchange interactions in half-metallic Heusler alloys using first-principles calculations in conjunction with the frozen-magnon approximation. The Curie temperature is estimated within both mean-field (MF) and random-phase-approximation (RPA) approaches. For the halfHeusler alloys NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In this case the MF and RPA estimations differ strongly. The RPA approach provides better agreement with experiment. The exchange interactions are more complex in the case of full-Heusler alloys Co2MnSi and Co2CrAl where the dominant effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms and between Co atoms at different sublattices. For these compounds we find that both MF and RPA give very close values of the Curie temperature slightly underestimating experimental quantities. We study the influence of the lattice compression on the magnetic properties. The temperature dependence of the magnetization is calculated using the RPA method within both quantum mechanical and classical approaches.

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We use the local density approximation (LDA) and LDA+U schemes to study the magnetism of (GaMn)As and (GaMn)N for a number of Mn concentrations and varying number of holes. We show that for both systems and both calculational schemes the presence of holes is crucial for establishing ferromagnetism. For both systems, the introduction of U increases delocalization of the holes and, simultaneously, decreases the p-d interaction. Since these two trends exert opposite influences on the Mn-Mn exchange interaction the character of the variation of the Curie temperature (TC ) cannot be predicted without direct calculation. We show that the variation of TC is different for two systems. For low Mn concentrations we obtain the tendency to increasing TC in the case of (GaMn)N whereas an opposite tendency to decreasing TC is obtained for (GaMn)As. We reveal the origin of this difference by inspecting the properties of the densities of states and holes for both systems. The main body of calculations is performed within a supercell approach. The Curie temperatures calculated within the coherent potential approximation to atomic disorder are reported for comparison. Both approaches give similar qualitative behavior. The results of calculations are related to the experimental data.

The paper is partly motivated by recent pump-probe experiments with ultrashort laser pulses on antiferromagnetic FeRh that have shown the generation of magnetization within a subpicosecond time scale. On the other hand, the physical mechanism of the thermal antiferromagnetic-ferromagnetic (AFM-FM) phase transition in FeRh, known for many decades, remains a topic of controversial discussions. The selection of the magnetic degrees of freedom as well as the treatment of the magnetic excited states differ strongly in recent models by different authors. We report a density functional theory (DFT) investigation of FeRh. For the study of excited states, DFT calculations with constraints imposed on the directions and values of the atomic moments are employed. We show that the formation of the Rh moment as a consequence of the AFM-FM phase transition cannot be described within the Stoner picture. Instead, an implicit spin splitting of the Rh states takes place in the AFM phase, resulting in the intra-atomic spin polarization of the Rh atoms. This property is a consequence of the strong hybridization between Rh and Fe states. The Fe-Rh hybridization is an important factor in the physics of FeRh. We demonstrate that the ferromagnetic Fe-Rh exchange interaction is robust with respect to the crystal volume variation, whereas the antiferromagnetic Fe-Fe exchange interaction is strongly volume dependent. These different volume dependencies of the competing exchange interactions lead to their strong compensation at certain crystal volume. We perform Monte Carlo simulations and show that the calculated thermodynamics depends on the way the magnetic degrees of freedom are selected. We argue that the excited states resulting from the variation of the value of the Rh moment treated as degree of freedom are important for both the equilibrium thermodynamics of FeRh and the femtomagnetic phenomena in this system. We also study the spin mixing caused by spin-orbit coupling. The obtained value of the Elliott-Yafet spin-mixing parameter is comparable with earlier calculations for the ferromagnetic 3d metals. We draw the conclusion that the Elliott-Yafet mechanism of the angular-momentum transfer between electrons and lattice plays an important role in the femtomagnetic properties of FeRh.

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Because of the large spatial separation of the Mn atoms in Heusler alloys ͑d Mn-Mn Ͼ 4 Å͒, the Mn 3d states belonging to different atoms do not overlap considerably. Therefore, an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi-and full-Heusler alloys, we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The Curie temperature is estimated within the mean-field approximation. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction sp electrons, their spin polarization, and the position of the unoccupied Mn 3d states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The magnetic phase diagram is determined at zero temperature. The results of the calculations are in good agreement with available experimental data. Anderson's s-d model is used to perform a qualitative analysis of the obtained results. The conditions leading to a diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied Mn 3d states lie well above the Fermi level, a Ruderman-Kittel-Kasuya-Yoshida-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied Mn 3d states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculation results are in good correlation with the conclusions made on the basis of the Anderson s-d model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.