Spin-fluctuation mechanism of anomalous temperature dependence of magnetocrystalline anisotropy in itinerant magnets

The origins of the anomalous temperature dependence of magnetocrystalline anisotropy in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are elucidated using first-principles calculations within the disordered local moment model. Excellent agreement with experimental data is obtained. The anomalies are associated with the changes in band occupations due to Stoner-like band shifts and with the selective suppression of spin-orbit "hot spots" by thermal spin fluctuations. Under certain conditions, the anisotropy can increase, rather than decrease, with decreasing magnetization due to these peculiar electronic mechanisms, which contrast starkly with those assumed in existing models.Comment: 9 pages, 10 figures (including supplemental material

Electronic Structure and Magnetic Exchange Coupling in Ferromagnetic Full Heusler Alloys

Density-functional studies of the electronic structures and exchange interaction parameters have been performed for a series of ferromagnetic full Heusler alloys of general formula Co$_2$MnZ (Z = Ga, Si, Ge, Sn), Rh$_2$MnZ (Z = Ge, Sn, Pb), Ni$_2$MnSn, Cu$_2$MnSn and Pd$_2$MnSn, and the connection between the electronic spectra and the magnetic interactions have been studied. Different mechanisms contributing to the exchange coupling are revealed. The band dependence of the exchange parameters, their dependence on volume and valence electron concentration have been thoroughly analyzed within the Green function technique.Comment: 9 figures, 6 table

Self-consistent local GW method: Application to 3\u3ci\u3ed\u3c/i\u3e and 4\u3ci\u3ed\u3c/i\u3e metals

The spectral densities for 3d and 4d transition metals are calculated using the simplified version of the self-consistent GW method employing the local (one-site) approximation and the self-consistent quasiparticle basis set. The results are compared with those given by the traditional local density approximation (LDA) and also with experimental x-ray photoemission and inverse photoemission spectra. While no systematic improvements over LDA are observed, this fully self-consistent many-body technique generates quite reasonable results and can serve as a practical prototype for further development of the many-body electronic structure theory

Effects of alloying and strain on the magnetic properties of Fe16_{16}N2_2

The electronic structure and magnetic properties of pure and doped {Fe$_{16}$N$_2$} systems have been studied in the local-density (LDA) and quasiparticle self-consistent {\emph{GW}} approximations. The {\emph{GW}} magnetic moment of pure {Fe$_{16}$N$_2$} is somewhat larger compared to LDA but not anomalously large. The effects of doping on magnetic moment and exchange coupling were analyzed using the coherent potential approximation. The theoretical Curie temperature in pure {Fe$_{16}$N$_2$} is significantly higher than the measured value, which is attributed to the quality of available samples and the interpretation of experimental results. We found that different Fe sites contribute very differently to the magnetocrystalline anisotropy energy (MAE), which offers a way to increase MAE by small additions of Co or Ti. MAE also increases under tetragonal strain

Prediction of Van Hove singularity systems in ternary borides

A computational search for stable structures among both $\alpha$ and $\beta$ phases of ternary ATB4 borides (A= Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or 4d transition elements) has been performed. We found that $\alpha$-ATB4 compounds with A=Mg, Ca, Al, and T=V, Cr, Mn, Fe, Ni, and Co form a family of structurally stable or almost stable materials. These systems are metallic in non-magnetic states and characterized by the formation of the localized molecular-like state of 3d transition metal atom dimers, which leads to the appearance of numerous Van Hove singularities (VHS) in the electronic spectrum. The closeness of these VHS to the Fermi level can be easily tuned by electron doping. For the atoms in the middle of the 3d row (Cr, Mn, and Fe), these VHS led to magnetic instabilities and new magnetic ground states with a weakly metallic or semiconducting nature. The magnetic ground states in these systems appear as an analog of the spin glass state. Experimental attempts to produce MgFeB4 and associated challenges are discussed, and promising directions for further synthetic studies are formulated.Comment: 9 figure

Magnetocrystalline anisotropy in cobalt based magnets: a choice of correlation parameters and the relativistic effects

The dependence of the magnetocrystalline anisotropy energy (MAE) in MCo5 (M  =  Y, La, Ce, Gd) and CoPt on the Coulomb correlations and strength of spin orbit (SO) interaction within the GGA  +  U scheme is investigated. A range of parameters suitable for the satisfactory description of key magnetic properties is determined. We show that for a large variation of SO interaction the MAE in these materials can be well described by the traditional second order perturbation theory. We also show that in these materials the MAE can be both proportional and negatively proportional to the orbital moment anisotropy (OMA) of Co atoms. Dependence of relativistic effects on Coulomb correlations, applicability of the second order perturbation theory for the description of MAE, and effective screening of the SO interaction in these systems are discussed using a generalized virial theorem. Such determined sets of parameters of Coulomb correlations can be used in much needed large scale atomistic simulations

Origin of the spin reorientation transitions in (Fe1−x_{1-x}Cox_{x})2_{2}B alloys

Low-temperature measurements of the magnetocrystalline anisotropy energy $K$ in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence $K(x)$ with a maximum near $x=0.3$ and a minimum near $x=0.8$ is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, $K$ depends strongly on the tetragonality and doubles under a modest 3% increase of the $c/a$ ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.Comment: 4 pages + supplementary material, 6 figures. Accepted in Applied Physics Letter