Spin excitations in K2_{2}Fe4+x_{4+x}Se5_{5}: linear response approach

Using \emph{ab initio} linear response techniques we calculate spin wave spectra in K$_{2}$Fe$_{4+x}$Se$_{5}$, and find it to be in excellent agreement with a recent experiment. The spectrum can be alternatively described rather well by localized spin Hamiltonian restricted to first and second nearest neighbor couplings. We confirm that exchange coupling between nearest neighbor Fe magnetic moments is strongly anisotropic, and show directly that in the ideal system this anisotropy has itinerant nature which can be imitated by introducing higher order terms in effective localized spin Hamiltonian (biquadratic coupling). In the real system, structural relaxation provides an additional source of the exchange anisotropy of approximately the same magnitude. The dependence of spin wave spectra on filling of Fe vacancy sites is also discussed

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

Identification of transverse spin currents in noncollinear magnetic structures

We show that the transverse components of spin current in a ferromagnet is linked to an off diagonal spin component of the transmission matrix at interfaces;it has little to do with the mismatch of band structures between dissimilar metals. When we take account of this component,not considered in prior analyses, we find spin torque comes from a region of at lease 3 nm around an interface.Comment: 4 pages, Submitted to Physical Review Letter

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

Low Energy, Coherent, Stoner-like Excitations in CaFe2_{2}As2_{2}

Using linear-response density-functional theory, magnetic excitations in the striped phase of CaFe$_{2}$As$_{2}$ are studied as a function of local moment amplitude. We find a new kind of excitation: sharp resonances of Stoner-like (itinerant) excitations at energies comparable to the N{\'{e}}el temperature, originating largely from a narrow band of Fe $d$ states near the Fermi level, and coexist with more conventional (localized) spin waves. Both kinds of excitations can show multiple branches, highlighting the inadequacy of a description based on a localized spin model

Metallization and Spin Fluctuations in Cu-doped Lead Apatite

An electronic structure and magnetic properties analysis of the recently proposed Cu-doped lead apatite is performed. We show that electronic structures of differently Cu-substituted structures are characterized by localized molecular Cu-O bands at or near the Fermi level. The Cu substitutions can happen at both Pb1 and Pb2 sites, leading to metallic and semiconducting states differently. The electronic states in these bands are highly unstable magnetically and form clusters of rigidly ferromagnetically coupled magnetic moments on Cu and neighboring oxygen atoms with a total moment of about 1 $\mu_B$. The ground state of uniformly Cu-doped lead apatite appears to be magnetic and semiconducting. The non-uniform distribution of two Cu atoms at the nearest Pb2 sites leads to an antiferromagnetic semiconducting state with formation energy close to uniformly distributed Cu configurations. The inclusion of quantum spin fluctuations confirms the stability of magnetic Cu-O clusters. Our calculations revealed the absence of the long-range magnetic order between uniformly distributed Cu-O clusters, creating the spin glass type of system