Magnons in disordered nonstoichiometric low-dimensional magnets

Abstract

We study spin excitation spectra of one-, two-, and three-dimensional magnets featuring nonmagnetic defects at a wide range of concentrations. Taking the Heisenberg model as the starting point, we tackle the problem by both direct numerical simulations in large supercells and using a semianalytic coherent-potential approximation. We consider the properties of the excitations in both direct and reciprocal spaces. In the limits of the concentration c of the magnetic atoms tending to 0 or 1 the properties of the spin excitations are similar in all three dimensions. In the case of a low concentration of magnetic atoms the spin excitation spectra are dominated by the modes confined in the real space to single atoms or small clusters and delocalized in the reciprocal space. In the limit of c tending to 1, we obtain the spin-wave excitations delocalized in the real space and localized in the reciprocal space. However, for the intermediate concentrations the properties of the spin excitations are strongly dimensionality dependent. We pay particular attention to the formation, with increase of c, of the Lorentzian-shaped peaks in the spectral densities of the spin excitations, which can be regarded as magnon states with a finite lifetime given by the width of the peaks. In general, low-dimensional magnets are more strongly affected by the presence of nonmagnetic impurities than their bulk counterparts. The details of the electronic structure, varying with the dimensionality and the concentration, substantially influence the spin excitation spectra of real materials, as we show in the example of the FeAl alloy.

Spin-density fluctuations and the fluctuation-dissipation theorem in 3d ferromagnetic metals

“…1 top) are overestimated. In order to cure this problem, several correction schemes have been designed based on a modification of the exchange-correlation kernel 13,37 or Kohn-Sham susceptibility 14 such that the zero-frequency Goldstone mode is recovered. While such a procedure is crucial for spin wave dispersion studies, in this work we focus on BZ-integrated quantities at much larger energy scales and, therefore, the presence of finite excitation gap of the order of few tens meV has a small effect on these results.…”

“…Theoretically, SDF in real materials can be explored from first principles using linear response technique based on density functional theory [10][11][12][13][14][15] or many-body perturbation methods. 16,17 However, these calculations considered only limited energy and wavevector ranges.…”

Extended spin model in atomistic simulations of alloys

“…Recently, a great deal of progress has been achieved to study the Stoner excitations through the transversal dynamical susceptibility from first principles within the framework of linear response time dependent density functional theory 21,22 , or from tight binding theory 23 . However, due to the complexity of such calculations one faces with difficulties applying the theory to complex magnetic compounds and alloys and to include temperature effects.…”

Methods of electron transport inab initiotheory of spin stiffness

“…Besides the mentioned problems in determination of the exchange interactions of random alloys, evaluation of magnon spectra of these systems is considered as another challenge for the solid-state theory 19 . The formulation of effective-medium approaches is rather sophisticated and it leads to nontrivial numerical implementation 20,21 . Various brute-force simulations using large supercells have thus been used as alternatives which may be efficient and reliable in a number of cases, including both the spinwave spectra and values of the spin-wave stiffness 19,[21][22][23][24] .…”

“…The formulation of effective-medium approaches is rather sophisticated and it leads to nontrivial numerical implementation 20,21 . Various brute-force simulations using large supercells have thus been used as alternatives which may be efficient and reliable in a number of cases, including both the spinwave spectra and values of the spin-wave stiffness 19,[21][22][23][24] . It was found that the spin stiffness of random diluted ferromagnets is reduced due to the sites without local magnetic moments as compared to the value obtained for a clean crystal with nonrandom concentration-weighted exchange interactions of the alloy.…”