1,721,002 research outputs found
Collinear and non collinear magnetic structures in anisotropic crystals with non primitive Bravais lattice: cubic case
No full textThe classical anisotropic exchange Hamiltonian is used for the analysis of the low-temperature magnetic structures of polyatomic crystals. The magnetic order is described by the wave-vector k of the Brillouin zone associated with the Bravais lattice in direct space. For cubic crystals it is shown that anisotropic terms of tetragonal symmetry in the exchange may determine the direction of the spins with respect to k and to the crystal axes. It follows that the experimental observation of longitudinal or transverse polarisation indicates that the spin-spin interaction is not of Heisenberg type. It is also shown that anisotropy is the basic mechanism for the formation of multiaxial non-helicoloidal spin configurations. The author has found two kinds of such spin arrangements, depending on the symmetry of k and on the configuration of the unit cell. Calculations of the exchange energy in the approximation of the next-nearest-neighbor interaction for crystals of cubic symmetry Oh with four atoms per unit cell are presented. A detailed discussion of multiaxial configurations of the triangular and multiple-k-type is given. Their occurrence in MnPt3 and in uranium monopnictides is accounted for in the framework of the present mode
Multiaxial magnetic structures in cubic compounds: Uranium monopnictides and Manganese alloys
No full textThe inclusion of anisotropic terms of tetragonal symmetry in the exchange interaction of polyatomic cubic crystals is investigated. This explains the two kinds of multiaxialstructures found in U monopnictides and Mn intermetallics on the basis of wave vector k point symmetry and configuration of the unit cel
Pressure induced magnetic moment reduction in CeAg
No full textWe report the first theoretical results on the magnetization behaviour of CeAg under pressure. According to our self-consistent electronic calculations the magneticmoment, of essentially Ce 4ƒ and 5d origin, decreases by about 50% at the highest pressure of 6 GPa. From the analysis of the electronic structure at several lattice constants we are able to exclude 4ƒ charge fluctuations and to assess the presence of the Anderson hybridization in the whole pressure rang
Role of coverage on the electron properties of sulfur chemisorbed on Cu(100): ab initio calculations
No full textWe present an ab initio study of the electronic properties of the S∕Cu(100) surface, with sulfur chemisorbed in the c(2×2) geometry, and compare it with the p(2×2) phase. We find that the two geometries, observed, respectively, at 0.5 ML and 0.25 ML of sulfur coverage, affect both the density of states and the dispersion and charge distribution of the hybrid S-Cu electronic states. The same superstructures occur under suitable experimental conditions also when organic molecules containing a sulfur atom as a headgroup, like MBO and CH3S, are chemisorbed on Cu(100). We show that the electronic properties of the S-Cu hybrid in simple S∕Cu(100) may be used to interpret succesfully the experimental results of these two complex molecular systems
EFFECT OF COMPOSITION ON THE ANDERSON HYBRIDIZATION IN TERNARY CUBIC ALLOYS OF CE
No full textThe cubic compounds of Ce with Zn, Ag and Cd are considered to behave as Kondo lattices depending on applied pressure or alloying. In particular their magnetic behaviour is ascribed to the possible competition between the RKKY interaction and the Kondo effect. We calculate the hybridisation width Δ of the Anderson model for the two ternary cubic compounds CeCd1−x Ag x , CeCd1−x Zn x . This quantity determines, along with the positionE f and correlationU of the Cef-state, the antiferromagnetic coupling constantJ s-f of the Kondo Hamiltonian. We start fromab initio electronic calculations performed with the FLAPW method in the LSDA and carry out an LCAO parametrisation of the band structure for the pure binary compounds. A virtual-crystal approximation is used to obtain the LCAO parameters of the ternary alloys. We check the accuracy of this approximation on CeCd0.5Zn0.5 by a self-consistent FLAPW calculation on the supercell Ce2CdZn, finding that it is satisfactory for the present purposes. We find that Δ, in the whole concentration range, is affected quantitatively much more by substitution of Cd by Ag than by Zn—isovalent to Cd—, although the latter one induces a larger volume reduction. Moreover, the Δ behaviour in CeCd1−x Zn x contrasts with the expected volume dependence. This shows that electronic effects may be more effective than electronic pressure alone (volume effects) in determining the Kondo properties of substitutional system
Band hybridization effect on indirect magnetic coupling of localized moments
No full textWe study the influence of interorbital hybridization γk on the indirect exchange interaction J(k) in a model of two bcc bands of s-type. We report two cases where the stable magnetic structure changes due to γk. However, γk generally leaves unchanged the structure, while reducing greatly its stability vs. temperatur
Volume Effects on the magnetic properties of cubic isostructural intermetallics of Ce
No full textWe compare the magnetic behavior of CeAg, CeZn, and CeCd in the ferromagnetic phase under pressure, using total-energy electronic-structure calculations within the local-spin-density approximation. While the calculated magnetic moments at ambient pressure reflect the trend of the unit-cell volumes as the pressure is increased, an instability of the magnetic phase occurs in isovalent CeCd and CeZn at much lower applied pressure than in CeAg, as found by experiments. Results with use of a rigid-band model to fill in the density of states of CeAg with the same number of electrons as CeCd show that the different magnetic behavior of the two compounds is essentially due to a more effective screening of the complete 6s shell of Cd. Analogous conclusions are valid for CeZn; the calculated equilibrium volume of the ferromagnetic phase of CeZn is found within the experimental pressure range in which the observed order-order magnetic phase transition takes plac
Theory of muon spin relaxation due to the hopping of positive muons in ferromagnets
No full textThe interaction of the magnetic moments of spin-polarized positive muons with the magnetic fields at the interstices in ferromagnetic (or ferrimagnetic) materials may be used to study the localization and the hopping motion of thermalized muons by observing the relaxation of the longitudinal or transverse spin polarization or the shift of the muon spin precession frequency as a function of the orientation of a strong applied magnetic field. The general theory is developed and applied to the special cases of muon diffusion between two types of interstice of tetragonal symmetry in body-centred-cubic metals (α-Fe) or of cubic symmetry in facecentred-cubic metals (Ni). It is shown that by a suitable choice of the strength and the crystallographic direction of the applied magnetic field, quite detailed information on the quantum diffusion of positively charged light particles in crystals may be obtaine
Electronic properties of isostructural intermetallics of Ce
No full textWe present self-consistent FLAPW-LSD electronic structure calculations of ferromagnetic CeAg, CeCd and CeZn, including the spin-orbit interaction in the spin-polarised formalism. The incomplete Ce 4f-state polarisation indicates the presence of Anderson hybridisation with conduction bands. The calculated magnetic moments are compared to previous theoretical and experimental results, for the three compound
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