Determination of magnetic parameters by means of brillouinscattering. Examples: Fe,Ni, Ni0.8Fe0.2

We have used the new technique of Brillouin scattering from spinwaves in thin films for a determination of their magnetic parameters. Films of Fe, Ni and Ni_0.8Fe_0.2 (permalloy) have been investigated and we find good agreement with the results obtained by other methods. We also display spectra from all three materials and discuss some of their characteristic features. For Fe epitaxial growth on epipolished sapphire is reported here for the first time.     

Magnetic properties of UT2Si2 and UT2Ge2 (T = Co,Ni, Cu) intermetallic systems

Neutron diffraction and magnetization measurements indicate that, at low temperatures, long-range magnetic order is present in UCO₂Si₂, UNi₂Si₂, UCu₂Si₂, UNi₂Ge₂, and UCo₂Ge₂. UCo₂Si₂ and UNi₂Ge₂ are simple collinear antiferromagnets of +-+- type, UCu₂Si₂ a simple collinear ferromagnet. In UNi₂Si₂, a magnetic phase transition from a LSW type structure to collinear antiferromagnetism of +-+- type was found, while in UCu₂Ge₂, the antiferromagnetic structure of ++-- transforms into collinear ferromagnetism. Crystal structure and magnetic parameters are given. No magnetic moment on transition metal ions was found within the accuracy of a powder neutron diffraction experiment. The stability of particular magnetic ordering schemes is discussed in terms of an isotropic RKKY mechanism.     

Electronic structures of ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe₂As₂ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin-orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t-J _H -J ₁-J ₂-type model [2008, arXiv: 0806.3526v2].     

Structure cristallographique et magnetique de Mn2N0.86 à basse temperature

The compound Mn₂N_0.86 which belongs to the crystallographic space group P6₃22 undergoes at 308K a paramagnetic-antiferromagnetic transition accompanied by a crystallographic distorsion. The magnetic structure is collinear with a magnetic moment of 1.7 μ_B per atom Mn. The Shubnikov group is Cp22'2'₁ (P_A2₁2₁2).     

The antiferromagnetic structure of ErCo2Si2 by neutron diffraction

The magnetic structure of the tetragonal ErCo₂Si₂ compound is determined by neutron diffraction on powder sample at 4.2 K. The magnetic ordering is connected with a symmetry lowering, magnetic space group P_2s$\text{1}$ (Sh⁷₂)k = 000. The structure is collinear antiferromagnetic with the erbium magnetic moments making an angle of 56.2° with the c axis. The magnetic moment value for erbium is 6.75μ_B.     

Etude par diffraction neutronique de la forme haute pression de FeOOH

A powder sample of iron oxyhydroxide (FeOOH_HP) synthesized under high pressure-high temperature conditions, has been studied by neutron diffraction. The magnetic structure has been determined. The magnetic and chemical unit cells are the same and the antiferromagnetic arrangement is collinear with the spins parallel to the c-axis. The crystallographic structure affinement by the Rietveld method has confirmed that FeOOH_HP is isostructural with InOOH (space group P2₁nm) and has established the positional atomic parameters. The magnetic moment of iron (4.7μ_B at 4.2 K) and the O-H distance in the hydrogen bond (1.11 Å) are discussed with regard to the values obtained for the other forms of iron oxyhydroxide.     

Electronic structures of ternary iron arsenides <Emphasis Type="Italic">A</Emphasis>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> (<Emphasis Type="Italic">A</Emphasis> = Ba,Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe₂As₂ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin-orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t-J _H -J ₁-J ₂-type model [2008, arXiv: 0806.3526v2].     

Quantitative calculations of thermal-expansion coefficient in Fe–Ni alloys: First-principles approach

The thermal expansion coefficients (α) of Fe_1-xNi_x alloys are calculated by means of the Debye-Grüneisen model, which uses the input parameters calculated from density functional theory (DFT) with collinear spin alignments. The various atomic configurations of fcc and bcc supercells with x being 0–0.5 are calculated to conduct the composition-specific analysis. Thermodynamic analysis is employed to facilitate the evaluation of α in bulk alloys, where the calculated supercells are used as the canonical ensemble. Such calculated α exhibits very similar composition-dependency in comparison with the experimental data from literature, particularly providing the well-known Invar effect at 65 wt% of Fe. The calculations also demonstrated that the pressure-derived magnetic frustration, i.e. the magneto-volume effect, is strongly correlated with the Invar effect. The present approach combining the Debye-Grüneisen formalism and the collinear DFT calculation is shown to be a comprehensive theoretical framework for analysis on the thermal expansion properties in metal alloys.     

Nuclear magnetic resonance of 55Mn in the antiferromagnet CsMnBr3 in a variable longitudinal magnetic field

The spectrum and intensities of NMR lines are investigated experimentally and theoretically for excitation by an alternating magnetic field h‖ parallel to a static field H in the quasi-one-dimensional, six-sublattice antiferromagnet CsMnBr₃. According to theory, two new NMR lines, which are not excited by a transverse magnetic field h _⊥, are observed near the phase transition from triangular to collinear structure (H=H _c ) [JETP 86, 197 (1998)]. Zh. éksp. Teor. Fiz. 115, 2228–2241 (June 1999)     

A new general R-matrix theory of collinear reactions and its application to the H + H2 reaction

A new general R-matrix theory of collinear reactions is developed by using the polar coordinate for the inner interaction region and the Cartesian coordinate for the two outer (reactants and products) asymptotic regions. The theory is applied to the collinear H + H₂ exchange reaction.     

Exchange interactions in BaFe12O19

In the scope of the mean field approximation the nine nearest neighbour exchange interactions existing among the ferric ions in BaFe₁₂O₁₉ hexagonal ferrite have been determined from the experimental sublattice magnetizations. It is shown that these exchange interactions are well explained by means of the superexchange theory. It is also concluded that some direct exchange interaction does exist among the octahedral 4f _VI ions. The calculated magnetization, paramagnetic susceptibility, and Curie temperature are in good agreement with the corresponding experimental values. The stability of the collinear Gorter-type magnetic structure of BaFe₁₂O₁₉ is discussed as well.     

The Magnetic structure of UIr

Low-temperature neutron-diffraction and magnetization measurements carried out on poly- and monocrystalline UIr are explained by a simple collinear ferromagnetic structure with magnetic moments of 0.6(3)μ_B/U atom oriented along [010] of the monoclinic cell described in space group P2₁. The bulk magnetic behavior of UIr (T_C = 46 K) is not substantially altered by a partial substitution of Ir by Rh, Pt (increase of T_C) or Os (decrease of T_C) and it is virtually reproduced in the isoelectronic compound UOs_0.5Pt_0.5.     

Neutron diffraction study of the magnetic ordering in EuAs3

The magnetic structure of one of the two ordered phases has been determined at 5 K. The magnetic structure is found to be antiferromagnetic with a cell doubling in the c direction of the monoclinic nuclear cell. The centering in the nuclear cell is replaced by anticentering. The collinear magnetic moments are parallel to the b axis. The magnetic moment per Eu atom has been found to be 5.74(6)μ_B which leads to the saturation magnetic moment of 6.5μ_B.     

Crystal field effects on the magnetic structures of the rare earth compounds with the FeB structure

In order to confirm the role of the crystalline electric potential on the stability of non collinear magnetic structures of the rare earth compounds with the FeB-type structure, the magnetic properties of the (Gd_0.5Y_0.5)Ni compound, where the rare earth orbital moment is nul, are studied. Below its Curie temperature (57 K) the compound is ferromagnetic. The spontaneous magnetization at 0 K reaches 7.05 μ_B per gadolinium atom. Yttrium and nickel atoms being not magnetic the gadolinium moments are parallel and the exchange interactions are positive. Then the non collinear magnetic structures observed when the alloyed rare earths have an orbital moment result from the competition between a multiaxial anisotropy due to the crystal field effects and isotropic exchange interactions of the Heisenberg type.     

Onset of non-collinear magnetism in small Fe clusters

The onset of noncollinear magnetism in small Fe_N clusters is investigated by using a rotational invariant tight-binding Hamiltonian. The ground-state local magnetic moments, magnetic order and average magnetic moments are calculated as a function of the Coulomb exchange integral J. Representative results for Fe₃ and Fe₅ show that the noncollinear magnetic solutions are the most stable. A variety of qualitatively different self-consistent solutions is obtained as a function of J. This includes magnetic solutions with collinear and noncollinear spin arrangements. Our calculations are compared with previous density-functional results. Extensions and limitations of our work are also pointed out.     

The antiferromagnetic structure of TbB4

The magnetic structure of the rare earth tetraboride TbB₄ (crystallographic space group P4/mbm) has been determined by neutron diffraction on a polycrystalline sample. Below the experimentally determined Néel temperature of T_N = (43±1) K TbB₄ is ordered antiferromagnetically. The data refinement yielded a magnetic moment value of (7.7 ± 0.2) μ_B/Tb ion at 4.2 K which we interpret as Tb⁴⁺. The magnetic structure is antiferromagnetic collinear with the moments perpendicular to the tetragonal axis.     

Structural and magnetic properties of Fe<Subscript>7−<Emphasis Type="Italic">n</Emphasis></Subscript>Pt<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> with <Emphasis Type="Italic">n</Emphasis> = 0, 1, 2, . . . 7, bimetallic clusters

An exhaustive study of the structural and magnetic properties of Fe_7?n Pt _n with n = 0, 1, 2, …7, bimetallic clusters is presented. Based on ab initio density functional theory that includes spin-orbit coupling (SOC) and graph theory, the ground state geometry, the local chemical order, and the orbital and spin magnetic moments are calculated. We show how the systems evolves from the 3-d Fe to the quasi-planar Pt clusters. These calculations show that SOC are necessary to describe correctly the composition dependence of the binding energy of these nanoalloys. We observe that the ground state geometries on the Fe rich side resemble the fcc structure adopted by bulk samples. Furthermore, we observe how the spin and orbital magnetic moments depend on the chemical concentration and chemical order. Based on these results, we estimated the magnetic anisotropy energy and found that the largest values correspond to some of the most symmetric structures, Fe₅Pt₂ and FePt₆. To determine the degree of non-collinearity, we define an index that shows that in FePt₆ the total magnetic moments, on each atom, are the less collinear.     

Magnetic properties of NdAu2Ge2

Physical properties of NdAu₂Ge₂, crystallising with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric and electrical transport measurements as well as by neutron diffraction. The compound exhibits antiferromagnetic ordering below T_N=4.5 K with a collinear magnetic structure of the AFI-type. The neodymium magnetic moments are parallel to the c-axis and amount to 1.04(4) μ_B at 1.5 K. The observed magnetic behaviour is strongly influenced by crystalline electric field effect.     

Constraints for anomalous dimensions of local light-cone operators in [ϕ 3]6 theory

Using the MS scheme, we derive in [? ³]₆ theory the collinear conformal Ward identity for the Green's functions of local light-cone operators of leading twist. The Ward identity for special collinear conformal transformations and renormalization group invariance give constraints for the off-diagonal part of the anomalous dimension matrix for the general case of β#0. We compute the anomaly of special conformal transformation in lowest loop order and obtain from the constraints the off-diagonal part of the anomalous dimension in 2-loop order.     

Antiferromagnetic resonance in NaCrS2

Antiferromagnetic resonance measurements in rhombohedral NaCrS₂ (T_N = 17 K) are reported. The data are compared with theory developed by Battles for a collinear easy-plane antiferromagnet with anisotropy in the basal plane. While there is reasonable agreement with this theory, some discrepancies occur. These are presumably related to low-temperature in-plane distortions from hexagonal symmetry, previously postulated by other workers.