Intralayer Exchange Coupling Research Articles

Magnetic and critical properties of models of magnetic superlattices

For theoretical simulation of critical behavior in multilayers, we have suggested a model of superlattice Fe/V. The calculations were performed using the Monte Carlo method. The relation between interlayer and intralayer exchange coupling was varied in a wide limit that allowed description of the systems with different concentration of adsorbed hydrogen. We have calculated spontaneous magnetization, susceptibility and specific heat as functions of the temperature and exchange parameters. The critical temperature and static critical exponents have been calculated and phase diagrams have been plotted.

Intralayer coupling in self-organized Fe nanoclusters grown on vicinal Si(111)

This paper reports on the magnetic properties of Fe thin film grown on vicinal Si(111). The surface analysis, performed via scanning tunneling microscopy, showed that the iron grows in a form of two distinct types of elongated nanosized grains, aligned parallel and perpendicular to the substrate steps, respectively. A phenomenological model was used to interpret the experimental magnetization data which considers the two types of Fe nanoclusters with both cubic and uniaxial anisotropies, where those of each type are all identical and aligned. Every particle of the first type is coupled to the adjacent particle of the other type only via direct exchange coupling through the corresponding surface atoms. The experimental hysteresis loops as well as the coercivity and remanent magnetization angular variations are very well reproduced by the model, demonstrating that the magnetic behavior of this system is mainly determined by the ferromagnetic intralayer exchange coupling.

Exchange interactions and anisotropic spin waves in bilayer manganites

We present a theory of the magnetic interactions in the bilayer manganites, based upon the manganite t-J model, and show that the ferromagnetic phase is an anisotropic orbital liquiddisordered state. By including the correlations of the e g electrons we explain why the intralayer exchange coupling increases with hole doping x. The transition from the ferromagnetic to A-type antiferromagnetic phase in La222xSr112xMn2O7 at x.0.45 occurs when the occupancy of 3z 2 2r 2 orbitals drops and the superexchange term exceeds the double exchange in the interlayer coupling.

Thermal properties of surface and bulk spin waves in metamagnets FeBr2 and FeCl2

Surface and bulk spin wave properties are studied in metamagnets, which consist of ferromagnetically ordered layers with the intralayer ferromagnetic exchange coupling being much stronger than the interlayer antiferromagnetic exchange. Using a Green's function method, dispersion relations are calculated for the S = 1 metamagnets FeBr2 and FeCl2 over a broad range of temperatures by considering layer dependent thermal averages for the spins. Results are also deduced for the temperature dependence of the integrated intensities of acoustic and optical spin waves close to the surface.

Sclérose latérale amyotrophique chez un patient atteint d’une granulomatose de Wegener

The thermodynamic properties of a nano-graphene bilayer, consisting of the upper layer A of spin-3/2 with antiferromagnetic intralayer exchange coupling and the bottom layer B of spin-5/2 with ferromagnetic intralayer exchange coupling, have been studied by the use of Monte Carlo simulation. We find a number of characteristic phenomena. The effects of the exchange coupling, the single-ion anisotropy and the longitudinal magnetic field on the internal energy, the specific heat and the blocking temperature of the mixed-spin bilayer system have been investigated in detail. The internal energy and the specific heat profiles are clarified. In particular, we have found that the specific heat curve may show two peaks phenomenon for appropriate values of the system parameters.

Variational mean-field approach for spin configurations in magnetic layered systems with competing anisotropies

A variational mean-field approach is established to describe the effects of competing anisotropies on spin configurations and their temperature dependencies in magnetic layered systems. Using thermal averaged magnitudes and thermal averaged auentation angles of magnetic moments at each layer as mean-field parameters, a set of self-consistent equations are derived. The effects of the competing anisotropies in the systems as well as the intralayer and interlayer exchange couplings and the magnetic dipole-dipole interactions, are described in a unified way by a set of self-consistent equations. The numerical results show that this variational mean-field approach can present a qualitative correct description for spin configurations and their temperature dependencies in magnetic layered systems with competing anisotropies

Spin-Wave Resonance in the Thin FeBr2 Field-Induced Metamagnet with Modified Surface Exchange Interactions

The spin-wave resonance spectrum is calculated for the thin film of an FeBr2 field-induced metamagnet, which consists of ferromagnetically ordered layers parallel to the (001)-plane, with intralayer ferromagnetic exchange coupling, interlayer antiferromagnetic exchange coupling as well as a strong three-ion anisotropy. The profiles of the spin-wave modes, the positions of the absorption lines and their intensities are analysed for standing spin waves existing in the paramagnetic phase of the FeBr2 metamagnet at low temperatures. It is found that the absorption spectrum is strongly affected by modifications of both surface exchange parameters. By allowing the surface parameters to deviate from the bulk values, the full richness of spin-wave resonance spectra is displayed. Also, the conditions for the appearance of various surface and bulk spin-wave features are discussed.

Quantum theory for the temperature dependences of magnetic configurations in magnetic ultrathin films

A lattice model and a corresponding quantum mean field approach are proposed to describe the temperature dependences of magnetic configurations in magnetic ultrathin films. This lattice model takes into account the layer-by-layer variations of the local microscopic magnetic parameters such as the exchange coupling constants and the magnetic anisotropy constants. A set of self-consistent equations are derived for the thermal averaged magnitudes and orientation angles of magnetic moments in each layer. The competing effects of the intralayer and interlayer exchange couplings, the different local magnetic anisotropies in different layers, and the long range magnetic dipole–dipole interactions, are studied within the mean field approximation. The theoretical results are in qualitative agreement with experimental observations. The general formulas established can also be applied to magnetic multilayers.

Cu-NQR Study in the Paramagnetic State in YBa2Cu3O6

The NQR measurements for the Cu(2) and the Cu(1) sites in antiferromagnetic insulator YBa 2 Cu 3 O 6 with T N =415 K were carried out in the paramagnetic state for the respective temperature ranges of 475–575 K and of 295–775 K. For the Cu(2) sites, the nuclear spin-lattice relaxation rate 1/ T 1 and the second moment of the indirect nuclear spin-spin interactions 1/ T 2G exhibit a similar behavior to those in La 2 CuO 4 not only in the temperature dependence but also in the magnitude at least below 550 K. These results suggest a possibility that the renormalized classical description for two-dimensional quantum Heisenberg antiferromagnet is also applicable to YBa 2 Cu 3 O 6 as to La 2 CuO 4 and that the upper limit of the bilayer coupling J ''/ J is as large as 10 -2 , where J and J '' are the intra-layer and bilayer exchange couplings, respectively. The Cu(1) NQR does not probe any spin dynamics in the CuO 2 plane except for the temperatures below 600 K. The rapid increase in 1/ T 1 above 600 K suggests...

Phase Transitions in Bilayer Heisenberg Model with General Couplings

The ground state properties and phase diagram of the bilayer square-lattice Heisenberg model are studied in a broad parameter space of intralayer exchange couplings, assuming an antiferromagnetic coupling between constituent layers. In the classical limit, the model exhibits three phases: two of these are ordered phases specified by the ordering wave vectors (pi,pi;pi) and (0,0;pi), where the third component of each indecates the antiferromagnetic orientation between layers, while another one is a canted phase, stabilized by competing interactions. The effects of quantum fluctuations in the model with S=1/2 have been explored by means of dimer mean-field theory, small-system exact diagonalization, and high-order perturbation expansions about the interlayer dimer limit.

Effect of magnetic anisotropies on spin waves of multilayers with antiferromagnetic coupling

We investigate how the spin waves spectra of Fe Cr finite superlattices with antiferromagnetic coupling depend on the interface anisotropy (in-plane and out-of-plane). The magnetic ground state, evaluated within the mean field approach, exhibits a canted configuration of the static magnetization. The spin waves are calculated by considering dipole interaction, intralayer and interlayer exchange coupling, uniaxial and interface anisotropies, in presence of an external field. The calculated spectra are compared with recent experimental Brillouin scattering data which show two spin waves peaks.

Spin Waves in Antiferromagnetically Coupled Multilayers

We investigate how the static and dynamical magnetic properties of Fe/Cr finite superlattices with antiferromagnetic coupling depend on the ratio between the uniaxial anisotropy constant and the interlayer exchange constant of Fe films. The spin waves are calculated by considering dipole interaction, intralayer and interlayer exchange coupling, uniaxial anisotropy, in presence of an external field. The calculated spectra are compared with recent experimental Brillouin scattering data which exhibit two spin waves peaks. PACS numbers: 75.70.—i, 75.30.Pd, 75.70.Cn

Properties of Standing Spin Waves in a Thin Film of Field-Induced Metamagnet with Modified Surface Exchange Interactions

Properties of surface spin waves are studied in a thin film of the field-induced metamagnet which consists of ferromagnetically ordered layers parallel to the (001) plane, with intralayer ferromagnetic exchange coupling, interlayer antiferromagnetic exchange coupling as well as a strong three-ion anisotropy. A special Rutherford mathematical procedure for the description of the surface properties of finite crystal structures is generalized and applied to solve the eigenvalue problem for standing spin waves existing in the paramagnetic phase of the metamagnet at low temperatures. Also, by allowing the surface exchange parameters to deviate from the bulk values the stability conditions for the system are taken under theoretical discussion.

Micromagnetic modelling of spin valve and GMR read heads

Spin valve and GMR read heads for ultra-high density storage applications are subject to strong self and inter-layer demagnetizing fields which dominate their behavior. A simple vector model for the description of the magnetic characteristics of macroscopic samples of spin valve material has been generalized and extended to describe finite width stripes by the incorporation of demagnetizing fields and intra-layer exchange coupling.

The structure and EPR properties of (CH2OHCH2NH3)2CuCl4

The title compound is a member of the (RNH3)2CuCl4 layer perovskite family, space group C2/c, witha=21.624(5),b=7.511(1),c=7.431(2)A, β=89.56(2)° andV=1206.9(4)A3 withZ=4. In the antiferrodistortive structure, the Cu(II) ion assumes an elongated octahedral coordination (unique Cu−Cl distances=2.274(2), 2.288(2), and 3.010(2)A). The octahedra share corners with four neighbors to form a two-dimensional network. The −NH3 moieties hydrogen bond to the layers, so that the −C2H5OH moieties form sheathes about each metal halide layer. A twofold disorder of the −C2H5OH groups is observed. The EPR spectrum is investigated to help understand the magnetic properties of the systems. Theg-values are consistent with strong intralayer exchange coupling. The EPR linewidths show evidence of spin diffusion effects at liquid nitrogen temperature. However, at room temperature, the spin anisotropies lead to broadening of the EPR lines through a phonon modulation mechanism.

Weak Inter- and Intralayer Exchange Coupling between Copper(II) Dimers and a Triplet Density Effect in EPR of Tris(ethylenediamine)cobalt(III) Bis(.mu.-chloro)bis[trichlorocuprate(II)] Dichloride Dihydrate

Weak Inter- and Intralayer Exchange Coupling between Copper(II) Dimers and a Triplet Density Effect in EPR of Tris(ethylenediamine)cobalt(III) Bis(.mu.-chloro)bis[trichlorocuprate(II)] Dichloride Dihydrate

The influence of exchange interactions on surface spin waves in a semi-infinite FeBr2 field-induced metamagnet

Properties of surface spin waves are studied in an FeBr2 field-induced metamagnet that consists of ferromagnetically ordered layers, with intralayer nearest-neighbour ferromagnetic exchange coupling, intralayer next-nearest-neighbour antiferromagnetic exchange coupling and interlayer antiferromagnetic exchange coupling as well as strong three-ion anisotropy. A Green function method is applied to calculate surface spin-wave modes for the (001) surface and for the paramagnetic phase at low temperatures. By allowing the surface exchange parameters to deviate from the bulk values, the full richness of the surface spin-wave spectra is found. The problem of the stability of the ground state of the system is also discussed.

Spin waves in the semi-infinite FeBr/sub 2/ field-induced metamagnet

Properties of surface spin waves are studied in the FeBr/sub 2/ field-induced metamagnet that consists of ferromagnetically ordered layers, with intralayer nearest-neighbour ferromagnetic exchange coupling, intralayer next-nearest-neighbour antiferromagnetic exchange coupling and interlayer antiferromagnetic exchange coupling as well as a strong three-ion anisotropy. Surface spin wave modes are calculated for the (001) surface and for the paramagnetic phase at low temperatures. By allowing the surface exchange parameters to deviate from the bulk values, full richness of the surface spin-wave spectra is displayed. The problem of the stability of the ground state of the system is also discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the magnetic phase transition of some layered copper compounds: I. Extrapolation to the ideal, two-dimensional, [formula omitted], Heisenberg ferromagnet

In this paper an analysis is presented of the dependence of the reduced critical temperature kT c/ J on the ratio R ≡ H′ E / H E of the interlayer ( H′ E) and intralayer ( H E) exchange coupling, as observed for a number of layer-type copper compounds. The analysis aims at a determination of the critical parameters of the susceptibility of the ideal ( R = 0), quadratic, S = 1 2 , Heisenberg ferromagnet. For the ideal system, the experimental data—when compared with the available theoretical calculations—seem to point to a critical behaviour, characterized by an exponential divergence of the susceptibility rather than to a power-law divergence. From the analysis an upper bound kT c (0)/ J⩽ 0.25 is inferred for the reduced critical temperature kT c(0)/ J of the ideal, quadratic, S = 1 2 , Heisenberg ferromagnet. However, kT c (0)/ J = 0 has to be taken into consideration, since the data are not inconsistent with the results of the S = 1 2 , Green's-function approach of Yamaji and Kondo, in which the susceptibility is predicted to diverge exponentially at kT c (0)/ J = 0, according to χ ̃ ≡ X J/(k C ̃ )= A(kT/J) −1 exp [C(kT/J) −1] , with C = π. Under the assumption kT c (0)/ J = 0, our analysis points to C=3.0. On the other hand, on basis of the presently available experimental data it is not possible to exclude a χ̃ divergence of the form χ ̃ =A exp{C[k(T−T c(0))/J] 0−v} , with kT c(0)/J > 0,0.5 $ ̌ ν < 1 . The latter expression, with ν⋍0.8, has been suggested by Camp and Van Dyke for the S=∝, triangular Heisenberg ferromagnet from an analysis of high-temperature series expansions.