Two-sublattice System Research Articles

Exchange scattering on ultrafast timescales in a ferromagnetic two-sublattice system

We investigate ultrafast spin dynamics due to exchange, electron–phonon and Elliott–Yafet spin-flip scattering in a model with a simple band structure and ferromagnetically coupled electronic sublattices (or more generally, subsystems). We show that this incoherent model of electronic dynamics leads to sublattice magnetization changes in opposite directions after ultrashort-pulse excitation. This prominent feature on an ultrafast timescale is related to a transfer of energy and angular momentum between the subsystems due to exchange scattering. Our calculations illustrate a possible incoherent mechanism that works in addition to the coherent optically induced spin transfer mechanism.

Magnetic properties of some rare-earth nanostuctured aluminates

Nanocrystalline single-phase RAlO3 (R = Nd, Sm, Eu, Dy, Gd) has been prepared by modified Pechini's method. Malic acid has been used for the first time as a new complexing agent in the sol-gel process. It has facilited a low temperature synthesis of the compound. The characterization of the nanoparticles has been carried out by different methods. Using Physical Property Measurement System (PPMS-9 QD) the temperature and magnetic dependency of the susceptibility and magnetization of the nanostuctured aluminates were measured. The obtained results were compared with the existing ones for the single crystals and powder specimens of the same aluminates. The differences observed have been discussed in the framework of the molecular field theory for a two-sublattice system.

OPTICAL CONDUCTIVITY OF TWO-DIMENSIONAL LATTICE

We investigate the optical conductivity in the two-dimensional (2D) square and triangular tight-binding lattice-electron model with staggered magnetic flux (SMF). The SMF results in a two-sublattice system with two branches of energy bands in both cases, and even generates new flux-dependent optical properties. Results for the flux parameter dependence of the mean kinetic energy, the Drude weight and the optical conductivity are discussed in detail. A comparison between the two cases has been done.

Magnetic second-harmonic generation of an antiferromagnetic film

Second-harmonic generation of an antiferromagnetic film in Voigt geometry is calculated and analyzed. This calculation is carried out with an assumption of weak nonlinearity in a two-sublattice antiferromagnetic system. Second-harmonic waves are analyzed by the conventional approach of ordinary nonlinear optics.

Elementary Excitations in Antiquadrupolar, Antiferrimagnetic and Antiferromagnetic Biquadratic Spin Systems (S = 3/2)

The magnon and quadron spectrum is calculated in a two-sublattice system of localized spins S = 3/2 with the bilinear Heisenberg and isotropic biquadratic interactions. The calculations are performed in the (1/z)0 approximation in the high-density scheme (z is the number of spins interacting with any given spin) using diagrammatic methods for the Racah and standard basis operators. In the antiquadrupolar phase two positive and two negative branches of magnons and quadrons are obtained. In the antiferrimagnetic phase there are two positive and one negative magnon branches as well as one positive and one negative quadron branch. In the antiferromagnetic phase there is one positive and one negative branch of magnons and quadrons. The negative spectra, more pronounced here than in the (considered elsewhere) ferrimagnetic and ferromagnetic phases, give rise to the zero-point corrections at T = 0 in higher-order approximations.

Elementary Excitations in Antiquadrupolar, Antiferrimagnetic and Antiferromagnetic Biquadratic Spin Systems (S = 3/2)

The magnon and quadron spectrum is calculated in a two-sublattice system of localized spins S = 3/2 with the bilinear Heisenberg and isotropic biquadratic interactions. The calculations are performed in the (1/z) 0 approximation in the high-density scheme (z is the number of spins interacting with any given spin) using diagrammatic methods for the Racah and standard basis operators. In the antiquadrupolar phase two positive and two negative branches of magnons and quadrons are obtained. In the antiferrimagnetic phase there are two positive and one negative magnon branches as well as one positive and one negative quadron branch. In the antiferromagnetic phase there is one positive and one negative branch of magnons and quadrons. The negative spectra, more pronounced here than in the (considered elsewhere) ferrimagnetic and ferromagnetic phases, give rise to the zero-point corrections at T = 0 in higher-order approximations.

High-frequency ferromagnetic resonance in Nd/sub 2/Fe/sub 14/B/α-Fe nanocomposite films

Thin-film magnets, composed of nanophase dispersed Nd/sub 2/Fe/sub 14/B and /spl alpha/-Fe have been synthesized by rf magnetron sputtering. With increasing /spl alpha/-Fe volume fraction V/sub Fe/, the saturation magnetization increased linearly, while the coercivity decreased. Ferromagnetic resonance measurements were made by using a high-frequency and high-field ESR apparatus, which exhibited two absorption peaks. A model calculation of the resonance field was made for a simplified hard/soft two-sublattice system to analyze these results. Estimated values of the exchange-coupling constant are in good agreement with those deduced from micromagnetic analysis of the magnetization curves.

強磁性共鳴によるナノコンポジット磁石の交換結合の評価

Thin-film magnets, composed of nanophase dispersed Nd2Fe14B and α-Fe, were synthesized by rf magnetron sputtering onto glass substrates in an Ar atmosphere. Magnetization curves for films with different α-Fe volume fractions (VFe) were single-phase-like, suggesting a firm exchange coupling between hard and soft phases. With increasing VFe, the saturation magnetization increased linearly, while the coercivity decreased. Ferromagnetic resonance measurements were made by using a high-frequency, high-field ESR apparatus, which exhibited two absorption peaks when the field was swept with a fixed frequency. A model calculation of the resonance field was made for a simplified hard/soft two-sublattice system to analyze these results. Estimated values of the exchange-coupling constant are in good agreement with those estimated from micromagnetic analysis of the magnetization curves.

Thermal effects on two-sublattice magnetic systems

We report on the thermal effects on a two-sublattice system in the mean field approximation using computer simulation. In particular we study the temperature dependence of magnetization, magnetic susceptibility, and magnetic heat capacity for a system in which the magnetic state is determined by the magnetic anisotropies of the two components of the compound and the exchange interaction energy between them. The partition function is first determined, subsequently the magnetic properties are derived from the partition function. In certain cases, the findings of the model are compared with experimental data on rare-earth transition metal alloy system.

First-Order Spin Reorientation Reform (FOSRR) in a Two-Sublattice Ferrimagnetic System

Based on a two-sublattice mean-field model, a calculation procedure was developed to investigate the temperature dependence of the spontaneous magnetization of the two-sublattice system. First-order spin reorientation reforms (FOSRRs) were found in a two-sublattice ferrimagnetic system, depending on the competition between the exchange and the opposite anisotropies of the two sublattices. At the temperature where the FOSRR transition occurs both the directions and the strengths of the sublattice moments change accompanied with alterations of the exchange fields and the anisotropies. One expects that the FOSRRs could be found experimentally in R-T compounds and/or in ultrathin magnetic films and magnetic monolayers, although the mean-field approximation used here is quite poor for the latter (2D) systems.

New phenomena in temperature dependence of spontaneous magnetization in a two-sublattice system

In a two-sublattice system, it has been found that even at zero field there exist two different spin configurations, i.e., the completely parallel (or antiparallel) coupling and the canting coupling of the sublattice moments in ferromagnetic (or ferrimagnetic) materials. The present work suggests, in a phenomenological way, that the ferromagnetic canting spin configuration can show a ferrimagnetic-like temperature dependence of spontaneous magnetization. Two new types of first-order spin reorientation (FOSR) transitions are reported depending on the competition among the exchange and the anisotropies of the two sublattices. These FOSRs show an unusual behaviour that at the transition temperature both the directions and the strengths of the sublattice moments change, which are therefore defined as first-order spin reorientation reform (FOSRR).

Spin configurations and magnetization processes in a free-sample two-sublattice system

Spin structures in the presence of an external magnetic field have been studied for the two-sublattice model. The field dependence of the spin configurations has been found to depend sensitively on the competition among the exchange and the anisotropies of the two sublattices. Four kinds of magnetization processes have been identified in the ferrimagnetic case, including first-order moment reorientations (FOMRs), taking only the second-order anisotropies into account. For this case, we present phase diagrams defining ranges of parameter values leading to different magnetization processes. The spin structures and the magnetization processes are also discussed for the two-sublattice model with sixth-order anisotropies.

Torque and magnetization study of the canting model

We present a study of the effect of the canting angle between the magnetization vectors of a two-sublattice system on the torque exerted by an external magnetic field on the magnetization of the system. The magnetization process is analyzed in the light of the canting model. There is strong evidence for the canting mechanism, although coherent rotation of the magnetization is present in certain cases. The effects of both the magnitude and the orientation of the external magnetic field on the canting, magnetization and equilibrium energy of the system are discussed.

A FIRST ORDER PHASE TRANSITION IN ISING FERRIMAGNETS

The mean field theory is used for the analysis of a two-sublattice system of Ising spins, which describes ferro-, antiferro-, and ferrimagnetic orderings. It is proven that the phase transition in these systems is of a first order when the exchange constants of the sublattices are different. The free energy, the order parameter profiles and the latent heat of the phase transition are calculated for almost equivalent sublattices.

Spin configurations in the absence of a magnetic field in a two-sublattice system

The spin configurations in the absence of an external magnetic field have been systematically investigated for a two-sublattice system. Based on a two-sublattice model, the conditions for the existence of collinear and non-collinear spin structures were derived for either ferromagnetic or ferrimagnetic materials and the phase diagrams of the spin configurations at zero field were drawn, taking into account the second-order anisotropy parameters of both sublattices. The inclusion of higher-order anisotropy is discussed and it is shown that in some cases the same phase diagrams result.

Local mean field study of two-sublattice frustrated Ising systems

One- and two-sublattice frustrated Ising systems are studied by the local mean field method. Our results for linear and nonlinear susceptibilities suggest that the reentrancy phenomenon is absent in these systems. A decrease of the irreversibility temperature with the field in the magnetically ordered phases of the two-sublattice system is found to be weaker than in the one-sublattice case. The field-cooled susceptibility of a spin glass is influenced by the two-sublattice structure, but not as much as the mean field theory predicts. The intersublattice frustration is shown to decrease the compensation point in frustrated ferrimagnets whereas the intrasublattice frustration increases it.

On the Critical Behaviour and Phase Diagram of Ising-like Metamagnet in a Transverse Field

A. behaviour of an Ising system in a transverse magnetic field T (applied to the easy direction) has been the subject of several theoretical [1-4] and experimental studies [5, 6]. An important feature of such system is a crossover near the zero-temperature from the quantum to the classical behaviour. It has been studied theoretically [2-4] and observed in a simple two-sublattice Ising-type antiferromagnet. MnCl2 •4H20 (MnCl2 •4D20) at temperatures in a range 100 < T < 350 mK [5, 6]. It is interesting, however, to consider the Ising-type two-sublattice system, when except of the transverse field T a field H parallel to the easy direction is applied. In such a case we have metamagnet, in which the phase transition can be driven by three parameters, namely, F, H, and T. This problem has been recently reported in Ref. [7], where the system in the quantum regime has been investigated. The aim of the present paper is to show that in the three-dimensional space of parameters F, H, and T not only critical lines, in F, H [7] and T, H [8]-planes but also a surface of critical points exist, which is bounded by the line of tricritical points. Consider an Ising-like metamagnet described by the following Hamiltonian [7] (cf. also Ref. [8]):

Magnetization Process in a Two-Sublattice System. II. Phase Diagram of the Second Derivatives of Magnetization for Ferromagnetic Materials

The magnetization process in a two-sublattice system is studied by a computer simulation program established in Part I. Phase diagrams of the second derivatives with respect to different magnetization processes are determined for some ferromagnetic materials. Effects of both exchange and anisotropy on these phase diagrams are discussed.

A low-field peak emerging in the second derivative of the magnetization curve in a two-sublattice system

The magnetization process in a two-sublattice system is studied by a computer simulation program, without using the assumption that there exists a small canting angle between the magnetic moments of the two sublattices. A peak in the low-field range of the second derivative of the magnetization curve is reported, which depends on the competition between the comparatively weak exchange and the competing anisotropies.

Magnetization Process in a Two-Sublattice System. I. Low-Field Peak and Effective Anisotropy in Ferromagnetic Materials

The magnetization process in a two-sublattice system is studied by a computer simulation program, without using the assumption of the existence of a small canting angle between magnetic moments of the two sublattices. Effects of the exchange and the anisotropy on the shape of the second-derivative curves of magnetization are discussed. In the present work, a peak in the low-field range of the second-derivative curve of magnetization is reported. This peak as well as a strong effective anisotropy are found to depend on the competition between the comparatively weak exchange and the opposite anisotropies.