Green function method study of the anisotropic ferromagnetic Heisenberg model on a square lattice

We study the phase diagram of the anisotropic ferromagnetic Heisenberg model on a square lattice. We use the double-time Green’s function method within the Callen decoupling approximation. The dependence of the Curie temperature T_c on the spin S and on the anisotropy parameter Δ (Δ=0 and 1 correspond to the isotropic Heisenberg and Ising model, respectively) is obtained explicitly. Our results are in agreement with results obtained from other theoretical approaches.     

Thermoelectric power of magnetic metals: II. Anisotropic metals

We present a theory of the thermoelectric power tensor of anisotropic ferromagnetic metals with localized magnetic moments starting from the Boltzmann equation and incorporating anisotropy effects due to the lattice structure through a parameter measuring the anisotropy in the sound velocity. Elastic and inelastic phonon and spin scattering contributions are taken into account through a linear superposition of scattering cross sections. A mean field approximation is used to describe the ordered magnetic phase. Spin wave and impurity scattering, phonon and magnon drag are not included. In a range encompassing the Curie temperature, i.e. at “moderate temperatures”, the theory quantitatively reproduces observed features except for specific details (e.g. rounding near T_c) needing other physical input. We compare our theory to data on single crystals of Gd and Tb₇₅Gd₂₅. The c-axis thermoelectric power is well recovered for very reasonable values of anisotropy and scattering strength parameters. A conjecture is given to explain the basal plane thermoelectric power positive slope at high temperature.     

Green function approach to an anisotropic to an anisotropic Heisenberg model with mixed spins

Green function technique with random phase approximation has been utilized to obtain the Curie temperature of a mixed spin system for an anisotropic Heisenberg model with uniaxial anisotropy. The results obtained have been compared with the isolated spin system of a regular lattice.     

Structure and magnetism of the Pr2Fe14-xCoxB system

Isostructural, tetragonal materials were synthesized for the entire composition range (x = 0 to 14). They were studied by X-ray and magnetometry methods. Magnetization measurements confirm a ferromagnetic arrangement of 3d and Pr magnetic moments and reveal a sharp increase of the Curie temperature with the increase of Co concentration. Saturation magnetization at 295 and 77 K reaches a slight maximum around x = 1 and gradually decreases for higher Co content. Anisotropy fields initially decrease and afterwards increase very quickly as more Co is introduced to the system. A spin reorientation occurs in alloys with x ⩾ 9.5 at temperature higher than 660 K. It marks a transition between uniaxial anisotropy and planar anisotropy. Spin reorientation temperature becomes lower for materials with higher Co content. A discussion of the Curie temperature change is conducted in terms of preferential substitution of Co into 16k₂ sites. The weakening of the exchange interaction between Pr and 3d sublattices, as well as the decreasing Pr-sublattice anisotropy with rising temperature, are considered to be responsible for anisotropy field changes. The interplay between crystal field and exchange field interactions is emphasized. A comparison with the Nd₂Fe_14-xCo_xB system is included.     

The Curie temperature of a spin-one Heisenberg ferromagnet with biquadratic exchange and uniaxial anisotropy

With the pair model approximation, the Curie temperature of a spin-one Heisenberg ferromagnet with biquadratic exchange interaction J' and uniaxial anisotropy term D is calculated for both positive as well as negative values of J' and D.     

Effect of magnetic field annealing on microstructure and magnetic properties of FePt films

FePt (20 nm) films were annealed in a magnetic field (along the normal direction of the films) at a temperature around the Curie temperature of L1₀ FePt. The influence of magnetic filed annealing on texture and magnetic properties of FePt films were investigated. The results indicate that preferential (0 0 1) orientation and perpendicular anisotropy can be obtained in L1₀ FePt films by using magnetic field annealing around the Curie temperature of L1₀ FePt. This is one of the potential methods to obtain (0 0 1) orientation and thus to improve the perpendicular anisotropy in FePt films.     

Anisotropic exchange contributions to the magnetic susceptibility of transition metal ions in hexagonal close packed metals

When the effect of anisotropic exchange, together with crystal-field potentials is included in the Hamiltonian for magnetic impurity ions in a non-cubic metal, an additional contribution to the magnetic susceptibility is obtained. This produces a change in the temperature dependence of the susceptibility at low temperatures and gives an anisotropy varying as 1/T². The anisotropy in exchange, deduced from experiment, is 0.0086 per cent for Zn: Mn and between 2.4 and 7.2 per cent for Zn : Cr with J₆ > J_⊥ in both cases.     

Influence on magnetic properties of substitution of Co for Fe in TbFe11.3Nb0.7

The magnetic properties of Tb(Fe_1−xCo_x)_11.3Nb_0.7 compounds with x=0, 0.05, 0.1, 0.15, 0.2 and 0.3 have been investigated. All compounds studied crystallize in the ThMn₁₂-type of structure. Substitution of Co for Fe leads to a contraction of the unit-cell volume. The Curie temperature clearly increases with increasing Co content from 551 K for x=0 to 831 K for x=0.3. The magnetic moment of the transition-metal sublattice increases with increasing Co content from 22.2 μ_B/f.u. for x=0 to 23.1 μ_B/f.u. for x=0.3. As the temperature increases, a spin reorientation from easy-plane to easy-cone is found in all compounds investigated. The spin-reorientation temperatures T_sr have been derived from the temperature dependence of the magnetization in a low field and decrease monotonously with increasing Co content. The easy magnetization direction at room temperature has been determined by X-ray diffraction on magnetically-aligned powder samples. The influence of the substitution of Co for Fe on the magnetic anisotropy is discussed in terms of crystal-field theory.     

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ～ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (T _C1), to the crystallization temperature (T _ac), and to the Curie temperature in the crystalline phase (T _C2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH～1?12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ～ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ～ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.     

Nd1.9M0.1Fe12Co2B,M = Ti or Hf as a material for permanent magnets

Magnetic properties (saturation magnetizations, anisotropy fields and Curie temperatures) of the alloys of the composition Nd_1.9M_0.1Fe₁₂Co₂B, with M = Ti and Hf are presented. All of the compounds crystallize in the tetragonal Nd₂Fe₁₄B structure. With substitution of Nd by Ti or Hf, the saturation moment and Curie temperature decrease, but the anisotropy fields, H_A, are found to increase significantly. Theoretical energy products are reduced from 65 to 63 MGOe when Nd is replaced by Ti or Hf to the extent of x = 0.1.     

Tb1-xGdxFe2的磁矩和矫顽力

测量了赝二元立方Laves相化合物TbGdFe₂(0≤x≤1)的磁化曲线、居里温度和矫顽力随温度的变化。发现化合物由TbFe₂向GdFe₂过渡时,样品的饱和磁化强度几乎直线下降。在所有成分下,Fe原子的磁矩都是常数(μ_Fe=1.60±0.04μ_B),但Tb原子的磁矩,由于受晶场影响,却小于自由离子的值(gJ=9.Oμ_B)。矫顽力由两部分组成:一为畴壁受Peierls势垒和稀土 关键词：     

Increase in the magnetostrictive susceptibility of Tb0.3Dy0.67Ho0.03Fe2−x Co x alloys upon substitution of cobalt for iron

This paper reports on the results of investigations into the concentration dependences of the magnetostrictive susceptibility, the magnetostriction, the magnetization, and the Curie temperature for Tb_0.3Dy_0.67Ho_0.03Fe_2?x Co _x alloys upon substitution of cobalt for iron. It is revealed that the temperature of the spin-reorientation transition shifts toward room temperature with an increase in the cobalt content in the range 0 ≤ x ≤ 1.3. Substitution of cobalt for iron in the alloys leads to a decrease in the contribution of the 3d transition metal sublattice to the magnetic anisotropy owing to the opposite signs of the single-ion anisotropy constants for iron and cobalt. The decrease observed in the magnetocrystalline anisotropy compensated in both rare-earth and 3d transition metal sublattices is responsible for the high magnetostrictive susceptibility of the studied compounds at a high cobalt content (x = 1.3) in the room-temperature range.     

Stochastic simulation of thermally assisted magnetization reversal in sub-100 nm dots with perpendicular anisotropy

Thermally assisted magnetization reversal of sub-100 nm dots with perpendicular anisotropy has been investigated using a micromagnetic Langevin model. The performance of the two different reversal modes of (i) a reduced barrier writing scheme and (ii) a Curie point writing scheme are compared. For the reduced barrier writing scheme, the switching field H_swt decreases with an increase in writing temperature but is still larger than that of the Curie point writing scheme. For the Curie point writing scheme, the required threshold field H_th, evaluated from 50 simulation results, saturates at a value, which is not simply related to the energy barrier height. The value of H_th increases with a decrease in cooling time owing to the dynamic aspects of the magnetic ordering process. Dependence of H_th on material parameters and dot sizes has been systematically studied.     

Spin-one Ising ferromagnet with three-site four-spin interaction

For the Ising ferromagnet with S = 1 the effects of the three-site four-spin interaction J′ on the magnetization and the Curie temperature have been calculated using the three-spin model approximation and its effect is shown to be significant for negative value of J′.     

Nitrogen-containing compounds <Emphasis Type="Italic">R</Emphasis>Fe<Subscript>11</Subscript>TiN<Subscript>x</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Gd or Lu)

The structure and magnetic properties of RFe₁₁TiN compounds (R=Gd or Lu) containing nitrogen are investigated. Magnetic measurements are performed on a magnetometer in magnetic fields up to 100 kOe in the temperature range from 4.2 to 750 K with the use of RFe₁₁TiN single crystals, RFe₁₁TiN powders placed in a ceramic cell, and samples oriented in an external magnetic field. It is found that the nitridation leads to an increase in the Curie temperature and the saturation magnetization. The samples studied are uniaxial over the entire temperature range of magnetic ordering. The magnetic anisotropy decreases upon nitridation. It is demonstrated that, within the local anisotropy model, the decrease in the magnetic anisotropy constant K₁ can be explained by the redistribution of the electron density in the vicinity of the crystallographic positions occupied by iron atoms.     

Structural,elastic and thermodynamic properties of the tetragonal structure of germanium carbonitride

The structural, mechanical, electronic and thermodynamic properties of the tetragonal structure germanium carbonitride (t-GeCN) were first investigated using the density function theory with the ultrasoft psedopotential scheme in the frame of the generalized gradient approximation and the local density approximation. The elastic constants have confirmed that the t-GeCN is mechanically stable and phonon spectra have confirmed that the t-GeCN is dynamically stable. The anisotropy studies show that t-GeCN exhibits a larger anisotropy in its Poisson's ratio, Young's modulus, shear modulus, sound velocities and universal elastic anisotropy index. Electronic structure study shows that t-GeCN is an indirect semiconductor with band gap of 0.628 eV. The thermodynamic properties of t-GeCN, including Debye temperature, heat capacity, Grüneisen parameter and thermal expansion coefficient are investigated utilizing the quasi-harmonic Debye model.     

Intersite correlation in anisotropic ferromagnetic spin one systems

The effect of intersite correlations, in Callen approximation, on the critical temperature T_C has been investigated for the anisotropic spin one system in the presence of uniaxial anisotropy. The effect is found to be important at small anisotropy.     

Magnetic-field-induced quantum criticality in a spin-<Emphasis Type="Italic">S</Emphasis> planar ferromagnet with single-ion anisotropy

The effects of single-ion anisotropy on quantum criticality in a d-dimensional spin-S planar ferromagnet is explored by means of the two-time Green’s function method. We work at the Tyablikov decoupling level for exchange interactions and the Anderson-Callen decoupling level for single-ion anisotropy. In our analysis a longitudinal external magnetic field is used as the non-thermal control parameter and the phase diagram and the quantum critical properties are established for suitable values of the single-ion anisotropy parameter D. We find that the single-ion anisotropy has sensible effects on the structure of the phase diagram close to the quantum critical point. However, for values of the uniaxial crystal-field parameter below a positive threshold, the conventional magnetic-field-induced quantum critical scenario remains unchanged.     

Magnetocrystalline anisotropy of Ni and Mn substituted Nd2Fe14B compounds

Measurements of the effects of Ni and Mn substitutions for Fe on the magnetic properties of Nd₂Fe₁₄B compounds are reported. The Curie temperature is slightly increased with Ni substitution whilst in the case of replacement of Fe by Mn it is reduced drastically. A monotonic decrease of both the lattice parameters a and c is observed. The saturation magnetization is decreased by both Ni and especially Mn substitutions. The composition dependence of both the reorientation spin transition temperature and the cone angle has been measured. The influence of the 3d metal substitution on the Nd anisotropy has been measured and discussed. The composition dependence of the room temperature anisotropy field values, which is an important figure of merit for permanent magnet applications, decreases slightly in the case of Ni and drastically for Mn substitution. A comparison with the case of Co substitution has been made.     

Crystal-field excitations in Pr2CuO4

Neutron spectroscopy has been employed to study the crystal-field excitations in Pr₂CuO₄. The energetic ordering of the crystal-field levels and the crystal-field parameters are determined from the observed transitions. Our results are consistent with the large magnetic anisotropy observed in single-crystal susceptibility data.