The mixed spin-1/2 and spin-1 Ising system on a two-layer Bethe lattice

Two layered magnetic Bethe lattice with varying coordination number q is introduced and numerically studied via exact recursion relations within a pairwise approach. The system is influenced by competing interlayer and intralayer nearest-neighbour (NN) coupling interactions and also by the crystal and external magnetic fields. Cases where both layers are ferromagnetic or one is ferro and the other antiferromagnetic are considered. System configurations’ energy calculations are used to devise some ground state phase diagrams that have proven useful for the investigation of the very low temperature behaviour of the model. Analysis of the thermal behaviours of the total magnetization within the model parameters’ space yield interesting phase diagrams which display fascinating properties, in particular the presence of tricritical points. Increasing negative values of the crystal field strength stabilizes the disordered paramagnetic phase and sometimes gives rise to wavy transition lines.     

Field induced magnetic moment and anisotropy of HoAG and TbAG

A theoretical and experimental study of the field induced magnetic moment and magnetic anisotropy of holmium aluminum garnet and terbium aluminium garnet is presented. The torque curves due to the induced field dependent anisotropy obtained at 4.2 K in applied fields up to 7 T are compared with the theoretical calculation. The results indicate that the two systems show remarkably similar magnetic behaviours, and the calculation based upon a simplified effective Hamiltonian with the crystal field parameters determined from spectroscopic data accounts for the experimental results with satisfactory accuracy.     

Experimental investigations of the relation between electrical conductivity and natural magnetic anisotropy in ferrites containing cobalt

A deep relation between the crystalline structure, the nature and energy of the interatomic interaction, and the different physicochemical properties is exhibited with special clarity in ferrites. Therefore a comprehensive discussion of the physical properties of ferrites with the peculiarities of magnetic transitions in ferrites taken into account, along with an investigation of the structure, is of great significance. In this connection, we posed the problem of investigating the effect of magnetic anisotropy on the electrical properties of ferrites containing various amounts of cobalt ions, which make, as is well known, an anomalously large contribution to the magnetic anisotropy constant of ferrite-spinels and lead to a change in sign. Investigations of the electrical properties of ferrites have shown that a change in the activation energy of the electrical conductivity [1] and an anomaly in the temperature behavior of the spontaneous Hall field [2] are observed upon the transition of ferritesfrom a ferri- to a paramagnetic state. It is natural to assume that these anomalous changes are related to the disappearance of magnetic order inside a ferromagnetic semiconductor, which indicates a relation of the conductivity electrons to the magnetic structure. However, the magnetic order in crystals of ferro- and ferrimagnets can be altered in other ways besides a transition through the Curie point. The magnetic structure can be altered in a definite way also upon a change in the sign of the constant of natural magnetocrystalline anisotropy ₁ (i.e., when the direction of weak magnetization is changed in a crystal). Investigations of the effects of these kinds of changes in the magnetic structure on the motion of current carriers in ferrites are of great interest. It is important in this connection to carry out investigations in the temperature region in which the investigated object is in the ferrimagnetic state, i.e., the spontaneousmagnetization is not destroyed, and its direction in the crystal was altered only as a result of a change in the sign of ₁. It is completely permissible to assume that there occurs in the ferrite a change in the intracrystalline field, which affects in a definite way the wave functions of the electrons participating in the conductivity. Thus, even these simple qualitative discussions indicate that such investigations can give valuable information on the effect of a magnetic subsystem on kinetic phenomena in magnetic semiconductors — ferrites. Our investigations of the effect of a magnetic subsystem on the transport phenomenon differ from the experiments of a number of authors [3, 4, 5] in that these authors have investigated the temperature dependences of electrical resistance in the temperature range in which the sign of the magnetic anisotropy constant changes, i.e., the direction of the magnetization vector is altered not due to the effect of external magnetic fields, but as a result of a change in the anisotropic properties upon being heated up.Deceased.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 7–13, July, 1979.     

Temperature dependence of the lattice parameters and magnetic properties of Er3Ir single crystals

Er₃Ir single crystals were grown by the Czochralski method from a levitated melt. The electrical resistivity thermal dependence exhibits ordering temperature of the erbium sublattice at 40 K and a spin reorientation process at 22 K. The DC and AC magnetic susceptibility show antiferromagnetic ordering in the form of an asymmetric peak. The magnetization in strong magnetic fields up to 140 kOe exhibits anisotropy. The lattice parameters’ thermal dependence of Er₃Ir and Er₃Ni show anisotropy and anomalous behaviour.     

Faraday rotation and the magnetocaloric effect in the Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> terbium gallium garnet at low temperatures in strong magnetic fields

The magnetocaloric effect and the Faraday rotation in a paramagnetic cubic crystal of terbium gallium garnet in strong magnetic fields oriented along different crystallographic directions are calculated theoretically. It is demonstrated that, in strong magnetic fields, the magnetocaloric effect and the Faraday effect are characterized by strong anisotropy, which disappears in weak magnetic fields.     

Ferromagnetic resonance in arrays of highly anisotropic nanoparticles

We present in this study computational simulations of the ferromagnetic resonance response of magnetic nanoparticles with a uniaxial anisotropy considerably larger than the microwave excitation frequency (in field units). The particles are assumed to be randomly oriented in a two dimensional lattice, and are coupled by dipolar interactions through an effective demagnetization field, which is proportional to the packing fraction. We have included in the model fluctuations in the anisotropy field (H_K) and allowed variations in the demagnetizing field. We then analyzed the line shape and line intensity as a function of both fields. We have found that when H_K is increased the line shape changes drastically, with a structure of two lines appearing at high fields. The line intensity has a maximum when H_K equals the frequency gap and decreases considerably for larger values of the anisotropy. The effects of fluctuations in H_K and variations in the packing fraction have been also studied. Comparison with experimental data shows that the overall observed behavior is dominated by the particles with lower anisotropy.     

1H NMR study of static and fluctuating internal Magnetic fields in Tb(C2H5SO4)3 · 9 H2O ising ferromagnet

The pulsed NMR method is applied to an analysis of a complicated structure of inhomogeneous internal fields in a ferromagnetic crystal. Proton magnetic resonance in the Ising ferromagnet TbES at a temperature range from 1.6 K down to 35 mK is studied at frequencies of 10–35 MHz. A complicated picture of static and fluctuating internal magnetic fields in the crystal is presented. Interatomic distances are shown to have an uncertainty of the order of 0.2% due to defects in the crystal lattice. The fluctuations of internal magnetic fields produced by thermal excitation and spin-spin relaxation of Tb³⁺ ions give rise to the effective nuclear magnetic relaxation: 1/T₁₍₂₎～exp (δ/kT), where δ is the energy splitting of the lowest Tb³⁺ quasi-doublet. The rate of these fluctuations in TbES at low temperatures is approximately equal to 2×10⁷ s^?1 being independent of temperature and magnetic field.     

Electron spin resonance investigation of the substitution of Fe3^＋ for Ti4^＋ ions in rutile TiO2 single crystal

A Fe doped rutile TiO 2 single crystal is grown in an O 2 atmosphere by the floating zone technique.Electron spin resonance (ESR) spectra clearly demonstrate that Fe 3+ ions are substituted for the Ti 4+ ions in the rutile TiO 2 matrix.Magnetization measurements reveal that the Fe:TiO 2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K.The Fe 3+ ions possess weak magnetic anisotropy with an easy axis along the c axis.The annealed Fe:TiO 2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.     

Evolution of structure and physical properties in Al-substituted Ba-hexaferrites

The investigations of the crystal and magnetic structures of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions have been performed with powder neutron diffractometry. Magnetic properties of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions have been measured by vibration sample magnetometry at different temperatures under different magnetic fields.The atomic coordinates and lattice parameters have been Rietveld refined. The invar effect is observed in low temperature range(from 4.2 K to 150 K). It is explained by the thermal oscillation anharmonicity of atoms. The increase of microstress with decreasing temperature is found from Rietveld refinement. The Curie temperature and the change of total magnetic moment per formula unit are found for all compositions of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.     

Magnetic and electrical properties of oxygen stabilized nickel nanofibers prepared by the borohydride reduction method

Fine nickel fibers have been synthesized by chemical reduction of nickel ions in aqueous medium with sodium borohydride. The thermal stability and relevant properties of these fibers, as-prepared as well as air-annealed, have been investigated by structural, magnetic and electrical measurements. As-prepared samples appear to have a novel crystal structure due to the presence of interstitial oxygen. Upon annealing in air, the fcc-Ni phase emerges out initially and develops into a nanocomposite subsequently by retaining its fiber-like structure in nano phase. The as-prepared sample is observed to be weakly magnetic at room temperature, but attains surprisingly high magnetization values at low temperatures. This is attributed to the modified spin structure, presumably due to the presence of interstitial oxygen in the lattice. Development of a weakly ferromagnetic and electrically conducting phase upon annealing in air is attributed to the formation of the fcc-Ni phase. The structural phase transformations corroborate well with magnetic and electrical measurements.     

Magnetic-field dependent phonon states in paramagnetic CeF3

Raman scattering experiments in paramagnetic uniaxial CeF₃ at helium temperature demonstrate a splitting of some degenerate (E_g)-phonon states in an external magnetic field parallel to the crystal axis. A linear splitting is observed in low fields, followed by a field independent (saturation) splitting in high fields. In addition, changes in the Raman scattering efficiencies and a reduction of the line width of phonon transitions are observed with increasing magnetic fields. No such effects are observed for magnetic fields perpendicular to the crystal axis. The splittings of degenerate phonon modes are related to the paramagnetic saturation 〈S_z〉.     

Correlation of temperature anomalies of the hyperfine field and thermal expansion of the crystal lattice

Consideration is given to the effect of thermal expansion of crystal lattice upon the partial contributions to the magnetic hyperfine field for non-magnetic atoms in metallic ferromagnets. The existence of correlation between the temperature anomalies of a hyperfine field and the thermal expansion of the crystal lattice is shown in a simple and illustrative way.     

Features of low-frequency spin dynamics in manganite LaMnO3 according to 139La NMR data

The spin-lattice and spin-spin relaxation times of ¹³⁹La are measured in manganite LaMnO₃. Analysis of the frequency dependence of the spin-lattice relaxation rate in the paramagnetic temperature range shows that this quantity is determined by magnetic fluctuations. The magnitude of the fluctuating field is estimated. It is shown that the correlation time for spin fluctuations varies with temperature in accordance with the Arrhenius law. The high value of the spin-spin relaxation rate in the paramagnetic region can be due to strong anisotropy of fluctuating magnetic fields at La nuclei.     

Deviations from the Curie-Weiss law in paramagnetic dysprosium and terbium-50% holmium

The magnetic susceptibilities of single crystal dysprosium and terbium-50% holmium have been measured in their paramagnetic phases. The effect of annealing has been investigated and heating the sample to ≈ 360 K for ≈ 18 h causes ≈ 4% reduction in χ₂₇₃. Abrupt departures from Curie-Weiss behaviour occur at ≈ 257, 227, 199 K in Dy, and at 250, 214 and 197 K in Tb-50% Ho, coinciding with previously observed anomalies in the thermal expansion. This behaviour is attributed to the presence of residual antiferromagnetic order in the paramagnetic phase which is perturbed when the periodicity of the helical spin structure, estimated from neutron diffraction data, is commensurate with the hexagonal crystal lattice.     

Effect of hydrogenation on spin-reorientation phase transitions and magnetic anisotropy constants of RFe11Ti single crystals (R=Lu, Ho, and Er)

The magnetic anisotropy and spin-reorientation phase transitions in single crystals of the RFe₁₁Ti (R=Lu, Ho, and Er) compounds and their hydrides are investigated. Measurements are carried out on capacitance and torque magnetometers. The magnetic anisotropy constants K ₁ and K ₂ are determined by the mathematical processing of experimental magnetization curves in terms of the phenomenological theory of the anisotropic ferromagnet magnetization. It is demonstrated that the hydrogenation strongly affects the magnitude and the sign of magnetic anisotropy constants, as well as the spin-reorientation phase transitions. The hydrogenation of the HoFe₁₁Ti compound leads to the change in sign of the magnetic anisotropy constant K ₁. The inference is made that a change in the atomic volume and the axial ratio c/a cannot result in the observed effects. A change in the magnetic anisotropy constants upon hydrogenation is primarily due to the change in the interaction of the quadrupole moment of a 4f electron subshell of rare-earth ions with surrounding ions of the crystal lattice and also with valence and conduction electrons.     

Domain wall oscillations in magnetic garnet films

Domain wall oscillations in magnetic garnet films have been observed in the stripe lattice and in the bubble lattice for applied in-plane magnetic fields approaching the saturation limit of the film. Observations are reported in (111) and (001) oriented films. An effective domain wall mass model is developed which allows the variation of the azimuthal angle of the spins in a moving wall when an in-plane field is applied in the plane of the wall. The new model gives results which are in much better agreement with the experimental results than previous models. Reasonable agreement is also observed between theory and experiment when the cubic anisotropy is included. Experimental evidence of the Hubert wall structure and its change for in-plane fields less than 8M is also reported.     

金属间化合物DyMn_2Ge_2的自发磁相变和场诱导的磁相变

在 10— 80 0K的温度范围内用X射线衍射方法测量了DyMn2 Ge2 化合物的晶格常数与温度的变化关系 ,观察到高温时DyMn2 Ge2 由顺磁状态到反铁磁状态的自发磁相变伴随着晶格常数a的负的磁弹性异常现象 .在4 2K— 2 0 0K的温度范围内测量了DyMn2 Ge2 的交流磁化率 .在交换相互作用的分子场模型近似下 ,从理论上分析讨论了DyMn2 Ge2 的低温自发磁相变和场诱导的磁相变 .计算了DyMn2 Ge2 单晶的磁化强度与温度的变化关系以及不同温度下外磁场沿晶轴c方向时的磁化曲线 .理论分析和计算结果表明 ,温度低于 33K时在DyMn2 Ge2 中观察到的场诱导的一级磁相变为由亚铁磁状态 (Fi)到中间态 (IS)相变 .     

On the origin of giant magnetocaloric effect and thermal hysteresis in multifunctional α-FeRh thin films

We report temperature and field dependent lattice structure, magnetic properties and magnetocaloric effect in epitaxial Fe₅₀Rh₅₀ thin films with (001) texture. Temperature-dependent XRD measurements reveal an irreversible first-order phase transition with 0.66% lattice change upon heating/cooling. First-principle calculation shows a state change of Rh from non-magnetic (0 μ_B) for antiferromagnetic phase to magnetic (0.93 μ_B) state for ferromagnetic phase. A jump of magnetization at temperature of 305 K and field more than 5 T indicates a field-assisted magnetic state change of Ru that contributes to the jump. Giant positive magnetic entropy change was confirmed by isothermal magnetization measurements and an in-situ temperature rise of 15 K. The magnetic state change of Rh between antiferromagnetic and ferromagnetic states is the main origin of giant magnetic entropy change and large thermal hysteresis observed.     

Helimagnetism in the cubic Laves phase YMn2

Neutron diffraction experiments on YMn₂ using a wave length of λ_N = 2.483 Å show a splitting of the magnetic peaks. The magnetic structure is helimagnetic consistent with an angle modulation of the previously reported antiferromagnetic structure. The NMR spectrum can be explained as arising from a perturbation of the helix by the magnetocrystalline anisotropy. Below T_N the observed frustration of the negative Mn interactions is inherent from the topology of the crystallographic structure. Above T_N, it creates short range ordering whose thermal decrease may explain the increase in the paramagnetic neutron scattering as the temperature is increased.     

EPR,optical absorption and superposition model studies of Fe3+-doped cesium chloride single crystals: a case of substitutional as well as interstitial sites

An electron paramagnetic resonance study of Fe³⁺-doped cesium chloride single crystals was carried out at room temperature. Three sites are observed. The spin Hamiltonian parameters were determined from the angular variation of the observed resonance lines. The hyperfine structure is observed due to the presence of Fe⁵⁷ centers. At site I, Fe³⁺ enters the lattice substitutionally, replacing Cs⁺ in the cubic symmetry of the crystal, whereas at sites II and III, Fe³⁺ enters the lattice interstitially. The local site symmetry of Fe³⁺ in the host lattice is considered to be orthorhombic. An optical absorption study of the crystal was also performed at room temperature. The observed bands were assigned and the Racah inter-electronic repulsion parameters (B and C) and the cubic crystal field splitting parameter (Dq) were determined. On the basis of EPR and optical data, the nature of the metal–ligand bonding in the crystal was determined. The crystal field parameters were evaluated using the superposition model and then used in the microscopic spin Hamiltonian and perturbation equations to determine the zero-field splitting parameters (ZFSPs) theoretically for all sites observed. The theoretical ZFSPs are in good agreement with the experimental values.