Spontaneous phase transitions in ferrite garnet films

Spontaneous phase transitions in ferrite garnet films have been studied. It has been shown that, with variations in the temperature, domain walls undergo phase transitions which cause spontaneous phase transitions in the lattice of cylindrical magnetic domains. The phase transition in a domain wall causes a spin-reorientation phase transition over the whole sample near the magnetic compensation point. The character of the phase transition in the domain wall determines the mechanism of the spin-reorientation phase transition.     

Specific features in thermal expansion of <Emphasis Type="Italic">R</Emphasis>Fe<Subscript>11</Subscript>Ti single crystals

Thermal expansion and its anomalies in the vicinity of spin-reorientation phase transitions in single crystals of RFe₁₁Ti (R=Y, Tb, Dy, Ho, and Er) compounds are investigated by the tensometric technique in the temperature range 77–400 K. The temperature dependences of the thermal expansion coefficient α(T) are obtained. It is found that the YFe₁₁Ti and HoFe₁₁Ti uniaxial magnetic materials exhibit pronounced anomalies in the α coefficient at T=200 and 290 K. For the TbFe₁₁Ti single crystal, the α coefficient is close to zero in the vicinity of the spin-reorientation phase transition (at T=325 K). For the DyFe₁₁Ti single crystal, which is characterized by two spin-reorientation phase transitions (at T=120 and 250 K), no features in the α(T) dependence are revealed in the region of the low-temperature spin-reorientation phase transition. In the ErFe₁₁Ti single crystal, the specific feature of thermal expansion is observed at T ～ 220 K.     

Effect of an external magnetic field on the dynamics of the nucleus of a new phase near the first-order phase transition in magnetics with defects

The evolution of the nucleus of a new phase near the point of a spin-reorientation first-order phase transition in its magnetics in an external magnetic field is investigated. The effect of the external field and one-dimensional magnetic anisotropy defects on the dynamics of such a nucleus is investigated. Critical fields that promote remagnetization of the sample are noted and evaluated.     

Stabilization of the Polarity of (NdSmDy)(FeCo)B Permanent Magnets for Application in Magnetic Undulators

The temperature dependences of the magnetization of polycrystalline samples of (NdSmDy)(FeCo)B sintered permanent magnets are measured in various magnetic fields by a SQUID magnetometer. Near T = 110 K, the spin-reorientation transition occurs. Bistable magnetic states with two equally possible orientations of the magnetization vector corresponding to different polarities of the permanent magnet are formed in the samples near the spin-reorientation transition. The polarity of the sintered magnets can be stabilized by a small external magnetic field of ～250 Oe. It provides new possibilities for the application of these magnets in cryomagnetic systems such as magnetic undulators.     

Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.     

Effect of an external magnetic field on the orientational phase diagrams of (011) crystalline garnet ferrite plates with combined anisotropy

The effect of an external magnetic field on uniform magnetic states in (011) crystalline garnet ferrite plates with combined anisotropy is theoretically studied. Under the action of the field, the magnetic symmetry of the crystals is reduced and the pattern of spin-reorientation phase transitions, which depends significantly on the magnetic field strength and direction, changes. Curves of critical fields are found, and the general law of magnetization reversal in the (011) plates due to rotation is derived.     

Structure and temperature dependence of the magnetization of the DyFe<Subscript>11</Subscript>Ti nanocrystalline compound

The coercive force, the temperature dependence of the magnetization, and the structure of a DyFeTi alloy based on the DyFe₁₁Ti compound with an excess content of α-Fe in the initial coarse-grained, nanocrystalline, and submicrocrystalline states are investigated experimentally. It is found that, in the submicrocrystalline sample, the coercive force is three times stronger and the temperature of the first spin-reorientation transition is 20 K higher than those in the coarse-grained sample. In the nanocrystalline sample, the coercive force is five times stronger than that in the coarse-grained sample, the first spin-reorientation transition is not revealed, and the transition at the Curie temperature is smeared. It is demonstrated that the changes observed in the magnetic properties are unrelated to the phase transformations but stem from the small size of crystal grains and high imperfection of the structure. The thermal instability of the DyFe₁₁Ti compound is observed in submicrocrystalline and nanocrystalline states.     

Simulation of magnetization reversal processes in finite ferromagnets with defects

The magnetization and magnetization reversal processes that occur through the mechanism of incoherent rotation of magnetic moments in cubic ferromagnets with limited sizes are investigated theoretically. It is established that the appropriate model representation of magnetic inhomogeneities arising in the region of defects is provided by 0° domain walls. The influence of the external magnetic field on the structure and the stability region of the 0° domain walls is determined. This makes it possible to reveal the characteristic features of the magnetization reversal of real crystals as a function of the material and defect parameters, in particular, in the vicinity of the spin-reorientation phase transition.     

化合物HoMn6Sn6的磁晶各向异性及自旋重取向相变研究

采用交换相互作用的分子场理论模型对金属间化合物HoMn₆Sn₆的自旋重取向相变进行了研究. 从理论上计算了HoMn₆Sn₆的易磁化方向以及Ho和Mn离子磁矩与c轴夹角随温度的变化. 基于单离子模型计算了Ho离子的一阶和二阶磁晶各向异性常数K_1R和K_2R随温度的变化. 研究表明，为了很好描述该化合物的自旋重取向相变，必须考虑Ho离子的四阶晶场项及相应的二阶磁晶各向异性常数K_2R，K_2R与K_1R和Mn离子磁晶各向异性常数K_1t之间的相互竞争是导致HoMn₆Sn₆自旋重取向相变的重要因素. 关键词： 稀土-过渡族金属间化合物 自旋重取向 磁晶各向异性     

Collective transition densities in neutron-rich nuclei

Quadrupole transition densities in neutron-rich nuclei in the vicinity of the neutron drip-line are calculated in the framework of the Random Phase Approximation. The continuum is treated by expansion in oscillator functions. We focus on the states which contribute to the usual Giant Quadrupole Resonance, and not on the low-lying strength which is also expected in such nuclei and whose collective character is still under debate. We find that, due to the large neutron skin in these nuclei, the isoscalar and isovector modes are in general strongly mixed. We further show that the transition densities corresponding to the GQR states can be reasonably well described by the collective model in terms of in phase and out of phase oscillations of neutron and proton densities which have different radii.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Theory of spin reorientation in rare-earth orthochromites and orthoferrites

Mechanisms of the temperature-induced spin-reorientation in rare-earth orthochromites (and orthoferrites) are examined. It is concluded that the anisotropic parts of the magnetic interactions between Cr³⁺ (or Fe³⁺) and rare-earth ions, the antisymmetric and the anisotropic-symmetric exchange interactions, are generally responsible for both the rotational and the abrupt types of the spin-reorientations. These anisotropic exchange interactions produce an effective field for the Cr³⁺ up-spins in the direction perpendicular to that of these spins and an effective field for the Cr³⁺ down-spins in the direction opposite to the above. These effective fields favor rotation of the Cr³⁺ spins, retaining their original antiferromagnetic configuration. Thus, as the temperature is lowered, this effective field increases due to the increase of the rare-earth magnetization, and when the interaction energy of the Cr³⁺ spins with these effective fields exceeds the anisotropy energy of the Cr³⁺ ion, spin-reorientation. takes place. At the beginning and ending of the spin-reorientation a second-order phase-transition occurs. The first-order nature of the abrupt spin-reorientation is stressed. Anisotropic exchange interactions between Cr³⁺ and rare-earth ions also play an important role in inducing the abrupt spin-reorientation.     

Effect of Dy and Co substitutions on the spin-reorientation phenomena in Tm2Fe14B alloys

Partial substitution of Fe by Co atoms (up to x = 4) in Tm₂Fe_14−xCo_xB alloys results in an increase of spin-reorientation temperature. This is believed to be due to a change in the magnetocrystalline anisotropy from a plane to an easy axis with increasing temperature. In contrast, gradual replacement of Tm by Dy results in a marked decrease in the spin-reorientation temperature. This is attributed to the stronger crystal field interaction in Dy-rich systems. Both Co and Dy substitutions in Tm₂Fe₁₄B enhance the magnetic ordering temperature. A magnetic phase diagram is proposed on the basis of results obtained in this study.     

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.     

On the magnetic phase transition in terbium ferrite-garnet

It is shown that anomalies on the magnetization curves of the samples of terbium ferrite-garnet (Tb₃Fe₅O₁₂) revealed by Tran Khanh Vien and Dormann [1] are due to the first order spin-reorientation phase transition in iron sublattices. This transition is satisfactorily described by the simple phenomenological theory taking into consideration only anisotropic interactions.     

Magnetic order and spin dynamics in ferroelectric HoMnO3

Hexagonal HoMnO3 is a frustrated antiferromagnet (T(N)=72 K) ferroelectric (T(C)=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin-wave dispersion for the S=2 Mn3+ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy D that is 0.28 meV above the spin-reorientation transition at 40 K and 0.38 meV below. For H parallel c the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically.     

Computer simulation of spin reorientations in polycrystalline materials - application to R2Fe14Ba)

The easy direction of magnetization (DOM) is an extremely important property of a magnetic material, in regards to both permanent magnet and recording applications. In many magnetic materials, most notably Nd₂Fe₁₄B, the DOM is temperature dependent. When utilizing methods which permit the orientation of either a single crystal or pseudo-single crystal to be rigidly constrained (e.g., VSM and extraction), it is possible to determine the temperature dependence of the DOM directly. However, the determination of spin-reorientation temperatures with powder samples is not always straighforward. In some cases, a peak is observed in the magnetization versus temperature curve, while in other either a drop or an increase is observed. Using a two-dimensional, two-crystallite grain as a model and assumed temperature dependencies of the anisottropy constants, K₁ and K₂, it is shown that these peaks and other features can be explained by polycrystalline particles (or, equivalently, imperfect alignment of the powder). It is further shown that spin-reorientation temperatures are invariably characterized by the inflection points in the low field magnetization versus temperature curve. Examples of such curves obtained from a Faraday balance for Nd₂Fe₁₂Si₂B and Pr₂Co₁₄B are given.     

The Influence of Hydrogen Adsorption on Magnetic Properties of Ni/Cu(001) Surface

Ni/Cu(001) is known as a unique system showing the spin-reorientation transition from an in-plane to out-of-plane magnetization direction when the Ni-overlayer thickness is increased. We investigate different relaxed multilayer structures with a hydrogen adlayer using the full-potential linearized augmented plane-wave method. The relaxed geometries, determined by total energy and atomic force calculations, show that H-monolayer strongly influences the interlayer distance between the Ni-surface and sub-surface layers yielding the outward relaxation of Ni-layer at H/Ni interface. Furthermore, large decrease of local magnetic moments at the top surface area is found for the surface covered by H. The magneto-crystalline anisotropy energies are calculated for fully relaxed H/Ni-films. The spin-reorientation transition critical thickness of 4 ML is found in good quantitative agreement with the experiment.     

Magnetic properties of HoFe3(BO3)4

The magnetic properties of an antiferromagnet with trigonal symmetry, namely, HoFe₃(BO₃)₄, have been investigated theoretically. The calculations have been performed in the molecular field approximation and in the framework of the crystal field model for the rare-earth subsystem. Extensive experimental data on the magnetic properties of HoFe₃(BO₃)₄ have been interpreted and good agreement between theory and experiment has been achieved using the obtained theoretical dependences. The spontaneous spin-reorientation transition and the spin-reorientation transition induced by a magnetic field B ‖ a from the easy-axis to easy-plane state, as well as the spin-flop transition in a magnetic field B ‖ c, have been described. It has been shown that the spontaneous spin-reorientation transition is a magnetic analog of the Jahn-Teller effect. The temperature dependences of the initial magnetic susceptibility at temperatures ranging from 2 to 300 K, the nonlinear curves of magnetization for B ‖ c and B ⊥ c in a magnetic field up to 1.2 T (which indicate the occurrence of first-order phase transitions), and their evolution with variations in the temperature have been described, as well as the temperature and field dependences of the magnetization in a magnetic field up to 9 T. The parameters of the trigonal crystal field for the rare-earth ion Ho³⁺ and the parameters of the Fe-Fe and Ho-Fe exchange interactions have been determined in the course of interpretation of the experimental data.     

Influence of one-dimensional “defects” on the new-phase nucleation dynamics near the first-order transition in magnets

The evolution of a new-phase nucleus near the first-order spin-reorientation phase transition in magnets has been investigated. A strong influence of one-dimensional magnetic anisotropy “defects” on the nucleation dynamics has been shown. The conditions of the localization of the new-phase nucleus in the region of the magnetic anisotropy “defect” have been determined.