Ultrasonic study of dysprosium in a magnetic field

The ultrasonic attenuation of longitudinal waves propagating along the c axis of single crystal dyprosium is reported, as a function of the applied basal plane field in the paramagnetic region, and as a function of the temperature, at constant applied basal plane field, in the spin-spiral region. In the paramagnetic region, anomalous attenuation behavior is explained on the basis of competing spin-polarization and spin-fluctuation effects. Two anomalous maxima in the temperature dependence of the attenuation were observed: one near T_N is attributed to spin fluctuations associated with short range ferromagnetic ordering; another one at 130 K is attributed to a magnetic phase transition from a fanstructure phase, intermediate between the spin-spiral and ferromagnetically ordered phase     

Mean field theory with correlations in space and time for the ferromagnetic phase transition

A mean field theory with correlations in space and time is proposed which implicitly takes into account the effect of thermal spin fluctuations. The theory describes the kinetic critical slowing down of the relaxation of the magnetization near the ferromagnetic phase transition in crystalline and amorphous ferromagnets. Furthermore, it provides an explanation for the large temperature range of apparent “critical” behaviour as observed experimentally for the paramagnetic susceptibility of amorphous ferromagnets.     

Dynamical mean-field theory of a simplified double-exchange model

Simplified double-exchange model including transfer of the itinerant electrons with spin parallel to the localized spin in the same site and the indirect interaction J of kinetic type between localized spins is comprihensively investigated. The model is exactly solved in infinite dimensions. The exact equations describing the main ordered phases (ferromagnetic and antiferromagnetic) are obtained for the Bethe lattice with (z is the coordination number) in analytical form. The exact expression for the generalized paramagnetic susceptibility of the localized-spin subsystem is also obtained in analytical form. It is shown that temperature dependence of the uniform and the staggered susceptibilities has deviation from Curie-Weiss law. Dependence of Curie and Néel temperatures on itinerant-electron concentration is discussed to study instability conditions of the paramagnetic phase. Anomalous temperature behaviour of the chemical potential, the thermopower and the specific heat is investigated near the Curie point. It is found for J=0 that the system is unstable towards temperature phase separation between ferromagnetic and paramagnetic states. A phase separation connected with antiferromagnetic and the paramagnetic phases can occur only at . Zero-temperature phase diagram including the phase separation between ferromagnetic and antiferromagnetic states is given. Received 28 May 1999 and Received in final form 14 July 1999     

Coupled spin, elastic, and electromagnetic waves in the magnetic of a ferromagnetic spiral phase

The spectrum of coupled spin, elastic, and electromagnetic waves in the magnetic of a ferromagnetic spiral phase is investigated. The frequency dependences of the reflection coefficient of electromagnetic waves from the surface of a semi-infinite magnet for different angles of the spiral are obtained. An acoustic analog of the Faraday effect in the magnetics of a ferromagnetic spiral phase is considered.     

Strong enhancement of superconducting T(c) in ferromagnetic phases

It is shown that the critical temperature for spin-triplet, p-wave superconductivity mediated by spin fluctuations is generically much higher in a Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling of the magnons to the longitudinal magnetic susceptibility. Together with the tendency of the low-temperature ferromagnetic transition in very clean Heisenberg magnets to be of first order, this qualitatively explains the phase diagram recently observed in UGe(2).     

Magnetic properties of degenerate atomic Fermi gases

Magnetic effects in a degenerate atomic Fermi gas, such as the exchange enhancement of the paramagnetic susceptibility and the existence of the phase transition to the ferromagnetic state with the spontaneous polarization of the atomic spins, are discussed. The propagation of spin waves in the atomic system is considered.     

Vortex quantum creation and winding number scaling in a quenched spinor Bose gas

Motivated by a recent experiment, we study nonequilibrium quantum phenomena taking place in the quench of a spinor Bose-Einstein condensate through the zero-temperature phase transition separating the polar paramagnetic and planar ferromagnetic phases. We derive the typical spin domain structure (correlations of the effective magnetization) created by the quench arising due to spin-mode quantum fluctuations, and we establish a sample-size scaling law for the creation of spin vortices, which are topological defects in the transverse magnetization.     

Domain-wall induced phase shifts in spin waves

We study the interaction between two important features of ferromagnetic nanoparticles: magnetic domain walls and spin waves. Micromagnetic simulations reveal that magnetostatic spin waves change their phase as they pass through domain walls. Similar to an Aharonov-Bohm experiment, we suggest to probe this effect by splitting the waves on different branches of a ring. The interference of merging waves depends on the domain walls in the branches. A controlled manipulation of spin-wave phases could be the first step towards nanoscaled ferromagnetic devices performing logical operations based on spin-wave propagation.     

Observation of a griffiths phase in paramagnetic La1-xSrxMnO3

We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.     

Effect of spin and charge fluctuations on the electronic structure of ferromagnetic compounds based on rare-earth metals (on an example of EuO)

The influence of spin and charge density fluctuations on the electronic structure is studied for the generalized fd-model. It is demonstrated that these fluctuations lead to electronic spectrum transformation and redistribution of electrons between f- and d-states, thereby giving rise to electronic phase transformations. It is established that these phenomena are observed in ferromagnetic and paramagnetic regions. A detailed analysis is performed on an example of the EuO ferromagnetic semiconductor.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 100–104, October, 2004.     

Coexistence of ferromagnetism and superconductivity close to a quantum phase transition: the Heisenberg- to Ising-type crossover

A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transition is analyzed and the generic phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet ZrZn2, which sheds light on the proposed phase diagram of this compound. A possible explanation of superconductivity in UGe2 is also discussed.     

Stochastic resonance and nonequilibrium dynamic phase transition of Ising spin system driven by a joint external field

The dynamic response and stochastic resonance of a kinetic Ising spin system （ISS） subject to the joint action of an external field of weak sinusoidal modulation and stochastic white-nolse are studied by solving the mean-field equation of motion based on Glauber dynamics. The periodically driven stochastic ISS shows that the characteristic stochastic resonance as well as nonequilibrium dynamic phase transition （NDPT） occurs when the frequency ω and amplitude h0 of driving field, the temperature t of the system and noise intensity D are all specifically in accordance with each other in quantity. There exist in the system two typical dynamic phases, referred to as dynamic disordered paramagnetic and ordered ferromagnetic phases respectively, corresponding to a zero- and a unit-dynamic order parameter. The NDPT boundary surface of the system which separates the dynamic paramagnetic phase from the dynamic ferromagnetic phase in the 3D parameter space of ho-t-D is also investigated. An interesting dynamical ferromagnetic phase with an intermediate order parameter of 0.66 is revealed for the first time in the ISS subject to the perturbation of a joint determinant and stochastic field. The intermediate order dynamical ferromagnetic phase is dynamically metastable in nature and owns a peculiar characteristic in its stability as well as the response to external driving field as compared with a fully order dynamic ferromagnetic phase.     

A renormalization-group study into the effects of competing biquadratic and crystal-field interactions in the Blume–Emery–Griffiths model

The Blume–Emery–Griffiths model, a spin-1 Ising model with bilinear, biquadratic, and crystal field interactions, provides a general system for the analysis of systems driven by fluctuations in density and magnetization. In this study, we consider an exactly solvable system in which frustration is present due to competing biquadratic and crystal-field interactions. Thus, this calculation models a dilute ferromagnetic material with two types of nearest-neighbor site pairs, distinguished by whether or not simultaneous occupation is energetically favored. To determine the effects of this competition, we have constructed exactly solvable frustrated hierarchical models similar to those introduced to study spin glasses. The resulting phase diagrams reveal two distinct paramagnetic phases separated by a plane in parameter space in which the biquadratic interaction and crystal-field strength rescale chaotically. Each paramagnetic phase has a ferromagnetic complement in which the unique distribution of occupied sites possesses a net magnetization.     

Electronic structure and magnetic phase transition in MnSi

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature T _S (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.     

Electronic structure and spin fluctuations in the helical ferromagnet MnSi

The influence of spin fluctuations on the magnetic properties of the ferromagnetic helimagnet MnSi has been studied in the Hubbard model taking into account the antisymmetric relativistic Dzyaloshinskii–Moriya interaction for band electrons. The obtained equations of the magnetic state indicate the correlation between the fine structure of the density of electronic states and the magnetization and coefficient of mode–mode coupling. It has been shown that the position of the Fermi energy in the immediate proximity on the point of the local minimum of the density of electronic states leads to large zero spin fluctuations at low magnetization of the helimagnet. When approaching from down the Néel point (approximately, at 0.9T_N), the zero fluctuation disappear, and the temperature rise of thermal spin fluctuation is accompanied by the change in the sign of the coefficient of mode–mode coupling. A magnetic field perpendicular to the helicoids plane brings about the formation and subsequent “collapse” of the helimagnetic cone. However, the condition of the change in the sign of the coefficient of mode–mode coupling divides the MnSi phase diagram into two parts, one of which corresponds to the ferromagnetic state induced by the field, and the other corresponding to the paramagnetic state. In this case, the h–T diagram has a specific region, inside which the paramagnetic and the ferromagnetic state are instable. The boundaries of the region agree with the experimental data on the boundaries of the anomalous phase (a phase). It has been found that the results of calculations of the temperature dependence of the magnetic susceptibility agree with the experimental data.     

Curie point singularity in the temperature derivative of resistivity in (Ga,Mn)As

We observe a singularity in the temperature derivative drho/dT of resistivity at the Curie point of high-quality (Ga,Mn)As ferromagnetic semiconductors with Tc's ranging from approximately 80 to 185 K. The character of the anomaly is sharply distinct from the critical contribution to transport in conventional dense-moment magnetic semiconductors and is reminiscent of the drho/dT singularity in transition metal ferromagnets. Within the critical region accessible in our experiments, the temperature dependence on the ferromagnetic side can be explained by dominant scattering from uncorrelated spin fluctuations. The singular behavior of drho/dT on the paramagnetic side points to the important role of short-range correlated spin fluctuations.     

Quantum fluctuation driven first-order phase transition in weak ferromagnetic metals

In a local Fermi liquid (LFL), we show that there is a line of weak first-order phase transitions between the ferromagnetic and paramagnetic phases due to purely quantum fluctuations. We predict that an instability towards superconductivity is only possible in the ferromagnetic state. At T?=?0 we find a point on the phase diagram where all three phases meet and we call this a quantum triple point (QTP). A simple application of the Gibbs phase rule shows that only these three phases can meet at the QTP. This provides a natural explanation of the absence of superconductivity at this point coming from the paramagnetic side of the phase diagram, as observed in the recently discovered ferromagnetic superconductor, UGe ₂.     

Magnetic phase diagram for HO-50%tb from anomalies in ultrasonic propagation

Measurements of the elastic constant c₃₃ for Tb-50% Ho, as a function of temperature and magnetic field, have been used to determine the magnetic phase diagram of the alloy. Evidence for the four phases:paramagnetic, spiral spin antiferromagnetic, fan and ferromagnetic are presented. The true ferromagnetic phase is only found below 100 K even in fields of up to 7 Tesla.     

Dynamical phase transitions in the two-dimensional ANNNI model

We study the phase diagram of the two-dimensional anisotropic next-nearest neighbor Ising (ANNNI) model by comparing the time evolution of two distinct spin configurations submitted to the same thermal noise. We clearly see several dynamical transitions between ferromagnetic, paramagnetic, antiphase, and floating phases. These dynamical transitions seem to occur rather close to the transition lines determined previously in the literature.     

Magnetic phase diagrams of axial third-nearest-neighbour Ising model

The magnetic phase diagrams of axial third-nearest-neighbour Ising (A3NNI) model under an external field have been studied by means of the molecular field approximation. The phase boundaries among paramagnetic, (anti)ferromagnetic and various modulated phases are determined by analysing the frequency-dependent susceptibility, or by solving the coupled equations for magnetisations for spins up to 17, iteratively. The resultant phase diagram is found to be very consistent with the exact results for the ground state spin ordering and to be far improved than existing finite temperature phase diagrams for the same model.