Magnetic properties of BiFeO3 micro-cubes synthesized by microwave agitation

In this article, we present results of field cooled (FC) and zero field cooled (ZFC) magnetization measurements and investigation of aging and memory effect in bismuth ferrite (BFO) multiferroic micro-cubes obtained by means of simple microwave synthesis procedure. It is found that difference between FC and ZFC magnetizations appear at the temperature of freezing of ferromagnetic domain walls. The decay of the magnetic moment versus time described by power-law relation and the absence of memory effect are caused by domain growth mechanism rather than by the spin-glass phase. The negligible value of remnant magnetic moment indicates that BFO compound exhibits low concentration of ferromagnetic domains and can be close to ferromagnetic to spin-glass transition.     

The magnetic and superconducting phase diagram of Gd-doped lanthanumsulfides

The system La_3-x( )_xS₄ [where ( ) denotes a vacancy] containing up to 25 at.% Gd is shown to be a model system in which magnetic interactions in metals can be studied. By measuring the temperature dependence of the low field a.c. susceptibility, the superconducting, spin-glass and ferromagnetic transitions can be determined as a function of the Gd concentration. The depression of the superconducting transition temperature mainly follows the theory of Abrikosov and Gor'kov, with a possibility for coexistence of superconductivity and spin-glass magnetic order near a critical concentration of 3 at.% Gd. The transition from spin-glass to ferromagnet is well-defined with the percolation limit for the long range ferromagnetic order at 14 at.% Gd.     

Magnetism in FeNiW disordered alloys: Experiment and theory

We report here an experimental study of magnetization of FeNiW alloys at different compositions. We have studied variation of magnetization with temperature (at low external fields) and magnetic field (at low temperatures). The alloy shows para to ferromagnetic transitions across the composition range. We do not find any indication of the spin-glass phase. We have supplemented the experimental work with theoretical analysis using the first-principles tight-binding linear muffin-tin orbitals based augmented space recursion method. Our theoretical estimates of magnetic moment and Curie temperatures agree well with experiment. Our mean-field phase analysis also does not indicate the possibility of a spin-glass phase.     

Induced random fields in the LiHoxY1-xF4 quantum Ising magnet in a transverse magnetic field

The LiHoxY1-xF4 magnetic material in a transverse magnetic field Bx x perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in a random disordered system. We show that the Bx-induced magnetization along the x direction, combined with the local random dilution-induced destruction of crystalline symmetries, generates, via the predominant dipolar interactions between Ho3+ ions, random fields along the Ising z direction. This identifies LiHoxY1-xF4 in Bx as a new random field Ising system. The random fields explain the rapid decrease of the critical temperature in the diluted ferromagnetic regime and the smearing of the nonlinear susceptibility at the spin-glass transition with increasing Bx and render the Bx-induced quantum criticality in LiHoxY1-xF4 likely inaccessible.     

Double-Cusp Susceptibility of Quantum Heisenberg Spin Glasses: Ferromagnetic Coupling and Anisotropy interaction

With the help of the generalized static replica symmetric spin-glass theory, a quantum Heisenberg S = 112 spin-glass model with the infinite-ranged random Dzyaloshinskii-Moriya (DM) interactions and ferromagnetic coupling is investigated. The dependence of the local susceptibility and the corresponding order parameters on temperature is calculated numerically for different ferromagnetic interactions and fixed anisotropy It has been found that the local susceptibility exhibits double-cusp features for different ferromagnetic coupfings (J_o). In addition, the dependence of the spontaneous moment on temperature is studied numerically, too.     

Study of the magnetic superconductor,Y9Co7, at high pressure and high magnetic field

We have measured the electrical resistance and magnetic susceptibility of the magnetic superconductor, Y_gCo₇, at pressures up to 20 kbar and in magnetic fields up to 6T. We have found that pressure suppresses the magnetism resulting in a higher superconducting transition temperature and conclude that Y₉Co₇ is an itinerant ferromagnet, not a spin-glass. Pressure also sharpens the superconducting transition and increases the critical magnetic field, signifying that the long range ferromagnetic and superconducting order parameters co-exist but vary spatially. For pressures greater than 6 kbar, the magnetoresistance is always positive, further indicating the suppresion of magnetism by high pressure.     

Magnets with random uniaxial anisotropy: Thermodynamic properties in the large-N limit

We show that the random-axis model lends itself to a systematic large-N calculation. The model shows different behavior below and above four dimensions. The equation of state is derived and discussed in terms of “Arrott” plots. Higher-order terms in the disorder, when summed, have a crucial effect on the susceptibility which is found to be finite below four dimensions (and above four dimensions for strong disorder). A spin-glass to paramagnetic phase transition is characterized by the vanishing of the Edwards-Anderson order parameter, which differs from zero in the spin-glass phase. A cusp in the specific-heat and susceptibility is seen across the transition. The cross-over exponent and other exponents of interest are calculated. Above four dimensions a third phase appears for weak disorder and low-temperature ferromagnetic in nature. The transverse and longitudinal susceptibilities are discussed. Whereas the ferromagnetic transition is characterized by mean-field exponents, the ferromagnetic to spin-glass exponents are equal to their counterparts in the non-random system in d ? 2 dimensions. This is shown to originate from an effective random field proportional to the EA order parameter. The flow equations in the large-N limit are also discussed.     

Quantum XY Spin-Glass Model with Ferromagnetic Couplings and Plane Dzyaloshinski-Moriya interactions

In the generalized quantum replica symmetric approximation, a quantum XY spin-glass model with ferromagnetic coupling and planar Dzyaloshinski-Mariya (DM) interactions is investigated theoretically. Entropy and specific heat are calculated numerically as well as spin self-interactions and spin-glass order parameters for spin S = 1. It is found that the doublecusp of the specific heat, which corresponds to the collinear ferromagnetic and the paramagnetic ferromagnetic transitions respectively, .coincides completely due to DM-anisotropy interactions. Additionally, the local susceptibility and the spontaneous magnetization are evaluated numerically.     

Quantum spin glass and the dipolar interaction

Anisotropic dipolar systems are considered. Such systems in an external magnetic field are expected to be a good experimental realization of the transverse field Ising model. With random interactions, this model yields a spin glass to paramagnet phase transition as a function of the transverse field. We show that the off-diagonal dipolar interaction, although effectively reduced, induces a finite correlation length and thus destroys the spin-glass order at any finite transverse field. We thus explain the behavior of the nonlinear susceptibility in the experiments on LiHo(x)Y(1-x)F(4), and argue that a crossover to the paramagnetic phase, and not quantum criticality, is observed.     

Thermoelectric power of RAgSb(2) (R?=?Y, La, Ce, and Dy) in zero and applied magnetic fields

We report the experimental results of measurements of the thermoelectric power on the ternary intermetallic compounds RAgSb(2) (R = Y, La, Ce, and Dy) over the temperature range from 2 to 300 K and in magnetic fields up to 140 kOe. In this work, we present the thermoelectric transport properties of four materials from the same family with different ground states: a non-moment bearing paramagnetic metallic system (YAgSb(2)), a non-moment bearing charge density wave system (LaAgSb(2)), a local moment bearing compound with XY-like antiferromagnetic order in the tetragonal basal plane as well as readily accessible metamagnetism (DyAgSb(2)), and a Kondo lattice system with ferromagnetic order below T(C) = 9.7 K (CeAgSb(2)). The thermoelectric power data from these materials exhibit complex temperature and magnetic field dependences, which are associated with modification of the electronic density of states and changes in magnetic scattering. At low temperatures, quantum oscillations in the thermoelectric power are also observed. These oscillations are associated with the Landau quantization of electronic energy in an applied magnetic field.     

Bifurcation of a periodic instanton and quantum－classical transition in the biaxial nano－ferromagnet with a magnetic field along hard axis

Crossover from classical to quantum regimes of the barrier transition rate in a biaxial ferromagnetic magnet with a magnetic field applied along hard anisotropy axis is investigated. We show that the type of action-temperature diagrams can he determined by counting the number of bifurcation points. The model possesses not only the known type I and Ⅱ, hut also the interesting type Ⅲ and Ⅳ of transition which do not occur in general.     

Gelation as a rod-glass transition

A sol-gel transition of rod-like macromolecules in solution is viewed as a rod-glass transition, similar to the spin-glass transition in dilute magnetic systems. A molecular field theory for the order parameters is presented. Varying the disorder parameter an ordered nematic-like phase is found for zero disorder, exhibiting first order transition, while a rod-glass second order transition is found for maximal disorder. A remanent nematic-like order always accompanies the glass phase, unlike the case of magnetic spin-glass.     

A quantum Monte Carlo study of the localizable entanglement in anisotropic ferromagnetic Heisenberg chains

We study the localizable entanglement in large one-dimensional anisotropic XYZ ferromagnetic Heisenberg chains interacting with a uniform magnetic field. With the use of quantum Monte Carlo simulations we calculate the bounds of localizable entanglement by means of the correlation functions. The present quantum Monte Carlo method has the advantage over existing methods that it can be readily applied to fully anisotropic magnetic chains.     

Quantum SK S = 1 Spin Glass with Uniaxial Interactions

The quantum Sherrington-Kirkpatrick (SK) S = 1 spin-glass model with uniaxial anisotropy in applied magnetic field is investigated numerically. The dependences of entropy, specific heat and susceptibility on temperature are calculated for different magnetic fields. The dependence of specific heat on temperature exhibits the typical spin-glass characteristics. It is found that the susceptibility depends strongly on the sign of uniaxial aniso tropy cons tan ts.     

Magnetic transition and Meissner shielding in ferromagnetic high-Tc superconductor RuSr2Gd1.4Ce0.6Cu2O10-δ

The magnetic superconductor RuSr₂Gd_1.4Ce_0.6Cu₂O_{10- δ} was studied by means of detailed magnetization measurements in order to clarify its complex magnetic and superconducting properties. Our experiments agree with a model indicating that the in-plane component of the Ru moment might transform from a spin-glass ground state to a ferromagnetically coupled excited state by application of a small magnetic field below the magnetic transition temperature. The experiments also show that a full Meissner state is present at the low temperature, the low magnetic field region of the phase diagram. This is in contrast to the previous prediction that no Meissner state exists in this material for an internal magnetic field of several hundred Oersteds in the CuO₂ planes exerted by the Ru sublattice and exceeding the lower critical field.     

THE POSSIBLE ORIGIN OF STRCNG MAGNETIC FIELD OF NEUTRON STAR:FERROMAGNETIC STATE

The magnetic susceptibility of neutron matter was calculated by means of Owen's LOCV method. The results showed that a transition to a ferromagnetic state can exist for HJ, IY and M-S potentials, but there is no transition for Reid soft-core potential .By analyses, we concluded that the ferromagnetic state is a possible origin of strong magnetic field within the neutron star. We also considered an influence of the ferromagnetic state on the mass, radius and moment of inertia of the neutron star.Finally we discussed the effect of magnetization on physical state in the neutron star.     

Mott-Anderson transition controlled by a magnetic field in pyrochlore molybdate

The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metallic and the spin-glass insulating state. This metal-insulator transition is governed on a large energy scale by the electron-correlation effect, while the geometrical frustration causes the random potential. The magnetic field can tune the randomness of the potential and control, under a suitable pressure, the continuous Mott-Anderson transition precisely. The critical exponent (mu = 1.04 +/- 0.1) of the Mott-Anderson transition has been determined for this ferromagnetic orbital-degenerate electron system.     

Induced moment antiferromagnet in an external magnetic field—I: Molecular field theory

The behavior of a two-level induced moment antiferromagnet in an external magnetic field is investigated in the molecular field approximation. A significant reduction in the critical field and in the sublattice magnetizations is shown. However, the total magnetization rises more rapidly with field and can remain at large value in an external field even at T = 0. The magnetic susceptibility also remains finite at T = 0 in contrast to the case of a permanent moment Ising antiferromagnet. The effects of a ferromagnetic next-nearest neighbor interaction are then examined. It is shown that, in contrast to the usual Ising antiferromagnets, the ferromagnetic coupling has to exceed a certain value depending on the crystal field strength and the antiferromagnetic interaction, to allow for a first order phase transition in a field to occur even at zero temperature.     

Monte Carlo simulation of a random-field Ising antiferromagnet

Phase transitions in the three-dimensional diluted Ising antiferromagnet in an applied magnetic field are analyzed numerically. It is found that random magnetic field in a system with spin concentration below a certain threshold induces a crossover from second-order phase transition to first-order transition to a new phase characterized by a spin-glass ground state and metastable energy states at finite temperatures.     

Two-dimensional quantum antiferromagnet in a strong magnetic field

The behaviour of the two-dimensional quantum antiferromagnet of arbitrary spin in a strong transversal magnetic field on a square lattice is studied in terms of the equivalent Bose gas problem. The existence of phase transition from the state characterized by “quasi-long-range” magnetic order to the disordered ferromagnetic state is demonstrated. The expressions for correlation functions, thermodynamical and magnetic characteristics are derived.