Spin glass transition in an Ising spin system with transverse field

The effect of a transverse field Δ on the spin glass freezing temperature for an Ising spin glass model is studied. The freezing temperature is calculated for all values of Δ and it is shown that for Δ greater than twice the exchange interaction no spin glass transition takes place.     

Absence of an equilibrium ferromagnetic spin-glass phase in three dimensions

Using ground state computations, we study the transition from a spin glass to a ferromagnet in 3D spin glasses when changing the mean value of the spin-spin interaction. We find good evidence for replica symmetry breaking up until the critical value where ferromagnetic ordering sets in, and no ferromagnetic spin glass phase. This phase diagram is in conflict with the droplet/scaling and mean field theories of spin glasses. We also find that the exponents of the second order ferromagnetic transition do not depend on the microscopic Hamiltonian, suggesting universality of this transition.     

Spin anisotropy and slow dynamics in spin glasses

We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.     

Fermionic Ising glasses in magnetic transverse field with BCS pairing interaction

We study a fermionic infinited-ranged Ising spin glass with a real space BCS interaction in the presence of an applied transverse field. The problem is formulated in the integral functional formalism where the SU(2) spins are given in terms of bilinear combinations of Grassmann fields. The problem is solved within static approximation and the replica symmetry ansatz combined with previous approaches used to study the critical behavior of the quantum Ising spin glass in a transverse field and the spin glass Heisenberg model with BCS pairing. Our results show that the transverse field has strong effect in the phase boundary of the spin glass phase and the PAIR phase in which there is a long range order corresponding to formations of pairs. The location of the tricritical point in the PAIR phase transition line is also affected.     

Absence of an Almeida-Thouless line in three-dimensional spin glasses

We present results of Monte Carlo simulations of the three-dimensional Edwards-Anderson Ising spin glass in the presence of a (random) field. A finite-size scaling analysis of the correlation length shows no indication of a transition, in contrast with the zero-field case. This suggests that there is no Almeida-Thouless line for short-range Ising spin glasses.     

Spin glass model based on the Onsager reaction field

Changes in the Onsager reaction field are used to account quantitatively for aging (the decrease in the magnetic susceptibility when cooling in zero field is halted below the glass temperature) and rejuvenation (the disappearance of aging phenomena on further cooling only to reappear at Tw on heating) that characterize spin glasses. These effects must be caused by interactions between the spins since, absent the interactions, the magnetic properties of N spins are just N times the magnetic property of a single spin that cannot display aging. A spin introduced at an empty site with a nonzero field becomes polarized, and the polarized spin in turn polarizes its neighbors, thereby changing the local field. This additional field is the Onsager reaction field. Ma's theory for the reaction field in spin glasses [PRB 22, 4484 (1980)10.1103/PhysRevB.22.4484] has been extended to provide a spin-glass model that can account for the experimental data.     

Properties of an adiabatically cooled SK spin glass

Some physical properties of an SK spin glass subject to slow cooling are investigated. Among those are Edwards-Anderson order parameter, energy, field cooled and zero field cooled susceptibilities. The results are compared with predictions from Parisi's solution. Different values are obtained for the field cooled susceptibility whereas other quantities are identical in both calculations. Ultrametricity results in a natural way if adiabatic temperature changes involving cooling and heating periods are considered. Some results are also given for adiabatic cooling in a finite external field and for adiabatic changes of the field at constant temperature. The present setup allows to investigate explicitly the dependence on history at least for adiabatic changes of temperature and external field.     

Thermal studies of the spin liquid state and analog to the He melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG) has an extraordinary low-temperature phase diagram. Although the Curie–Weiss temperature of GGG is −2 K, GGG shows no long-range order down to T0.4 K. At low temperatures GGG has a spin glass phase at low fields (0.1 T), a field-induced long-range ordered antiferromagnetic state at fields of between 0.7 and 1.3 T, and, at intermediate fields, an apparent spin-liquid state without long-range order. We have characterized the intermediate field (IF) state through heat capacity, thermal conductivity, and magnetocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T0.2 K, in analogy to the minimum in the melting curve of ⁴He. The existence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature.     

Spin transverse force on spin current in an electric field

As a relativistic quantum mechanical effect, it is shown that the electron field exerts a transverse force on an electron spin 1/2 only if the electron is moving. The spin force, analogue to the Lorentz for an electron charge in a magnetic field, is perpendicular to the electric field and the spin current whose spin polarization is projected along the electric field. This spin-dependent force can be used to understand the Zitterbewegung of the electron wave packet with spin-orbit coupling and is relevant to the generation of the charge Hall effect driven by the spin current in semiconductors.     

Spin dynamics in the reentrant spin glass ( Fe0.65 Ni0.35)1-x Mnx

The spin dynamics in the reentrant spin glass ( Fe_0.65 Ni_0.35)_1-x Mn_x has been studied by zero, longitudinal and transverse field μSR. In the ferromagnetic reentrant and pure spin glass regimes (x\leqslant 0.175), zero field experiments reveal a stretched exponential muon relaxation with a universal behaviour of the dynamic exponent \beta above the spin glass transition. There are no qualitative differences between the ferromagnetic and paramagnetic phases. In transversal field μSR experiments the divergence of the relaxation rate close to the spin glass transition is suppressed for manganese doping up to x=0.113 but enhanced for slightly higher doping (x\geqslant 0.12). We understand this behaviour as a crossover from an itinerant to a more localized state of the 3d electron system. This is also supported by the fact that in the highly doped regime with dominant antiferromagnetic interactions the muon relaxation rate diverges above the antiferromagnetic transition temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamic glass transition in finite dimensions

Using an effective potential method, a replica formulism is set up for describing supercooled liquids near their glass transition. The resulting potential is equivalent to that for an Ising spin glass in a magnetic field. Results taken from the droplet picture of spin glasses are then used to provide an explanation of the main features of fragile glasses.     

Theory of an induced-moment spin glass

We review the derivation of generalised TAP equations for general quantum spin systems and apply the theory to a simple induced-moment spin glass model. We consider two-level systems with a singlet ground state and a triplet excited state, which interact via long range random exchange couplings. For not too large energy splitting the spin glass state can be stabilised.     

Quantum XY spin glass model with planar Dzyaloshinskii-Moriya interactions in longitudinal field

In the replica symmetric approximation and static limit in Matsubara “imaginary time”, the quantum XY spin glass model with planar Dzyaloshinskii-Moriya interaction in longitudinal field is investigated. Several thermodynamic quantities are calculated numerically as well as spin self-interaction and spin glass order parameter for spin S=1/2. It is shown that the entropy is not independent of the field. A crossover behavior of the specific heat depending on temperature is found. There is a deviation from the parabolic approximation, C/T=A+Bh ² . Received 11 March 1998     

自旋玻璃的重入行为及CoxZn1-x(FeyCr1-y)2O4尖晶石系统的磁相图

本文研究了Co_xZn_1-x(Fe_yCr_1-x2O₄尖晶石系统的磁性,测量了不同成份样品的低频弱场交流磁化率与低场直流磁化强度的温度关系。根据实验结果,给出了该系统可能的磁相图。发现在该系统中相当宽的成份范围内,都存在自旋玻璃的重入现象。同时发现,自旋玻璃的重入温度随磁性离子浓度增加而增加。这些行为是磁性离子浓度含量较高和多种磁性离子共存系统的共同特性。还讨论了自旋玻璃重入行为 关键词：     

PARABOLIC APPROXIMATION OF THE SPECIFIC HEAT IN QUANTUM XY SPIN GLASS MODEL WITH PLANAR DZYALOSHINSKII-MORIYA INTERACTIONS IN LONGITUDINAL FIELD

The quantum infinite range XY spin glass model with the infinite range planar Dzyaloshinskii-Moriya interaction in external field for spin S=1/2 is investigated theoretically. Numerical calculations show that, the specific heat depending on the temperature has crossover behavior; the weak field dependence of the specific heat can be fitted by the parabolic approximation, C/T=A+Bh², the coefficient B is peaked near the transition point.     

Photo-induced Magnetic bubbles in the spin glass Alloy Fe2SnTe4

The ternary material Fe₂SnTe₄ is an amorphous metallic alloy that exhibits spin glass behavior with a freezing temperature of T_f = 12 K. When this material is cooled well below the spin glass freezing temperature (T = 5 K) and placed in a magnetization field, the zero-field-cooled spin glass state has been observed to absorb ultraviolet radiation with the concomitant generation of a magnetic bubble on the surface of the material. The photo-induced magnetic bubbles are detected via magnetization measurements with a superconducting SQUID susceptometer. Photomagnetic experiments are reported on the zero-fieldcooled and field-cooled specimens. Each of these experiments results in the generation of a magnetic bubble in the material.     

Direct observation of chiral susceptibility in the canonical spin glass AuFe

The extraordinary Hall resistivity rho(xy) and the magnetization M of a canonical spin glass AuFe (8 at.% Fe) were measured simultaneously as functions of temperature with the best care to the thermal and the magnetic field hysteresis. The data of rho(xy) show an anomaly at the spin glass transition temperature T(g) and have different zero field cooling (ZFC) and field cooling (FC) measurements below T(g). Moreover, the value of rho(xy)/M, which represents the chiral susceptibility of the system in the present case, also shows the difference between ZFC and FC measurements. The results are consistent with the predictions of the chirality scenario of canonical spin glasses by Kawamura.     

Computer simulation of an amorphous antiferromagnet

Computer simulation has been used to study the ground state properties of a system of antiferromagnetically interacting Ising spins located at the centres of close-random-packed spheres. Significant quasi-degeneracy and low energy barriers are found, analogous to the case of a non-ordering periodically frustrated magnet. The local effective field distribution has a zero-field hole, analogous to a spin glass.     

Magnetic relaxation phenomena in a CuMn spin glass

Experiments on the temperature and time dependence of the response function and the field cooled magnetisation of a Cu(Mn) spin glass at temperatures below the zero field spin glass temperature are used to explore the non-equilibrium nature of the underlying spin configuration. The results imply that a certain spin configuration is imprinted on the system as the temperature is decreased at a constant cooling rate. The cooling rate governs the magnitude of the FC magnetisation ((H,T)). Any intermittent halt at a constant temperature, , imprints an extended spin configuration, a process that is reflected e.g. in a downward relaxation of . On continued cooling at the same rate, the magnitude of (T) remains at a lower level than that of a continuous cooling curve. These results are put into the context of the corresponding behaviour of the response function as observed in measurements of the relaxation of the zero field cooled magnetisation. Received 27 October 1998 and Received in final form 30 November 1998     

Zero-temperature nonequilibrium dynamics of the spherical Scherrington-Kirkpatrick model of a spin glass in an external field

The decay of the nonequilibrium spin-spin autocorrelation function is calculated analytically and numerically based on the Langevin dynamics for the spherical Scherrington-Kirkpatrick model (p = 2) of a spin glass in a constant magnetic field at zero temperature. The behavior of the spin system is analyzed for different amplitudes of the external constant magnetic field and different waiting times. In the mean-field approximation, the critical value of the external magnetic field is found. The occurrence of the spin-glass phase in weak magnetic fields is demonstrated. For stronger fields, the stationary behavior of the correlation function is revealed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 10–16, April, 2007.