Susceptibility,hysteresis and magnetization states in the CuMn spin glass

AC susceptibilities have been measured at T ⪡ T_f on a quenched Cu-8.25 at% Mn sample along the hysteresis curve and especially in remanent magnetization states. Along the M(H)-curve _ϰ∥ starts to decrease already below the S-point field H_w, when dM/dM is still increasing. After reduction of the external field from H ⪢ H_w to zero ϰ_⊥ by far exceeds ϰ _∥, which itself is a little increased above its starting (ZFC) value. Reversing the external field ϰ_∥ develops distinct maxima at the step fields H_st. A model is developed considering the spin glass at M_r to consist of magnetic moments bound by different anisotropies to preferential directions which are oriented partly at random (anisotropy field H¹_w), partly along M_r (anisotropy field H H¹_st ⪡ H¹_w). The model connects both susceptibilities with characteristic parameters of the hysteresis and quantitatively explains the data.     

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     

Lattice location of Mn in the spin glass CuMn

The lattice location of Mn in a Cu10 at.% Mn single crystal has been determined using ⁴He ion channeling and ion-induced X-rays. It is found that the Mn atoms are 100% in substitutional sites.     

Mesoscopic effects in adiabatic spin pumping

We show that temporal shape modulations (pumping) of a quantum dot in the presence of spin-orbital coupling lead to a finite dc spin current. Depending on the strength of the spin-orbit coupling, the spin current is polarized perpendicular to the plane of the two-dimensional electron gas, or has an arbitrary direction subject to mesoscopic fluctuations. We analyze the statistics of the spin and charge currents in the adiabatic limit for the full crossover from weak to strong spin-orbit coupling.     

Nonlinear static susceptibility of the Ising spin glass in chaotic and transverse fields

The static volume (linear and nonlinear cubic) magnetic susceptibility is calculated for the Ising spin glass in random longitudinal and transverse magnetic fields. The phase transition from the paramagnetic state to the spin glass state is investigated. Divergence of the nonlinear susceptibility indicative of the phase transition is established. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–6, October, 2005.     

Theory and calculations for a spin glass

We obtain the properties of a mean-field spin-glass (in which the bonds connecting each spin to every other spin are “frozen-in” with random signs), by locating the zeros of the partition function in the complex T plane. For N = 5 and 9 spins, we obtain the relevant polynomials and zeros explicitly, and the resulting thermodynamic properties (free energy, specific heat, magnetic susceptibility, etc.). We then analyze the properties of such a system in the thermodynamic limit N → ∞, where it is impossible to obtain the polynomials directly but where the presumed location of the zeros can be usefully construed. In this limit, the thermodynamic functions are obtainable as functions of the distribution functions of monopoles, quadrupoles, and possibly higher-order poles.     

Numerical results for spin glass ground states on Bethe lattices: Gaussian bonds

The average ground state energies for spin glasses on Bethe lattices of connectivities r = 3,...,15 are studied numerically for a Gaussian bond distribution. The Extremal Optimization heuristic is employed which provides high-quality approximations to ground states. The energies obtained from extrapolation to the thermodynamic limit smoothly approach the ground-state energy of the Sherrington-Kirkpatrick model for r ↦ ∞. Consistently for all values of r in this study, finite-size corrections are found to decay approximately with ~N^-4/5. The possibility of ~N^-2/3 corrections, found previously for Bethe lattices with a bimodal ± J bond distribution and also for the Sherrington-Kirkpatrick model, are constrained to the additional assumption of very specific higher-order terms. Instance-to-instance fluctuations in the ground state energy appear to be asymmetric up to the limit of the accuracy of our heuristic. The data analysis provides insights into the origin of trivial fluctuations when using continuous bonds and/or sparse networks.     

Investigation of the NMR enhancement factor in CuMn spin glasses

The NMR enhancement factor of a frozenCuMn spin glass has been measured at a temperatureT?T _g /5. The measurements were performed as function of static magnetic fields of different directions. A two component model of a spin glass has been outlined. One component being a “system of single spins” and the other one being a “subsystem of clusters”. Both components were attributed to different kinds of interaction being RKKY and dipole interaction respectively. The effective anisotropy field of the single spin system consists of two unidirectional contributionsH _a ^s andH _a ^c , which have been measured for different conditions. A second anisotropy fieldH _d binds the cluster system to the system of single spins. All anisotropy fields depend on the annealing temperature of the alloys.     

Mesoscopic superposition states in relativistic Landau levels

We show that a linear superposition of mesoscopic states in relativistic Landau levels can be built when an external magnetic field couples to a relativistic spin 1/2 charged particle. Under suitable initial conditions, the associated Dirac equation produces unitarily superpositions of coherent states involving the particle orbital quanta in a well-defined mesoscopic regime. We demonstrate that these mesoscopic superpositions have a purely relativistic origin and disappear in the nonrelativistic limit.     

Ground states and defect energies of the two-dimensional XY spin glass from a quasiexact algorithm

We employ a novel algorithm using a quasiexact embedded-cluster matching technique as minimization method within a genetic algorithm to reliably obtain numerically exact ground states of the Edwards-Anderson XY spin-glass model with bimodal coupling distribution for square lattices of up to 28 x 28 spins. Contrary to previous conjectures, the ground state of each disorder replica is nondegenerate up to a global O(2) rotation. The scaling of spin and chiral defect energies induced by applying several different sets of boundary conditions exhibits strong crossover effects. This suggests that previous calculations have yielded results far from the asymptotic regime. The novel algorithm and the aspect-ratio scaling technique consistently give theta(s)=-0.308(30) and theta(c)=-0.114(16) for the spin and chiral stiffness exponents, respectively.     

Twist free energy in a spin glass

The field theory of a short range spin glass with Gaussian random interactions, is considered near the upper critical dimension six. In the glassy phase, replica symmetry breaking is accompanied with massless Goldstone modes, generated by the breaking of reparametrization invariance of a Parisi type solution. Twisted boundary conditions are thus imposed at two opposite ends of the system in order to study the size dependence of the twist free energy. A loop-expansion is performed to first order around a twisted background. It is found, as expected but it is non trivial, that the theory does renormalize around such backgrounds, as well as for the bulk. However two main differences appear, in comparison with simple ferromagnetic transitions: (i) the loop expansion yields a (negative) anomaly in the size dependence of the free energy, thereby lifting the lower critical dimension to a value greater than two (ii) the free energy is lowered by twisting the boundary conditions. This situation is common in spin glasses, reflecting the non-positivity of mode multiplicity in replica symmetry breaking, but its physical meaning is still unclear. Received 12 April 2002 / Received in final form 30 July 2002 Published online 19 November 2002     

Invariant states of a thermally conducting barrier

For an infinite one-dimensional system representing a thermally conducting barrier and two semi-infinite reservoirs which it separates, we prove the existence of a unique stationary probability distribution, to which essentially any initial distribution converges for large times.Research partially supported by NSF grant No. PHY-8003298.Research partially supported by NSF grant No. PHY-8201708.     

Off-equilibrium fluctuation-dissipation relation in a spin glass

We report new experimental results obtained on the insulating spin glass CdCr_2-2x In_2x S₄. Our experimental setup allows a quantitative comparison between the thermo-remanent magnetisation and the autocorrelation of spontaneous fluctuations of magnetisation, yielding a complete determination of the fluctuation-dissipation relation. The dynamics can be studied both in the quasi-equilibrium regime, where the fluctuation-dissipation theorem holds, and in the deeply ageing regime. The limit of separation of time-scales, as used in analytical calculations, can be approached by use of a scaling procedure.Received: 2 March 2004, Published online: 9 September 2004PACS: 05.70.Ln Nonequilibrium and irreversible thermodynamics - 75.50.Lk Spin glasses and other random magnets - 07.20.Dt Thermometers - 07.55.Jg Magnetometers for susceptibility, magnetic moment, and magnetization measurementsD. Hérisson: Present address: Department of Engineering Sciences - Division of Solid State Physics, Uppsala University, 751 21 Uppsala, SwedenM. Ocio: deceased 21 December 2003