Dynamics of topological defects in a spiral: a scenario for the spin-glass phase of cuprates

We propose that the dissipative dynamics of topological defects in a spiral state is responsible for the transport properties in the spin-glass phase of cuprates. Using the collective-coordinate method, we show that topological defects are coupled to a bath of magnetic excitations. By integrating out the bath degrees of freedom, we find that the dynamical properties of the topological defects are dissipative. The calculated damping matrix is related to the in-plane resistivity, which exhibits an anisotropy and linear temperature dependence in agreement with experimental data.     

Clusters and the spin-glass transition

In this letter, we show that above the spin-glass transition T_G, we have clusters of linked spins. As the temperature is decreased, the clusters grow. T_G is defined by the occurrence of an infinite cluster. The susceptibility is shown to have a kink and the specific heat to be continuous. The frequency dependence of T_G follows a Fulcher law.     

Quantum Ising spin-glass

A random, quantum model of interacting spins, where the spin operators are represented by bilinear combinations of fermions, is studied in the Ising limit and in the static approximation. Assuming replica symmetry we recover basically the results of Sherrington and Kirkpatrick. Effects of quantum fluctuations remain in the determination of the static susceptibility χ as an independent order parameter and the breakdown of the Fischer relation.     

Condensation in the quantum spherical spin-glass

The quantum spherical spin-glass is analyzed by using a replica symmetric theory (RS) and the physical properties of the system can be explained by the existence of a zero frequency condensate below the critical temperature. The free energy coincides with previous results obtained with an annealed average of the partition function. A calculation with one step replica symmetry breaking (1S-RSB) shows that the only solution is the replica symmetric one.     

Dynamics of superparamagnetic filaments with finite magnetic relaxation time

In this paper we formulate a model of superparamagnetic filaments with internal dissipative torques due to the action of a rotating magnetic field. It is shown that spirals are formed at both ends of the filament due to the action of the internal torques. These spirals propagate to the center of the filament and collide, forming a compact cluster that rotates in accordance with the rotating magnetic field. These results are in agreement with recent experiments with chains of superparamagnetic beads in a rotating magnetic field.     

On the spin-glass state of some CoAlT alloys

Low-field, low-temperature static susceptibility measurements are reported for some CoAl_1-xT_x alloys with small concentrations of T = Cr, Fe, Co, Ni or Cu. All these alloys show field cooling effects. With increasing amount of T atoms spin-glass, mictomagnetic or ferromagnetic behaviour is observed. The Cr, Co and Cu alloys are in a spin-glass regime and pass subsequently through a cluster glass and reentrant regimes at the examined concentrations whereas the Fe and Ni alloys are mictomagnetic.     

Resonance-induced cluster-mobility: Dynamics of a finite Frenkel-Kontorova model

Statics and dynamics of a finite incommensurate Frenkel-Kontorova chain are studied in the framework of both continuum and discrete models. Since the ground state of the finite chain is not degenerate it cannot freely slide along the substrate, i.e., the finite chain does not possess the “supermobility” of the infinitely long incommensurate chain. Instead, the finite chain shows a “resonance mobility”. Simulating the behaviour of the finite chain in an external field we observe resonances around the lowest frequency in the acoustic branch of the chain spectrum. This frequency depends on the chain size and the degree of incommensurability and corresponds to vibration “in-phase” of all solitons in the chain. The resonance leads to a significantly increased diffusive mobility of clusters of a specific size. This selectively enhanced cluster-mobility persists also in the presence of stochastic forces and friction.     

Acoustic susceptibility of an insulating spin-glass

High frequency acoustic measurements have been performed at temperatures around the freezing pointT _c in a cobalt insulating spin-glass. Assuming a Waller coupling mechanism between the spins and the acoustic wave, the data are interpreted in the framework of a dynamical model which takes into account the modulation of the reaction field. As a result, at the freezing temperature the characteristic relaxation time has no anomaly while the relevant susceptibility presents a maximum. The peculiar frequency dependence (as ^1/2) atT _c of the complex elastic constant is experimentally put in evidence.Associated with the Centre National de la Recherche Scientifique (UA 789)     

Effect of the metal-insulator transition on the spin-glass interaction

The effect of the metal-insulator transition on the spin-glass interaction was studied by measuring the magnetic properties of (MnSi)O_X as a function of oxygen content X. As X varies from 0 to 3.6 one changes from a metallic to an insulating spin-glass. The transition at X ≲ 1 is marked by a sharp decrease in the susceptibility and a change of the Curie-Weiss temperature (θ) from ferromagnetic to antiferromagnetic.     

Investigation of the spin-glass state in γ-Fe-Ni-Mn alloys

The composition region where there are no long-range ferro- and antiferromagnetic order has been determined in γ-Fe-Ni-Mn alloys. In these alloys strong small-angle magnetic neutron scattering has been obtained. The intensity has an unusual temperature dependence which is discussed in the framework of the critical neutron scattering thermodynamic theory. The ground state of these alloys is identified with the spin-glass state in which there are also regions of both short-range ferro- and antiferromagnetic order.