Mechanism for resistivity minimum in amorphous ferromagnets

A mechanism for electron spin flip scattering in amorphous ferromagnets is proposed and shown to give rise to the observed resistivity minimum in these materials.     

The limit diffusion mechanism of relaxation for spin systems

The diffusion limit theorem for stochastic differential equations is applied to analyse the dynamical evolutions of spin systems. Bloch equations are derived and the stability of asymptotic evolutions is proved. The theory is applied to nuclear magnetic relaxation of two spins.     

Why spin ice obeys the ice rules

The low-temperature entropy of the spin ice compounds, such as and , is well described by the nearest-neighbor antiferromagnetic Ising model on the pyrochlore lattice, i.e., by the "ice rules." This is surprising since the dominant coupling between the spins is their long ranged dipole interaction. We show that this phenomenon can be understood rather elegantly: one can construct a model dipole interaction, by adding terms of shorter range, which yields precisely the same ground states, and hence entropy, as the nearest-neighbor interaction. A treatment of the small difference between the model and true dipole interactions reproduces the numerical work by Gingras et al. in detail. We are also led to a more general concept of projective equivalence between interactions.     

Anisotropy of the low temperature resistivity of hexagonal single crystalline spin glass systems

The impurity contribution to the resistivity in zero field (T) of dilute hexagonal single crystals of ZnMn, CdMn and MgMn has been studied in the mK range on samples cut parallel () and perpendicular () to thec-axis, using a SQUID technique for the measurements. Typical spin glass behavior is found in (T) as well as (T) for all alloys, with Kondo like logarithmic increases at higher temperatures and maxima atT _m at lower temperatures, indicating the influence of impurity interactions. The differences in the corresponding isotropic resistivity _poly(T) between the three systems can qualitatively be understood within the framework of a theoretical model by Larsen, describing (T) as a function of universal quantitiesT/T _K and _RKKY/T _K , where _RKKY is the RKKY-interaction strength andT _K the Kondo temperature. With respect to the two lattice directions studied, the behavior of (T and (T is anisotropic in the Kondo regime as well as in the range where ordering becomes important. While the anisotropy in the Kondo slope can be understood by an anisotropic unitarity limit, the understanding of the anisotropy in region where impurity interactions are important remains problematic.Dedicated to Prof. Dr. S. Methfessel on the occasion of his 60th birthday     

Proton spin relaxation in hexagonal ice

Measurements of the rotating frame proton spin relaxation timeT _1p in hexagonal ice single crystals as a function of temperature ? for various rotating magnetic field strengths reveal the expectedT _1p minimum at the lowest practicable field values. This allows a very precise determination of the proton correlation (? molecular jump) time τ_c and the related activation energy ΔE by means of the theoretical reasoning of relaxation spectroscopy. We find the Arrhenius-law temperature dependenceτ _c=1.99×10^?17exp(0.603/8.61×10^?5 ?)sec, which is in good agreement with our earlier indirect derivation.     

On the resistivity minimum in amorphous ferromagnetic alloys

We present a theoretical approach which, taking into account the amorphous nature of the system and the correlations between nearest-neighbour magnetic ions, gives a plausible explanation of some features present in the electrical resistivity of amorphous ferromagnetic alloys with transition metals.     

Kondo effect in spin glasses: Electrical resistivity

We investigate the influence of the Kondo effect on the electrical resistivity of spin glasses. Our approximation reduces in the limit of vanishing impurity concentrations to the Suhl-Nagaoka theory for the Kondo effect. The magnetic impurity interactions are taken into account in the form of time dependent two-spin correlation functions which can be measured by neutron scattering. The dynamics of the impurity spins leads to a partial destruction of the Kondo effect. For the resistivity this can be described by a temperature dependent effective spinS _eff ² (T) withS _eff ² 0 forT0 andS _eff ² S(S+1) forT, and by a reduction of the Kondo temperatureT _K. Sufficiently strong interactions lead toT _K=0. We obtain a resistivity maximum at a temperatureT _m due to the interplay of the Kondo effect and the spin dynamics;T _m depends onT _K and on the excitation spectrum, and therefore on parameters such as impurity concentration or pressure. The ratioT _m/T_f (T _f is the freezing temperature) is calculated for a single relaxation time and for a square density of relaxation modes and is compared with experimental data forAuCr,AuMn,AgMn, andCuMn. The influence of other possible modes on various spin glass properties is discussed.SFB 125 Aachen-Jülich-Köln     

Defect equilibria and conduction mechanisms in ice

Dielectric and elastic relaxation processes in ice may be explained by means of two types of point defects. At the crossover they interchange their roles as majority and minority mechanism. From conductivity measurements at the crossover it is possible to determine the defect mobilities, without knowing the effective charges. Measurements from different laboratories lead to the same results and to the conclusion that in contrast to earlier theoretieal ideas the ion states as such do not play an essential role in the Debye relaxation. The two mechanisms responsible for this process are Bjerrum defects as envisaged in earlier concepts and Bjerrum-ion aggregates. These defects allow an explanation of the temperature dependence of the conductivity in pure and doped ice.     

Nonmonotonic zero-point entropy in diluted spin ice

Water ice and spin ice are important model systems in which theory can directly account for "zero-point" entropy associated with quenched configurational disorder. Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with nonmagnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90%. Investigations of both Ho and Dy spin ices reveal that the zero-point entropy depends nonmonotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice.     

Non-universality for ising-like spin systems

Renormalisation calculations are carried out which reasonably reproduce the interaction-dependent critical point exponents of the Baxter model. Similar calculations indicate non-universal behaviour of a nearest and next-nearest neighbour pair-interaction Ising model in two dimensions.     

Microscopic mechanism for manipulating thermal conduction

Science China Physics, Mechanics & Astronomy - The concentrating efficiency of a thermal concentrator can be reflected in the ratio of its interior to exterior temperature gradients, which,...     

Dynamics of magnetic charges in artificial spin ice

Artificial spin ice has been recently implemented in two-dimensional arrays of mesoscopic magnetic wires. We propose a theoretical model of magnetization dynamics in artificial spin ice under the action of an applied magnetic field. Magnetization reversal is mediated by domain walls carrying two units of magnetic charge. They are emitted by lattice junctions when the local field exceeds a critical value Hc required to pull apart magnetic charges of opposite sign. Positive feedback from Coulomb interactions between magnetic charges induces avalanches in magnetization reversal.     

Diagonalization-free implementation of spin relaxation theory for large spin systems

The Liouville space spin relaxation theory equations are reformulated in such a way as to avoid the computationally expensive Hamiltonian diagonalization step, replacing it by numerical evaluation of the integrals in the generalized cumulant expansion. The resulting algorithm is particularly useful in the cases where the static part of the Hamiltonian is dominated by interactions other than Zeeman (e.g. in quadrupolar resonance, low-field EPR and Spin Chemistry). When used together with state space restriction tools, the algorithm reported is capable of computing full relaxation superoperators for NMR systems with more than 15 spins.     

Independence of spin and phonon resistivity in some kondo alloys

The temperatures of the resistivity minima (T_min) of several AlMn, AlCr, ThU, ZnMn and ZnCr alloys are calculated by using the empirical expressions for the phonon resistivities. The comparison between the calculated and observed T_min values seems to indicate the approximate additivity of spin and phonon resistivities in these alloys.     

Spinach--a software library for simulation of spin dynamics in large spin systems

We introduce a software library incorporating our recent research into efficient simulation algorithms for large spin systems. Liouville space simulations (including symmetry, relaxation and chemical kinetics) of most liquid-state NMR experiments on 40+ spin systems can now be performed without effort on a desktop workstation. Much progress has also been made with improving the efficiency of ESR, solid state NMR and Spin Chemistry simulations. Spinach is available for download at http://spindynamics.org.     

The resistivity maximum in the ternary spin glass system Au-Cu-Mn

The temperature of the resistivity maximum, T_m, in the ternary spin glass system Au-Cu-Mn has been analysed in terms of Larsen's theory in order to highlight the contribution from the Kondo effect and the RKKY interaction energy to the resistivity maximum in spin glasses. The competition between these two contributions has been effectively illustrated and a good agreement with theory is obtained for samples with varying magnetic and nonmagnetic atom concentration. A comparison of the dependence of T_m and the RKKY interaction energy on the lattice pressure generated due to addition of Au with reported pressure studies on Mn alloys shows that there is a close relation between the lattice pressure and the externally applied pressure.     

Probability estimates for continuous spin systems

Probability estimates for classical systems of particles with superstable interactions [1] are extended to continuous spin systems.