Tricritical point in classical spin systems with random transverse fields

The existence of a tricritical point is demonstrated for a classical n-vector model with annealed random transverse fields, and the corresponding tricritical temperature T_t and distribution width σ_t are evaluated within Landau theory for continuous spin dimensionality -2 < n < 10.     

Hyperfine fields in dilute Cr-Rh systems

The hyperfine fields on rhodium nuclei in a chromium matrix have been investigated for rhodium concentrations ranging from 39 to 1000 at. p.p.m. For dilute alloys the hyperfine fields measured at the maximum of the spin density wave of chromium areH _o=28.9±0.4 kOe at 308 K just below the Néel temperature andH _o=45.2±1.0 kOe at 272.6 K. It is shown, in a simple model, that the hyperfine field is proportional to the local magnetization of chromium. Concentration effects have been found which prove the existence of long-range interactions between the impurities in the chromium matrix.     

Aggregation and intermediate phases in dilute spin systems

We study a variety of dilute annealed lattice spin systems. For site diluted problems with many internal spin states, we uncover a new phase characterized by the occupation and vacancy of staggered sublattices. In cases where the uniform system has a low temperature phase, the staggered states represent an intermediate phase. Furthermore, in many of these cases, we show that (at least part of) the phase boundary separating the low-temperature and staggered phases is a line of phase coexistence-i.e. the transition is first order. We also study the phenomenon of aggregation (phase separation) in bond diluted models. Such transitions are known, trivially, to occur in the large-q Potts models. However, it turns out that phase separation is typical in bond diluted spin systems with many internal states. (In particular, a bond aggregation transition is not tied to a discontinuous transition in the uniform system.) Along the portions of the phase boundary where any of these phenomena occur, the prospects for a Fisher renormalization effect are deemed to be highly unlikely or are ruled out altogether.Partly supported by the NSF grant DMS-93-02023 (L.C.), the grants GAR 202/93/0449 and GAUK 376 (R.K.), and the NSF grant DMS-92-08029 and the Russian Fund of Fundamental Investigations grant 93-01-01470 (S.B.S.).     

Dynamics and transport in random antiferromagnetic spin chains

We present the first results on the low-frequency dynamical and transport properties of random antiferromagnetic spin chains at low temperature (T). We obtain the momentum and frequency dependent dynamic structure factor in the random singlet (RS) phases of both spin-1/2 and spin-1 chains, as well as in the random dimer phase of spin-1/2 chains. We also show that the RS phases are unusual "spin-metals" with divergent low-frequency conductivity at T = 0, and follow the spin conductivity through "metal-insulator" transitions tuned by the strength of dimerization or Ising anisotropy in the spin-1/2 case and by the strength of disorder in the spin-1 case.     

Numerical study on Anderson transitions in three-dimensional disordered systems in random magnetic fields

The Anderson transitions in a random magnetic field in three dimensions are investigated numerically. The critical behavior near the transition point is analyzed in detail by means of the transfer matrix method with high accuracy for systems both with and without an additional random scalar potential. We find the critical exponent ν for the localization length to be 1.45 ± 0.09 with a strong random scalar potential. Without it, the exponent is smaller but increases with the system sizes and extrapolates to the above value within the error bars. These results support the conventional classification of universality classes due to symmetry. Fractal dimensionality of the wave function at the critical point is also estimated by the equation-of-motion method.     

On the convergence of the magnus expansion for spin systems in periodic magnetic fields

An analytically solvable single-spin problem is employed to investigate the convergence of the Magnus expansion for spin systems in periodic magnetic fields. The slow decay of quasi-stationary magnetization under multiple-pulse spin-locking conditions is explained within the framework of a theory based on a time-independent effective hamiltonian.     

Dynamics of vector spin glasses with random anisotropy

The dynamics of vector spin glasses with additional random anisotropy is investigated in the mean field approximation. We find a cross-over of the upper critical line from a behaviour of Heisenberg spins with a field-temperature dependence , for large fieldsH, to Ising like behaviourT _f –TT ^2/3 , for small fields and fixed anisotropy, in agreement with results of Kotliar and Sompolinsky. Here, andT _f are characteristic spin glass temperatures. In addition, one has a second line with reversed behaviour which presumably represents a cross-over line from weak to strong non-ergodicity. The local transverse susceptibility _T () varies for large fields and 0 along the upper critical line as ^vT , with a critical exponentV _T = 1/2 – 11D/60 J, whereD andJ are the anisotropy and exchange coupling constants, respectively. On the Ising-like part of the upper critical line one has isotropic spin glass parameters,q _L =q _T , and susceptibilities, and a critical exponent, which is similar to that of Ising spins along the de Almeida-Thouless line.Dedicated to B. Mühlschlegel on the occasion of his 60th birthdaySFB Aachen-Jülich-Köln     

Neutrino flavor conversion in random magnetic fields

If massive neutrinos possess magnetic moments, a magnetic field can cause a spin flip. In the case of Dirac neutrinos the spin flip converts an active neutrino into a sterile one and vice versa. By constrast, if neutrinos are Majorana particles, a spin flip converts them to a neutrino of a different flavor. We examine the behavior of neutrinos in a random magnetic field as it occurs, for instance, in certain astronomical objects, such as an active galactic nucleus. Both Dirac and Majorana neutrinos behave ergodically: independently of their initial density matrix, they tend towards an equipartition of the helicity states. As a result, about half of the Dirac neutrinos produced becomes sterile. For Majorana neutrinos, there will be an approximate equipartition of flavors, independently of the production mechanism.     

Solvable spin systems with random interactions

It is shown that if the bonds connecting spins on a lattice are separable functions of random variables, the thermodynamic and magnetic parameters may be obtained using the known properties of a spin system with non-random bonds.     

Dynamics of nuclear spins appearing in transport measurements of an inter-edge spin diode in tilted magnetic fields

In a wide range of magnetic fields nonlinear transport between spin polarized edge channels is studied. The observed hysteresis of the I–V characteristic is attributed to the dynamic nuclear spin polarization due to the electronic spin-flip processes. We find extremely long nuclear spin relaxation times in the regime where the hyperfine interaction with electrons is switched off.     

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.     

Transverse spin relaxation of spin carriers diffusing in spatially periodic magnetic fields

An exact solution of the Bloch–Torrey equation that covers the entire range of relative diffusivities D between the spin carriers and the magnetic structure (due to, e.g., spin‐density waves) is given for the transverse relaxation of an initally uniformly polarized spin system under the influence of a magnetic field varying sinusoidally in space. Explicit closed‐form results for the short‐time relaxation are obtained making use of Laplace transforms, the three‐term recurrence relations associated with Mathieu’s equation, and novel sum rules. At intermediate diffusivities the transverse polarization exhibits a novel long‐time behaviour as a function of D. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon spin relaxation for Ar+NO2 systems in transverse and longitudinal magnetic fields

Surface muons produced in UT-MSL were introduced into argon gas of 4.0±0.2 atm with NO₂ (0–30 ppm), and muonium signals were detected in the presence of a transverse (1.7–3.4 G) and a longitudinal magnetic field (0–3.5 kG) at 295±1 K. The cross section for the transverse relaxation was (11.0±1.0)×10⁻¹⁶ cm². The relaxation rates in different longitudinal magnetic fields show that the rate does not follow the conventional equation which assumes that the relaxation occurs mainly by spin-exchange interaction. Similar measurements were performed for the Mu+O₂ system. These findings indicate that chemical reactions contribute to these relaxation rates.     

Dynamics of fields of island systems

An investigation is made of the dynamical characteristics of fields produced by an island axisymmetric distribution of charged sources from the point of view of the theory of gravitoinertial frames of reference.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 56–62, July, 1976.It is a pleasant duty to thank all the participants of the seminars led by D. D. Ivanenko and V. I. Rodichev for interesting discussions on this work.     

Quenching of spin fluctuations by high magnetic fields

The quenching of spin fluctuations by magnetic fields has been observed in heat capacity and electrical resistivity measurements at low temperatures for a series of highly exchange enhanced magnetic materials. These include: the weak itinerant electron ferromagnets Sc₃In, Zr_1−xHf_xZn₂ (0 x 0.2) and Ni₃Al; the strong Pauli paramagnets RCo₂ (R = Sc, Y and Lu), TiBe₂ and Pd_1−xNi_x (0 x 0.01); and the heavy fermion systems CeSn₃, CeSi_x (x ≈ 1.85) an d UAl₂. The reported quenching of spin fluctuations in scandium and palladium by magnetic fields is reviewed, and it appears that the initial observations and conclusions are incorrect, and that fields greater than 10 and 40 T, respectively, will be necessary to quench spin fluctuations in these metals. The behaviors of these spin fluctuators have been grouped into six classes.     

Low temperature behavior of the spin susceptibility in nearly magnetic dilute alloys

We show that the low temperature susceptibility of a dilute alloy containing nearly magnetic impurities varies like T² well below the characteristic temperature, (the spin fluctuation temperature or the Kondo one) in agreement with experiments.