What can we learn about critical magnetic phenomena from muon spin rotation experiments?

Measurements of the spin precession frequencies ω_μ and transvers relaxation rates Γ₂ of positive muons (μ⁺) magnetic phase of several iron-cobalt alloys and coabalt, and in amorphous Fe_o.91Zr_0.09 alloys are discussed with particular reference to the influence of critical phenomena on ω_μ and Γ₂. Although the temperature dependence ω_μ(T) is well described by scaling law ω_μ ∝ (T_c?T)^β′ where α′ varies strongly with the co concentration in the FeCo alloys, it is shown that the exponent α′ is not identical to the static scalling parameter β describing the critical behaviour of the spontaneous magnetization M. In paramagnetic iron we find Γ₂∝(T-T_c)^0.72, which is attributed to the effect of dynamical spin fluctuations on the μ⁺. Due to the apparent difference between the temperature dependence of ω_μ and that of M below T_c the frequency shifts observed in the paramagnetic regime are probably not directly proportional to the magnetic susceptibility. In amorphous Fe_0.91Zr_0.09 allosy frequency shifts and transverse relaxation rates are found up to nearly 2 T_c. Due to the shape of the Fe_0.91Zr_0.09 sample, demagnetization inhomogeneities certainly play an important rôle.     

Monte Carlo computer experiments on critical phenomena and metastable states

Ising and Heisenberg models are studied by the Monte Carlo method. Several hundred up to 60 000 spins located at two- and three-dimensional lattices are treated and various boundary conditions used to elucidate various aspects of phase transitions. Using free boundaries the finite size scaling theory is tested and surface properties are derived, while the periodic boundary condition or the effective field-like ‘self-consistent’ boundary condition are used to derive bulk critical properties. Since Monte Carlo averages can be interpreted as time averages of a stochastic model, ‘critical slowing down of convergence’ occurs. The critical dynamics is investigated in the case of the single spin-flip kinetic Ising model. Also non-equilibrium relaxation processes are treated, e.g. switching on small negative fields the magnetization reversal and nucleation processes are studied. The metastable states found can be understood in terms of a scaling theory and the droplet model. Using a spin exchange model the phase separation kinetics of a binary alloy is simulated.     

Hyperfine magnetic field on Pt in Gd

The hyperfine field on Pt nuclei in Pt-Gd alloys has been studied using the integral perturbed angular correlation technique (IPAC). Results are given for applied fields between 0.2 T and 4 T in the temperature range 13 K to 300 K.     

Hyperfine magnetic field on Pd in Pd-Fe alloys

The hyperfine field on Pd nuclei in Pd-Fe alloys has been studied using the integral perturbed angular correlation technique (IPAC). Results are given for samples with iron contents from 3 to 60 at.% Fe and the temperature dependence of the hyperfine field is studied for a sample with 10 at.% Fe.     

Phase transitions and critical phenomena in multiferroic films with orthorhombic magnetic structure

Phase transitions and critical phenomena in materials with strongly correlated magnetic, ferroelectric, and elastic subsystems are studied. A theoretical study of the influence external fields (dc and ac) have on the dynamics of phase transitions in perovskite-type crystals is performed.     

Hyperfine magnetic field of Zn in iron

Precise hyperfine field value of zinc in iron has been determined by nuclear magnetic resonance on oriented nuclei (NMR/ON): B_hf (ZnFe)=−18.785 (35) T at 7 mK. The relaxation constant of Zn in iron is established C_K=14(3) Ks. The new hyperfine field value of zinc in iron allows a more precise reevaluation of the magnetic moments of^69mZn and^71mZn measured with NMR/ON. and the NICOLE Collaboration, CERN     

Hyperfine magnetic fields in substituted Finemet alloys

Transmission Mössbauer spectroscopy was used to determine the hyperfine fields of Finemet-type alloys in form of ribbons, substituted alternatively by Mn, Ni, Co, Al, Zn, V or Ge of various concentration. The comparative analysis of magnetic hyperfine fields was carried out which enabled to understand the role of added elements in as-quenched as well as annealed samples. Moreover, the influence of the substitution on the mean direction of the local hyperfine magnetic field was examined.     

On the investigation of magnetic critical phenomena in ferromagnets by μSR and PAC

On the basis of current theoretical views on the critical phenomena in isotropic Heisenberg ferromagnets the power temperature behavior Λ=c(τ)λ₀τ^-w has been derived for the muon spin relaxation rate Λ as π-T _c ⁻¹ (T-T _c ) → 0⁺. It is shown that the crossover from an exchange critical regime to a dipolar one is accompanied not only with the change in the critical exponentw in the above law, but also with the reduction of the coefficientc(π). A comparison with the temperature behaviour of the inverse nuclear relaxation timet _R ⁻¹ measured in the PAC experiment is carried out.     

Entanglement in quantum critical phenomena

Entanglement, one of the most intriguing features of quantum theory and a main resource in quantum information science, is expected to play a crucial role also in the study of quantum phase transitions, where it is responsible for the appearance of long-range correlations. We investigate, through a microscopic calculation, the scaling properties of entanglement in spin chain systems, both near and at a quantum critical point. Our results establish a precise connection between concepts of quantum information, condensed matter physics, and quantum field theory, by showing that the behavior of critical entanglement in spin systems is analogous to that of entropy in conformal field theories. We explore some of the implications of this connection.     

Hyperfine fields and magnetic properties of barium monoferrite

First we studied the kinetics of the formation of Ba monoferrite by means of Mössbauer spectroscopy. The most important result is the considerable portion of hexaferrite existing after treatment of the starting materials between 800 and 900°C, caused by a higher rate of diffusion of the Ba²⁺ ions in comparison to the Fe³⁺ ions. Second we report susceptibility measurements recorded in a temperature range of 4.3 K to 280 K and Mössbauer spectra at room temperature and at 4.2 K without and in the presence of an applied magnetic field of 6 T. The experimental data demonstrate that the magnetic behaviour of Ba monoferrite deviates to some extent from a simple antiferromagnetic.     

Hyperfine fields and magnetic coupling in Gd metal

New hyperfine field measurements on transition metal impurities in Gd are reported. The data cannot be explained on a simple RKKY model, but are consistent with a mechanism in which magnetic coupling is through the d electrons.     

Quantum field theories and critical phenomena on defects

We derive a nonlinear integral equation (NLIE) for some bulk excited states of the sine-Gordon model on a finite interval with general integrable boundary interactions, including boundary terms proportional to the first time derivative of the field. We use this NLIE to compute numerically the dimensions of these states as a function of scale, and check the UV and IR limits analytically. We also find further support for the ground-state NLIE by comparison with boundary conformal perturbation theory (BCPT), boundary truncated conformal space approach (BTCSA) and the boundary analogue of the Lüscher formula.     

Hyperfine critical exponents β and δ of tantalum in nickel

The hyperfine critical coefficients β and δ on dilute ¹⁸¹Ta nuclei in a nickel matrix have been measured using the time differential perturbed angular correlation technique. We find: β = 0.041₇ ± 0.01₀, δ = 4.1 ± 0.3. They are somewhat different from the values deduced from the bulk magnetization measurements on pure nickel.     

A density-matrix approach to critical phenomena

A density-matrix approach to the constrained Hartree-Fock problem is proposed as an alternative to Kümmel's maximum overlap method for the study of critical phenomena, with reference to two different quasi-spin models. Excellent results are obtained.     

Conformal field theories and critical phenomena

In this article we present a brief review of the conformal symmetry and the two-dimensional conformal quantum field theories. As concrete applications of the conformal theories to the critical phenomena in statistical systems, we calculate the value of central charge and the anomalous scale dimensions of the Z ₂ symmetric quantum chain with boundary condition. The results are compatible with the prediction of the conformal field theories.