Effect Of Random Magnetic Field Research Articles

Ising spin glass under continuous-distribution random magnetic fields: Tricritical points and instability lines

The effects of random magnetic fields are considered in an Ising spin-glass model defined in the limit of infinite-range interactions. The probability distribution for the random magnetic fields is a double Gaussian, which consists of two Gaussian distributions centered, respectively, at +H0 and -H0, presenting the same width sigma . It is argued that such a distribution is more appropriate for a theoretical description of real systems than its simpler particular two well-known limits, namely, the single Gaussian distribution (sigma>>H0) and the bimodal one (sigma=0) . The model is investigated by means of the replica method, and phase diagrams are obtained within the replica-symmetric solution. Critical frontiers exhibiting tricritical points occur for different values of sigma , with the possibility of two tricritical points along the same critical frontier. To our knowledge, it is the first time that such a behavior is verified for a spin-glass model in the presence of a continuous-distribution random field, which represents a typical situation of a real system. The stability of the replica-symmetric solution is analyzed, and the usual Almeida-Thouless instability is verified for low temperatures. It is verified that the higher-temperature tricritical point always appears in the region of stability of the replica-symmetric solution; a condition involving the parameters H0 and sigma , for the occurrence of this tricritical point only, is obtained analytically. Some of our results are discussed in view of experimental measurements available in the literature.

Study of the low-temperature behavior of a disordered antiferromagnet with random fields by the parallel-tempering method

The parallel-tempering method has been applied to numerically study the thermodynamic behavior of a three-dimensional disordered antiferromagnetic Ising model with random fields at spin concentrations corresponding to regions of both weak and strong structural disorder. An analysis of the low-temperature behavior of the model convincingly shows that in the case of a weakly disordered samples there is realized an antiferromagnetic ordered state, while in the region of strong structural disorder the effects of random magnetic fields lead to the realization of a new phase state of the system with a complex domain structure consisting of antiferromagnetic and ferromagnetic domains separated by regions of a spin-glass phase and characterized by a spinglass ground state.

Some aspects of polarized line formation in magneto-turbulent media

Observations and numerical simulations of magneto-convection show a highly variable solar magnetic field. Using a statistical approach, we analyze the effects of random magnetic fields on Stokes profiles of spectral lines. We consider the micro and macro-turbulent regimes, which provide bounds for more general random fields with finite scales of variations. The mean Stokes parameters are obtained in the micro-turbulent regime, by first averaging the Zeeman propagation matrix Φ ^ over the probability distribution function P( B ) of the magnetic field and then solving the concerned radiative transfer equation. In the macro-turbulent regime, the mean solution is obtained by averaging the emergent solution over P( B ). It is assumed that B has a Gaussian distribution defined by its mean field B 0, angular distribution and dispersion. Fluctuations parallel and perpendicular to B 0 are considered. Spectral lines are parameterized by their strength β, which is varied over the range 1–10 4. A detailed comparison of micro and macro-turbulent limit with mean field solution shows that differences are important for β ⩾ 10. When β increases, the saturation behavior of micro-turbulent profiles are significantly different from that of mean field profiles. The Stokes profiles shapes are explained in terms of the non-linear β-dependence of the Unno–Rachkovsky solution using approximate expressions for the mean absorption coefficients. These expressions when inserted in the Unno–Rachkovsky solution can predict Stokes profiles that match with the numerical result to a good approximation.

Random magnetic fields inducing solar neutrino spin-flavor precession in a three generation context

We study the effect of random magnetic fields in the spin-flavor precession of solar neutrinos in a three generation context, when a non-vanishing transition magnetic moment is assumed between the second and third families. We have analyzed the high energy solar neutrino data and the KamLAND experiment to constrain the solar mixing angle, θ⊙, and solar mass difference, Δm2⊙, and we have found a large shift of allowed values. Also sizable effects in Borexino experiment are expected which can discriminate this scenario and standard Large Mixing Angle (LMA) solution to the solar neutrino problem.

Random magnetic fields inducing solar neutrino spin-flavor precession in a three generation context

We study the effect of random magnetic fields in the spin-flavor precession of solar neutrinos in a three generation context, when a non-vanishing transition magnetic moment is assumed. While this kind of precession is strongly constrained when the magnetic moment involves the first family, such constraints do not apply if we suppose a transition magnetic moment between the second and third families. In this scenario we can have a large non-electron anti-neutrino flux arriving on Earth, which can lead to some interesting phenomenological consequences, as, for instance, the suppression of day-night asymmetry. We have analyzed the high energy solar neutrino data and the KamLAND experiment to constrain the solar mixing angle, and solar mass difference, and we have found a larger shift of allowed values.

Dissociation of ultracold molecules via Feshbach resonances: The effect of magnetic-field fluctuations

The effect of random magnetic fields on the dissociation of ultracold molecules with Feshbach resonances is studied analytically. The dissociation spectrum and the evolution of the molecule fraction are obtained for a general form of the magnetic-field ramp and different noise properties relevant to standard experimental conditions. The results uncover the robustness against noise of some characteristics of the dissociation process, in particular, of the dependence of the mean atomic kinetic energy on the system parameters. Implications for the applicability of a method proposed to determine the width of the Feshbach resonances from the measurement of the dissociation spectrum are analyzed. Moreover, we discuss how the presence of fluctuations can affect the use of particular ramp shapes to tailor the properties of the generated atomic distribution.

Investigation of the Disordered Antiferromagnetic Ising Model with the Effects of Random Magnetic Fields

Computer simulation of phase transitions is made by the Monte Carlo method using a three-dimensional disordered antiferromagnetic Ising model in the external magnetic field. It is found that in the case where the spin concentration in a system is lower than a threshold one, the effects of random magnetic fields destroy the second-order phase transition and lead to the first-order phase transition into a new phase state of the system characterized by a ground spin-glassy state and metastable energy states at finite temperatures. The dependence of the threshold concentration on the external magnetic field is revealed.

Effect of a random magnetic field on active-sterile neutrino conversion in the supernova core

The active-sterile neutrino conversion is studied for neutrino propagating in the axial potential generated by magnetised electron plasma in the supernova medium. We consider the effect of random magnetic field $B_{rms}$ on the average neutrino conversion probability. We obtained the constraint on $\Delta m^2$ and $\sin^2 2\theta$ for different strength of the random magnetic fields, by considering the positive definiteness of the average neutrino conversion probability inside the supernova core. Our calculation shows that, $B_{rms}\le 0.63\times 10^{14}$ Gauss is preferable so that small values of $\Delta m^2$ can not be excluded.

Effect of random magnetic fields on the hyperfine structure of M�ssbauer spectra in paramagnetic single crystals

Under certain conditions, the57Fe Mossbauer spectra of Al(NO3)3·9H2O∶Fe3+ single crystals exhibit new lines [1,2] (so-called z-lines) at an unusually high velocity of about ∓10.5 mm/s. The dependence of the positions and intensities of these z-lines on the magnitude and direction of an external magnetic field has been investigated. The appearance of the z-lines is explained by small random magnetic fields arising from magnetic nuclei in the crystal.

Critical behaviour of random compressible magnets

The critical properties of a compressible random magnet are studied using renormalization group methods. Then-component orderparameter is coupled to quenched disorder and to the elastic fluctuations of the anisotropic solid. It is shown, that the critical behaviour of a compressible random magnet is in general the same as that of a random magnet on a rigid lattice. However, if the specific heat exponent of the ideal magnet is positive and the disorder is sufficiently weak, a macroscopic instability may prevent the system in reaching the critical point. The resulting first-order transition may be preceded by pseudocritical behaviour characteristic to pure compressible magnets. The effect of random magnetic fields on the critical properties of compressible magnets is also discussed.