We have studied both the dynamic response and the relevant nonequilibrium dynami cal p hase transition of an Ising spin system subject to three sorts of oscillating fi eld i.e. sinusoidal, square and sawtooth waves. The above three sorts of externa l field drive dynamically the Ising spin system in either simply gradual ( abrupt) way or their combination respectively. In the case of both sinusoidal and squar e ways, it was observed that the Ising spin system displays a low_temperature sy mmetr y-breaking ordered phase and a high_temperature symmetric disordered phase as well as the dynamic transition between two dynamical phases above. We also detected the tri_critical point separating high_temperature continuous dynamic transition and low_temperature discontinuous one on the boundary of dynamic transition. The trend of dynamic transition boundary and the dependence of tri_critical point upon the system temperature, the frequency and amplitude of the driving field we re revea led as well. In contrast, nodynamic transition occurs any longer and the system always stays in symmetry-breaking ordered state if the external field takes the form of sawtooth wave. The preceding discrepancy in dynamic response and transi tion is attributed to the perturbative characteristic of different oscillating f ields.
The low-frequency internal friction in the magnetization process, where magnetic field increases linearly, is measured for (Bi0.8Pb0.2SrCaCuOy) in superconducting state by using a frequency variable inverted torsion pendulum system. Experimental conditions are as follows: measure temperature: 95 K, at which specimen is in resistance- free state; frequency: 0.5 Hz to 5 Hz. The characteristics of internal friction (IF) peak observed in curve of IF vs magnetic field strength H are as follows. 1. The peak height increases with an increasing of H (variance rate of magnetic field) but decreases as frequency increasing. 2. Magnetic field strength at which IF peak appears is 4-6 mT. 3. When H falis down to zero from a given value suddenly, IF peak disappears. 4. IF peak mentioned above disappears also when specimen turns to normal state.It is considered that, the eigen-IF of superconductor mentioned above during magnetization process stems from motion of quantum flux.
The study on interesting stochastic resonance of the Ising spin system (ISS), subject to a joint external field of weak sinusoidal modulation and stochastic white noise, was carried out through solving the meanfield equation of motion ba sed on Glauber dynamics, and some new features in the dynamic response and stochasti c resonance of the above ISS were also revealed. The periodic ally driven stochastic ISS shows the occurrence of characteristic nonequilibrium d y namic phase transition when the frequency and amplitude of driving field, the te mperature of system and noise intensity attain a specific accordance in quantity . Somewhat similar to the situation in sole sinusoidal field, our primary result indicates that the dynamic transition of ISS in an applying stochastic fieldre tains probably the feature of both discontinuous transition at low temperatures and continuous transition at high temperatures. The analysis was made concerning the nonequilibrium dynamic transition induced in the Ising spin system by exter nal stochastic force in detail.
The Monte Carlo method was employed to perform the numerical simulation for classical discrete diluted Heisenberg spin system driven by an oscillating external magnetic field. To form the diluted spin system based on isotropic Heisenberg model, we introduced in the Hamiltonian of a typical Heisenberg model both random anisotropy energy term characterizing amorphous state with a proportion X and uniaxial anisotropy energy term representing crystalline state with a proportion 1-X. The dynamical transition behavior of the Spin system mentioned above, hysteresis loop scaled with parameter X and random as well as uniaxial anisotropy constant D and A, respectively, was studied in detail. It has been put forward a scaling relationship between hysteresis loop area and parameter X, A and D as below: Aarea-AδDηXσ . The main conclusions can be summarized below:(1)At a specific value X(defined as Xmin), the investigated system gains minimal hysteresis which has been proved experimentally in our another investigation. (2) The exponents δ,η and σ of the diluted spin system are universal constants, and the sum of δ+η(~0.9) equals the exponent δ,η of single either uniaxial or random anisotropy spin system. (3)The specific value Xmin versus logarithm of the ratio of A/D shows a peculiar sigmoidal trend.
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