Department of Physics

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Browsing Department of Physics by Author "Acharyya, Muktish"

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    Nonequilibrium Phase Transition in Ferromagnetic Model Systems
    Acharyya, Muktish
    In this thesis, I have mainly discussed the dynamical responses and behaviours of a ferromagnetic system under different kinds of magnetic field variations that are varied in time as well as in space, especially in the form of propagating magnetic waves and standing magnetic wave using Monte-Carlo simulation. These magnetic field waves cause the ferromagnet to undergo nonequilibrium phase transition; depending on the temperature, amplitude and wavelength of the magnetic field and the strength of anisotropy. In an Ising ferromagnet driven by standing magnetic field wave the high temperature phase is found to be quite different from that observed when propagating magnetic wave was passing through the Ising ferromagnet. Dynamical phases having similar attributes are formed in Blume-Capel (S = 1) ferromagnet when placed under these kinds of waves. However, in BC ferromagnet the strength of anisotropy affects the transition temperature as well as the morphological structure of the ferromagnetic spins. The studies regarding general characteristics of spin-S Ising ferromagnet excited by magnetic field waves revealed that the nature of dynamic phase transition is similar to the earlier studies but the transition temperature is found to decrease towards a limiting value with increase in the value of S. The values of critical exponents of spin - 1/2 Ising ferromagnet driven by propagating magnetic field waves are found to be very close to those obtained in Onsager’s solution for equilibrium ferro-para phase transition. This indicates that the nonequilibrium phase transition in an Ising ferromagnet driven by propagating magnetic wave belongs to the same Universality Class of equilibrium Ising ferromagnet.
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    Statistical studies of metastable Lifetimes in magnetic model Systems
    Acharyya, Muktish
    In my thesis, I have focused on metastable behaviours of a ferromagnetic system. The studies have been carried out particularly on Ising and Blume-Capel ferromagnet in presence of externally applied magnetic field using Monte Carlo simulation technique based on Metropolis algorithm. The metastable lifetime is found to decrease in presence of quenched random field compared to that in presence of uniform field only. The strength of the random field is observed to play a similar role as played by temperature on metastable lifetime. Becker-D¨oring theory of classical nucleation (originally proposed for the spin-1/2 Ising system), as well as Avrami’s law, have been verified in the random field Ising model. However, the nucleation regime is found to be affected by the stronger random field. The metastable lifetime of both the surface and bulk have been studied separately by introducing a relative interfacial interaction strength (R). Surface reversal time is found to be different from the bulk reversal time. Depending on R, temperature and applied field, a competition between the surface reversal and bulk reversal is noticed. The effect of anisotropy (D, both positive and negative) on the metastable lifetime has been investigated. The linear dependency of the mean macroscopic reversal time on a suitably defined microscopic reversal time has been observed. The saturated magnetisation Mf, after the reversal, is noticed to be strongly dependent on D. That Mf, D, and h (field) are found to follow a proposed scaling relation. The metastable behaviours under the influence of graded and step-like variation of both the applied field and anisotropy have been explored. Motion of an interface, arising due to the presence of a stronger gradient of either field or anisotropy, has been studied. A competitive reversal by the field and anisotropy is observed when I consider the similar spatial modulation of both the field and anisotropy.