Thursday, May 19 2022
10:00 - 11:15

IMSc Webinar

Statistical studies of metastable lifetimes in magnetic model systems

Moumita Naskar

Presidency University, Kolkata

In this study, 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 a quenched random field. The strength of the random field plays an equivalent role to temperature on metastable lifetime. Becker Döring’s 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 competitive nature of the metastable lifetime of surface and bulk has been studied 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 the applied field, a competition between the surface reversal and bulk reversal is noticed based on faster reversal. The effect of uniaxial 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. Finally, Becker-Döring’s theory is also verified in spin-s (s = 1/2, 1, 3/2, 2, 5/2, 3, 7/2) Ising and Blume-Capel models. Moreover, the decay of metastable volume fraction is found to follow Avrami’s law nicely for all values of s and both models considered.

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