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Role of magnetic inertia in damped macrospin dynamics

Abstract : The goal of this thesis is to introduce the concept of inertia in damped macrospin dynamics. Following the work performed by T. L. Gilbert in this direction, a mass is associated to the macrospin, not related to the displacement of a real mass, but to macrospin's inertia. As a consequence, a generalised form of the Gilbert dynamics equation accounting for macrospin's inertia is derived. At the scale of a nanostructure ferromagnet, fluctuations are relevant. The macrospin undergoes a Browninan motion in the corresponding configuration space (a sphere of radius Ms). In order to make a description of the macrospin, a simple, rigorous and new theory, that of mesoscopic nonequilibrium thermodynamics (MNET), is used as an alternative to the quite heavy formalism of stochastic processes used by W. F. Brown Jr. The state of the magnetisation is described by a number of degrees of freedom that exert an influence in the overall dynamics of the system: the orientation m of the magnetisation of angles (θ, φ) and the velocity u = dm/dt , or the angular momentum L. A distribution function f is introduced related to the probability of finding the magnetisation in a particular state (m, u) or (m,L). The combination of the statistical definition of the entropy as a function of the probability, together with the systematic methodology of nonequilibrium thermodynamics, results in a powerful theory describing not only the average dynamics of the macrospin, but also the fluctuations about it. Hence, a generalised dynamic Gilbert and a generalised stochastic Brown equation are derived within the MNET theory. A relaxation time is defined dependent on macrospin's inertia and damping. The magnetisation behaviour determines two regimes: the inertial regime or the short time scale limit t<
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Contributor : Mihaela-Cristina Ciornei <>
Submitted on : Tuesday, March 2, 2010 - 6:05:34 PM
Last modification on : Friday, July 31, 2020 - 9:24:06 AM
Long-term archiving on: : Friday, June 18, 2010 - 10:10:12 PM

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  • HAL Id : tel-00460905, version 1

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Mihaela-Cristina Ciornei. Role of magnetic inertia in damped macrospin dynamics. Condensed Matter [cond-mat]. Ecole Polytechnique X, 2010. English. ⟨tel-00460905⟩

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