Non-equilibrium Dynamics In Multi-freedom Magnetic Systems

Spintronics is one of the hot spots in modern physical research.It is of great theoretical value and application prospect to study the manipulation and modulation of the spin degree of freedom of the magnetic system by applying the external electric field or temperature gradient.In the process of fluctuations or field driving,considering the interaction among the magnetic system,such as spin,orbit,charge and lattice,the subsystem will change with the disturbance adiabatically only if the direct disturbance compared to its intrinsic couplings of the freedom concerned can be regarded as perturbation.However,when the condition is different,the response of the subsystem will not change with the dis-turbance adiabatically,corresponding to non-equilibrium dynamics of the freedom concerned.From the view of spatial,if the?thermal?fluctuations are spatially inhomogeneous,a spatial non-equilibrium distribution will be expected.By the“ultrafast”requirement of Quantum spin device,the non-equilibrium dynamics and non-equilibrium statistics are becoming an unavoidable research object.To exploring the question mentioned,we study the couplings between the subsystem-s and the non-equilibrium dynamics systematically.The work is studied by two parts:One is about the non-equilibrium dynamics in Mutiferro interface;Anoth-er is about the non-equilibrium distributions of temperatures on one dimensional magnetic lattice,and the general properties of the non-equilibrium statistics.The first part of the thesis is about the magnetoelectric?ME?couplings and the dynamic behaviors in a FM/FE Composite structure.Firstly,we investigate the adiabatic response of the magnetization M to an applied field E as a result of the ME couplings of the screening effect of the spin polarized electrons and the strain effect in the BTO/Fe interface.We found that both of the mechanisms can induce a magnetic response to an applied electric field in the ferromagnetic system.The results can be draw as follows:1.they are all suitable for high frequency feedback but cannot be extended to the whole ferromagnetic system;2.strain effect is related to phonons,as the velocity of phonons in BTO at room temperature is about?1000m/s,the feedback time of the magnetoelectric cou-pling is about 1ps,with a faster speed of the response;Only when the strength of the ME couplings more than 1s/f,and in working frequency?1ns,the spin polarization effect induces a significant magnetic response in the FM subsystem.The main feedback is along the FM/FE chain,in the direction perpendicular to it,there is no hysteresis changes.Secondly,We discussed the influence of non-adiabatic spin excited to the ME couplings near the FM/FE interface.Detailed theoretical analysis reveals that:coherent spin wave excitations with low energy will be induced by a non-zero vertical polarization in the bulk of FM subsys-tem.The spiral order of magnetization with particular symmetry of chiral will be established.Meanwhile,a direct strong coupling of linear ME interactions can be obtained.Based on the ME couplings mentioned,we realize the dynamics precession of magnetization GHz and relaxation dynamics by an applied electric field.The phenomenon revealed in this work provide a reliable basis of reserve theory and support for the future applications of low energy consumption RAM,micro-antenna,electro-optic modulator and other new spintronic devices.The second part is about the non-equilibrium thermodynamics of magnon-phonon system.In chapter IV,we have studied one dimensional Cherenkov pro-cesses in ferromagnetic isolators with perturbation theory.Conservation of energy and momentum have been used and the transition channels have been founded.It is found that the transition probabilities is proportional to k²under the long wave limit at room temperatures.The relaxation time of long wave magnons to the rest of magnons is proportional to k⁴,and 1/?_pmis linear dependent to temperatures?T>70K?.The distribution of temperatures have been solved by SW model,the temperature distributions of long wave magnons will separate with the ones of the short wave magnons,and the critical wave vector of the separation has been deduced,which of YIG has been estimated as 0.066?/a.In chapter V,the sta-tistical properties of thermal non-equilibrium systems have been researched.The statistical weight of the phase space has been introduced to describe the state of a non-equilibrium system.Based on the orthonormal property and the Gibbs criterion?if the statistical weight of the subsystem satisfies Gibbs distribution,the system containing the subsystem concerned is in a state of thermal equilibrium?,the concept of temperature states and density of temperatures are introduced.The principle of temperature measurement based on non-equilibrium statistics has been established.A general method to calculate the density of temperatures of the system has been proposed.As a sample,the density of temperatures in one dimensional lattice that the mean free path of the phonons is infinite has been studied.In the last chapter,we summarize the research contents of this thesis and prospect the future research.