Research Over Magnetic Granular Film Based On Monte Carlo Simulation

The nature is full of disorder and random structure.Percolation theory is one of the best ways to deal with strong disorder and random geometry.It can be applied to a wide range of physical phenomena,and the scope of its application is gradually expanding.Percolation model is a physical model for describing the random process of spatial distribution,which is more intuitively reflected in the process of gradual infiltration of disordered media.Monte Carlo method is a statistical simulation method guided by probability and statistics,and it's very effective for the numerical study of percolation process.Metal-insulator granular film is a composite conductive material which is composed of nanoscale metal particles distributed evenly in the insulator.With metal particles proportion f gradually increased to the percolation threshold f_c,the transition of insulator to metal occur in metal-insulator granular film,and the solution of percolation threshold is the key of the study in the percolation system.Similarly,in the nanometer magnetic granular films,as the temperature gradually increases to the Curie temperature or the proportion of magnetic particles gradually increases to the threshold,the percolation transition of ferromagnetic to paramagnetic system gradually occurs in the system.In this paper,the percolation theory is applied to the metal-insulator granular films and magnetic nanometer granular films,using Monte Carlo method to simulate the magnetization behavior analyses of magnetic nanometer granular films and the electrical transport properties of metal-insulator granular films.The main work and research results are as follows:(1)The Monte Carlo method is used to simulate the geometric percolation threshold of the two-dimensional and the three-dimensional percolation model,and the results are 0.6 and 0.32respectively.The finite temperature induced jump conductance between particles,the percolation threshold of two-dimensional percolation model is reduced from 0.6 to 0.32,and the percolation threshold of three-dimensional percolation model is reduced from 0.32 to 0.12.(2)The percolation threshold of different particle geometries is different.The rotating ellipsoid which has large semi axis is more likely to form a conducting connected channel through the whole composite system,and the percolation threshold is also smaller.Based on the effective medium theory,the thin film samples that near the percolation point were verified,and the relationship between the effective resistivity and temperature is consistent with the experimental data.(3)Building two-dimensional and three-dimensional magnetic granular film system.With the increase of temperature,the hysteresis phenomenon of magnetic nanoparticles system gradually decreases,and the system gradually changes from ferromagnetic to paramagnetic.With the enhancement of intergranular exchange and dipole interaction in the system,the remanence and coercivity increase,and the exchange interaction effect between particles has great influence on the magnetization behavior of the system.The larger the exchange interaction coefficient,and the hysteresis loop appears more and more"Rectangle".(4)The influence of demagnetization field on the magnetization of the system is also considered.When the half axis ratio of the long ellipsoid is getting larger,then the axial demagnetization factor is getting smaller,and the system is easier to be magnetized.Conversely,when the oblate ellipsoid becoming more flat,then the axial demagnetization factor is getting larger,and the demagnetization field is getting larger,so the system is more difficult to be magnetized.Furthermore when the particles of the cylindrical shape is getting finer,the demagnetization factor at the axial center of the cylinder is getting smaller,and the system is more easily magnetized.(5)The doping of non-ferromagnetic particles has a great influence on the magnetization behavior of the system.The magnetization behavior of the system is simulated under the condition of different proportion magnetic particles,and it is found that the percolation transition of the paramagnetic to ferromagnetism occurs when the proportion of magnetic particle composition is up to 0.2,and the percolation threshold is less than the geometric percolation threshold in the system.