FePt NPs: Preparation Of MgO Nanocomposite Film And Its Magnetic And Optical Properties

Due to the extremely high uniaxial magnetocrystalline anisotropy,small superparamagnetic critical size and good chemical stability of the L10-phase FePt alloy,it is considered as the first choice of ultra-high density magnetic storage materials in the future.With the development of science and technology,smaller size quantum devices are bound to replace microelectronic devices.Therefore,how to ensure the excellent performance of FePt nanoparticles and adjust their structure and properties according to different application requirements has become an urgent problem to be solved.In this paper,FePt nanoparticles were embedded in single crystal MgO matrix by pulsed laser deposition(PLD)method,and FePt NPs:MgO nanocomposite films were obtained.The magnetic and optical properties were studied.The research content is summarized as follows:Firstly,FePt NPs:MgO nanocomposite films were prepared by PLD method and their compositions and structures were characterized.During the growth of the film,FePt nanoparticles and MgO are three-dimensional island growth and two-dimensional layer-by-layer growth mode,respectively.The embedding of FePt nanoparticles does not affect the epitaxial growth of MgO layer.The epitaxial relations of FePt and MgO are[200]FePt ||[100]MgO.At fixed laser energy density,the roughness of the films increases with the increase of pulse deposition number(100-900 pulses)during the growth of FePt and the spacing of FePt nanoparticles becomes smaller(10-2 nm).When the number of pulsed number of FePt is constant,the ratio of Pt atoms to Fe atoms in the FePt alloy increases from 0.48 to 1.73 as the laser energy density used for the growth of FePt increases from 3 J/cm2 to 7 J/cm2.In addition,when the laser energy density is low,the prepared FePt nanoparticles are quasi-spherical,which increase the laser energy density,and the nanoparticles become quasi-elliptical.And as the laser energy density increases,the aspect ratio of the quasi-elliptical gradually increases.When the laser energy density increases from 4 J/cm2 to 6 J/cm2,the statistical aspect ratio increases from 1 to 1.65.Secondly,the magnetic properties of FePt NPs:MgO nanocomposite films were studied.After the growth,the FePt nanoparticles in the samples were disordered A1 phase.After annealing the samples in 5%H2+95%Ar atmosphere,the ordered transformation of the FePt nanoparticles took place,resulting in the formation of the L10 phase.This is due to the stirring phenomenon caused by the infiltration of H atoms into FePt,which increases the diffusion rate of Fe atoms and Pt atoms,thereby promoting the orderly transformation of FePt.Science a FePt nanoparticle can be considered as a large spin,we have also studied the effect of FePtnanoparticles spacing on the interaction between particles.With the increase of the FePt pulsed number,the FePt nanoparticle spacing decrease,the Blocking Temperature increases from 4.1 K to 34.2 K,the FePt NPs:MgO nanocomposite film changed from superparamagnetic phase to super-spin-glass phase.In addition,by controlling the ratio of Pt atoms and Fe atoms in the FePt alloy,the ratio of the soft magnetic phase(A1 phase FePt,Fe3Pt or FePt3)in the FePt nanoparticles can be adjusted between 0%and 47.5%.And as the soft magnetic phase content increases,the saturation magnetization of the nanocomposite film decreases from 234.1 emu/cr2 to 60.7 emu/cm2.Finally,we studied the linear and nonlinear optical properties of FePt NPs:MgO nanocomposite films.It is calculated by Mie theory that embedding FePt nanoparticles in MgO matrix will produce plasmon resonance absorption effect,and its wavelength range should be between 237-922 nm.This is in good agreement with what was observed in the experiment.The quasi-spherical FePt nanoparticles were embedded in the MgO matrix and the plasmon resonance absorption occurred at 234 nm.Changing the aspect ratio of FePt nanoparticles by increasing FePt growth laser energy density,the plasmon resonance absorption peak splits into two peaks.And as the aspect ratio increases,a red-shift and a blue-shift are observed for the two peaks.This is because that changing the shape of FePt nanoparticles will change the distribution of surface charge density.The third-order nonlinear optical properties of FePt NPs:MgO nanocomposite thin films were further measured by fs laser.The Z-scan closed-hole curves show the shape of valley-peak,indicating that the nanocomposite films have self-focusing effect.With the increase of the deposition number of FePt,the z-scan opened-hole curve changes from peak to valley.It indicates that two-photon absorption takes the place of saturation absorption and becomes the mechanism of the third-order nonlinear optical properties of the sample.In addition,by fitting the curve obtained by Z-scan test,the third-order nonlinear absorption coefficient and the third-order nonlinear refractive index of the film are obtained.The third-order nonlinear polarizability of FePt NPs:MgO nanocomposite films obtained by further calculation is about 10-10,which indicates that the FePt NPs:MgO nanocomposite films can be applied to the third-order nonlinear optical devices.