The Study Of Nano-composite Soft Magnetic Films Prepared By PLD

The rapid development of electronic and information technology has significantly promoted magnetic devices that can be miniaturized,integrated and capable of high-frequency operation.As the core material of magnetic devices,it is extremely urgent to develop new-type soft magnetic materials with large saturation magnetization,high resistivity and high working frequency.Composite magnetic thin films containing both metal(Fe,Co,Ni)and insulator components are the most promising candidates.The traditional method to prepare composite magnetic thin film is magnetron sputtering,which is difficult to acquire large saturation magnetization and high resistivity simultaneously,and always lead to large coercivity.In this work,a series of FeAlO,FeCoAlOand FeCoAlON composite magnetic thin films have been prepared via the reactive pulsed laser deposition.By tuning the deposition atmosphere,target composition and annealing temperature,films with granular structure have been successfully obtained.The main results are as follows:The influence of deposition atmosphere,target composition and annealing temperature on the structure and physical properties of FeAlO film has been investigated systematically.Under the base pressure of 5×10-7 torr,Fe can maintain its metallic state,while Al is oxidized.Amorphous FeAlO film with granular structure can be obtained with a FeAl alloy target containing 20 at.%Al.The film exhibits high electrical resistivity of 3700 ?? · cm and negative temperature coefficient of resistivity.When subjected to annealing,the Fe nanoparticles is crystallized and interconnects with each other,while Al2O3 exists as inclusions in the Fe matrix.The electrical resistivity declines rapidly to 250 ?? · cm,which is similar to the value of metallic films.FeCoAlO granular film is successfully fabricated with(Fe0.65Co0.35)0.sAl0.2 target.Although the diameter of FeCo nanoparticles(2.69 nm)is much smaller than the superparamagnetic transition limit,they can be exchange coupled through the ultrathin Al2O3 insulating layers.When the temperature is above 623 K,the win-out of thermal fluctuation over exchange coupling will turn the film into superparamagnetic state.The FeCoAlO film will exhibits the optimal properties after 573 K annealing with resistivity of 2500??·cm,saturation magnetization of 10.5 kGs,initial permeability of 120,cut-off frequency of 2.8 GHz and ?f of 3.0 GHz.FeCoAlON granular films are successfully fabricated under different nitrogen pressures.When increasing the nitrogen pressure,the insulating phase is transformed from Al2O3 to AIN and the oxidation of FeCo is effectively suppressed,which is beneficial to simultaneous high resistivity and large saturation magnetization.The optimal nitrogen pressure of 3 mtorr leads to the best physical properties:resistivity of 3380 ?? · cm,saturation magnetization of 12.5 kGs,coercivity of 3 Oe,initial permeability of 149,cut-off frequency of 2.1 GHz and ?f of 3.5 GHz,which is a promising candidate for microwave absorption application.Further increasing the nitrogen pressure to 5 mtorr,the formation of nonmagnetic Fe2N leads to smaller FeCo nanoparticle and thicker nonmagnetic layer.The win-out of thermal fluctuation over exchange coupling turns the film into superparamagnetic state,resulting in the deteriorated high frequency response.After post annealing at 573 K,the permeability spectra is transformed from relaxation type to resonant type.The films also exhibit higher cut-off frequency,due to the increased in-plane anisotropy after post annealing.For the film prepared under 3 mtorr nitrogen pressure,the magnetic properties have been greatly improved,with saturation magnetization of 13.0 kGs,initial permeability of 170,and cut-off frequency of 3.0 GHz.