High Frequency Soft Magnetic Properties And Magnetic Anisotropy Modulation Of FeCo-B₂O₃ Nanogranular Films

In order to satisfy the demand of miniaturization,integration and high frequency application of electro-magnetic devices,the basic demands for soft magnetic films operated in the GHz range include high resistivityρ,high permeability(μ),high saturation magnetization M_s and appropriate anisotropy field H_k so as to effectively suppress eddy current loss and to possess high ferromagnetic resonance(FMR) frequency f_r(which determines the cut-off frequency for high frequency application).Granular films composited by magnetic metal nano-particles distributed uniformly in insulator matrix are one of the best candidates for these demands.These films have the advantages of high permeabilityμ(hence low coercivity H_c) of magnetic metal and high resistivityρof insulator,making it possible for highμand highρin same materials. Consequently,these new kinds of granular films have been one of the cut-edge fields.In this work,soft magnetic nanogranular films FeCo-B₂O₃ system was fabricated by rf-magnetron sputtering.The structures of the samples were characterized by X-ray Diffraction (XRD) and high resolution transmission electron microscope(HRTEM) while the magnetic properties and electrical properties were systematically studied through vibrating sample magnetometer(VSM) and four points method.Meanwhile,the high-frequency characteristics of complex permeability were obtained by using the shorted microstrip transmission-line perturbation method.In order to explain the origin of soft magnetic properties,the interaction among particles was explored byδM(H) plot.Moreover,we attempted to explore the method of modulation magnetic anisotropy.The main contents are briefly introduced as follows:â… .StructureFe₆₅Co₃₅-B₂O₃ films were fabricated by rf-magnetron sputtering.The HRTEM bright field shows the metal particles with size of 1～8 nm are embedded uniformly in insulating B₂O₃ matrix. The particle size decreases with the decrease of the metal volume fraction.The electronic diffraction pattern in further confirms that these samples are bcc structures.In addition,the results of XRD and HRTEM indicate that the crystallite size is close to that of particles. â…¡.Magnetism(1) The dependence of coercivity H_c on metal volume fraction x was systematically studied.In the range 0.55＜x＜0.66,H_ce does not exceed 15.6 Oe,namely,good soft magnetic properties are obtained in this x range.These good soft magnetic properties in the Fe₆₅Co₃₅-B₂O₃ system can be understood by exchange coupling between Fe₆₅Co₃₅ particles.For FeCo alloy,the exchange length L_ex can be calculated out to be about 26 nm,which is much larger than the sum of metal particle size and the distance between them.Therefore,exchange interaction between particles may take place in the wide x range,which averages out the magneto-crystalline anisotropy of individual particles and overcomes the demagnetization effect.As a result,the coercivity H_c diminishes effectively.(2) The dependence of anisotropy H_k on metal volume fraction x was also systematically studied. With decreasing x,the anisotropy H_k increases first,then decreases when x＜0.61,the trend is contrary to that of H_c.Also in the range of 0.55＜x＜0.66,the magnitude of H_k is obviously bigger than that of other samples,which indicates that the value of H_k is bigger in this x range with strong exchange coupling.(3)δM(H) plot was employed to study the intergranular interaction.Research results show that the origin of good soft magnetic properties is the exchange coupling between granules.â…¢.High frequency properties of granular films(1) For Fe₆₅Co₃₅-B₂O₃ system,in the range of 0.55＜x＜0.66,highμ′and high FMR resonance frequency were obtained,and the electric resistivityρin this range is 3 orders higher than that of pure metal,so the Fe₆₅Co₃₅-B₂O₃ granular films can be applied in high frequency range.(2) For the sample with x=0.61,when f＜1.3 GHz,the real partμ′of complex permeability is larger than 180 and then decreases with frequency while the imaginary partμ″is visible when f＞1 GHz and the FMR frequency is as high as 2.65 GHz. â…£.Modulation anisotropy field of FeCo-B₂O₃ systemTwo methods of modulation anisotropy field are as follow:(1) A series of(Fe_100-yCo_y)_0.61-(B₂O₃)_0.39(35.0＜y＜80.9) granular films were fabricated.For 35.0＜y＜77.0,the H_k almost do not change,but the He decrease.When y＞77.0,H_c increases rapidly and H_k decreases to zero steeply.(2)(Fe_100-yCo_y)_0.61-(B₂O₃)_0.39 was fabricated by oblique sputtering.The relathionship of Hk with angleθwere investigated.When the oblique angleθchanges from 0°to 30°,Hk increases from 53 Oe to 115 Oe.Based on our experiment results,two origins of magnetic anisotropy,namely, anisotropic magnetic coupling and orientation order of atomic pairs were suggested.