Investigation On The Structure And Magnetic Properties Of Al₂O₃ And B Doped FeCo Based Films

With the rapid developments of electronic information and communication technologies, electromagnetic devices operating in high frequency range are becoming more and more miniaturized and integrated. Meanwhile, we need microwave absorption materials with high performance which eliminate the increasing electromagnetic interference and pollution. Therefore, soft magnetic films with high permeability, high saturation magnetization, large resistivity, and good high-frequency characteristics have attracted more attentions.FeCo alloy has high saturation magnetization. It is difficult to achieve low coercivity for the FeCo films due to its high magnetocrystalline anisotropy and magnetostriction. The coercivity of the FeCo films prepared by a conventional sputtering method is usually higher than 12 kA/m. Therefore, it is a challenge to improve the soft magnetic properties of FeCo films while maintain its high saturation magnetization. Adding a third element and using a suitable underlayer for the FeCo films are the usual methods in which the resistivity of the films is still small. Furthermore, nanogranular films consisting of FeCo nanoparticles distributed in insulator matrix are promising candidates owing to the high saturation magnetization of FeCo alloy and high resistivity of insulator. So, these nanogranular films have been one of the current hot topics.In this work, a series of FeCo-based soft magnetic nanogranular films were fabricated by magnetron sputtering. The effect of Al2O3 and B additions into the Fe65Co35 films on the structure, electrical properties, and magnetic properties was systematically studied. We also studied the effect of the Al2O3 or B additions, the simultaneous introduction of Al2O3 and B, and the increased Co/(Fe+Co) ratio on the microstructure, magnetic softness, and high frequency characteristics of Fe50Co50B-Al2O3 films. The main results are briefly shown as follows:Ⅰ. Fe65Co35-Al2O3 films(1) For Al2O3 volume fraction x<0.18, the microstructure of the Fe65Co35-Al2O3 films is a random mixture of the Fe65Co35 particles and the amorphous Al2O3 phase. Some particles are connected with each other. For x>0.18, the films form clear nanogranular structures, in which the Fe6sCo35 nanoparticles are uniformly distributed in the amorphous Al2O3 matrix. Furthermore, as x increases, the Fe65Co35 particles are mainly surrounded and isolated by insulating Al2O3. The thickness of the intergranular Al2O3 layer increases with increasing Al2O3 content.(2) Theμ0Ms decreases monotonically with increasing Al2O3 content. The resistivity increases slowly with x increasing from 0 to 0.18, resulting from the metallic conductivity in the films. Then the p increases rapidly with increasing x for x>0.18, reaching 6.8 mΩ·cm at x=0.36. This results from a combined effect between the metallic conductivity and the tunneling conductivity.(3) By introducing Al2O3 into the Fe65Co35 films, the magnetic softness can be improved. The Al2O3 addition into the films is beneficial to refine the Fe65Co35 particles. The exchange coupling among particles may occur and overcome the magnetocrystalline anisotropy. When the Fe65Co35-Al2O3 films form clear nanogranular structures, the thickness of the intergranular Al2O3 layer increases with increasing x, resulting in the occurence of intergranular decoupling. Using a modified G. Herzer model, we systematically investigated the variation of Hc with x in the Fe65Co35-Al2O3 films. We have calculated the Hc and the calculated results agree well with the experimental data.(4) The 8m(H) plots confirm the existence of exchange coupling between the Fe65Co35 particles and reveal the strength of exchange interaction initially increases with increasing x, reaching a maximum at x=0.18, then decreases with further increasing x. The Hch and Hce of the film with x=0.18 reach 0.56 and 2.91 kA/m, respectively.Ⅱ. Fe65C035B-Al2O3 films(1) The introduction of B into the Fe65Co35-Al2O3 films with low content of Al2O3 causes the formation of the nanogranular structure. The B addition leads to a refinement of Fe65Co35B particles and an amorphization of the films.(2) Theμ0Ms decreases monotonically and the p increases with increasing B content. As the B content increases, the Hc markedly decreases and the Hk initially increases and then decreases with increasing B content. The best comprehensive performance can be achieved in the (Fe65Co35)90B7(Al2O3)3 film, for example,μ0Ms=1.75 T, Hch=0.28 kA/m, Hk=5.57 kA/m,ρ=224μΩ·cm, fr=3.2 GHz,μ>250,Δf=1.5 GHz.(3) The good magnetic softness of the (Fe65Co35)97-yBy(Al2O3)3 films are mainly attributed to exchange coupling among (Fe65Co35)97-yBy nanoparticles which overcomes the magnetocrystalline anisotropy and the demagnetization effect of individual particle. As a result, the effective magnetocrystalline anisotropy is reduced significantly. The Henkel plots prove the existence of exchange coupling. The strength of exchange coupling between particles increases with increasing B content, causing to the decrease in the Hc.(4) The Mossbauer results show that the mean Hhf gradually decreases with the increasing B content, which is consistent with the reduction of theμ0Ms with increasing B content. This results from that the mean magnetic moment of the Fe atoms decreases by increasing B addition into the films. The line width increases due to the enhanced disordered local atomic environment of Fe atoms with increasing B content. The intensity ratio of the lines reveals that the magnetic moments are preferentially oriented in the plane of the films. (5) For y≥7, the dynamic characteristics of the (Fe65Co35)97-yBy(Al2O3)3 films exhibit the natural resonance mode. According L-L equation, we have calculated the theoretical permeability spectra, the calculated permeability spectra of the films fit well with the measured data. With y increasing from 7 to 20, the resonant frequencies decrease from 3.2 GHz to 1.9 GHz; theμ' increases from 250 to 500 below 1 GHz; the maximum ofμ" increases from 600 to 1600; and theΔf decreases from 1.5 to 0.7 GHz.Ⅲ. Fe5oCo5oB-Al2O3 films(1) The XRD analyses of the Fe50Co50B and Fe50Co50B-Al2O3 films show that B addition leads to a refinement of Fe50Co50B particles and the B atoms enter the interstitial sites of the a-Fe50Co50 lattice, leading to an expansion of the lattice. The high B content leads to an amorphization of the particles and destroys the alignment of local atom pair ordering induced by applying an external magnetic field.(2) The Al2O3 or B additions lead to a refinement of the particles. The insoluble amorphous Al2O3 phase is segregated on the growth surface of the Fe50Co50 islands; the Al2O3 layers covering the growth surface of Fe50Co50 islands provide a barrier for the diffusion of Co and Fe atoms and limit the growth and coalescence of the islands. The active B atoms as an impurity can be absorbed on the growing island surfaces and dissolved in the crystal lattice. Then, the B atoms may inhibit the absorption of other deposited atoms and suppress the growth and coalescence of the Fe50Co50 islands.(3) The analyses of XRD and XPS show that a majority of B atoms may enter into the a-Fe50Co50 lattice and the intergranular Al2O3 layers, the B content in the Fe50Co50B particles should be higher than that in the intergranular Al2O3 layers; a minority of B atoms may bond with O to form B2O3 on the interfaces between the Fe50Co50B particles and the intergranular Al2O3 layers.(4) Compared with the (Fe65Co35)97-yBy(Al2O3)3 films with the same B content y, the (Fe50Co50)94.8-yBy(Al2O3)5.2 films have the Hk values are 0.8～1.6 kA/m larger. The larger Hk in the (Fe50Co50)94.8-yBy(Al2O3)3 films is mainly ascribed to the increased Co/(Fe+Co) ratio.(5) The Fe50Co50B-Al2O3 films show good magnetic softness and excellent high frequency characteristics, the values of fr are higher than 2.3 GHz and theΔf is larger than 1 GHz when the B content is in range from 7.8 to 19.2. Especially, the (Fe50Co50)87B7.8(Al2O3)5.2 film exhibitsμ0Ms=1.73 T, Hch=0.3 kA/m, Hk=7.2 kA/m,ρ=350μΩ·cm,fr=3.5 GHz,μ'>200 (-3.2 GHz), and the maximum ofμ"～560,Δf=1.5 GHz.