The Influence Of Doping And Magnetic Anisotropy On GMI Effect In Fe-based Films

The giant magnetoimpedance (GMI) effect has become an attractive topic in magnetic materials research mainly owing to its technological applications in highly sensitive field sensor, magnetic recording heads, automobile and industrial robots. The effect has been reported to be the strongest for an amorphous wire of Co-based alloy having a slight negative magnetostriction of the order of ～10^-7. In the low-frequency range l～10kHz, which is typical of the magneto-inductive effect, the inductive component of an AC wire voltage decreases by 50% for a longitudinal field of 0.16～0.4kA/m. At higher frequencies (0.1～10MHz) where the skin effect is essential the giant magneto-impedance (GMI) effect occurs; the amplitude of the total wire voltage decreases by 40～60% under the influence of a longitudinal field of 0.24～0.8kA/m. GMI devices probe the value of flux of magnetic field instead of its change in flux, they do not need coil, and their sensitivity is one order higher than the giant magnetoresistance response. GMI effect has been explained in terms of the change of skin depth when the permeability of magnetic material is varied by a dc magnetic field associated with the circumferential or transverse field induced by the high-frequency ac current. In the low-frequency range, the GMI effect is dominated by the magnetoreactance, while in the high-frequency range, the skin effect is essential for the GMI effect to occur. Since the impedance behavior as a function of magnetic field and frequency depends on both magnetic anisotropy and overall domain configuration, appropriate annealing under stress or a transverse magnetic field, or a dc joule heating has been done to induce transverse anisotropy. As a result, large GMI effects with very high sensitivity can be obtained.In fact, GMI is a skin effect of the radio-frequency (RF) current passing through the sample, which is related to the transverse permeability of the sample and changes sensitively with the applied magnetic field and driving current frequency. However, the permeability in soft magnetic materials is a very sensitive parameter and can be influenced by many factors, such as the composition of the materials, sample geometry, domain structure, stress distribution, preparation technique, as well as the internal configuration of the magnetization of the materials. Therefore, it would be useful to investigate the influence of the above-mentioned factors. The GMI effect has been studied extensively only in Co-based and Fe-based amorphous or nanocrystalline wires, ribbons. However, thin film magnetic sensors are very useful in micromagnetic techniques and the microelectronics. Thin films are also the preferred media for exploring the MI phenomena, which remain to be understood in detail. In short, it is very important to explore the influence of above-mentioned factors, such as the composition of the films, the structure of magnetic domain, anisotropy and sputtering conditions, on the GMI effect in Fe-based amorphous soft magnetic films.In this work, appropriate magnetic anisotropy was induced by a dc magnetic field applied during the deposition, and soft magnetic property was further tuned by the conventional annealing. The effects of the induced anisotropy and annealing on GMI have been studied for FeCuNbSiB, FeCuCrVSiB and FeZrB films by measuring the frequency and magnetic field dependence of the GMI.The main works and important results are as follows.1,The GMI effect in Fe-Zr-B based soft magnetic filmsA systematical study on the influence of doping different elements, such as Ni and Cu, on magnetic properties and giant magneto-impedance(GMI) effect was done. The effects of the RF sputtering power and annealing were also studied. Following are summaries of our works:Because of the incompatibility between Cu and Fe atoms, doping Cu atoms of a few percent can prevent the Fe atoms from cluster, and avoid the precipitation of microcrystalline. As a result, the soft magnetic properties are improved. Doping suitable Nb can weaken the transversal anisotropy, while the magnetic properties and GMI effects are reduced simultaneously. It is indicated that appropriate anisotropy is an important precondition for GMI effect.The giant magneto-impedance (GMI) effect has been observed at room temperature in soft magnetic alloy films of FeZrBCu prepared by RF sputtering. The dependence of the soft magnetic properties and the GMI effect on the sputtering power has been studied. Electron probe micro-analysis (EPMA) reveals that the as-deposited films sputtered at 240W possess 2.9at% Cu and 87.32at% Fe. This sample possesses excellent soft magnetic properties and the maximum values of GMI effect, the coercive force is about 68A/m, the saturation magnetization is about 1.36MA/m, and the maximum GMI ratios are 17% and 10.5% in longitudinal and transverse fields at the frequency of 13MHz, respectively. In addition, the dependence of resistance R, inductance X and effective permeability μ_e on the frequency has been investigated. And the results show that the GMI effect is mainly a magneto-inductive effect at low frequencies. Such large GMI ratios at so low frequencies in as-deposited films have not been reported before.The influence of different annealing process on the giant magneto-impedance (GMI) effect has been studied at room temperature in soft magnetic alloy films of (Fe₈₈Zr₇B₅)_0.96Cu_0.04 prepared by RF sputtering. It was found that the longitudinal GMI ratios are decreased after the natural annealing and current annealing. But the sensitivity of the sample annealed at 800mA can reach a value about 7%/kA/m. Furthermore, the magnetic field annealing effects on GMI effects were discussed. The results show 250°C is the optimum annealing temperature, at which GMI ratios reach the maximum values of about 17.5% in longitudinal and 17.8% in transverse dc magnetic fields, at 13 MHz. The transverse magnetic responses of GMI ratios show two peaks around the magnetic field values ±0.4kA/m. 2,The GMI effect in FeCuCrVSiB soft magnetic filmsMagnetic properties and giant magneto-impedance (GMI) effects of FeCuCrVSiB single layered and (FeCuCrVSiB/SiO₂)₃/Ag/( FeCuCrVSiB/SiO₂)₃ multilayered films with Ag as the central layer and SiO₂ as the isolated layers, which were prepared by RF sputtering without field and with a magnetic field about 72kA/m, are investigated. The hysteresis loops of single layered films show that the samples deposited with a magnetic field possess excellent soft magnetic properties and the coercive force is only about 64A/m. After annealing at an optimum temperature of 230°C for 1.5h, the maximum GMI ratios of single layered films are 37.5% in transverse field at the frequency of 13MHz, and the maximum GMI ratios of multilayered films is 277% at the frequency of 8.6MHz. The frequency dependencies of the magneto-impedance indicate that the GMI effect in multilayered films can be very large even at relatively low frequencies.In this structure, the inner conductor provides the main path for the alternating current while the outer enveloping magnetic layers provide a closed path for the magnetic flux induced by the alternating current. The SiO₂ layers play an important role in multilayered films. First, they prevent atomic diffusion between the magnetic layers and the Ag layer, and the conductivity of the Ag layer is increased. Second, they decrease the eddy current losses obviously within the magnetic films, especially at higher frequencies, and therefore the multilayered film has a higher effective permeability at higher frequencies.For the films prepared with a magnetic field along the film length, a larger GMI ratio has been obtained as compared with those prepared with a magnetic field along the film width. It is because the transversal direction is the direction of easy axis for multilayered films, and perpendicular to the direction of the applied magnetic field during the deposition process. 3, The GMI effect in FeCuNbSiB soft magnetic filmsThe giant magnetoimpedance (GMI) effects in RF sputtered FeCuNbSiB films with a dc magnetic field applied in the process of deposition are presented. Dependences of domain structure and GMI ratios on the annealing temperatures are investigated. The results show that after annealing at 300°C, the GMI ratios reach the maximum values of about 38% in longitudinal and 27% in transverse dc magnetic fields, at a frequency of 13MHz. The longitudinal magnetic responses of GMI ratios show very sharp peaks around the transverse anisotropy field, and the sensitivity can reach 47.5%/kA/m in the field range 0.4～6kA/m, at 13MHz. The transverse anisotropy is 0.4kA/m, larger than that of 0.08kA/m in the film prepared without magnetic field, because an additional transverse anisotropy is induced by the field applied in the fabrication process. This may be the reason that a large GMI ratio was obtained.The frequency dependencies of R and X indicate the existence of critical frequencies f_x^* and f_R^* for the change of R and X, respectively, and f_X^*r^*. It shows that at lower frequencies the magnetoinductance is dominant, while at higher frequencies the magnetoresistance effect becomes obvious. The GMI effect is determined by the change of both the resistance and reactance components.Compared the films deposited in a longitudinal magnetic field with these deposited in a transversal magnetic field, the films which was prepared with a transversal magnetic field has a larger GMI ratios. It is because the magnetic field perpendicular to the longitudinal direction which is the easy axis of the films.