Giant Magnetoimpedance And Microwave Absorption Properties In Some Soft Magnetic Materials

In this thesis, the giant magnetoimpedance effect （GMI） has been investigated in FeCuNbVSiB nanocrystalline metallic ribbons and sol-gel La_0.7Ba_0.3MnO₃. The microwave absorbing properties of mixture of La0.85K0.15MnO3 and carbon nanotubes have been also studied.With the rapid development of the worldwide science and technology, metal soft magnetic materials have played a fundamental role in the process of the world’s industrialization and informatization. They have been exploited and used more extensively. Due to the excellent soft magnetic properties, nanocrystalline FeCuNbSiB has absorbed much attention. In 1992 Mohri et al found the giant magnetoimpedance （GMI） effect in FeCoSiB amorphous wire:at high frequencies, the risistance and reactance as well as the impedance sensitively vary with the magnetic field. The GMI effect has been later found in FeCuNbSiB, FeZrBCu, FeZrB, FeNbB nanocrystalline ribbons, wires and thin films. Yanzhong Zhang et al also found that through part substitution of Nb by V in FeCuNbSiB, the nanocrystalline FeCuNbVSiB also exhibits excellent soft magnetic behaviors. Since the price of V is cheaper than that of Nb, the nanocrystalline FeCuNbVSiB is also significant. One of the aims of this thesis is to explore the GMI effect in FeCuNbVSiB nanocrystalline metallic ribbons.The influences of annealing temperature on structure, magnetic properties and GMI effect have been studied for Fe_73.5Cu₁Nb₂V₁Si_13.5B₉ and Fe ribbons. The following conclusions have been found:After annealing. Fe_73.5Cu₁Nb₂V₁Si_13.5B₉ and Fe ribbon crystalizes gradually and nano bcc α-Fe phase precipitates. When the annealing temperature is above 550℃. the hardmagnetic Fe2B occurs in the annealed ribbons.The optimal annealing temperature for largest GMI effect in Fe_73.5Cu₁Nb₂V₁Si_13.5B₉ and Fe ribbon is 550℃. Below 550℃, the amount of nano bcc α-Fe phase increases, thus increasing GMI. When the the annealing temperature is above 550℃, the precipitation of Fe2B decrease the GMI of ribbons.For Fe_73.5Cu₁Nb₂V₁Si_13.5B₉ ribbon annealed at 550℃ for 30 min., △X/X（90Oe） is-340% at 50 kHz. The △Z/Z（90Oe） reaches its maximum of-73% at 700 kHz. Meanwhile, AR/R（90Oe）is-90% at 2 MHz.For Fe_72.5Cu₁Nb₂V₂Si_13.5B₉ ribbon annealed at 550℃ for 30 min., △X/X（90Oe） is-500% at 50 kHz. The △Z/Z（90Oe） reaches its maximum of-121% at 700 kHz. Meanwhile, △R/R（90Oe） is-150% at 3 MHz.For the impedance, resistance plays a dominant role at low frequencies. The skin effect increases with an increase of ac frequency. The reactance and resistance increases with an increase of frequency. The magnetoresistance plays an important role in GMI at low frequency, while magnetoreactance plays an important role at high frequencies.We measured the effective inductance of annealed ribbons. The ribbon annealed at 550℃ has the largest effective inductance among all the annealed ribbons. The effective inductance is proportional to the real part of magnetic permeability. The ribbon annealed at 550℃ has the largest real part of magnetic permeability among all the annealed ribbons. This is consistent with the finding of largest GMI at 550℃. The origin of GMI is due to the change of permeability with frequency, which induces the change of penetration depth and impedance. We measured the change of inductance with the transverse magnetic field. We found the change ratio of the effective permeability decreases with frequency. The GMI is function of not only permeability but also the skin effect.It can be found that the GMI value of Fe_73.5Cu₁Nb₂V₁Si_13.5B₉ and Fe nanocrystalline ribbon is function of both magnetic field and ac frequency. At low frequency, the impedance decreases with an increase of frequency, due to the movement of domain wall. At high frequency, the impedance increase at first, undergoes a peak, and then drop again. The peak phenomenon is connected with the transverse magnetic anisotropy. There is an optimal frequency for GMI effect.Due to the colossal magnetoresistance （CMR）, the perovskite manganites have attracted much attention. However, there are some shortcomings for CMR:The large CMR occurs near the Curie temperature or transition temperature between metal and insulator. The magnetosistance is not large at room temperature. In addition. CMR needs a very large magnetic field. In 2000, the GMI effect has been found in La0.67Ba0.33MnO3 sintered manganites. Its magnetic response is more sensitively than that of DC magnetoresistance, which open a novel field of the application of manganites. As the origins of conduction and magnetism of manganites are different from those of metallic materials, there are some novel things for GMI in manganites. In this thesis, the GMI of La_0.7Ba_0.3MnO₃ materialis prepared by sol-gel technique is investigated. The following conclusions have been made:（1） The sol-gel La_0.7Ba_0.3MnO₃ crystallizes as rhombohedaral perovskite structure. The Curie temperature is 350 K, higher than room temperature.（2） The intrinsic properties of La_0.7Ba_0.3MnO₃ at room temperature is metallic. With an increase of frequency, the impedance increases showing the skin effect. Under H=500 Oe, the magnetoresistance △R/R0 is-59% at 3 MHz. The magnetoreactance △X/X0 is-61.4% at 100 kHz. The GMI △Z/Z0 is-46.5% at f=1 MHz. Meanwhile, dc magnetoresistance △R/R0 is only-11.7% under H=8000 Oe.（3） Up to f=20 MHz, we have not found the peak of GMI. This means that the transverse magnetic anisotropy is very small for this manganite.（4） At low temperature and under zero magnetic field, both resistance and reactance increase with increasing temperature. However, under H=500 Oe, the reactance decreases with an increasing temperature.（5） At low temperature and low frequency, magnetoresistance decreases at first, undoes a valley with an increase of temperature. However, at high frequency the magnetoresistance increases with an increase of temperature. The magnetoreactance increases with an increase of temperature. However, the magnetoreactance decreases with an increase of frequency.The microwave absorbing effect, （absorbing the electromagnetic wave and translating it into the thermal energy）, has many application in stealth and other field. It has been found that manganites and carbon nanotubes exhibit microwave absorbing effect, respectively. In this thesis, the microwave absorbing effect of the mixture of La_0.85K_0.1sMnO₃ and carbon nanotubes composite with different thickness D has been investigated. The microwave absorbing effect at D=2.8 mm is very good. The RL values are 12-18 dB within wide frequencies from 9 to 16 GHz.