| High sensitive magnetic sensors and large magnetic memory devices require materials with sensitive magnetic response properties. Such materials have been explored by researchers ever since the discovery of the Hall Effect and the intrinsic magnetoresistance effects, which are based on the Lorentz force. The Hall Effect materials have been widely applied on the magnetic sensors, such as Hall Effect displacement transducer, Hall Effect compass and Hall Effect tachometric transducer. However, the sensitivity of the Hall Effect materials is still too low to perform in some applications. Meanwhile, the dimension of semi-conductor devices has been reduced to several nanometers, a series of quantum interference effects in the mesoscopic physics could disable the classical physics laws. Therefore, it is necessary to develop new magnetic induction materials.The Giant Magnetoresistance (GMR) effect was discovered on the Fe/Cr multilayer films. With the application of magnetic fields, the Fe layers could overcome the anti-ferromagnetic coupling, and parallel the magnetic moments, thus reduce the resistance of the multilayer film. Such GMR effects have been applied in tremendous storage capacities magnetic memory devices.Perovskite manganites such as La1-xAxMnO3(A=Ca, Sr, Ba, Pb) could exhibit ferromagnetic because of the double-exchange interaction, There are complex interplay among charge, spin, and lattice within the manganites, which could lead to a rich spectrum of unique physical properties, such as the colossal magnetoresistance (CMR) effect discovered in1993. The CMR effect could exhibit even large field induced change of resistance than the GMR effect near Curie temperature (TC). However, the CMR effect is only very strong near Tc under large applied magnetic fields of several Teslas, but usually very weak at room temperature under low fields, which limits the potential applications. In2000, Hu and Qin demonbstrate that the Giant magnetoimpedance effect at radio frequencies on the bulk manganites and caught much attention due to the sensitive response to low magnetic fields. The GMI effects was firstly found in soft magnetic metallic materials, such as amorphous Co-Fe-Si-B or Fe-based nanocrystalline wires and ribbons, and mainly connected with the field induced change of transverse permeability via the penetration depth. Similar to the metallic materials, the manganite materials have high Ms and low Hc, which make it possible to achieve GMI effect on manganite materials. In2000, the GMI effect on manganites was reported on La0.67Ba0.33Mn03polycrystalline bulk samples. Later on, more reports about the GMI effect on manganites were published. For the manganites, the GMI effect could provide much more sensitivity to the applied magnetic fields than the CMR effect. Compared with the metallic materials, the manganite materials could approach the saturation of GMI effect under larger fields. These properties make the GMI effect on manganites have a potential application on some magnetic sensors.As the origin of the magnetism and transport properties for the manganites and metallic materials are quite different, it is necessary to investigate the study the correlation between the GMI effect and field induced permeability change for the manganites. Meanwhile, to develop new measurement methods which could obtain larger GMI effect, could also improve the application prospect for manganites. Miniature magnetic memory devices require small sample size, where the manganite film samples should also be considered.In this dissertation, we investigated the GMI effects for the manganites, and the following results are included:1. GMI effect for sol-gel Lao.65Ba0.35MnO3at room temperature.We prepared the La0.65Ba0.35MnO3manganite samples by sol-gel method. GMI measurements were carried out with both four-terminal method and coil method. With the four-terminal method, the maximum of ac magnetoresistance△R/R0=-53.9%, magnetoreactance△X/X0=-36.0%and GMI effect△Z/Z0=-27.8%could be obtained under H=500Oe. By comparing the GMI effect and field induced permeability change ratio, we conformed that the GMI effect on manganites are connected to there field induced change of transverse permeability via the penetration depth. The results measured with the coil method on the same sol-gel La0.65Bao.35MnO3manganite indicated that with the coil method we could obtain much larger GMI effect on the same sample. Under H=500Oe, the maximum of ac magnetoresistance△R/R0=-95.0%, magnetoreactance△X/X0=-80.7%and GMI effect△Z/Z0=-80.6%could be obtained, which are the largest magnetoimpedance effect among all GMI reports for manganites at radio frequency range so far. When measuring with the coil method, it is the longitudinal permeability that is connected with the GMI effect. By studying the classical electrodynamics equations, and comparing the GMI results and field induced longitudinal permeability change ratio, we can infer that the there is approximative linear relation between the GMI results and the longitudinal permeability change ratio. Thus larger GMI effect could be obtained with the coil method through the longitudinal permeability change.2. Influence of sample thickness and measurement geometry on GMI effect.We measured the GMI effects of sol-gel Lao.7Sro.3Mn03manganite samples with different thickness. The results indicated that the thicker sample prefers to have a stronger skin effect and GMI effect at low frequencies. Meanwhile, the results obtained by coil method with different number of turns shows that larger GMI effect could be gained with more turns. Such results could help us seeking larger GMI effects.3. The effects of grain boundaries on GMI effect.For the sol-gel La0.7Sr0.3MnO3manganite with average grain size of about100nm, a classic low field magnetoresistance effect was observed at room temperature. However, under ac case, there is no obviously effect of the grain boundaries on the GMI effect at room temperature. As for the La0.65Bao.35MnO3sol-gel manganite with average grain size of about1um, the existence of grain boundaries could bring a secondary resistance-temperature peak at low temperature below Tc under dc case and low frequency range of ac case. With increasing ac frequency, skin effect gradually enhances, the peak associated with the grain boundaries are less pronounced. The effects of grain boundaries on ac resistance are not obviously. At a very low frequency of100Hz, we observed a very small negative reactance of about-10mcΩ, which is connected with the capacitance of insulator state of the grain boundaries. At high frequencies, an X peak is pronounced at200K, which is connected with the inductance behavior of the grain boundaries. Thus we can conclude that the grain boundaries should not be simply considered as tunneling insulator state, but a mixture of weak ferromagnetic metal and paramagnetic insulating state, and exhibits different behaviors as the frequency or temperature changes. Meanwhile, with the affect of grain boundaries, the maximum of GMI effect take place near room temperature below Tc, which brings a special advantage in potential applications.4. GMI effects on manganite films.We prepared the La0.67Sr0.33MnO3film on LaAlO3(100) substrate and La0.75Sr0.25MnO3polycrystalline film on Si (100) substrate using by PLD, respectively. Both of the films exhibit ferromagnetic metal at room temperature, and intrinsic CMR effects. Under ac case, both the films are affected by the capacitive substrate. The La0.67Sr0.33MnO3film with thinner thickness could not exhibit the GMI effect due to the stronger affect from the substrate. However, GMI effect with△Z/Z0=-8.1%could be observed on the thicker La0.75Sr0.25MnO3polycrystalline film with less effect from the substrate in spite of the grain boundary effect. It can be concluded that the existence of grain boundaries is not a decisive factor for gaining GMI effect on manganite films. To fit the requirement of applications on Miniature magnetic memory devices, thicker film with thickness larger than1μm should be considered. |
