Transport Character In Manganese Oxides, FM Multilayer Structure

Since Baibich's discovery of the giant magnetoreistance (GMR) in multilayer films, the magnetoresistance effect has attracted the attention in condensed physics and materials fields due to its potential application. Under the stimulation of the GMR effect, the larger tunneling magnetoresistance (TMR) effect was also discovered in 1995. Furthermore, since the discovery of the colossal magnetoresistacne (CMR) in doped perovskite manganese oxides in 1993, it has become one of the most active research fields because the manganese oxides belong to the strongly correlation system, exhibits the different electrical,magnetic response and has the abundant physical phenomena, such as photo-induced insulator-metal transition, charge order state, orbital order and phase separation, etc. In this thesis, based on manganites epitaxial film, manganites/FM heterostrucure and FM/manganites/FM sandwich structure, the transport and field response characters in the mangantes film and the structure have been invistaged.Our main works are as follows:1. The La_1-xSr_xMnO₃ (x=0.015, 0.5, and 0.3) bulk materials have been prepared by the sol-gel method and the x-ray diffraction analysis of the samples shows that the targets have the perovskite structure.2. The epitaxial La_1-xSr_xMnO₃(x=0.015, 0.5 and 0.3) films have been deposited on LaAlO₃(100) substrate by the magnetron sputtering method. The transport character indicates that the films exhibit the typical metal-semiconductor transition. The samples have the similar response for the magnetic field and laser irradiation. The magnetic field tends to uniform the spin core of the Mn³⁺ and Mn⁴⁺, therefore the double exchange becomes strong and it favors the e_g hopping. So the resistance increases. The laser causes the photo-induced demagnetization at ferromagnetic phase and excites the carriers at paramagnetic phase. The maximum MR is about 21% at T=243 K for the sample x=0.015 when the magnetic field is 0.5T. The maximum photo-induced relative change in resistance is about 9.62% at 360 K for the sample x=0.5.3. The LSMO (LSMO x=0.2) heterostructure has been prepared on Co substrate by the sol-gel process. Both the measured CIP and CPP resistance～T relations favor the semiconductor-type conduction in the whole temperature range and there is no distinct resistance peak for CIP geometry. The changes in resistance with increasing the temperature are more than three orders of magnitude. The measured resistance is stable and repeatable. The CIP geometry below 300 K exhibits the variable-range-hopping mechanism and the current-voltage characteristic shows that the dominant transport seems to be the quasiparticle tunneling. The Schottky barrier forms at the LSMO/Co interface.4. The La_1-xSr_xMnO₃Fe (x=0.015, 0.5, and 0.3) heterostructures have been prepared on Si (100) substrate. The sample for x=0.015 exhibits the typical ferromagnetic metallic (FM)-paramagnetic insulating (PI) phase transition in CIP geometry at T=130K near to the peak resistance temperature (T_p). The samples for x=0.5 and 0.3 favor the semi-conductive type conduction regardless of the CIP and CPP geometry. During the formation of the interface between the LSMO film and Fe, the oxygen of LSMO film is absorbed by the Fe film below due to the strong oxidizability of Fe, which results in the LSMO film in highly oxygen deficiency. The disorder due to the oxygen deficiency in LSMO film induces the sample in energy band structure of the amorphous state with the localized state in the band tails and extended state. The hopping between the localized states and the exciting between the extended states of the carriers form the conduction. These can be verified by the current-voltage relations. We have observed the transient photoconductivity in the LSMO/Fe heterostructure, which is caused by high disorder. The photo injection creates electron-hole pair and increases the density of carriers, which moves the Fermi level through the mobility edge and accordingly provokes the giant change in resistance. The maximum photo-induced relative change in resistance is about 7500% at 220 K for the sample at x=0.3. The photo-induced relative change in resistance is about 2400% for x=0.015 at T=292K.5. The Fe/ La_1-xSr_xMnO₃/Fe(x=0.015, 0.5, and 0.3) /Fe sandwich structures have been prepared by magnetron sputtering and the CPP resistance linearly increases with increasing temperature, indicating that it has the metallic conduction. The sandwiches exhibit the positive and negative magnetoresistance effect as the temperature is increased, which can be attributed to the ferromagnetic-paramagnetic transition of the manganites layer.