Preparation And Electrical And Magnetic Properties Of La_0.67(Ca_1-xSr_x)_0.33MnO₃ (x=0, 1/3) Polycrystals And Thin Film

As a research focus Perovskite manganite has a great application prospect.For La0.67(Ca1-xSrx)0.33MnO3,it has attracted a lot of attention because of its steep metal-insulator transition near the curie temperature and colossal magnetic resistance(CMR)under a external magnetic field.In this thesis,we use the sol-gel and solid state sintering methods to synthesize the LCMO polycrystalline targets and optimize the sintering process by adjusting the calcination and sintering temperature and time.The highest TCR value of the pure LCMO sample has reached to 22%,which is considerably larger than that of the reported pristine LCMO.Then the Sr was doped into LCMO to replace some of the Ca so that the metal-insulator transition temperature(Tp)can be furter enhanced.After enhancing the properties of the polycrystalline we use the pulsed laser deposition(PLD)technique to prepare the LCSMO thin films.By optimizing the deposition conditions,the TP of the film is improved to above the room temperature(TP=350 K),and the large TCR value(7.3%)is obtained at the same time.This result makes La1-xAxMnO3 materials have great potential in application of room-temperature bolometer.Finally we further optimize the polycrystalline sintering process and improve the TCR value to 26%.The experimental results show that the surface morphology,grain size,grain boundary and resistivity have important effects on the TCR value.Especially the TCR is directly related to the resistivity.Next we doped Ag into the LCMO material.According to the previous reports it will significantly improve the microscopic appearance of the crystal and reduce the resistivity,as a result the TCR will greatly increase.In this experiment,althogh the resistivity is greatly reduced,there is no significant improvement for the microscopic appearance of the crystal and the TCR value.We think there are three reasons:First of all,the function of Ag is to improve sintering.Since the sintering progress is already optimized,the influence of Ag is small.Secondly,although Ag has improved the connection of the grains,the intrinsic resistivity is not change.Finally,according to the transition temperature ΔT,TCR is also depends on degee of materials’order.However Ag cannot change that since the preparation process is the same.Through magnetic measurements we found that the sample LCMO:Ag has the largest MR values 40%under the low magnetic field(0.1 T).But under high magnetic field(2T)the MR values of the sample LCMO and LCMO:Ag is almost the same.These phenominaare due to the connection improvement of the grain boundaries with the doping of Ag.The connectivity improvement will reduce the impact of the boundary effect,so the intrinsic magnetic resistance is outstanding.It is also found that the MR increase dramatically with the increasing of the number of grain boundary at low temperature range.When the number of grain boundary is large enough,the MR at low temperatures range is even larger than the MR near the TP.