A-Site Dopant Effect Of Perovskite Manganite/Cobaltites

The perovskite manganites and cobaltites have attracted a lot of research due to their interesting structural and magnetic properties, and potential applications in the spintronic devices.In this dissertation, we have studied the half-doped perovskite manganese La0.5Sr0.5MnO3/Dy0.5Sr0.5MnO3 and cobaltites LaCoO3 phase as the research object. With the A-site elements doping, it is found the physical properties of materials, mainly the magnetoelectric properties are change a lot by adjusting the doping level and different elements. Details are as follows:In chapter one: we introduced some basic concepts related to the perovskite manganites and cobaltites by beginning from the basic structure and properties.Meanwhile, the recent developments and studys on the basic structure and properties of manganites and cobaltites have been reviewed and discussed.In chapter two: the perovskite manganese oxides La0.3Ce0.2CaxSr0.5-xMnO3(0?x?0.25) have been synthesized by a solid-state reaction,and their transport and magnetic properties have been systemically studied. It is found that the temperature dependent resistivity shows a maximum at Tmax below Curie temperature Tc with temperature decreasing, which is correlated to the Ce3+ spin-dependent scattering.Moreover,Tmax is almost unchanged when x?0.15,but decreases quickly and is closing on the temperature of Tc while x>0.15, which indicates the increase of Ce ionic valence.It is suggested that Ca dopant changes not only the chemical pressure,but also the valence of Ce ion. There is a threshold value for the chemical pressure corresponding to Ca-doping content at around x=0.15 (or <rA>tv=1.2395A), which is the starting point of charge transfer appearance in Ca-doping La0.3Ce0.2Sr0.5MnO3 compound.In chapter three: the Dy0.5-xLaxSr0.sMnO3 (0?x?0.4) manganite compounds have been synthesized by solid state reaction, and the effect of nonmagnetic La ion doping on the crystal structure, electrical and magnetic properties are systematically investigated.It is found that with successive doping of La element, A-site cationic arrangement transforms from disorder (random, x?0.25) to order structure (x>0.25),and the crystal structure undergoes a transition from cubic to orthogonal distorted perovskite structure. When 0?x<0.25, the compounds show a similar semiconducting behavior and magnetic property with a low-temperature spin-glass state. When x=0.25,however, long-range ferromagnetic (FM) order begins to form and coexists with anti-ferromagnetic (AFM) phase in the compound. With x further increases, FM interactions strengthen and become predominant in the magnetic properties of the compound.On the other hand, in contrast to the semiconducting behavior of the compounds with x<0.25, when x>0.25, the resistivity decreases quickly in the corresponding temperature range.It is suggested that not only the A-site cationic disorder turning, closely related to the cation radius distribution (mismatch), ?2,controlled by A-site cation radius <rA>, but the doping of nonmagnetic La ions, or A-site magnetic change, is an effective way to adjust the crystal structure and the related magnetic and electrical properties of the perovskite manganite oxides for the applications.In chapter four: In this work, the magnetic properties of polycrystalline La1-xCaxCoO3 (0?x?0.25) (LCCO) are systemicallystudied. It is found that all the Ca-doped samples exhibit the FM ground states with two FM transitions. Above the high-temperature FM transition, a short-range FM state is also found to be present in this system. However, quite different from the Sr and Ba-doped cobaltites, but similar to the doped manganites, the short-range FM state herein can be well viewed as the Griffiths phase. The possible origin of this important difference is discussed.In chapter five: the transport properties of polycrystalline LCCO with are systemically studied.Magnetotransport measurements reveal that besides the two types of negative MR as reported in the Sr-doped cobaltites, a positive MR is interestingly found in the LCCO, which increases with doping. As x is increased to 0.25, the MR at low temperatures is dominated by the positive MR. Moreover,it is isotropic and nearly linear with the magnetic field above 20kOe. The possible origin of the positive MR is observed.