Study On Structure And Magnetoelectrical Transport Properties Of The Doping Effect Of La-Sr-Mn-O And Related Composites

Manganites are a kind of important functional material which has always stimulated strong interests from 1950s.From 1950 to 1990,most researches had been focused on the magnetic and electrical properties of divalent cation doped manganese oxides.The material has the strongest ferromagnetic and metallic properties at the 33%doping level,which had been elucidated by the double exchange model where the carders hop through neighboring Mn³⁺and Mn⁴⁺ion with strong Hund coupling.From 1990s,some new effects such as colossal magnetoresistance effect, grain-boundary effect and magnetocaloric effect have triggered new increasing surge.The materials will show larger applying prospect in the fields such as hard disc head-reading and room-temperature refrigeration.La_2/3Sr_1/3MnO₃(LSMO)compound that exhibits a paramagnetic insulator to ferromagnetic metallic around 360K above room temperature,has a prospect in application.The material, however,is challenging many researchers in many fields including too low room-temperature magnetoresistance,too high external field(several tesla)and too low low-field magnetoresistance.The related magnetotransport mechanism remains obscure.In this paper, The effect of divalent Zn²⁺substitution for La site on the structure and magnetotransport properties has been investigated.Moreover,different functional materials including ferroelectric BaTiO₃,semiconducting materials with different bandgap(CuO,ZnO,Al₂O₃),ferdte with ferromagnetic insulator such as soft-magnetic ferrite and hard-magnetic ferrite,have been incorporated into LSMO matrix to investigate the coupling effect between these materials and its effect on spin-dependent scattering and tunneling effect.Some novel results were obtained in the following and related mechanisms have been interpreted.1 ZnMnO₃ compound shows insulator with spinel structure.If La is partially substituted by Zn²⁺,the La_1-xZn_xMnO₃ compound exhibits perovskite structure and the resistivity reaches the minimal at x=1/3.If Sr²⁺is partially substituted by Zn²⁺and the Mn³⁺/Mn⁴⁺ratio is fixed at 2, the resulted compound maintains perovskite structure,the Curie temperature is shifted towards lower temperature and the magnitude of magnetoresistance(MR)near Curie temperature increases. These results show that the codoping of Zn²⁺and Sr²⁺is an important way to improve room temperature magnetoresistance. 2 The structure and magnetotransport properties of LSMO/BaTiO₃ composites have been investigated by introducing different doping level of BaTiO₃(BTO)into LSMO matrix.The results show that there exists two kinds of grain boundaries:LSMO/LSMO and LSMO/BTO/LSMO.The resistivity increases by the increase of BaTiO₃ doping level and increases by two orders of magnitude at the doping level of 5%.If Ba is partially by La to enhance the electrical conductivity and lower the potential barrier of grain boundaries,there is a around 50%decrease in the resistivity of the composite with x=5%.These results show that LSMO/BTO/LSMO governs the magnetotransport properties of LSMO/BTO composites.Low temperature resistivity decreases dratically in a strong magnetic field of 5T,indicating that the spin-dependent tunneling effect is dominated.High-temperature resistivity decreases little in 5T field indicating that thermal tunneling process is dominated.The temperature dependence of magnetoresistance of BTO doped composites exhibits different behavior from that of LSMO compound.The MR decreases monotonously with increasing temperature.Moreover,the low temperature MR increases and high temperature decreases by the increase of the doping level of BTO.The magnetic-field dependence of MR shows that low field MR decreases and high-field MR increases with increasing the BTO doping level at temperatures below 280K.3 The BaTiO₃ powders were prepared by hydrothermal method.The results show that the Ba/Ti ratio and alkaline have an important effect on the quality of the powder product.It is beneficial to form high-quality powder at high Ba/Ti ratio(＞2)and high alkaline.If the powder was introduced to LSMO matrix,the resistivity is small even at high BaTiO₃ doping level(20%). Other research is in progress.4 The structure,magnetic and magnetotransport properties of Ls_2/3Sr_1/3MnO₃(LSMO)/0.33(CuO,ZnO,Al₂O₃)composites have been investigated to explore the role of second introduced phase.The microstructural analysis shows two kinds of grain boundaries:LSMO/LSMO and LSMO/Second phase/LSMO.Two maximal resistivities appear in LSMO/0.33CuO and LSMO/0.33ZnO composites while the resistivity of LSMO/0.33Al₂O₃ decreases monotonically with increasing the temperature from 200K to 400K.Moreover,the temperature dependence of magnetoresistance(MR)of LSMO/0.33Al₂O₃ that decreases monotonically with increasing the temperature is different from that of LSMO/0.33CuO and LSMO/0.33ZnO.Potential-barrier scattering and spin-dependent scattering play an important role on the magnetotransport behavior of LSMO-based composites.Low bandgap leads to low resistivity and low magnetoresistance at the temperatures below room temperature.Although ZnO has a wide bandgap of 3.2ev at room temperature,ZnO is easy to collapse to form Zn donors or O vacancys during sintering,leading to weak potential-barrier scattering and low resistivity of LSMO-based composites.5 Effects of soft-magnetic ferrite and hard-magnetic ferrite on the structure and magnetic transport properties in La_2/3Sr_1/3MnO₃(LSMO)/xferrites(x is the doping content, ferrites=Mn_0.5Zn_0.5Fe₂O₄,BaO·6Fe₂O₃)composites have been investigated.It was found that the inclusion of MnZn ferrite(Mn_0.5Zn_0.5Fe₂O₄,MZF)phase reduces magnetization and ferromagnetic-paramagnetic transition temperature(T_c).With increasing the content of dopants, the high-temperature magnetoresistance(MR)decreases while low-temperature MR increases and attains 42%at 150K and x=0.1.However,for LSMO/BaO·6Fe₂O₃(BaM)composites, magnetization and ferromagnetic-paramagnetic transition temperature(T_c)decrease firstly with x＜5%,and then increase with x＞5%.The resistivity increases by five orders of magnitude at x=1%and is out of measured range x=5%.High magnetic field has little effect on the resistivity and magnetoresistance for the composites with x＞=5%,which should be attributed to the pining effect of BaM grains.