LaMnO_3的A位二元掺杂对磁电阻效应的调制和提高

Modulating and Promoting of Magnetoresistance Effect by Two-Element-Doped in A-site of LaMnO_3

基于探讨钙钛矿型锰氧化物磁电阻效应的目的,样品采用固相反应法制备,主要就LaMnO3的A位二元掺杂对磁电阻效应的影响作比较研究,发现(La1-xRx)0.67T0.33MnO3(T为Ca,Sr;R为Sm,Gd,Tb,Y)随x的增加,Tp和Tc逐渐降低,ρm和磁电阻值迅速增加,均可用晶格效应来解释;La0.67Sr0.33-xCuxMnO3(0≤x≤0.33)的Tp随x的增加而下降,室温磁电阻值明显增大还提高了其温度稳定性,是因A位平均离子半径<rA>和A位离子的失配度起主要作用;La0.67(Ba1-xCax)0.33MnO3(x=0,0.40,0.45,0.55,0.60,1.00)的ρ-T变化呈现双峰特性,在0.4 T的磁场下,在77 K^230 K的温区内,磁电阻随T的升高而单调下降,可用晶界效应来解释,在Tc附近出现峰值磁电阻,这种高温磁电阻变化行为可用DE模型及非磁无序来说明;La0.5Ca0.5-xBaxMnO3(0≤x≤0.50)系列样品,当x≥0.14时随x的增加Tp升高,Tp处的峰值电阻逐渐减小,在5 T的磁场下,x≤0.20时磁电阻随T的降低先增大后减小再增大,峰值在Tp附近,x≥0.20时磁电阻随T降低而线性增加,其原因是相邻Mn离子间存在铁磁性和反铁磁性两种效应相互竞争。

Samples were prepared by conventional solid-state reaction method to study magnetoresistance effect of perovskite Mn oxide. The influence of two-element-doped in A-site of LaMnO3 on the magnetoresistance effect was investigated comparatively. It is shown that with increasing the doping amount of x in （La1-aRx）0.67T0.33MnO3 （T = Ca, Sr; R = Sm, Gd, Tb, Y） , Tp and Tc decreased monotonically, ρm and magnetoresistance value increased dramatically, which can be explained by the lattice effects. With increasing the doping amount of x, Tp of La0.67Sr0.33-xCuxMnO3 （0 ≤ x ≤ 0. 33） decreased, room-temperature magnetoresistance value and temperature stability increased, the mismatch of the average ionic radius of A-site and ionic of A-site plays a main role in these phenomena. It can be explained by grain boundary effects that the double peaks appeared in the ρ-T of La0.67 （Ba1-xCax）0.33 MnO3 （x = 0,0.40,0.45,0.55,0.60, 1.00） and magnetoresistance dropped monotonously from 77 K up to 230 K in the surrounding 0.4 T with the increasing of temperature. Around Tc, they had magnetoresistance peaks, this behaviors can be explained by DE model and nomagnetic randomness. For a series of samples of La0.5 Ca0.5-xBaxMnO3 （0 ≤x ≤ 0. 50）, when x ≥0. 14, Tp increased and resistivity peaks droped monotonically with increasing the doping amount of x. In the surrounding 5 T, for x ≤ 0.20, the magnetoresistanee increased along with the decreasing of temperature and reached a maximum near the transition temperature, then it decreased and reincreased with the decreasing temperature; for x ≥0.20, the magnetoresistanee increased linearly with the decreasing of temperature, which can he explained by the competition between the ferromagnetic coupling and antiferromagnetic coupling in this system.