Temperature stability of magnetoresistance (MR) and MR enhancement in La_1−x (Sr_1−y Ag _y ) _x MnO₃ system

Polycrystalline samples of La_{1− x} (Sr_{1− y} Ag _y ) _x MnO₃ (y = 0.0, 0.2, 0.4, 0.6, 1.0) were prepared by the solid-state reaction method. The temperature stability of magnetoresistance and magnetoresistance enhancement in La_1−x (Sr_1−y Ag _y ) _x MnO₃ system with both univalent and bivalent elements doped at A site and with unchanged value of Mn³⁺/Mn⁴⁺ ratio were explored through the measurements of X-ray diffraction patterns, magnetization–temperature (M–T) curves, resistivity–temperature (ρ–T) curves and magnetoresistance–temperature (MR–T) curves. The results are as follows: there are two peaks in the ρ–T curves of the samples with Ag doping, one is caused by resistance change during the paramagnetism–ferromagnetism transition, and the other is from boundary-dependent scattering of conduction electrons on the boundaries of grains. The peak value of MR increases with increasing Ag doping content, and it increases from 8.2 % for y = 0.2 to 29.6 % for y = 1.0 under the magnetic field of B = 0.8 T; MR remains a constant of 12 % in the temperature range of 218–168 K for the sample with y = 1.0, and the temperature stability of MR is in favor of the practical application of MR.

There are two peaks in the ρ–T curves of the samples with Ag doping; A sharp MR peak exists in high temperature range, this is the character of intrinsic magnetoresistance which increases continuously with temperature decreasing in the low temperature range, and this is the character of low-field magnetoresistance, the competition between them leads to temperature stability of magnetoresistance (MR) in intermediate temperature range. MR basically remains the constant of 12 % in the temperature range of 218–168 K for the sample with y = 1.0, so the temperature stability of MR has been realized.