Temperature stability of magnetoresistance (MR) and MR enhancement in La_(1-x)(Sr_(1-y)Ag_y)_x MnO_3 system

Polycrystalline samples of Lal-x（Srl-yAgy）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 Lal_x（Srl_yAgy）~MnO3 system with both univalent and bivalent elements doped at A site and with unchanged value of Mn~＋/Mn4＋ ratio were explored through the measurements of X-ray diffraction patterns, magnetiza- tion-temperature （M-T） curves, resistivity-temperature （p-T） curves and magnetoresistance-temperature （MR-T） curves. The results are as follows： there are two peaks in the p-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.

Effect of cobalt on the magnetoresistance characteristics of rare-earth doped manganites

The effect of cobalt-doping on the magnetic,transport and magnetoresistance characteristics of La1-xSrxMnO3 was investigated.The results show that the magnetoelectric property of rare-earth doped manganites is greatly affected by substitution of Co for Mn sites.The Curie temperature as well as the magnetic moment decreases with the increase of doping concentration,and the samples exhibit obvious characteristics of the spin glass state.Moreover,the magnetoresistance is evidently modulated by doping concentration,and the relevant temperature dependence is also suppressed.In addition,low-temperature magnetoresistance is significantly promoted as doping concentration increases,which renders a value of approximately 50% in the temperature range of 5-200 K and varies within 12.5%.It can be attributed to the effect of spin scattering, induced by cobalt doping,on the itinerant electrons of Mn ions,thus introducing a spin-disorder region into the ferromagnetic region of double-exchange interaction between neighbouring Mn^3＋ and Mn^4＋ ions.

Electric transport mechanism and magnetoresistance of La_(0.80)Sr_(0.15)Ag_(0.05)MnO_3/x(CuO)

The samples of La0.80Sr0.15Ag0.05MnO3/x（CuO） （x = 0, 0.05, 0.10, 0.15, 0.20） were prepared by the solid-state reaction method, and the structure of the sampies was detected by X-ray diffraction （XRD）, scanning electron microscope （SEM） equipped with energy dispersive spectroscopy （EDS）, electric transport mechanism, and magnetoresistance enhancement, and the temperature stability of magnetoresistance of the samples was studied through resistivity-temperature （ρ-T） curves, ρ-T fitted curves, and magnetoresistance-temperature （MR-T） curves. The results indicate that ρ-T data can be fitted by the formula ρ = ρ0 ＋ AT^2 very well, and the electric transport mechanism of all the samples in metal-like area is the scattering of single magneton upon spin electron; the magnetoresistance of composite samples is far larger than that of the original material, and the MR peak value of the sample with x = 0.20 is nearly 4 times as large as that of the sample with x = 0; composite samples have comparatively good temperature stability of magnetoresistance in the temperature range of 200-260 K, and the magnetoresistance of the sample with x = 0.15 almost does not change with temperature and keeps at （5.03 ± 0.20） % in the temperature range of 210-260 K.