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We reported the synthesis, characterization, electrical transport, and magnetic properties of polycrystalline La0.7Ca0.24Na0.06MnO3:Agy (LCNMO:Agy) with y = 0.0, 0.1, 0.2, 0.3, and 0.4. The combination of EDS and XPS proved that Ag not only entered the perovskite phase structure but there would also be a small amount of Ag enrichment at the grain boundaries. The magnetoresistance (MR) and temperature coefficient of resistivity (TCR) improved significantly with the addition of Ag as a dopant. At y = 0.2, a multi-functional perovskite material with high TCR (30.57% K−1) near room temperature and large MR (72.28%) at the low magnetic field (1 T) was obtained. This enhancement of MR and TCR near room temperature was attributed to the good connectivity between the grains and the decrease in electron scattering at the grain boundaries. Overall, these findings look promising for future applications in infrared detection bolometers.

Manganites with large temperature coefficient of resistance (TCR) have been considered as potential candidate for infrared detecting. To date, A-site doping with alkaline-earth metal (+2 oxidation state) has been widely used to improve the TCR. However, doping with alkali ions (+1 oxidation state) was less explored. In this study, 2% K doped La0.7Ca0.28K0.02MnO3 (LCKMO) ceramics were prepared by sol-gel method and sintered within the temperature range of 1000–1300 °C for structural and electrical transport properties investigation. X-ray diffraction (XRD) analysis demonstrated that all samples exhibited an orthorhombic perovskite structure with space group Pnma. Elevated sintering temperature would lead to an increase in grain size, which could account for the concurrent decrease of the resistivity (ρ). By optimizing the sintering process, the magnetoresistance (MR) and TCR reached their maxima of 60.95% and 13.54% K−1, respectively, for sintering temperature of 1200 °C. By fitting the temperature dependence of resistivity (ρ-T), grain boundary scattering and small-polaron hopping (SPH) were found to dominate in the low-temperature ferromagnetic and high-temperature paramagnetic regions, respectively. In addition, the ρ-T in the whole temperature range of 100–300 K can also be explained by the percolation model.

A series of La0.71Ca0.29Mn1−xVxO3 (x = 0.00, 0.01, 0.03, 0.05, 0.10) ceramic samples were synthesized via sol–gel method. Structural, electric, and magnetic transport properties of samples were characterized. Results show that all samples have perovskite phases, and no impurity phases were detected. Slight doping with V on B sites does not change perovskite structure of samples, but the doping leads to lattice distortion through modifying Mn–O bond lengths and Mn–O–Mn bond angles. The cell volume increased, melting point decreased, and abnormal grain growth occurred as the content of V increased. Temperature dependence of sample resistivities (ρ–T) was tested using standard four-probe method. Results show that slight doping with V can effectively increase temperature coefficient of resistance (TCR) up to 37.04% K−1 (x = 0.01). However, this decreases metal–insulator transition temperature (TMI). Meanwhile, ρ–T dependence in magnetic field (1 T) shows that V substituting for Mn can significantly increase magnetoresistance of La0.71Ca0.29MnO3 near room temperature by ~ 78.3%, showing the promise of this material for use in magnetoresistive sensors, contactless magnetoresistive switches, and memories.

La0.67−xDyxSr0.33MnO3 (x = 0.1, 0.12, 0.15, 0.20, 0.25, 0.30, 0.32) ceramics were prepared by sol–gel method. The samples were characterized by scanning electron microscopy, X-ray diffractometry, and resistance–temperature test (ρ–T) method. The experimental results showed that all of the samples were composed of a single phase with the orthorhombic perovskite structure and Pnma space group. The samples had high density, the average size of the grain decreased from 11.83 μm to 7.24 μm, while the number of grain boundaries increased. The one reason why resistance (ρmax) increases was supposed to be the boundary scattering enhancement, while the temperature corresponding to the peak resistivity (TP) decreases. The average cation radius of the A-site decreased as the amount/concentration of doping with Dy3+ increased, changing the bond length and bond angle. Meanwhile, the double exchange was suppressed, this being another and more important reason why the resistance increased and the TP decreased. The metal–insulator transition temperature wide degree (ΔT) and resistivity (ρ) are the important factors in the temperature coefficient of resistance. In the competition of the two factors, the ρ occupied an advantageous place, therefore the electrical properties of the materials can be further optimized by adjusting the average cation radius of the A-site.

La0.67Ca0.33MnO3 (LCMO) attracts considerable attention as a quintessential example for colossal magnetoresistance (CMR), metal-insulator transition and related temperature coefficient of resistance (TCR) studies. Here, co-precipitation method was utilized to prepare the LCMO ceramics, whose magnetotransport properties as a function of calcination temperature (Tcal) and calcination time (tcal) were investigated. The magnetotransport properties of these LCMO ceramics were significantly enhanced compared with LCMO derived by sol-gel methods. The TCR of LCMO increased firstly and then decreased as the Tcal increased, whereas the metal-insulator transition temperature (TMIT) shifted towards to the lower temperature. Magnetoresistance (MR) increased as Tcal rose and reached 82.4 % at Tcal = 800 ℃. The mechanism of such magnetotransport properties with different temperature ranges was discussed. The optimal TCR of 32.3%·K-1 in LCMO was prepared with Tcal = 500 ℃ and tcal = 8 h, showing that co-precipitation method would facilitate the potential application of LCMO in infrared detecting and magnetoresistive switching.