Structure, magnetic and electrical properties of La-Sr manganite doped with cerium and zinc

Background and Objectives: Doped manganites of La-Sr system exhibiting effects of colossal magnetoresistance, giant magnetostriction, and electrical switching are promising functional materials for various technical applications, in particular for sensor and information devices. However, the influence of characteristics of doping ions, especially those introduced into various sublattices, on the properties of manganites and their frequency dependences has not been sufficiently studied. The aim of this work is to establish the influence of Ce$^{4+}(5p^6)$ and Zn$^{2+}(3d^{10})$ ions, which are donors and acceptors, on crystal lattice parameters of manganites, their magnetization, Curie point, magnetoresistance, and dependences of the resistance on the frequency and amplitude of electric field. Materials and Methods: For the first time, single-phase ceramic La$_{0.625}$Sr$_{0.35}$Ce$_{0.025}$Mn$_{0.975}$Zn$_{0.025}$O$_3$manganite was synthesized. Parameters of crystal structure were determined from powder X-ray diffractograms. Specific magnetization measurements were performed in magnetic field with an induction of 0.56 T. Curie point was determined by the maximum modulus of the derivative of magnetic permeability with respect to temperature. Measurements of the resistance were performed in the frequency range from 100 kHz to 4 MHz at two values of measuring voltage. Dependences of magnetoresistance on the temperature were measured in transverse field with an induction of 0.92 T. Results: The synthesized manganite has a rhombohedral crystal structure. Specific magnetization (91.2 A $\cdot$ m$^2$/kg at 80 K) is higher, and Curie temperature (222 K) is lower than that of the manganite of basic composition La$_{0.65}$Sr$_{0.35}$MnO$_3$. A significant width of the temperature range of ferromagnetic–paramagnetic transition (about 45 K) indicates a high inhomogeneity of obtained manganite. At the temperatures exceeding 285 K, manganite exhibits semiconductor properties. The modulus of negative magnetoresistance varies with temperature nonmonotonically and reaches a maximum (18$\%$) at the temperature of 231 K. With an increase in frequency in the range from 100 kHz to 4 MHz, the resistance of manganite decreases with a slowdown of the fall process in the region of 1.5–2 MHz. Conclusion: The results obtained are explained by diamagnetic dilution of octahedral sublattice with zinc ions, formation of Mn$^{2+}$ ions, increase in spin fluctuations near Curie point, relationship of electronic band structure with magnetic subsystem of manganite, phase stratification and formation of the clusters of different-valence ions, competition and change of charge transfer mechanisms (hopping, tunneling, percolation) depending on temperature and frequency. Established dependencies are important for understanding the mechanisms of the effect of simultaneous introduction of quadrivalent and divalent ions into various sublattices of manganites on their properties, and are also of interest for obtaining manganites with required parameters.