Effects of Ca2+ doping on dielectric, ferromagnetic properties and magnetic phase transition of SmFeO3 ceramics

In this paper we deal with the preparation of Sm1-xCaxFeO3 (x = 0-0.3) ceramics by the solid stat reaction and study the influences of Ca2+ doping on the dielectric, ferromagnetic properties and magnetic phase transition of SmFeO3. The crystalline structures of the Sm1-xCaxFeO3 (x = 0-0.3) samples are characterized by X-ray diffraction. The dielectric property is measured by a precisive impedance analyzer (HP4294A) in a frequency range from 40 to 110 MHz. The microstructures of Sm1-xCaxFeO3 are imaged with scanning electron microscope under an operating voltage of 20 kV. The coexistence of Fe3+/2+ ions in Sm1-xCaxFeO3 samples is investigated with X-ray photoelectron spectroscopy (XPS). The magnetic properties of Sm1-xCaxFeO3 are measured with the physical property measurement system. The result shows that all the peaks for Sm1-xCaxFeO3 samples can be indexed according to the crystal structure of pure SmFeO3 and their fine crystal structures are obtained by XRD. The lattice parameter a value of SmFeO3 gradually increases, while the values of b and c decrease, and the unit cell volume (V) shrinks slightly with the increase of x. The scan electron microscope images indicate that Ca2+ doping significantly increases the grain size of SmFeO3 ceramic. The average grain sizes of Sm1-xCaxFeO3 samples range from 0.5 to 2 mu m with Ca2+ doping. The epsilon(r) values of Sm1-xCaxFeO3 (x = 0.1, 0.2, 0.3) measured at 1 kHz are about 5, 3 and 2.6 times greater than that of SmFeO3, respectively, and dielectric loss increases by an order of magnitude. The increase of epsilon(r) is mainly caused by the interaction between the dipole and the space charge orientation polarization. Both the conductance current and the space charge limiting current are the main factors to increase the dielectric loss. The magnetic measurements show that the M-H curves of Sm1-xCaxFeO3 (x = 0-0.3) samples exhibit saturated magnetic hysteresis loops with the increase of Ca2+, and the M-r values of Sm1-xCaxFeO3 (x = 0.1, 0.2, 0.3) are 20, 31, and 68 times that of SmFeO3, respectively, indicating the weakly ferromagnetic behavior. The XPS spectrum indicates that the Fe2+ and Fe3+ co-exist in each of Sm1-xCaxFeO3 samples. The ratio of Fe2+/Fe3+ increases with doping Ca2+ increasing, and the magnetic preparation of SmFeO3 is enhanced. It can be attributed to the structural distortion and the formation of Fe2+-O2--Fe3+ super exchange. The spin recombination temperature (T-SR) and the Neel temperature (T-N) are obtained, respectively, to be 438 K and 687 K by measuring the M-T curves. It is noted that both T-SR and T-N of SmFeO3 samples move toward low temperature with the increase of x, and the spin recombination disappears when x = 0.3. This is mainly due to the stability of the magnetic structure of SmFeO3 sample and the interactions of Fe2+-O2--Fe3+ and Sm3+-O2--Fe3+ super-exchange.