Study Of Electromagnetic Transport Properties In The Pr_1-xCa_xMnO₃Ceramics

Based on the potential application in the magnetic storage and magnetic sensors, the alkaline-earth doped perovskites R1-xMxMnO3(R=rare earth element, M=alkaline earth element) have become the focus of study in condensed matter physics. Early, people have found the colossal magnetoresistance (CMR) effect in the Pro.7Cao.3Mn03thin film. However, the origin of the CMR effect has not formed the consentaneous view. The aim of this text is to discuss the origin of CMR effect.In this paper, the series ceramic samples Pr1-xCaxMnO3(x=0.1,0.3,0.5,0.7) prepared by solid-state reaction have investigated by the direct current (DC) and alternating current (AC) methods under different magnetic and electric fields, respectively. For the sample with doping concentration x=0.3, it shows remarkable CMR effect due to the two-phase coexistence with ferromagnetic insulating state and the charge ordered antiferromagnetic insulating state in the electronic phase diagram, and the Curie temperature is145K, which is consistent with that reflected from M-T measurement by Vibrating Sample Magnetometer (VSM). The AC measurement shows that the grain boundary resistance reduces, while that of grain keeps the same values even under the same magnetic field, which well indicates that the CMR effect originates from the grain boundary but not the grain itself. The grain boundary barrier height is117meV obtained by fitting the temperature spectrum of impedance under different frequency, which is coincident with that obtained from the DC-RT data. The "trap state" model is proposed to explain all the measuring data.For the samples with doping concentration x=0.1,0.5,0.7, the same measuring methods have carried out. And the results show that, only the sample with x=0.1has the Curie temperature of125K, while the other two components (x=0.5,0.7) do not show the characteristic temperature. Additionally, for all the samples, the magnetic fields have not effect on the grain boundary resistance. Besides, the activation energy is obtained by fitting the emperature spectrum of impedance t under different frequency using Arrhenius formula, and they are149meV,55meV and59meV for the samples with doping concentration x0.1,0.5and0.7, respectively.