Study On Structure And Magnetic Ground State In Sm_0.5Ca_0.5Mn_1-xCo_xO₃

At present, the perovskite manganites are the most representative materials system which can show versatile unconventional electronic-lattice structural changes or insulator-metal transitions upon stimulation by external stimuli, not only by magnetic field but also by irradiation with light, x-rays and electron-beams as well as current injection. The underlying mechanism for such electronic phase transitions is believed to be common to that of colossal magnetoresistance (CMR).In this paper, we started from the half-doped manganite Sm0.5Ca0.5MnO3-in other words, the density of the hole carrier is 0.5-to study the effect of Co3+ doping on the coupling among spin, orbital, charge and lattice degrees of freedom. The pure-phase polycrystalline Sm0.5Cao.5Mn1-xCox03 were prepared using a standard ceramic process. The evolution of crystal structure were investigated using powder x-ray diffraction (XRD), dc magnetization and ac susceptibility measurements under different conditions have been performed by physical property measurement system (PPMS). We calculated the orthorhombic deformation and the normalized slope for all samples. The critical slowing down model and the dynamic scaling equation were applied to some samples.The pattern of XRD illustrates that the pure orthorhombic perovskite phase has been obtained for all samples. The lattice parameters were refined by the least square fitting method using the Jade program. The decrease of unit cell volume and orthorhombic deformation with the doping of Co3+is consistent with the variation of the radius of substitution ions and the temperature of charge order respectively.Dc magnetization measurements show that the charge and magnetic order of the parent phase can be strongly impact by doping of Co3+. The charge order peak of Smo.5Cao.5MnO3 is so sensitive to the ratio of Mn3+ and Mn4+ that the charge order phenomenon almost disappeared at the Co3+ doping ratio of 2.5%. Ferromagnetic components does exist in the parent phase. For now, we cannot distinguish whether it is an antiferromagnet with little ferromagnetic clusters or a canted antiferromagnet. All the samples except parent phase display the characteristics existing in both the glassy state and phase separation system, i.e., the occurrence of freezing temperature and irreversibility temperature. Negative magnetization behavior was observed in the zero-field-cooled curves of high-doped samples (x=0.15 and 0.20), we considered x=0.15,0.20 samples to be ferromagnetic after eliminating the artifact might be caused by the residual field trapped in the superconducting magnet. The M-H loops present ferromagnetic behavior for all samples.There is no detectable peak in the χ"(T) curves of parent phase, it means that the ground state is homogeneous without clusters and the FM behavior is indeed caused by the effect of canted antiferromagnetism. The results of P indicated that these samples are not canonical spin glass. The x=0.025,0.05,0.075 and 0.10 samples are considered to be cluster spin glass based on the more reasonable fitting results of critical slowing down model and dynamic scaling equation. Considering the broad bend of the zero-field-cooled curve and the dependent on temperature below freezing temperature in the field-cooled curve, the transition state from antiferromagnetism to spin glass should provide the background of the x=0.025 sample. The x =0.15 and 0.20 samples are phase separation ferrimagnet with different size distribution of ferromagnetic clusters. Moreover, the small peaks at～8 K in χ"(T) curves which have no obvious shift depend on frequency are believed to be induced by the canted moments of Mn ions.