Magnetic and transport properties of cobalt doped La0.7Sr0.3MnO3

Abstract

The magnetic and transport properties of La0.7Sr0.3Mn1−xCoxO3 (0 ≤ x ≤ 1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x > 0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3 < x ≤ 0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ∼0.1 for the typical La0.7Sr0.3Mn0.6Co0.4O3 sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co3+ ions are in the intermediate spin state with the increase of Co-doping level (x ≥ 0.3), while the Co4+ ions show the spin state transition from the high spin state to the intermediate spin state at x = 0.6. The temperature dependence of resistivity shows a metal-insulator transition character for the intermediate Co-doping samples, which can be well fitted by the three dimensional variable-range-hopping model in the whole temperature range. These facts suggest that the double exchange interaction in the perovskite La0.7Sr0.3MnO3 is destroyed by the B site doping, due to the strong competition between the double exchange and superexchange interactions in Co/Mn ions as well as the frustration of spins in the system.