Competing Magnetic Interactions in Co-Doped La0.7Sr0.3MnO3

The crystallographic structural and magnetic properties of polycrystalline La_0.7Sr_0.3Mn_{1_x}Co_xO₃ (x = 0.4, 0.5, 0.6, 1.0) have been systematically studied in detail by neutron powder diffraction and magnetic measurements. The introduction of Co dopant to the Mn network has decreased the bandwidth (W) and the overlap between the O-2p and the Mn-3d orbitals. In addition, it in turn weakened the fragile double exchange coupling of Mn³⁺-O-Mn⁴⁺ and reduced the magnetization M as well as magnetic ordering temperature Tc. The magnetic measurements indicate the presence of mixed magnetic phases in this system. When the temperature is above Tc, the jump of field-dependent magnetization disappeared, which is related to the competition of ferromagnetic and antiferromagnetic interactions. The average atom magnetization extracted from M-H curve at 5 K using the approach-to-saturation method, is -.1.70 μB/Co for La_0.7Sr_0.3MnO₃. This value approaches to the neutron refined data 1.93(8) μB. However, as for x = 0.4, x = 0.5 and 0.6 samples, the calculated data from magnetization at 5 T are all smaller than the refined one, due to the existence of spin-glass-like phase. The weak magnetic peaks are responsible for the small magnetic moment data (0.48 μB for LSMO-Co_0.5 and 0.65 μB for LSMO-Co0.6). These results indicate the competing magnetic interactions in the system of La_0.7Sr_0.3Mn_{1_x}Co_xO₃ compounds.