Magnetic Properties of the Double Perovskites (Sr1-xLax)2CoMO6 with M = Sb, Nb, and Ta

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We have investigated the crystalline structure and the magnetic properties of (Sr1-xLax)2CoMO6 with M = Sb, Nb, and Ta. For M = Sb, an almost completely ordered double perovskite structure was realized for all x. On the other hand for M = Nb and Ta, the ionic disorder of Co and M ions occurs for x \leq 0.1, while the ionic order increases with increasing Co2+ content resulting from La3+ substitution. The magnetic susceptibility (\chi) for M = Sb showed a Curie--Weiss-like temperature dependence for all x values, suggesting the high-spin (HS) or the intermediate-spin (IS) state as the ground state of Co-3d electrons. For M = Nb and Ta with x \leq 0.1, the spin state of Co ions is interpreted as a mixture of Co ions with LS and IS (and/or HS) states, and a spin-glass-like freezing of the magnetic Co ions occurs below 20 K. For x \geq 0.2, HS or IS states becomes dominant with increasing the ionic order of Co and M. We infer that the difference in the covalent character between M^{5+}--O2- bond for M = Sb and that for M = Nb or Ta is the origin of both the difference of the ionic order and the difference of the spin state of Co ions through the crystalline field around them.
2012-04-15