Magnetic Properties of the Double Perovskites (Sr1-xLax)2Co$M$O6 with $M$ = Sb, Nb, and Ta

We have investigated the crystalline structure and the magnetic properties of (Sr 1-x La x ) 2 Co$M$O 6 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 Co 2+ content resulting from La 3+ 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 $M^{5+}$--O 2- 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.