Magnetic frustration in (LaA)CoNbO6 (A = Ca, Sr, and Ba) double perovskites

The crystal and magnetic structures of the (LaA)CoNbO6 (A = Ca, Sr, and Ba) double perovskites have been investigated. The A = Ca and Sr compounds crystallize with a monoclinic P2(1)/n superstructure while the larger A = Ba gives a tetragonal I4/m superstructure. These materials have a rocksalt ordered arrangement of Co and Nb with almost no inversion (<1%) for A=Ca and 4% inversion for A=Sr and Ba. Magnetic susceptibility measurements reveal antiferromagnetic ordering transitions with Neel temperatures of 17 K (A=Ca), 16 K (A=Sr), and 10 K (A=Ba). The low temperature antiferromagnetic ordering is described by the magnetic propagation vector k = (1/2 0 1/2) for all materials. The saturated cobalt magnetic moment decreases from 2.97(2) &mu;(B) to 2.52(3) &mu;(B) to 1.85(5) &mu;(B) for A=Ca, Sr, and Ba, respectively. The decrease of ordering temperature and moment with increasing size of A evidences magnetic frustration due to competition between 90&DEG; superexchange pathways. This is shown to be a general feature in antiferromagnetic double perovskites.