Magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAl2-xGaxO4: The role of lattice expansion and site disorder

The spinels CoB2O4 with magnetic Co2+ ions on the diamond lattice A site can be frustrated because of competing near-neighbor (J(1)) and next-near-neighbor (J(2)) interactions. Here we describe attempts to understand these interactions by substitution on the nonmagnetic B site. The system we employ is CoAl2-xGaxO4, where Al is systematically replaced by the larger Ga, ostensibly on the B site. Ga substitution has the effect of expanding the lattice and pushes Co atoms on the A-site further away from one another weakening the magnetic interactions. We also find, however, that Ga distributes between the B and the A site in a concentration dependent manner displacing an increasing amount of Co from the A site for larger values of x. This site mixing, confirmed by powder neutron diffraction studies carried out at room temperature, affects magnetic properties very significantly and changes the nature of the ground state. We discuss the role that both structural changes play in changing the degree of magnetic frustration on the diamond lattice. We also use classical Monte Carlo modeling of the magnetic coupling to illustrate the complexity of the interactions that arises from site mixing.