Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAl$_{2-x}$Ga$_x$O$_4$: Lattice expansion and site disorder

TL;DR

This study investigates how substituting Ga for Al in CoAl2-xGaxO4 spinels affects lattice expansion, site disorder, and magnetic frustration on the diamond lattice, revealing complex magnetic ground states.

Contribution

It demonstrates how chemical substitution and resulting site disorder can tune magnetic frustration and ground states in A-site magnetic spinels.

Findings

Lattice expansion with Ga substitution weakens magnetic interactions.

Increased inversion displaces Co from the A site, altering magnetic order.

Site disorder significantly impacts the magnetic ground state.

Abstract

The spinels CoB$_2$O$_4$ with magnetic Co$^{2+}$ 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 tune the relative strengths of these interactions by substitution on the non-magnetic B-site. The system we employ is CoAl$_{2-x}$Ga$_x$O$_4$, where Al is systematically replaced by the larger Ga, ostensibly on the B site. As expected, Ga substitution expands the lattice, resulting in Co atoms on the A-site being pushed further from one other and thereby weakening magnetic interactions. In addition, Ga distributes between the B and the A site in a concentration dependent manner displacing an increasing amount of Co from the A site with increasing $x$. This increased inversion, which is confirmed by neutron diffraction studies carried out at room temperature, affects magnetic ordering very significantly, and changes the nature of the ground state. Modeling of the magnetic coupling illustrates the complexity that arises from the cation site disorder.