Evolution of Magnetic and Orbital Properties in the Magnetically-Diluted A-Site Spinel Cu\(_{1-x}\)Zn\(_x\)Rh\(_2\)O\(_4\)

In frustrated spinel antiferromagnets, dilution with non-magnetic ions can be a powerful strategy for probing unconventional spin states or uncovering interesting phenomena. Here, we present X-ray, neutron scattering and thermodynamic studies of the effects of magnetic dilution of the tetragonally-distorted A-site spinel antiferromagnet, CuRh\(_2\)O\(_4\), with non-magnetic Zn\(^{2+}\) ions. Our data confirm the helical spin order recently identified at low-temperatures in this material, and further demonstrate a systematic suppression of the associated Néel temperature with increasing site dilution towards a continuous transition with critical doping of \(x_{spin} \sim 0.44\). Interestingly, this critical doping is demonstrably distinct from a second structural critical point at \(x_{JT} \sim 0.6\), which is consistent with the suppression of orbital order on the A-site through a classical percolative mechanism. This anomalously low value for \(x_{spin}\) is confirmed via multiple measurements, and is inconsistent with predictions of classical percolation theory, suggesting that the spin transition in this material is driven by an enhancement of pre-existing spin fluctuations with weak dilution.