Magnetic ordering in the $J_{\rm eff}$ = 0 Nickelate NiRh$_2$O$_4$ prepared via a solid-state metathesis

TL;DR

This study demonstrates that magnetic ordering in NiRh$_2$O$_4$ can be induced by a specific low-temperature synthesis method, revealing complex spin-orbit entanglement and excitonic mechanisms in this $J_{eff}=0$ nickelate.

Contribution

The paper shows that synthesis route critically influences magnetic properties in NiRh$_2$O$_4$, highlighting the role of $J_{eff}$=0 exciton condensation in emergent magnetism.

Findings

Magnetic ordering observed at T_N = 45 K in low-temperature synthesized samples.

Suppression of magnetic order in previous high-temperature samples due to cation-mixing.

Enhanced effective magnetic moment consistent with $J_{eff}$=0 physics.

Abstract

In spinel-type nickelate NiRh$_2$O$_4$, magnetic ordering is observed upon the sample synthesized via kinetically controlled low-temperature solid-state metathesis, as opposed to previously-reported samples obtained through conventional solid-state reaction. Our findings are based on a combination of bulk susceptibility and specific heat measurements that disclose a N$\'e$el transition temperature of $T_N$ = 45 K in this material, which might feature spin-orbit entanglement in the tetragonally-coordinated $d^8$ Mott insulators. The emergence of magnetic ordering upon alteration of the synthesis route indicates that the suppression of magnetic ordering in the previous sample was rooted in the cation-mixing assisted by the entropy gain that results from high-temperature reactions. Furthermore, the $J_{\rm eff}$ = 0 physics, instead of solely the spin-only $S = 1$, describes the observed enhancement of effective magnetic moment well. Overseeing all observations and speculations, we propose that the possible mechanism responsible for the emergent magnetic orderings in NiRh$_2$O$_4$ is the condensation of $J_{\rm eff}$ = 0 exciton, driven by the interplay of the tetragonal crystal field and superexchange interactions.