Hidden spin-orbital order in the Kitaev hyperhoneycomb $\beta$-Li$_2$IrO$_3$

TL;DR

This paper uncovers a high-temperature phase transition in $eta$-Li$_2$IrO$_3$ where orbital moments order without freezing local Ir moments, revealing a new magnetic frustration mechanism in Kitaev systems.

Contribution

It demonstrates the existence of a bulk, intrinsic phase transition involving orbital order separate from local moment ordering in a 3D Kitaev candidate.

Findings

High-temperature phase transition with tiny magnetic moment

Orbital origin of the ordered moment coupled to spin correlations

Coexistence of orbital order and local moment order at different temperatures

Abstract

We report the existence of a phase transition at high temperature in the 3D Kitaev candidate material, $\beta$-Li$_2$IrO$_3$. We show that the transition is bulk, intrinsic and orders a tiny magnetic moment with a spatially anisotropic saturation moment. We show that even though this transition is global, it does not freeze the local Ir moments, which order at much lower temperatures into an incommensurate state. Rather, the ordered moment has an orbital origin that is coupled to spin correlations, likely of a Kitaev origin. The separate ordering of spin-correlated orbital moments and of local Ir moments reveals a novel way in which magnetic frustration in Kitaev systems can lead to coexisting magnetic states.