Triple-$\mathcal{Q}$ partial magnetic orders induced by quadrupolar interactions: Triforce order scenario for UNi$_4$B

TL;DR

This paper proposes a new magnetic order called triforce order in UNi4B, induced by quadrupolar interactions, which explains recent experimental observations and differs from the previously assumed toroidal order.

Contribution

The study introduces the triforce order as a novel triple-Q magnetic structure driven by quadrupolar interactions, expanding understanding of symmetry-broken states in UNi4B.

Findings

Triforce order has the same spin structure factor as toroidal order.

Triforce order is consistent with current-induced magnetization observations.

Quadrupole moments significantly influence magnetic structures.

Abstract

We theoretically investigate possible symmetry-broken states in ${\rm UNi_4B}$, constructing a localized pseudo triplet crystalline-electric field model. For a long time, its low-temperature symmetry-broken phase in ${\rm UNi_4B}$ has been considered to be a magnetic toroidal order forming atomic-scale vortices lattice with disordered sites at each center of the vortices. However, recent observation of current-induced magnetizations offers a reinvestigation about the validity of this order parameter because of the contradiction in their anisotropy. Our model takes into account the quadrupole degrees of freedom, whose importance is recently evidenced by the sound-velocity softening. We find that the quadrupole moments play an important role in determining the magnetic structure in the ordered states. For a wide range of parameter space, we obtain two triple-$\mathcal{Q}$ magnetic orders in our 36-site mean-field analysis: toroidal order and another one with the same number of disordered sites as in the toroidal order. We name the latter ``triforce'' order after its magnetic structure. Importantly, the triforce order possesses exactly the same spin structure factor as the toroidal order does, while the phase factors in the superposition of the triple-$\mathcal{Q}$ structure are different. We show that the triforce order is consistent with the observed current-induced magnetization when the realistic crystal structure of ${\rm UNi_4B}$ is taken into account. We compare the predictions of the triforce order with the experimental data available at present in detail and also discuss possible applications of the present mechanism of triple-$\mathcal{Q}$ orders to anisotropic correlated systems.