Spin-orbital order and excitons in magnetoresistive HoBi

TL;DR

This study investigates the magnetic properties, excitations, and structural distortions of HoBi, a candidate topological material with extreme magnetoresistance, revealing complex spin-orbital interactions and antiferromagnetic order below 5.72 K.

Contribution

It provides a detailed analysis of HoBi's magnetic order, excitonic behavior, and lattice coupling, highlighting the role of crystal electric fields and exchange interactions in its magnetism.

Findings

HoBi exhibits type-II antiferromagnetic order below 5.72 K.

Weakly dispersive excitons are explained by a spin Hamiltonian including crystal electric field effects.

The easy magnetic axis aligns with the tetragonal distortion axis.

Abstract

The magnetism of the rock-salt $fcc$ rare-earth monopnictide HoBi, a candidate topological material with extreme magnetoresistance, is investigated. From the Ho$^{3+}$ non-Kramers $J$=8 spin-orbital multiplet, the cubic crystal electric field yields six nearly degenerate low-energy levels. These constitute an anisotropic magnetic moment with a Jahn-Teller-like coupling to the lattice. In the cubic phase for $T>T_N~=~5.72(1)~K$, the paramagnetic neutron scattering is centered at $\mathbf{k}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ and was fit to dominant antiferromagnetic interactions between Ho spins separated by $\{100\}$ and ferromagnetic interactions between spins displaced by $\{\frac{1}{2}\frac{1}{2}0\}$. For $T<T_N$, a type-II AFM long-range order with $\mathbf{k}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ develops along with a tetragonal lattice distortion. While neutron diffraction from a multi-domain sample cannot unambiguously determine the spin orientation within a domain, the bulk magnetization, structural distortion, and our measurements of the magnetic excitations all show the easy axis coincides with the tetragonal axis. The weakly dispersive excitons for $T<T_N$ can be accounted for by a spin Hamiltonian that includes the crystal electric field and exchange interactions within the Random Phase Approximation.