Canted antiferromagnetic order and spin dynamics in the honeycomb-lattice Tb2Ir3Ga9

TL;DR

This study investigates the magnetic order and spin dynamics of honeycomb-lattice Tb2Ir3Ga9 using neutron scattering, magnetization, and first-principles calculations, revealing canted antiferromagnetic order and complex exchange interactions.

Contribution

It provides the first detailed characterization of the magnetic structure and spin interactions in Tb2Ir3Ga9, highlighting the role of Dzyaloshinskii-Moriya interactions and bond-dependent exchange.

Findings

Tb$^{3+}$ ions form a canted antiferromagnetic structure below 12.5 K.

Long-range interactions up to second nearest neighbors are essential.

The spin Hamiltonian exhibits large in-plane anisotropy and bond-dependent exchange.

Abstract

Single crystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements, and first-principles calculations are used to investigate the magnetic properties of the honeycomb lattice $\rm Tb_2Ir_3Ga_9$. While the $R\ln2$ magnetic contribution to the low-temperature entropy indicates a $\rm J_{eff}=1/2$ moment for the lowest-energy crystal-field doublet, the Tb$^{3+}$ ions form a canted antiferromagnetic structure below 12.5 K. Due to the Dzyalloshinskii-Moriya interactions, the Tb moments in the $ab$ plane are slightly canted towards $b$ by $6^\circ$ with a canted moment of 1.22 $\mu_{\rm B} $ per formula unit. A minimal $xxz$ spin Hamiltonian is used to simultaneously fit the spin-wave frequencies along the high symmetry directions and the field dependence of the magnetization along the three crystallographic axes. Long-range magnetic interactions for both in-plane and out-of-plane couplings up to the second nearest neighbors are needed to account for the observed static and dynamic properties. The $z$ component of the exchange interactions between Tb moments are larger than the $x$ and $y$ components. This compound also exhibits bond-dependent exchange with negligible nearest exchange coupling between moments parallel and perpendicular to the 4$f$ orbitals. Despite the $J_{{\rm eff}}=1/2$ moments, the spin Hamiltonian is denominated by a large in-plane anisotropy $K_z \sim -1$ meV. DFT calculations confirm the antiferromagnetic ground state and the substantial inter-plane coupling at larger Tb-Tb distances.