Crystal field excitation in the chiral helimagnet YbNi$_3$Al$_9$

TL;DR

This study determines the crystal field level scheme of YbNi$_3$Al$_9$, revealing its anisotropic magnetic properties and the nature of its magnetic excitations, which are crucial for understanding its chiral magnetic soliton lattice state.

Contribution

The paper provides the first detailed crystal field level scheme for YbNi$_3$Al$_9$ using inelastic neutron scattering, elucidating its anisotropic magnetic behavior and exchange interactions.

Findings

Ground and first excited doublets separated by 44 K

Easy axis of anisotropy is the c axis at high temperatures and fields

Low-temperature magnetism described by an $S=1/2$ system with anisotropic $g$-factor

Abstract

Crystal field level scheme of a uniaxial chiral helimagnet YbNi$_3$Al$_9$, exhibiting a chiral magnetic soliton lattice state by Cu substitution for Ni, has been determined by inelastic neutron scattering. The ground and the first excited doublets are separated by 44 K and are simply expressed as $\alpha|\pm 7/2\rangle + \beta |\mp 5/2\rangle$ with $\alpha$ and $\beta$ nearly equal to $\pm 1/\sqrt{2}$. The easy axis of the crystal field anisotropy is the $c$ axis when the excited levels are populated at high temperatures and high magnetic fields. On the other hand, the magnetism at low temperatures and low magnetic fields, where only the ground doublet is populated, is described by an easy plane anisotropy which may be treated as an $S=1/2$ system with an anisotropic $g$-factor, $g_{xy}=3.02$ and $g_z=1.14$. An orbital dependent exchange interaction is also discussed to explain the temperature dependence of the magnetic susceptibility based on this level scheme.