Refined magnetic structure of VI$_3$

TL;DR

This study refines the magnetic structure of VI₃ using muon spin relaxation and DFT calculations, revealing unquenched orbital angular momentum and a unique short-range order above the Curie temperature.

Contribution

The paper presents a new magnetic structure for VI₃ based on combined $^+$SR and DFT, resolving inconsistencies from neutron data and highlighting orbital contributions.

Findings

Confirmed long-range ferromagnetic order below 50 K.

Identified unquenched orbital angular momentum in V$^{3+}$.

Discovered an unusual short-range order above $T_C$.

Abstract

The van der Waals ferromagnet (FM), VI$_3$, was studied by muon spin relaxation ($\mu^+$SR) and first principle calculations based on density functional theory (DFT). Temperature dependent zero field muon spin relaxation ($\mu^+$SR) measurements confirm the onset of long range FM order and the time spectra exhibits clear muon spin precession frequencies for $T<T_{\rm C}=50.03(1)$~K. The calculated internal magnetic fields at the predicted muon sites, based on the established magnetic structure from neutron diffraction, is inconsistent with the measured one. This inconsistency is because of strong incoherent neutron scattering and absorption originating from the elements V and I. Instead, a new and a more accurate magnetic structure is derived based on a combined study using $\mu^+$SR and DFT. These results suggest strong contritions from orbital angular momentum, providing experimental evidence for the existence of unquenched orbital angular momentum of V$^{3+}$ in VI$_3$. Finally, an unusual form of a short range ordering is present above $T_{\rm C}$. Its temperature dependence is unlike previously reported cases in other layered compounds and its microscopic origin is discussed.