Crystal field effects in the zig-zag chain compound SrTm$_2$O$_4$

TL;DR

This study investigates the crystal field effects in SrTm$_2$O$_4$, revealing anisotropic single ion properties and complex magnetic behavior through various experimental techniques and theoretical models.

Contribution

It applies and compares two crystal field models to experimental data, identifying anisotropies and dynamic magnetic states in SrTm$_2$O$_4$.

Findings

Effective charge model describes experimental results well.

Identifies easy-axis and easy-plane anisotropies for Tm$^{3+}$ sites.

Reveals dynamic magnetic behavior with extended critical regime.

Abstract

The single ion properties of the zig-zag chain compound SrTm$_2$O$_4$ have been investigated using heat capacity, magnetic susceptibility, magnetization, inelastic neutron scattering, and polarized muon spectroscopy. Two crystal field models are employed to estimate the single ion properties; a Density Function Theory based model and an effective charge model based on the Hutchings point charge model. The latter describes our experimental results well. This model estimates an easy-axis anisotropy for one of the Tm$^{3+}$ sites and an easy-plane anisotropy for the second site. It also predicts a mixed ground state with dominating $J = 0$ characteristics for both sites. Additionally, muon spin rotation/relaxation ($\mu^+$SR) spectra reveal oscillations, typically a sign of long-range magnetic order. However, the temperature dependence of the precession frequency and the relaxation rates indicate that the system is in an extended critical regime and the observed relaxation is actually dynamic.