Terahertz spectroscopy of crystal-field transitions in magnetoelectric TmAl3(BO3)4

TL;DR

This study uses terahertz spectroscopy to investigate crystal-field transitions in the magnetoelectric TmAl3(BO3)4, revealing local distortions and their effects on magnetic properties and susceptibilities.

Contribution

It provides new insights into the crystal-field transitions and local site distortions in TmAl3(BO3)4 using terahertz spectroscopy, highlighting their impact on magnetic behavior.

Findings

Observation of crystal field transitions within Tm3+ ions.

Detection of fine structure due to local site distortions.

Identification of two types of distorted sites and their effects.

Abstract

Dynamic magnetic properties of magnetoelectric TmAl3(BO3)4 borate have been investigated by terahertz spectroscopy. Crystal field (CF) transitions within the ground multiplet 3H6 of Tm3+ ions are observed and they are identified as magnetic-dipole transitions from the ground singlet A1 to the next excited doublet E of Tm3+ ions. Unexpected fine structure of the transitions is detected at low temperatures. The new modes are assigned to local distortions of the sites with D3 symmetry by Bi3+ impurities, which resulted in the splitting of A1 -> E transition. Two types of locally distorted sites are identified and investigated. The main contribution to the static magnetic susceptibility is shown to be determined by the matrix elements of the observed magnetic transitions. We demonstrate that even in case of local distortions the symmetry of the undistorted crystal is recovered for magnetic and for quadratic magnetoelectric susceptibilities.