X-Band ESR Determination of Dzyaloshinsky-Moriya Interaction in 2D SrCu$_2$(BO$_3$)$_2$ System

TL;DR

This study uses X-band ESR measurements to analyze the Dzyaloshinsky-Moriya interactions in SrCu$_2$(BO$_3$)$_2$, revealing the importance of in-plane components and spin-phonon coupling in linewidth anisotropy.

Contribution

It introduces a comprehensive model including in-plane and out-of-plane DM interactions and spin-phonon coupling to explain ESR linewidth anisotropy in SrCu$_2$(BO$_3$)$_2$.

Findings

Inclusion of in-plane DM components explains angular dependence.

Linewidth increases linearly at high temperatures due to spin-phonon coupling.

Dynamical mechanisms are active above the structural phase transition at 395 K.

Abstract

X-band ESR measurements on a single crystal of SrCu$_2$(BO$_3$)$_2$ system in a temperature range between 10 K and 580 K are presented. The temperature and angular dependence of unusually broad ESR spectra can be explained by the inclusion of antisymmetric Dzyaloshinsky-Moriya (DM) interaction, which yields by far the largest contribution to the linewidth. However, the well-accepted picture of only out-of-plane interdimer DM vectors is not sufficient for explanation of the observed angular dependence. In order to account for the experimental linewidth anisotropy we had to include sizable in-plane components of interdimer as well as intradimer DM interaction in addition to the out-of-plane interdimer one. The nearest-neighbor DM vectors lie perpendicular to crystal anisotropy c-axis due to crystal symmetry. We also emphasize that above the structural phase transition occurring at 395 K dynamical mechanism should be present allowing for instantaneous DM interactions. Moreover, the linewidth at an arbitrary temperature can be divided into two contributions; namely, the first part arising from spin dynamics governed by the spin Hamiltonian of the system and the second part due to significant spin-phonon coupling. The nature of the latter mechanism is attributed to phonon-modulation of the antisymmetric interaction, which is responsible for the observed linear increase of the linewidth at high temperatures.