Spin wave excitations in the tetragonal double perovskite Sr$_2$CuWO$_6$

TL;DR

This study investigates spin wave excitations in Sr$_2$CuWO$_6$ using inelastic neutron scattering, revealing highly two-dimensional magnetic interactions and a weak anisotropy, with experimental and theoretical analyses providing new insights into its magnetic properties.

Contribution

The paper combines experimental neutron scattering data with theoretical calculations to show that next nearest neighbour interactions dominate in Sr$_2$CuWO$_6$, contrasting previous ab-initio results.

Findings

Dispersive spin wave modes observed at 6 K

Partial survival of spin waves above T_N=24 K

Next nearest neighbour interactions are strongest

Abstract

Sr$_2$CuWO$_6$ is a double perovskite proposed to be at the border between two and three dimensional magnetism, with a square lattice of $S=\frac{1}{2}$ Cu$^{2+}$ ions. We have used inelastic neutron scattering to investigate the spin wave excitations of the system, to find out how they evolve as a function of temperature, as well as to obtain information about the magnetic exchange interactions. We observed well defined dispersive spin wave modes at $6$~K, which partially survive above the magnetic ordering temperature, $T_N=24$~K. Linear spin wave theory is used to determine the exchange interactions revealing them to be highly two-dimensional in nature. Density functional theory calculations are presented supporting this experimental finding, which is in contrast to a previous \emph{ab-initio} study of the magnetic interactions. Our analysis confirms that not the nearest neighbour, but the next nearest neighbour interactions in the tetragonal $ab$ plane are the strongest. Low incident energy measurements reveal the opening of a $0.6(1)$~meV gap below $T_N$, which suggests the presence of a very weak single ion anisotropy term in the form of an easy axis along $\hat{\mathbf{a}}$.