# Splitting of antiferromagnetic resonance modes in the quasi-two-dimensional collinear antiferromagnet Cu(en)(H$_2$O)$_2$SO$_4$

## TL;DR

This study investigates the antiferromagnetic resonance modes in Cu(en)(H$_2$O)$_2$SO$_4$, revealing mode splitting and anisotropy characteristics through low-temperature magnetic resonance measurements and theoretical analysis.

## Contribution

It provides the first detailed analysis of resonance mode splitting and anisotropy parameters in this quasi-two-dimensional antiferromagnet, highlighting the role of dipolar interactions.

## Key findings

- Resonance mode splitting observed below 0.6K.
- Dipolar coupling is the main anisotropy source.
- Non-monotonous frequency dependence of mode splitting.

## Abstract

Low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H$_2$O)$_2$SO$_4$ (en = C$_2$H$_8$N$_2$) was performed down to 0.45~K. This compound orders antiferromagnetically at 0.9K. The analysis of the resonance data within the hydrodynamic approach allowed to identify anisotropy axes and to estimate the anisotropy parameters for the antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main contribution to the anisotropy of the antiferromagnetic phase. The splitting of the resonance modes and its non-monotonous dependency on the applied frequency was observed below 0.6K in all three field orientations. Several models were discussed to explain the origin of the nontrivial splitting and the existence of inequivalent magnetic subsystems in Cu(en)(H$_2$O)$_2$SO$_4$ was chosen as the most probable source.