# Low-Temperature Ferromagnetic Order in a Two-Level Layered Co2+ Material

## TL;DR

This paper reports the discovery of low-temperature ferromagnetic order in a layered cobalt material.

## Contribution

The study reveals ferromagnetic interactions in a Co2+ material at temperatures below 2 K, challenging previous assumptions about its magnetic behavior.

## Key findings

- Ferromagnetic interactions between Co2+ cations become significant below 2 K.
- A long-range ordered ferromagnetic state emerges below 246 mK.
- Electron paramagnetic resonance and heat capacity measurements support the presence of a two-level model with weak interactions.

## Abstract

The magnetic properties of a 2D layered material consisting
of
high-spin Co2+ complexes, [Co(NH3NH2)2(H2O)2Cl2]Cl2 (CoHyd2Cl4), have been extensively characterized using
electron paramagnetic resonance, magnetic susceptibility, and low-temperature
heat capacity measurements. Electron paramagnetic resonance spectroscopy
studies suggest that below 50 K, the J = 3/2 orbital
triplet state of Co is gradually depopulated in favor of the J = 1/2 spin state, which is dominant below 20 K. In light
of this, the magnetic susceptibility has been fitted with a two-level
model, indicating that the interactions in this material are much
weaker than previously thought. This two-level model is unable to
fit the data at low temperatures and, combined with electron paramagnetic
resonance spectroscopy, suggests that ferromagnetic interactions between
Co2+ cations in the J = 1/2 state become
significant approaching 2 K. Heat capacity measurements suggest the
emergence of a long-range ordered state below 246 mK, which neutron
diffraction confirms to be ferromagnetic.