Subtle competition between ferromagnetic and antiferromagnetic order in a Mn(II) - free radical ferrimagnetic chain

TL;DR

This study reveals a subtle competition between ferromagnetic and antiferromagnetic orders in a Mn(II)-free radical chain, showing a fragile antiferromagnetic state that can switch to ferromagnetic order under weak magnetic fields, highlighting complex magnetic interactions.

Contribution

The paper provides detailed neutron diffraction analysis demonstrating the coexistence and competition of magnetic orders in a molecular chain, emphasizing the importance of microscopic techniques over macroscopic measurements.

Findings

Below 3 K, the material exhibits antiferromagnetic order with canted moments.

Weak magnetic fields can induce a transition to ferromagnetic interchain order.

Dipolar and exchange interactions compete, influencing the magnetic state.

Abstract

The macroscopic magnetic characterization of the Mn(II) - nitronyl nitroxide free radical chain (Mn(hfac)2(R)-3MLNN) evidenced its transition from a 1-dimensional behavior of ferrimagnetic chains to a 3-dimensional ferromagnetic long range order below 3 K. Neutron diffraction experiments, performed on a single crystal around the transition temperature, led to a different conclusion : the magnetic Bragg reflections detected below 3 K correspond to a canted antiferromagnet where the magnetic moments are mainly oriented along the chain axis. Surprisingly in the context of other compounds in this family of magnets, the interchain coupling is antiferromagnetic. This state is shown to be very fragile since a ferromagnetic interchain arrangement is recovered in a weak magnetic field. This peculiar behavior might be explained by the competition between dipolar interaction, shown to be responsible for the antiferromagnetic long range order below 3 K, and exchange interaction, the balance between these interactions being driven by the strong intrachain spin correlations. More generally, this study underlines the need, in this kind of molecular compounds, to go beyond macroscopic magnetization measurements.