Low-dimensional metal-organic magnets as a route towards the S=2 Haldane phase

TL;DR

This study explores a new low-dimensional metal-organic magnet with S=2 spins, aiming to realize the Haldane phase, and finds promising structural features despite current limitations preventing its observation.

Contribution

The paper reports the synthesis and detailed magnetic characterization of a novel S=2 quasi-1D MOM, advancing understanding of conditions needed to realize the S=2 Haldane phase.

Findings

CrCl₂(pym) has antiferromagnetic chains with weak ferromagnetic coupling.

The material exhibits easy-axis anisotropy.

Current interactions prevent the observation of the Haldane phase.

Abstract

Metal-organic magnets (MOMs), modular magnetic materials where metal atoms are connected by organic linkers, are promising candidates for next-generation quantum technologies. MOMs readily form low-dimensional structures, and so are ideal systems to realise physical examples of key quantum models, including the Haldane phase, where a topological excitation gap occurs in integer-spin antiferromagnetic (AFM) chains. Thus far the Haldane phase has only been identified for $S=1$, with $S \geq 2$ still unrealised because the larger spin imposes more stringent requirements on the magnetic interactions. Here, we report the structure and magnetic properties of CrCl$_2$(pym) (pym=pyrimidine), a new quasi-1D $S=2$ AFM MOM. We show, using X-ray and neutron diffraction, bulk property measurements, density-functional theory calculations and inelastic neutron spectroscopy (INS) that CrCl$_2$(pym) consists of AFM CrCl$_2$ spin chains ($J_1=-1.13(4)\;$meV) which are weakly ferromagnetically coupled through bridging pym ($J_2=0.10(2)\;$meV), with easy-axis anisotropy ($D=-0.15(3)\;$meV). We find that although small compared to $J_1$, these additional interactions are sufficient to prevent observation of the Haldane phase in this material. Nevertheless, the proximity to the Haldane phase together with the modularity of MOMs suggests that layered Cr(II) MOMs are a promising family to search for the elusive $S=2$ Haldane phase.