Experimental Evidence of a Haldane Gap in an S = 2 Quasi-linear Chain Antiferromagnet

TL;DR

This study provides experimental evidence for a Haldane gap in an S=2 quasi-linear chain antiferromagnet through magnetic susceptibility and magnetization measurements, confirming theoretical predictions for such quantum spin systems.

Contribution

The paper reports the first experimental observation of a Haldane gap in an S=2 chain antiferromagnet, expanding understanding of quantum spin chains beyond S=1.

Findings

No long-range magnetic order observed.

Critical fields identified at ~1.2 T and ~1.8 T for different orientations.

Magnetization increases continuously above critical fields, indicating a gap.

Abstract

The magnetic susceptibility of the $S = 2$ quasi-linear chain Heisenberg antiferromagnet (2,$2'$-bipyridine)trichloromanganese(III), MnCl_{3}(bipy), has been measured from 1.8 to 300 K with the magnetic field, H, parallel and perpendicular to the chains. The analyzed data yield $g\approx 2$ and $J\approx 35$ K. The magnetization, M, has been studied at 30 mK and 1.4 K in H up to 16 T. No evidence of long-range order is observed. Depending on crystal orientation, $M\approx 0$ at 30 mK until a critical field is achieved ($H_{c\|} = 1.2\pm 0.2 T$ and $H_{c\bot} = 1.8\pm 0.2 T), where M increases continuously as H is increased. These results are interpreted as evidence of a Haldane gap.