Properties of an organic model $S=1$ Haldane chain system

TL;DR

This paper introduces a new organic $S=1$ antiferromagnetic chain system, BoNO, exhibiting properties consistent with a Haldane chain, including low anisotropy and specific magnetic interactions, opening avenues for further quantum phase studies.

Contribution

The study reports the discovery and characterization of BoNO as the first known organic Haldane system with nearly isotropic $g$ factor and small magnetic anisotropy.

Findings

BoNO exhibits strong ferromagnetic coupling within molecules forming effective $S=1$ states.

The system shows antiferromagnetic intrachain interactions with $J_{1D}/k_B \,=\, 11.3$ K.

Critical magnetic fields are approximately 2 T and 33 T, indicating accessible quantum phases.

Abstract

We present the properties of a new organic $S=1$ antiferromagnetic chain system $m$-NO$_2$PhBNO (abbreviated BoNO). In this biradical system two unpaired electrons from aminoxyl groups are strongly ferromagnetically coupled ($|J_\text{FM}| /k_B \gtrsim 500$ K) which leads to the formation of an effective $S=1$ state for each molecule. The chains of BoNO biradicals propagate along the crystallographic $a$ axis. Temperature dependence of the $g$ factor and electron paramagnetic resonance (EPR) linewidth are consistent with a low-dimensional system with antiferromagnetic interactions. The EPR data further suggest that BoNO is the first known Haldane system with an almost isotropic $g$ factor ($2.0023 \pm 2 \unicode{x2030}$). The magnetization measurements in magnetic fields up to $40$ T and low-field susceptibility, together with $^1$H nuclear magnetic resonance (NMR) spectra, reveal a dominant intrachain antiferromagnetic exchange coupling of $J_\text{1D}/k_B = (11.3\pm0.1)$ K, and attainable critical magnetic fields of $\mu_0 H_\text{c1} \approx 2$ T and $\mu_0 H_\text{c2} \approx 33$ T. These measurements therefore suggest that BoNO is a unique Haldane system with extremely small magnetic anisotropy. Present results are crucial for a future in-depth NMR study of the low-temperature Tomonaga-Luttinger liquid (TLL) and magnetic field-induced phases, which can be performed in the entire phase space.