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

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 $J_{\text{1D}}/k_B=(11.3\pm 0.1)$ 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.

13 pages, 12 figures

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