Magnetic order in the purely organic quasi-one-dimensional ferromagnet 2-benzimidazolyl nitronyl nitroxide

We show that the quasi-one-dimensional organic ferromagnet 2-benzimidazolyl nitronyl nitroxide (2-BIMNN) undergoes a phase transition to long-range magnetic order below $T_{\text{C}}=1.0\left(1\right)\text{K}$. The intrachain exchange interactions are $J/k_{\text{B}}=22\text{K}$ and so the low-temperature ordering demonstrates the weakness of the interchain interactions. Our electron-spin-resonance measurements provide further evidence for the one-dimensional character of the magnetic fluctuations in 2-BIMNN and, using muon-spin rotation, we are able to characterize the broad phase transition in the material. We discuss these results in the context of other spin-$\frac{1}{2}$ quasi-one-dimensional Heisenberg ferromagnets and demonstrate that 2-BIMNN is the best realization of a one-dimensional Heisenberg ferromagnet among purely organic compounds yet discovered.

Figure 1

(Color online) The crystal structure of 2-BIMNN projected along the (a) $a$ axis, (b) $b$ axis, and (c) $c$ axis. The dotted lines show the dominant hydrogen bonds.

Figure 2

Angular dependence of the peak-to-peak ESR linewidth $B_{\text{pp}}$ within the $ac$ and $bc$ planes at 292 K. The angle between the one-dimensional chain axis (the $c$ axis) and the applied field is $\theta$. The open circles and crosses are experimental data and the solid and dotted lines are fitted curves (see text).

Figure 3

(Color online) Example ${\mu }^{+}\text{SR}$ spectra for 2-BIMNN measured above and below the magnetic transition at $T_{\text{C}}=1.0\left(1\right)\text{K}$. Inset: the molecular structure of 2-BIMNN.

Figure 4

Temperature evolution of the properties of the ${\mu }^{+}\text{SR}$ spectra for $T<T_{\text{C}}$ from fits to Eq. (2). (a) Precession frequency (the line is a guide to the eye); (b) amplitudes of the relaxing components $A_1$ and $A_2$; and (c) relaxation rates $\lambda$ and $\sigma$.