A suite of experimental tools, including high-field magnetization and electron spin resonance (ESR) studies in magnetic fields of up to 50 T and heat capacity studies up to 9 T, have revealed antiferromagnetic order in single crystals of the Heisenberg $S=2$ chain compound ${\text{MnCl}}_3$(bpy), where bpy is $2,2^{\prime }\text{-bipyridine}$. The Néel temperature, which depends on the strength of the applied magnetic field and its orientation with respect to the crystalline axes that was revealed by heat capacity measurements, is near 11.5 K in zero field. The spin-flop transition is identified in the magnetization curve acquired at 1.7 K and at ${\mu }_{\mathrm{o}}{H}_{\mathrm{SF}}^c=24$ T along the $c$ axis. The transition field $H_{\mathrm{SF}}$ is lower than that expected from the previous antiferromagnetic resonance (AFMR) studies on a powder sample. The identification of the long-range antiferromagnetic order resolves an earlier report by Granroth et al. [Phys. Rev. Lett. 77, 1616 (1996)] that identified ${\text{MnCl}}_3$(bpy) as an $S=2$ Haldane system down to 40 mK. The ESR studies identify a wide range of antiferromagnetic resonance modes that provide additional microscopic information about the $g$ values ($g_{a^{*}}=2.09, g_b=1.92$, and $g_c=2.07$), the zero-field splitting constants, $D/k_{\mathrm{B}}=-1.5$ K and $E/k_{\mathrm{B}}=-0.17$ K when the nearest-neighbor spin interaction $J/k_{\mathrm{B}}=31.2$ K, which is evaluated from fitting the susceptibility, and the anisotropy of this compound (easy axis is the $c$ axis, the second easy-axis is the $b$ axis, and the hard axis is the $a^{*}$ axis), when using a standard (two-sublattice) AFMR analysis that does not quantitatively reproduce the observed $H_{\mathrm{SF}}^c$ value. The observed resonance mode indicates the frequency minimum at $H_{\mathrm{SF}}^c$.

• Received 2 November 2015

DOI:https://doi.org/10.1103/PhysRevB.93.014407