CoMOF 5 (pyrazine)(H 2 O) 2 (M = Nb, Ta): Two-Layered Cobalt Oxyfluoride Antiferromagnets with Spin Flop Transitions.

Two cobalt oxyfluoride antiferromagnets CoMOF ₅ (pyz)(H ₂ O) ₂ (M = Nb 1 , Ta 2 ; pyz = pyrazine) have been synthesized via conventional hydrothermal methods and characterized by thermogravimetric (TGA) analysis, FTIR spectroscopy, electron spin resonance (ESR), magnetic susceptibility, and magnetization measurements at both static low field and pulsed high field. The single-crystal X-ray diffraction indicates both compounds 1 and 2 are isostructural and crystallize in the monoclinic space group C 2/ m with a two-dimensional Co ²⁺ triangular lattice in the ab plane, separated by the nonmagnetic MOF ₅ (M = Nb 1 , Ta 2 ) octahedra along the c -axis with large intertriangular-lattice Co ··· Co distance. Because of low dimensionality together with frustrated triangular lattice, compounds 1 and 2 exhibit no long-range antiferromagnetic order until ∼3.7 K. Moreover, a spin flop transition is observed in the magnetization curves at 2 K for both compounds, which is further confirmed by ESR spectra. In addition, the ESR spectra suggest the presence of a zero-field spin gap in both compounds. The high field magnetization measured at 2 K saturates at ∼7 T with M _s = 1.55 μ _B for 1 and 1.71 μ _B for 2 , respectively, after subtracting the Van Vleck paramagnetic contribution, which is usually observed for Co ²⁺ ions with pseudospin spin of 1/2 at low temperature. Powder-averaged magnetic anisotropy of g = 3.10 for 1 (3.42 for 2 ) and magnetic superexchange interaction J / k _B = -3.2 K for 1 (-3.6 K for 2 ) are obtained.