Critical Field Determination in Organic Antiferromagnet F4BImNN

A combination of magnetic heat capacity measurements versus temperature under multiple fixed external fields using a polycrystalline sample, plus easy and hard axis aligned magnetization versus field measurements using a single crystal of the radical 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl) at multiple fixed temperatures below 0.75 K, confirmed a zero-field Neel temperature of TN(0) = 0.73 K, with extrapolated 0 K critical field parameters of Hc(parallel) = 170 Oe and Hc(perpendicular) = 306 Oe. Assuming the previously determined 1D intrachain ferromagnetic exchange interaction of J = +22 K between radicals (Hamiltonian H = -2J {\Sigma}Si . Sj), , the interchain exchange in antiferromagnetic with 2zJinter = -0.09 K.