Ab-initio XRPD crystal structure and giant hysteretic effect (H-c=5.9 T) of a new hybrid terephthalate-based cobalt(II) magnet

A new terephthalate-based cobalt hydroxide, Co-2(OH)(2)(C8H4O4), was synthe sized by the hydrothermal method. Its crystal structure has been determined by ab-initio XRPD methods (monoclinic, C2/m, a = 19.943(1), b = 3.2895(1), c = 6.2896(3) Angstrom, beta = 95.746(3)degrees) and fully refined by the Rietveld technique down to R-p = 0.15 for 9301 observed data (178 independe nt reflections). The terephthalates are coordinated and pillared directly t o the cobalt hydroxide layers and thus a three-dimensional framework is for med. Because of the bonds with the terephthalates, two crystallographically inequivalent cobalt sites are found inside the hydroxide layers, with diff erent octahedral orientations. Magnetic studies show that the intralayer ex change interaction between Co(II) ions is ferromagnetic but the whole syste m orders antiferromagnetically at 48 K with a metamagnetic transition above a threshold field of 0.2 T. The existence of conjugated pi electrons in te rephthalates explains the antiferromagnetic interactions between the layers . Below 45 K, the compound exhibits a hysteretic metamagnetic loop and a re mnant moment that is small down to about 30 K, and then rises suddenly reac hing a plateau below 15 K. However, at low temperatures the remnant moment is still only a fraction of the full Co(II) moment, which is a sign of cant ed antiferromagnetism associated with a non-collinear orientation of the mo ments between the layers. The magnetization loop shows a giant coercive fie ld of 5.9 T at 4.2 K, which must be related to an extremely large single-io n anisotropy on the Co sites.