Ab-Initio XRPD Crystal Structure and Giant Hysteretic Effect (<i>H</i>_c = 5.9 T) of a New Hybrid Terephthalate-Based Cobalt(II) Magnet

Abstract

A new terephthalate-based cobalt hydroxide, Co2(OH)2(C8H4O4), was synthesized 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) Å, β = 95.746(3)°) and fully refined by the Rietveld technique down to Rp = 0.15 for 9301 observed data (178 independent reflections). The terephthalates are coordinated and pillared directly to the cobalt hydroxide layers and thus a three-dimensional framework is formed. Because of the bonds with the terephthalates, two crystallographically inequivalent cobalt sites are found inside the hydroxide layers, with different octahedral orientations. Magnetic studies show that the intralayer exchange interaction between Co(II) ions is ferromagnetic but the whole system orders antiferromagnetically at 48 K with a metamagnetic transition above a threshold field of 0.2 T. The existence of conjugated π electrons in terephthalates explains the antiferromagnetic interactions between the layers. Below 45 K, the compound exhibits a hysteretic metamagnetic loop and a remnant moment that is small down to about 30 K, and then rises suddenly reaching 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 canted antiferromagnetism associated with a non-collinear orientation of the moments between the layers. The magnetization loop shows a giant coercive field of 5.9 T at 4.2 K, which must be related to an extremely large single-ion anisotropy on the Co sites.