Geometric Frustration on the Trillium Lattice in a Magnetic Metal-Organic Framework

In the dense metal-organic framework Na[Mn(HCOO)3], Mn2+ ions (S = \(5\over2\)) occupy the nodes of a ‘trillium’ net. We show that the system is strongly magnetically frustrated: the Neel transition is suppressed well ´ below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Neel ´ temperature; and the magnetic susceptibility exhibits a pseudo-plateau at \(1\over3\)-saturation magnetisation. A simple model of nearest-neighbour Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground-state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground-state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions.