Geometric frustration on the trillium lattice in a magnetic metal-organic framework
Résumé
In the dense metal-organic framework Na[Mn(HCOO)3], Mn 2+ ions (S = 5 2) occupy the nodes of a 'trillium' net. We show that the system is strongly magnetically frustrated: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at 1 3-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.
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