Low dimensional quantum behavior in magnetic metal-organic frameworks

Materials with reduced dimensionality of 2D layers and 1D chains provide a promising route to enhance quantum behavior. This is exemplified by graphene and beyond-graphene materials where the bulk compound has 2D layers weakly coupled through van der Waals bonding that can often be exfoliated or otherwise isolated down to a few or single monolayers. In these inorganic compounds numerous non-trivial topological and quantum behaviors manifest. Conversely, the tunability of coordination polymers, or equivalently magnetic metal-organic frameworks (MOFs), offers a powerful but less explored material space to achieve analogous physics when magnetic metal ions are added to well isolated coordination structures. An extremely large variety of magnetic MOF structures are available through often highly predictable organic chemistry routes to combine metal and organic linker building blocks. The ability to control the lattice motif of the magnetic metal ions from 1D chains to 2D layers to bulk 3D networks and the potential to introduce hybrid functionality on the organic linkers affords multiple intriguing research avenues for magnetic MOFs in the realm of quantum materials. In this talk we will present elastic and inelastic neutron scattering investigations on low dimensional magnetic MOFs. One such intriguing compound is M(N₂H₅)₂(SO₄)₂, with 1D chains of magnetic ions. Investigating the series with M=Mn, Co, Cu to follow the influence of reducing the spin from S=5/2 to S=1/2 revealed indications of short-range ordering and non-classical magnetic excitations. Polarized neutron diffraction explored the local site susceptibility, which provides a well suited technique to investigate anisotropy in a wide variety of magnetic MOFs, even in the presence of large hydrogen content that would make unpolarized studies unfeasible without deuteration. As a further material example we have explored the [Mn(pyrazinecarboxylate)2]n (Mn-pyrazine), empirical formula C₁₀H₆MnN₄O₄. This has distorted hexagonal 2D layers with dimer Mn-Mn interactions. Through a comprehensive investigation we show the magnetic ordering undergoes a two-stage transition, which can be associated with low dimensional behavior. Collectively the unique role of neutron scattering to investigate magnetic MOFs in general is emphasized.