Tunable magnetism of Boron Imidazolate-based Metal-Organic Frameworks

TL;DR

This study explores the magnetic behaviors of four boron imidazolate-based metal-organic frameworks, revealing diverse magnetic interactions and phenomena such as paramagnetism, antiferromagnetism, and spin-glass states, driven by their distinct lattice structures.

Contribution

It introduces a new set of MOFs with varied magnetic properties using a common organic linker, highlighting the influence of lattice structure on magnetism.

Findings

Cu-BIF exhibits simple paramagnetic behavior.

Co-BIF shows antiferromagnetic interactions and spin-crossover effects.

Ni-BIF demonstrates cluster spin-glass behavior.

Abstract

Magnetic metal-organic frameworks (MMOFs), where magnetic metal nodes are connected into a crystal structure by organic linkers, have a potential to host exotic magnetic states. We present a study of bulk magnetic properties of four metal-organic frameworks with the same boron imidazolate linkers, Cu-BIF, Co-BIF, Ni-BIF, and newly synthesized Zn-BIF, displaying a variety of lattice structures and nontrivial magnetic behaviors. While non-magnetic Zn-BIF provides an offset of magnetic response, magnetic susceptibility measurements of the other three magnetic materials demonstrate the presence of weak magnetic interactions in these MOFs, which differ between materials by sign and size. Cu-BIF, where magnetic nodes are connected into octahedral cages, shows simple paramagnetic behavior. Triangular lattice Co-MOF shows antiferromagnetic interactions on the order of 1 K, and a spin-crossover-like effect in magnetic susceptibility due to thermal depopulation of excited crystal electric field levels. Magnetic properties of Ni-BIF suggest sizable ferromagnetic interactions. Using DC/AC susceptibility and variable-field DC magnetization, we detect cluster spin-glass behavior in Ni-BIF and discuss possible microscopic origins of this behavior. This work demonstrates the variety of magnetic properties that are possible with a single organic ligand, and establishes the low energy scale of magnetic interactions through superexchange in boron imidazolate frameworks.