Ferromagnetism in a Monolayer 2D Metal-Organic Framework

Motivated by applications in information and energy technology, the search for new materials with stable long-range magnetic ordering continues. In this respect, metal-organic magnets, being built by coordination of metal-atoms with suitable ligands, have attracted significant attention in the recent years. Nanoscale magnetic materials with reduced dimensionality promise very interesting applications in information technology, e.g. information storage, information processing (spintronics) or quantum computing. The Mermin-Wagner theorem establishes that in systems with dimensions D ≤ 2, governed by the isotropic Heisenberg exchange with short-range interactions, continuous symmetries cannot break spontaneously at finite temperatures. Only in presence of a significant magnetic anisotropy that counteracts random spin reorientations (or non-Heisenberg exchange interactions), ferromagnetism at low dimensions can occur. In this work, we show the existence of ferromagnetic coupling in a 2D metal-organic framework (MOF) consisting of Ni atom centers and tetracyanoethylene (TCNE) ligands on a Au(111) surface. Scanning tunneling microscopy (STM) shows a well-ordered MOF after co-deposition of Ni and TCNE. Analyses based on x-ray magnetic circular dichroism (XMCD) measurements reveal a strong out-of-plane magnetic anisotropy and a square like hysteresis loop with a coercive field of 1 T. Origin of such 2D ferromagnetism will be discussed