We report on micro-focused Raman spectroscopy of encapsulated vanadium tetracyanoethylene (V­[TCNE] x , x ≈ 2) films as they age under ambient conditions to understand the structural changes that accompany aging and how those changes influence the magnetic properties. V­[TCNE] x is an organic-based ferrimagnet with a high magnetic ordering temperature T C > 600 K, low magnetic damping, and growth compatibility with a wide variety of substrates. However, similar to other organic-based materials, it is sensitive to air. Although encapsulation of V­[TCNE] x with glass and epoxy extends the film lifetime from hours to weeks, its aging processes remain poorly understood. We identify the relevant features in the Raman spectra in agreement with ab initio theory, reproducing CC and CN stretching vibrational modes. We correlate changes in the Raman intensity and in photoluminescence to changes in the magnetic properties of the sample as measured using ferromagnetic resonance and magnetometry. Based on changes in the Raman spectra, we hypothesize structural changes and aging mechanisms in V­[TCNE] x . We also find that we can introduce similar changes using focused laser illumination at high intensity, enabling a new mechanism for “erasing” magnetism in this material through selective modification of local bonding. These findings enable a local optical probe of V­[TCNE] x film quality, which is invaluable in experiments where assessing film quality with local magnetic characterization is not possible, and they enable patterning of V­[TCNE] x by laser writing.

Materials that simultaneously exhibit permanent porosity and high-temperature magnetic order could lead to advances in fundamental physics and numerous emerging technologies. Herein, we show that the archetypal molecule-based magnet and magnonic material V(TCNE)2 (TCNE = tetracyanoethylene) can be desolvated to generate a room-temperature microporous magnet. The solution-phase reaction of V(CO)6 with TCNE yields V(TCNE)2·0.95CH2Cl2, for which a characteristic temperature of T* = 646 K is estimated from a Bloch fit to variable-temperature magnetization data. Removal of the solvent under reduced pressure affords the activated compound V(TCNE)2, which exhibits a T* value of 590 K and permanent microporosity (Langmuir surface area of 850 m2/g). The porous structure of V(TCNE)2 is accessible to the small gas molecules H2, N2, O2, CO2, ethane, and ethylene. While V(TCNE)2 exhibits thermally activated electron transfer with O2, all the other studied gases engage in physisorption. The T* value of V(TCNE)2 is slightly modulated upon adsorption of H2 (T* = 583 K) or CO2 (T* = 596 K), while it decreases more significantly upon ethylene insertion (T* = 459 K). These results provide an initial demonstration of microporosity in a room-temperature magnet and highlight the possibility of further incorporation of small-molecule guests, potentially even molecular qubits, toward future applications.

The molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene (V[TCNE] x, x [Formula: see text] 2) has garnered interest from the quantum information community due to its excellent coherent magnonic properties and ease of on-chip integration. Despite these attractive properties, a detailed understanding of the electronic structure and mechanism for long-range magnetic ordering have remained elusive due to a lack of detailed atomic and electronic structural information. Previous studies via x-ray absorption near edge spectroscopy and the extended x-ray absorption fine structure have led to various proposed structures, and in general, V[TCNE] x is believed to be a three-dimensional network of octahedrally coordinated V2+, each bonded to six TCNE molecules. Here, we elucidate the electronic structure, structural ordering, and degradation pathways of V[TCNE] x films by correlating calculations of density functional theory (DFT) with scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) of V[TCNE] x films. Low-loss EELS measurements reveal a bandgap and an excited state structure that agree quantitatively with DFT modeling, including an energy splitting between apical and equatorial TCNE ligands within the structure, providing experimental results directly backed by theoretical descriptions of the electronic structure driving the robust magnetic ordering in these films. Core-loss EELS confirms the presence of octahedrally coordinated V+2 atoms. Upon oxidation, changes in the C1s- π* peak indicate that C=C of TCNE is preferentially attacked. Furthermore, we identify a relaxation of the structural ordering as the films age. These results lay the foundation for a more comprehensive and fundamental understanding of magnetic ordering and dynamics in these classes of metal–ligand compounds.

Vanadium tetracyanoethylene (V [TCNE]x, x ≈ 2) is an organic-based ferrimagnet with a high magnetic ordering temperature TC > 600 K, low magnetic damping, and growth compatibility with a wide variety of substrates. However, similar to other organic-based materials, it is sensitive to air. Although encapsulation of V [TCNE]x with glass and epoxy extends the film lifetime from an hour to a few weeks, what is limiting its lifetime remains poorly understood. Here we characterize encapsulated V[TCNE]x films using confocal microscopy, Raman spectroscopy, ferromagnetic resonance and SQUID magnetometry. We identify the relevant features in the Raman spectra in agreement with ab initio theory, reproducing C = C, C ≡ N vibrational modes. We correlate changes in the effective dynamic magnetization with changes in Raman intensity and in photoluminescence. Based on changes in Raman spectra, we hypothesize possible structural changes and aging mechanisms in V [TCNE]x. These findings enable a local optical probe of V [TCNE]x film quality, which is invaluable in experiments where assessing film quality with local magnetic characterization is not possible.