Quantum information science and engineering require novel low-loss magnetic materials for magnon-based quantum-coherent operations. The search for low-loss magnetic materials, traditionally driven by applications in microwave electronics near room temperature, has gained additional constraints from the need to operate at cryogenic temperatures for many applications in quantum information science and technology. Whereas yttrium iron garnet (YIG) has been the material of choice for decades, the emergence of molecule-based materials with robust magnetism and ultra-low damping has opened new avenues for exploration. Specifically, thin films of vanadium tetracyanoethylene (V[TCNE]x) can be patterned into the multiple, connected structures needed for hybrid quantum elements and have shown room-temperature Gilbert damping (α = 4 × 10−5) that rivals the intrinsic (bulk) damping otherwise seen only in highly polished YIG spheres (far more challenging to integrate into arrays). Here, the authors present a comprehensive and systematic study of the low-temperature magnetization dynamics for V[TCNE]x thin films, with implications for their application in quantum systems. These studies reveal a temperature-driven, strain-dependent magnetic anisotropy that compensates the thin-film shape anisotropy and the recovery of a magnetic resonance linewidth at 5 K that is comparable to room-temperature values (roughly 2 G at 9.4 GHz). The authors can account for these variations of the V[TCNE]x linewidth within the context of scattering from very dilute paramagnetic impurities and anticipate additional linewidth narrowing as the temperature is further reduced.

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.

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.

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