Raman Spectroscopy and Aging of the Low-Loss Ferrimagnet Vanadium Tetracyanoethylene

Abstract

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 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 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 CN 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.