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

TL;DR

This study uses Raman spectroscopy and other techniques to analyze the aging process of encapsulated V[TCNE]$_{x}$ ferrimagnetic films, revealing structural changes and providing a local optical method to assess film quality.

Contribution

It correlates Raman spectral features with magnetic properties and aging mechanisms, offering new insights into the stability of organic ferrimagnets.

Findings

Raman spectra match ab initiobb vibrational modes.

Changes in Raman intensity relate to magnetic property variations.

Identifies potential structural aging mechanisms in V[TCNE]$_{x}$ films.

Abstract

Vanadium tetracyanoethylene (V[TCNE]$_{x}$, $x\approx 2$) is an organic-based ferrimagnet with a high magnetic ordering temperature $\mathrm{T_C>600 ~K}$, 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 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 \textit{ab initio} theory, reproducing $\mathrm{C=C,C\equiv N}$ 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.