Impact of hidden structural order on low temperature magnetic resonance in V[TCNE]x*

The coordination compound V[TCNE]x is a promising magnetic material for integration into solid-state quantum information systems due to its low damping (α = (3.98 ± 0.22) × 10^-5), high quality factor (Q up to 8,000) microwave resonance, ease of patterning, and compatibility with a wide variety of substrates. Since current quantum information systems require low temperature operation, a comprehensive study of the low temperature magnetic behavior of V[TCNE]x is essential for realizing this promise. Here we report the observation of anomalous temperature dependent anisotropy in thin films of V[TCNE]x at temperatures down to 5 K. The resonance linewidth at 5K is larger by a factor of 3 than at room temperature, competitive with the very best liquid phase epitaxy (LPE) yittrium iron garnet (YIG) films. Further, we observe temperature-dependent changes in the magnetic anisotropy that suggest that this modest increase in linewidth may in fact arise from inhomogeneous thermally-induced strain in the thin film. This result promises further reductions in the low temperature linewidth in structures engineered to minimize differences in thermal contraction between the sample and the substrate.