Ferrimagnetic resonance in films of vanadium [tetracyanoethanide]x, grown by chemical vapor deposition

Ferrimagnetic resonance (FMR) measurements in thin films of V[TCNE]x grown by chemical vapor deposition exhibit a series of sharp lines at 300 K. The orientational dependence of these lines is a result of the sample geometry provided the magnetization tracks the applied magnetic field. The FMR intensities scale with the temperature dependence of the magnetization as measured by SQUID magnetometry. The temperature dependence of the FMR at various orientations yields an estimate of the local, negative anisotropy field, which is approximately 200 Oe at zero temperature and decreases linearly with increasing temperature and is well fit by a model developed for spin glasses. The widths of the FMR lines track the temperature dependence of the magnetization, which suggests that they are determined by density fluctuations. The spacings between the individual FMR lines and their temperature dependence are consistent with the presence of nonlinear spin waves, whose critical microwave field is approximately 10-2 Oe. A rough estimate of the exchange stiffness constant is A=10-10 erg/cm. The small value of A may be due to the magnitude of the antiferromagnetic exchange between V and adjacent TCNE and the frustration of AFM exchange between adjacent TCNE radicals.