Millimetre-wave studies on the high-spin molecules Cr10(OMe)20(O2CCMe3)10 and Cr12O9(OH)3(O2CCMe3)15

We report millimetre-wave electron spin resonance (ESR) measurements on single crystals of the high-spin molecules Cr10(OMe)20(O2CCMe3)10 and Cr12O9(OH)3(O2CCMe3)15 within a temperature range of 1.4 K to 50 K and in magnetic fields of up to 5 Tesla. In our experiments it is possible to vary the orientation of the magnetic field with respect to the crystal axes, and thus to study the ESR lineshapes as a function of both temperature and angle. Our results confirm that Cr10(OMe)20(O2CCMe3)10 behaves as a single-molecule magnet with S = 15 and D = -0.03 K, while Cr12O9(OH)3(O2CCMe3)15 has S = 6 and D ∼ 0.1 K. A comparison of the experimental spectra with numerical simulations gives good agreement at low temperatures. At higher temperatures, we observe a narrowing of the ESR spectrum that is not explained by simple models.