Determination of the Voigt constant of phthalocyanines by magneto-optical Kerr-effect spectroscopy

Thin films of organic molecular semiconductors from the class of phthalocyanines are investigated by means of magneto-optical Kerr effect (MOKE) spectroscopy at room temperature. A numerical analysis of the energy dispersion of the real and imaginary part of the complex magneto-optical Kerr rotation angle in the visible to near ultraviolet spectral range allows the first determination of the magneto-optical material constant, the so-called Voigt constant, for a molecular material and the results are compared to density-functional theory calculations. The Voigt constant is only 1-2 orders of magnitude smaller compared with soft ferromagnetic materials such as Ni. The amplitude of the Voigt constant is exploited to determine the molecular orientation in thin films with high accuracy.