Effect of H+ irradiation on magneto-optical properties of Co-doped ZnO thin films

We present an investigation on the strong magneto-optical response exhibited by ferromagnetic Codoped ZnO films upon hydrogen ions irradiation. Magneto-optical thin film materials with high figure of merit (i.e. high ratio of Faraday rotation per unit length to the optical loss per unit length) can pave the way for development of new integrated on-chip non-reciprocal photonic devices. Most efforts are currently directed to the growth of films of ferromagnetic garnet oxides which are however very difficult to integrate in optoelectronic technologies. In such a context, we show that ZnO-based diluted magnetic semiconductors obtained by incorporating transition metal elements (e.g. Fe, Co) are suitable for the purpose. Ferromagnetic behaviour of Co-doped ZnO films can be improved when treated by a suitable atomic hydrogen treatment [1]. We investigated the role of hydrogen in the improvement of the magneto-optical properties by comparing the magnetic circular dichroism spectra of H-irradiated ZnCoO films. Optical and microstructural analyses indicate that Co fully substitutes for Zn ions without phase segregation. In addition to a large increase in the saturation magnetization we observe larger values of the magnetic circular dichroism upon hydrogen incorporation, especially for the samples processed with high H-doses which also show a large value of the Faraday rotation at magnetic saturation. These results give a direct indication of the intrinsic nature of the ferromagnetism in this class of materials as arising from the spin polarized carriers magnetically coupled to Co ions. The values of the Faraday rotation and hence the figure of merit of the hydrogenated films in the vis range can be tuned by a suitable choice of the H-dose and are promising in the perspective of non-reciprocal magneto-optical devices integrated on semiconductors.