Superconducting properties and the interplay between magnetism and superconductivity in 1111 Fe arsenides as revealed by torque magnetometry

We performed a study of the angular dependence of the magnetic torque in LaFeAsO$_{0.9}$F$_{0.1}$ single crystals. We developed a method to separate the magnetic and the superconducting components inherent to the FeAs layers and which are superimposed onto the reversible torque signal $\tau_{\rm{rev}} (\theta, H, T)$. We show that by exploring the amplitude of the superconducting component in $\tau_{\rm{rev}} (\theta)$ as a function of $H$, it is possible to extract the thermodynamic value of the superconducting upper critical field $H_{c2}$. This so obtained value can be used to extract the field and the temperature dependencies of respectively, the superconducting anisotropy and the superfluid density through the Kogan formalism. We observe a strong temperature and field dependence of the superconducting anisotropy as expectable within a multi-band superconducting scenario. The resulting $T$-dependence of the superfluid-density resembles the behavior previously reported for LaFePO and which was ascribed to nodal superconductivity.