Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque \(\tau(\theta)\) in the mixed state of LaO\(_{0.9}\)F\(_{0.1}\)FeAs single crystals as a function of temperature \(T\) and magnetic fields \(H\) up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter \(\gamma=(m_c/m_{ab})^{1/2}\) from \(\tau(\theta)\). We show that \(\gamma\) depends strongly on both \(T\) and \(H\), reaching a maximum value of \(\sim\) 10 followed by a decrease towards values close to 1 as \(T\) is lowered. The observed field dependencies of the London penetration depth \(\lambda_{ab}\) and \(\gamma\) suggest the onset of suppression of a superconducing gap at \(H \approx H_{c2}/3\).