Conduction electron spin-lattice relaxation time in the MgB2 superconductor

The spin-lattice relaxation time, $T_1$, of conduction electrons is measured as a function of temperature and magnetic field in MgB${}_2$. The method is based on the detection of the $z$ component of the conduction electron magnetization under electron spin resonance conditions with amplitude modulated microwave excitation. Measurement of $T_1$ below $T_c$ at 0.32 T allows to disentangle contributions from the two Fermi surfaces of MgB${}_2$ as this field restores normal state on the Fermi surface part with $\pi$ symmetry only.