Spin-flip mechanism in Ag nanowires with MgO surface protection layers has been investigated by means of nonlocal spin valve measurements using Permalloy/Ag lateral spin valves. The spin flip events mediated by surface scattering are effectively suppressed by the MgO capping layer. The spin relaxation process was found to be well described in the framework of Elliott-Yafet mechanism and then the probabilities of spin-filp scattering for phonon or impurity mediated momentum scattering is precisely determined in the nanowires. The temperature dependent spin-lattice relaxation follows the Bloch-Grüneisen theory and falls on to a universal curve for the monovalent metals as in the Monod and Beuneu scaling determined from the conduction electron spin resonance data for bulk.Spin injection, transport and detection are key ingredients in spintoronics, which have drawn a great deal of attention in recent years due to possible application for magnetic memories as well as fundamental interests concerning the interplay between charge and spin transport. 1,2 Lateral spin valves (LSVs) offer an effective means to study transport properties of a pure spin current, i.e., a diffusive flow of spin angular momentum accompanying no charge currents. A large number of the spin injection experiments have been reported since the pioneering work by Johnson and Silsbee in 1985. 3 More recently non-local spin valve experiments by Jedema et al. 4 brought renewed interests in LSVs in response to the timely development in both micro-fabrication technology and emergent interest in the pure spin current. Having a betterinsight into the spin transport and relaxation mechanism in nano-scaled devices is important to enhance the performance for the spintronic application.The spin relaxation mechanism in nonmagnetic metals (NM) has originally been discussed by Elliott and Yafet. 5,6 According to their theory, the spin-orbit interaction (SOI) in NM lifts the spin degeneracy of Bloch electrons, and results in two different energy states for up or down spin.The spin relaxation, i.e., the transition between the opposite spin states, can therefore be caused by the spin independent momentum scatterings due to impurities, grain boundaries, surfaces and phonon. [5][6][7] The earlier experimental works on the spin relaxation mechanism were mainly performed by conduction electron spin resonance (CESR) measurements and the results were

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