We study the stellar population properties of the IRAC-detected $6 lesssim z lesssim 10$ galaxy candidates from the Spitzer UltRa Faint SUrvey Program (SURFS UP). Using the Lyman Break selection technique, we find a total of 16 new galaxy candidates
at $6 lesssim z lesssim 10$ with $S/N geq 3$ in at least one of the IRAC $3.6mu$m and $4.5mu$m bands. According to the best mass models available for the surveyed galaxy clusters, these IRAC-detected galaxy candidates are magnified by factors of $sim 1.2$--$5.5$. We find that the IRAC-detected $6 lesssim z lesssim 10$ sample is likely not a homogeneous galaxy population: some are relatively massive (stellar mass as high as $4 times 10^9,M_{odot}$) and evolved (age $lesssim 500$ Myr) galaxies, while others are less massive ($M_{text{stellar}}sim 10^8,M_{odot}$) and very young ($sim 10$ Myr) galaxies with strong nebular emission lines that boost their rest-frame optical fluxes. We identify two Ly$alpha$ emitters in our sample from the Keck DEIMOS spectra, one at $z_{text{Ly}alpha}=6.76$ (in RXJ1347) and one at $z_{text{Ly}alpha}=6.32$ (in MACS0454). We show that IRAC $[3.6]-[4.5]$ color, when combined with photometric redshift, can be used to identify galaxies likely with strong nebular emission lines within certain redshift windows.
