  We investigated the temperature dependence of the upper critical field
[$H_{c2}(T)$] of fluorine-free SmFeAsO$_{0.85}$ and fluorine-doped
SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by measuring the resistive transition
in low static magnetic fields and in pulsed fields up to 60 T. Both crystals
show that $H_{c2}(T)$'s along the c axis [$H_{c2}^c(T)$] and in an $ab$-planar
direction [$H_{c2}^{ab}(T)$] exhibit a linear and a sublinear increase,
respectively, with decreasing temperature below the superconducting transition.
$H_{c2}(T)$'s in both directions deviate from the conventional one-gap
Werthamer-Helfand-Hohenberg theoretical prediction at low temperatures. A
two-gap nature and the paramagnetic pair-breaking effect are shown to be
responsible for the temperature-dependent behavior of $H_{c2}^c$ and
$H_{c2}^{ab}$, respectively.
