  We report the observation of tunable spin-orbit coupling (SOC) for ultracold
$^{87}$Rb atoms in hyperfine spin-1 states. Different from most earlier
experiments where atomic SOC of pseudo-spin-1/2 are synthesized with Raman
coupling lasers, the scheme we demonstrate employs a gradient magnetic field
(GMF) with ground state atoms and is immune to atomic spontaneous emission. The
effect of the SOC is confirmed through the studies of: 1) the collective dipole
oscillation of an atomic condensate in a harmonic trap after the synthesized
SOC is abruptly turned on; and 2) the minimum energy state at a finite
adiabatically adjusted momentum when the SOC strength is slowly ramped up. The
coherence properties of the spinor condensates remain very good after
interacting with modulating GMFs, which prompts the enthusiastic claim that our
work provides a new repertoire for synthesized gauge fields aimed at quantum
simulation studies with cold atoms.
