  We present a joint experimental and theoretical study of spin dynamics of a
single $^{88}$Sr$^+$ ion colliding with an ultracold cloud of Rb atoms in
various hyperfine states. While spin-exchange between the two species occurs
after 9.1(6) Langevin collisions on average, spin-relaxation of the Sr$^+$ ion
Zeeman qubit occurs after 48(7) Langevin collisions which is significantly
slower than in previously studied systems due to a small second-order
spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange
rate was observed as the magnetic field was increased. Interestingly, we found
that, while the phases acquired when colliding on the spin singlet and triplet
potentials vary largely between different partial waves, the singlet-triplet
phase difference, which determines the spin-exchange cross-section, remains
locked to a single value over a wide range of partial-waves which leads to
quantum interference effects.
