  We report coherent manipulation of S=10 Fe$_{8}$ single-molecule magnets. The
temperature dependence of the spin decoherence time $T_2$ measured by high
frequency pulsed electron paramagnetic resonance indicates that strong spin
decoherence is dominated by Fe$_{8}$ spin bath fluctuations. By polarizing the
spin bath in Fe$_{8}$ single-molecule magnets at magnetic field $B$ = 4.6 T and
temperature $T$ = 1.3 K, spin decoherence is significantly suppressed and
extends the spin decoherence time $T_2$ to as long as 712 ns. A second
decoherence source is likely due to fluctuations of the nuclear spin bath. This
hints that the spin decoherence time can be further extended via isotopic
substitution to smaller magnetic moments.
