  We use muon spin relaxation (muSR) to investigate the magnetic properties of
a bulk form diluted ferromagnetic semiconductor (DFS) Li1.15(Zn0.9Mn0.1)P with
T_C ~ 22 K. MuSR results confirm the gradual development of ferromagnetic
ordering below T_C with a nearly 100% magnetic ordered volume. Despite its low
carrier density, the relation between static internal field and Curie
temperature observed for Li(Zn,Mn)P is consistent with the trend found in
(Ga,Mn)As and other bulk DFSs, indicating these systems share a common
mechanism for the ferromagnetic exchange interaction. Li1+y(Zn1-xMnx)P has the
advantage of decoupled carrier and spin doping, where Mn2+ substitution for
Zn2+ introduces spins and Li+ off-stoichiometry provides carriers. This
advantage enables us to investigate the influence of overdoped Li on the
ferromagnetic ordered state. Overdoping Li suppresses both T_C and saturation
moments for a certain amount of spins, which indicates that more carriers are
detrimental to the ferromagnetic exchange interaction, and that a delicate
balance between charge and spin densities is required to achieve highest T_C.
