  We present a study of the structural and physical properties of directly hole
doped LaFe1-xMnxAsO (x = 0.0-0.2) and the influence of charge compensation /
electron-doping by additional F doping in LaFe0.9Mn0.1AsO1-yFy (y = 0.1-0.5).
High quality polycrystalline samples were prepared using a solid state
metathesis reaction. The unit cell increases upon Mn doping, but decreases
again when additional F is inserted. The semiconducting character of
LaFe1-xMnxAsO decreases with additional F doping. Muon spin relaxation (muSR)
measurements reveal short range magnetic order in LaFe1-xMnxAsO and a
suppression of magnetism by additional electron-doping with fluoride in
LaFe0.9Mn0.1AsO1-yFy. Superconductivity remains absent even though the
electronic preconditions are fulfilled in electron-doped LaFe0.9Mn0.1AsO1-yFy
at x > 0.1, which is suggestive of effective pair breaking by Mn in this
system.
