The de Haas-van Alphen (dHvA) effect has been investigated on high-quality
single crystals of FeP in a double helical magnetic state with a 5c period
in the magnetic fields up to 14T and at temperatures down to 0.42 K. Single
crystals were grown by the chemical transport technique using iodine as th
e transport agent. Nineteen branches were observed in three crystallographi
c planes (100), (010) and (001), and analyzed with simple analytic equation
s for the Fermi surfaces. Many of the dHvA branches have a characteristic a
ngular dependence and are found to correspond to the orbits on surfaces bey
ond the Brillouin zone in the magnetic state which is reduced to 1/5 of the
non-magnetic one with the MnP-type along the [001] direction. These branch
es originate from the orbits caused by a magnetic breakdown phenomenon unde
r high fields. Cyclotron mass ratios were obtained to be up to 5.9, whereas
the electronic specific heat coefficient is rather small. 2.80 mJK(-2) mol
(-1).