  We have investigated the magnetic ground-state of the multiferroic relaxor
ferroelectric \pfn with $\mu$SR spectroscopy and neutron scattering. We find
that a transition to a partially disordered phase occurs below T=20 K that
coexists with long-range antiferromagnetic ordering. The disordered phase
resembles a spin-glass. No clustering of magnetic ions could be evidenced by
$\mu$SR so that the coexistence appears homogeneous in the sample.
