We have investigated the magnetic ground state of the multiferroic relaxor ferroelectric $\text{Pb}\left({\text{Fe}}_{1/2}{\text{Nb}}_{1/2}\right){\text{O}}_3$ with $\mu \text{SR}$ spectroscopy and neutron scattering. We find that a transition to a partially disordered phase occurs below $T=20\text{K}$ that coexists with long-range antiferromagnetic ordering. The disordered phase resembles a spin glass. No clustering of magnetic ions could be evidenced by $\mu \text{SR}$ so that the coexistence appears homogeneous in the sample.

• Received 13 January 2009

DOI:https://doi.org/10.1103/PhysRevB.79.184430