Magnetization dynamics and frustration in the multiferroic double perovskite Lu₂ MnCoO₆

We investigate the magnetic ordering and the magnetization dynamics (from kHz to THz time scales) of the double perovskite Lu2MnCoO6 using elastic neutron diffraction, muon spin relaxation and micro-Hall magnetization measurements. This compound is known to be a type II multifer- roic with the interesting feature that a ferromagnetic-like magnetization hysteresis loop couples to an equally hysteretic electric polarization in the bulk of the material despite a zero-field mag- netic ordering of the type ↑↑↓↓ along Co-Mn spin chains. Here we explore the unusual dynamics of this compound and find extremely strong fluctuations, consistent with the axial next-nearest- neighbor Ising (ANNNI) model for frustrated spin chains. We identify three temperature scales in Lu2MnCoO6 corresponding to the onset of highly fluctuating long-range order below TN = 50 ± 3 K identified from neutron scattering, the onset of magnetic and electric hysteresis, with change in kHz magnetic and electric dynamics below a 30 K temperature scale, and partial freezing of ∼MHz spin fluctuations in the muon spin relaxation data below 12 ± 3 K. Our results provide a framework for understanding the multiferroic behavior of this compound and its hysteresis and dynamics.