Coexistence of long-range cycloidal order and spin-cluster glass state in the multiferroic BaYFeO4

We report the presence of spin glass state below the cycloidal spin ordering in the multiferroic BaYFeO₄. This compound is known to crystallize in an orthorhombic structure with a centrosymmetric space group Pnma and exhibits two successive antiferromagnetic phase transitions. Upon cooling, it undergoes a spin density wave (SDW)-like antiferromagnetic ordering at T _N1 ~ 48 K and a cycloidal ordering at T _N2 ~ 35 K. Using dc magnetic memory effect and magnetization relaxation studies, we have shown that this oxide undergoes a reentrant spin glass transition below T ^* ~ 17 K. Our analysis suggests the presence of spin clusters in the glassy state. The coexistence of spin-cluster glass and long-range cycloidal ordered states results in an exchange bias effect at 2 K. The origin of the glassy state has been attributed to freezing of some Fe³⁺ moments, which do not participate in the long-range ordering.