  We report structural, magnetic and dielectric properties of layered
perovskite materials LnBaCuFeO5 (Ln = La and Lu). LaBaCuFeO5 shows magnetic
cluster glass behavior below 60 K owing to the competing ferromagnetic and
antiferromagnetic exchange interactions. Glassy dynamics of electric dipoles
has also been observed in the vicinity of the magnetic glass transition
temperature. The presence of significant coupling between spin and polar
degrees of freedom results in the multiglass feature in LaBaCuFeO5. The
LuBaCuFeO5 compound undergoes YBaCuFeO5 like commensurate to incommensurate
antiferromagnetic transition at 175 K. Large magnetic irreversibility below 17
K in this compound suggests the presence of strong spin anisotropy. In
addition, in this compound the interaction between the dipoles is not strong
enough, which results in the absence of glassy dynamics of electric dipoles.
The contrasting behavior of two compounds is possibly due to variation in the
ferromagnetic and antiferromagnetic interactions along c-axis, which is the
manifestation of structural modification arising out of the difference in the
ionic radii of La and Lu.
