Small polaron transport and glassy ferromagnetism in (LaSr)Co1−xFexO4

We have studied transport and magnetic properties of (LaSr)Co1−xFexO4 system. The conduction of (LaSr)CoO4 is three-dimensional variable range hopping at low temperatures below ∼300 K and is expected to be thermally activated hopping above ∼900 K. With increasing x the electronic conduction changes from a variable range hopping type to a band-gap type between x = 0.2 and 0.6. Substitution of Co by Fe induces ferromagnetism with large hysteresis in the temperature dependence of magnetization below the transition temperature. This glassy ferromagnetism appears in the concentration region where the variable range hopping of small polaron is dominant. In the samples with x ≥ 0.6 the temperature dependence of magnetization shows an antiferromagnetic behavior. The lattice parameters a and c increase with increasing x. However, the increasing rate of c-axis is small in the variable hopping region below x ∼ 0.2. The values of resistivity and ferromagnetic moment become maximum around x = 0.2. The transport and magnetic properties have a close correlation in this system.