Lattice variation, reentrant electric properties, and magnetic order in bilayered La1.02Sr1.98Mn2O7

Neutron diffraction, dc electric resistivity, and ac magnetic susceptibility measurements were performed to study the structural, electric, and magnetic properties of naturally bilayered La1.02Sr1.98Mn2O7. An insulator to metal transition was clearly seen at ∼170 K. Below 60 K, the system reentered the insulating state. Three-dimensional (3D) long-range order of the Mn spins was found, with an ordering temperature of Tm≈180 K and a saturated moment of 〈μZ〉=2.70(2)μB. The in-plane couplings were ferromagnetic. Within each bilayer, simple antiferromagnetic couplings between the ferromagnetic sheets were found, whereas the neighboring bilayers were coupled ferromagnetically. A structural change that results in a reduction of the thermal contraction rate occurs at ∼250 K as well.