Reentrant behavior in Cr doped bilayer manganite LaSr2Mn2O7.

We have studied the effect of replacing Mn 3+ by Cr 3+ on the structure, transport and magnetism in the bilayered manganite LaSr 2 Mn 2 O 7 . Although no structural transition was observed in LaSr 2 Mn 2−y Cr y O 7 (0.1 ≤ y ≤ 0.6), the electrical transport and the magnetic properties were found to be affected significantly by this substitution. Substitution of Cr 3+ reduces the conductivity by restricting the hopping of small polarons. Magnetization increases with increasing Cr 3+ concentration suggesting that Cr 3+ -ions induce ferromagnetic moments . The ferromagnetic and an antiferromagnetic phase observed above ∼60 K merge into an inhomogeneous phase below this temperature. Thermopower yields an essentially concentration independent charge density nearly equal to its value for chromium free composition inspite of its expected decrease with this substitution suggesting that the small charge density of the insulating AFM phase is supplemented by the free carriers in the FM phase. The inhomogeneous phase shows a relaxor type behavior which contrasts with the spin glass behavior seen in La 0.46 Sr 0.54 Mn 0.98 Cr 0.02 O 3 having an identical AFM magnetic state. The difference is attributed to the non-JT character of Cr-ions which reduce the distortion of the Mn O octahedra located within the FM domains. With a higher lattice strain in the surrounding AFM matrix the carriers remain confined within the FM domains leading to the relaxor type behavior. [ABSTRACT FROM AUTHOR]