Reentrant behavior in Cr doped bilayer manganite LaSr2Mn2O7

We have studied the effect of replacing Mn3+ by Cr3+ on the structure, transport and magnetism in the bilayered manganite LaSr2Mn2O7. Although no structural transition was observed in LaSr2Mn2−yCryO7 (0.1 ≤ y ≤ 0.6), the electrical transport and the magnetic properties were found to be affected significantly by this substitution. Substitution of Cr3+ reduces the conductivity by restricting the hopping of small polarons. Magnetization increases with increasing Cr3+ concentration suggesting that Cr3+-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 La0.46Sr0.54Mn0.98Cr0.02O3 having an identical AFM magnetic state. The difference is attributed to the non-JT character of Cr-ions which reduce the distortion of the MnO 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.