Unconventional magnetostriction in layered La1.2Sr1.8Mn2O7: Evidence for spin-lattice coupling above TC

The crystal structure of La1.2Sr1.8Mn2O7 has been determined as a function of temperature in magnetic fields of 0 and 0.6 T. There is a significant magnetostriction at the Curie temperature TC; the a axis contracts (Δa/a=-0.018%) while the c axis expands (Δc/c=0.05%). The latter value is larger than that found for perovskite manganites (Δl/l=0.02% at 1 T). No such effect occurs at lower temperatures. The magnetostriction results from changes in individual Mn-O bond lengths. From TC to ∼300 K, bulk magnetization and magnetoresistance measurements indicate the presence of magnetic order not observed in the neutron-diffraction data. This suggests short-range magnetic order in this temperature range. Our structural measurements show that within this temperature interval there is also a weak magnetostriction. copyright 1997 The American Physical Society