Unconventional magnetostriction in layeredLa1.2Sr1.8Mn2O7: Evidence for spin-lattice coupling aboveTC

The crystal structure of La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} 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 T{sub C}; the a axis contracts ({Delta}a/a={minus}0.018{percent}) while the c axis expands ({Delta}c/c=0.05{percent}). The latter value is larger than that found for perovskite manganites ({Delta}l/l=0.02{percent} at 1 T). No such effect occurs at lower temperatures. The magnetostriction results from changes in individual Mn-O bond lengths. From T{sub C} to {approximately}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} {ital 1997} {ital The American Physical Society}