Observation of modulated magnetic long-range order

Magnetic superlattice peaks are observed in single-crystal neutron-diffraction measurements on orthorhombic (Formula presented) at reciprocal points of (1/2±ε,1/2,0) and (1/2,1/2±ε,0) in the tetragonal notation where (Formula presented)0.126(Formula presented)0.003. The La NMR measurement reveals a broadening of the field-swept spectrum below (Formula presented)45 K corresponding to the existence of magnetic order. The remarkable softening of longitudinal sound waves along [110] is observed in the same crystal. The features observed in the neutron diffraction, NMR, and ultrasonic measurements suggest that the dynamical incommensurate spin correlation is pinned by a lattice instability toward the low-temperature tetragonal phase.