Anisotropic Magnetoelastic Phenomena in Layered Manganite Crystals: Implication of Change in Orbital State

Magnetoelastic phenomena correlated with magnetoresistive ones have been investigated for bilayered manganite crystals, ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ( $0.3\le x\le 0.45$), by measurements of the striction parallel and perpendicular to the ${\mathrm{MnO}}_2$ plane in magnetic fields. The magnetization process causes not only a large negative magnetoresistance but also a gigantic lattice striction which is highly anisotropic with respect to the crystal axes. The magnitude and the sign of the lattice striction systematically varies with the doping level ( $x$), which should reflect variation of the orbital-dependent occupancy in the two-dimensional conduction band with temperature, magnetic field, and doping level.