  We report a steplike lattice transformation of single crystalline
(La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$bilayered manganite
accompanied by both magnetization and magnetoresistive jumps, and examine the
ultrasharp nature of the field-induced first-order transition from a
paramagnetic insulator to a ferromagnetic metal phase accompanied by a huge
decrease in resistance. Our findings support that the abrupt magnetostriction
is closely related to an orbital frustration existing in the inhomogeneous
paramagnetic insulating phase rather than a martensitic scenario between
competing two phases.
