Field-induced orbital order-disorder transition in an A-type antiferromagnetic manganite: High-field study of ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_3$

Magnetization and magnetoresistance were measured in A-type antiferromagnet ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_3$ utilizing pulsed magnetic fields up to 45 T. We have observed a ferromagnetic transition accompanied by a discontinuous decrease of resistivity. The temperature dependence of the resistivity in the ferromagnetic state showed characteristics of a three-dimensional metal. The observed phenomena are explained in terms of simultaneous destruction of the $d_{x^2-y^2}$ orbital ordering and the A-type antiferromagnetic spin ordering by magnetic field.