Orbital order and a canted phase in the paramagnetic and ferromagnetic states of 50% hole-doped colossal magnetoresistance manganites

We report neutron scattering results on the spin dynamics in the paramagnetic (PM) and ferromagnetic (FM) states of the 50% hole-doped manganites ${\mathrm{Pr}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_3$ and ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_3.$ In the PM phase, these systems exhibit two kinds of diffuse scatterings: an isotropic quasielastic diffuse scattering and a dynamical ridge-type, i.e., two-dimensional FM diffuse scattering. With decreasing temperature, both systems enter the pure FM metallic state, but the spin dynamics in the FM state possess a strong A-type antiferromagnetic feature. In fact, ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_3$ enters a canted antiferromagnetic phase at lower temperature through a second order phase transition. These behaviors in spin dynamics demonstrate the existence of the static $d_{x^2-y^2}$-type orbital ordering in the PM and FM states of 50% hole-doped manganites which have relatively larger one-electron bandwidth.