Long-scale phase separation versus homogeneous magnetic state in(La1−yPry)0.7Ca0.3MnO3:A neutron diffraction study

Abstract

The magnetic structure of the series $({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ for y from 0.5 to 1.0 has been studied by neutron powder diffraction in the temperature range from 10 to 293 K and in external magnetic fields up to 4 T. The phase diagram has a border region of concentrations $0.6<~y<~0.8$ separating the homogeneous ferromagnetic (FM) metallic and canted antiferromagnetic (AFM) insulating states. In this region the low-temperature magnetic state is macroscopically $(>{10}^{3} \AA{})$ separated into AFM and FM phases. The FM phase has a small noncollinearity, presumably due to interfaces to the AFM phase. The macroscopical clusters can be induced by disorder on the carrier's hopping amplitude caused by natural dispersion of the A cation radius near the metal-insulator transition around $y=0.7.$ For the concentrations $y>~0.9$ the long-range ordered magnetic state is homogeneous with a canted AFM structure. The total long-range ordered magnetic moment of the Mn ion shows a steplike decrease from ${\ensuremath{\mu}}_{\mathrm{Mn}}=3.4{\ensuremath{\mu}}_{B}$ to $2.5{\ensuremath{\mu}}_{B}$ as a function of Pr concentration at the transition to a homogeneous canted antiferromagnetic (CAF) state. The spatial inhomogeneities can still be present for $y>~0.9,$ according to the reduced ${\ensuremath{\mu}}_{\mathrm{Mn}}$ value, but the Mn spins between the homogeneously CAF-ordered moments have to be either short-range ordered or paramagnetic. In addition, a ferromagnetic contribution of the Pr moments parallel to the ferromagnetic component of Mn moments is found for $y>0.6.$ The moment of Pr scales with the ferromagnetic Mn moment rather than with the Pr concentration and thus presumably induced by Mn.