Observation of anomalous magnetism in the low-temperature monoclinic phase of single-crystallinePrAlO3perovskite

Abstract

We have investigated unusual magnetism of the ${\text{PrAlO}}_{3}$ perovskite system by performing measurements of the magnetization, magnetic susceptibility, and $^{27}\text{A}\text{l}$ NMR spectra on single crystals, prepared by the Czochralski method. Two kinds of samples were investigated, an as-grown crystal containing a mixture of ${\text{Pr}}^{3+}$ and ${\text{Pr}}^{4+}$ ions in the approximate ratio 4:1 and a crystal annealed in a reducing ${\text{H}}_{2}/{\text{N}}_{2}$ atmosphere, which contained the ${\text{Pr}}^{3+}$ ions only. Within the high-temperature rhombohedral and orthorhombic phases, the ${\text{PrAlO}}_{3}$ crystals behave as simple paramagnets, whereas in the low-temperature monoclinic phase below 152 K, anomalous magnetic properties were observed. The zero-field-cooled (zfc) magnetization vs the magnetic field $M(H)$ curves exhibit nonlinear behavior in the low-field part of the virgin field sweep, whereas they become linear paramagnetic at fields larger than the ``polarizing'' field ${H}_{\text{p}}$ and remain locked to the linear line for all subsequent field sweeps. The nonlinear part is absent in the field-cooled (fc) $M(H)$ curves. The zfc and fc susceptibilities exhibit very different temperature dependences as a function of the magnetic field. At low temperatures, the zfc and fc susceptibilities become temperature independent, consistent with the Van Vleck-type paramagnetism. The $^{27}\text{A}\text{l}$ NMR spectra show unusual tendency of increasing crystal symmetry toward cubic upon cooling. The experimental results can be explained by considering the monoclinic phase to contain tetragonally distorted domains whose fourfold axes can be reoriented by a magnetic field into the field direction. The tetragonal distortion is temperature dependent and decreases upon cooling.