Noncollinear commensurate antiferromagnetic structure in metallic Pr2PdAl7Ge4

Abstract

We report the crystal structure, magnetic structure, magnetic properties, electrical- and magnetotransport properties, and heat capacity of the rare-earth transition-metal compound ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$. ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ crystallizes in the noncentrosymmetric tetragonal structure $P\overline{4}{2}_{1}m$ (No. 113) with unit cell parameters $a=6.0059(3)\phantom{\rule{4pt}{0ex}}\AA{}$ and $c=15.2278(13)\phantom{\rule{4pt}{0ex}}\AA{}$. The N\'eel temperature ${T}_{\mathit{N}}$ of ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ was determined to be 6 K by the temperature dependences of magnetization, heat capacity, and electrical resistivity. The magnetic-field-induced antiferromagnetic to ferromagnetic transition occurs for magnetic fields $H$ along both the $ab$ plane and the $c$ axis. The electrical resistivity shows a metallic behavior with an upturn at the ordering temperature due to a superzone gap. Negative linear magnetoresistance is observed since the magnetic field increases the order degree of the antiferromagnetic state, reducing the magnetic correlation scattering. Neutron powder diffraction experiments reveal that metallic ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$ has an unusual noncollinear commensurate antiferromagnetic structure with the propagation vector $\mathbit{k}=(0,\phantom{\rule{0.28em}{0ex}}0,\phantom{\rule{0.28em}{0ex}}0.5)$. Magnetic Pr atom layers stack with the ABBA sequence along the $c$ axis. Within the layers, the Pr moments lie in the $ab$ plane, and the angle between the nearest-neighbor spins is $67.{1}^{\ensuremath{\circ}}$. This magnetic structure can well explain the magnetic properties of ${\mathrm{Pr}}_{2}{\mathrm{PdAl}}_{7}{\mathrm{Ge}}_{4}$.