Magnetic spin-flop transition and interlayer spin-wave dispersion inPrCaFeO4revealed by neutron diffraction and inelastic neutron scattering

Abstract

We present a comprehensive study on ${\mathrm{PrCaFeO}}_{4}$ using macroscopic methods, neutron and x-ray diffraction, as well as inelastic neutron scattering. One polycrystalline and two single-crystal samples were investigated exhibiting structural phase transitions from a high-temperature tetragonal phase to an intermediate orthorhombic phase (space group $Bmeb$) at $510{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ (783 K). At approximately 240 K a second structural phase transition takes place into the space group $Pccn$ where the tilt axis of the ${\mathrm{FeO}}_{6}$ octahedra changes from the [100] to the $\ensuremath{\langle}110\ensuremath{\rangle}$ directions. Due to strong diffuse scattering at high temperatures neutron powder diffraction can only safely state that ${T}_{N}$ is above 330 K. ${\mathrm{PrCaFeO}}_{4}$ exhibits a magnetic spin-flop phase transition where the magnetic moments turn from the $b$ axis to the $c$ axis upon cooling. However, the transition temperatures and the width of this magnetic transition are strikingly different between the investigated samples, suggesting a strong influence from the real structure. Indeed, a significant difference in the oxygen content was deduced by single-crystal x-ray diffraction. The magnon dispersion was studied by inelastic neutron scattering revealing a nearest-neighbor interaction comparable to that in ${\mathrm{LaSrFeO}}_{4}$ but with smaller anisotropy gaps. A clear interlayer dispersion was observed resulting from the structural distortions and the relief of geometrical frustration due to the orthorhombic splitting.