Magnetic ordering in La1.5Ca0.5CoO4 revealed by resonant magnetic x-ray scattering and neutron diffraction

We report a comprehensive study on the magnetic ground state of La1.5Ca0.5CoO4 combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the Co L-2,L-3 edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at T-N approximate to 52 K. Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic A-centered lattice but with a and b axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group A2mm where two nonequivalent compressed and expanded CoO6 octahedra are ordered showing a checkerboard pattern in the ab plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as (1/4, 1/4, 0)(t), (1/4, 1/4, 1)(t), and (1/4, 1/4, 1/2)(t) in the parent tetragonal cell were observed at low temperature at the Co L-2,L-3-edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the Co L-2 edge, indicates that only Co2+-like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the ab plane of the parent tetragonal cell with a slight tilt in the c axis. Despite the onset temperatures for the three resonant magnetic reflections being the same, approximate to 55 K, different thermal behavior is observed between (1/4, 1/4, 1/2)(t) and (1/4, 1/4, L)(t) (L = integer) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the (1/4, 1/4, 1/2)(t) magnetic reflection is at least ten times larger than that of the (1/4, 1/4, L)(t) (L = integer) ones. On the other hand, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector k = (1/4, 1/4, 1/2)(t) that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in La1.5Ca0.5CoO4 corresponds then to this propagation vector k = (1/4, 1/4, 1/2)(t) while (1/4, 1/4, 0)(t) and (1/4, 1/4, 1)(t) magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.

Automated summary

The study provides a comprehensive investigation on the magnetic ground state of La1.5Ca0.5CoO4, revealing magnetic phase transitions and structural changes in the crystal lattice. Combined single crystal neutron diffraction and resonant magnetic x-ray scattering techniques offer detailed insights into the magnetic ordering and structural evolution of the material, enhancing our understanding of its magnetic properties.