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 CoL2,3 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 TN≈52K. 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 CoL2,3-edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the CoL2 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, ≈55K, 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.
For financial support, we thank the Spanish Ministerio de Ciencia, Innovación y Universidades (Project No. RTI2018-098537-B-C22 co-funded by ERDF from EU) and Diputación General de Aragón (Project No. E12-20R).