No abstract

The nuclear and magnetic structures of La 1Ϫx Ca x MnO 3 have been studied by neutron powder diffraction methods for the compositions xϭ0. 06, 0.15, 0.175, 0.25, and 0.33. At low concentrations of Ca (xϭ0.06) the oxidized sample contains cation vacancies and the structure is ferromagnetic, while in reduced samples all atomic sites are fully occupied and the structure is antiferromagnetic. This result explains the discrepancies found in published phase diagrams. In the samples with compositions xу0.15, the structural distortions associated with the magnetic and electronic transitions increase with increasing Ca content. These distortions consist mainly of an increase in the tilting of the MnO 6 octahedra and a consequent sharp decrease of the Mn-Mn separations as the system becomes metallic and ferromagnetic. These readjustments of the structure may be responsible for the metallic character of the bonds at the transition from insulating to metallic behavior. The magnetic-field dependence of the lattice parameters, studied in sample xϭ0.33 at two fixed temperatures ͑270 and 240 K͒, demonstrates that there is strong coupling between the magnetization and the structural properties in this system.

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.