Magnetic and neutron diffraction studies of long-range ferromagnetic order in monolayered manganites

The structural, magnetic, and electrical properties of single crystals of Ru-doped monolayered manganites La_0.5Sr_1.5Mn_1-xRu_xO₄ (0<∼x<∼0.5) were investigated by neutron-diffraction, magnetization, and transport measurements. With increasing Ru content, the magnetic ground state changed from the parent antiferromagnetic (AFM) to the spin-glass to the ferromagnetic (FM) phase. The temperature-dependent neutron-diffraction measurements at x=0.35 clearly revealed long-range FM order in the monolayered manganite, which was confirmed by bulk magnetometry. The uniaxial magnetic anisotropy, in which the Ru spins are aligned along the c axis, was also observed in the compounds. This unique magnetic behavior might be ascribed to the magnetic anisotropy of the Ru ions stabilized under the structural distortion, as well as the FM ordering of Mn spins induced by AFM coupling among Mn/Ru pairs. The magnetoresistance of the Ru-doped system, which exhibits conduction confinement within a layer, diminishes with increasing Ru doping because of an increase in the proportion of FM domains at the expense of the magnetic glassy state.