Neutron diffraction study of parasitic Nd-moment order in the checkerboard-type phase Nd_1.3Sr_0.7NiO₄

The Nd-moment order in the layered nickelate Nd_2-xSr_xNiO₄ (x = 0.7) has been investigated by performing a neutron diffraction experiment using a single crystal sample. First, the checkerboard (CB)-type charge order was confirmed by observing the temperature dependence of the nuclear superlattice peak at Q = (5, 0, 0) between 1.9 and 300 K, which indicates that the transition temperature of the CB-type charge order is above 300 K. Magnetic superlattice peaks with the propagation vector k = (1 - ε, 0, 1) appear below 67 K, and the value of ε was determined to be 0.455 in good agreement with previous studies. The intensity of the magnetic superlattice peaks appearing below 67 K shows a sharp increase below ≈20 K. This behavior indicates that the Nd moments freeze under the influence of the Ni ordering. The CB-type antiferromagnetic (AFM) Ni order in the NiO₂ layers is stacked antiferromagnetically in the c-axis direction, while the Nd moments in the Nd/SrO₂ layers are coupled antiferromagnetically with the Ni moments. The Nd moments are parallel to the c-axis, while the Ni moments are canted towards the c-axis direction from the basal ab-plane at low temperatures where the Nd moments are well ordered.