Magnetic Structures of Sr(Co₁_−xPd_x)₂As₂

We present neutron-diffraction results to determine the magnetic structures of Sr(Co_1−xPd_x)₂As₂ single crystals. Although the parent compound (x = 0) does not order down to 50 mK, it shows magnetic fluctuations due to ferromagnetic interactions between nearest neighbors (NN) that compete with antiferromagnetic interactions between next-NN. At a moderate Pd concentration (x ∼ 0.1), the system is magnetically ordered, where ferromagnetic (FM) Co-Pd layers in the ab-plane stack with an incommensurate (IC) spiral pattern along the c-direction. At higher Pd concentrations (x ∼ 0.15), the system undergoes a phase transition to a fragile ferromagnetic state below T_C such that the moments in the FM layers are initially in the plane and, in a somewhat chaotic way, flip to align along the crystallographic c-axis, as the temperature is lowered. The lattice constants show positive thermal expansion coefficients (CTE) in the plane and negative CTE out-of-plane, while the overall unit cell volume shows positive CTE. The CTE for volume has a prominent peak below the Curie temperature (T_C), which, we reason, is due to a robust magnetoelastic coupling (MEC) mediated by Pd spin-orbit coupling.