Magnetic field dependence of parallel spin stripe order in La_1.6-xNd_0.4Sr_xCuO₄ at optimal doping

Elastic neutron scattering measurements have been carried out on a single crystal of La_1.6-xNd_0.4Sr_xCuO₄ at optimal doping for superconductivity (x = 0.19), in magnetic fields up to 8 T, applied ∥ c. At zero magnetic field, these measurements of the intensity of parallel spin stripe quasi-Bragg peaks show a departure from a weak sloping background below T_N = 40 K, with an approximately linear growth in intensity that is bounded between superconducting Tc(onset) = 26 K and Tc(mid) = 13 K. Below Tc(mid), the order parameter plateaus and subsequently displays a strong increase below T_Nd ~ 8 K. This strong, low-temperature increase is due to weak coupling between the magnetism in the CuO₂ planes and that of Nd³⁺ moments between the planes. A magnetic field ∥ c > 1.5 T suppresses this low-temperature rise of the order parameter, as the Nd³⁺ moments polarize in the field, and the coupled Nd³⁺-CuO₂ magnetism can no longer participate in the incommensurate, parallel spin stripe structure. At T = 10 K, above T_Nd but below Tc(mid), the stripe intensity weakly decreases with field, consistent with vortex-induced disorder of short-range stripes in d-wave superconducting regions. For T ≥ 17 K, no measurable field dependence is observed, showing that a uniform H ∥ c < 8 T does not perturb CuO₂ stripe magnetism. A plateau at Tc(mid) persists across all fields. These results are discussed in the context of earlier studies of the magnetic-field dependence of parallel spin stripes in La_1.6-xNd_0.4Sr_xCuO₄, La_2-xSr_xCuO₄, and La_2-xBaxCuO₄.