Diagonal static spin correlation in the low-temperature orthorhombic $\mathrm{Pccn}$ phase of ${\mathrm{La}}_{1.55}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_4$

Elastic neutron-scattering measurements have been performed on ${\mathrm{La}}_{1.55}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_4,$ which exhibits a structural phase transition at $T_s\sim 60\hspace{0.5em}\mathrm{K}$ from the low-temperature orthorhombic $\mathrm{Bmab}$ phase (labeled LTO1) to the low-temperature orthorhombic $\mathrm{Pccn}$ phase (labeled LTO2). At low temperatures, well below $T_s,$ elastic magnetic peaks are observed at the “diagonal” incommensurate (IC) positions $(0,1\mathrm{}\pm 0.055,0),$ with the modulation direction only along the orthorhombic b axis, just as in Nd-free ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_4.$ In the present study, the one dimensionality of the IC modulation, which is naturally explained by a stripe model, is clearly demonstrated with our “single-domain” crystal. The temperature dependence of the IC peak intensity suggests a substantial contribution from the ${\mathrm{Nd}}^{3+}$ spins below $\sim 3\hspace{0.5em}\mathrm{K}.$ Consistent with this, the L dependence of the magnetic scattering is accurately accounted for by a model in which the contribution of the ${\mathrm{Nd}}^{3+}$ spins is explicitly included.