Neutron polarization analysis has been carried out in order to clarify the magnetic structures of multiple order parameter $f$-electron system ${\mathrm{NdB}}_{4}$. We confirmed the noncollinear ``all-in all-out'' structure (${\mathrm{\ensuremath{\Gamma}}}_{4}$) of the in-plane moment, which is in good agreement with our previous neutron powder diffraction study. We found that the magnetic moment along the $c$-axis ${m}_{c}$ showed diagonally antiferromagnetic structure (${\mathrm{\ensuremath{\Gamma}}}_{10}$), inconsistent with previously reported ``vortex'' structure (${\mathrm{\ensuremath{\Gamma}}}_{2}$). The microscopic mixture of these two structures with ${\stackrel{P\vec}{q}}_{0}=(0,0,0)$ appears in phase II and remains stable in phases III and IV, where an incommensurate modulation coexists. The unusual magnetic ordering is phenomenologically understood via Landau theory with the primary order parameter ${\mathrm{\ensuremath{\Gamma}}}_{4}$ coupled with higher-order secondary order parameter ${\mathrm{\ensuremath{\Gamma}}}_{10}$. The magnetic moments were estimated to be $1.8\ifmmode\pm\else\textpm\fi{}0.2$ and $0.2\ifmmode\pm\else\textpm\fi{}0.05{\ensuremath{\mu}}_{\mathrm{B}}$ at $T=7.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{\ensuremath{\Gamma}}}_{4}$ and ${\mathrm{\ensuremath{\Gamma}}}_{10}$, respectively. We also found a long-period incommensurate modulation of the ${\stackrel{P\vec}{q}}_{1}=(0,0,1/2$) antiferromagnetic structure of ${m}_{c}$ with the propagation ${\stackrel{P\vec}{q}}_{s1}\phantom{\rule{0.16em}{0ex}}=(0.14,0.14,0.1)$ and ${\stackrel{P\vec}{q}}_{s2}=(0.2,0,0.1)$ in phase III and IV, respectively. The amplitude of sinusoidal modulation was about ${m}_{c}=1.0\ifmmode\pm\else\textpm\fi{}0.2{\ensuremath{\mu}}_{\mathrm{B}}$ at $T=1.5$ K. The local ($0,0,1/2$) structure consists of in-plane ferromagnetic and out-of-plane antiferromagnetic coupling of ${m}_{c}$, opposite to the coexisting ${\mathrm{\ensuremath{\Gamma}}}_{10}$. The ${m}_{c}$ of ${\mathrm{\ensuremath{\Gamma}}}_{10}$ is significantly enhanced up to $0.6{\ensuremath{\mu}}_{\mathrm{B}}$ at $T=1.5$ K, which is accompanied by the incommensurate modulations. The Landau phenomenological approach indicates that the higher-order magnetic and/or multipole interactions based on the pseudoquartet $f$-electron state play important roles.
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