Impurity-induced orthogonal double sinusoidal magnetic structure in the triangular lattice antiferromagnet CuFe 1 − x Al x O 2

Magnetic ordering has been investigated in detail in the low-temperature (LT) phase of the triangular lattice antiferromagnet ${\mathrm{CuFe}}_{0.9845}{\mathrm{Al}}_{0.0155}{\mathrm{O}}_{2}$. The LT phase characterized by the original Heisenberg character of the orbital singlet ${\mathrm{Fe}}^{3+}$ ($S=\frac{5}{2}$, $L=0$) is induced from the quasi-Ising four-sublattice $(\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow})$ state in ${\mathrm{CuFeO}}_{2}$, by a small amount of nonmagnetic impurity; nevertheless, the magnetic structure was not identified in our previous neutron diffraction measurements with triple-axis spectrometer [J. Phys. Soc. Jpn. 74, 2604 (2005)]. In the present study, we could determine orthogonal double sinusoidal (ODS) structure in the LT phase, using a lifting-counter two-axis neutron diffractometer. The ODS structure consists of two sinusoidal spin modulations that are characterized by slightly different wave numbers and whose polarization axes are orthogonal to each other. Moreover, keeping the determined structure parameters of the ODS structure, we also discuss the stabilization of the four-sublattice state in ${\mathrm{CuFeO}}_{2}$.