$o-{\mathrm{HoMnO}}_3$ crystallizes in the ${\mathrm{GdFeO}}_3$-type structure (Pbnm) with no crystal structure anomalies below 295 K and with the Mn-O-Mn angle as low as 144.1°. The ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ magnetic moments are ordered below $T_N=38\mathrm{}\mathrm{K}$ and $T_{\mathrm{Ho}}=11\mathrm{}\mathrm{K},$ respectively. The ordering is incommensurate with a propagation vector $\mathbf{k}{=k}_y{\mathbf{b}}^{*},$ where $k_y$ is between 0.395 and 0.431 depending on temperature. Symmetry analyses have been carried out for both magnetic sites. The ordering was determined to be $A_y$ for the ${\mathrm{Mn}}^{3+}$ moments, modulated in a longitudinal sinusoidal wave along the propagation vector. The ${\mathrm{Ho}}^{3+}$ moments order with the same propagation vector as ${\mathrm{Mn}}^{3+},$ but the site is split in two orbits of two ions, each ordered in an $a_x{f}_y$ pattern with a phase factor between them. At the lowest temperatures an antiferromagnetic (commensurate) magnetic phase appears in competition with the main (incommensurate) magnetic phase. The arrangement of the moments is discussed in terms of the magnetic exchange interactions and compared with the closely related magnetic structures of ${\mathrm{LaMnO}}_3,\hspace{0.5em}{\mathrm{TbMnO}}_3,$ and $o-{\mathrm{YMnO}}_3.\hspace{0.5em}{k}_y$ is found to increase with decreasing size of the rare-earth component.

• Received 18 August 2000

DOI:https://doi.org/10.1103/PhysRevB.63.094411