The magnetic and crystal structures of the magnetocaloric ${\text{Ho}}_5{\left({\text{Si}}_x{\text{Ge}}_{1-x}\right)}_4$ ($x=1$, 0.75, 0.5 and 0) have been studied by neutron powder diffraction experiments. The room-temperature crystal phases of the compositions $x=1$, 0.75, and 0.5 are preserved in the whole temperature range 2–300 K, i.e. the ${\text{Gd}}_5{\text{Si}}_4$-type $Pnma$ O(I) structure for ${\text{Ho}}_5{\text{Si}}_4$, the ${\text{Gd}}_5{\text{Si}}_2{\text{Ge}}_2$-type $P{112}_1/a$ M state in $x=0.75$, and the ${\text{Sm}}_5{\text{Ge}}_4$-type $Pnma$ O(II) phase in $x=0.5$. ${\text{Ho}}_5{\text{Si}}_4$ undergoes a second order ferromagnetic (FM) transition at $T_{\text{C}}=77\text{K}$ into a noncollinear FM structure with the magnetization oriented mainly along the $a$ axis and weak antiferromagnetic (AFM) coupling along $b$ and $c$ (magnetic space group $Pn{m}^{\prime }{a}^{\prime }$). ${\text{Ho}}_5{\text{Si}}_3\text{Ge}$ becomes FM at $T_{\text{C}}=50\text{K}$ (magnetic space group $P{112}_1^{\prime }/a$). At $T_{\text{C}}^{\prime }\sim 16\text{K}$ an incipient second incommensurate magnetic phase appears which becomes commensurate below 8 K with a propagation vector $\mathbf{k}=\left(00\frac{1}{4}\right)$. The magnetic structure of the main phase at 30 and 2 K shows the dominance of a FM coupling along the $a$ axis with an AFM canting along $c$. ${\text{Ho}}_5{\text{Si}}_2{\text{Ge}}_2$ stands out for the complexity of its low-temperature magnetic structure. In addition to the Néel transition of the O(II) phase at $T_{\text{N}}\sim 30\text{K}$ (magnetic space group $P{n}^{\prime }{m}^{\prime }{a}^{\prime }$), two additional magnetic phases with propagation vectors $\mathbf{k}=\left(00\frac{1}{4}\right)$ and $\mathbf{k}=\left(\frac{1}{2}00\right)$ appear at 15 and 12 K, respectively. In ${\text{Ho}}_5{\text{Ge}}_4$ the main O(II) structure orders AFM with $\mathbf{k}=\left(000\right)$ at $T_{\text{N}}\sim 30\text{K}$ in the same magnetic space group $P{n}^{\prime }{m}^{\prime }{a}^{\prime }$. Below 25 K a complete structural transition from high temperature O(II) $Pnma$ to $P{2}_1/m$ takes place within the AFM state. The magnetic structure of this new nuclear phase stays AFM with $\mathbf{k}=\left(000\right)$, but sees two out of six independent Ho sites non magnetic. A second magnetic transition takes place at about 18 K characterized by the appearance of a second propagation vector $\mathbf{k}=\left(00\frac{1}{2}\right)$ which magnetically couples the formerly non magnetic Ho sites. Magnetic-field dependent neutron diffractograms demonstrate that FM sets in in ${\text{Ho}}_5{\text{Ge}}_4$. The onset of ferromagnetism is associated with the previously reported nucleation of a new high field O(II) $Pnma$ phase. Contrary to the intensity of the magnetic coupling with the propagation vector $\mathbf{k}=\left(000\right)$, which disappears quickly with the onset of FM, a progressive decrease of the intensity associated with the state $\mathbf{k}=\left(00\frac{1}{2}\right)$ suggests a possible relationship between the extent of the magnetic coupling $\mathbf{k}=\left(00\frac{1}{2}\right)$ and the percentage of remnant $P{2}_1/m$ phase.

• Received 31 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.104427