Magnetic and crystal structure ofHo5(SixGe1−x)4studied by neutron diffraction

Abstract

The magnetic and crystal structures of the magnetocaloric ${\text{Ho}}_{5}{({\text{Si}}_{x}{\text{Ge}}_{1\ensuremath{-}x})}_{4}$ ($x=1$, 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\char21{}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 ${T}_{\text{C}}=77\text{ }\text{K}$ 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 $Pn{m}^{\ensuremath{'}}{a}^{\ensuremath{'}}$). ${\text{Ho}}_{5}{\text{Si}}_{3}\text{Ge}$ becomes FM at ${T}_{\text{C}}=50\text{ }\text{K}$ (magnetic space group $P{112}_{1}^{\ensuremath{'}}/a$). At ${T}_{\text{C}}^{\ensuremath{'}}\ensuremath{\sim}16\text{ }\text{K}$ an incipient second incommensurate magnetic phase appears which becomes commensurate below 8 K with a propagation vector $\mathbf{k}=(0\text{ }0\text{ }\frac{1}{4})$. 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 N\'eel transition of the O(II) phase at ${T}_{\text{N}}\ensuremath{\sim}30\text{ }\text{K}$ (magnetic space group $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}{a}^{\ensuremath{'}}$), two additional magnetic phases with propagation vectors $\mathbf{k}=(0\text{ }0\text{ }\frac{1}{4})$ and $\mathbf{k}=(\frac{1}{2}\text{ }0\text{ }0)$ appear at 15 and 12 K, respectively. In ${\text{Ho}}_{5}{\text{Ge}}_{4}$ the main O(II) structure orders AFM with $\mathbf{k}=(0\text{ }0\text{ }0)$ at ${T}_{\text{N}}\ensuremath{\sim}30\text{ }\text{K}$ in the same magnetic space group $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}{a}^{\ensuremath{'}}$. 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 $\mathbf{k}=(0\text{ }0\text{ }0)$, 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 $\mathbf{k}=(0\text{ }0\text{ }\frac{1}{2})$ 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 $\mathbf{k}=(0\text{ }0\text{ }0)$, which disappears quickly with the onset of FM, a progressive decrease of the intensity associated with the state $\mathbf{k}=(0\text{ }0\text{ }\frac{1}{2})$ suggests a possible relationship between the extent of the magnetic coupling $\mathbf{k}=(0\text{ }0\text{ }\frac{1}{2})$ and the percentage of remnant $P{2}_{1}/m$ phase.