We investigated the magnetic ordering, crystal lattice distortion, and spin dynamics in the magnetocaloric material $\mathrm{Ho}{\mathrm{B}}_2$ using neutron and x-ray diffraction (XRD), along with AC susceptibility measurements. Although $\mathrm{Ho}{\mathrm{B}}_2$ exhibits a ferromagnetic phase transition at $T_1=15\mathrm{K}$, the phase transition at $T_2=11\mathrm{K}$ has not yet been elucidated. In high-resolution powder neutron diffraction and single-crystal XRD experiments, we observed a structural phase transition from hexagonal ($P6$/mmm) to monoclinic ($C2/m$) at $T_2$. The structural change occurred concomitantly with a change in the moment size and the fixing of the spin orientation angle of the $\mathrm{H}{\mathrm{o}}^{3+}$ moments below $T_2$. Analyzing the structural distortion mode coupled to the spin direction, we found that the ferromagnetically ordered moments lie in the hexagonal ($x$, −$x, z$) plane. In the AC susceptibility measurements, we observed a large enhancement in susceptibility along the $ab$ plane for $T_2<T<T_1$, indicating that the ferromagnetic state in the intermediate-temperature region is characterized by large spin fluctuations. Considering the observed symmetry lowering in the crystal structure combined with the previously reported crystal electric field levels for the $4f$ orbitals of $\mathrm{H}{\mathrm{o}}^{3+}$, we propose that the lower-temperature phase transition can be understood by the rearrangement of the $4f$ orbital state with monoclinic symmetry.

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• Ferromagnetism
• Magnetocaloric effect
• Structural phase transition
• Ferromagnets
• Magnetic refrigeration
• AC susceptibility measurements
• Neutron diffraction
• X-ray diffraction