A uranium-based metallic compound ${\mathrm{UNi}}_4\mathrm{B}$ with an orthorhombic structure exhibits antiferromagnetic (AFM) ordering at $T_{\mathrm{N}}=20.4$ K. Although an electric current-induced magnetization below $T_{\mathrm{N}}$ has been observed in this compound [H. Saito et al., J. Phys. Soc. Jpn. 87, 033702 (2018)], its underlying mechanism remains controversial. In this study, we measured thermal expansion and magnetostriction along the $a$ axis under a magnetic field $H$ up to 14 T applied within the $bc$ plane to investigate the rotational symmetry of a magnetic field–temperature $\left(H\text{−}T\right)$ phase diagram in ${\mathrm{UNi}}_4\mathrm{B}$. From the phase diagrams constructed under various $H$ angles between $b$ and $c$, it is suggested that the $H\text{−}T$ phase diagram practically exhibits sixfold rotational symmetry within the $bc$ plane despite an orthorhombic crystal structure, which would be attributed to a quasihexagonal structure formed by U ions with small orthorhombic distortion.

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• Antiferromagnetism
• Frustrated magnetism
• Magnetic order
• Magnetostriction
• Phase diagrams
• Thermal expansion
• Actinides
• Antiferromagnets