We have studied the magnetization, dielectric permittivity, thermal expansion, and magnetostriction of ${\mathrm{Ni}}_2{\mathrm{NbBO}}_6$ single crystals. Different from that of the $ab$ plane, the $c$ axis magnetization approaches zero at low magnetic fields and a spin-flop transition happens around $H_{\mathrm{SF}}=3.65\mathrm{T}$. The results indicate that the $c$ axis is the hard axis and the $ab$ plane is the easy plane. The high-field isothermal magnetization measured at $T=4.2\mathrm{K}$ reveals that the saturation magnetic field is $H_s=17\mathrm{T}$, above which the magnetic structure changes from canted-antiferromagnetic order to ferromagnetic order with the saturation magnetic moment ${\mu }_{\mathrm{eff}}=2.40 {\mu }_{\mathrm{B}}/{\mathrm{Ni}}^{2+}$ Below antiferromagnetic transition temperature $T_N$, an unusual negative thermal expansion phenomenon is detected along the $b$ axis and it can be interpreted by the magnetic structural variation. Based on the magnetization and the magnetostriction data, the magnetic field–temperature phase diagram of ${\mathrm{Ni}}_2{\mathrm{NbBO}}_6$ is constructed. The magnetodielectric effect confirms the existence of magnetoelectric coupling in ${\mathrm{Ni}}_2{\mathrm{NbBO}}_6$.

• Received 13 September 2021
• Revised 20 December 2021
• Accepted 18 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.054408