Spin-flop transition in Co$_2$B$_2$O$_5$ pyroborate

Cobalt pyroborate Co$_2$B$_2$O$_5$ single crystals have been obtained by spontaneous crystallization from solution-melt. Powder X-ray diffraction measurements have revealed the triclinic symmetry $P\overline 1 $ with the lattice parameters $a=3.1666(7)$ Å, $b=6.1543(6)$ Å, $c=9.2785(12)$ Å, $\alpha=104.240(5)^\circ$, $\beta=90.841(14)^\circ$, $\gamma=92.064(16)^\circ$, and $V=175.10(5)$ Å$^3$. Magnetic properties have been studied in the temperature range of $4.2$–$300$ K and in magnetic fields up to $90$ kOe by measuring the static magnetization and molar heat capacity. A transition to an antiferromagnetic state has been detected at $T_N=45$ K. A spin-flop transition occurs in the sample in strong magnetic fields.