${\mathrm{GdCo}}_2{\mathrm{B}}_2$ is a member of the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ family of compounds which exhibits good magnetocaloric properties associated with antiferromagnetic orderings in the temperature range for hydrogen liquefaction applications. As antiferromagnetic compounds typically do not exhibit a large magnetocaloric effect, understanding the magnetic orderings in ${\mathrm{GdCo}}_2{\mathrm{B}}_2$ could help uncover more magnetocaloric compounds that rely on antiferromagnetic ordering. ${\mathrm{GdCo}}_2{\mathrm{B}}_2$ exhibits four antiferromagnetic phase transitions at 22 K, 18.5 K, 13 K, and 7 K. Due to the difficulty of conducting neutron diffraction measurements on compounds containing affordable natural Gd, the microscopic magnetic structures have so far not been investigated. In this study, by using high energy neutrons and devising special sample preparations for lower energy neutrons, powder neutron diffraction patterns of ${\mathrm{GdCo}}_2{\mathrm{B}}_2$ have successfully been measured. A long-period incommensurate magnetic ordering with ${\mathit{k}}_1=\left(0.053100\right)$ was seen at 20 K. The propagation vector of the incommensurate structure shifted to ${\mathit{k}}_1=\left(0.07100\right)$ at 16 K, and an additional $\mathit{k}=\left(000\right)$ ferromagnetic component was observed. At 10 K, two incommensurate propagation vectors of ${\mathit{k}}_1=\left(0.073100\right)$ and ${\mathit{k}}_2=\left(0.07310.07310\right)$, which shifted to ${\mathit{k}}_1=\left(0.074100\right)$ and ${\mathit{k}}_2=\left(0.07410.07410\right)$ at 6 K, were observed to coexist. By considering only the ${\mathit{k}}_1$-modulation, two possible magnetic structures below 10 K are reported to describe the data, either $\mathit{ac}$-cycloid structure with constant moment at each Gd site or ellipsoidal $\mathit{ab}$-cycloid structure. Exchange interaction energy calculations indicated that the long-period antiferromagnetic ordering in ${\mathrm{GdCo}}_2{\mathrm{B}}_2$ is energetically close to the ferromagnetic state due to the strong competition between ferromagnetic and antiferromagnetic exchange interactions, which plays an essential role for the emergence of the large magnetocaloric effect in ${\mathrm{GdCo}}_2{\mathrm{B}}_2$.

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• Magnetic phase transitions
• Magnetocaloric effect
• Antiferromagnets
• Crystal structures
• Neutron diffraction