Unambiguous determination of the commensurate antiferromagnetic structure of HoNi 2 B 2 C in the superconducting ground state

Crystal and magnetic structures of the intermetallic HoNi2B2C have been investigated using different single-crystal diffraction techniques at both room and low temperatures (superconducting phase). Combined refinement of the neutron and x-ray diffraction data performed at room temperature shows that the crystal structure of HoNi2B2C is well described in the tetragonal space group I4/mmm, in agreement with the literature. The magnetic ordering at low temperatures is accompanied by a structural phase transition to the orthorhombic space group Fmmm. However, only noncharacteristic orbits are occupied in this space group, and the symmetry reduction is entirely due to a lifting of the symmetry-induced restrictions on the anisotropic displacement parameters. Based on magnetic symmetry analysis, two types of commensurate antiferromagnetic structures with different magnetic space groups (MSGs) are found below TN≈5.2 K: MSG PInnm (Ho spins are along the [100] direction of the parent structure) and MSG CAmca (Ho spins are along [110] of the parent structure). Both models agree well with conventional unpolarized neutron diffraction data at low temperature (2.4 K) with ordered magnetic moments of 9.16(9) μB/Ho3+ atom. Spherical neutron polarimetry, on the other hand, allowed us to unambiguously solve the magnetic structure (MSG CAmca) as well as to determine the population of two types of 90∘ antiferromagnetic domains (0.45/0.55)