Incommensurate magnetic structure and magnetism under pressure in Ho3Co

The bulk magnetic measurements (dc magnetization, specific heat, ac-susceptibility) on Ho₃Co suggest that its magnetic structure changes continuously below the N\'eel temperature TN=21 K until another antiferromagnetic (AFM) transition takes place at Tₜ=9 K. In the temperature range 34pt0exK≤T<TN, the magnetic structure is described by an incommensurate propagation vector k=(α,0,0) with 0.1605<α<0.1585 and a commensurate component k₀=(0,0,0). For Tₜ<T≤TN, the spin arrangement consists of fanlike and cycloidal modulations for the Ho and Co sublattices, respectively. Below Tₜ, the presence of the 3k and 5k harmonics results in a highly complex anharmonic ground state associated with the squaring up of the modulation. The evolution of the magnetic modulations in Ho₃Co within 34pt0exK≤T<TN is explained by two active irreducible representations under the Pnm^'a(α00)000 magnetic superspace group, which allows the change of the structure without further symmetry breaking. A significant moment contribution is detected in Co. Experiments reveal the coexistence of glassy states, probably related to the complex noncollinear AFM structure at low temperature. In addition, externally applied pressure shifts both transitions at TN and Tₜ to higher temperatures. Eventually, high pressure strengthens the AFM interactions at low temperature while the weak glassy states persist up to 1.1 GPa.