W<inf>3</inf>CoB<inf>3</inf>-type {Y, Gd - Ho}<inf>3</inf>Co<inf>4−x</inf>Al<inf>x</inf> (x=0.5−1) rare earth compounds: Specific features of crystal structure and magnetic ordering

The crystal structure of new W3CoB3-type {Y, Gd - Ho}3Co3.25Al0.75, Gd3Co3.5Al0.5 and Tb3Co3Al compounds (Cmcm. N 63, oC28) has been established using powder X-ray diffraction studies. The magnetic properties of Gd3Co3.5Al0.5, Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 were determined by bulk magnetization measurements and neutron diffraction studies. Gd3Co0.5Al0.5, Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 exhibit ferrimagnetic ordering below 196 K, 161 K and 151 K, respectively. Tb3Co3.25Al0.75 shows a spin-reorientation transition at ~42 K. Below the ferrimagnetic ordering temperature Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 are soft ferrimagnets, meanwhile Tb3Co3.25Al0.75 shows magnetic hardness below the spin-reorientation transition with remanent magnetization per formula unit of 9.7 μB and coercive field of 15 kOe at 2 K. The magnetocaloric effects of Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 were calculated in terms of isothermal magnetic entropy change and they reach maximum values of −4.9 J/kg·K at 135–145 K and −3.7 J/kg·K at 115–135 K, respectively, for a field change of 0–50 kOe. Low temperature magnetic ordering in Tb3Co3.25Al0.75 is accompanied by a positive magnetocaloric effect with isothermal magnetic entropy changes of +13.6 J/kg·K at 10 K for a field change of 0–50 kOe and +0.9 J/kg K at 45 K for a field change of 0–10 kOe. Neutron diffraction study in zero applied field shows mixed ferro-antiferromagnetic ordering of Tb3Co3.25Al0.75 with a wave vector K0=[0, 0, 0]. Below ~ 137 K Tb3Co3.25Al0.75 exhibits non-collinear ferrimagnetic ordering of terbium and cobalt sublattices with resulting of b-axis ferromagnetic and c-axis antiferromagnetic components of Cmc′m={1, mx}×{1, mz/[0, 0, 1/2]}×{1, i′}×{1, 1/[1/2, 1/2, 0]} magnetic space group. The spin-reorientation transition in Tb3Co3.25Al0.75 below ~38 K corresponds to appearance of additional a-axis ferromagnetic component and decreasing of symmetry of magnetic ordering down to C2′/c={1, mz/[0, 0, 1/2]}×{1, i′}×{1, 1/[1/2, 1/2, 0]} magnetic space group.