Magnetic ordering of Ho6Co2Ga-type {Gd, Tb, Dy}6Co2.2 Al0.8 and Tb6Co2Al compounds by magnetization and neutron diffraction study

The magnetic ordering of polycrystalline Ho6Co2Ga-type Gd6Co2.2Al0.8, Tb6Co2.2Al0.8, Tb6Co2Al and Dy6Co2.2Al0.8 (space group Immm, N 71, oI36) has been established using magnetic measurements and the magnetic structure of Tb6Co2.2Al0.8 has been resolved by the neutron diffraction study. Gd6Co2.2Al0.8 exhibits field sensitive ferro-antiferromagnetic ordering, meanwhile Tb6Co2.2Al0.8, Tb6Co2Al and Dy6Co2.2Al0.8 are antiferromagnets with metamagnetic transformation of magnetic ordering. Gd6Co2.2Al0.8 is soft ferromagnet, whereas Tb6Co2.2Al0.8, Tb6Co2Al and Dy6Co2.2Al0.8 exhibit low-temperature weak permanent magnet properties. From these compounds, Gd6Co2.2Al0.8 demonstrates largest magnetocaloric effect with magnetic entropy change of -10.89 J/kg.K at 75 K for a field change of 50 kOe (relative cooling power of similar to 760 J/kg). Neutron diffraction study indicates non-collinear abc-antiferromagnetic ordering of Tb6Co2.2Al0.8 in zero applied magnetic field. Below 63 K and down to 52 K, Tb6Co2.2Al0.8 exhibits antiferromagnetic ordering with magnetic space group I'22'2' (wave vector K-0 = [0, 0, 0]). Below 52 K and down to 1.5 K, the magnetic structure of Tb6Co2.2Al0.8 is sum of magnetic component with P21'212' magnetic space group (wave vector K-0 = [0, 0, 0]) and square-modulated antiferromagnetic component along c-axis of unit cell with K-1 = [+/- 1/4, 0, +/- 1/4] wave vector.