Change of easy magnetization direction in TmMn6Sn6−xGax (0.00 < x < 1.22) studied in high magnetic fields

Single crystals of TmMn6Sn6−xGax compounds (0 ≤ x ≤ 1.22) obtained in a tin flux have been studied by thermomagnetic and high-field magnetization measurements. A collinear-antiferro to heli-magnetic transition at 324 K has been observed in TmMn6Sn6. Spin-reorientation transitions at low temperature are found in the compounds with x = 0.66 and 1.22. The magnetization curves in high fields at 4.2 K are interpreted to result from the competition between the Tm–Mn exchange interaction and the magnetocrystalline anisotropy of the Tm and Mn sublattices. A strong easy-plane anisotropy is favored for the Ga-poor compounds and a strong easy-axis anisotropy for the Ga-rich compounds while the compounds of intermediate Ga content (0.51 ≤ x ≤ 0.66) display rather weak magnetocrystalline anisotropy.