Magnetic ordering in TmGa.

We have determined the magnetic structure of the intermetallic compound TmGa by high-resolution neutron powder diffraction and 169Tm M?ssbauer spectroscopy. This compound crystallizes in the orthorhombic (Cmcm) CrB-type structure and its magnetic structure is characterized by magnetic order of the Tm sublattice along the a-axis. The initial magnetic ordering occurs at 15(1)?K and yields an incommensurate antiferromagnetic structure described by the propagation vector k1 = [0?0.275(2)?0]. At 12?K the dominant ferromagnetic ordering of the Tm sublattice along the a-axis develops in what appears to be a first-order transition. At 3?K the magnetic structure of TmGa is predominantly ferromagnetic but a weakened incommensurate component remains. The ferromagnetic Tm moment reaches 6.7(2)??B at 3?K and the amplitude of the remaining incommensurate component is 2.7(4)??B. The 169Tm hyperfine magnetic field at 5?K is 631(1)?T.