Neutron diffraction study of the magnetic ordering in the Ho(Mn1-xAlx)(2) system

The magnetic and crystal structure of the as-cast cubic Ho(Mn1-xAlx)(2) com pounds (x less than or equal to 0.1) was studied using neutron powder diffr action. The samples showed substantial inhomogeneity and consisted of at le ast two C15-type crystal phases. In the first phase predominating in HoMn2 (96.3%) ferrimagnetic long range order is stable. The second phase with the cell parameter lying within the narrow range 7.49-7.52 Angstrom shows an i ncommensurate magnetic structure with a wave vector k approximate to [0.18, 0.18, 0.08] weakly dependent on x. Short range correlations of an antiferr omagnetic type along with long range ordering were observed. The diffuse ma xima lie at the positions corresponding to the Bragg reflections with half- integer indices and are ascribed to short range correlations based on the M nO-type antiferromagnetic structure developed within clusters of similar to 25 Angstrom in size. No volume effect associated with the onset of Mn magn etism was observed up to x = 0.1.