Evolution of the magnetic order in the Ho(Mn, Al)2 system; neutron diffraction study

The neutron diffraction study of Ho(Mn1-xAlx)2 shows the coexistence of two cubic Laves phases with different unit-cell parameters and substantially different magnetic behaviours. The first phase combines ordered ferrimagnetic and disordered antiferromagnetic components of the magnetic moments. With increasing Al content, starting from the long-range ferrimagnetic order with the induced Mn moment in HoMn2, the progressive formation of spontaneous Mn moments yields short-range order, which in turn transforms to ferromagnetic order in HoAl2. The second phase with incommensurate magnetic structure is driven by the spontaneous Mn moments and exists only over limited ranges of Al content and unit-cell parameter. It has a finite correlation length and appears from a second-order transition without a magneto-volume effect