The magnetic structure and phase transitions of holmium-yttrium alloys

Films of holmium-yttrium alloys were grown by molecular beam epitaxy (MBE), and found to have a mosaic spread of about 0.2 degrees , a concentration uniformity of better than 0.5% and a concentration accuracy of better than 2%. Neutron scattering techniques at the Brookhaven HFBR were used to study the magnetic structures and phase transitions of four samples Ho_xY _1-x with x=1.0, 0.9, 0.7 and 0.5 in zero field, and one sample with x=0.7 as a function of applied magnetic field. In zero field the structures are all helical structures with the moments in the basal planes and the wavevectors of the helix along the hexagonal c-axis. At T_N, the wavevectors are 0.282c*, and decrease on cooling, until at low temperatures the structures become commensurate with wavevectors of 1/5c*(x=1), 2/9c*(x=0.9), 1/4c*(x=0.7) and 4/15c*(x=0.5). The critical exponent beta was measured for each of the films and is consistent for all of the materials with beta =0.50+or-0.05. There is some rounding at T_N which may be due to the second and long length scale that has been found in pure holmium. When a magnetic field between 0 and 6 T is applied in the basal plane, the phase diagram for x=0.7 has six phases. On cooling in a low field the sequence is paramagnetic, incommensurate helix, q=1/4 helix, while for fields above 4 T, the sequence is paramagnetic, incommensurate fan, q=1/4 fan. On increasing the field the helical phase goes directly to the fan phase, but on decreasing the field from the q=1/4 fan phase, the system shows a q=1/4 helifan phase between the fan and helical phases.