Magnetic and structural properties of DyMn2Hx (0 <= x <= 4.2)

The magnetic properties of Dy((Fe0.01Mn0.99)-Fe-57)(2)H-x compounds were in vestigated by means of Fe-57 Mossbauer spectroscopy and temperature depende nt magnetic susceptibility (DC, AC) and magnetisation measurements. Their s tructural properties in the temperature range 70 K less than or equal to T less than or equal to 400 K were studied by means of powder X-ray diffracti on. It was found that, at room temperature, the lattice parameters increase continuously upon hydrogen absorption in the 0.8 less than or equal to x l ess than or equal to 3.5 range and the cubic C15 crystal structure is prese rved. At 70 K the system differentiates into several distinct hydride phase s and two-phase ranges. The lattice expansion together with the chemical ef fect of hydrogen results in dramatic changes in the properties of the hydri des as compared to the host compound. By means of Fe-57 Mossbauer spectrosc opy, values of the average isomer shifts at 4.2 and 300 K and the average h yperfine fields at 4.2 K were determined. From isomer shift changes at room temperature, the relative importance of the volume expansion and band effe cts due to hydrogen absorption were deduced. A nonlinear increase of the ma gnetic ordering temperatures T-M versus x and an abrupt increase of the ave rage hyperfine fields for x greater than or equal to 2.5 were also found. B ased on T-M values for Y((Fe0.005Mn0.995)-Fe-57)(2)H-x as reference the mag nitude of the 4f-3d indirect interaction was inferred. In addition, a stron g decrease of this interaction with increasing x (resulting in its cancella tion for x>2.8) and a weak dependence on the Dy-Mn distance are clearly sho wn. (C) 1999 Published by Elsevier Science S.A. All rights reserved.