Since the discovery of a giant magnetocaloric effect in Gd5Si 2Ge2 that is associated with a first-order magnetostructural transition, the R5SixGe4- x materials (where R=Y,La-Lu) have come under intensive study. Systematic structural data has now been obtained for most of the R5Si xGe4-x compounds (Y, Pr, Nd, Gd, Tb, Er, and recently Yb), and from this structural data, trends have been begun to emerge. The Yb5SixGe 4-x system however defies all conventional wisdom and adopts the orthorhomic Gd5Si4 structure, independently of x, a unique feature in the R5SixGe 4-x family. The magnetic transition temperatures of the Yb5SixGe4- x compounds have been found to be monotonically increasing from 2.4 K to 3.2 K with increasing Ge content contrary to trend measured in other R5SixGe4- x compounds. Moreover, magnetic susceptibility suggests the presence of both Yb2+ and Yb3+ in all compounds at a 60:40 ratio. / In this study, we have performed cold-source 170Yb Mossbauer spectroscopy on samples that have been previously fully characterized, and analyzed our data with an exact solution to the full Hamiltonian. / We conclusively confirm the presence of both Yb2+ and Yb3+ ions in all samples, however with a ratio closer to 50:50. Previous attempts to assign specific crystallographic sites for them fail. We also find the magnetic transition temperature to be far lower than was indicated by the susceptibility data. More strikingly we find that the magnetic behavior of the x = 0 and 4 members is similar, with a single transition temperature at similar temperatures, whereas the middle members, x = 2, 3, have not only considerably different magnetic behaviour (we find two separate magnetic components with different ordering temperatures), but also lower transition temperatures. This system is considerably more complex than was originally thought.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.98511 |
| Date | January 2005 |
| Creators | Voyer, Christian. |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Master of Science (Department of Physics.) |
| Rights | © Christian Voyer, 2005 |
| Relation | alephsysno: 002491034, proquestno: AAIMR24818, Theses scanned by UMI/ProQuest. |
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