<p> LiHoxY1-xF4 is an insulating system where the Ho ions are magnetic. The crystal field gives an Ising character to the Ho ions, and the dominant interaction between them is through magnetic dipolar fields. For x=1, the system is a ferromagnet with a critical temperature of 1.54 K. The critical temperature decreases as x is decreased until x~0.25. At that point the long range order is destroyed, giving rise to a state with some characteristics of spin glass behavior.</p> <p> The study of this system is important because its Ising nature makes it a test ground for theoretical models. Interestingly, not only experiments and theory diverge in many points, but experimental data from different research groups are contradictory. In order to contribute to a clearer experimental picture, we performed μSR measurements in this system and we present our results in this thesis.</p> <p> In the ferromagnetic samples (x=1 and 0.45) we found that the internal field distribution at the muon site is quite broad, even in the x=1 system. Then it is possible that the magnetic field distribution at the Ho ions is broad as well. We show qualitatively how this broadening could explain a feature of the field-temperature phase diagram which is still not understood.</p> <p> For the more diluted samples (x=0.25, 0.12, 0.08, 0.045 and 0.018), we
found that they all have the same qualitative behavior as a function of temperature and magnetic field. Our analysis of the data did not show any feature of canonical spin glass behavior. Instead, an analysis using dynamical Kubo-Toyabe functions show a temperature independent fluctuation rate of the Ho moments below ~0.5 K for all these samples. This behavior is characteristic of cooperative paramagnetic systems. We also found that the low temperature fluctuation rate decreases as x is decreased.</p> <p> To analyze the data from LiHoxY1-xF4 we developed microscopic models of μSR signals. These models are presented in this thesis and they correspond to stochastic models of signals using stretched magnetic field distributions; and the modeling of signals from systems with F-μ-F bond formation where the set of bonds does not have cubic symmetry.</p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/17352 |
| Date | 08 1900 |
| Creators | Rodriguez, Jose |
| Contributors | Luke, Graeme M., Physics and Astronomy |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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