Long-range order of magnetic nanocluster lattices and surface acoustic wave applications of lithium niobate thin films
Robert, Marc A.; Rabson, Thomas A.
Doctor of Philosophy thesis
Part I. We have investigated theoretically the interactions and phase transitions of arrays of magnetic nanoclusters embedded in nonmagnetic metals and semiconductors. Two nanoclusters of ferromagnetic elements embedded in a nonmagnetic semiconducting material are expected to be coupled magnetically if the nanoclusters are large enough and the semiconductor is sufficiently doped to provide enough electrons, since the latter mediate the indirect magnetic interactions between the nanoclusters. Apart from its potential technological applications, such as giant magnetoresistance for non-volatile memory applications, and DMS (diluted magnetic semiconductors) for combined process and storage devices, this study is of fundamental interest. As a consequence of the temperature dependence of the indirect interactions, it is shown that for semiconductors several ferromagnetic phase transitions are possible depending on the size of the nanoclusters, whereas for metals a single ferromagnetic phase transition occurs independently of the size of the nanoclusters. The critical temperature and spontaneous magnetization of the network of nanoclusters are evaluated in all cases. Part II. We have found evidence for surface acoustic wave (SAW) propagation of a lithium niobate (LiNbO3)/diamond/silicon multilayer structure. Devices with higher-frequency response and low insertion loss as well as smaller sizes and weight are required for telecommunications. Theoretical calculations had predicted that LiNbO3 thin films on diamond/silicon substrates have both high SAW propagation velocities and high electromechanical coupling coefficients. Metallo-organic decomposition (MOD) and rf-sputtering techniques are used here to grow LiNbO3 thin films on diamond/silicon substrates. These thin films are characterized by XRD, AFM, and interferometer. SAW filters are fabricated by depositing interdigital transducers (IDTs) onto the LiNbO3/diamond/silicon films. Microwave characterizations, such as frequency response, are done by using network analyzer test sets. The impulse response in the time domain is calculated by fast Fourier transforms. Evidence for SAW activity is found for this multilayer structure. Integrating a composite structure of a piezoelectric layer and a high acoustic-velocity layer onto silicon substrates thus shows promise as a way to increase the operating frequency for wireless-telecommunication applications.
Chemical engineering; Condensed matter physics