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Title: Automated ultrasound studies of magnetoelastic effects in rare earth metals and alloys
Author: Lim, Chee Ming
ISNI:       0000 0001 3610 0589
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1998
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Ultrasound studies of single crystals of Er, Tm and alloys of Er-Tm were carried out as a function of temperature (4.2 - 300 K) and applied magnetic field (0 – 5 T). The elastic constants of these materials were measured and anomalies in the elastic constants were observed. The ultrasound data were compared with reported results from other material characterisation techniques and the magnetic phases and transition temperatures of the materials were then identified. The effects of the application of magnetic field on the magnetic ordering of the materials were studied using the ultrasound method. In Er-Tm there was evidence of applied field (a-axis field and c-axis field) induced ordering in the cycloid phase and c-axis applied magnetic field of > 3 T resulted in the ferrimagnetic to ferromagnetic transition in Tm. A commercial ultrasound measurement system was modified and adapted for use in this work. The modified system enables the ultrasonic velocity and attenuation to be measured as a function of: (a) temperature, (b) applied magnetic field and (c) frequency. The present system was enhanced to work with less efficient ultrasonic transducers such as quartz and electromagnetic acoustic (EMAT) transducers. This work looked at the design and feasibility of using EMATs to generate ultrasound in single crystals of the rare earth metals and alloys. EMATs generating (a) in-plane radially polarised shear waves and (b) longitudinal waves were made and shown to work on these materials. The use of EMATs meant that ultrasound measurements could be conducted in the non-contact regime, i.e. no acoustic couplant is required between the sample and transducer. EMATs are particularly useful in this work where the sample and transducer are subjected to repeated temperature cycles over a wide temperature range (4.2 to 300 K) and acoustic couplant can fracture. The EMAT acoustic coupling efficiency in these samples were studied as a function of temperature and applied magnetic field. Large increases in the EMAT acoustic coupling efficiency (combination of generation and detection efficiencies) often occur close to the magnetic phase transition temperatures of the samples.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)