Page 11 - Spring 2010

Fig. 1. Room temperature Resonant Ultrasound Spectroscopy transducers with mounted sample
Fig. 2. Resonant Ultrasound Spectroscopy probe for measurements at low tempera- tures and in high magnetic fields.
ity of RUS to work with small samples. Whereas convention- al techniques can demand a sample dimension up to a cen- timeter, RUS measurements can be made on samples a frac- tion of a millimeter in size.
Since its development, RUS has proven to be a valuable technique for materials research. Not only can the magnitude of the elastic constants—and sound velocity—be measured at room temperature with high accuracy (the shear modulus is usually determined with a precision of 0.05% or better), RUS can easily be performed as a function of temperature. Such measurements are very useful for studying thermodynami- cally-driven changes in the free energy such as phase transi- tions and local modes. In such cases, RUS has established
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sample in place even when high magnetic fields are applied. For samples with irregular shape or symmetry lower than orthorhombic, the procedure for calculating the elastic moduli from the resonances becomes more involved, but RUS measurements can give important information even
without an absolute value for the elastic constants. Any devi- ation from “normal” thermodynamic behavior will be reflected in the temperature-dependence of the resonant fre- quencies, and these can be measured and interpreted without long computation.
Extensions of RUS that were motivated by interesting samples
Typically, RUS samples are polished into rectangular
parallelepipeds, cylinders or spheres, a few millimeters or
larger in size, and these shapes and sizes are readily handled
with the conventional RUS technique. However, very new
materials may pose some problems that require modification
of the conventional method. For example, single-crystal sam-
ples of very new materials often are available only in small
sizes—crystalline samples of new exotic materials may have
dimensions of only a few hundred microns, and the sample
Figure 2 shows a low-temper- ature probe and transducers set for RUS in high magnetic fields. This allows RUS measurements to be taken as a func- tion of temperature between 300 mK and 350 K and in mag- netic fields up to 15 Tesla. The magnetic forces exerted by the field on ferromagnetic samples tend to launch such speci- mens into the surrounding exchange-gas space, because the samples are typically only loosely held between the transduc- ers, as illustrated in Fig. 1. The newly designed transducers use a “flat-mount” approach, making it possible to keep the
itself as a very powerful tool.
masses may be less than 100 μg (one sample measured with 8
RUS was only 70 μg). Such small samples virtually appear as Listening to materials 7