The structural changes exhibited by materials in response to their surroundings is of fundamental importance across countless disciplines in science and engineering. In the particular field of transducer technologies, strain-inducing phenomena – such as magnetostriction, electrostriction, and piezoelectricity – are utilised in their own right, and form the physical foundation for sensor and actuator devices. Knowledge of the origin and mechanics of such phenomena, and how they manifest themselves in different materials, underpins the development of new transducers and the optimisation of existing technologies. As such, considerable research is undertaken each year to develop an understanding of the physical processes at work. Yet this is not without its limitations.

Theoretical work may study the fundamental properties of a given material, and develop models for its behaviour at an atomic scale. However, experimental observation of strain on a similarly microscopic scale, and thus verification of such models, has presented some serious technical challenges. This has essentially been due to the difficulty in detecting and quantifying atomic motion with sufficient precision. In the case of magnetostriction, inter-atomic displacements in response to an external magnetic field typically saturate at a few femtometres. Here, even commonly employed local probes, such as X-ray Absorption Spectroscopy (XAS), lack the resolution to observe this motion by some two orders of magnitude. As a result, work to date has almost exclusively dealt with magnetostriction in a scaled-up form. Common experiments employ strain gauges on large, macroscopic samples, where the strain is easier to detect, but where atomic information is lost.

However, by exploiting the intrinsic stability and time resolution of the dispersive XAS set-up, the recent development of Differential XAS (DiffXAS) on ID24 at the ESRF has made such direct, atomic-scale measurements possible [1].

This talk will introduce the Differential XAS technique, and chart its development through some of the most significant results obtained to date. These include the first DiffXAS measurements, which demonstrated the technique’s sensitivity to femtometre scale motion [1]; studies of the technologically important Fe(1-x)Gax system, where the development of DiffXAS data analysis procedures allowed chemically-selective, atomic magnetostriction coefficients to be quantified [2][3]; studies of the magneto-elastic coupling of FeCo through measurements under applied hydrostatic pressure [4]; and studies of structural changes as a function of temperature with Thermal DiffXAS [5].

References
[1] - R.F. Pettifer, O. Mathon, S. Pascarelli et al., Nature 435, 79 (2005)
[2] - M. P. Ruffoni, S. Pascarelli, R. Grössinger et al., Phys. Rev. Lett. 101, 147202 (2008)
[3] - S. Pascarelli, M. P. Ruffoni, R. Sato-Turtelli et al., Phys. Rev. B 77, 184406 (2008)
[4] - S. Pascarelli, M. P. Ruffoni, A. Trapananti et al., Phys. Rev. Lett. 99, 237204 (2007)
[5] - M. P. Ruffoni, R. F. Pettifer, S. Pascarelli et al., J. Synchrotron Radiat. 14 421 (2007)