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Investigating Curie Point Transformations with Linkam

The Linkam HFS91 probe stage Market leaders in temperature controlled microscopy, Linkam Scientific Instruments, report on the work of Dr Sharath Sriram, a Research Fellow at the Royal Melbourne Institute of Technology University. 

The Functional Materials and Microsystems Research Group at RMIT comprises of researchers adept at thin film synthesis, microfabrication, and materials characterisation. Post-Doctoral Research Fellow Dr. Sharath Sriram is a key researcher of the Group with expertise in the characterisation of complex thin films for electronic applications.

Dr Sriram's research in characterising piezoelectric thin films led him to choose a temperature controlled stage from Linkam for use in combination with a Renishaw inVia Raman microscope. Dr Sriram commented that "the ability of the Linkam stage with its accurate control of temperature and ramp rates, conveniently incorporated with a micro-Raman spectrometer, enabled these experiments".

The Group's main area of work has been the investigation of the reversal and pinning of Curie point transformation in thin film piezoelectrics. The crystalline thin film used for this study is strontium-doped lead zirconate titanate (PSZT). This is a high performance piezoelectric, pyroelectric and ferroelectric material that has multiple industrial uses as an electrical, pressure and temperature sensor. Using a Renishaw inVia micro-Raman system equipped with a CCD camera, and a long focus 50× objective, the PSZT thin films are heated to 350°C and cooled at 10°C/min in the Linkam HFS91-PB4 (HFSE600-PB4) stage with corresponding real-time collection of Raman spectra. With an argon 514 nm laser used as the excitation source and a line grating of 1800 lines/mm, a spectral resolution of approximately 1 cm-1 was achieved. Data was averaged over 10 accumulations of 20 seconds.

This enabled the researchers to determine two main Raman peaks for the film at room temperature, ~575 cm-¹ and ~744 cm-¹ (at which point the film had a rhombohedral structure). Controlled heating and cooling of the thin film caused peaks and intensity changes at the Curie point. This was indicative of a phase change occurring at the Curie point, where the film changed from a rhombohedral to a cubic arrangement. This phase change coincided with loss of piezoelectric charge and structure. With controlled cooling the cubic phase reverses back to the rhombohedral phase and piezoelectrical potential with minimum hysteresis. 

This transformation can be disrupted by uncontrolled cooling, which results in the Raman peak positions and intensities being fixed in place. This is indicative of a permanent phase change with the material remaining "locked" in the cubic phase. Therefore fast cooling can permanently remove the piezoelectric charge within a material (in this case PSZT).

Future research will make use of the probe capability of the Linkam stage to apply an electrical bias and study Raman spectrum changes in various piezoelectric thin film samples.

Visit Linkam at http://www.linkam.co.uk/ and learn about the broad range of applications in the field of temperature controlled microscopy.


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