Optical Spectroscopy - Characterization of Phase Transitions in Martensites by Ellipsometry

Ellipsometric study of a martensitic single crystal NiMnGa was carried out in a wide temperature range and allowed to detect all the critical temperatures of the sample.

               

Temperature dependence of the refractive index at wavelength 620 nm. The points (a), (b), and (c) show the temperatures where cooling/heating was stopped to analyze relaxation. The arrows indicate the isothermal relaxation of the refractive index at temperatures 120 K, 220 K, and 280 K during times t_a = 3 h, t_b = 7 h, and t_c = 15 h, respectively.

The performed optical measurements revealed the earlier onset of premartensitic transition at the sample surface. The giant isothermal creep (relaxation up to 20%) of the refractive index was found below room temperature. It was shown that the creep amplitude is temperature dependent. The underlying mechanisms of the observed effects are revealed [1].

References
_{1. A. Dejneka, V. Zablotskii, M. Tyunina, L. Jastrabik, J. I. Pérez-Landazábal, V. Recarte, V. Sánchez-Alarcos, V. A. Chernenko, Ellipsometry applied to phase transitions and relaxation phenomena in Ni2MnGa ferromagnetic shape memory alloy. Appl. Phys. Lett. (2012) 101: 141908.}