Optical Properties of Bulk Nickel as a Function of Temperature

Nickel is a silvery-white metal, element number 28 in the first row of the transition metals. It is ferromagnetic below the Curie temperature (TC=630 K), i.e., it retains a net magnetization in the absence of a magnetic field. When heated above the Curie temperature, it becomes paramagnetic and loses its magnetization. The literature reports a distinct change in the slope of the DC electrical resistivity of nickel versus temperature near the Curie transition. Similarly, a change in the optical reflectivity of nickel has been reported at TC. We therefore used spectroscopic ellipsometry to measure the complex refractive index of nickel with very high precision from 77 to 800 K. In our data for n(T), we find deviations from linearity near 150 K and near 590 K. The refractive index also changes again at even higher temperatures. We believe that our data are affected by two factors: (1) The native oxide on the Ni surface disintegrates above 700 K, leading to a change in the surface condition of our sample. (2) The optical constants also change near the Curie temperature. Work is in progress to separate the two observed phenomena.