Structural and temperature dependence of the optical and magneto-optical properties of the HeuslerNi2MnGaalloy

Abstract

The influence of martensitic and magnetic phase transformations on the optical and magneto-optical ~MO! properties of polycrystalline Ni 2MnGa alloy samples, in bulk and thin films, has been investigated. The parameters of charge carriers ~plasma and relaxation frequencies! were determined for all the aforementioned states. By the analogy with the band structure and optical properties of the Ni 2MnSn alloy the nature of interband absorption peaks in the optical-conductivity ~OC! spectrum of Ni2MnGa alloy, located at 1.78 and 3.40 eV, is discussed. It was shown that a cooling of substrate with liquid nitrogen during the film deposition leads to the formation of an amorphous phase in the Ni2MnGa alloy, which, unlike the bulk sample, is not ferromagnetically ordered at 293 K. An annealing of the amorphous film at 680 K for 60 min restores its crystallinity and also the ferromagnetic order. The off-diagonal components of the dielectric function ~DF! for the cubic phase of the Ni2MnGa alloy were determined. Like in the interband OC spectrum, the absorptive part of the off-diagonal components of the DF exhibits a two-peak structure, in which the low-energy peak is located at the same energy as in the OC spectrum. Both optical and MO properties of the Ni2MnGa alloy show a noticeable structural dependence; the more intense the peaks, the better the crystallinity of the sample. It was shown that the value of the MO response for Ni2MnGa alloy was strongly influenced by the optical properties, indicating a close connection between them.