The influence of martensitic and magnetic phase transformations on the optical and magneto-optical ͑MO͒ properties of polycrystalline Ni 2 MnGa alloy samples, in bulk and thin films, has been investigated. The parameters of charge carriers ͑plasma and relaxation frequencies͒ were determined for all the aforementioned states. By the analogy with the band structure and optical properties of the Ni 2 MnSn alloy the nature of interband absorption peaks in the optical-conductivity ͑OC͒ spectrum of Ni 2 MnGa 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 Ni 2 MnGa 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 ͑DF͒ for the cubic phase of the Ni 2 MnGa 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 Ni 2 MnGa 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 Ni 2 MnGa alloy was strongly influenced by the optical properties, indicating a close connection between them.
Optical properties of ferromagnetic half-metallic full-Heusler Co 2 FeGe alloy are investigated experimentally and theoretically. Co 2 FeGe thin films were obtained by DC magnetron sputtering and show the saturation magnetization at T =10 K of m ≈5.6 µ B /f.u., close to the value predicted by the Slater-Pauling rule. First-principles calculations of the electronic structure and the dielectric tensor are performed using the fullpotential linearized-augmented-plane-wave method in the generalized gradient (GGA) and GGA+U approximations. The measured interband optical conductivity spectrum for the alloy exhibits a strong absorption band in the 1 -4 eV energy range with pronounced fine structure, which agrees well with the calculated half-metallic spectrum of the system, suggesting a near perfect spin-polarization in the material.
Fe/Si multilayered films (MLF) exhibiting a strong antiferromagnetic (AF) coupling were investigated by optical and magneto-optical (MO) spectroscopies. The results were compared with the computer-simulated spectra based on various structural models of MLF. It was shown that neither semiconducting FeSi2 nor ε-FeSi can be considered as the spacer layers in the Fe/Si MLF for the strong AF coupling. The optical properties of the spacer extracted from the effective optical response of the MLF strongly support its metallic nature. A reasonable agreement between experimental and simulated equatorial-Kerr-effect spectra was obtained with the fitted optical parameters of the spacer with the FeSi stoichiometry. Comparison of the extracted optical properties of the spacer with the calculated ones by using the first principles showed that a B2-phase metallic FeSi compound is spontaneously formed at the interfaces during deposition. For the Fe/Si system with ultrathin Fe and Si sublayers (thinner than 1 nm), our optical data indicate that the structure of the whole MLF is close to the amorphous and semiconducting ε-FeSi.
Optical properties of ordered and disordered FeAl alloy films have been measured in the 0.5 - 5.0 eV energy range. The influence of the order - disorder structural transition on the optical properties of FeAl compounds has been studied. Experimental results for ordered FeAl are compared with the results of ab initio semi-relativistic extended linear augmented-plane-wave calculations, and explained in terms of the electronic energy band structures.
Shape memory and ferromagnetic shape memory effects in single-crystal Ni 2 MnGa thin filmsOff-stoichiometric Ni 2 MnGa polycrystalline films, deposited by the flash-evaporation technique on the heated mica substrates, show a martensitic phase transformation at 310 K. At room temperature, the films have a tetragonal structure (aϭbϭ0.598 nm, cϭ0.576 nm) close to the bulk Ni 2 MnGa with c/aϭ0.96. The austenite to martensite transformation brings about an anomalous minimum in the effective magnetization and a strong increase in the ferromagnetic resonance linewidth in the martensitic phase just below the transformation temperature. The results are discussed in terms of a qualitative model that combines the ferromagnetic resonance response with a specific microstructure of the polycrystalline Ni 2 MnGa film.
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