Y. V. Kudryavtsev

The optical properties of

The optical properties of have been deduced using spectrophotometry and ellipsometry and calculated within the extended linear augmented plane wave framework in the energy range 0 - 6 eV. Good agreement amongst all approaches is obtained up to about 2 eV, beyond which there is some divergence between theory and experiment where, for reasons discussed, the theory needs further refinement.

Martensitic transformation in Ni2MnGa films: A ferromagnetic resonance study

Off-stoichiometric Ni2MnGa 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 (a=b=0.598 nm, c=0.576 nm) close to the bulk Ni2MnGa with c/a=0.96. 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 Ni2MnGa film.

Electronic structure and magnetic properties of Ni2MnGa alloy films with different structural orders

The structural dependence of magnetic and transport properties of nearly stoichiometric Ni2MnGa alloy films prepared by flash evaporation is investigated. The films deposited at 720 K have the L21 Heusler structure and behave nearly the same as the bulk Ni2MnGa alloy, while the films deposited at 150 K have an almost amorphous structure. Disordered Ni2MnGa alloy films do not exhibit magnetic ordering down to 4 K, but rather have the characteristics of highly resistive Pauli paramagnets. Annealing of disordered films restores the L21 structure. The magnetic and transport properties of the annealed films are similar to those of the films deposited at 720 K. A possible explanation of the loss of magnetic moment in the disordered film is discussed with the aid of first-principles electronic-structure calculations.

Structural dependence of some physical properties of the Ni2MnGe heusler alloy films

Structural ordering of the flash-evaporated Ni2MnGe Heusler alloy films has been studied by magneto-optical (MO), optical, and magnetic methods. The properties of the films have been compared with the corresponding properties of the bulk Ni2MnGe samples with L21 and tetragonally distorted L21 structures, respectively. Depending on a substrate temperature (Ts) and/or postdeposition annealing temperature (Ta) the films have been amorphous (Ts=150K), disordered (Ta 720K). The ordered films have exhibited optical and MO properties close to those of the bulk Ni2MnGe, while the amorphous films showed no ferromagnetic ordering. The optical and MO properties of Ni2MnGe with L21 structure have been proved almost the same as those of Ni2MnGa and Ni2MnIn. Changes in the optical properties of Ni2MnGe alloy films have been discussed in terms of Ni2MnGe band structure.

Effect of structural disorder on some physical properties of the Cu2MnAl Heusler alloy films

The structural dependence of the magnetic, transport, optical, and magneto-optical (MO) properties of Cu2MnAl Heusler alloy (HA) thin-film samples was investigated. Disordered thin films of Cu2MnAl HA fabricated by vapor-quenching deposition onto substrates cooled by liquid nitrogen behave as highly resistive Pauli paramagnets down to 5 K, while an annealing of the disordered films at 580 K for 1 h restores the L21-type ordered structure, as well as the ferromagnetic order and other physical properties typical for the ordered Cu2MnAl HA film. The film deposition onto substrates held at T⩾690K leads to the formation of a multiphase crystalline structure. A significant influence of the structural disorder on the optical and MO properties of Cu2MnAl was experimentally observed and discussed in terms of the electronic structure of the alloy.

Electronic structure, optical, and magneto-optical properties of Co2CrGa Heusler alloy films: Experimental and theoretical study

The optical and magneto-optical properties of bulk L21 ordered Co2CrGa Heusler alloy (HA) have been experimentally investigated for the first time below and above the Curie temperature (T≈1.2TC) and explained in terms of the band structure. It is shown that the optical properties of Co2CrGa in paramagnetic phase look similar to those in the ferromagnetic phase. Crystallization of amorphous Co2CrGa HA films with the formation of B2-type of order causes significant increase in alloy resistivity due to formation of the energy gap at the Fermi level in the minority bands. Unlike bulk L21-type ordered Co2CrGa HA sample B2 structural order of Co2CrGa HA films exhibit negative temperature coefficient of resistivity for 80–750 K temperature range. Atomic disorder causes redshift in the interband absorption peak by about 0.6 eV (and hence corresponding reduction in the energy distance between initial and final energy bands responsible for this peak formation).

Magnetic, magneto-optical, and transport properties of ferromagnetic shape-memory Ni2MnGa alloy

The magnetic, transport, and magneto-optical properties of Ni2MnGa alloy in the martensitic and austenitic states were investigated. The temperature dependence of resistivity shows kinks at the structural and ferroparamagnetic transitions. The temperature dependence of magnetization was also investigated for both the bulk and the film. It was shown that the lack of crystallinity in the amorphous Ni2MnGa alloy films leads to the loss of ferromagnetic order above 4 K. The nature of the interband absorption peaks in the optical-conductivity spectra and in the absorptive part of off-diagonal components of the dielectric function for the authentic state of Ni2MnGa alloy was elucidated.

Magneto-optical spectroscopy study of the solid-state reaction in Ti'Ni multilayered films

A comparative study of the solid-state reaction (SSR) in a series of Ti/Ni multilayered films (MLF) with a bilayer period of 0.65–22.2 nm and constant Ti to Ni sublayer thickness ratio has been performed by magneto-optical (MO) spectroscopy as well as x-ray diffraction (XRD). The spectral and sublayer-thickness dependences of the MO properties of the Ti/Ni MLF were explained on the basis of the electromagnetic theory. A threshold nominal Ni-sublayer thickness of about 3 nm in the as-deposited Ti/Ni MLF is necessary for observing the equatorial Kerr effect. Such a fact is explained by the formation, by solid-state reaction, of nonmagnetic alloyed regions between pure components. The SSR in the Ti/Ni MLF caused by the low temperature annealing leads to a formation of an amorphous Ti/Ni alloy and takes place mainly in Ti/Ni MLF with “thick” sublayers. In the case of Ti/Ni MLF, MO turns out to be more sensitive in determining the thickness of the reacted zone, while XRD is more useful for the structural analy...

Effect of the structural disorder on the magnetic, transport, and optical properties of B2-phase Ni0.50Al0.50 alloy films

The influence of structural disorder on the magnetic, transport, and optical properties of Ni0.50Al0.50 alloy films has been investigated. A significantly disordered state was obtained by using vapor-quenching deposition onto substrates cooled by liquid nitrogen. The loss of translational invariance in the disordered state leads to a prominent increase of the magnetic moment of alloy below 50 K. This growth can be explained by the appearance of antistructure Ni atoms (or their clusters) in the disordered state of alloy and their ferromagnetic coupling below 50 K. The temperature dependences of resistivity for both ordered and disordered states of the Ni0.50Al0.50 alloy films exhibit the resistivity minimum at 17–18 K, which has a nonmagnetic nature and can be related mainly to the quantum corrections to the electron–electron interactions in the presence of weak localization. It was shown that the resistivity of the ordered Ni0.50Al0.50 alloy films in 50–300 K temperature range originates mainly from the e...

Influence of a structural disorder in the Co0.62Al0.38 alloy film on the magneto-optical, magnetic, and optical properties

The influence of the order-disorder structural transition on the magneto-optical, magnetic, and optical properties of Co-rich β-phase Co-Al alloy films has been investigated. The disordered state in these alloy films was obtained by means of vapor quenching deposition onto glass substrates cooled by liquid nitrogen. The experimental studies of the magneto-optical and optical properties of the ordered and disordered Co-Al alloy films have been carried out in 0.5–5.0 eV energy range at room temperature. The influence of the order-disorder structural transition on the magneto-optical, magnetic, and optical properties was discussed by using the results of the first-principles calculations of the electronic structure of the CoAl compound and the structural defect approach.