V. V. Marchenkov

Magnetic and electrical properties of the half-metallic ferromagnets Co2CrAl

This paper presents the results of measurements of the magnetic and electrical properties of the ferromagnetic alloy Co2CrAl in two structural states: (i) after severe plastic deformation and (ii) after shortterm high-temperature annealing of the deformed specimens. The experiments have been performed at temperatures in the range from 2 to 900 K in magnetic fields H ≤ 50 kOe. The ferromagnetic Curie temperature TC and the paramagnetic Curie temperature Θ have been determined (TC = 305 K and Θ = 326 K), as well as the spontaneous magnetic moment μS and the effective magnetic moment μeff per molecule of the alloy (μS = 1.62 μB and μeff2 = 8.2 μB2). It has been shown that the magnetic crystalline anisotropy energy of the alloy is on the order of ∼5 × 105 erg/g. The specific features of the electrical properties are associated with the presence of an energy gap in the electronic spectrum near the Fermi level EF and with the change in the parameters of the energy gap as a function of the temperature.

Effect of severe plastic deformation by torsion on the properties and structure of the Ni54Mn21Ga25 and Ni54Mn20Fe1Ga25 alloys

The effect of one-percent substitution of iron for manganese on the physical (magnetic, electrical, thermal, and galvanomagnetic) properties and the crystal structure of the Ni54Mn21Ga25 alloy has been investigated. It has been demonstrated that the deviation of the alloy composition from the stoichiometric composition Ni50Mn25Ga25 leads to the formation of a mixed ferromagnetic-antiferromagnetic state. The atomic disordering and nanostructuring of the alloys under investigation due to the severe plastic deformation by torsion in Bridgman anvils to sizes of 10–20 nm result in the suppression of reversible magnetically controlled shape memory effects.

Experimental validation of the anomalies in the electron density of states in semiconductor iron-vanadium-aluminum alloys

The temperature dependences of the resistivity, Hall coefficient, and magnetic susceptibility of iron-vanadium-aluminum alloys have been investigated. It has been established that the alloy Fe1.9V1.1Al exhibits semiconductor behavior for the method used to obtain uniform alloys. It is shown that at temperatures below 30K the semiconductor alloy possesses the characteristic low-temperature scale of the dependences observed, which could be responsible for the appearance of a narrow pseudogap in the electron density of states. A simple theoretical description of the effects of a pseudogap is proposed. A consistent fit of the theoretical to the experimental relations made it possible to determine the effective width of the pseudogap (∼1MeV) and its relative depth (∼102).

Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe2MnAl, Fe2MnSi, and Co2MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe2MnAl, Fe2MnSi, and Co2MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L21 structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.

Specific features of the electrical resistivity of half-metallic ferromagnets Fe2MeAl (Me = Ti, V, Cr, Mn, Fe, Ni)

The transport properties of half-metallic ferromagnetic Heusler alloys Fe2MeAl (where Me = Ti, V, Cr, Mn, Fe, and Ni are 3d transition elements) have been measured in the temperature range of 4–900 K. The specific features in the behavior of the electrical resistivity have been considered in terms of the two-current conduction model, which takes into account the presence of an energy gap in the electron spectrum of the alloys near the Fermi level.

Specific features of the electrical resistance of half-metallic ferromagnetic alloys Co2CrAl and Co2CrGa

It has been shown by comparing the results of studying the electrical and magnetic properties of the half-metallic ferromagnetic Heusler alloys Co2CrAl and Co2CrGa with the calculations of their electronic structure that high values of the electrical resistivity ρ are caused by a disordered distribution of atoms over the sites of the L21 cubic structure, and the anomalous behavior of ρ(T) is associated with the transformation of the electronic spectrum due to the ferromagnetic-to-paramagnetic transition.

Electrical properties of ferromagnetic Ni2MnGa and Co2CrGa Heusler alloys

The electrical properties of ferromagnetic Ni2MnGa and Co2CrGa Heusler alloys are measured in the temperature range 4–900 K. The effect of the energy gap near the Fermi level in the electronic spectrum on the behavior of electrical resistivity and absolute differential thermopower is discussed.

Electrical and Optical Properties of X2YZ (X = Co, Fe; Y = Cr, Mn, Ti; Z = Ga, Al, Si) Heusler Alloys

We studied the electrical and optical properties of X2YZ (X = Co, Fe; Y = Cr, Mn, Ti; Z = Ga, Al, Si) Heusler alloys. We showed that the electrical and optical properties of Co2TiAl and Fe2TiAl are typical of metals. An abnormal behavior of the resistivity and the optical properties in the IR range of the other alloys was observed, i.e., the resistivity exhibits a semiconductor-like behavior and the optical conductivity has no contribution from the intraband absorption of the conduction electrons. The experimental data analysis, based on existing band calculations, allowed us to conclude that the anomalous behavior of their properties is determined by electronic states near to and at the Fermi level. The electronic states in the subband with spin up give the main contribution to the interband transitions in the IR range, but they practically do not contribute to the static conductivity (resistivity). There are almost no charge carries in the other electronic subband (with spin down), and hence, the interband transitions occur only at the energies above the semiconductor gap values.

Effect of cobalt doping on thermoelastic martensitic transformations and physical properties of magnetic shape memory alloys Ni50 − xCoxMn29Ga21

The magnetic and thermoelastic martensitic transformations and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of multicomponent magnetic shape memory alloys Ni50 − xCoxMn29Ga21 (x = 0, 1, 2, 3, 10 at %) have been investigated. The critical temperatures of thermoelastic martensitic transformation and magnetic transitions have been determined. It has been found that the alloy with 10 at % Co undergoes a martensitic transformation in the temperature range of 6–10 K.

Crystal structure and physical properties of magnetic shape memory alloys Ni50 − xCuxMn29Ga21

The phase composition, crystal structure, and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of the stoichiometric alloy Ni50Mn25Ga25 and nonstoichiometric alloys Ni50 − xCuxMn29Ga21 (x = 0, 1, 2) with the thermoelastic martensitic transformation have been investigated. The influence of the chemical composition on the transformations and physical properties of the alloys has been determined.