V. N. Prudnikov

Direct measurements of field-induced adiabatic temperature changes near compound phase transitions in Ni–Mn–In based Heusler alloys

The adiabatic temperature changes (ΔTad) in the vicinity of the Curie and martensitic transition temperatures of Ni50Mn35In15 and Ni50Mn35In14Z (Z=Al and Ge) Heusler alloys have been studied using an adiabatic magnetocalorimeter of 250–350 K temperature interval for applied magnetic field changes up to ΔH=1.8 T. The largest measured changes were ΔTad=−2 and 2 K near the martensitic (first-order) and ferromagnetic (second-order) transitions for ΔH=1.8 T, respectively. It was observed that |ΔTad|≈1 K for relatively small field changes (ΔH=1 T) for both types of transitions. The results indicate that these materials should be further explored as potential working materials in magnetic refrigeration applications.

Magnetic, transport, and magnetocaloric properties of boron doped Ni-Mn-In alloys

The impact of B substitution in Ni50Mn35In15−xBx Heusler alloys on the structural, magnetic, transport, and parameters of the magnetocaloric effect (MCE) has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (H) up to 5 T, and in the temperature interval 5–400 K). Direct adiabatic temperature change (ΔTAD) measurements have been carried out for an applied magnetic field change of 1.8 T. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5.

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − xSix Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni50Mn35In15 − xSix (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220–280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150–200 μΩ cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.

Magnetocaloric effect in Ni50Mn35In15 Heusler alloy in low and high magnetic fields

The magnetic and magnetocaloric characteristics of Ni50Mn35In15 Heusler alloy are studied in low and high applied magnetic field. At a magnetic field of 14 T, the adiabatic temperature change ΔTad measured by the sample extraction technique near the martensitic transformation (≈315 K) is as large as 11 K. This value is an order of magnitude larger than the corresponding change measured at 1.6 T. The observed giant values of the magnetocaloric effect could be related to the suppression of antiferromagnetic correlations.

Quasi-Diamagnetism and Exchange Anisotropy in Ni–Mn–In–Co Heusler Alloys

The quasi-diamagnetism effect that manifests itself in the negative magnetic susceptibility observed in weak magnetic fields during cooling of samples below some critical temperature has been revealed in Ni48Co2Mn35In15 Heusler alloys. The effect has been attributed to a strong nonequilibrium of the system due to the presence of magnetic and structural disorders and exchange anisotropy.

Logarithmic temperature dependence of electrical resistivity of (Co41Fe39B20)x(Al–O)100 – x nanocomposites

The temperature dependence of the electrical conductivity σ(T) of (Co41Fe39B20)x(Al–O)100–x of nanocomposite films for different concentrations x of amorphous ferromagnetic metal (56 > x > 30) has been studied in the temperature range of 4.2–300 K. It has been shown that, for concentrations in the interval 56 > x > 49, the conductivity obeys the logarithmic law σ(T) = A(1 + αlnT), where A and α depend on the concentration. According to the theory developed by Efetov et al., this logarithmic dependence is connected with specificities of the Coulomb interaction in nanogranulated alloys on the intergranule tunneling in the transient region of concentrations from metallic conduction to the dielectric regime. The comparison of the theory with the experiment has revealed only qualitative agreement. The reasons of the quantitative disagreement have been discussed. The resistivity of samples with the concentrations lying in the range 49 > x > 30 obeys the 1/2 power law.

Magnetic, Magnetocaloric, Magnetotransport, and Magneto-optical Properties of Ni–Mn–In-Based Heusler Alloys: Bulk, Ribbons, and Microwires

In this review, we will survey recent experimental results on magnetic, magnetocaloric, magnetotransport, and magneto-optical properties of Ni–Mn–In-based Heusler alloys in bulk polycrystalline samples, melt-spun ribbons, and glass-coated microwires. These ternary Ni–Mn–In and doped, quaternary alloys comprise a novel class of multifunctional magnetic materials with exceptional properties related to the magnetostructural martensitic transformation. We will focus on recent developments that have led to a better understanding of properties that are promising for applications, possible routes for improvements, and the identification of unsolved problems.

Magneto-optical spectroscopy of the martensitic transition in Fe48Mn24Ga28 Heusler alloys

Magneto-optical spectra of polycrystalline samples of the Fe48Mn24Ga28 Heusler alloy undergoing martensitic transformation from the high-temperature paramagnetic austenitic to ferromagnetic martensitic phase have been studied at 50–320 K in the transversal Kerr effect geometry. A comparison of magnetooptical spectra with data obtained in magnetic measurements has demonstrated that the martensitic transition on the surface of a sample and in its bulk takes place in the same temperature interval. Magnetic anisotropy has been found in the martensitic phase driven by large multidomain inclusions of martensite in austenite. The magneto-optical signal of Fe48Mn24Ga28 differs strongly in spectral shape from that measured in Ni-Mn-Ga.

Correlation between magnetoresistance and magnetic entropy at first-order and second-order phase transitions in Ni-Mn-In-Si Heusler alloys

A correlation between the magnetic part of the entropy and magnetoresistance has been studied using measurements of the resistivity, magnetoresistance, and magnetization of the Ni50Mn35In12Si3 and Ni50Mn35In11Si4 Heusler alloys. It has been shown that although the variations of the entropy and the magnetoresistance are observed to be maximal within the same temperature intervals in the vicinity of the first-order and second-order phase transitions, there is no universal correlation between these effects.

Anomalous Hall effect in (Co41Fe39B20)x(Al-O)100 − x nanocomposites

The concentration dependence of the coefficient Rs characterizing the anomalous Hall effect (AHE) has been studied by measuring the electrical resistivity ρ, magnetoresistance, and the magnetic field dependence of magnetization and Hall resistivity of (Co41Fe39B20)x(Al-O)100 − x nanocomposite thin films. It has been demonstrated that the AHE coefficient increases by more than an order of magnitude with a decrease in the percentage x of the amorphous ferromagnetic metal from 60 to 30 and its behavior is described by the relation Rs ∼ ρm, where m = 0.46 ± 0.1. At the same time, the coefficient characterizing the normal Hall effect grows by a factor of less than 10. The mechanisms underlying the giant Hall effect in nanocomposites have been discussed.