A. P. Kazakov

The comparison of direct and indirect methods for determining the magnetocaloric parameters in the Heusler alloy Ni50Mn34.8In14.2B

The magnetocaloric properties of the Ni50Mn34.8In14.2B Heusler alloy have been studied by direct measurements of the adiabatic temperature change (ΔTAD(T,H)) and indirectly by magnetization (M(T,H)), differential scanning calorimetry, and specific heat (C(T,H)) measurements. The presence of a first-order ferromagnetic-paramagnetic transition has been detected for Ni50Mn34.8In14.2B at 320 K. The magnetocaloric parameters, i.e., the magnetic entropy change (ΔSM = (2.9-3.2) J/kgK) and the adiabatic temperature change (ΔTAD = (1.3-1.52) K), have been evaluated for ΔH = 1.8 T from CP(T,H) and M(T,H) data and from direct ΔTAD(T,H) measurements. The extracted magnetocaloric parameters are comparable to those of Gd.

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.

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.

Hall effect in a martensitic transformation in Ni-Co-Mn-In Heusler alloys

The Hall effect, transverse magnetoresistance, and magnetization of Ni48Co2Mn35In15 Heusler alloys have been studied at T = 77–300 K in magnetic fields up to 15 kOe. It has been shown that a martensitic transformation is accompanied by a change in the sign of the constant of the ordinary Hall effect, which means a strong change in the electronic spectrum in the martensitic transformation, while the anomalous Hall effect (AHE) constant is positive in both the austenite and martensite phases. In both phases, there are no correlations between the AHE constant and the square of the resistivity, which are characteristic of the side jump mechanism in the AHE theory. In the near vicinity of the martensitic transformation, the field dependences of the Hall resistance are complex and nonmonotonic, indicating a change in the relative concentrations of the austenite and martensite phases in strong fields.

The Adiabatic Temperature Changes in the Vicinity of the First-Order Paramagnetic-Ferromagnetic Transition in the Ni-Mn-In-B Heusler Alloy

The magnetocaloric properties of Ni-Mn-In-B Heusler alloy have been studied using direct measurements of the adiabatic temperature change (Δ<i>T</i>_AD(<i>T</i>,<i>H</i>)) , and indirectly by magnetization (<i>M</i>(<i>T</i>,<i>H</i>)), differential scanning calorimetry, and specific heat (<i>C</i>_P(<i>T</i>,<i>H</i>)) measurements. The presence of a first-order ferromagnetic-paramagnetic transition has been detected for Ni₅₀Mn_34.8In_14.2B at 320 K. The magnetocaloric parameters, i.e., the magnetic entropy change, Δ<i>S</i>_M = (2.9-3.2) J/kgK, and the adiabatic temperature change, Δ<i>T</i>_AD = (1.3-1.52) K, have been evaluated for Δμ₀<i>H</i> = 1.8 T from <i>C</i>_P(<i>T</i>,<i>H</i>) and M(T,H), and from <i>C</i>_P(<i>T</i>,<i>H</i>) and direct Δ<i>T</i>_AD(<i>T</i>,<i>H</i>) measurements, respectively, in the vicinity of the first-order transition temperature. The extracted magnetocaloric parameters are comparable to that of Gd.

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.