Search

The magnetic and magnetotransport properties of thin Heusler alloy Ni49.7Fe17.4Co4.2Ga28.7 films deposited onto MgO(100) substrates are studied over a wide temperature range, which includes a martensitic transition (MT). For this composition, the MT is not accompanied by a magnetic phase transition, since the martensitic and austenitic phases are ferromagnets with similar magnetizations. The electrical resistivity does not undergo sharp changes during the MT. The magnetoresistance is negative, decreases in magnitude with increasing temperature in the range 100–250 K corresponding to the MT, and then increases to –1%. The field dependences of the Hall effect resistivity have the shape that is characteristic of homogeneous ferromagnetic alloys. The coefficients of the normal and anomalous Hall effects are determined. The anomalous Hall effect coefficient is shown to be described by the relation Rs = αρ + βρ2, where ρ is the electrical resistivity and the second term is lower than the first, which indicates an important role of the interference impurity–phonon scattering mechanism.

Magnetic metamaterials made of non-magnetic resonators – metaatoms – are an essential part of metamaterial world reacting to the ac magnetic field and being able to demonstrate negative permeability. We describe local permeability distribution in the vicinity of the resonant frequency where effective medium theory is not applicable because of metasurfaces’ finite sizes and the excitation of magnetoinductive (MI) waves. We show that inter-element coupling significantly affects not only MI waves passband but the local permeability distribution.

The magnetic and magneto-optical properties of (Со40Fe40B20)x(SiO2)100x nanocomposites with x = 30–72 at % are studied. The results reveal the inhomogeneous structure of the nanocomposites, which exhibit both large granules and small particles that make independent contributions of different nature to the magnetic properties of the materials. Specific features of the coercive force near the percolation threshold indicate superferromagnetic ordering in the composites at low temperatures.

Magnetoelastic transitions (METs) in bulk in nearly equiatomic Fe-Rh alloys produced by arc melting may show poor reproducibility related to insufficient chemical homogeneity and presence of impurity phases in variable concentrations. To better understand the synthesis conditions that reliably yield bulk FeRh materials with reproducible MET characteristics, Fe100-xRhx alloys with x = 50, 50.5 and 51 at. % were prepared by induction melting and thermal annealing under identical conditions. The fabricated samples were cut into several slices, followed by characterization of METs in each of the slices using isothermal and isofield magnetization measurements, differential scanning calorimetry, and direct measurements of the magnetocaloric effect. All of the slices exhibit METs between the AFM and FM states, but the transitions are abrupt with nearly the same change of magnetization, ΔM, when x = 50.5 and 51, whereas for the x = 50 alloy the transition spreads over a wide temperature interval and ΔM may fluctuate by as much as 10 % from one specimen to another. A comparison of the magnetocaloric responses of x = 50 and 51 materials is presented. The clearly different effect of the magnetic field on the transition in both directions leads to significant differences in the reversibility and maximum values of the magnetic field-induced entropy and adiabatic temperature changes, as well as average hysteresis losses. In terms of reproducibility, our results suggest that induction melting is a more appropriate technique to prepare these binary alloys.

The magnetic properties and reversibility of the magnetocaloric effect of Ni 50 Mn 35 In 15 have been studied in the vicinity of the phase transition using magnetization and direct adiabatic temperature change (ΔT ad ) measurements in magnetic fields up to 14 T. The magnetostructural phase transitions (MSTs) between a martensitic phase (MP) with low magnetization (paramagnetic or antiferromagnetic) and a nearly ferromagnetic austenitic phase were detected from thermomagnetic curves, M(T,H), at the applied magnetic fields up to 5 T. The MST temperature was found to be nearly independent of magnetic field for H ad ∼ −11 К for a magnetic field change of ΔH = 14 T was observed in the vicinity of the MST. The irreversibility of ΔT ad was found to be 1 K. A direct ΔT ad of +7 K for ΔH = 14 T was detected at the second order ferromagnetic-paramagnetic phase transition. The obtained results have been discussed in terms of the suppression of antiferromagnetic correlations with the application of a strong magnetic field, and a reversibility of the initial magnetic state of the MP with applied magnetic field when the MST coincides with T C .

The isothermal magnetic entropy changes are studied in Ni80Fe20/Ni67Cu33/Co90Cu10/Mn80Ir20 stacks at temperatures near the Curie point of the Ni67Cu33 spacer by applying magnetic fields in a few tens of Oersted. Such low fields were sufficient for toggling magnetic moments in the soft ferromagnetic (FM) layer (Ni80Fe20). It is found out that this switching provides the magnetic entropy change, which is up to 20 times larger than that achievable in a single Ni67Cu33 film subjected to such low fields. Our finding holds promise to be utilized in the magnetocaloric devices that would be based on FM/PM/FM heterostructures and would operate with moderate magnetic fields., Comment: 7 pages, 4 figures

We present our last results on anomalous Hall effect (AHE) in (Co41Fe39B20)x(Al–O)100-x nanocomposites focusing on the possible correlation between temperature dependence of AHE and resistivity. It is shown that the temperature dependence of conductivity G=1/Rxx, where Rxx is resistivity, for compositions with x=49-56% at 10K

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 ΔT ad 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.

Adiabatic temperature change, resulting from MCE, is a complex effect depending on in-field behavior of both lattice and magnetic subsystem of the sample. For studying MCE, the understanding of their interplay is crucial. Comparing ΔT ad in different fields (e.g. below, above and far above saturation field) it should be possible to derive contributions from lattice and magnetic subsystems. In this present work we present our study of the MCE in Ni 50 Mn 35 In 15 Heusler alloy in the vicinity of the magnetostructural transition in 1.6 T and 14 T field.

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.

Results from an experimental study of the interaction between elements of discrete magnetic metamaterials in the GHz range, depending on the mutualorientation of meta-atoms in the same plane, are presented. The increased complexity of the mechanism of interaction between meta-atoms is discussed for when the size of the elements is reduced as optical frequencies are approached.

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.

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.

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.

The properties of magnetic metamaterials composed of microresonators have been studied. Coupling of meta-atoms induces magnetoinductive waves, which determine the features of propagation of electromagnetic radiation in metamaterials. The complication of the coupling mechanism due to the radiation when the element sizes decrease as frequencies tend to optical ones is discussed.

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni50Mn35In15 − x Si x (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.

The magnetocaloric properties of Ni-Mn-In-B Heusler alloy have been studied using direct measurements of the adiabatic temperature change (ΔTAD(T,H)) , and indirectly by magnetization (M(T,H)), differential scanning calorimetry, and specific heat (CP(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 Δμ0H = 1.8 T from CP(T,H) and M(T,H), and from CP(T,H) and direct ΔTAD(T,H) measurements, respectively, in the vicinity of the first-order transition temperature. The extracted magnetocaloric parameters are comparable to that of Gd.

The dispersion of magnetic inductive waves in biatomic metamaterials in the MHz range is studied experimentally for the first time. The biatomic material properties is the result of interaction between meta-atoms. It is shown how to obtain structures in which dispersion is split into two branches separated by a stop band in which the propagation of magnetic inductive wave is impossible.

We present the results of the studies of magnetic and magnetocaloric (MCE) properties of quaternary Heusler alloys Ni48.5Mn35In15Co1.5 and Ni50Mn35In13.5Al1.5 in temperature range between 80 K and 400 K. While doping initial ternary Ni-Mn-In alloy with 1.5 at.% Al does not change TCA as well as MS, doping with 1.5 at.% Co significantly increases both TCA and MS. The direct magnetocaloric measurements around martensitic transition and austenite Curie temperature reveal the competition between the contributions of different signs. The latter is related to the vicinity of transition temperatures of austenite and martensite, resulting in mixed state, where both phases undergo magnetic order-disorder transformation in the same temperature region. Additionally temperature dependencies of adiabatic temperature change ΔTad and magnetic entropy ΔSM of the system exhibit additional peak feature which we attribute to intermartensitic transition occurring in these alloys.

The transverse magnetoresistance and the Hall effect of Fe100 −xAlx ordered alloys with x = 26.5−34.1 at. % are studied at temperatures of 77 and 295 K. No saturation is found in the field dependences up to 17 kOe. The magnetoresistance is negative, depends nonmonotonically on the Al concentration, and reaches a maximum |ΔR/R| = 1.6% in the concentration range from 28 to 30 at. %. The coefficient of the anomalous Hall effect depends nonlinearly on the Al concentration and resistivity: it increases to a lesser extent as compared to the resistivity at x = 26–30 at. % and decreases with increasing resistivity at x > 30 at. %. An explanation is proposed for the observed anomalies in the magnetotransport properties in the framework of the model developed for the inhomogeneous magnetic microstructure of the Fe100 −xAlx ordered alloys.

Magnetotransport and magnetic properties of Co66Fe4B15Si15 amorphous ribbons, annealed in vacuum and in open air for various times, are studied. The air-annealed samples exhibit the presence of the near-surface hard magnetic layer. The anomalous Hall effect coefficient and the resistivity change simultaneously under heat treatment with slightly non-linear correlation between them. Positive magnetoresistance in low fields changes to negative magnetoresistance with increase of magnetic field. We explain both positive and negative magnetoresistance in terms of spin-dependent scattering.

The magnetotransport and magnetostatic properties of synthesized dilute magnetic semiconductors (DMS) In x Mn 1− x Sb were studied. The samples with x =0.75–1.33 at% Mn manifested the hysteresis at room temperature with the coercive force below 50 Oe and the saturation field about 5 kOe. Both the magnetoresistance and Hall effect were positive. With the Mn concentration decrease the magnetoresistance significantly increased. The co-doping of these DMS with Zn enhances the ferromagnetic properties according to the kinematic exchange theory.

Magnetic properties, the Hall effect and crystallic structure of atomic-disordered γ-MnGa near the stoichiometric γ-Mn 3 Ga composition were studied within the temperatures 300–1000 K. Heisenberg spin glass with random competing anisotropy have been obtained where the longitudinal L, transverse T and mixed LT-phases of spin glass, the Q-phase with nonzero parameter of order have been experimentally observed. Successive transitions in spin glass phase have been found. The anomalous Hall effect coefficient is temperature independent and equals R a =0.8×10 −9 Ω cm G −1 both in the paramagnetic region and in the spin glass state.

The experimental data of morphological, structural, transport and magnetic properties of sintered Co (100− x ) (CuO) x composites are reported. The positive transverse magnetoresistance was observed at low fields. This effect is due to the strong spin-dependent scattering of conduction electrons on the surface-oxidized layers of Co particles.

Heisenberg spin glass with random competing anisotropy is studied, where longitudinal (L), transverse (T), mixed (LT) of spin glass, Q-phase with non-zero quadrupole parameter of order were been experimentally observed. Consecutive transitions into spin glass have been observed.

The Hall effect and transverse magnetoresistance (MR) in amorphous ferromagnetic alloys Fe100−xBx with x = 12.2–18.1 at % have been studied at 77–300 K. It was found that the ordinary and extraordinary Hall coefficients, Ro and Rs, are equal to Ro = (20–70) × 10−12 W cm G−1 and Rs = (6–9) × 10−10 W cm G−1, depend on temperature (d Ro/dT < 0, dRs/dT < 0) and display a non-monotonic dependence on composition, with extrema at x = 14.4 at %. It was established that for amorphous and crystalline alloys the typical relation between Rs and the resistivity is not fulfilled in amorphous alloys Fe-B. In the case of a magnetic field perpendicular to the ribbon plane, the transverse MR is positive up to 8–10 kOe. The most pronounced positive effect exists in alloys with 13.1 at % at ∼ 180 K. Possible reasons for the unusual behaviour of Rs and the MR are discussed.

Influence of anxiety (tested by Luscher) on the spatial organization of brain electrical activity was studied using multichannel EEG recording. In subjects with a high-level of anxiety, the level of coherence and spectral power of some high-frequency EEG components was higher in the parieto-temporal regions of both hemispheres than that in the opposite group. Solving the inverse problem (classification of individuals with one of the groups) made it possible to work out the criteria of automated evaluation of the anxiety level at the preclinic state of neuropsychic disorders. The EEG signs of the influence of increased anxiety on psychophysiological and cognitive processes and autonomic functions were analyzed. The EEG signs of increased anxiety in animals (rats) and humans were compared and some similarity of the spatial-frequency organization was revealed. The main difference consists in a greater activation of the left parieto-temporal area in humans, which is interpreted as a morphofunctional basis of verbalization of anxiogenic stimuli.