O. D. Chistyakov

Genesis of the anomalous Hall effect in CeAl2

The Hall coefficient RH, resistivity ρ, and Seebeck coefficient S of the CeAl2 compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences RH(ϕ T, H≤70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic RHam and main RHa components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component RHam were used to analyze the complex magnetic phase diagram H-T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution RHa (H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl2 intermetallic compound. The results led us to conclude that taking into account spin-polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl2 matrix were estimated to be (55–90)m0 and 6–10 Å, respectively. The behaviors of the parameters RH, S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl2.

Anomalous thermopower in heavy-fermion compounds CeB6, CeAl3, and CeCu6 − x Au x

High-precision measurements of thermopower have been performed in a wide temperature range (2–300 K) for a series of cerium-based heavy-fermion compounds, including CeB6, CeAl3, CeCu6, and substitutional solid solutions of the CeCu6 − xAux system (x = 0.1, 0.2). All compounds exhibit an unusual (logarithmic) asymptotic behavior of the temperature dependence of the Seebeck coefficient: S ∝ −lnT. In the case of cerium hexaboride, this anomalous behavior of S(T) is accompanied by the appearance of weak-carrier-localization-mode asymptotics in the conductivity (σ(T) ∝ T0.39), while the paramagnetic susceptibility χ(T) and the effective mass of charge carriers meff(T) vary according to a power law (χ(T), meff(T) ∝ T−0.8) in the temperature interval T = 10–80 K. This behavior corresponds to renormalization of the density of states at the Fermi level. The observed anomalous behavior of thermopower in CeB6 and other cerium-based intermetallic compounds is attributed to the formation of heavy fermions (many-body states in the metal matrix) at low temperatures.

Hall-effect anomalies near the quantum critical point in CeCu6−xAux

The results of Hall-effect and resistance measurements on the substitutional solid solutions CeCu6−xAux with concentrations 0⩽x⩽0.3, corresponding to a wide neighborhood of the quantum critical point (QCP) at x=0.1, are presented. The measurements are made by rotation of the sample in a constant magnetic field of up to 70kOe in a temperature interval of 1.8–300K. For the classic heavy-fermion compound CeCu6 the temperature dependence of the Hall coefficient RH(T) exhibits a complex activational form with activation energies Ea1∕kB≈110K and Ea2∕kB≈1.5K in the temperature intervals 50–300K and 3–10K, respectively. It is shown that the anomalous behavior of the Hall effect can be explained in a spin-polaron approach, in which the values Ea1,2 can be associated to the binding energy of many-body and one can obtain estimates of the effective mass (meff1,2≈130–150m0) and localization radius (ap1,2*≈1.7 and 14A) of the charge carriers in CeCu6. For the compound CeCu5.9Au0.1, corresponding to the QCP, one observe...

Effect of hydrogenation on the magnetic properties of the intermetallic compound Er2Fe14B with single-crystal and nanocrystalline structures

Hydrogenated single crystals Er2Fe14Bhx with different hydrogen contents are grown and their magnetic properties are studied for the first time. It is established that both the Curie temperature and the temperature of the spin-reorientation phase transition increase with an increase in the hydrogen content. In the Er2Fe14B single crystal, the contributions of the rare-earth metal and iron sublattices to the magnetic anisotropy decrease upon hydrogenation. However, their compensation occurs at a temperature higher than that in the initial compound Er2Fe14B due to the enhancement of the Fe-Fe and R-Fe exchange interactions. The effect of hydrogenation on the magnetic characteristics of the Er2Fe14B compound with a nanocrystalline structure is investigated. It is revealed that the hydrogenation leads to an increase in the coercive force and the residual magnetization of these alloys.

Structural, magnetic, and magnetothermal properties of the Tb0.3Dy0.7Co2 compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb0.3Dy0.7Co2 compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb0.3Dy0.7Co2 compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb0.3Dy0.7Co2 have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.

Hysteresis properties of nanostructured Nd-Ho-Fe-Co-B alloys

The structure and magnetic properties of a rapidly quenched (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2. alloy are investigated by the X-ray and electron diffraction and by the standard magnetometry techniques. It is found that using the quenching velocity of Vq = 30 m/s, it is possible to attain the high-coercive state (iHc > 20 kOe) at room temperature as a result of formation of nano-sized grains of the main 2-14-1 phase. The low-temperature study of hysteresis properties of the material obtained is performed for the first time.

Magnetic and magnetostrictive properties of Tb-Dy-Ho-Fe-Co alloys

In this work, the structural and magnetic properties of single-phase TbxDyyHoz(Fe,Co)2 (x + y + z = 1) alloys have been investigated by means of X-ray diffraction, 57Fe Mossbauer spectroscopy, magnetization measurements and a standard strain gage technique. The magnetostriction of TbxDyyHoz(Fe,Co)2 was examined against an applied magnetic field up to 10 kOe in 80 - 400 K temperature range. The Co-containing compounds are found to have high values of magnetostrictive susceptibility due to compensation of magnetic anisotropy in both the rare-earth and 3d transition metals sublattices.

Hall effect in the Ce(Al1 − x Co x )2 heavy-fermion system

The influence of substitution on the binding energy of many-body states and the formation of the magnetically ordered state in a heavy-fermion compound (CeAl2) have been studied by measuring the transport characteristics (Hall effect, resistivity) in intermetallic compounds of the Ce(Al1−xMx)2 system (M = Ni, Co; x ≤ 0.08). It is established that the Hall coefficient RH in Ce(Al1−xCox)2 intermetallides with x = 0.05 and 0.08 grows by more than an order of magnitude as the temperature decreases from 1.8 to 300 K. The experimental data are used to estimate the effective mass of charge carriers, the relaxation time, and the localization radius of many-body states.

The influence of interatomic distances on magnetic ordering in RMnSi compounds (R=La, Y, Sm, and Gd)

Magnetic ordering in the RMnSi (R=La, Y, Sm, and Gd) compounds is investigated. It is found that the type of magnetic ordering depends on the dMn-Mn distance between manganese atoms inside the magnetic layers located in the planes perpendicular to the c axis. This inference is based on the results of studies performed with SmMnSi and GdMnSi compounds in which the distances between manganese atoms are close to the critical value dMn-Mn that corresponds to the crossover between ferromagnetic and antiferromagnetic ordering in RMnSi compounds. The introduction of lanthanum and yttrium atoms into the rare-earth sublattice leads to an increase and a decrease in the unit cell size, respectively, and brings about magnetic phase transitions in the compounds under investigation.

The Effect of Structural State on Magnetic and Magnetocaloric Properties of Micro-and Nanocrystalline Gd

The effect of micro-and nanocrystalline structural state on magnetic properties and magnetic entropy (ΔSM) of Gd is investigated. The marked influence of severe plastic deformation on ΔSM is demonstrated.