Anisotropic k-ƒ interactions from spin-orbit coupled 5d intermediate states

The anisotropy of the conduction electron-4? local moment interaction ($\text{k-?}$) gives rise to anisotropic transport effects. The contribution from the orbital character of the 4? moment has been well documented by magnetotransport studies on noble metals containing non-S state rare-earth ions. To determine whether there is an additional contribution to the anisotropy from the spin-orbit coupled 5d electron states we have studied the magnetoresistivity of Ag: Gd alloys. We have not found any significant anisotropy of the magnetoresistivity. By comparing this result and previous Hall effect and magnetoresistivity data on noble metal-rare earth alloys with our model calculation, we come to the conclusion that the spin-orbit splitting of the 5d virtual bound state is relatively small. We discuss the details of the resulting anisotropy of the $\text{k-?}$ interaction.     

Electrical resistivity and magneto-resistance of very dilute Cu-Cr alloys

Measurements of the electrical resistivity from 1.5–80 K and the longitudinal magnetoresistivity from 0–95 kOe at 1.9, 4.2 and 25 K for every dilute Cr in Cu alloys are reported.     

Magnetic field dependence of resistivity minima in amorphous LaGdAu alloys with high Gd content

Magnetoresistivity measurements have been performed in fields up to 40 kOe on some concentrated amorphous LaGdAu alloys exhibiting characteristics of a spin-glass. The negative magnetoresistivity at low temperature is found to be roughly proportional to the square of the magnetization. The resistivity minima in these alloys are attributed to a mechanism of electron scattering from magnetic clouds coupled by RKKY interactions, in qualitative agreement with analysis of our remanent magnetization data.     

Resistivity and magnetoresistivity of amorphous rare-earth alloys

The resistivity and magnetoresistivity of amorphous rare-earth alloys are studied starting from the general approach of Van Peski-Tinbergen and Dekker. The random axial crystal-field and the magnetic correlations between the rare-earth ions are consistently taken into account. The characteristic features of the available experimental data are explained both of the case of random ferromagnetic and antiferromagnetic order.     

Magnetization and magnetoresistance of amorphous FexB1−x in high magnetic fields

The magnetization and magnetoresistance of ferromagnetic melt-quenched amorphous Fe_xB_1?x has been measured. The magnetization indicates no saturation to fields above 30 tesla. The absolute value of the negative magnetoresistivity increases by a factor of five with decreasing B content from x = 0.25 to 0.15. These results are discussed in terms of different models of the resistivity minimum behaviour of amorphous alloys.     

Some transport properties in martensitic iron-nickel alloys

Summary Resistivity, Hall effect and magnetoresistivity measurements in Fe−Ni alloy specimens with Ni content ranging from 29 to 32 wt.% and subjected to martensitic transformation have been performed. The results have been interpreted by considering both the mechanisms of the martensitic transformation, different in 29 and 30 wt.% Ni alloys from that in 31 and 32 wt.% Ni ones, and the theories of Smith and Berger on the Hall effect in dilute ferromagnetic alloys, which have been applied to the above-described alloys. The Hall data, analysed by means of Kohler polts, have allowed us to identify the charge carrier scattering mechanisms prevailing in the different alloys,i.e. the skew scattering in 29 and 30 wt.% Ni alloys and the side jump scattering in 31 and 32 wt.% Ni ones. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Kondo effect in a magnetic field and the magnetoresistivity of kondo alloys

The effect of a magnetic field on the spectral density of a S = 1/2 Kondo impurity is investigated at zero and finite temperatures by using Wilson's numerical renormalization group method. A splitting of the total spectral density is found for fields larger than a critical value H(c)(T = 0) approximately 0.5T(K), where T(K) is the Kondo scale. The splitting correlates with a peak in the magnetoresistivity of dilute magnetic alloys which we calculate and compare with the experiments on CexLa1-xAl2,x = 0.0063. The linear magnetoconductance of quantum dots exhibiting the Kondo effect is also calculated.     

Magnetoelastic effects in soft nanocrystalline Fe-based alloys

Nanocyrstalline Fe-based alloys of Finemet are here investigated. These ferromagnets were prepared by annealing amorphous ribbons and have excellent soft magnetic properties. Ribbons prepared by zero-field annealing and by annealing in longitudinal or transverse magnetic fields are considered. A comparison is made between the anisotropic effects induced at room temperature by applying external stresses and those induced by the magnetic fields applied during the production process. The analysis is performed through the characteristics of the hysteresis loops, the distribution function of the anisotropy fields together with measurements of the initial complex permeability and of the magnetoresistivity. The results have been interpreted on the basis of the random anisotropy model.     

Magnetic properties of amorphous MnxPd82−xGe18 alloys

Magnetoresistivity, magnetic, susceptibility and high field magnetization measurements were performed on the amorphous Mn _x Pd_82–x Ge₁₈ alloys for 1x7. These amorphous alloys were prepared by radio frequency (R. F.) sputtering using argon, depositing onto fused quartz substrates to a thickness of about 20 microns. The negative magnetoresistivity is approximately proportional to the square of the magnetization and the susceptibility obeying the Curie-Weiss law between 18 and 293 K. These results lead to the conclusion that a Kondo-type s-d exchange interaction exists in the amorphous Mn-Pd-Ge alloys and the d-d, spin correlation between magnetic atoms is, in general, weaker in the amorphous alloys than in the corresponding crystalline alloys.     

Negative Magnetoresistivity of ErBi and Its Crystal-Field Levels

A theoretical approach is generalized and employed in this work to evaluate the magnetoresistivity of ErBi in external magnetic fields. The calculated results and theoretical analyses show that when an external magnetic field is applied in the z-direction, the magnetoresistivity can be reduced considerably due to the degeneracy lifting of the crystal-field levels. However, when the magnetic field is exerted along the x-axis, the magnetoresistivity will be increased because of the formations of new magnetic states of the Er ion and its transitions within and between these new states.     

Magnetotransport in a semconductor superlattice with transverse magnetic field

Magnetotransport in a semiconductor superlattice (SL) under transverse magnetic field has been investigated. It is shown that in weak magnetic and electric fields there is negative magnetoresistivity along the SL layers and positive magnetoresistivity along the SL axis. The Hall resistivity is much less than the usual semiconductor value. With an increase of electric field, there appears a negative differential conductivity (NDC) along the SL layers, and the Hall voltage depends nonlinearly on current density. In higher electric field, destroying the miniband structure, the magnetoresistivity along the SL axis is negative. The magnetoresistivity along the SL axis in strong magnetic field is positive for any current density. The Hall resistivity in strong magnetic (electric) field equals the classical value.     

探针法测量磁电阻效应

阐述了各向异性磁电阻效应测量原理，提出采用对称电流六探针来改善测量误差，以及八探针法测量多层膜巨磁阻效应。     

Magnetic Structure of FexCu100−x Magnetoresistive Alloys Produced by Mechanical Alloying

Fe _x Cu_100–x magnetoresistive alloys were produced by mechanical alloying. X-ray diffraction shows fcc structure. The room-temperature Mössbauer spectra evolves from an asymmetrical doublet below x=25%, to a broad magnetic hyperfine field distribution above this concentration. Quadrupole splitting of the doublet varies between 0.48 and 0.57 mm/s, and its isomer shift from 0.16 to 0.29 mm/s. Low-temperature Mössbauer spectroscopy displays a B _hf distribution. Magnetization measurements display different features depending on concentration, from mictomagnetism to ferromagnetism. Low-temperature magnetoresistance is measured. Samples with x20% exhibit larger magnetoresistivity ratios. Bulk and hyperfine magnetic properties are correlated in order to explain magnetoresistivity features of these samples.     

Effect of the four-sheet Fermi surface on magnetoresistivity of MgB2

Recent experimental data of anisotropic magnetoresistivity measured in MgB₂ films have shown an intriguing behaviour: the angular dependence of magnetoresistivity changes dramatically with temperature and disorder. In order to explain such phenomenology, in this work, we extend our previous analyses on multiband transverse magnetoresistivity in magnesium diboride, by calculating its analytic expression, assuming a constant anisotropic Fermi surface mass tensor. The calculation is done for arbitrary orientation of the magnetic field with respect to the crystalline axes and for the current density either perpendicular or parallel to the magnetic field. This approach allows to extract quite univocally the values of the scattering times in the σ- and π-bands by fitting experimental data with a simple analytic expression. We also extend the analysis to the magnetoresistivity of polycrystalline samples, with an arbitrary angle between the current density and the magnetic field, taking into account the anisotropy of each randomly oriented grain. Thereby, we propose magnetoresistivity as a very powerful characterization tool to explore the effect of disorder by irradiation or selective doping as well as of phonon scattering in each one of the two types of bands, in single crystals and polycrystalline samples, which is a crucial issue in the study of magnesium diboride.     

Theory of magnetoresistance due to lattice dislocations in face-centred cubic metals

A theoretical model to describe the low temperature magneto-resistivity of high purity copper single and polycrystals containing different density and distribution of dislocations has been developed. In the model, magnetoresistivity tensor is evaluated numerically using the effective medium approximation. The anisotropy of dislocation-induced relaxation time is considered by incorporating two independent energy bands with different relaxation times and the spherical and cylindrical Fermi surfaces representing open, extended and closed electron orbits. The effect of dislocation microstructure is introduced by means of two adjustable parameters corresponding to the length and direction of electron orbits in the momentum space, which permits prediction of magnetoresistance of FCC metals containing different density and distribution of dislocations. The results reveal that dislocation microstructure influences the character of the field-dependent magnetoresistivity. In the orientation of the open orbits, the quadratic variation in magnetoresistivity changes to quasi-linear as the density of dislocations increases. In the closed orbit orientation, dislocations delay the onset of magnetoresistivity saturation. The results indicate that in the open orbit orientations of the crystals, the anisotropic relaxation time due to small-angle dislocation scattering induces the upward deviation from Kohler’s rule. In the closed orbit orientations Kohler’s rule holds, independent of the density of dislocations. The results obtained with the model show good agreement with the experimental measurements of transverse magnetoresistivity in deformed single and polycrystal samples of copper at 2 K.     

Anomalous magnetoresistivity of bismuth with dilute ionized impurities in quantizing magnetic field

Transverse magnetoconductivity tensor components have been measured in dilute Bi:Sn and Bi:Te alloys up to magnetic field/temperature = 13T / 0.5 K and compared with those of high purity sample. As was expected, perfect phase inversion and enhancement of the quantum oscillation of the magnetoresistivity component were observed. It was also found that peaks of Hall resistivity tensor component due to the hole Fermi surface in the highest magnetic field region revealed dispersion-type line shape in Bi:Sn alloy in contrast to those in Bi:Te alloy or in pure Bi, which may be ascribed to the difference between the screening of anions and that of cations by electrons and holes in these semimetals.     

The effect of plastic deformation on the transverse magnetoresistivity of potassium

A number of pure potassium samples have been plastically deformed under tension at low temperatures and the effect of this on the transverse magnetoresistivity at fields up to 6 T has been investigated. Although the influence of the dislocations so introduced is readily visible in the change of the zero field resistivity ?₀, there is no appreciable effect on the magnetoresistivity other than a shift by an amount equivalent to the change in ?₀.     

Electron-electron interaction in the magnetoresistance of graphene

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.     

Magnetic and electrical properties of amorphous PdCoP alloys

Magnetic susceptibility and electrical resistivity measurements were performed (Pd_100?xCo_x)₈₀P₂₀ alloys where 15 < x < 50. The magnetic properties show that these alloys undergo a ferromagnetic transition between 272 and 399 K as the cobalt concentration increases from 15 to 50 atomic %. Below 20 atomic % Co the short-range exchange interactions which produce the ferromagnetism are unable to establish a long-range magnetic order and a peak in the magnetization shows up at the lowest temperature range under an applied field of 6.0 kOe. The electrical resistivity of these alloys has been measured from 4.2 K up to the vinicity of the melting point (900 K). The electrical resistivity data could be interpreted by the coexistence fo a Kondo-like minimum and ferromagnetism. The minimum becomes less important as the transition metal concentration increases. The coefficients of In T and T² become smaller and concentration dependent. The spin ordering in such alloys can be simulated as either the ordering due to an applied “external field” or as an increase in “internal fields”. These are due to an increase in transition metal concentration. The negative magnetoresistivity is a strong indication of the existence of localized moment.     

Galvanomagnetic properties of Heusler alloys Co<Subscript>2</Subscript>Fe<Emphasis Type="Italic">Z</Emphasis> (Z = Al,Si, Ga,Ge, In,Sn, Sb)

The magnetization, Hall effect, and resistivity of Heusler alloys Co₂FeZ (where Z = Al, Si, Ga, Ge, In, Sn, and Sb are s- and p-elements) have been studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. In strong fields (H > 20 kOe), magnetization can be described by the Stoner model. The normal R ₀ and anomalous R _S Hall effect coefficients have been determined. The coefficient RS is positive for almost all the studied alloys and represents a “linearly quadratic” resistivity function incorporating linear and quadratic terms. The constant R ₀ is negative for most alloys, and its absolute value is two or three orders of magnitude smaller than for R _S . The magnetoresistivity of the studied alloys does not exceed several percent and may be both positive and negative for different specimens.