Hall constants and magnetoresistance in aluminium single crystals,studied by means of helicon resonances

The Hall coefficient and the transverse magnetoresistance in aluminium single crystals were determined from measurements on standing helicon waves, in the three main crystallographic directions [100], [110] and [111]. The magnetic fields ranged from 1 to 7T and the temperature from 4.2 to 20 K. A Legendy-type theory with parameters adapted to the separation of the satellites as measured at high fields gave good fittings over the whole field range. The Hall coefficients, determined in this work, decrease with magnetic field as mentioned in earlier publications, but are generally lower than those published previously for comparable field ranges.The transverse magnetoresistance as a function of temperature was determined from a continuous recording of the helicon response at resonance. The temperature dependent part of the resistivity varies according to a T³ law for all fields. The field dependence is close to linear, but indications of quadratic and saturation behaviour are also present.     

Electrical conduction of a quasi-one dimensional Nb3S4 single crystal

With respect to single crystals of Nb₃S₄ the electrical resistivity from 2.8 K to 300 K and the magnetoresistance at 4.2 K were measured. The resistivity is represented as a sum of a temperature independent and an intrinsic temperature dependent component. The temperature dependence of the intrinsic resistivity subjects to T₃ form between 7 and 50 K above which it becomes weaker than T₃ approaching a T linear from. This behaviour is discussed in terms of the electron-electron Umklapp scattering. The ratio of the resistivities perpendicular and parallel to the c-axis takes about 15 between room temperature and 50 K. The transverse magnetoresistance is proportional to the magnetic field. The longitudinal magnetoresistance is too small to be measured.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

Pulsed magnetic field study of the spin gap in intermediate valence compound SmB6

In this work, we report the behavior of electrical resistivity of SmB₆ at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at T_C~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB₆ proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.     

Hall resistivity in ferromagnetic manganese-oxide compounds

The transport properties of manganese-oxides are studied using the spin correlation fluctuation scattering mechanism. It is shown that the Hall resistivity in a small magnetic field exhibits a maximum near the Curie point, and a strong field shifts the peak position to high temperature and suppresses the peak value; the dependence of the Hall resistivity on the magnetic field above T_c and below T_c is different. These results agree with the experimental curves qualitatively, but disagree quantitatively, which indicates that the spin correlation fluctuation scattering might not be the dominant mechanism of the colossal magnetoresistance. The double polaron mechanism due to strong electron-phonon and electron-spin coupling is proposed to be responsible for the colossal magnetoresistance in manganese-oxides.     

Anisotropic electron spin-wave scattering in terbium

The magnetoresistance in Tb single crystals has been investigated in magnetic fields up to 75 kOe, and in the temperature range 4–36 K. The longitudinal magnetoresistance is negative, in contrast to the results in the transverse field which are positive and show a maximum for T > 20K. The results suggest an anisotropy in scattering of electrons by spin waves, and the ratio of the anisotropic to the total spin wave resistivities are approximately 18 and 12% for a- and b-axis crystals respectively.     

Magnetoresistance of lanthanum manganites with activation-type conductivity

The temperature dependence of the resistivity and magnetic moment of La_0.85Ba_0.15MnO₃ and La_0.85Sr_0.15MnO₃ manganite single crystals in magnetic fields up to 90 kOe is investigated. Analysis of the experimental results shows that the magnetoresistance of lanthanum manganites far from the Curie temperature T _C can be described quantitatively by the s-d model normally used for ferromagnets and taking into account only the exchange interaction between the spins of charge carriers and magnetic moments. These data also show that the features of lanthanum manganites responsible for colossal magnetoresistance (CMR) are manifested in a narrow temperature interval δT ≈ 20 K near T _C. Our results suggest a CMR mechanism analogous to the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr-type multilayers with nanometer layer thickness. The nanostratification observed in lanthanum manganites and required for GMR can be described taking into account the spread in T _C in the CMR range δT.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

High-temperature properties of the manganites: Manifestation of a paramagnetic-phase inhomogeneity?

An analysis was made of the magnetic susceptibility, electrical resistivity, and magnetoresistance of (La_1?yPr_y)_0.7Ca_0.3MnO₃ samples differing in Pr content and enriched in the oxygen isotope ¹⁸O. At high temperatures, all samples were paramagnetic insulators, while below 60 K, part of them transferred to a ferromagnetic metallic state. All the samples exhibit practically identical behavior of the susceptibility, resistivity, and magnetoresistance in the high-temperature region, despite a noticeable difference between their properties at low temperatures; more specifically, the magnetoresistance grows quadratically with magnetic field within a broad range of temperatures and magnetic fields and scales with increasing temperature close to 1/T⁵. A combined analysis of the magnetic susceptibility and magnetoresistance indicates the possible existence of an inhomogeneous state with considerable ferromagnetic correlations in the paramagnetic region.     

Influence of magnetic field on the superconducting transition in a Nd1.82Ce0.18CuO4−δ film

The temperature and magnetic-field dependences of the resistivity ρ and Hall effect R(j‖ab, B‖c) in a Nd_1.82Ce_0.18CuO_4−δ single crystal film (T _c =6 K) is investigated at temperatures 1.4≤T≤20 K and magnetic fields 0≤B≤5.5 T. At the lowest temperature T=1.4 K the resistive state (exhibiting resistivity and the Hall effect) arises in a magnetic field B=0.5 T. A transition to the normal state is completed at B _c 2≃3 T, where the Hall coefficient becomes nearly constant. The negative magnetoresistance due to the weak-localization effect in the normal state is observed for B>3 T. The nonmonotonic behavior and the inversion of the sign of R(B) in the mixed state are explained in a reasonable way by the flux-flow model with the effect of pinning taken into account. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 407–411 (25 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Thermoelectric properties and ferromagnetism of diluted magnetic semiconductors Sb<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>3</Subscript>

Thermoelectric properties of single crystals of a new dilute magnetic semiconductor p-Sb_{2 ? x} Cr _x Te₃ are studied in the temperature interval 7–300 K. The temperature dependences of the thermal conductivity are measured. The Seebeck coefficient S is found to increase upon doping with Cr. At low temperatures, a ferromagnetic phase with Curie temperature T _C ≈ 5.8 K exists for a Cr concentration x = 0.0215, its easy magnetization axis being parallel to the crystallographic axis C ₃. At T = 4.2 K, a negative magnetoresistance and anomalous Hall effect are observed; in strong magnetic fields, the Shubnikov-de Haas effect is manifested.     

Physical properties and anisotropies of the RNiGe3 series (R = Y, Ce-Nd, Sm, Gd-Lu)

We have studied RNiGe₃ (R=Y, Ce-Nd, Sm, Gd-Lu) single crystals by measuring crystal structure and stoichiometry, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, and specific heat. Clear anisotropies as well as antiferromagnetic ordering in the RNiGe₃ series (R=Ce-Nd, Sm, Gd-Tm) have been observed above 1.8 K from the magnetic susceptibility. A metamagnetic transition in this family (except for R=Sm) was detected at 2 K for applied magnetic fields below 70 kOe. The electrical resistivity of this series follows metallic behavior in the high temperature region. Below the antiferromagnetic ordering temperature a significant anisotropy is exhibited in the resistivity and magnetoresistance for different current directions. The anisotropic magnetic, transport, and thermal properties of RNiGe₃ compounds are discussed in terms of Ni site occupancy as well as a combination of the effect of formation of a magnetic superzone gap and the crystalline electric field.     

Angular dependence of the upper critical field in Bi2Sr2CuO6+δ

The angular dependence of the upper critical magnetic field was investigated in a wide range of temperatures in very high-quality Bi₂Sr₂CuO_6+δ single crystals with critical temperature T _c (midpoint) ? 9 K in magnetic fields up to 28 T. Although the typical value of the normal state resistivity ratio ρ_c/ρ_ab≈10⁴, the anisotropy ratio H _c2∥ab/H _c2⊥ab of the upper critical fields is much smaller and shows an unexpected temperature dependence. A model based on strong anisotropy and small transparency between superconducting layers is proposed.     

Magnetic properties, magnetoresistance, and magnetic-field-induced phase transitions in Tb0.95Bi0.05MnO3 and Eu0.8Ce0.2Mn2O5 multiferroics

This paper reports on a study of the magnetic properties, magnetoresistance, and phase transitions in the semiconducting manganite multiferroics Tb_0.95Bi_0.05MnO₃ and Eu_0.8Ce_0.2Mn₂O₅ whose dielectric properties have been a subject of an earlier study. An analysis of these properties has led us to the conclusion that the above crystals at temperatures T ≥ 180 K undergo phase separation with the formation of a dynamic periodic alternation of quasi-2D layers of manganese ions in different valence states, i.e., charge-induced ferroelectricity. This state exhibits a giant permittivity and ferromagnetism in the layers containing Mn³⁺ and Mn⁴⁺ ions. At low temperatures (T < 100 K), the phase volume is occupied primarily by the dielectric phase. Studies of the magnetic properties and the effect of the magnetic field on the dielectric properties of crystals substantiate the scenario of the formation of a state with giant permittivity put forward by us. At low temperatures, Tb_0.95Bi_0.05MnO₃ exhibits features at the points of phase transitions in pure TbMnO₃. A ferromagnetic moment is observed to exist at all the temperatures covered. At the boundaries of the quasi-2D layers, magnetic-field-induced jumps of the electrical resistivity caused by metamagnetic transitions in the layers with Mn³⁺ and Mn⁴⁺ ions are observed. At temperatures T ≥ 180 K, the electrical resistivity undergoes a periodic variation in a magnetic field which is a manifestation of carrier self-organization. A high magnetic field is capable of shifting the phase transition from 180 K to higher temperatures and inducing additional phase transitions.     

Magnetic polarons,clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La_1?xA_xMnO₃(A≡Ca,Sr;x=0.07–0.1) single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy ΔEρ, its temperature coefficient γ, the thermopower activation energy ΔE _S , the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of 275±25 K, which is approximately twice as high as their Curie point T_C and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10–12 Å in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to T _C of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.     

Negative magnetoresistance and nonlinear conduction in Ti0.99V0.01Se2

Both magnetic and electric field dependences of transport coefficients are investigated on the layered material Ti_1-xV_xSe₂ (x = 0.01). In contrast to semimetallic TiSe₂, the resistivity of the V-doped samples increases with decrease of temperature even in the low temperature region. At liquid helium temperatures it is found that the resistivity is strongly dependent on electric field strength. The behaviour of the nonlinear conduction is similar to that observed in 1T-TaS₂. In the low field (Ohmic) region anomalously large negative mangetoresistance is observed, Δ?/?₀ = -80% at 1.6 K and 60 KOe. Moreover the Hall coefficient is also found to depend on both magnetic and electric fields. All the experimental data suggests that mobile carriers are excited by the applied fields.     

Hall effect and negative magnetoresistance in thin crystals of NbSe<Subscript>3</Subscript>

We have measured the Hall effect and the transverse magnetoresistance in NbSe₃ single crystals. In the liquid helium temperature range we observed an absolute negative magnetoresistance (NMR) — the value of the resistance under magnetic field being much lower than that at zero field — in NbSe₃ single crystals with a thickness less than 5 μm with the magnetic field oriented in the (b, c) plane. We show that this NMR effect is observed in the magnetic field range in which the Hall constant changes its sign. The results are qualitatively explained by the change of the surface scattering contribution to the magnetoconductance in the magnetic field range near the Hall voltage zero crossing.     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Magnetism in a UNi2/3Rh1/3Al single crystal

We report a magnetization, magnetostriction, electrical resistivity, specific heat and neutron scattering study of a UNi_2/3Rh_1/3Al single crystal, a solid solution of an antiferromagnet UNiAl and a ferromagnet URhAl. The huge uniaxial magnetic anisotropy confining the principal magnetic response to the c axis in the parent compounds persists also for the solid solution. The magnetization curve at 1.6 K has a pronounced S shape with an inflection at 12 T. The temperature dependence of magnetic susceptibility exhibits a maximum around 10 K and is magnetic history dependent at lower temperatures where the resistivity increases linearly with decreasing temperature. The low-temperature ρ(T) anomaly is removed in a magnetic field applied along c, which yields a large negative magnetoresistance amounting to m46 zin 14T (at 2 K). The C/T values exhibit a minimum around 12 K and below 8 K they become nearly constant (about 250 mJ mol^?1 K^?2), which is strongly affected by magnetic fields. Neutron scattering data confirm a non-magnetic ground state of UNi_2/3Rh_1/3Al. The bulk properties at low temperatures are tentatively attributed to the freezing of U magnetic moments with antiferromagnetic correlations. The additional intensities detected on top of nuclear reflections in neutron diffraction in a magnetic field applied along c are found to be proportional to the field-induced magnetization, which reflects field-induced ferromagnetic coupling of U magnetic moments. This scenario is corroborated also by finding low-temperature magnetostriction data that also scale with the square of magnetization.