High-field Magnetoresistance Measurements Research Articles

High-field magnetoresistance measurements on Ni75Co25 and Ni40Co60 alloys at 3 K and 300 K

In the present work, a detailed field dependence of the resistivity of Ni75Co25 and Ni40Co60 bulk alloys was measured at T = 3 K and 300 K up to high magnetic fields. The focus of the study was to determine the anisotropic magnetoresistance (AMR) and to get a quantitative description of the field-induced resistivity change, the latter not yet being available for Ni-Co alloys. The AMR parameters were derived from the resistivity data in the magnetically saturated (monodomain) state by using the Kohler analysis. The values of the AMR ratio were found to be close to the relevant previous data both at low and high temperatures. Due to the measurement precision and careful data evaluation, our AMR data obtained on well-characterized samples can be considered as reference values for the bulk state of the investigated compositions. In addition, also the resistivity anisotropy splitting was determined. The experimentally found field dependence of the resistivity at both T = 3 K and 300 K turned out to be at variance with the current theoretical descriptions both for the resistivity increase due to the ordinary magnetoresistance effect being significant at T = 3 K and for the resistivity decrease due to the magnon suppression process at T = 300 K, invoking for a refinement of theory in both cases.

Structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy: Comparison with stoichiometric SrRuO3

We investigate structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy on (001) SrTiO3 substrates. To distinguish the influence of the two types of disorders in the films—Ru vacancies within lattices and disorders near the interface—SrRu0.7O3 thin films with various thicknesses (t = 1–60 nm) were prepared. It was found that the influence of the former dominates the electrical and magnetic properties when t ≥ 5–10 nm while that of the latter does when t ≤ 5–10 nm. Structural characterizations revealed that the crystallinity, in terms of the Sr and O sublattices, of SrRu0.7O3 thin films is as high as that of the ultrahigh-quality SrRuO3 ones. The Curie temperature (TC) analysis elucidated that SrRu0.7O3 (TC ≈ 140 K) is a material distinct from SrRuO3 (TC ≈ 150 K). Despite the large Ru deficiency (∼30%), the SrRu0.7O3 films showed metallic conduction when t ≥ 5 nm. In high-field magnetoresistance measurements, the fascinating phenomenon of Weyl fermion transport was not observed for the SrRu0.7O3 thin films irrespective of thickness, which is in contrast to the stoichiometric SrRuO3 films. The (magneto)transport properties suggest that a picture of carrier scattering due to the Ru vacancies is appropriate for SrRu0.7O3 and also that proper stoichiometry control is a prerequisite to utilizing the full potential of SrRuO3 as a magnetic Weyl semimetal and two-dimensional spin-polarized system. Nevertheless, the large tolerance in Ru composition (∼30%) to metallic conduction is advantageous for some practical applications where SrRu1−xO3 is used as an epitaxial conducting layer.

Weak antilocalization and low-temperature characterization of sputtered polycrystalline bismuth selenide

We report a thorough crystal and transport characterization of sputtered polycrystalline BixSe1−x (20 nm), grown on a thermally oxidized silicon substrate. The crystal and grain structures of the sample are characterized by transmission electron microscopy. Selected-area electron diffraction shows a highly polycrystalline structure. Transport measurements suggest semiconducting behavior of the BixSe1−x film with a very high carrier concentration (∼1020 cm3) and low mobility [∼8 cm2/(V s)]. High-field magnetoresistance measurements reveal weak antilocalization, to which both the low mobility and the angular dependence suggest an impurity-dominated contribution. Fitting parameters are obtained from 2D magnetoconductivity using the Hikami-Larkin-Nagaoka equation. The variation of the phase coherence length with temperature suggests electron–electron scattering for phase decoherence. Electron–electron interaction theory is used to analyze the low-temperature conductivity.

Lifshitz Transitions in the Ferromagnetic Superconductor UCoGe.

We present high field magnetoresistance, Hall effect and thermopower measurements in the Ising-type ferromagnetic superconductor UCoGe. A magnetic field is applied along the easy magnetization c axis of the orthorhombic crystal. In the different experimental probes, we observed five successive anomalies at H≈4, 9, 12, 16, and 21T. Magnetic quantum oscillations were detected both in resistivity and thermoelectric power. At most of the anomalies, significant changes of the oscillation frequencies and the effective masses have been observed, indicating successive Fermi surface instabilities induced by the strong magnetic polarization under a magnetic field.

Giant magnetoresistance and field-induced phase transitions in Tb7Rh3 single crystal

High-field magnetization and magnetoresistance measurements have been performed to investigate the field-induced magnetic phase transitions in Tb7Rh3 single crystals. Metamagnetic transitions were observed along the a-, b- and c-axes of the hexagonal crystal lattice in the three magnetic states at temperatures below the Neel temperature TN = 91 K; magnetic field H — temperature T phase diagrams were constructed for b- and c-axes. At temperatures below TN, the magnetic structure transformations under application of a magnetic field along the b-axis are observed to be accompanied by the giant magnetoresistance effect (|Δρ/ρ| up to 65%). Magnetization curves measured at temperatures above TN show inflection points, which may be indicative of phase transitions within regions with an antiferromagnetic short-range order persisting in a paramagnetic matrix.

Conductivity crossover in nano-crystalline diamond films: Realization of a disordered superlattice-like structure

We present the electrical transport characteristics of a batch of nano-crystalline diamond films of varying nitrogen concentrations and explain the conduction mechanism by the disordered quasi-superlattice model applied to semiconductor heterostructures. Synthesized by the hot-filament chemical-vapour deposition technique, the degree of structural disorder in the films, confirmed from Raman spectroscopy, is found to be controllable, resulting in the transition of conduction mechanism from localized and activated to the metallic conduction regime. Hence through high-field magneto-resistance measurements at low temperatures we firmly establish a conductivity crossover from hopping to 3D weak localization. The long electronic dephasing time and its weak temperature dependence suggest the possibility for diamond-based high-speed device applications.

High-field magnetoresistance in nanowire organic spin valves

In this paper, we report high-field magnetoresistance measurements on template-grown nanowire organic spin valves in which tris-8 hydroxyquinoline aluminum (Alq${}_{3}$) is sandwiched between two ferromagnetic electrodes: nickel (Ni) and cobalt (Co). We report a magnetoresistance effect in which the resistance changes considerably in the field range where both ferromagnets have parallel magnetizations. We show that this effect does not originate from organic magnetoresistance or the anisotropic magnetoresistance effect of the ferromagnets. Instead, the observed effect can be explained by invoking magnetic-field-dependent spin-diffusion length, where spin diffusion takes place primarily via the Elliott-Yafet mode. These data support the view expressed by some previous reports that Elliott-Yafet is the dominant spin-relaxation mode in template-grown organic nanowires.

Enhanced weak localization effect in few-layer graphene

We have investigated the weak localization correction to magnetoresistance in one to six layer graphene structures. The magnetoresistance measurements have revealed that, in addition to the known transition from weak anti-localization in monolayer graphene to weak localization in bilayer graphene, the weak localization effect becomes more pronounced as the number of graphene layers increases. The obtained results substantiate that because few-layer graphene suppresses mesoscopic corrugations and increases intervalley scattering it leads to the observed enhancement of negative resistance, resulting in the restoration of the weak localization in graphene materials. High field magnetoresistance measurements show non-linear behavior, which indicates the breaking of sub-lattice symmetry and the formation of excitonic gap in the Landau level.

High magnetic field magnetoresistance anomalies in the charge density wave state of the quasi-two dimensional bronze KMo6O17

We report high magnetic field magnetoresistance measurements performed in pulsed fields up to 55T on the quasi-two dimensional charge density wave conductor KMo 6 O 17 . Magnetoresistance curves show several anomalies below 28 T. First order transitions to smaller gap states take place at low temperature above 30T. A phase diagram T(B) has been obtained. The angular dependence of the anomalies is reported.

Magnetoresistance and Hall effect measurements of Ni to 6 GPa

High field magnetoresistance (MR) and Hall effect measurements have been conducted on modular Ni 0.985O 0.015 thin film samples in the diamond anvil cell between 280 and 316 K at applied magnetic fields up to 6 T and at pressures up to 6 GPa. The magnetic resistivity above the technical saturation of the magnetization is found to be negative, linear and nonsaturating while the magnitude of the field derivative, 0.010 – 0.018 μ Ω cm T - 1 , increases with temperature and decreases with pressure. The decrease in the MR is attributed to spin wave damping of electron magnon scattering processes at high fields and the zero temperature magnon mass, 535(49) meV Å 2 at 1.1 GPa, is found to be a slightly increasing function of pressure. Hall effect measurements indicate that skew scattering becomes increasingly important with pressure. Resistivity measurements at zero applied field, normalized to 1.7 GPa, yield a pressure derivative of - 0.018 ( 1 ) GPa - 1 . The effects of pressure and oxygen on the density of states (DOS) at the Fermi level are investigated using total energy electronic band structure calculations and reveal that the DOS of Ni 0.985O 0.015 increases with pressure but remains significantly less than for Ni at 10 GPa. The average magnetic moment of Ni 0.985O 0.015 decreases from 0.675 μ B at 0 GPa to 0.648 μ B at 10 GPa. The magnon mass and magnetic moment are increasing and decreasing functions of pressure, respectively. These effects dominate over the increase in the DOS at the Fermi level and, correspondingly, the field derivative of the magnetic resistivity is found to decrease relative to zero pressure measurements.

Magnetoresistance characterization of nanometer Si metal-oxide-semiconductor transistors

We report on the high-field (up to 10T) magnetoresistance measurements performed on the short (down to 75-nm gate length) n-type Si metal-oxide-semiconductor field-effect transistors. The electron magnetoresistance mobility of these nanometer devices was determined for a wide range of the electron concentration (107–1013cm−2, i.e., from a weak to a strong inversion) and gate length (10μm–75nm). In the case of long samples, the magnetoresistance mobility was compared to the effective mobility obtained by the standard parameter extraction and the split C–V techniques. The results are discussed in terms of the scattering power-law two-dimensional transport analysis. The data clearly indicate a significant decrease of the mobility with the gate length reduction below 100nm.

High critical current YBa2Cu3O7 artificial joints using Ag foils as welding agent

A new high quality joining technique of melt processed YBCO has been developed based on interfacial melting induced by a metallic silver foil inserted between two single domain YBCO tiles. This simple technique allows joining large monoliths to form the complex-shaped pieces required for many power applications. Hall probe magnetic flux profile analysis under magnetic field and high field magnetoresistance measurements demonstrate that the superconducting behaviour of the joint is indistinguishable from the bulk.

Magnetisation and magnetoresistance of YMn 6Sn 6− xIn x compounds ( x=0.10, 0.18, 0.21, 0.38)

Polycrystalline samples of HfFe 6Ge 6-type YMn 6Sn 6− x In x compounds (0.10≤ x≤0.38) have been studied by X-ray diffraction, high field magnetisation and magnetoresistance measurements. The low In content compounds order antiferromagnetically ( T N=328, 321 and 309 K for x=0.10, 0.18 and 0.21, respectively) while YMn 6Sn 5.62In 0.38 orders ferromagnetically ( T c=329 K) and undergoes a ferro- to antiferromagnetic transition on cooling ( T t=240 K). All compounds display metamagnetic behaviour at 10 K associated with giant magnetoresistance (Δ R/ R∼−30% for x=0.10). The different magnetoresistance effects measured in the x=0.18 and 0.21 compound seems to be related to different textures suggesting a more pronounced magnetoresistance along the [001] direction. Results are discussed and compared with properties of isotypic series.

Electron-magnon scattering and magnetic resistivity in3dferromagnets

The determination of collective spin excitations and their contribution to the intrinsic resistivity via spin-flip electronic scattering are addressed for $3d$ ferromagnets using magnetotransport experiments. We present longitudinal high-field magnetoresistance (MR) measurements from 4 to 500 K and up to 40 T on Fe-, Co-, and Ni-patterned thin films. Well above the technical saturation of the magnetization\char22{}i.e., in the paraprocess regime\char22{}we report an almost linear and nonsaturating negative MR of around 0.01\char21{}0.03 \ensuremath{\mu}\ensuremath{\Omega} cm ${\mathrm{T}}^{\mathrm{\ensuremath{-}}1}$ at 300 K for the three magnets. We demonstrate its magnetic origin, and we assign this high-field resistivity decrease to the electron-magnon scattering and the spin-wave damping in high fields. We propose a theoretical calculation of the magnetic resistivity originating from spin-flip intraband $s\ensuremath{-}s$ and $d\ensuremath{-}d$ and interband $s\ensuremath{-}d$ transitions via electron-magnon diffusion including both the high-field effect on the magnon spectrum and the magnon mass renormalization. Convincing agreements between the high-field measurements and our model provide a unique estimate of the pure magnetic resistivity in $3d$ ferromagnets. Our analysis also gives an insight into the low-energy spin waves\char22{}i.e., the theoretical magnon saturation field and the magnon mass renormalization consistent with neutron scattering results for the three magnets.

High field magnetoresistance and ESR measurements on Ni stripes on GaAs substrate

Ni nanowires fabricated on a GaAs substrate, which have width narrower than 500 nm and thickness 50 nm, showed an antiferromagnetic-like domain structure. Many Ni nanowires fabricated on GaAs using the electron beam lithography technique have been studied by electron spin resonance (ESR). Splitting of ESR signals in the frequency-field diagram was observed around 57 GHz, although only a ferromagnetic resonance mode was observed for other frequencies. This mode is suggested to be an antiferromagnetic resonance related to the domain structure on the Ni nanowires.

Quantum lifetime in the 2DEG from high magnetic field magnetoresistance measurements

We measured the diagonal component of the transverse magnetoresistance in two pseudomorphic In xGa 1−xAs HEMT structures. The quantum lifetime is obtained from the variation of the amplitude of the Shubnikov–de Hass oscillations with the magnetic field, from Landau levels index between 22 and 5, using a modified Dingle plot method which takes into account the quadratic relation between the magnetoresistance and the density of states at the Fermi level. The quantum lifetimes found in this manner are consistent with quantum lifetimes determined at low magnetic field using Dingle plots and at high magnetic field with a peak magnetoresistance calculation.

High-Field Magnetization and Magnetoresistance of Single-Crystal CeRhSb

We report the high-field magnetization and longitudinal magnetoresistance measurements in magnetic fields of up to 55 T, and Hall resistivity and transverse magnetoresistance measurements in fields of up to 15 T for single crystals of CeRhSb with a pseudogap in the density of states. The magnetization at 1.3 K shows rather weak anisotropy along the orthorhombic principal axes, M a ≃ M b > M c , and the initial straight lines deviate upward near magnetic field values of 20, 30 and 25 T, respectively. The longitudinal magnetoresistance at 1.3 K is largely negative for both B // a and B // b and saturates near 30 T, while that for B // c is positive. The transverse magnetoresistance is positive and increases in proportion to B 1.5 for B // c , while those for B // a and B // b saturate at around 10 T. The Hall coefficient for B // a changes sign from positive to negative below 4 K, while that for B // c progressively increases with decreasing temperature. The Hall resistivity for B // a and B // b possesses,...

Field Induced Quenching of the Energy Gap in Single Crystal CeNiSn

High-field longitudinal magnetoresistance and magnetization measurements up to 36 T have been carried out on a single crystal CeNiSn which shows metallic behavior in the resistivity along the a -axis. Large negative magnetoresistances are found along the a -, b - and c -axes in high fields above 10 T as previously reported in a semiconducting sample. A metamagnetic behaviour in the magnetization is observed along the a -axis at around 15 T. The linear field dependence of log ρ( H ) indicates that the Zeeman energy of the 4 f -electron moment quenches the pseudo energy gap on the Fermi surface in CeNiSn.

Crossover from antiferromagnetic Kondo metal to Kondo insulator in Ce(Pt 1− xNi x)Sn alloys

Transport, magnetic, inelastic neutron scattering and high field magnetoresistance measurements have been made on Ce(Pt 1− x Ni x )Sn alloys, in order to investigate the crossover from the antiferromagnetic Kondo metal (CePtSn) to the Kondo insulator (CeNiSn). Inelastic neutron scattering from CePtSn shows two well-defined crystal field excitations. These broaden with increasing Ni content x in Ce(Pt 1− x Ni x )Sn. CeNiSn shows a single broad inelastic excitation. High field magnetoresistance of CePtSn and CePt 0.5Ni 0.5Sn shows good agreement with Schlottmann's J = 1 2 Kondo impurity model. CeNiSn exhibits a large negative magnetoresistance (−41% at 20 T and 1.7 K) arising from the suppression of the energy gap with field.

High-field magnetoresistance oscillations in alpha -

High-field magnetoresistance measurements have been performed on single-crystal samples of the charge-transfer salt \ensuremath{\alpha}-${\mathrm{ET}}_{2}$KHg (SCN${)}_{4}$ [where ET is bis(ethylenedithio)tetrathiafulvalene] using fields of up to 30 T. We examine the character of the Shubnikov--de Haas oscillations for a range of samples, temperatures, and field regimes to understand the role played by spin splitting and magnetic breakdown effects. Angle-dependent magnetoresistance oscillations (AMRO) in magnetic fields below the kink transition (22 T) indicate that the low-magnetic-field band structure is dominated by a one-dimensional (1D) open section of Fermi surface (FS) inclined at \ensuremath{\sim}21\ifmmode^\circ\else\textdegree\fi{} to the crystallographic ${\mathit{b}}^{\mathrm{*}}$c plane; above the kink, the AMRO change in character, indicating that the FS now possesses a 2D closed section in the form of a distorted cylinder. The data are discussed in terms of a recent model of the low-field band structure and give definitive proof that the kink is the signature of a magnetic-field-induced change in the FS.