Low-temperature Magnetoresistance Research Articles

Spin transport in p-type germanium

We report on the spin transport properties in p-doped germanium (Ge-p) using low temperature magnetoresistance measurements, electrical spin injection from a ferromagnetic metal and the spin pumping-inverse spin Hall effect method. Electrical spin injection is carried out using three-terminal measurements and the Hanle effect. In the 2–20 K temperature range, weak antilocalization and the Hanle effect provide the same spin lifetime in the germanium valence band (≈1 ps) in agreement with predicted values and previous optical measurements. These results, combined with dynamical spin injection by spin pumping and the inverse spin Hall effect, demonstrate successful spin accumulation in Ge. We also estimate the spin Hall angle in Ge-p (6–) at room temperature, pointing out the essential role of ionized impurities in spin dependent scattering.

Giant magnetoresistance in the cluster glass regime of Co-Ga alloys

A detailed study of low temperature electrical transport properties of CoxGa100−x (x = 54, 55.5, 57) alloy has been carried out. The origin of the resistivity anomalies and correlation between magnetic and electrical transport properties are identified through an elaborate analysis. The weak localization and enhanced electron-electron interaction effects partially support the electrical transport properties of the system. Further, the observed magnetoresistance can be well represented by localized model along with quantum corrections. The low temperature magnetoresistance value near critical composition is comparable to that reported in giant magnetoresistance materials.

Weak antilocalization in Cd3As2 thin films.

Recently, it has been theoretically predicted that Cd3As2 is a three dimensional Dirac material, a new topological phase discovered after topological insulators, which exhibits a linear energy dispersion in the bulk with massless Dirac fermions. Here, we report on the low-temperature magnetoresistance measurements on a ~50 nm-thick Cd3As2 film. The weak antilocalization under perpendicular magnetic field is discussed based on the two-dimensional Hikami-Larkin-Nagaoka (HLN) theory. The electron-electron interaction is addressed as the source of the dephasing based on the temperature-dependent scaling behavior. The weak antilocalization can be also observed while the magnetic field is parallel to the electric field due to the strong interaction between the different conductance channels in this quasi-two-dimensional film.

Influence of dislocation microstructure on low temperature magnetoresistance in copper single- and polycrystals

Angular and field dependent transverse magnetoresistance (TMR) have been studied in copper single- and polycrystals containing different density and distribution of dislocations. In the open-orbit orientation, the quadratic variation of TMR with the applied magnetic field changes to the quasi-linear dependence, as the density of dislocations increases. In the closed-orbit orientations, TMR shows a linear dependence with the magnetic field, the slope of the characteristic decreases with increasing the density of dislocations. The signatures of closed and extended orbits are preserved until the very high dislocation densities stored in the single crystals. The analysis of the Kohler's plots indicates that in the open-orbit orientation TMR violates Kohler's rule, but in the closed-orbit orientation Kohler's rule is obeyed. The results suggest that at low dislocation density, the magneto-transport of electrons is strongly anisotropic and is dominated by small-angle scattering, irrespective of the distribution of dislocations. In the crystals with higher densities of dislocations containing dislocation walls and cells, the electron transport is dominated by a high-angle scattering process. The magnetoresistance data are consistent with the two-band model and support the theory that the dislocation-induced relaxation time is anisotropic due to the non-sphericity of the Fermi surface. In the spherical portion of the Fermi surface the scattering is isotropic, associated with the constant relaxation time. Strongly anisotropic scattering in the aspherical portion of the Fermi surface is associated with a much smaller relaxation time.

Conduction mechanism in mesoporous hematite thin films using low temperature electrical measurements and theoretical electronic band structure calculations

Low temperature four-probe electrical resistivity and magnetoresistivity measurements in mesoporous hematite thin films imply that the operative conduction mechanism involves impurity band with intrinsic donor defects. The surface morphology revealed the porous nanostructures with necked particle size ranging from 17 nm to 23 nm. Due to the presence of ionized and high magnetic spin configuration of Fe 3d-electrons states in α-Fe2O3, it shows spin magnetic moment per ion 4.17 μB as described by a Hubbard-type on-site Coulomb repulsion (GGA+U approach), which was found to provide an accurate description of the electronic properties which is good agreement with reported results. It has an estimated band gap about 2.2 eV with enhanced photocurrent (16.73%) properties and could promise photoelectrochemical applications.

Two-dimensional quantum transport in highly conductive carbon nanotube fibers

Measurements of the electrical resistivity, from 1.5 to 300 K, and of the low temperature magnetoresistance of highly conductive carbon nanotube (CNT) fibers, obtained by wet-spinning from liquid crystalline phase (LCP), are reported. At high temperature the results obtained on the raw CNT fibers show a typical metallic behavior and the resistivity levels without postdoping process were found to be only one order of magnitude higher than the best electrical conductors, with the specific conductivity (conductivity per unit weight) comparable to that of pure copper. At low temperature a logarithmic dependence of the resistivity and the temperature dependence of the negative magnetoresistance are consistent with a two-dimensional quantum charge transport---weak localization and Coulomb interaction---in the few-walled CNT fibers. The temperature dependence of the phase-breaking scattering rate has also been determined from magnetoresistance measurements. In the temperature range $Tl100\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, electron-electron scattering is found to be the dominant source of dephasing in these highly conductive CNT fibers. While quantum effects demonstrate the two-dimensional aspect of conduction in the fibers, the fact that it was found that their resistance is mainly determined by the intrinsic resistivity of the CNTs---and not by intertube resistances---suggests that better practical conductors could be obtained by improving the quality of the CNTs and the fiber morphology.

Tunnel magnetoresistance of compacted CrO2 powders with particle shape anisotropy

The effect the shape anisotropy of particles of compacted CrO2 powders have on tunnel magnetoresistance is investigated in the temperature range of 4.2–20 K. Measurements are made at different orientations of the magnetic field relative to the sample plane. It is shown that the anisotropy of the low-temperature tunnel magnetoresistance of compacted as a powders, result of the orientation of the magnetic field relative to the pressing axis, depends on the particle shape anisotropy.

Observation of large low temperature magnetocaloric effect in HoCu2

We report the observation of large low temperature magnetocaloric effect and magnetoresistance in the rare-earth based intermetallic compound HoCu2. The compound undergoes an antiferromagnetic type ordering below about TN = 10.5 K, which is second order in nature. The magnetocaloric effect in terms of entropy change under the application of 50 kOe of field is found to have a maximum value of −19.3 J kg−1 K−1 peaking around TN, and an appreciable value of relative cooling power of 268 J kg−1 was associated with it. The sample also shows giant negative magnetoresistance with its value as high as −36.5% around TN for 50 kOe of field. Field induced second order metamagnetic transition is found to be responsible for the observed magnetocaloric and magnetoresistance behaviors in the sample. The sample is devoid of any thermal or field hysteresis by virtue of the second order nature of the transitions, which enables us to exploit large reversible magnetic cooling at cryogenic temperatures.

Low-temperature resistivity, magnetoresistance, and domain-wall resistance of highly pure single- and polycrystalline iron

On a set of 12 bulk and highly pure [residual resistance ratio (RRR) up to 6000] iron single crystals with different crystallographic orientation and on polycrystalline material the ``hidden'' basic resistivity $\ensuremath{\rho}(T,B\ensuremath{\rightarrow}0)$ and magnetoresistive effects (anisotropic magnetoresistance, longitudinal and transverse Lorentz resistance) are quantitatively disentangled. The temperature-dependent basic resistivity for $T\ensuremath{\le}30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ follows a $\ensuremath{\Delta}\ensuremath{\rho}(T,B\ensuremath{\rightarrow}0)=a{T}^{2}+b{T}^{5}$ law, proving that electron-electron and electron-phonon scattering (with a resistive Debye temperature ${\ensuremath{\Theta}}_{\mathrm{R}}\ensuremath{\cong}450\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) are dominant also in iron, similar to nonferromagnetic transition metals. Here the crystal-orientation dependence of the intrinsic longitudinal magnetoresistance in the single-domain state could be quantitatively evaluated in accordance with the symmetry of the Fermi surface. The transverse magnetoresistance (TMR) turns out to be the sum of the so-called ``two-band conduction term'' and the unboundedly growing TMR term in compensated metals. It is proven that the second term is not restricted to the high-field limit (as usually discussed up to now) but acts down to lowest TMR or ${\ensuremath{\omega}}_{\mathrm{C}}\ensuremath{\tau}$ values. This conclusion is verified by TMR measurements on highly pure molybdenum single crystals (RRR \ensuremath{\cong} 100 000). Supported by Kerr microscopy, a small positive domain wall resistance (DWR) could be isolated from the dominating negative DWR in the multidomain state of iron, resulting in a relation of larger than 5:1 for the electrons to pass a domain wall with spin tracking compared to scattering with spin conservation.

Post-growth purification of Co nanostructures prepared by focused electron beam induced deposition

In the majority of cases nanostructures prepared by focused electron beam induced deposition (FEBID) employing an organometallic precursor contain predominantly carbon-based ligand dissociation products. This is unfortunate with regard to using this high-resolution direct-write approach for the preparation of nanostructures for various fields, such as mesoscopic physics, micromagnetism, electronic correlations, spin-dependent transport and numerous applications. Here we present an in situ cleaning approach to obtain pure Co-FEBID nanostructures. The purification procedure lies in the exposure of heated samples to a H2 atmosphere in conjunction with the irradiation by low-energy electrons. The key finding is that the combination of annealing at 300 °C, H2 exposure and electron irradiation leads to compact, carbon- and oxygen free Co layers down to a thickness of about 20 nm starting from as-deposited Co-FEBID structures. In addition to this, in temperature-dependent electrical resistance measurements on post-processed samples we find a typical metallic behavior. In low-temperature magnetoresistance and Hall effect measurements we observe ferromagnetic behavior.

Observation of magnetically hard grain boundaries in double-perovskite Sr2FeMoO6

Unusual low-temperature magneto-resistance (MR) of ferromagnetic Sr2FeMoO6 polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr2FeMoO6 by utilizing the different probing depths of the different detection modes of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), namely, the total electron yield (TEY) mode (probing depth ) and the total fluorescence yield (TFY) mode (probing depth ). At 20 K, the magnetic coercivity detected in the TEY mode was several times larger than that in the TFY mode , indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft and and were nearly the same. From the line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe2+ or well-screened signals, while the non-magnetic component is dominated by Fe3+ or poorly screened signals.

Influence of Mn Site Substitution on Electrical Resistivity and Magnetoresistance Properties of Rare Earth Manganite

We investigate the effects of Cr, Ru and Sn substitution on electrical resistivity and magnetoresistance property of polycrystalline samples La0.67Sr0.33Mn1-xBxO3(x = 0, 0.05; B= Ru, Cr and Sn) compounds. The value of M-I transition temperature (TP) decreases while resistivity increases with Cr, Ru and Sn substitution, moreover, the largest low-temperature magnetoresistance (MR %) is found at magnetic field dependent (Isotherm), which suggest that the spin-dependent scattering from internal grain regions is also responsible for the low-temperature MR %. Resistivity data have been fitted with the variable range hopping model to estimate the density of state at Fermi level. It was observed that the substitution of various transition metals in the Mn-site leads to a decrease in conductivity of the doped manganite samples, with conduction being controlled by the disorder induced localization of charge carriers.

Catalytic purification of directly written nanostructured Pt microelectrodes.

In the majority of cases, nanostructures prepared by focused electron beam induced deposition employing an organometallic precursor contain predominantly carbon-based ligand dissociation products. This is unfortunate with regard to using this high-resolution direct-write approach for the preparation of nanostructures for various fields, such as mesoscopic physics, micromagnetism, metaoptical phenomena in the visible spectral range, or others. Following early attempts of postprocessing Pt-based structures prepared by focused electron beam induced deposition at several hundred degrees Celsius in a reactive gas atmosphere, recent work has focused on developing in situ purification processes by using a stationary O2 flux in combination with electron irradiation to oxidize the carbonaceous component of the deposits. Here we show that this purification process is driven by the catalytic activity of Pt and in fact does not rely on the parallel electron irradiation process to function, if the O2 exposure is done in a pulsed fashion. We suggest a multistep cleaning mechanism which results in pure, nanoporous Pt. By suitably chosen beam parameters, high-resolution Pt dot and line structures with dimensions below 10 nm can thus be conveniently obtained. In temperature-dependent resistance measurements, we find the typical metallic behavior of Pt. In low-temperature magnetoresistance measurements, we see clear evidence for weak antilocalization effects and deduce a dephasing length of 234 nm at 1.2 K. We consider this to be a promising starting point for developing this approach into a versatile preparation technique for Pt-based mesoscopic structures, in particular since the purification process can be run in parallel on different deposits. We furthermore anticipate that our results will spur further research on purification approaches for nanostructures prepared by focused electron beam induced deposition containing a catalytically active metal species such as Pd-, Fe-, or Co-based deposits.

Invited: In Situ Characterization of Plasma Assisted Hydrogenation/Fluorination of Graphene and CFx Battery Applications

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in-situ. Low temperature transport properties, viz., electrical resistance (R), thermopower (S), Hall mobility (m), and magneto-resistance (MR) were measured for each sample and correlated with ex-situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak-hydrogenation, the transport is seen to be governed by electron diffusion and low temperature transport properties show metallic behavior (conductance G remains non-zero as 0). For strong-hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (0 as 0). Weak localization (WL) behavior is seen with a negative MR for weakly-hydrogenated graphene and this WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly-hydrogenated graphene. Multi-walled carbon nanotubes synthesized using fluidized bed chemical vapor deposition technique were fluorinated sequentially to prepare a series of CFx battery electrodes. Primary battery performance was tested using CFx as a cathode against Li. Fully fluorinated MWNTs showed capacity exceeding 815 mAh/g while partially fluorinated samples showed systematically lowered capacity with decreasing x (in CFx). However, fully fluorinated MWCNTs showed distinctly low rechargeable capacity compared to the subfluorinated samples when used as an anode against Li. Mildly fluorinated MWNTs show high capacity and better stability during charge/discharge cycles. High concentrations of fluorine seem to affect capacity retention due to the increased defect densities and reduced electronic conduction. These defects of nanotubes will provide additional pathways for lithium ions to diffuse within the core of the fluorinated structure and to access the electrochemically active C-F sites. XRD, XPS, and Raman spectroscopy were utilized to characterize the samples. Finally the electrochemical performance of fluorinated MWNTs was compared with that of Natural Chinese Graphite (NCG).

Magnetic Properties of Cobalt Nanowires and Nanotubes

Development of novel magnetic materials has played an important role in modern day science and technology. Nanomagnetism research not only sparks interests in fundamental physics, but also renders promising technological applications. Ferromagnetic Co nanowires, synthesized via low voltage electrodeposition method. High resolution transmission electron microscopy and x-ray diffraction results show that the nanowires are uniform in size, and consist predominantly hcp structure with the magnetocrystalline easy axis (c-axis) perpendicular to the wire axis. SQUID measurement illustrates the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire axis. Furthermore, the magnetic domain structures of individual, segmented or multiple nanowires are studied via magnetic force microscopy. It shows a strong dipole at the ends of the wire, together with a spatial magnetization modulation along the wire with a period around 700 nm. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis. Low temperature magnetoresistance measurement shows irreversible magnetization switching in addition to the anisotropic behavior due to magnetization orientation dependent electron scattering.On the other hand, Cobalt nanotube arrays embedded in hexagonally ordered anodic aluminum oxide templates are fabricated by direct electrodeposition with low current density under several mA/cm2. The wall thickness is about 15 nm and outer diameters are in accordance with pore diameter around 80 nm. Nearly 100% filling rate is achieved which benefits from the through pores which enable the efficient penetration of electrolyte. Transmission electron microscopy and selected-area electron diffraction are performed to show that the nanotubes are predominantly hcp single crystals with c-axis (easy axis) perpendicular to the tube axis. SQUID and MFM results indicate that the magnetization follows a circumferential direction around the tube, which is manifested in the weak magnetic signal in the MFM imaging as well as a sheared hysteresis dependence for field applied along the tube axis. This phenomenon has been confirmed by theoretical modeling taking into account the magnetocrystalline, shape demagnetization and magnetic exchange energies.

Magnetic tunnel junctions with single-layer-graphene tunnel barriers

We report on the fabrication and characterization of magnetic tunnel junctions consisting of a single layer of graphene as the tunnel barrier, sandwiched between two metallic ferromagnetic electrodes. We employ a fabrication process chosen to minimize oxidation of the electrode materials at the ferromagnet/graphene interfaces. The devices have low resistance-area products of 1.5--6 \textohm{} \ensuremath{\mu}m${}^{2}$, with low-temperature magnetoresistances of 1.5--3.4%. The temperature and bias dependencies of the resistance confirm that transport is dominated by tunneling processes rather than by any unintended pinholes.

Magnetoresistance of a copolymer: FeCl3-doped poly(2,5-dioctyloxy-p-phenylene vinylene-alt-3,4-ethylenedioxythiophene vinylene)

An experimental study of the low temperature magnetoresistance (MR) in a FeCl3-doped copolymer in which ethylenedioxythiophene and phenylene are alternatively linked by vinylene units is reported. The temperature dependence of conductivity followed characteristics of Coulomb gap variable range hopping (VRH) at low temperatures. The MR showed a crossover from negative to positive values. The crossover was explained by competition of negative and positive contributions which originated from quantum interference and shrinkage of wave function, respectively.

Surface state dominated transport in topological insulator Bi2Te3 nanowires

We report on low temperature magnetoresistance measurements on single-crystalline Bi2Te3 nanowires synthesized via catalytic growth and post-annealing in a Te-rich atmosphere. The observation of Aharonov-Bohm oscillations indicates the presence of topological surface states. Analyses of Subnikov-de Haas oscillations in perpendicular magnetoresistance yield extremely low two-dimensional carrier concentrations and effective electron masses, and very high carrier mobilities. All our findings are in excellent agreement with theoretical predictions of massless Dirac fermions at the surfaces of topological insulators.

In Situ Study of Hydrogenation of Graphene and New Phases of Localization between Metal–Insulator Transitions

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in situ. Low-temperature transport properties, namely, electrical resistance (R), thermopower (S), Hall mobility (μ), and magnetoresistance (MR), were measured for each sample and correlated with ex situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak hydrogenation, the transport is seen to be governed by electron diffusion, and low-temperature transport properties show metallic behavior (conductance G remains nonzero as T → 0). For strong hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G → 0 as T → 0). Weak localization (WL) behavior is seen with a negative MR for weakly hydrogenated graphene, and these WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly hydrogenated graphene.

Large low-temperature magnetoresistance in SrFe2As2 single crystals

We present the first report on a large low-temperature magnetoresistance (MR) of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca ion-implanted SrFe2As2 single crystal that occurs before the emergence of crystallographic strain-induced bulk superconductivity arising from a sample aging effect. In accordance to band structure calculations from the literature, which consitently show that more than 2 bands are involved in the transport, we have modeled this large MR at high fields using a 3-carrier scenario rather than solely on quantum linear MR model generally used to explain the MR in iron-pnictides. At and below 20 K the large MR may be due to 3-carrier transport in an inhomogeneous state where there are superconducting and metallic regions.