Magnetoresistance Data Research Articles

Weak localization and mobility in ZnO nanostructures

We conduct a comprehensive investigation into the electronic and magnetotransport properties of ZnO nanoplates grown concurrently with ZnO nanowires by the vapor-liquid-solid method. We present magnetoresistance data showing weak localization in our nanoplates and probe its dependence on temperature and carrier concentration. We measure phase coherence lengths of 50--100 nm at 1.9 K and, because we do not observe spin-orbit scattering through antilocalization, suggest that ZnO nanostructures may be promising for further spintronic study. We then proceed to study the effect of weak localization on electron mobility using four-terminal van der Pauw resistivity and Hall measurements versus temperature and carrier concentration. We report an electron mobility of $\ensuremath{\sim}100\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ at 275 K, comparable to what is observed in ZnO thin films. We compare Hall mobility to field-effect mobility, which is more commonly reported in studies on ZnO nanowires and find that field-effect mobility tends to overestimate Hall mobility by a factor of 2 in our devices. Finally, we comment on temperature-dependent hysteresis observed during transconductance measurements and its relationship to mobile, positively charged Zn interstitial impurities.

Measurement of spin memory lengths in PdNi and PdFe ferromagnetic alloys

Weakly ferromagnetic alloys are being used by several groups in the study of superconducting/ferromagnetic hybrid systems. Because spin-flip and spin-orbit scattering in such alloys disrupt the penetration of pair correlations into the ferromagnetic material, it is desirable to have a direct measurement of the spin memory length in such alloys. We have measured the spin memory length at 4.2 K in sputtered Pd0.88Ni0.12 and Pd0.987Fe0.013 alloys using methods based on current-perpendicular-to-plane giant magnetoresistance. The alloys are incorporated into hybrid spin valves of various types, and the spin memory length is determined by fits of the Valet-Fert spin-transport equations to data of magnetoresistance vs. alloy thickness. For the case of PdNi alloy, the resulting values of the spin memory length are lsf(PdNi) = 2.8 +/- 0.5 nm and 5.4 +/- 0.6 nm, depending on whether or not the PdNi is exchange biased by an adjacent Permalloy layer. For PdFe, the spin memory length is somewhat longer, lsf(PdFe) = 9.6 +/- 2 nm, consistent with earlier measurements indicating lower spin-orbit scattering in that material. Unfortunately, even the longer spin memory length in PdFe may not be long enough to facilitate observation of spin-triplet superconducting correlations predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of magnetic inhomogeneity.

Temperature Response of BaZrO3-Inclusion to YBa2Cu3O7−δ Probed by Magneto-Transport Measurements

This paper reflects a simple and practically scalable technique, in which attempts have been made for preparation of fine BaZrO3 (BZO) as defects in YBa2Cu3O7−δ (YBCO) superconductor, to enhance superconducting properties. The texture growth of YBCO + xBZO (x=1.0,2.5,5.0 and 10.0 wt.%) composites has been made through solid-state reaction route by employing calcined YBCO and sub-micron sized powders of BZO. The phase formation, texture and grain alignment were analyzed using XRD and SEM. The magnetoresistivity data obtained as a function of temperature (T) in the tail region shows two key features: first an anomalous secondary peak at \(T_{c_{\mathrm{on}2}}\) well below 92 K as a function of BZO content with varying magnetic fields, and secondly a drop in global resistivity transition temperature (Tc0) following the incorporation of excess BZO to the grain boundaries. Impurities appear to reduce \(T_{c_{\mathrm{on}2}}\) greatly to the lower-temperature values as a function of BZO content and to influence the onset of \(T_{c_{\mathrm{on}1}}\) in presence of varying magnetic fields. The findings of this investigation suggest the presence of multiple transition temperatures in the composites.

Spin dynamics control of recombination current in organic semiconductors

The recombination rate of coulombically bound electron–hole pairs depends on their spin configuration. Because of low spin–orbit coupling the spin dynamics of these well separated coulombic pairs is determined by weak hyperfine and exchange interactions. In this case a weak magnetic field produces strong effect on the spin dynamics and hence on the recombination rate of e–h pairs. We have shown that the recombination current in organic semiconductors may have a maximum as a function of recombination constant. For high recombination constants the current is space charge limited and decreases with increasing the e–h recombination constant. This decay of current is due to decrease of the region where the recombination takes place. At a low recombination constant the recombination takes place in the whole volume and the current increases with increasing the recombination constant. The characteristic recombination constant separating those two regimes depends on the thickness of sample, applied voltage, and charge carrier mobilities. The model predictions are consistent with experimental data for magnetoresistance of organic semiconductors.

Superparamagnetism and giant magnetoresistance in sputtered FeCuAg granular films

Abstract Magnetization and magnetoresistance studies were carried out on the granular FexCuyAgz films prepared by dc magnetron sputtering. In this series of samples, we have substituted Ag in the parent Fe–Cu system such that the Ag concentration increases from zero while that of the Cu decreases to zero when we go from one sample to another. The blocking temperatures of the films lie in the range 20–50 K. By using a model, the room temperature magnetization data have been analyzed by determining the contributions from the smaller superparamagnetic particles and larger ferromagnetic particles. Size distributions of the superparamagnetic particles are obtained. The magnetoresistance and (M/Ms)2 data do not agree for most of the samples investigated. This discrepancy is due to the fact that the (M/Ms)2 behavior is valid for purely superparamagnetic particles of only one size whereas there are both superparamagnetic and ferromagnetic particles in our samples. When we analyzed the magnetoresistance data using a superparamagnetic model, a good agreement between the experimental data and the simulated data were obtained for some samples. Significant discrepancy observed for other samples may be due to the spin dependent scattering of the electrons traveling from superparamagnetic to ferromagnetic particles or vice versa.

Investigation of Li-doped MgB2

We present a systematic characterization ofMg1−xLixB2 polycrystalline samples with nominal Li contentx up to 0.30. We explore the effectiveness of the substitution and investigate its influence onsuperconducting and normal state properties. The structural analyses by neutron and x-raydiffraction indicate that, despite the lattice parameters remaining unchanged within theexperimental accuracy, around 30% of the nominal Li content actually enters the structure.Also the transition temperature is only weakly affected by Li substitution, but its relationshipwith the residual resistivity and magnetoresistivity data is compatible with a picture whereπ bands are filled with holes and/or affected by disorder, as predictedby theory. We also measured the magnetic penetration depthλ by an inductance technique. Data fits based on the standardBardeen–Cooper–Schrieffer two-band model yield the zero-temperature limit of bothλ and superconductinggaps: we find that λ(0) increases weakly and quickly saturates with increasingx, whereasΔπ(0) andΔσ(0) decreasewith x. This analysis suggests that lithium substitution induces disorder mainly in theπ band. Point contact spectroscopy results on samples obtained from the same batch are infull agreement with magnetic penetration depth data on the undoped sample, but givesuperconducting gaps almost insensitive to Li substitution, showing at most a small increase inΔπ(0) that may be related to an inhomogeneous distribution of Li content.

Superconducting characteristics of 4-A carbon nanotube-zeolite composite.

We have fabricated nanocomposites consisting of 4-A carbon nanotubes embedded in the 0.7-nm pores of aluminophosphate-five (AFI) zeolite that display a superconducting specific heat transition at 15 K. MicroRaman spectra of the samples show strong and spatially uniform radial breathing mode (RBM) signals at 510 cm(-1) and 550 cm(-1), characteristic of the (4, 2) and (5, 0) nanotubes, respectively. The specific heat transition is suppressed at >2 T, with a temperature dependence characteristic of finite-size effects. Comparison with theory shows the behavior to be consistent with that of a type II BCS superconductor, characterized by a coherence length of 14 +/- 2 nm and a magnetic penetration length of 1.5 +/- 0.7 mum. Four probe and differential resistance measurements have also indicated a superconducting transition initiating at 15 K, but the magnetoresistance data indicate the superconducting network to be inhomogeneous, with a component being susceptible to magnetic fields below 3 T and other parts capable of withstanding a magnetic field of 5 T or beyond.

Magnetic and transport properties of Pr 2Pd 3Si 5

We have investigated the magnetic and transport properties of a new ternary intermetallic compound Pr 2Pd 3Si 5 which forms in U 2Co 3Si 5-type orthorhombic structure (space group Ibam). At low field (0.01 T) magnetic susceptibility exhibits an abrupt increase below 7 K and peaks at 5 K, revealing a magnetic phase transition. The onset of magnetic order is also confirmed by well defined anomalies in the specific heat and electrical resistivity data. Apart from the sharp λ-type anomaly, magnetic part of specific heat also shows a broad Schottky-type hump due to crystal field effect. Magnetoresistance data as a function of temperature exhibits a pronounced peak in paramagnetic state which could be interpreted in terms of crystal field effect and short-range ferromagnetic correlations.

Magnetic, transport and thermal studies in the zero magnetization alloy Nd0.75Ho0.25Al2

The result of dc and ac magnetization, heat capacity and magneto-resistance measurements in the polycrystalline and single crystal samples at the stoichiometry, Nd0.75Ho0.25Al2, are presented. Magnetic compensation between the antiferromagnetically coupled Nd and Ho moments in nominal zero field, and the field induced reorientation of the Nd/Ho moments in H = 5 kOe are observed to happen at the same temperature for H ∥ [100], [110] and [111] directions. The ac magnetization data also appear to imprint the field-induced moment reversal process in polycrystalline as well as in a single crystal sample (for H ∥ [100]). Intriguingly, the magneto-resistivity data in H = 10 kOe show three sign reversals below Tc.

Complex conductivity of UTX compounds in high magnetic fields

We have performed rf-skin depth (complex-conductivity) and magnetoresistance measurements of antiferromagnetic UTX compounds (T=Ni and X=Al, Ga, Ge) in applied magnetic fields up to 60 T applied parallel to the easy directions. The rf penetration depth was measured by coupling the sample to the inductive element of a resonant tank circuit and then, measuring the shifts in the resonant frequency Δf of the circuit. Shifts in the resonant frequency Δf are known to be proportional to the skin depth of the sample and we find a direct correspondence between the features in Δf and magnetoresistance. Several first-order metamagnetic transitions, which are accompanied by a drastic change in Δf, were observed in these compounds. In general, the complex-conductivity results are consistent with magnetoresistance data.

The magnetic state of 1212-type ruthenocuprate in magnetocaloricand magnetoresistivity measurements of polycrystalline samples ofRuSr2Gd1−xCexCu2O8 and Ru1−xSr2GdCu2O8

The magnetic properties of superconductingRu1−xSr2GdCu2O8 (x = 0, 0.02) andnon-superconducting RuSr2Gd1−xCexCu2O8 (x = 0.07,0.1) were investigated by means of magnetocaloric experiments with complementarymagnetoresistivity and ac susceptibility measurements. The isothermal magnetocaloric coefficientMT(B) assumes positive values in a broad range of temperatures(20 K≤T≤231 K) andmagnetic fields (0 T≤B≤13 T), i.e. also in the magnetically ordered state(Tm = 132 K forRuSr2GdCu2O8 andTm = 150 K forRuSr2Gd0.93Ce0.07Cu2O8) whichindicates no gain in the system’s magnetic entropy with increasing magnetic field. The maximum in theMT(B) dependence wasobserved for RuSr2GdCu2O8 in thetemperature vicinity of Tm, which indicates a ferromagnetic character of the accessed magneticcorrelations. No spontaneous ferromagnetic order was revealed as theMT assumes limiting zero values at zero magnetic field for the whole rangeof investigated temperatures. Temperature dependences of the specificheat reveal the magnetic-field-induced positive temperature shift of theanomaly associated with the magnetic transition in the Ru spin system. TheMT(B) dependences and the magnetoresistivity data suggest that the magnetic system may beinhomogeneous.

Multidiscipline Modeling in Materials and Structures

A simple interaction‐potential model has been established to calculate the higher order elastic constants of intermetallic YbAl2 in the temperature range from 10‐300K. Temperature dependent second and third order elastic constants are used for the determination of the ultrasonic attenuation, velocity, Grüneisen numbers, Acoustic‐coupling constants, and thermal relaxation time at the different temperatures. Temperature dependency of the ultrasonic properties of YbAl2 is similar at low temperatures to that of pure metals and the low carrier heavy fermion systems ‐ LaSb, YbAs and YbP having simple NaCl‐type structures. Thermal energy density makes significant contribution to the total attenuation in the compound at the higher temperatures from 100‐300K. Effect of the magnetic field on the ultrasonic attenuation is also evaluated using the magneto resistance data. At 100K, the effect of the magnetic field becomes insignificant. The attenuation decreases with the field at 3K to 50K.

Ultrasonic Study of Intermediate-Valent Intermetallic YbAl<SUB>2</SUB> at Different Physical Conditions

A simple interaction-potential model has been established to calculate the higher order elastic constants of intermetallic YbAl 2 in the temperature range from 10-300K. Temperature dependent second and third order elastic constants are used for the determination of the ultrasonic attenuation, velocity, Gruneisen numbers, Acoustic-coupling constants, and thermal relaxation time at the different temperatures. Temperature dependency of the ultrasonic properties of YbAl 2 is similar at low temperatures to that of pure metals and the low carrier heavy fermion systems- LaSb, YbAs and YbP having simple NaCl-type structures. Thermal energy density makes significant contribution to the total attenuation in the compound at the higher temperatures from 100-300K. Effect of the magnetic field on the ultrasonic attenuation is also evaluated using the magneto resistance data. At 100K, the effect of the magnetic field becomes insignificant. The attenuation decreases with the field at 3K to 50K.

High-quality quantum point contact in two-dimensional GaAs (311)A hole system

We studied ballistic transport across a quantum point contact (QPC) defined in a high-quality GaAs (311)A two-dimensional hole system using shallow etching and top gating. The QPC conductance exhibits up to 11 quantized plateaus. The ballistic one-dimensional subbands are tuned by changing the lateral confinement and the Fermi energy of the holes in the QPC. We demonstrate that the positions of the plateaus (in gate voltage), the source-drain data, and the negative magnetoresistance data can be understood in a simple model that takes into account the variation, with gate bias, of the hole density and the width of the QPC conducting channel.

Thin Cr2O3 Films for Magnetoelectric Data Storage Deposited by Reactive E-beam Evaporation

Magnetoelectric materials are proposed for use in magnetic memory applications. The main idea of the Magnetoelectric Random Access Memory (MERAM) is to enable electrical data writing in a magnetoelectric antiferromagnet, while the principle of the magnetoresistive data reading remains. As a possible medium for magnetoelectric data storage Cr 2 O 3 is proposed. Thin films of Cr 2 O 3 have been grown on a [0001] sapphire substrate by reactive e-beam evaporation of Cr 2 O 3 in an O 2 atmosphere. The out-of plane orientation of the thin Cr 2 O 3 films and the chemical environment of Cr 3+ ions are evidenced by X-Ray diffractometry and absorption spectroscopy. Surface characterization has been done by atomic force microscopy. At decreasing temperature the conductivity shows an exponentional decrease, which is probably due to hopping via small amounts of Cr 3 O 4 .

Indications for intrinsic superconductivity in highly oriented pyrolytic graphite

High-resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest nonhomogenous superconductivity with critical temperature ${T}_{c}\ensuremath{\sim}25\text{ }\text{K}$ and higher temperature. Our claim is based mainly in the observation of anomalous hysteresis loops of resistance versus magnetic field that cannot be assigned to magnetic irreversibility but indicates the existence of Josephson-coupled superconducting grains. In addition we observe quantum resonances that can be assigned to Andreev reflections and the absence of Schubnikov de Haas oscillations. The results indicate that graphite is a system with nonpercolative superconducting domains immersed in a semiconductinglike matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different type of carriers with different masses are present.

Magnetotransport of manganite superlattices: Investigating the role of a magnetic insulating spacer

Magnetotransport data are presented for a set of manganite superlattices comprising La2∕3Sr1∕3MnO3 as the ferromagnetic (FM) metallic layers (thickness 11.9nm) and Pr2∕3Ca1∕3MnO3 as the antiferromagnetic insulating spacer (IS) with thicknesses tIS between 0.8 and 7.6nm. Previous studies on this system showed the presence of FM inclusions within the IS, with a notable magnetization peak at tIS∼3nm. Now, we show that this peak is also accompanied by a maximum in the magnetoresistance (MR). Moreover, the MR data of all the samples scale in a single curve, MR∝MIS2, where MIS is the magnetization of the IS. These results provide a quantitative demonstration of the feasibility to improve the response of engineered MR devices by including magnetically active IS instead of the usual nonmagnetic compounds.

Inhomogeneity effects on magnetic and magnetotransport properties of CoxCu100−x films produced by ultrashort pulsed laser deposition

Films produced by means of ultrashort pulsed laser deposition (uPLD) and constituted of Co nanoparticles (NPs) mixed with Cu ones exhibit peculiar morphological and topological properties. In particular, the NPs obtained by uPLD retain their individuality with a moderate coalescence which permits interparticle discontinuities. Due to this condition, the NP interface contributes significantly to the electron transport mechanisms depending on the magnetizing field. As a consequence, magnetoresistance effects are evidenced which do not saturate up to external magnetizing fields much higher than the value required for the saturation of the macroscopic magnetization. Moreover, when the cobalt volume fraction (x) is lower than 35%, the set of magnetic, resistive and magnetoresistive data shows that the magnetic percolation among the Co particles is not completely obtained and a transport mechanism of the giant magnetoresistance (GMR) kind is active in the uPLD films, notwithstanding the fact that the average particle size is higher in comparison with that expected for GMR optimization. On the other hand, if the contact among Co particles is obtained (x ⩾ 50%) the anisotropic magnetoresistance (AMR) becomes the prominent effect. In this circumstance, the presence of interparticle discontinuities is still deducible by the experimental evidence of a negative derivative of the magnetoresistance ratio versus the applied magnetic field. In the same x range (x ⩾ 50%), the AMR increases with the Co content, while zero field resistivity does not change.

Hopping Conductivity Spectroscopy of Carbon Nanocluster Materials

Procedure of the hopping conductivity ρ∼exp[(T 0/T)α] and magnetoresistance data analysis, which takes into account spin‐polarization contribution and allows finding correct parameters of the localized states, is suggested. This general approach was checked for the nanocluster material synthesized under high (up to 8 GPa) pressure of carbon SWNT mat containing 5% Ni. Experiments for the temperature/field domain 1.8–300 K/70 kOe have confirmed theoretical results, including universal magnetoresistance scaling ln[ρ(H)/ρ(0)] = (T 0/T)α F(H/T). It is found that the interaction with magnetic impurities renormalizes the electron magnetic moment: μ≈0.7μB. In conclusion, a possible way of creation of the carbon nanocluster material with colossal magnetoresistance is suggested.

Evidence for cluster glass behavior inFe2VAlHeusler alloys

Detailed magnetic measurements on intermetallic ${\text{Fe}}_{2}\text{VAl}$ Heusler type alloys are presented. Annealed ${\text{Fe}}_{2}\text{VAl}$ sample is structurally better ordered and exhibits higher magnetization, resistivity and magnetoresistance values compared to as cast samples. From the systematic analysis of magnetization and electrical transport data it has been established that the ${\text{Fe}}_{2}\text{VAl}$ behaves more like a cluster glass. Magnetization as a function of magnetic field at various temperatures suggests the presence of superparamagnetic clusters along with paramagnetic background. Arrot plots confirm the absence of spontaneous magnetic moment and hence long-range ferromagnetic order down to 5 K. The field cooled and zero-field-cooled magnetization curves suggest the superparamagnetic entities freeze below 10 K. Our magnetoresistance data corroborate the magnetization results and confirm the presence of magnetic clusters.