Magnetic Field Dependence Of Resistivity Research Articles

Magneto-transport Properties of Gd-doped In2O3 Thin Films

AbstractDilute Magnetic Semiconductors (DMS) are a rare group of promising materials that utilize both the electronic charge - a characteristic of semiconductor materials - and the electronic spin - a characteristic of magnetic materials. Oxide based DMS show promise of ferromagnetism (FM) at room temperature. It has been found that doping metal oxides such as ZnO, TiO2, and In2O3 with magnetic ions such as Fe, Co, Mn, and Cr produces DMS, which exhibit FM above room temperature. In2O3, a transparent opto-electronic material, is an interesting prospect for spintronics due to a unique combination of magnetic, electrical, and optical properties. High quality thin films of rare earth magnetic gadolinium (Gd) doped oxide-based DMS materials have been grown by pulsed laser deposition (PLD) technique on various substrates such as single crystal of sapphire (001) and quartz under suitable growth conditions of substrate temperature and oxygen pressure in the PLD chamber. The effect of rare earth magnetic doping on the structural and electro - magnetic properties of these films has been studied using Raman Spectroscopy, X-Ray Diffraction, Scanning Electron Microscopy, and Magneto - Transport. An X- ray diffraction study reveals that these films are single phase and highly oriented. Characteristic Raman peaks typical of indium oxide are observed at 496 and 627 cm−1. We have observed high magnetoresistance (∼18 %) at a relatively small field of 1.3 Tesla for the films with 10 % gadolinium. A detailed study of temperature and magnetic field dependent resistivity, magnetoresistance, and Hall Effect will be presented.

Anomalous Magnetoresistance in the π-d System (DIETSe)2FeCl4

The low temperature transport properties of the π-d system (DIETSe)2MCl4 [M = Fe, Ga] are investigated under the high magnetic fields and high pressures. Anomalous behaviors in the magnetic field dependence of resistance are observed in the magnetic Fe salt below about 2 K, while such anomalous behaviors are absent in the nonmagnetic Ga salt, suggesting the strong interaction between the itinerant π-electrons and the localized moments of d-electrons in (DIETSe)2FeCl4.

Interplay of charge and spin ordering in Pr 0.65Ca 0.35− xSr xMnO 3

Different phase transitions are observed in polycrystalline Pr 0.65Ca 0.35− x Sr x MnO 3 ( x = 0–0.35) manganites from the temperature (10–300 K) and magnetic field (0–8 T) dependent resistivity and magnetization data. Replacement of Ca by Sr induces the ferromagnetic metallic (FMM) phase by the melting of the antiferromagnetic/charge ordered insulating (AF/CO-I) phases. The magnetoresistance ratio (MRR) is found to be maximum in samples with coexistent FMM and AF/CO-I phases and minimum in samples with either FMM or AF/CO-I phase. Thermal and magnetic hysteresis of resistivity as well as magnetization can be well explained on the basis of the interplay of charge and spin ordering.

Kinetic properties of an La0.85Ba0.15MnO3 single crystal

We report the temperature and magnetic field dependence of resistivity, Hall effect, and thermopower ofan La0.85Ba0.15MnO3 single crystal. It is found that the Fermi level lies in the energy gap and conduction is byhopping. In the ferromagnetic state the variable range hopping prevails, while in theparamagnetic state hopping between nearest neighbours dominates. The small value of theactivation energy of thermopower is explained by a weak asymmetry of density of statesnear the Fermi energy.

Radiation damage effects in CMR manganite materials

Polycrystalline La0.5Pb0.5Mn1−xCrxO3 (x=0.075 and 0.15) samples have been irradiated with 50MeV Li3+ ions with different fluences and the effects on the transport properties have been studied by means of the temperature and magnetic field dependent resistivity measurements. Due to Li3+ ion irradiation, the resistivity increases and the metal–insulator transition temperature (Tmi) decreases. At low temperatures (below Tmi), a dominant contribution of the electron–magnon scattering process is observed for all the irradiated and unirradiated samples. The low temperature resistivity behavior as well as the magnetoresistance is modified due to irradiation. The changes in the magnetotransport properties due to irradiation have been compared with the changes caused due to Mn site substitution.

Observation of a near 100% magnetoresistance in (LaBi)CaMnO

Abstract The electrical transport properties of (La 0.8 Bi 0.2 ) 0.67 Ca 0.33 MnO 3 was studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T, respectively. An obvious difference in temperature and magnetic field dependence of resistivity between the (La 0.8 Bi 0.2 ) 0.67 Ca 0.33 MnO 3 and pure La 0.67 Ca 0.33 MnO 3 was observed. Especially, an enhanced magnetoresistance effect was obtained in the (La 0.8 Bi 0.2 ) 0.67 Ca 0.33 MnO 3 , such as, a near 100% magnetoresistance plateau within a temperature ranging from 10 to 180 K was observed in a 3 T field. The unusual large magnetoresistance value is encouraging as regard to the potential application of CMR materials. On the other hand, a significant hysteresis both in temperature and magnetic field dependence resistivity is simultaneously observed in the Bi-doped sample.

Electronic transport in ferromagneticLa1−xSrxMnO3single-crystal manganites

We report results of integrated study of resistivity, magnetoresistance, Hall effect, thermopower, and magnetothermopower in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals with $x=0.15,0.20,0.25.$ The focus is on the vicinity of the Curie temperature. It is shown that near ${T}_{C}$ where the colossal magnetoresistance is observed the crystals are in insulator state. It is established that the conductivity in the manganites with different level of doping differs in nature. Near ${T}_{C},$ temperature and magnetic-field dependences of resistivity arise due to change of activation energy, which is linear in squared magnetization. The metal-insulator transition in the $x=0.20$ and 0.25 manganites occurs not at a certain temperature but in a temperature interval about 80 K wide.

Evidence for the formation of GaAs homojunction far-infrared detection by magnetic fielddependent Hall analysis

Experimental magnetic field dependent Hall and resistivity data are presented for n-GaAshomojunction far-infrared (FIR) detector structures in the temperature rangefrom 1.8 to 200.0 K and with a magnetic field up to 15 T. The carrier transportproperties of the multilayer/multicarrier structure have been separated and quantifiedindividually by a hybrid approach consisting of mobility spectrum analysis followed by amulticarrier fitting procedure. The observed temperature dependence of mobility andconcentration is explained using classical band theory, which unambiguously revealshighly doped degenerate contact layers and semiconductor–metal transition inemitter layers. The detailed analysis of the concentration and the Fermi level in theemitter layers gives clear evidence for the formation of GaAs homojunction FIRdetection occurring at the interfaces between the emitter and intrinsic layers.

Are the interaction effects responsible for the temperature and magnetic field dependent conductivity in Si‐MOSFETs?

AbstractWe compare the temperature and in‐plane magnetic field dependences of resistivity ρ (T, B∥) of Si MOSFETs with the recent theories. In the comparison we use the effective mass m* and g*‐factor determined independently. An anomalous increase of ρ with temperature, which has been considered a signature of the “metallic” state, for high conductivities (σ > e2/h) can be described quantitatively by the interaction effects in the ballistic regime. ρ(B∥) is consistent with the theory only qualitatively; it is found to be more susceptible than ρ(T) to details of disorder, possibly associated with magnetic moments of localized states. Description of the transport in the “critical” regime of ρ ∼ h/e2 is still lacking. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetotransport properties of alkali metal doped La–Ca–Mn–O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase

Pulsed magnetic field (0–4.4 T) was used to study the magnetic field dependent resistivity (12–350 K) and thermoelectric power (0–1.5 T) of Na and K-doped La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80–300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (Tp<T<θD/2, θD being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>θD/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (T<Tp) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein’s condition of the adiabatic “small polaron” hopping conduction mechanism for the region T>Tp.

Quantitative analysis of interface resistance in Co/Nb multilayers for normal and superconducting Nb

The thickness and magnetic field dependence of resistance in Co/Nb multilayers with current perpendicular to plane geometry were measured at 4.2 K. When the number of bilayers increased, the resistance increased linearly. This linear behavior enables us to use a series resistance model to analyze our data for the thicknesses we used, 20 and 80 nm for Nb. When sandwiched between Co, Nb is a normal metal when thinner than 30 nm, otherwise it is superconducting. Assuming bulk resistivities for normal Nb and Co, we got unit area resistance values of 6.3 and 3.5 fΩ m2 for superconducting and normal Nb/Co interfaces. There was no magnetoresistance observed, at a resolution of about 0.1%. The applicability of a diffusive regime model is discussed.

Effect of low Fe3O4 doping in La0.67Ca0.33MnO3

The electrical and magnetic transport behaviour of composite samples of (1−x)La0.67Ca0.33MnO3+xFe3O4 with x = 0, 0.1%, 1% and 2% were studied in a temperature interval 10–300 K and for magnetic fields H = 0, 0.3, 0.5, 1 and 3 T. The temperature and magnetic field dependence of resistivity of composites show that the low Fe3O4 doping levels have important effects on electrical and magnetic transport behaviour of La0.67Ca0.33MnO3. Especially, compared to pure La0.67Ca0.33MnO3, a new insulator–metal (I–M) transition was observed at a lower temperature TP2 in all Fe3O4 doped La0.67Ca0.33MnO3 composites, which may result from the existence of a new phase related to Fe3O4 dopant at grain boundaries and surfaces of La0.67Ca0.33MnO3 phases. It is of interest to note that the new I–M transition temperature, TP2, is almost independent of the magnetic field. Meanwhile, an obvious magnetoresistance effect was observed between temperatures TP1 and TP2. The results were discussed by considering the results of scanning electron microscopy, x-ray diffraction, dielectric relaxation and susceptibility analysis.

Temperature dependences of resistivity and magnetoresistivity for half-metallic ferromagnets

Peculiarities of transport properties of three- and two-dimensional half-metallic ferromagnets are investigated, which are connected with the absence of spin-flip scattering processes. The temperature and magnetic field dependences of resistivity in various regimes are calculated. The resistivity is proportional to T 9/2 for T < T * and to T 7/2 for T > T *, T* being the crossover temperature for longitudinal scattering processes. The latter scale plays also an important role in magnetoresistance. The contribution of non-quasiparticle (incoherent) states to the transport properties is discussed. It is shown that they can dominate in the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity.

Substitution dependence of magnetoresistance in sol–gel-derived La 2/3− xTl xCa 1/3MnO 3− δ

We have fabricated a series of bulk samples of La 2/3− x Tl x Ca 1/3MnO 3− δ (0.0⩽ x⩽0.20) by sol–gel process and studied their magnetization M and magnetic field dependence of resistivity in the temperature range 77⩽ T⩽300 K. The MR properties of these specimens were characterized to be broad, rather than a sharp as in a typical CMR material, which was explained by magnetic inhomogeneity.

Electronic properties of heteroepitaxial undoped and n-InSb epilayers using SnTe source by molecular beam epitaxy

We report on the electrical characteristics of InSb and n-type doping of InSb layers grown on GaAs substrates using a SnTe captive source by molecular beam epitaxy (MBE). The undoped epilayers are n-type in the temperature range of 10 to 300 K investigated. Doped layer with carrier concentrations ranging from 2×1016/cm3 to 3.2×1018/cm3 with corresponding x-ray full width at half maxima varying from 170–200 arcsec have been achieved. High carrier mobility of 94 098 cm2/V s on lightly doped samples has been achieved. These results suggest SnTe source as being one of the donor dopants of choice for MBE grown InSb epilayers. Temperature and magnetic field dependent Hall and resistivity measurements with various multicarrier conduction analysis techniques indicate three conduction channels for undoped InSb and two conduction channels for doped InSb. They have been used successfully to explain the temperature and thickness dependence of the electrical properties of MBE grown undoped and doped InSb epilayers.

Particle size and magnetic field dependent resistivity and thermoelectric power of La0.5Pb0.5MnO3 above and below metal–insulator transition

The effect of particle size on the transport properties (resistivity and thermopower) of La0.5Pb0.5MnO3 has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature Tp of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T&amp;gt;θD/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy WH decreases but polaron radius rp increases with the increase of grain size. In the metallic regime (for T&amp;lt;Tp), resistivity data fit well with ρ=ρ0+ρ2.5 T2.5 and the transport mechanism is attributed mainly to the magnon-carrier scattering (∼T2.5). In all the samples with different grain sizes, S changes sign below Tp. In contrast to magnetoresistance, application of magnetic field increases S at low temperature (T&amp;lt;Tp) for these samples. Thermopower data in the metallic phase (both for B=0.0 and 1.5 T) can be analyzed by considering a spin-wave fluctuation term (∼T4) in addition to the magnon-scattering term similar to the case of resistivity data. Although the variable range hopping mechanism is supported from the resistivity data (for Tp&amp;gt;T&amp;gt;θD/2), it is hard to justify this model from the temperature dependent thermopower data.

Development and physical properties of magnetic organic superconductors based on BETS molecules [BETS=Bis(ethylenedithio)tetraselenafulvalene

Owing to the coupling between π and d electron systems, organic conductors based on BETS molecules and magnetic anions such as FeCl 4 − and FeBr 4 − exhibit very unique physical properties. The orthorhombic κ-(BETS) 2FeBr 4 system is the first antiferromagnetic organic superconductor. Due to the metamagnetic nature of Fe spin system, the magnetic field dependence of resistivity shows a strong anisotropy in the ac conduction plane below T c. On the other hand, the triclinic modification, λ-(BETS) 2FeCl 4 undergoes coupled antiferromagnetic and metal–insulator (MI) transitions at 8.5 K. The easy axis of the antiferromagnetic spin structure was found to be deviated from the c axis (∥ needle axis of the crystal) by 30–35°. The field-induced insulator-to-metal transition and pressure-induced superconducting transition of λ-(BETS) 2FeCl 4 were briefly described from the viewpoint of the role of coupling between π and d electron systems.

Adiabatic and non-adiabatic hopping conduction in La–Pb–Mn–O type system

Magnetic field dependent resistivity ( ρ) and Seebeck coefficient (S) of La 1− x Pb x MnO 3 (x=0.0–0.5), prepared by sol–gel method, have been reported. One set (LaMnO 3+ δ ) sample, termed as LCON, was found to show insulator (semiconductor) to metal transition (MIT) around T p∼265 K and the other set (LaMnO 3), termed as LSEM, did not show such transition. T p shifts to the lower temperature under magnetic field. It has been shown, for the first time, that both in presence or in absence of magnetic field, two different transport mechanisms (adiabatic and non-adiabatic) are followed, respectively, by LSEM and LCON (for T> T p). Like resistivity, S of the sample was also found to depend on the applied magnetic field particularly around T p. The samples showing no MIT indicated larger Debye temperatures and electron–phonon interaction constants ( γ p) than those of the sample showing MIT. Both ρ and S of La 1− x Pb x MnO 3 (x=0.0–0.5) behave similarly to those of LCON.

Nonlinear microwave response of Nb/sub 3/Sn films: a case study of granular superconductors

We have investigated the microwave magnetic field dependent surface resistance, R/sub s/(B/sub s/), of Nb/sub 3/Sn films of different film thickness and on different substrates at 19 GHz. The film thickness controls the grain size of the polycrystalline films. It provides a tool to study the role of grain boundaries on the nonlinear microwave response of granular superconductors. We find a non-monotonic dependence of the onset field, B/sub on/, of nonlinear surface resistance on film thickness, with a maximum of 25 mT at 1.2 /spl mu/m. This behaviour reveals that magnetic losses at grain boundaries dominate in thin films, and microwave heating in thick films. The universal scaling behaviour of B/sub on/ with temperature, its dependence on the thermal conductivity of the substrate, and the good agreement between the quadratic field dependence of R/sub s/ and a weakly-coupled grain model indicate heating at grain boundaries as a possible mechanism. The correlation between the nonlinear microwave response and microstructure of the granular films indicates possible improvements and intrinsic limitations.

Thermoelectric and Thermogalvanomagnetic Properties of NiBi3

We present some results of an experimental study on transport properties of poly-crystalline NiBi3. Resistivity, thermoelectric power and the Nernst coefficient were measured in a magnetic field up to 1.2 T between 100 and 300 K. A large magnetoresistance was observed and it was revealed that the behavior of thermoelectric power changed markedly upon the application of a magnetic field. The carrier mobility was determined to be 0.446 m2V-1s-1 at room temperature and the Fermi energy was estimated to be 0.643 eV. We obtained an expression ρ= ρ0 + d(H) exp (-(T0(H) / T)1.36H2+1) as the temperature and magnetic field dependence of resistivity.