Low-temperature Resistivity Minimum Research Articles

Effect of A-site disorder and reduced grain size on low temperature resistivity minima and magnetoresistance behaviour in Nd0.7-xGdxSr0.3MnO3 (0 ≤ x ≤ 0.3)

ABSTRACT The present work provides a detailed investigation of the electronic- and magneto-transport properties of bulk and nanostructured Nd0.7-xGdxSr0.3MnO3 (x = 0, 0.1, 0.2 and 0.3) polycrystalline manganites. We have specially addressed the issue of the observed minimum in the temperature dependent resistivity of these manganites in the low temperature regime (<50 K). It is observed that increase of A-site disorder due to progressive substitution of Gd and decrease of grain size enhances the observed resistivity minima. Different theoretical models have been employed to analyze the resistivity data in the low temperature regime. It has been established that the quantum interference effect due to electron–electron interaction, most likely, is the dominant factor of the observed resistivity minimum and upturn in resistivity in the lower temperature regime. The effect of phase separation is also found to play a key role in the electronic-transport of the materials. Further the magnetoresistance is found to increase with the increase of Gd concentration as well as with the decrease of grain size in the low temperature regime. The origin of magnetoresistance has been critically analyzed.

The Origin of the Low-Temperature Minimum of Electrical Resistivity in Strontium Ferromolybdate Ceramics

In this work, we analyze the electrical behavior of strontium ferromolybdate below room temperature. We demonstrate that in SFMO ceramics, SFMO thin films deposited by pulsed laser deposition including (100) and (111) textured thin films, as well as in nonstoichiometric SFMO ceramics, an intergrain tunneling mechanism of charge carrier conduction leads to a decrease in resistivity with increasing temperature in the low-temperature region. This intergrain tunneling can be attributed to fluctuation-induced tunneling. On the other hand, bulk metallic resistivity of the grains, which increases with temperature, becomes dominant at higher temperatures and magnetic fluxes. The interplay of these conduction mechanisms leads to a resistivity minimum, i.e., a resistivity upturn below the temperature of minimum resistivity. Several mechanisms have been discussed in the literature to describe the low-temperature upturn in resistivity. Based on available literature data, we propose a revised model describing the appearance of a low-temperature resistivity minimum in SFMO ceramics by an interplay of fluctuation-induced tunneling and metallic conductivity. Additionally, we obtained that in the region of metallic conductivity at higher temperatures and magnetic fluxes, the pre-factor Rm of the temperature-dependent term of metallic conductivity written as a power law decreases exponentially with the temperature exponent m of this power law. Here, the value of m is determined by the charge scattering mechanism.

Interfacial quantum interference effect and dual magnetoresistance in La0.7Sr0.3MnO3 thin films grown on (001) Si

Transmission electron microscope image and electronic transport of La0.7Sr0.3MnO3 (LSMO) films grown on (001) oriented Si using the sputtered pulsed plasma method confirmed the presence of around 8 nm thick, less dense, and highly resistive LSMO at the interface below the conducting phase. Thicker LSMO films, in addition to metal-insulator transition, show an anomaly around the Curie temperature in temperature-dependent resistivity and magnetoresistance (MR), which is a unique observation. The conduction in these LSMO films at temperatures below low-temperature resistivity minimum is dominated by Kondo-like scattering over electron–electron scattering, established using the phenomenological model. At 20 K, the maximum positive MR is ∼ 12% for the in-plane field, while it is ∼ 7.2% for the out-of-plane field. The maximum negative in-plane MR is found to be ∼ 42.5%, which becomes ∼ 30% when the orientation of the field changes to the out-of-plane direction. The two-dimensional field-dependent change in the magneto-conductance model evidenced the quantum interference effect (QIE). The existence of QIE is associated with magnetic scattering and scattering due to spin–orbit coupling. These results may be used to modulate the electrical properties of future electronic devices and can encourage scientists to explore the multi-functionalities of complex oxides grown on bare Si substrates.

Nature of electrons from oxygen vacancies and polar catastrophe at LaAlO3/SrTiO3 interfaces

The relative significance of quantum conductivity correction and magnetic nature of electrons in understanding the intriguing low-temperature resistivity minimum and negative magnetoresistance (MR) of the two-dimensional electron gas at LaAlO3/SrTiO3 interfaces has been a long outstanding issue since its discovery. Here we report a comparative magnetotransport study on amorphous and oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures at a relatively high-temperature range, where the orbital scattering is largely suppressed by thermal fluctuations. Despite of a predominantly negative out-of-plane MR effect for both, the magnetotransport is isotropic for amorphous LaAlO3/SrTiO3 while strongly anisotropic and well falls into a two-dimensional quantum correction frame for annealed crystalline LaAlO3/SrTiO3. These results clearly indicate that a large portion of electrons from oxygen vacancies are localized at low temperatures, serving as magnetic centers, while the electrons from the polar field are only weakly localized due to constructive interference between time-reversed electron paths in the clean limit and no signature of magnetic nature is visible.

Low-Temperature Tunneling Electroresistance in Ferromagnetic Metal/Ferroelectric/Semiconductor Tunnel Junctions.

Ferroelectric tunnel junctions (FTJs) as artificial synaptic devices are promising candidates for the building block of nonvolatile data storage devices. However, a small ON/OFF ratio of FTJs limits their application in low-temperature operations. In this work, the influence of quantum interference effects on tunneling electroresistance in the La0.7Sr0.3MnO3/BaTiO3/Nb:SrTiO3 (ferromagnetic metal/ferroelectric/semiconductor) FTJ at low temperatures is investigated. The Current-voltage curves are observed in the tunnel junction from 300 to 10 K with a six-unit-cell thick BaTiO3 film by the ferroelectric polarization effect. First, the ON/OFF current ratio increases from 300 to 30 K due to the increase of polarization in the ferroelectric barrier, and then, it gradually decreases when the temperature drops below 30 K. An anomalous ON/OFF current ratio of ∼105 is obtained at 30 K. The low-temperature tunneling properties in the FTJ are associated with a low-temperature resistivity minimum in the ferromagnetic metal layer by the electron-electron interaction, which increases the La0.7Sr0.3MnO3/BaTiO3 interface resistance, leading to a higher resistance state and lower IOFF for the OFF state. As a result, the ON/OFF current ratio is abruptly enhanced at 30 K. Our results emphasize the crucial role of transport properties of La0.7Sr0.3MnO3 in FTJs and pave the way for the design and application of FTJs at low temperatures.

A comparative study in different rare earth ions in multiferroic nanocomposites: Low temperature resistivity minima and low field magnetoresistance

Low temperature magnetoresistance and resistivity properties are systematically studied in rare earth based multiferroic nanocomposites La0.7Sr0.3MnO3-TMnO3 (T = Er,Ho) under magnetic fields up to 5 Tesla. Resistivity with temperature exhibits metal-insulator transition. At very low temperature, the sharp upturn of resistivity is observed, which is found to be suppressed by field. The upturn of resistivity is characterized with spin fluctuation of the system. The reduction of the upturn of resistivity with magnetic field ensures the progress of the electrons tunneling. Effective reduction of charging energy is obtained upon an applied magnetic field. Temperature dependent resistivity shows minima at low temperature under different applied magnetic fields. The resistivity fit at low temperature is made with consideration of equally electron-electron interactions concerning (temperature)1/2 term and the Kondo-identical spin dependent scattering for ln(temperature) dependence. Lessening of the spin fluctuation by an applied magnetic field plays a dynamical role in defining the magnetoresistive behavior in samples. Experimental data are analyzed and demonstrated through a simple proposed phenomenological model to provide an insight on the physical mechanism behind the low temperature transport phenomena. The derived surface spin susceptibility is accordance with the temperature dependence of magnetoresistance involving an intergranular surface magnetization. Temperature dependent field cooled and zero field cooled dc magnetization curve having a large bifurcation between them, is a suggestion of a glassy like phase that may be originated from the challenging magnetic exchange interactions. The low temperature glassy magnetic behavior is endorsed to a spin-glass identical frozen state for the surface of the nanoparticles. Magnetic field dependent magnetization study shows an increase in magnetization without saturation in decreasing temperature. The multiferroic nanocomposites exhibit the magnetoelectric coupling, which arises due to the strain mediated magnetostriction behavior of the piezomagnetic phase (La0.7Sr0.3MnO3) in the nanocomposites.

Transport properties and electroresistance of manganite based heterostructure

In the present communication, ZnO/La0.7Sr0.3MnO3/Al2O3 (ZnO/LSMO/Al2O3) heterostructure was grown using chemical solution deposition (CSD) technique. X–ray diffraction (XRD) measurement confirms the (100) crystallographic oriented growth of LSMO manganite and polycrystalline hexagonal growth of ZnO layer. Temperature dependent resistivity behavior under different applied magnetic fields, performed for two different geometries [current in plane (CIP) and current perpendicular to plane (CPP)], suggests the modifications in metal to insulator transition temperature TP and alterations in resistivity with applied magnetic field and measurement geometry. This also implies the better charge conduction across the ZnO/LSMO interface (CPP) as compared to LSMO film layer (CIP). Various charge conduction mechanisms have been employed to understand the charge transport for observed low temperature resistivity minimum, metallic behavior, insulating state and magnetoresistance (MR) of LSMO film and ZnO/LSMO interface. Electroresistance (ER) and field effect configuration (FEC) have been investigated across the ZnO/LSMO interface by recording the LSMO channel resistance, at room temperature, that suggest the tuning of signature and value of ER by considering forward and reverse bias modes across the interface. FEC studies show the large ER ∼ +750% and −85% across the LSMO channel with its tunability under different interface biases.

Role of antimony in the charge transport mechanisms for La0.67Ca0.33Mn1–xSbxO3 manganites

Single phasic La0.67Ca0.33Mn1–xSbxO3 (LCMSO; x = 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10) samples were characterized by performing temperature and magnetic field dependent resistance measurements. Present study, mainly, aims for the better understanding of possible charge conduction mechanisms responsible for the low temperature resistivity and high temperature [well above metal to insulator transition temperature (TP)] semiconducting regions. Variation in resistivity and TP with Sb5+ content (x) and applied magnetic field has been discussed in the light of the modifications in structural and magnetic lattices of smaller diamagnetic Sb5+ doped LCMSO system. Various models and mechanisms have been theoretical employed to fit obtained experimental resistivity data for the low temperature resistivity and semiconducting regions of all LCMSO manganites. It is found that low temperature resistivity minima follows the coulomb blockade model while charge conduction in the semiconducting region obeys the variable range hopping (VRH) mechanism. Variation in low temperature blocking energy, activation energy in semiconducting region and magnetoresistance (MR) with Sb5+ content (x) and applied magnetic field has been discussed in detail.

Evidence of weak localization in quantum interference effects observed in epitaxial La0.7Sr0.3MnO3 ultrathin films.

Quantum interference effects (QIEs) dominate the appearance of low-temperature resistivity minimum in colossal magnetoresistance manganites. The T1/2 dependent resistivity under high magnetic field has been evidenced as electron-electron (e-e) interaction. However, the evidence of the other source of QIEs, weak localization (WL), still remains insufficient in manganites. Here we report on the direct experimental evidence of WL in QIEs observed in the single-crystal La0.7Sr0.3MnO3 (LSMO) ultrathin films deposited by laser molecular beam epitaxy. The sharp cusps around zero magnetic field in magnetoresistance measurements is unambiguously observed, which corresponds to the WL effect. This convincingly leads to the solid conclusion that the resistivity minima at low temperatures in single-crystal manganites are attributed to both the e-e interaction and the WL effect. Moreover, the temperature-dependent phase-coherence length corroborates the WL effect of LSMO ultrathin films is within a two-dimensional localization theory.

Magnetic and transport studies of core–shell nanorods based on metallic oxide ferromagnet SrRuO3

This study presents the synthesis of perovskite metal-semiconductor core–shell heterostructures by sputtering SrRuO3 (SRO) shell layer on vertically aligned hydrothermally produced ZnO nanorods. Compared to the two-dimensional SRO films, the magnetic behaviors of the SRO shells on ZnO nanorods are morphology and thickness dependent, as reflected by the magnetic isotropy effects and the appearance of double-step magnetic hysteresis loops caused by nanograin-induced disorder and uncompensated spin at the surface and interface. The appearance of low-temperature resistance minimum and the good fitting of the low-temperature resistance data to a theoretical model establish the emergence of weak localization effect in the SRO shells, whose strength can be reinforced by a magnetic field. In addition, an apparent low-resistance Ohmic contact was realized in the ZnO/SRO heterojunctions due to the lower work function of the SRO. This, together with the absence of the Schottky barrier at the interface, demonstrates that the ZnO/SRO nanostructures could hold great promise for applications in advanced electron field emitters.

Magnetic and magnetoresistance in half-doped manganite La0.5Ca0.5MnO3 and La0.5Ca0.4Ag0.1MnO3

La0.5Ca0.5MnO3 and La0.5Ca0.4Ag0.1MnO3 ceramics have been prepared by sol–gel methods and its magnetic properties, and electronic transport properties have been experimentally investigated. X-ray diffraction data reveal that all samples crystallize in a orthorhombic structure with Pnma space group. Magnetic measurements of La0.5Ca0.5MnO3 show multiple magnetic transitions, a paramagnetic to ferromagnetic and a ferromagnetic to charge-ordered antiferromagnetic. Two metal–semiconductor transitions (Tρ1 and Tρ2) are observed in the electrical resistivity. The electrical resistivity decreases when an external magnetic field is applied rendering a magnetoresistive effect of the order of 75% at the charge-ordered temperature. All the samples clearly reveal the unusual low temperature resistivity minimum, presumably due to the combined effect of weak localization, electron–electron and electron–phonon scattering. The present results are discussed and possible explanations were given based on the related theory and previous reported results.

Kondo scattering inδ-dopedLaTiO3/SrTiO3interfaces: Renormalization by spin-orbit interactions

We present a study of delta ($\delta$) doping at LaTiO$_3$/SrTiO$_3$ (LTO/STO) interface with iso-structural antiferromagnetic perovskite LaCrO$_3$ (LCO) that dramatically alters the properties of the two dimensional electron gas (2-DEG) at the interface. The effects include a reduction in sheet-carrier density, prominence of the low temperature resistivity minimum, enhancement of weak antilocalization below 10 K and observation of a strong anisotropic magnetoresistance (MR). The positive and negative MR for out-of-plane and in-plane field respectively and the field and temperature dependencies of MR suggest Kondo scattering by localized Ti$^{3+}$ moments renormalized by spin-orbit interaction at T $<$ 10 K, with the increased $\delta$-layer thickness. Electron energy loss spectroscopy and density functional calculations provide convincing evidence for blocking of electron transfer from LTO to STO by the $\delta$-layer.

Electrical transport studies on the La0.7−xPrxBa0.3MnO3 (x=0, 0.1 and 0.2) manganite: Double metal–insulator transitions and low-temperature resistivity minimum

The electrical transport properties of the polycrystalline samples La0.7−xPrxBa0.3MnO3 (0≤x≤0.2) have been investigated. Two metal–insulator transitions (TP1 and TP2) are observed in the electrical resistivity. The abnormal transport properties are considered to be a sign of a martensitic transition in the TP2 temperature region. Besides, all the samples clearly reveal the unusual low temperature resistivity minimum, presumably due to the combined effect of weak localization, electron–electron and electron–phonon scattering. The present results are discussed and possible explanations were given based on the related theory and previous reported results.

Magnetoelectric transport and quantum interference effect in ultrathin manganite films

The magnetoelectric transport behavior with respect to the thicknesses of ultrathin La0.9Sr0.1MnO3 films is investigated in detail. The metal-insulator phase transition, which has never been observed in bulk La0.9Sr0.1MnO3, is found in ultrathin films with thicknesses larger than 6 unit cells. Low-temperature resistivity minima appeared in films with thicknesses less than 10 unit cells. This is attributed to the presence of quantum interference effects. These data suggest that the influence of the weak localization becomes much pronounced as the film thickness decreases from 16 to 8 unit cells.

Readdressing the issue of low-temperature resistivity minimum in La0.7Ca0.3MnO3 thin films

We have investigated the origin of low-temperature resistivity minima observed in epitaxial thin films of La0.7Ca0.3MnO3 (thicknesses—300 A and 3000 A) using electrical and magneto-transport property measurements. We observe considerably smaller hysteresis in the magnetoresistance measurements for the thicker film than the thinner film. 300 A film shows meta-stability in the resistivity measurements at low temperature and for this film the sample current ‘I’ shows large effect on the resistivity and its minima temperature. These observations suggest that the strain induces electronic intra grain inhomogeneity in these samples and these inhomogeneities consist of regions of different resistive phases. It appears that the high resistive phase prevents the transport of charge carriers between two low resistive regions thus giving rise to the resistivity minimum in these samples.

Crossover effect of inter-granular transport and quantum correction in Co-doping La2/3Ca1/3MnO3 manganites

The dependence of electrical transport on temperature and magnetic field is studied systemically in La2/3Ca1/3Mn1−xCoxO3 (x=0–0.15) systems between 2.5 and 300K in magnetic field ranging from 0 to 6T. The substitution of Co at Mn site results in weakening the ferromagnetism, a deterioration of the metallic conductivity. The PM–FM transition is broadened with increasing Co doping content x, and the magnetic spontaneous state has changed into a cluster glass state for the sample of x=0.15. The resistivity minima are clearly observed at low temperature (T<50K) for all the samples. An unusual modulation effect under magnetic field for low temperature resistivity minimum of the Co-doping samples is observed. The abnormal behavior of Co-doped samples is that the resistivity minimum temperature (Tmin) shifts to higher temperatures with the increase in magnetic field. It is distinctly different from undoped and low doped samples where Tmin shifts to lower temperature under higher magnetic fields. In other words, the resistivity upturn of Co-doped samples is enhanced by magnetic field instead of the general field suppression effect. The observed resistivity minimum at low temperatures is partly due to the coexistence of electron–electron interaction enhanced by an intrinsic magnetic inhomogeneity and spin polarization tunneling through grain boundaries. The electron–electron scattering due to the Coulomb interaction is the intrinsic mechanism for the low temperature resistivity upturn. Electrical resistivity behavior of the compounds as function of temperature at paramagnetic state could be explained on the basis of variable-range hopping model.

Low-temperature resistance minimum in granular hole-doped cobaltites

It is for the first time that in three-dimensional granular cobaltites (La1−x Srx)1−yAgyCoO3 with a grain size of about 1 μm and replacement ratios x = 0.35 and y = 0, 0.05 a “metal–nonmetal” conductivity transition was observed, presumably associated with the AFM ordering of granule moments. This transition manifests itself as a minimum in the temperature dependence of resistance of the samples. An explanation of the nature of the minimum is proposed, based on the intragranular mechanism of electron correlation on the basis of the Zener double-exchange and on the intergranular mechanism of spin-polarized electron tunneling between the nearest neighbors under the antiferromagnetic exchange interaction of granule moments. Numerical calculation of the system conductance is made via the model based on summation of intragranular phase conductivities of the electronic system and on the computation of the total impedance of the system as a sum of resistances of individual granules with a given resistive contribution of intergranular tunneling. It is found that the external magnetic field of up to 10 T does not affect the depth of the minimum and its position on the temperature scale, suggesting that the intergranular antiferromagnetic interaction is stable to external magnetic fields.

Spin correlations and Kondo effect in a strong ferromagnet

The spin structure and electron-transport behavior of Pt-substituted MnBi thin films have been investigated. The electrical resistivity of these ferromagnetic films shows an unusual low-temperature resistance minimum or Kondo effect accompanied by positive magnetoresistance. First-principles calculations show that Mn atoms displaced to the bipyramidal interstitial sites are antiferromagnetically coupled to the Mn atoms on their regular lattice sites. We explain the observed Kondo effect and the positive magnetoresistance as the consequences of local spin correlations involving Mn atoms displaced to interstitial sites by Pt doping.

Electrical resistivity of the μ-Al4Mn giant-unit-cell complex metallic alloy

The μ-Al4Mn complex intermetallic phase with 563 atoms in its giant unit cell exhibits a complicated temperature dependence of electrical resistivity that has a broad maximum at about 175 K and a minimum at 13 K. The temperature dependence of the resistivity was reproduced by employing the theory of quantum transport of slow charge carriers, which predicts a crossover from the metallic (Boltzmann-type) positive-temperature-coefficient electrical resistivity at low temperatures to the insulator-like (non-Boltzmann) negative-temperature-coefficient resistivity at elevated temperatures. The low-temperature resistivity minimum was reproduced by considering it as a magnetic effect due to increased scattering of the conduction electrons by the Mn spins on approaching the spin glass phase that develops below the spin freezing temperature Tf = 2.7 K.

Modulation of Insulator-Metal Transition Temperature by Visible Light in La7/8Sr1/8MnO3 Thin Film

Effects of magnetic field and light illumination on the electrical transport properties of La7/8Sr1/8MnO3 thin film grown on a Si substrate are investigated. The film shows an insulator-metal transition at TP ≈ 191.9 K and a low-temperature resistance minimum at Tmin ≈ 48 K in darkness. Both magnetic field and light illumination shift the insulator-metal transition temperature T P to be higher, while the low-temperature transport properties of the film induced by them show different trends. That is, the magnetic field and light illumination make the Tmin shift to lower and higher temperatures, respectively. The enhancement of both TP and Tmin under light illumination could be explained in terms of photoinduced hole-doping and demagnetization effects of La7/8Sr1/8MnO3.