Low-field Magnetoresistance Effect Research Articles

Enhanced Low-Field Magnetoresistance in La0.71Sr0.29MnO3 Nanoparticles Synthesized by the Nonaqueous Sol–Gel Route

In colossal magnetoresistive (CMR) materials, magnetic fields of several tesla are typically required to exhibit large changes in electrical resistance, and hence, materials should be engineered to provide a more sensitive MR response at lower fields for their viability in practical applications. Enhanced low-field magnetoresistance (LFMR) was observed in highly ferromagnetic ∼20 nm La0.71Sr0.29MnO3 particles synthesized by the nonaqueous sol–gel route. The enhanced LFMR of the nanoparticles (NPs) reaches 29.8% at 30 K with 50 mT, and the high-field magnetoresistance was 56% with a 5000 mT applied field. The large LFMR effect can be attributed to the spin-polarized tunneling across the ∼1.3 nm thick natural grain boundaries. The weaker MR effect below 30 K was attributed to the re-entrant spin glass at the core of the NPs and surface spin glass phase that could be eliminated with lower and higher applied fields, respectively. The magnetic and MR properties of the NPs were compared to those of the correspo...

Giant magnetoimpedance and grain boundary effect in La0.75Ba0.25MnO3 sol–gel manganite

The La0.75Ba0.25MnO3 sol–gel manganite shows a typical dc low field magnetoresistance (LFMR) effect at low temperature. Besides the sharp resistance peak representing the metal–insulator transition around Curie temperature, another broad peak associated with the effect of grain boundaries could be observed around 260K. At low frequencies, such double peaks in ac resistance also occur. With increasing ac frequency, skin effect gradually enhances, the peak associated with the grain boundaries becomes less pronounced. At 0.5MHz, beside the peak of ac magnetoresistance ΔR/R0 at Curie temperature, the ac ΔR/R0 increases with decreasing temperature, since the ac resistance is strongly dependant on dc resistivity at low frequency. With increasing ac frequency, the peak of ac ΔR/R0 at Curie temperature becomes weak and almost disappears, as the ac ΔR/R0 strongly depends upon variation of magnetic-permeabilities at high frequencies. The magnetoreactance originates mainly from the change of permeability induced by the field. At 300K, the value of magnetoimpedance ΔZ/Z0 for La0.75Ba0.25MnO3 sol–gel polycrystalline sample can reach −49% at f=4MHz under a very small field of 500Oe.

Role of boundaries on low-field magnetotransport properties of La0.7Sr0.3MnO3-based nanocomposite thin films

Abstract

Influence of the covalent grafting of organic radicals to graphene on its magnetoresistance

Graphene was obtained by direct exfoliation of graphite in o-dichlorobenzene (oDCB) or benzylamine, and further functionalized with 4,4′-[(1,3-dioxo-1,3-propanediyl)bis(oxy)]bis[2,2,6,6-tetramethyl-1-piperidinyloxy] (1-TEMPO) organic radicals by using the Bingel–Hirsch cyclopropanation reaction. Here, the use of different solvents permits variation of the density of radicals anchored to the carbon layers. Covalent grafting is unambiguously demonstrated by TGA, μ-Raman, XPS and EPR measurements, which also rule out spurious physisorption. Our transport measurements indicate that the conduction mechanism varies as a function of the density of radicals grafted to the carbon layers. Moreover, the presence of paramagnetic moieties influences the magnetoresistive behaviour of the system by the appearance of a low field magnetoresistance (LFMR) effect. Finally, the derivatization of graphene with diamagnetic ethylmalonate molecules, by following an analogous route, permits us to discard the chemical derivatization, which is responsible for the observed differences in the MR response that must be rather ascribed to the grafting of organic spin carriers.

Studies on Low-Field and Room-Temperature Magnetoresistance in La2/3(Ca1−x Sr x )1/3MnO3 Perovskites

The polycrystalline perovskites La2/3(Ca1−x Sr x )1/3MnO3 (x=0,0.2,0.4,0.6,0.8,1) were successfully prepared by a modified method of solid-state reactions, in which the ingredient mixture with ethanol as a liquid milling medium to form suspension was milled by high-energy ball milling for 10 h and sintered in air at 1400 °C for 10 h. The microstructure, electrical transport, and low-field magnetoresistance (LFMR) of the perovskites were investigated to study the room-temperature magnetoresistance (RTMR) behavior. The results reveal that the metal-to-insulator transition temperature (T MI) increased with increasing doping level x, and the peak values of the magnetoresistance (MR) near T MI dropped with the more Ca2+ substituted. A single-phase La2/3Ca1/3MnO3 showed T MI at 263 K and the peak MR of 23 % in the applied field of 3 kOe near T MI. The LFMR effect at room temperature could be obtained by controlling the doping Sr2+-substituted Ca2+ level. When x=0.29, transition temperature (T MI) was 305.30 K, and the MR effect was recorded up to 12.9 % at 298.55 K and 3 kOe. Finally, the possible mechanism is discussed.

Spin-dependent transport properties of oleic acid molecule self-assembled La0.7Sr0.3MnO3 nanoparticles

Spin-dependent transport property through molecules is investigated using a monolayer of oleic acid molecule self-assembled half metallic La0.7Sr0.3MnO3 (LSMO) nanoparticles, which was synthesized using a coprecipitation method. Fourier transform infrared spectroscopy was used to confirm that one-monolayer oleic acid molecules chemically bond to the LSMO nanoparticles. The transport properties and magnetoresistance (MR) effect of the oleic acid molecule coated LSMO nanoparticles were measured by a direct current four probes method using a Cu/nanoparticle assembly/elargol electrode sandwich device with various temperatures and bias voltages. The non-linear I–V curve indicates a tunneling type transport properties. The tunnel barrier height around 1.3±0.15eV was obtained by fitting the I–V curve according to the Simmons equation. The magnetoresistance curves can be divided to high-field MR and low-field MR (LFMR) parts. The former is ascribed to the influence of spin disorder or canting within the LSMO nanoparticle surface and the latter one with strong bias dependence is attributed to the spin-dependent tunneling effect through the insulating surface layer of LSMO and oleic acid molecules. The enhanced LFMR effect for oleic acid coated LSMO with respect to the bare LSMO was attributed to the enhanced tunneling transport and weak spin scattering in oleic acid molecule barrier.

Low-field magnetoresistance of (1−x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) composite system under different sintering temperatures of matrix

A series of (1−x)La0.6Dy0.1Sr0.3MnO3/0.5x(Sb2O3)(x=0.15) samples were prepared by the solid-state reaction method, and the influence of sintering temperature of the matrix on low-field magnetoresistance of (1−x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) was studied through the measurements of X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) image, resistivity-temperature (ρ−T) curves, and magnetoresistance-temperature (MR-T) curves. The results indicate that for the samples with low sintering temperature of the matrix, low-field magnetoresistance effect appears on the whole temperature range and can be explained by grain boundary effect; for the sample with high sintering temperature of the matrix, intrinsic magnetoresistance peak appears on the high-temperature range, low-field magnetoresistance effect appears on low temperature range, and the magnetoresistance in the magnetic field of 0.2 T and on the comparatively large temperature range between 280 K and 225 K hardly changes with temperature and remains at 4.8%, which can be explained by the competition between the intrinsic magnetoresistance induced by double-exchange function inside grains and the tunneling magnetoresistance (TMR) induced by grain boundary effect. The temperature stability of magnetoresistance is beneficial to the practical applications of MR.

Characterization of La0.67Sr0.33MnOz thin films synthesized by metal-organic decomposition on different substrates

La0.67Sr0.33MnOz (LSMO) thin films were synthesized by means of metal-organic decomposition on the substrates including amorphous quartz, (100) Si chip, (100) MgO single crystal and polycrystalline Al2O3 ceramic plate. The structure and magnetotransport properties of the films were characterized. X-ray diffraction spectra show that all samples are polycrystalline with (202) preferred orientation. All films present metal–insulator transition and enhanced magnetoresistance (MR) effect below metal–insulator transition peak temperature (Tp). At room temperature (RT) low-field magnetoresistance effect (LFMR) and linear change of resistivity under applied field are exhibited by all the films. These magnetotransport properties were first ascribed to the porous structural characteristics in the films observed by atomic force microscope. Furthermore, the LSMO film synthesized on (100) MgO substrate presents a bit different magnetic properties and magnetotransport from the other samples, including broad ferromagnetic–paramagnetic transition zone, lower Tp and weaker LFMR at RT. However, for the samples synthesized on the other substrates, the LFMR effect is very similar to each other and their MR ratio reaches near 5% under 10kOe field. Thus the substrate effect of LSMO film on (100) MgO is more intensive than that of the other samples.

Low‐Field Magnetoresistance Effect in Core–Shell Structured La0.7Sr0.3CoO3 Nanoparticles

Core-shell structured La(0.7) Sr(0.3) CoO(3) nanoparticles are synthesized and their magnetic and magneto-transport properties are investigated. In these core-shell La(0.7) Sr(0.3) CoO(3) nanoparticles, the cores are single-crystalline and ferromagnetic, whereas the shells are noncrystalline and predominantly paramagnetic. Moreover, the insulating-like shells can serve as a good spin tunneling barrier. Therefore, in such a special core-shell structure, the spin-polarized interparticle tunneling is improved due to the existence of shells, which thus induces an enhanced low-field magnetoresistance effect.

Tilted Aligned Epitaxial La0.7Sr0.3MnO3 Nanocolumnar Films with Enhanced Low-Field Magnetoresistance by Pulsed Laser Oblique-Angle Deposition

Single-phase epitaxial La0.7Sr0.3MnO3 (LSMO) thin films with significantly enhanced low-field magnetoresistance (LFMR) properties (23% at 20 K) are demonstrated in this work. The LSMO films on SrTiO3 (001) substrates exhibit tilted and well-aligned nanocolumn structure achieved by pulsed laser oblique-angle deposition (PLOAD) followed by subsequent postannealing. The tilted aligned nanocolumnar (TAN) arrays have been achieved at relative high deposition angles (≥30o) and low deposition temperatures (≤450 °C). More attractively, the tilted grain boundaries (GBs) can be systematically manipulated by the postannealing process and so can the LFMR values of the LSMO TAN films. These results demonstrate that the tilted nanocolumnar films achieved by PLOAD and the GB tailoring by postannealing may provide a new approach to control and manipulate the magnetotransport properties of single-phase manganite perovskite films for device applications that require large LFMR effects, high epitaxial quality, and low resistivity.

Sample thickness dependence of giant magnetoimpedance under low fields for La 0.67Sr 0.33Mn 0.98Co 0.02O 3 manganites

The La 0.67Sr 0.33Mn 0.98Co 0.02O 3 plate sintered at 1250 °C shows a low field magnetoresistance (LFMR) effect under fields H < 4.6 kOe. However, the DC magnetoresistance Δ R/ R 0 is very small, only −1.84% under H = 4.6 kOe. The character of helical growth was observed on grain surface of La 0.67Sr 0.33Mn 0.98Co 0.02O 3 phase. The LFMR is connected with the interface of grain or grain boundary, while the giant magnetoimpedance under low fields for La 0.67Sr 0.33Mn 0.98Co 0.02O 3 sintered plates strongly depends upon the plate thickness. With an increase of sample thickness, the magnetoimpedance increases and the frequency, where the maximum magnetoimpedance occurs, shifts to low frequencies. A giant magnetoimpedance of −15.6% and a large AC magnetoresistance of −30.5% could be obtained under a very small field H = 600 Oe for the plate with a thickness of 3 mm.

Effect of NiFe2O4 doping on the structure, magnetic properties and electrical properties of La0.7Ca0.3MnO3

The (La0.7Ca0.3MnO3)1x/(NiFe2O4)x (x = 0 to 0.09) composites were prepared using a conventional solid state reaction method. The structural, magnetic properties, and electrical properties of LCMO/NFO composites were investigated using X-ray diffraction, scanning electron microscopy, field cooled DC magnetization, and magnetoresistance (MR) measurements. The resistivity measured as a function temperature demonstrates that the pure LCMO and x = 0.01 samples display metal to semiconductor transitions. However, the composites of x > 0.03 samples clearly present the electrical behavior as an insulator/semiconductor type behavior. It was observed that the resistivity of the samples increased systemically with an increase of the NFO content. From the MR measurements, it was found that the MR effect is enhanced for x = 0.01 with a NFO composition. In all, the spin-polarized tunneling and the spin-dependent scattering may be beneficial for an improved low-field magnetoresistance effect. These phenomena can be explained by the segregation of a new phase related to NFO at the grain boundaries or surfaces of the LCMO grains.

Low-field Magnetoresistance, Specific Heat and Magnetocaloric Effect in Sr Substituted Pr0.7Ca0.3MnO3

We investigate the effect of ionic size variation on the electrical and thermodynamic properties in a series of Pr0.7Ca0.3−xSrxMnO3 (PCSMO) samples. The increase in Sr content results in an increase of the unit cell volume, as a bigger Sr2+ ion replaces the smaller Ca2+ ions. Resistivity measurements show that the increase in the Sr content also results in the induction of a metal–insulator transition (TMI), which increases with increasing Sr content. The activation energy (Ea), calculated from the resistivity data, decreases with increasing Sr content confirming the metallic character. The effect of the magnetic field on resistivity and specific heat has also been studied.

Fabrication and Properties of La-Manganite/SnO&lt;sub&gt;2&lt;/sub&gt; Composites

Composites with compositions La0.7Ca0.3MnO3(LCMO) and La0.7Sr0.3MnO3(LSMO)/SnO2 were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between La based maganite and SnO2 grains, and that SnO2 segregates mostly at the grain boundaries of La based maganite. The variation in resistivity with temperature has been studied and shows a semiconducting behavior, furthermore the composites exhibit metallic percolation. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 0.5 Tesla. The spin-polarized tunneling and the spin-polarized tunneling may be attributed to the enhanced low-field magnetoresistance (LFMR) effect.

Enhancement of the magnetoresistive effect in nanocomposites based on manganite La0.67Sr0.33MnO3 and magnetite Fe3O4

The effect of a magnetite Fe3O4 nanopowder in a nanocomposite with manganite La0.67Sr0.33MnO3 on its magnetoresistive effect and conductivity is studied. The addition of a magnetite nanopowder to the initial material leads to a substantial (twofold) increase in the low-field magnetoresistive effect. The low-field magnetoresistive effect in substituted lanthanum manganite nanopowders is based on spin-dependent electron tunneling through grain boundaries. The transport mechanisms in such systems are analyzed, and noise spectroscopy is also used for this purpose.

ELECTRICAL TRANSPORT PROPERTIES AND MAGNETORESISTANCE OF (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x COMPOSITES

The ( La 0.7 Ca 0.3 MnO 3)1-x/( CuFe 2O4)x (x = 0–0.15) composites were prepared by conventional solid state reaction method. We have investigated the structural, magnetic and electrical properties of ( La 0.7 Ca 0.3 MnO 3)1-x/( CuFe 2 O 4)x composites using X-ray diffraction (XRD), scanning electron microscopy, field cooled DC magnetization and magnetoresistance (MR) measurements. The resistance measured as a function of temperature demonstrates that pure LCMO samples display metal to semiconductor transition. On the other hand, all the LCMO/CFO composites samples clearly present the electrical behavior of insulator/semiconductor. It indicates that resistivity of the samples increase systemically with the increase of CFO content. The MR was measured in the presence of 0.5 T field. At low temperatures, it is clearly observed that the MR effect is enhanced with x = 0.03 of CFO composition. In summary, the spin-polarized tunneling and the spin-dependent scattering may be helpful to the improved low-field magnetoresistance (LFMR) effect. These phenomena can be explained by the segregation of a new phase related to CFO at the grain boundaries or surfaces of the LCMO grains.

Electrical properties and magnetoelectric effect measurement in La 0.7Ca 0.2Sr 0.1MnO 3/ xCoFe 2O 4 composites

Composites with compositions La 0.7Ca 0.2Sr 0.1MnO 3 (LCSMO)/ xCoFe 2O 4 (CFO) were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and CFO grains, and that CFO segregates mostly at the grain boundaries of LCSMO. The variation in resistivity with temperature has been studied and shows a semi-conducting behavior, furthermore the composites exhibit metallic percolation threshold at x = x P = 8%. The magnetic moment ( σ S) changed with the increase of CFO. Curie temperature ( T C) is a dividing point; T < T C, σ S decreases with CFO content increasing, while T > T C, σ S changes contrarily with the addition of CFO. The magnetoresistance (MR) effect is enhanced at a wide temperature range in an applied magnetic field (3 kOe). The spin-polarized tunneling and the spin-dependent scattering may be attributed to the enhanced low-field magnetoresistance (LFMR) effect.

Observation of large low field magnetoresistance and large magnetocaloric effects in polycrystalline Pr0.65(Ca0.7Sr0.3)0.35MnO3

Magnetotransport and magnetocaloric properties of polycrystalline Pr0.65(Ca0.7Sr0.3)MnO3 have been studied. Large low field magnetoresistance (∼100% at 75K for 1T field) with high magnetic field sensitivity (∼5000%T−1) has been observed due to the melting of charge ordered state. The system also shows large magnetocaloric effect at ∼215 and 90K. The magnetic entropy change is ∼8 and 5.5J∕kgK, respectively, around these temperatures for a field change from 0to7T. Such huge low field magnetoresistance along with large magnetocaloric effect exhibited by the material is interesting from the application point of view.

Influence of polyparaphenylene on the magnetotransport of manganite/polymer composites

The ferromagnetic, polycrystalline manganese oxide La 0.7Sr 0.3MnO 3 (LSMO) and the conjugated polymer polyparaphenylene (PPP) were prepared by conventional solid-state reaction processing in air and synthesis by cationic polarization of benzene, respectively. The so-obtained compounds were individually characterized after their electrical, magnetic, structural or thermal properties by using conventional analysis methods. By mixing the pre-prepared LSMO and PPP powders, [LSMO] 1− x [PPP] x ( x stands for the weigh fraction of PPP) two-phase manganite/polymer granular composites were obtained and their microstructural, transport and magnetic features carefully investigated. The resistance of composites calcined at 400 °C for 1 h in air atmosphere increased with x on the whole temperature range. In turn, these samples displayed a pronounced low-field magnetoresistance (LFMR) effect within a wide temperature range. Spin-polarized tunneling of conduction electrons across interfaces or grain boundaries was invoked as the dominant mechanism controlling this effect. The addition of the polymer may modify the LSMO grain surfaces adjusting the conduction carrier's spin-dependent tunneling distance, which sensitively depends on the nature of the interface or on interface-related disorder. Concretely, the PPP should provoke spin disorder at the LSMO interface.

Low-field positive magnetoresistance near room temperature in three-component perovskite-type artificial superlattices

Perovskite-type superlattices consisting of La0.9Sr0.1MnO3, LaAlO3, and SrTiO3 were fabricated on (001) SrNb0.01Ti0.99O3 substrates by laser molecular beam epitaxy. The three-component superlattice exhibited desirable low-field magnetoresistance effects. At a given current bias of 10μA, positive magnetoresistance ratio was enhanced from 2% to 15% and 22% with increasing temperatures from 15 to 300 and 325K at 300Oe, at which a maximum value occurred from the external magnetic field dependence of magnetoresistance. The nature of the enhanced magnetoresistance is discussed in connection to the interfacial effect.