Large Low-field Magnetoresistance Research Articles

High-mobility spin-polarized quasi-two-dimensional electron gas and large low-field magnetoresistance at the interface of EuTiO3/SrTiO3 (110) heterostructures

High-mobility spin-polarized two-dimensional electron gas (2DEG) at the interfaces of complex oxide heterostructures provide great potential for spintronic device applications. Unfortunately, the interfacial ferromagnetism and its associated spin polarization of mobile electrons and negative magnetoresistance (MR) are too weak. As of now, obtaining enhanced interfacial ferromagnetism and MR and strong spin-polarized 2DEG is still a great challenge. In this paper, we report on the realization of strong spin-polarized 2DEG at the interface of EuTiO3/SrTiO3 (110) heterostructures, which were prepared by directly depositing 39-nm EuTiO3 films onto as-received SrTiO3 (110) substrates. Hall and Kondo effects, low-field MR, Shubnikov–de Haas (SdH) oscillation, and magnetic hysteresis loop measurements demonstrate that high mobility electrons (1.4 × 104 cm2 V−1 s−1) accumulate at the interface of the heterostructures, which are not only highly conducting and show SdH oscillations with a non-zero Berry phase but also show a large out-of-plane and in-plane butterfly-like negative low-field MR whose magnitude is unprecedentedly large (46%–59% at 500 Oe and 1.8 K), approximately one to two orders higher than those of previously reported spin-polarized 2DEG systems. The strong spin polarization of the interfacial 2DEG is attributed to the presence of interfacial Eu2+ 4f (3.6–4 μB/f.u.) and Ti3+ 3d moments. Our results may provide guidance for exploring strong spin-polarized 2DEG at the interface of rare-earth titanate–strontium titanate heterostructures.

Large Magnetoresistance of Isolated Domain Walls in La2/3 Sr1/3 MnO3 Nanowires.

Generation, manipulation, and sensing of magnetic domain walls are cornerstones in the design of efficient spintronic devices. Half-metals are amenable for this purpose as large low field magnetoresistance signals can be expected from spin accumulation at spin textures. Among half metals, La1- x Srx MnO3 (LSMO) manganites are considered as promising candidates for their robust half-metallic ground state, Curie temperature above room temperature (Tc = 360 K, for x = 1/3), and chemical stability. Yet domain wall magnetoresistance is poorly understood, with large discrepancies in the reported values and conflicting interpretation of experimental data due to the entanglement of various source of magnetoresistance, namely, spin accumulation, anisotropic magnetoresistance, and colossal magnetoresistance. In this work, the domain wall magnetoresistance is measured in LSMO cross-shape nanowires with single-domain walls nucleated across the current path. Magnetoresistance values above 10% are found to be originating at the spin accumulation caused by the mistracking effect of the spin texture of the domain wall by the conduction electrons. Fundamentally, this result shows the importance on non-adiabatic processes at spin textures despite the strong Hund coupling to the localized t2g electrons of the manganite. These large magnetoresistance values are high enough for encoding and reading magnetic bits in future oxide spintronic sensors.

Enhancement of room-temperature magnetoresistance in polyvinyl acetate encapsulated Fe3O4 nanoparticles

We report a facile method to enhance the magnetoresistance (MR) of magnetite (Fe3O4) nanoparticles assemblies using encapsulating with polyvinyl acetate (PVAc). Fe3O4 nanoparticles of various size distributions have been synthesized by a chemical co-precipitation method. X-ray diffraction study revealed the formation of cubic inverse spinel structure with Fd3¯m space group. Morphology and the particle size distributions were studied using FESEM and TEM. The films of smaller sized nanoparticles show higher resistivity, and a significant drop in resistivity was observed after the PVAc encapsulation over the film. Negative MR of 1.7–2.13% was observed with the decrease of mean particle size from ∼ 50 nm to 21 nm at room temperature under 0.5 T, which was increased to 4 – 8.4 % after the PVAc encapsulation. The large low field magnetoresistance is mainly attributed to the tunnelling of high spin polarized electrons in Fe3O4 grains through the spin disordered grain boundaries. The encapsulation of PVAc over the nanoparticles film reassembles the nanoparticles and reduce the interparticle distances by producing a compressive stress, which reduces the tunnelling barrier for the interparticle charge transport and improves the magnetoresistance.

Large temperature coefficient of resistivity (TCR) of La1-xCaxMnO3 films prepared by spin-coating method

La1-xAxMnO3 (A = alkaline-earth or alkali metal) perovskites with large temperature coefficient of resistivity (TCR) offer immense opportunity for the application in high-performance infrared bolometer. However, numerous studies have revealed that the manganite films with large TCR values are not easily obtained by various fabrication technologies. Herein, a facile sol-gel spin-coating technique was used to prepare La1-xCaxMnO3 (LCMO, 0.280 ≤ x ≤ 0.375) films. All the samples were highly oriented along the out-of-plane direction of La0.3Sr0.7Al0.65Ta0.35O3 (001) substrates. With the rise in Ca doping amount, the resistivity of the films first decreased, and then, increased. This change can be attributed to the competition between the carrier concentration and lattice distortion. The LCMO films displayed large TCR and low-field magnetoresistance (MR) values, which exceeded those of previously reported thin films. The maximum TCR and low-field MR values were observed at x = 0.330, reaching 30.13% K−1 (266.43 K) and 68.6% (270.08 K), respectively. The La0.67Ca0.33MnO3 film with large TCR and MR values presents potential for the applications in high-sensitivity infrared bolometers and magnetic sensors.

Large Low-Field Magnetoresistance (LFMR) Effect in Free-Standing La0.7Sr0.3MnO3 Films.

The realization of a large low-field magnetoresistance (LFMR) effect in free-standing magnetic oxide films is a crucial goal toward promoting the development of flexible, low power consumption, and nonvolatile memory devices for information storage. La0.7Sr0.3MnO3 (LSMO) is an ideal material for spintronic devices due to its excellent magnetic and electronic properties. However, it is difficult to achieve both a large LFMR effect and high flexibility in LSMO films due to the lack of research on LFMR-related mechanisms and the strict LSMO growth conditions, which require rigid substrates. Here, we induced a large LFMR effect in an LSMO/mica heterostructure by utilizing a disorder-related spin-polarized tunneling effect and developed a simple transfer method to obtain free-standing LSMO films for the first time. Electrical and magnetic characterizations of these free-standing LSMO films revealed that all of the principal properties of LSMO were sustained under compressive and tensile conditions. Notably, the magnetoresistance of the processed LSMO film reached up to 16% under an ultrasmall magnetic field (0.1 T), which is 80 times that of a traditional LSMO film. As a demonstration, a stable nonvolatile multivalue storage function in flexible LSMO films was successfully achieved. Our work may pave the way for future wearable resistive memory device applications.

Martensitic transformation and magnetoresistance in Ni40Mn44-xCoxAl16 Heusler alloys

In this work, we focus on the magnetic properties and martensitic transformation behaviors in Ni40Mn44-xCoxAl16 (x = 5, 7) Heusler alloys. By partial substitution of Co for Mn, the martensitic transformation (MT) can be tuned in a wide temperature range and shifts to the higher temperature with the increase of Co content in these alloys. The reverse MT can be induced by magnetic field from the weak-magnetic martensite phase to ferromagnetic austenite phase in these alloys. The large magnetoresistance effect of −46.39% and −41.75% are acquired under 90 kOe for Ni40Co5Mn39Al16 and Ni40Co7Mn39Al16 alloys, respectively. The Ni40Mn44-xCoxAl16 alloys with large low-field magnetoresistance and low cost show promising applications for multifunctional materials.

Large low-field magnetoresistance of Fe3O4 nanocrystal at room temperature

Superparamagnetic magnetite (Fe3O4) nanoparticles with an average size of 6.5nm and good monodispersion were synthesized and investigated by X-ray diffraction, Raman spectrometer, transmission electron microscopy and vibrating sample magnetometer. Corresponding low-field magnetoresistance (LFMR) was tested by physical property measurement system. A quite high LFMR has been observed at room temperature. For examples, at a field of 3000Oe, the LFMR is −3.5%, and when the field increases to 6000Oe, the LFMR is up to −5.1%. The electron spin polarization was estimated at 25%. This result is superior to the previous reports showing the LFMR of no more than 2% at room temperature. The conduction mechanism is proposed to be the tunneling of conduction electrons between adjacent grains considering that the monodisperse nanocrystals may supply more grain boundaries increasing the tunneling probability, and consequently enhancing the overall magnetoresistance.

Electrical transport and magnetic properties of Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 bilayer film on (001) SrTiO3

We have grown Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3 bilayer on (001) single crystal SrTiO3 substrate by pulsed laser deposition technique. The bilayer film is well (001) orientation grown with high crystalline quality. When the applied magnetic field increases up to 9 T, only metal-like transport behavior can be observed below room temperature. The Low-field MR (LFMR) as large as ~−73% has been achieved at an applied field of 1 T and the magnetoresistance (MR) can reach a value of ~−641% at an applied field of 9 T. Ferromagnetic (FM) clusters percolation accompanied with the collapse (melting) of charge-ordered (CO) state in La0.5Ca0.5MnO3 may be responsible for the enhancement of MR. Moreover, the disappearance of exchange bias (EB) in the bilayer demonstrates the existence of FM clusters percolation at the PSMO/LCMO interface. Large LFMR provides possibilities in developing manganite-based FM/CO film devices.

Colossal magnetoresistance of Pr0.7Sr0.3MnO3 layer grown on charge-ordered La0.5Ca0.5MnO3 manganite layer

We present a report of the interfacial reaction between ferromagnetic (FM) Pr0.7Sr0.3MnO3 (PSMO-3) in proximity to charge ordered (CO) La0.5Ca0.5MnO3 (LCMO-5) in a bilayer structure. PSMO-3/LCMO-5 bilayer film with high crystalline quality was epitaxially grown on SrTiO3 (001) substrate by pulsed laser deposition. The bilayer loses 20% of its resistance for only 0.5 T, and loses 50% of its resistance for 2 T. This large low-field magnetoresistance (LFMR) is comparable to some FM/CO multilayers and other mixed compounds including strongly competing phases. LFMR of ∼−73% has been achieved at H = 1 T and T = 185 K. The maximum MR reaches a value of ∼−641% at H = 9 T and T = 205 K and the corresponding MR at room temperature is ∼−30%. The formation of CO anti-ferromagnetic state in LCMO-5 and its collapse (melting) under the applied magnetic field may be responsible for the enhanced MR mentioned above. In other words, the enhancement of MR can be explained by a FM phase percolation. Moreover, exchange bias (EB) cannot be observed in the bilayer at low temperature of 3 K, further confirming the occurrence of FM phase percolation at the PSMO-3/LCMO-5 interface. The disappearance of EB can be explained by a combination of the preferable distribution of FM clusters and FM clusters percolation at the PSMO-3/LCMO-5 interface. Large MR can be realized by using a CO material as a sublayer, which can provide potential application in the used devices.

Observation of large low‐field magnetoresistance in spinel cobaltite: A new half‐metal

Low‐field magnetoresistance is an effective and energy‐saving way to use half‐metallic materials in magnetic reading heads and magnetic random access memory. Common spin‐polarized materials with low field magnetoresistance effect are perovskite‐type manganese, cobalt, and molybdenum oxides. In this study, we report a new type of spinel cobaltite materials, self‐assembled nanocrystalline NiCo2O4, which shows large low field magnetoresistance as large as –19.1% at 0.5 T and –50% at 9 T (2 K). The large low field magnetoresistance is attributed to the fast magnetization rotation of the core nanocrystals. The surface spin‐glass is responsible for the observed weak saturation of magnetoresistance under high fields. Our calculation demonstrates that the half‐metallicity of NiCo2O4 comes from the hopping eg electrons within the tetrahedral Co‐atoms and the octahedral Ni‐atoms. The discovery of large low‐field magnetoresistance in simple spinel oxide NiCo2O4, a non‐perovskite oxide, leads to an extended family of low‐field magnetoresistance materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Large low-field positive magnetoresistance in nonmagnetic half-Heusler ScPtBi single crystal

High-quality nonmagnetic half-Heusler ScPtBi single crystals were synthesized by a Bi self-flux method. This compound was revealed to be a hole-dominated semimetal with a large low-field magnetoresistance up to 240% at 2 K in a magnetic field of 1 T. Magneto-transport measurements demonstrated that the large low-field magnetoresistance effect resulted from the coexistence of field-induced metal-semiconductor transition and weak-antilocalization effect. Moreover, Hall measurements indicated that ScPtBi single crystal showed a high mobility over a wide temperature region even up to room temperature (4050 cm2V−1s−1 at 2 K–2016 cm2V−1s−1 at 300 K). These findings not only suggest the nonmagnetic ScPtBi semimetal a potential material candidate for applications in high-sensitivity magnetic sensors but also are of great significance to comprehensively understand the rare-earth based half-Heusler compounds.

Quantum Transport Detected by Strong Proximity Interaction at a Graphene-WS2 van der Waals Interface.

Magnetotransport measurements demonstrate that graphene in a van der Waals heterostructure is a sensitive probe of quantum transport in an adjacent WS2 layer via strong Coulomb interactions. We observe a large low-field magnetoresistance (≫ e(2)/h) and a -ln T temperature dependence of the resistance. In-plane magnetic field resistance indicates the origin is orbital and nonclassical. We demonstrate a strong electron-hole asymmetry in the mobility and coherence length of graphene demonstrating the presence of localized Coulomb interactions with ionized donors in the WS2 substrate, which ultimately leads to screening as the Fermi level of graphene is tuned toward the conduction band of WS2. This leads us to conclude that graphene couples to quantum localization processes in WS2 via the Coulomb interaction and results in the observed signatures of quantum transport. Our results show that theoretical descriptions of the van der Waals interface should not ignore localized strong correlations.

Enhanced tunable magnetoresistance properties over a wide temperature range in epitaxial (La0.7Sr0.3MnO3)1−x:(CeO2)x nanocomposites

Vertically aligned nanocomposite (VAN) (La0.7Sr0.3MnO3)1−x:(CeO2)x (LSMO:CeO2) thin films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. Tunable magnetoresistance properties as well as microstructures are demonstrated in these VAN films by modulating the film composition (x = 0, 0.3, 0.4, 0.45, 0.5, and 0.55). The sample of x = 0.3 shows a large low-field magnetoresistance (LFMR) in a high temperature range, i.e., over 10% at the range of 280 K to 320 K under 1 T and with a peak value of ∼13.5% at 310 K. In addition, a vast enhancement of LFMR in a low temperature range of 20–150 K with peak of ≈34.3% at 45 K for 1 T could be achieved with x = 0.5. The enhanced LFMR properties can be attributed to both the phase boundary induced spin fluctuation and the magnetic tunneling effect through vertical ferromagnetic/insulator/ferromagnetic structures. The observed enhanced LFMR performance, especially at high temperatures, as well as its simple growth method, offers a great potential for LSMO:CeO2nanocomposites to be used in spintronic devices in a large temperature range.

Influence of Molar Ratio of Citric Acid to Metal Ions on Structure, Microstructure and Electrical Transport Properties on Nanosized La<sub>0.85</sub>Na<sub>0.15</sub>MnO<sub>3</sub>

In this paper, nanosized La0.85Na0.15MnO3 (LNMO) has been synthesized via sol-gel method by involving two major steps, first the complexation of citric acid (CA) with metal ions (MI) and second the polyesterification between CA and ethylene glycol (EG). The effect of molar ratio CA:MI varying from 2-4 on structure, microstructure and electrical transport properties of LNMO have been investigated by constant the amount of EG. All samples show single perovskite phase with hexagonal structure and space group R3c after sintering at 800°C for 10h. Sample of molar ratio 2.5 is observed to possess smallest grain sizes which yield high resistivity value compared with others, is suggested to originate from the increase of tunneling barriers (grain boundaries). The large low field magnetoresistance (LFMR) of about ~ -16% at 0.1T and low temperature confirmed the important role of grain boundaries in the nanosized LNMO.

Excellent low-field magnetoresistance effect in Ga-doped MnZn ferrites

An excellent low field magnetoresistance (LFMR) property was achieved from the Ga-doped (Mn0.8Zn0.2)Fe2O4 (MnZn) ferrites at room temperature (RT). For this study, undoped and Ga-doped MnZn ferrites with the nominal compositions of (Mn0.8Zn0.2)1−xGaxFe2O4 (x = 0 ∼ 0.1) were prepared by the conventional solid state reaction at 1400°C for 2 h in air. From the magneto-transport measurements, Ga-doped MnZn ferrites were found to have not only much lower resistivity values but also greatly improved LFMR ratios in comparison with undoped sample. The highest maximum LFMR ratio of 2.5% at 290 K in 0.5 kOe was achievable from 2 mol% Ga-doped MnZn ferrite. This large LFMR effect is attributable to an increase in spin electrons by Ga3+ ion substitution for the (Mn, Zn)2+ site.

Large low field magneto-resistance and temperature coefficient of resistance in La0.8Ca0.2MnO3 epitaxial thin film

An epitaxial La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film was fabricated using the pulsed laser deposition (PLD) technique to evaluate the magnetoresistance (MR) and temperature coefficient of resistance (TCR). The LCMO film was about 200nm in thickness and appeared to have a strong out-of-plane texture. A giant value of MR 73% and 57% for both the ab-plane and in the c-directions respectively are obtained at 1.5T applied field. The values of TCR are about 14.2% K−1 and 11.5% K−1 for both the ab-plane and in the c-directions respectively in a wide temperature range. These results proved that La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film is a promising candidate of perovskites for novel electronic applications.

Large, Temperature‐Tunable Low‐Field Magnetoresistance in La0.7Sr0.3MnO3:NiO Nanocomposite Films Modulated by Microstructures

Magnetic properties and low‐field magnetoresistance (LFMR) in La0.7Sr0.3MnO3 (LSMO):NiO nanocomposite films grown on SrTiO3 (001) substrates, which are shown to be tunable with different microstructures, are investigated. The LSMO:NiO nanocomposite films with NiO volume ratio of 50% have a checkerboard‐like structure and show a large LFMR in a temperature range from 200 to 300 K (≈17% at 250 K with a magnetic field of 1 T). As the NiO volume ratio is increased to 70%, a nano‐columnar structure formed in the films. Their LFMR is significantly enhanced at a wide temperature range of 10–210 K. The highest value of LFMR with 41% is achieved at 10 K in a magnetic field of 1 T. The enhanced LFMR can be considered to result from the electron scattering at the ferromagnetic LSMO/NiO interfaces and magnetic tunnel junctions (MTJs) of LSMO/NiO/LSMO at the nanometer scale. These results demonstrate that large and tunable LFMR from low temperature to room temperature can be realized by controlling the microstructures in the epitaxial La0.7Sr0.3MnO3:NiO nano­composite thin films, which will be expected to be applied in the devices using for a wide temperature range.

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...

Hopping Magnetotransport via Nonzero Orbital Momentum States and Organic Magnetoresistance

In hopping magnetoresistance of doped insulators, an applied magnetic field shrinks the electron (hole) s-wave function of a donor or an acceptor and this reduces the overlap between hopping sites resulting in the positive magnetoresistance quadratic in a weak magnetic field, B. We extend the theory of hopping magnetoresistance to states with nonzero orbital momenta. Different from s states, a weak magnetic field expands the electron (hole) wave functions with positive magnetic quantum numbers, m>0, and shrinks the states with negative m in a wide region outside the point defect. This together with a magnetic-field dependence of injection/ionization rates results in a negative weak-field magnetoresistance, which is linear in B when the orbital degeneracy is lifted. The theory provides a possible explanation of a large low-field magnetoresistance in disordered π-conjugated organic materials.

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.