Low-field Magnetoresistance Effect Research Articles

Multilevel magnetoresistance states in La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 heterostructure grown on MgO

Magnetic tunnel junction (MTJ) is one of the cornerstones of modern information technologies. Bringing MTJ's operation beyond the conventional binary regime, enabled by tunneling magnetoresistance (TMR) effect, is highly promising for prospective memory technologies and neuromorphic hardware development. In this paper, we demonstrate multilevel magnetoresistance states in an all-perovskite-oxide La0.7Sr0.3MnO3 (LSMO)/BaTiO3/LSMO heterostructure grown on MgO substrates. Unlike traditional TMR, we observe four distinct regions of increased magnetoresistance, which result in three magnetic field-induced resistance states in total. We show that the observed phenomenon arises from the low-field magnetoresistance effect, which occurs in the two epitaxial LSMO layers, independently and at different values of the magnetic field. The effect is well simulated by a model based on the presence of structural defects and non-uniform deformations in the LSMO layers, induced by the large lattice mismatch of the LSMO with the MgO substrate. We believe that our findings contribute to the understanding of complex magnetoresistance effects in MTJs and can be taken into consideration for the design of multi-bit memory cells or neuromorphic devices.

High-Pressure Synthesis of Semiconducting PbCu3Mn4O12 with Near-Room-Temperature Ferrimagnetic Order.

A transition-metal oxide of PbCu3Mn4O12 was prepared at 1523 K and 10 GPa. An A-site-ordered quadruple perovskite structure with the space group Im3̅ is assigned for this compound. Based on bond-valence-sum calculations and X-ray absorption spectroscopy, the charge combination is determined to be PbCu32+Mn44+O12. Due to Cu2+(↑)-Mn4+(↓) antiferromagnetic coupling, a near-room-temperature ferrimagnetic phase transition is observed at approximately 287 K. PbCu3Mn4O12 exhibits a semiconducting electric transport property with the energy band gap Eg ≈ 0.2 eV. In addition, considerable low-field magnetoresistance effects are observed at lower temperatures. This study provides an intrinsic near-room-temperature ferrimagnetic semiconductor that exhibits potential applications in next-generation spintronic devices.

Nonequilibrium Green’s function approach to multi-band Cooper-pair transport: linear magnetoresistance effect due to nonunitary superconductivity

Many-body transport has emerged as an efficient tool for understanding interaction effects in quantum materials with a multi-band electronic structure. This paper proposes a formula for the two-particle transmission coefficient for Cooper-pair transport between multi-band normal and superconducting materials. The approach employs a tight-binding nonequilibrium Green’s function technique, allowing a direct calculation of the two-particle current, without invoking the paradigm of Andreev reflection. As an application of the theory, we demonstrate a low-field linear magnetoresistance effect for superconductors with an induced nonunitary order parameter. These results uncover an unexplored route for detecting unconventional nonunitary superconductivity in quantum materials of current theoretical and experimental interest.

Effect of Boundary Scattering on Magneto-Transport Performance in BN-Encapsulated Graphene

For conductors in the ballistic regime, electron-boundary scattering at the sample edge plays a dominant role in determining the transport performance, giving rise to many intriguing phenomena like low-field negative magnetoresistance effect. We systematically investigate the magneto-transport behaviors of BN-encapsulated graphene devices with narrow channel width W, wherein the bulk mean free path L mfp can be very large and highly tunable. By comparing the magnetoresistance features and the amplitude of L mfp in a large parameter space of temperature and carrier density, we reveal that the boundary-scattering-dominated negative magnetoresistance effect can still survive even when the ballistic ratio (L mfp/W) is as low as 0.15. This striking value is much smaller than the expected value for achieving (quasi-) ballistic transport regime (L mfp/W ≥ 1), and can be attributed to the ultra-low specularity of the sample edge of our graphene devices. These findings enrich our understanding of the effects of boundary scattering on channel transport, which is of vital importance for future designs of two-dimensional electronic devices with limited lateral sizes.

Flexible La0.67Sr0.33MnO3:ZnO Nanocomposite Thin Films Integrated on Mica

The integration of functional oxide thin films on flexible substrates is critical for their application in flexible electronics. Here, to achieve flexible perovskite manganite oxide film with excellent low-field magnetoresistance (LFMR) effect, textured La0.67Sr0.33MnO3 (LSMO):ZnO nanocomposite film was deposited on a flexible mica substrate with ZnO buffer using pulsed laser deposition (PLD). Compared to the polycrystalline LSMO:ZnO nanocomposite film directly deposited on mica without buffer, the LSMO:ZnO/ZnO/mica sample exhibits larger saturation magnetization (164 emu/cm3) and higher Curie temperature (∼319 K), which results from the crystallinity and strain in the LSMO phase. In addition, the LSMO:ZnO/ZnO/mica film presents a high MR value of ∼39% at 10 K under 1 T. Furthermore, the good mechanical stretchability and property stability of the nanocomposite thin films have been demonstrated with mechanical bending.

Structural, electrical, and magneto-transport properties of Pr0.7Sr0.3MnO3: Al2O3 composites

Incorporation of insulating secondary phase into manganite composites can enhance the low-field magnetoresistance (LFMR). This work reports the structural and electrical transport properties of (1-x) Pr0.7Sr0.3MnO3 (PSMO): x Al2O3 composites synthesised by solid-state reaction method. X-ray diffraction (XRD) patterns indicated that Mn-O-Mn bond distance and angle were disturbed due to the substitution of Al3+ at Mn-site of PSMO system. This subsequently increased the resistivity of composite samples and shifted the metal-insulator transition temperature (TMI) to lower temperatures as increasing content of Al2O3. LFMR effect was observed as the magnetoresistance values increase monotonically with the decreasing temperature.

Magnetic, magnetotransport and critical properties of polycrystalline Pr0.7Sr0.3MnO3 located at the tricritical point

In this work, we present a detailed investigation on the electrical and magnetic effects in a Pr0.7Sr0.3MnO3 ceramic, which was synthesized by a solid state reaction method. The Rietveld refinement analysis suggests that the sample is single phase of Pr0.7Sr0.3MnO3, has an orthorhombic structure with space group of Pbnm. Based on magnetic field and temperature dependences of the magnetization, M(H,T), we have estimated the critical parameters of Pr0.7Sr0.3MnO3 near TC (β ≈ 0.20–0.23, γ ≈ 0.87–1.10, δ ≈ 5.54–6.01, and TC ≈ 256–258 K), suggesting that its ferromagnetic-paramagnetic phase transition is located at the tricritical point. Our results also suggest that the critical behavior of this material is significantly influenced by the selected magnetic field range and the contribution of the demagnetization field. The magnetocaloric and the magnetoresistance effects associated to the double-exchange interactions in the sample have been investigated via the temperature and magnetic field dependences of the resistivity, ρ(T,H), and M(H,T) data. Under an applied field of 10 kOe, the maximum values of the magnetic entropy change (ΔSm) and the magnetoresistance (MR = 100%×[ρ(H)-ρ(0)/ρ(H)) are found to be about − 3.1 J/kgK and − 123.8%, respectively. The suitability of the value as well as the tendency of ΔSm(T) determined through the data sets M(H), M(T) and ρ(T) shows that these phenomenological models can be completely applied in the study of the thermal effects of materials. Additionally, the low-field magnetoresistance effect, the field dependences of the ordering temperature of Pr and the maximum value of ΔSm of Pr0.7Sr0.3MnO3 ceramic have also been considered.

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.

Approaches to enhance low-field magnetoresistance effect at room temperature of self-assembled manganite nanocomposite films via microstructure design

The low-field magnetoresistance (LFMR) effect of perovskite manganite nanocomposite films can be enhanced by controlling the microstructure. However, improving the LFMR effect at room temperature is still a challenge. Therefore, it is necessary to design unique nanostructures for manganite nanocomposite films and accurately control the composition, morphology, size, and geometric distribution of the second phase. In this article, we first summarize the research progress of the LFMR effect in the perovskite manganite composite films and then discuss the main factors affecting the LFMR effect. Finally, new approaches to enhance the LFMR effect at room temperature of self-assembled manganite nanocomposite films via nanostructure design are discussed.

Room-temperature TCR and low-field MR of La0.7Ca0.3-xSrxMnO3 (0.06 ≤ x ≤ 0.1) polycrystalline ceramics

In this study, La0.7Ca0.3-xSrxMnO3 (LCSMO, 0.06 ≤ x ≤ 0.1) ceramics were synthesized via a sol-gel method. The perovskite microstructure, electrical transport properties, low-field magnetoresistance (MR) and peak temperature coefficient of resistivity (TCR) behaviors of the as-obtained materials were all studied and the results were discussed. Grain size of LCSMO ceramics expanded as more Sr was added but grain boundaries (GBs) decreased. Field emission scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and vibrating sample magnetometry (VSM) analyses suggested that addition of Sr greatly enhanced the electrical/magnetic properties of the prepared specimens. The metal-insulator transition temperature rose as doping concentration x enhanced, and increased Ca2+ substitution led to reduced peaks of MR and TCR. For single-phase LCSMO, large room-temperature TCR and low-field MR effects (0.8 T magnetic field) were obtained by controlling the level of Sr2+ substituted Ca2+. For x = 0.095, the optimized room-temperature TCR and MR reached 13.78% K−1 (299.2 K) and 26.41% (302.6 K), respectively. Overall, LCSMO ceramics have broad application prospects in photoelectronic or magnetic devices at room-temperature.

Influence of Er doping on La site on lattice effect of low field magnetoresistance of manganite La0.7Ca0.3MnO3

When La in perovskite (ABO3) manganite is partially substituted by rare earth element Er, average ionic radius on A site decreases, resulting in influence of internal pressure generated in a lattice on electronic activity energy band overlapped between the d orbit on B site and the p orbit on O site. Manganite polycrystalline samples La0.7-xErxCa0.3MnO3(x = 0, 0.1, 0.2) were prepared by solid-state reaction technique. X-ray diffraction spectra, resistance-temperature (R-T) curves and magnetoresistance-temperature (MR-T) curves of the samples were measured. The influence of different substitution of Er for La on low field magnetoresistance effect of the samples was analyzed. Experimental results showed that crystal structure of the samples was cubic structure. Lattice constant dMn-Mn, and transition temperature between ferromagnetic state and paramagnetic state TC of the samples decreased with the increase of Er doping. Tolerance factor t of the samples was less than 1 and Mn-O-Mn bond angle of the samples was less than The samples showed obvious low field magnetoresistance effect in experimental temperature range in 0.25 T magnetic field.

Effect of Ho-doping on structural, electrical and magnetic properties of La0.7Sr0.3MnO3 ceramics prepared by Plasma-Activated Sintering

Different components of La0.7−x Ho x Sr0.3MnO3 (LHSMO, x = 0, 0.1, 0.2, 0.3) ceramics were fabricated by Plasma-Activated Sintering (PAS), so as to study the correlation between the contents of Ho3+ and the structural, electrical, magnetic properties. XRD and SEM confirmed that LHSMO ceramics prepared by PAS exhibited high-purity phase and dense microstructure. The measurement of electrical resistivity showed that the resistivity of LHSMO ceramics increased, and the metal–insulator transition temperature decreased with the increasing Ho-doping content. The resistivity data were then fitted using various empirical equations, and the conduction mechanism of LHSMO ceramics was found to be in accord with the electron–magnon scattering process in the low-temperature region and the small polaron hopping model in the high-temperature region. Lastly, we calculated the values of magnetoresistance of the LHSMO ceramics, which increased with increasing Ho-doping content, from 3.5% for x = 0 to 14.6% for x = 0.3. Therefore, the doping of Ho3+ into La0.7Sr0.3MnO3 can effectively enhance the low-field magnetoresistance effect.

Pulsed laser deposition of Sr2FeMoO6 thin films grown on spark plasma sintered Sr2MgWO6 substrates

Sr2FeMoO6 (SFMO) films were deposited on polycrystalline spark plasma synthesized Sr2MgWO6 (SMWO) substrates. Films were grown using pulsed laser deposition at temperatures (Tdep) between 720 °C and 820 °C in a vacuum environment of pressure mbar (0.1 mPa); after deposition they were cooled either in a pressure or mbar (10 mPa) O2. Despite the use of an isostructural substrate, the growth and cooling conditions play the primary role in determining details of the films’ structures and properties, similarly to single-crystals. Grazing x-ray and electron back-scatter diffraction indicate that vacuum-cooled films were pure perovskite-structured SFMO exhibiting grain-over-grain growth that aligned the perovskite sub-cells. SrMoO4 impurities were observed in the x-ray patterns for the oxygen-cooled films similarly to single-crystal substrates. Magnetic, electronic and magnetoresistive properties were all a function of growth and cooling environments. The Curie temperature and magnetization of the films increased with Tdep up to 800 °C. The vacuum-cooled films had low-resistivities with essentially metallic conductivity (small resistivity increases occurred at low-T), while the conductivity of oxygen-cooled films were consistent with variable range hopping. The oxygen-cooled films had higher low-field magnetoresistance effects at 5 K than the vacuum-cooled films, which seems consistent with SrMoO4 forming at grain boundaries. This work opens the route to tailor the electronic properties by engineering the grain boundaries in thin films.

Applying Capacitive Energy Storage for In Situ Manipulation of Magnetization in Ordered Mesoporous Perovskite-Type LSMO Thin Films.

Mesostructured nonsilicate materials, particularly mixed-metal oxides, are receiving much attention in recent years because of their potential for numerous applications. Via the polymer-templating method, perovskite-type lanthanum strontium manganese oxide (La1-xSrxMnO3, LSMO, with x ≈ 0.15 to 0.30) with a continuous 3D cubic network of 23 nm pores is prepared in thin-film form for the first time. Characterization results from grazing incidence X-ray scattering, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and electron microscopy and tomography show that the dip-coated sol-gel-derived films are of high quality in terms of both composition and morphology and that they are stable to over 700 °C. Magnetic and magnetotransport measurements demonstrate that the material with the highest strontium concentration is ferromagnetic at room temperature and exhibits metallic resistivity behavior below 270 K. Besides, it behaves differently from epitaxial layers (e.g., enhanced low-field magnetoresistance effect). It is also shown that carriers (electrons and holes) can be induced into the polymer-templated mesostructured LSMO films via capacitive double-layer charging. This kind of electrostatic doping utilizing ionic liquid gating causes large relative changes in magnetic susceptibility at room temperature and is a viable technique to tune the magnetic phase diagram in situ.

Enhancement of Low-field Magnetoresistance in Self-Assembled Epitaxial La0.67Ca0.33MnO3:NiO and La0.67Ca0.33MnO3:Co3O4 Composite Films via Polymer-Assisted Deposition.

Polymer-assisted deposition method has been used to fabricate self-assembled epitaxial La0.67Ca0.33MnO3:NiO and La0.67Ca0.33MnO3:Co3O4 films on LaAlO3 substrates. Compared to pulsed-laser deposition method, polymer-assisted deposition provides a simpler and lower-cost approach to self-assembled composite films with enhanced low-field magnetoresistance effect. After the addition of NiO or Co3O4, triangular NiO and tetrahedral Co3O4 nanoparticles remain on the surface of La0.67Ca0.33MnO3 films. This results in a dramatic increase in resistivity of the films from 0.0061 Ω•cm to 0.59 Ω•cm and 1.07 Ω•cm, and a decrease in metal-insulator transition temperature from 270 K to 180 K and 172 K by the addition of 10%-NiO and 10%-Co3O4, respectively. Accordingly, the maximum absolute magnetoresistance value is improved from −44.6% to −59.1% and −52.7% by the addition of 10%-NiO and 10%-Co3O4, respectively. The enhanced low-field magnetoresistance property is ascribed to the introduced insulating phase at the grain boundaries. The magnetism is found to be more suppressed for the La0.67Ca0.33MnO3:Co3O4 composite films than the La0.67Ca0.33MnO3:NiO films, which can be attributed to the antiferromagnetic properties of the Co3O4 phase. The solution-processed composite films show enhanced low-field magnetoresistance effect which are crucial in practical applications. We expect our polymer-assisted deposited films paving the pathway in the field of hole-doped perovskites with their intrinsic colossal magnetoresistance.

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.

Electrical resistance and magnetoresistance of highly oriented and polycrystalline La0.67Sr0.33MnO3/MgO(001) thin films

La0.67Sr0.33MnO3 thin films exhibiting a highly (001)-plane oriented and polycrystalline structure with a variable amount of (011)-textured crystallites have been grown in situ by RF magnetron sputtering on crystalline MgO(001) substrates by changing deposition temperature from 550 to 800 °C. Competing contribution of grains and grain boundaries to resistivity and magnetoresistance of the films has been investigated at T = (78–330) K. A model based on two parallel channels of current flow across grain boundaries has been applied to explain coexistence of low (LFMR) and high field (HFMR) magnetoresistance effects in the polycrystalline films at low temperatures. The LFMR effect has been understood assuming tunnelling of spin-polarized carriers via magnetic field-driven tunnelling barriers formed naturally between neighbouring misoriented grains. Meanwhile, the HFMR phenomenon has been associated with magnetic field-dependent hopping of carriers via the intergrain regions with reduced carrier density. Importance of the phase separation phenomenon on possible inhomogeneity of the material at grain boundaries has been discussed.

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.

A half-metallic A- and B-site-ordered quadruple perovskite oxide CaCu3Fe2Re2O12 with large magnetization and a high transition temperature.

Strong correlation between spins and conduction electrons is key in spintronic materials and devices. A few ferro- or ferrimagnetic transition metal oxides such as La1-(x)Sr(x)MnO3, Fe3O4, CrO2 and Sr2FeMoO6 have spin-polarized conduction electrons at room temperature, but it is difficult to find other spin-polarized oxides with high Curie temperatures (well above room temperature) and large magnetizations for spintronics applications. Here we show that an A- and B-site-ordered quadruple perovskite oxide, CaCu3Fe2Re2O12, has spin-polarized conduction electrons and is ferrimagnetic up to 560 K. The couplings between the three magnetic cations lead to the high Curie temperature, a large saturation magnetization of 8.7 μB and a half-metallic electronic structure, in which only minority-spin bands cross the Fermi level, producing highly spin-polarized conduction electrons. Spin polarization is confirmed by an observed low-field magnetoresistance effect in a polycrystalline sample. Optimization of CaCu3Fe2Re2O12 and related quadruple perovskite phases is expected to produce a new family of useful spintronic materials.