MnO3 Nanowires Research Articles

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.

Structural characterization and magnetic properties of single‐crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires

AbstractHerein, structural and magnetic properties of single‐crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires synthesized via a hydrothermal process are reported. The nanowires are crystallized in an orthorhombic structure (Pnma space group). Their lattice parameters follow the relationship a ≈ c ≈ b/√2 These nanowires exhibited a clean and smooth surface with diameters of 60‐120 nm and an average length of approximately 2.0 μm. High‐resolution transmission electron microscopy images confirmed the single‐crystalline nature of the nanowires growing along the [100] direction. The nanowires demonstrated magnetic hysteresis loops at low temperatures and a weak exchange bias (EB) effect. Paramagnetic (PM)–ferromagnetic (FM) phase transition occurred at a Curie temperature (TC) of 224 K, and strong irreversibility between zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization (MZFC and MFC, respectively) curves was observed at 273 K. The MZFC curve exhibited a significantly broad peak with a maximum at a freezing temperature (Tf) of 134 K. Relative difference between MFC and MZFC in the nanowires [(MFC − MZFC)/MFC] rapidly increased below Tf and reached approximately 50% below 35 K. The effective magnetic moment deduced from the Curie constant is larger than the theoretical value, indicating short‐range FM interactions in the nanowires. A positive PM TC (θp) implies dominant FM interactions in the nanowires, and θp > TC observed herein indicates the existence of short‐range ordered states above TC.

Microstructure analysis of electrospun La0.8Sr0.2MnO3 nanowires using electron microscopy and electron backscatter diffraction (EBSD)

The microstructural properties of electrospun La0.8Sr0.2MnO3 (LSMO) nanofibers were investigated using electron microscopy and electron backscatter diffraction (EBSD). By means of EBSD, it is possible to measure the crystallographic orientation of the LSMO grains within an individual nanofiber. As the LSMO grains within the nanofibers are in the 10-nm range, we employ here parts of the recently developed transmission Kikuchi diffraction technique in order to enhance the Kikuchi pattern quality to enable an automated mapping of the crystallographic data. The diffraction results demonstrate that the grain orientation is not random, but there is a texture induced by the shape of the polymer nanofiber formed after the electrospinning step. Within an individual nanofiber section, the dominating grain boundaries are high-angle ones, which play an important role in the current flow through the sample (low- and high field magnetoresistance). The data obtained allow further an analysis of the grain shape aspect ratio, and elucidate the grain and grain boundary arrangement within electrospun LSMO nanofibers.

Synthesis and enhanced electrocatalytic mechanism of mesoporous La0.8Sr0.2MnO3 nanowires as high-active electrocatalysts for Zn-air batteries

The one-dimensional mesoporous La0.8Sr0.2MnO3 catalysts with excellent electronic conduction were successfully prepared by molten salt template method to accelerate the kinetics of ORR for metal-air batteries. The as-synthesized mesoporous La0.8Sr0.2MnO3 nanowires with relatively high oxygen vacancy concentration and ratio of Mn4+/Mn3+ show high specific surface area of 31.03 m2 g−1, higher than the most of reported perovskite oxides. More importantly, more catalytic active sites can be exposed at the three-phase interface of ORR due to high specific surface area. The mesoporous La0.8Sr0.2MnO3 nanowires exhibit high half-wave potential and catalyze near 4 e− ORR process. Besides, the one-dimensional mesoporous La0.8Sr0.2MnO3-based Zn-air batteries show high peak power density of 113.86 mW cm−2. Simultaneously, density functional theory (DFT) calculation reveals that strengthened electrocatalytic mechanism of La1-xSrxMnO3 catalysts originates from lower formation energy of oxygen vacancy and more positive O 2p band center because of Sr dopants, which is consistent with the experimental results. Consequently, this investigation not only provide a new idea for developing mesoporous perovskite oxides nanowires but also reveals enhanced electrocatalytic mechanism of La1-xSrxMnO3 catalysts.

Effect of Size Reduction on Structure, Magnetic Memory and Relaxation of La0.7Sr0.3MnO3 Nanowires

We have demonstrated here the study of memory effect, relaxation and interaction of hydrothermally grown La0.7Sr0.3MnO3 (LSMO-0.3) nanowires (NWs) and its bulk form synthesized by using polymeric precursor route. Magnetic relaxation measurement has been performed and the analysis shows the existence of two relaxation times in both the cases. It has been observed that both the samples show signature of magnetic memory effect and prefer magnetized state. Curie temperature of La0.7Sr0.3MnO3 (LSMO-0.3) NWs is decreased compared to its bulk form due to finite size effects.

Critical behaviors of ferromagnetic-paraferromagnetic transition in La0.5Sr0.5MnO3 nanowires bundles under low applied field

The critical behaviors of the perovskite La0.5Sr0.5MnO3 nanowires bundles around the ferromagnetic-paraferromagnetic (FM-PM) transition through modified Arrott plot and critical isothermal analysis is investigated. The obtained critical exponents are determined by modified Arrott Noakes equation (H/M)1/γ=(T−Tc)/Tc+(M/M1)1/β under a low external magnetic field of 0–0.8T, where β = 0.93 and γ = 2.25 were obtained at Tc ≈ 328 k. The availability of the parameters is proved by fulfilling the Widom scaling relation δ=1+γ/β which obeys the single scaling equation of M(H,|ε|)=εβf±(H/|ε|β+γ). The critical exponents determined in this system is unique and not following theories in any published literature. It may be attributed to the nano-size effect and surface magnetism of this system.

Detection of phase separation and nano-droplet states in La_033Pr_034Ca_033MnO_3 nanowires via near-infrared reflection experiments

We present an optical investigation on La0.33Pr0.34Ca0.33MnO3 (LPCMO) nanowires by using standard near-infrared (NIR) reflection experiments. It is found that the samples can respond strongly to the NIR radiation so that the temperature dependence of the reflectivity can be measured accordingly. Interestingly, the temperature induced phase separation and nano-droplet states in the nanowire samples with different diameters can be identified and observed optically without making ohmic contact electrodes on the samples. The onset temperature of the nano-droplet state measured optically is in line with that obtained from previous transport measurements. Importantly, via examining the dependence of the NIR reflectivity upon radiation wavelength, we find that the electronic localization effect exists in the LPCMO nanowires in both electronic states. Such information cannot be directly obtained from electric transport and magneto-transport experiments. The interesting and important experimental findings from this study demonstrate that the simple and contactless NIR reflection measurements can be employed to conveniently study and characterize the basic physical properties of perovskite manganites based nanostructure systems.

Synthesis and magnetic properties of complex oxides La0.67Sr0.33MnO3 nanowire arrays

As a new approach of melt-injection-decomposition method, it has been successfully adopted for the synthesis of the complex oxides La0.67Sr0.33MnO3 nanowire arrays. X-ray diffraction studies confirmed the formation of perovskite manganite phase of the sample. Transmission electron microscope and scanning electron microscope characterizations showed a large quantity of one-dimensional nanowires is obtained and the nanowires are continuous, concrete, oriented and rather uniform with an average diameter of 170nm and length of several tens of micrometers. Magnetic measurements exhibited good ferromagnetic properties at the temperature of 10K and 300K respectively. The transition temperature of the complex oxides La0.67Sr0.33MnO3 nanowire arrays is about 350K, which will endow their great potential applications in magnetoresistance, spintronics or sensor fields at room temperature.

Electrical transport properties of (La,Pr,Ca)MnO3 nanowires investigated using terahertz time domain spectroscopy

The electrical transport properties of a 100-nm-width (La,Pr,Ca)MnO3 nanowire sample were investigated using terahertz (THz) time domain spectroscopy. When the electric field of incident THz pulses was parallel to the nanowires, we obtained their intrinsic THz conductivity. The temperature-dependent dc conductivity and metallic fraction were simultaneously estimated by analyzing the THz conductivity using a metal-insulator composite model. The evaluated dc conductivity closely reproduced that measured by electrical probe measurement. The metallic fraction showed the evolution of electric domains from the metallic state at temperatures below 100 K to the insulating state at temperatures above 150 K through a coexistence region, which was in consistence with the phase-separated scenario.

Strange critical behaviors of ferromagnetic to paramagnetic transition in La0.5Ca0.5MnO3 nanowires bundles

La0.5Ca0.5MnO3 nanowire bundles were synthesized by a hard template method, followed by further investigation on their strange critical behaviors.

Highly homogeneous YBCO/LSMO nanowires for photoresponse experiments

By using nanolithography and a soft etching procedure, we have realized Y Ba2Cu3O7−x/La0.7Sr0.3MnO3 (YBCO/LSMO) nanowires, with cross sections down to 100 × 50 nm2 that ensure the cover age of areas up to 10 × 30 μm2. The LSMO layer acts as a capping for YBCO, minimizing the degradation of the superconducting properties taking place during the patterning; moreover, as a ferromagnetic manganite, it is expected to accelerate the relaxation dynamics of quasiparticles in YBCO, making such a system potentially attractive for applications in superconducting ultrafast optoelectronics. The reproducibility of the values of the critical current densities measured in different devices with the same geometry makes our nanowires ideal candidates for photoresponse experiments. First measurements have shown a satisfactory photoresponse from YBCO/LSMO devices.

Inverse magnetocaloric and exchange bias effects in single crystalline La0.5Sr0.5MnO3 nanowires

We report the first observation of inverse magnetocaloric effect (IMCE) in hydrothermally synthesized single crystalline La0.5Sr0.5MnO3 nanowires. The core of the nanowires is phase separated with the development of double exchange driven ferromagnetism (FM) in the antiferromagnetic (AFM) matrix, whereas the surface is found to be composed of disordered magnetic spins. The FM phase scales with the effective magnetic anisotropy, which is directly probed by transverse susceptibility experiments. The surface exhibits a glassy behavior and undergoes spin freezing, which manifests as a positive peak (TL ∼ 42 K) in the magnetic entropy change (−ΔSM) curves, thereby stabilizing the re-entrance of the conventional magnetocaloric effect. Precisely at TL, the nanowires develop the exchange bias (EB) effect. Our results conclusively demonstrate that the mere coexistence of FM and AFM phases along with a disordered surface below their Néel temperature (TN ∼ 210 K) does not trigger EB, but this develops only below the surface spin freezing temperature.

Synthesis of La<SUB>0.6</SUB>Nd<SUB>0.2</SUB>Na<SUB>0.2</SUB>MnO<SUB>3</SUB> Nanowire and Its Magnetism

The colossal magnetoresistance (CMR) manganite La0.6Nd0.2Na0.2MnO3 nanowires were synthesized in porous anodic alumina oxide (AAO) membrane via the sol-gel template route. X-ray diffraction (XRD) results of La0.6Nd0.2Na0.2MnO3/AAO composites verified the purity of the perovskite structure of La0.6Nd0.2Na0.2MnO3. Field emission scanning electron microscopy (FE-SEM) confirmed that the monodisperse cylindrical pores of alumina membrane were filled with manganite nanowire arrays. The nanowires were found to be essentially polycrystalline materials by high-resolution transmission electron microscope (HRTEM) images and selected electron diffraction (SAED) analysis. The Curie temperature of the La0.6Nd0.2Na0.2MnO3 nanowires determined by superconducting quantum interference device (SQUID) was much less than the the Curie temperature of the bulk material. According to our experimental results, we proposed the single domain size should be less than 50 nm conservatively. The result suggested that the finite size effect may play an important role in the reduction of the Curie temperature of the La0.6Nd0.2Na0.2MnO3 nanowires.

Large low field magnetoresistance in La0.67Sr0.33MnO3 nanowire devices

Large low field magnetoresistance (LFMR) of about 28% is observed in La0.67Sr0.33MnO3 nanowires with 80 nm in diameter at T=300 K. A gradual decrease in the LFMR has been found with increase in wire diameter. The LFMR drops to zero for wires above 280 nm in diameter. The nanowires are grown by means of electrospinning process and exhibit distorted orthorhombic crystal structure. The large LFMR is considered as a grain boundary effect as observed in several perovskite systems. The large LFMR observed in these manganites with reduced dimensions may be useful for room temperature device applications.

Exchange Bias Effect and Suppression of Charge Ordering Induced by a Surface Phase Separation in La0.25Ca0.75MnO3 Nanowires

The microstructure and magnetic properties of the polycrystalline La0.25Ca0.75MnO3 nanowires with diameter of about 60 nm prepared by a sol−gel template method were studied with use of X-ray diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry. Compared with the robust charge ordering (CO) state with CO transition temperature TCO ≈ 240 K in the bulk counterpart, the CO is suppressed with TCO ≈ 138 K and CO transition becomes wide, at the same time the ferromagnetic (FM) cluster glass state and the exchange bias effect are detected in La0.25Ca0.75MnO3 nanowires. This exotic behavior is attributed to the uncompensated surface spins which change the spin configuration of nanowires and enhance the eg charge density at the surface compared to bulk compound and thereby induce the FM cluster glass state as well as exchange bias effect, and weaken the CO.

Magnetic oxide nanowires with strain-controlled uniaxial magnetic anisotropy direction

While magnetic nanowires generally have a preferential magnetization direction along the wire axis to minimize magnetostatic energy, it is shown here for epitaxial magnetic oxide nanowires that substrate-induced strain can be used to tailor the magnetic easy axis in any direction. La0.67Sr0.33MnO3 (LSMO) nanowires were prepared by pulsed laser deposition of LSMO thin films on NdGaO3 (NGO) substrates of two different orientations [NGO(110)o and NGO(010)o], followed by patterning into arrays of nanowires by laser interference lithography. The uniaxial compressive strain from the substrate induces a strong uniaxial magnetic anisotropy in the LSMO that dominates the anisotropy. Hence, one obtains LSMO nanowires having a magnetic easy axis that can lie in any direction, including perpendicular to the wire axis. In marked contrast, similar nanowires on SrTiO3(001) substrates without significant uniaxial strain exhibit the usual preferential magnetization direction along the wire axis, as dictated by magnetostatic shape anisotropy. The tunable magnetic anisotropy direction is a useful feature for applications of magnetic nanowires in magnetic memory, sensor, and logic devices.

Single‐Crystalline La0.7Sr0.3MnO3 Nanowires by Polymer‐Template‐Directed Chemical Solution Synthesis

A facile polymer-template-aided synthesis is adopted to prepare single crystalline La0.7Sr0.3MnO3 nanowires by chemical solution deposition. The La0.7Sr0.3MnO3 nanowires crystallize in a novel monoclinic phase and exhibit ferromagnetic ordering at room temperature.

Synthesis and Structural Characterizations of Multi-Crystalline La0.5Ba0.5MnO3 Nanowires: A "Multi-Nuclei" Growth Mode

Multi-crystalline La0.5Ba0.5MnO3 Nanowires have been synthesized with cationic surfactant at 220 degrees C. The structure, composition and physical property of the as-prepared nanowires have been characterized by a series of techniques including XRD, SEM, HRTEM, electron energy-loss spectroscopy (EELS) and magnetic measurements. It is found that the La0.5Ba0.5MnO3 nanowire has the typical cubic perovskite structure, each nanowire is composed of many small crystalline grains with different orientations. Valance state of manganese ion in La0.5Ba0.5MnO3 nanowires is determined by EELS technique. A "multi-nuclei" growth mode is proposed based on the microstructure analysis. The temperature dependence of magnetization has been briefly discussed.

Polaron Activation Energy as Evidenced by EMR in Colossal Magnetoresistive Nanowires

In the present work an X-band electron magnetic resonance (EMR) investigation in the paramagnetic regime of colossal magnetoresistive manganite La2/3Ca1/3 MnO3 (LCMO) nanowires was carried out. The temperature dependence of the EMR line width has been analyzed in terms of the small polaron hopping scenario. From this analysis, the polaron activation energy Ea in LCMO nanowires was evaluated. A discussion is given concerning the factors which could explain the smaller value of Ea as compared with the corresponding ones for LCMO nanoparticles.

Synthesis and electronic properties of transition metal oxide core–shell nanowires

This paper reviews our recent progress on the synthesis andtransport studies of novel transition metal oxide nanowires, includingYBa2Cu3O6.66,Fe3O4,La0.67Sr0.33MnO3 and PbZr0.58Ti0.42O3. Key to our success is the growth of standing single-crystalline MgO nanowires, which workedas excellent templates for epitaxial deposition of the desired transition metal oxides and ledto high-quality core–shell nanowires. Transport studies have been conducted on bothFe3O4 and La0.67Sr0.33MnO3 core–shell nanowires. Devices based on individualFe3O4 nanowires exhibited room-temperature magnetoresistance of 1.2% under a magnetic field ofB = 1.8 T, asa result of tunnelling of spin-polarized electrons across anti-phase boundaries. In addition, devices madeof La0.67Sr0.33MnO3 nanowires showed a remarkable metal–insulator transition at 325 K,accompanied by room-temperature colossal magnetoresistance (CMR)∼10% under a 1 T magnetic field. More interestingly, shape-inducedmagnetoresistance was also observed at room temperature with theLa0.67Sr0.33MnO3 nanowires. Our technique could enable various in-depth studies such as phase transition innanoscale oxides and may pave the way for novel applications of these fascinatingmaterials.