Mixed-valence Manganites Research Articles

The magnificent manifestation of Nd0.7Sr0.3MnO3 ceramics: A comprehensive exploration of different wet chemical routes via sol-gel and thermal treatment methods

The mixed-valence manganite has sparked immense interest owing to its intriguing colossal magnetoresistance (CMR) effect. However, a research gap exists in the current literature regarding Nd-based manganites with intermediate bandwidth. This paper addresses this gap by investigating Nd0.7Sr0.3MnO3 (NSMO) ceramics prepared through two different synthesis approaches: the newly developed thermal treatment method (TT) and the well-established sol-gel method (SG). The XRD analysis revealed a single high-purity NSMO phase with insignificant alteration in Mn-O bond length and Mn-O-Mn bond angle for both samples. Nevertheless, SG exhibited larger particle size (209.3 nm) and better crystallinity in comparison to TT (178.6 nm). These factors significantly affect the magnetic transition width while preserving the Curie temperature, TC ≈ 246 K. Regarding electrical behaviour, TT showed a robust suppression of the metal-insulator transition temperature, TMI from 224 K (SG) to 182 K. This difference can be attributed to the existence of magnetic disordered layer at the grain boundaries. Consequently, this resulted in an increased scattering process and elevated resistivity from 8 Ω (SG) to 22 Ω (TT). Various theoretical models were employed to elucidate the transport mechanism at different temperature ranges. Interestingly, both samples displayed distinct magneto-transport behaviour despite negligible differences in their structural parameters. The occurrence of magnetoresistance peak in SG suggests a pronounced intrinsic CMR effect, while its absence in TT implies a dominant influence of extrinsic CMR. These findings highlight that the physical properties of the manganite are not only governed by their intragrain properties (TC) but are also influenced by intergrain contributions (TMI) when various synthesis routes were utilised.

Investigations on structural and electrical properties of YMnO3 based mixed valent manganites

In this communication, the effect of doping of divalent cations (Sr, Ca) at Y–site in pure YMnO3 on their structural and electrical properties has been investigated. X–ray diffraction reveals dual phase nature of Ca–doped sample. The dielectric behaviours have been fitted using cole–cole relaxation. Role of secondary phase of 41% hexagonal YMnO3 coexists with 59% orthorhombic Y0.9Ca0.1MnO3 for Ca–doped YMnO3 compound has been discussed where orthorhombic phase provides major contribution for large dielectric constant. The ac conductivity has been explained by correlated barrier hopping mechanism. The current–voltage characteristics have been understood using Simmon’s model.

Tuning the metal-insulator transition temperature by the controlled generation of oxygen vacancies on La0.7Ca0.3MnO3-x epitaxial thin films

This report presents a study on the effect of the controlled generation of oxygen vacancies on the electronic transport properties of La0.7Ca0.3MnO3-x epitaxial thin films. Oxygen defects cause significant changes in the crystalline structure of mixed-valence manganites and lower the Mn valence, resulting in a shift of the metal-insulator transition to lower temperatures due to modifications in the angles and lengths of Mn-O bonds. Experiments using Polarized Extended X-Ray Absorption Fine Structure Spectroscopy have provided strong evidence that oxygen vacancies are mainly formed in the basal plane of the MnO6 octahedra. The accurate control of oxygen vacancies generation opens possibilities for managing and enhancing the magneto-electronic properties of epitaxial manganite thin films with potential in industrial applications.

Fully Magnetically Polarized Ultrathin La0.8Sr0.2MnO3 Films.

We report the observation of fully magnetically polarized ultrathin La0.8Sr0.2MnO3 films by using LaMnO3 and La0.45Sr0.55MnO3 buffer layers grown epitaxially on SrTiO3(001) substrates by molecular beam epitaxy. Specifically, we show that La0.8Sr0.2MnO3 films grown on 12-unit-cell LaMnO3 have bulk-like magnetic moments starting from a single unit cell thickness, while for the 15-unit-cell La0.45Sr0.55MnO3 buffer layer, the La0.8Sr0.2MnO3 transitions from an antiferromagnetic state to a fully spin-polarized ferromagnetic state at 4 unit cells. The magnetic results are confirmed by X-ray magnetic circular dichroism, while linear dichroic measurements carried out for the La0.8Sr0.2MnO3/La0.45Sr0.55MnO3 series show the presence of an orbital reorganization at the transition from the antiferromagnetic to ferromagnetic state corresponding to a change from a preferred in-plane orbital hole occupancy, characteristic of the A-type antiferromagnetic state of La0.45Sr0.55MnO3, to preferentially out of plane. We interpret our findings in terms of the different electronic charge transfers between the adjacent layers, confined to the unit cell in the case of insulating LaMnO3 and extended to a few unit cells in the case of conducting La0.45Sr0.55MnO3. Our work demonstrates an approach to growing ultrathin mixed-valence manganite films that are fully magnetically polarized from the single unit cell, paving the way to fully exploring the unique electronic properties of this class of strongly correlated oxide materials.

Effect of intrinsic vacancies on the electromagnetic properties of half-doped Sm0.5Ca0.5MnO3 manganites studied by positron annihilation

Mixed valence manganites are potential functional materials due to their unique electromagnetic properties. In this work, half-doped ceramics with the perovskite structure Sm0.5Ca0.5MnO3 polycrystalline samples are synthesized by the solid-state reaction method in open air at different final sintering temperatures. Structures and particle sizes are studied by x-ray diffraction and scanning electron microscopy, respectively. Positron annihilation spectroscopy and density-functional theory calculations are used to characterize the intrinsic vacancies in the bulk. Thereafter, the effect of vacancies on the magnetic and magnetoresistance (MR) properties is investigated. We find that Mn monovacancies (VMn) exist in the bulk among all the samples, and the concentration of VMn is different. We suggest a possible defect model that can be well applied to explain the phenomena of their electromagnetic properties. The transition temperature of the charge-order state (TCO) and ferromagnetic and anti-ferromagnetic (TN) are most likely related to the concentration of VMn and the particle sizes or pore sizes, respectively. The glass spin state transition temperature seems to be independent of the defect concentration and type. The metal conductive behavior does not appear in a high magnetic field and at low temperatures, which may be caused by the presence of a high concentration of VMn in the bulk and oxygen-related defects in the boundary, and the double exchange interaction is suppressed. At temperatures below TN and under a weak magnetic field, the MR is related to the total defect concentration. In addition, the high concentration of VMn is unfavorable for obtaining a high MR value.

An investigation on strain and magnetic properties of perovskite manganite superlattice films

The mixed-valence perovskite manganites attracted much attention because of their interesting electro-magnetic properties, and strain modulation on magnetic properties of perovskite manganites is worth exploring. In this paper, [La[Formula: see text]Ca[Formula: see text]MnO3/La[Formula: see text]Ca[Formula: see text]MnO3][Formula: see text] superlattice films with different La[Formula: see text]Ca[Formula: see text]MnO3 layer thicknesses are prepared by pulsed laser deposition (PLD). The crystal structures (lattice constants) of samples are measured by X-ray diffraction (XRD). The strain of the films is determined according to their crystal structures. The magnetic hysteresis loops (M-H loops) and magnetization versus temperature (M-T) curves of these superlattice films are measured by the physical property measurement system (PPMS). From the M-H loops, the coercive field (H[Formula: see text] of the samples can be measured. From the M-T curves, the Curie temperature (T[Formula: see text] of the samples can be obtained. All samples show a ferromagnetic to paramagnetic transition at TC, and the [La[Formula: see text]Ca[Formula: see text]MnO3/La[Formula: see text]Ca[Formula: see text]MnO3][Formula: see text] sample with the La[Formula: see text]Ca[Formula: see text]MnO3 layer thicknesses of 72[Formula: see text]Å has an antiferromagnetic Néel transition at TN. According to the strain state and magnetic phases, the magnetic properties are comprehensively analyzed. It is found that with the increase of ferromagnetic La[Formula: see text]Ca[Formula: see text]MnO3 layer thickness, the ferromagnetic phase is increased, and the double exchange effect can also be enhanced, resulting in the increase of TC. When the thickness of La[Formula: see text]Ca[Formula: see text]MnO3 reaches 96[Formula: see text]Å, the uneven strain distribution in the superlattice can induce the reduction of ferromagnetic phase compared with antiferromagnetic phase (even antiferromagnetic phase starts to appear), and the double exchange effect can be weakened, and finally leading to the decrease of TC. In addition, with increasing the La[Formula: see text]Ca[Formula: see text]MnO3 layer thickness, HC gradually decreases. The change of HC is related to strain states and magnetic phase interactions in the samples. The analysis of the strain and magnetism can contribute to the understanding of perovskite manganite superlattice films.

Role of cationic size mismatch and effect of disorder in mixed valent manganites

Comparative studies of structure, magnetism, and magnetoresistance (MR) have been carried out in A-site ordered NdBaMn2O6 (O-NB), A-site disordered NdBaMn2O6 (D-NB) and A-site disordered NdCaMn2O6 (D-NC). O-NB, where A-site cations, Nd3+ and Ba2+ (of different ionic sizes) are arranged periodically, undergoes structural transition with temperature, while no structural change is present in D-NB where A-site cations are arranged randomly. However, structural transitions are observed in D-NC where Nd3+ and Ca2+ have similar ionic sizes. Magnetization (M) data shows O-NB has AFM ground state associated with a lower structurally symmetric phase and an FM ground state is observed for D-NB with higher structural symmetry. However, AFM ground state is observed in D-NC similar to that of O-NB. Both the disorder systems exhibit semiconductive transport characteristics over the entire temperature range. The resistivity data of disorder compounds have been fitted with different theoretical models to elucidate the conduction process in these systems. Further, MR studies depict a three times higher value of MR in both disorder compounds compared to that of order one. However, the behavior of MR with H is different for D-NB and D-NC, implying a different origin of this large MR in these compounds. We believe that the different magnetic ground state of D-NB and D-NC is the possible origin of their distinct MR behavior to the magnetic field.

Variation of structural and magnetic properties of mixed-valent manganites through A-site cationic ordering

Effect of A-site cationic ordering on structural, transport and magnetic properties of mixed-valent perovskite manganite NdBaMn2O6 has been studied. Three samples of same chemical composition NdBaMn2O6 but with different degrees of A-site ordering (92 %, 60 %, 28 %) were prepared by tuning the synthesis conditions. At higher temperatures, all the compounds possess the same crystallographic phase (P4/mmm). However, temperature (T) dependent X-ray diffraction data reveals that different types of crystallographic phase transitions depends on the degree of ordering of the A-site cations Nd3+ and Ba2+. For example, the maximum ordered compound undergoes multiple structural phase transitions (from P4/mmm to Cmmm to P21am) with decreasing T, whereas the disordered compound retains the high-T crystal structure (P4/mmm) throughout the studied temperature range. Resistivity measurements shows that although both the compounds (ordered and disordered) are insulating, the mechanism of charge transport is different for the two compounds. Similarly the magnetic properties and colossal magnetoresistance also depend on the degree of ordering. Cationic disorder suppresses the long-range antiferromagnetic order and enhances ferromagnetic correlations at lower temperature; which (ferromagnetic correlation) is accompanied with larger value of saturation magnetic moment while ferromagnetic TC too shifts to higher temperature with increasing disorder. Disordered system also exhibits a larger value of magnetoresistance at low temperature compared to the ordered counterpart.

Magnetic field control of insulator-metal crossover in cobaltite films via thermally activated percolation

Competition between the double-exchange hopping and Coulomb repulsion underlies fertile phenomena in mixed-valent manganites, such as colossal magnetoresistance associated with a magnetic-field-driven insulator-metal transition (IMT). In contrast, an analogous double-exchange system, perovskite cobaltites, however, shows much smaller magnetoresistance, and the insulator-to-metal phase evolution with field stimulus remains elusive. Here, we unveil a submicrometer-scale phase separation and magnetic-field-controlled crossoverlike IMT in a tensile-strained ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ film. Our transport, magnetization, and magnetic-force-microscopy measurements reveal that, although the IMT is barely induced under an isothermal field sweep, it emerges under field cooling through a thermally activated ferromagnetic domain percolation. Such thermodynamic-history-dependent properties signify a nonergodic feature associated with the microscopic phase separation. By further comparing the transport properties among ${\mathrm{La}}_{0.7}\mathrm{A}{\mathrm{E}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ films with AE from Ca, Sr, to Ba, we reveal that the nonergodic behavior prevails in these films and that the mixed spin states are a key factor to enhance the energy barrier for domain-wall motions during domain percolation. Such a spin-state degree of freedom is absent in manganites, probably resulting in the large difference of the phase evolution kinetics in the magnetic-field-induced IMT between cobaltites and manganites.

Linear Magnetodielectric Effect and Huge Negative Magnetization in a Mixed-Valent Cobalt Manganite Spinel Oxide

Linear Magnetodielectric Effect and Huge Negative Magnetization in a Mixed-Valent Cobalt Manganite Spinel Oxide

Impact of NiO nano-particles on colossal magneto-resistance of La0.70Ca0.30MnO3 composite

Mixed-valent perovskite manganite has gained considerable research interest due to the increasing demand for energy storage materials. Herein, we show that colossal magneto-resistance properties of La0.7Ca0.3MnO3 (LCMO) can be changed via nickel oxide (NiO) doping. A two-step conventional solid-state reaction method is used to synthesize LCMO-10% NiO composite. The phase purity, structural, electrical (resistivity), and magneto-transport properties of as-synthesized LCMO-10% NiO composite are detailed. NiO doping shifts metal–insulator transition temperature (TMI) from 263 K to 195 K and increases resistivity. The magneto-resistance (MR) measurements showed 10% NiO doping resulted in a maximum MR of ∼ -70.32% at 200 K (near TMI) and 10 T applied magnetic field. However, -0.0849% MR is seen at 290 K (RT) and 0.3 T. The increased ∼ -63.85% MR is found at 100 K and at 10 T, which is comparatively much higher than the pristine LCMO sample. This is due to the addition of NiO in LCMO that improves physical properties (i.e., grain size).

Electric Field Effects on Charge Conduction for LaMnO3 Controlled La0.7Ca0.3MnO3 Manganite

Control over the different electronic states in mixed valent manganites advances the development of oxide based electronic devices. In this context, carrier density is one of the key aspects that controls the electronic ground states of manganites in a sparkling way since it does not create any impurity within the manganite lattice, unlike chemical doping process. LaMnO3–δ / La0.7Ca0.3MnO3 / LaAlO3 (LMO / LCMO / LAO) manganite based heterostructure was fabricated by low cost chemical solution deposition (CSD) method where thickness of LMO ∼ 50 nm and LCMO ∼ 100 nm were grown on (100) single crystalline substrate. Structural quality and epitaxial nature of heterostructure was verified by performing X–ray diffraction (XRD) ϕ–scan measurement on grown La0.7Ca0.3MnO3 / LaAlO3 (LCMO / LAO) 100 nm thin film and LMO / LCMO / LAO heterostructure. Understanding of few other structural, morphological and chemical aspects of presently studied LMO / LCMO interface and LMO / LCMO / LAO heterostructure has been presented through grazing incidence X–ray diffraction (GIXRD), Rocking curve XRD, cross sectional scanning electron microscopy (SEM), atomic force microscopy (AFM) and X–ray photoelectron spectroscopy (XPS) measurements. Electric field modulated charge conduction for LCMO manganite channel has been understood in the context of field effect configuration (FEC) based transport measurements where temperature dependent LCMO channel resistance was recorded under different control electric fields applied across LMO / LCMO interface in LMO / LCMO / LAO heterostructure. Thermal hysteretic resistance measurements for LCMO manganite channel under zero control electric field exhibit metal to insulator phase transition at TP with finite width of the hysteretic resistance behaviours between two thermal cycles, i.e. cooling and heating. This has been understood on the basis of phase separation scenario, freezing effect and non–reversible electronic phase transformation. Applied control electric field induced improvement in resistance of LCMO manganite channel and reduction in value of TP have been understood on the basis of charge injection process, strain induced disorder dependent charge conduction, phase separation based movements of metal–insulator interface and negligibly small Joule heating effect. Realization of insulating phase fraction, arrested during thermal cycles, has been discussed for the explanation of reduced width of the hysteretic resistance behaviours of LCMO channel upon increase in control electric field applied across LMO / LCMO interface. Possible charge conduction mechanisms have been understood on the basis of Zener double exchange (ZDE) polynomial law for obtained metallic state of LCMO channel. Variation in electroresistance (ER) of LCMO manganite channel with temperature under different control electric fields (applied across LMO / LCMO interface) has been discussed on the basis of charge injection process with an effective role of phase separation scenario of LCMO channel in LMO / LCMO / LAO heterostructure.

Variation of Structural and Magnetic Properties of Mixed-Valent Manganites Through A-Site Cationic Ordering

Variation of Structural and Magnetic Properties of Mixed-Valent Manganites Through A-Site Cationic Ordering

Variation of Structural and Magnetic Properties of Mixed-Valent Manganites Through A-Site Cationic Ordering

Variation of Structural and Magnetic Properties of Mixed-Valent Manganites Through A-Site Cationic Ordering

Effects of Cr,Co,Ni substitution at Mn-site on structural, magnetic properties and critical behaviour in Nd0.67Ba0.33MnO3 mixed-valent manganite

This work investigates on the crystal structure, magnetic nature and critical behaviour with the partial replacement of transition (Tr) elements (ie., 10% of Cr, Co, Ni) at the Mn-site of Nd0.67Ba0.33MnO3 (NBMO) synthesised via conventional solid-state-method. NBMO compound is crystallised into an orthorhombic crystal structure and possess a magnetic transition temperature (Tc) at 145 K. Crystal structure of NBMO remains the same with partial substitution of Tr elements but, the Tc is found to be slightly increased with the doping of Cr, while decreases with the doping of Co and Ni. The magnetic isotherms are traced, and critical exponents are studied using modified Arrott plots (MAP) and power law. All the substituted samples show a second-order nature of Tc similar to that of the parent compound. The critical parameters are identified and verified through MAP, critical isotherms, Kouvel-Fisher method and field dependence of entropy change. All the substituted compounds govern the 3D Heisenberg with short-range order.

Charge transport mechanisms in monovalent doped mixed valent manganites

In this communication, we report the results of the studies on structural and transport properties of monovalent Na+ doped La1–xNaxMnO3 (LNMO; x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) manganites synthesized by conventional ceramic method. X-ray diffraction (XRD) and Rietveld refinements reveal the single phasic nature of LNMO manganites without any detectable impurity within the measurement range. Temperature dependent resistivity, under different applied magnetic fields, has been performed on LNMO samples. Samples understudy exhibit metal to insulator (semiconductor) transition at temperature TP which is strongly influenced by the substitution of Na+ at La3+ site. r – T plots also exhibit resistivity upturn behavior at low temperature well below 40K under all the applied fields. Variation in TP and resistivity has been discussed in the context of the competition between the transport favoring tolerance factor and zener double exchange (ZDE) mechanism and transport degrading Jahn–Teller (JT) and size variance effects. In order to understand the mechanisms responsible for the charge transport in metallic and semiconducting regions and to explore the possible electronic processes responsible for the observed low temperature resistivity minima in all the presently studied LNMO manganites, various models have been employed. It has been found that VRH mechanism gets successfully fitted to the resistivity data in the semiconducting region while ZDE polynomial law is responsible for the charge conduction in metallic region for all the presently studied LNMO samples. A strong dependence of activation energy on the Na+ – content as well as applied magnetic field has been discussed in the context of variation and interrelations between the structural parameters. Charge conduction in metallic region has been discussed in the light of electron–phonon interactions which is influenced by the Na+ – content and applied magnetic field. Electrostatic blockade model has been employed to understand the low temperature resistivity minima behavior. Blocking energy for the charge carriers shows a dependence on the magnetic energy provided to the charge carriers. Present study can be useful to understand and to control the charge conduction in the manganites and hence to design the manganite based thin film devices for various spintronic applications.

Electronic phase derived impedance spectroscopic behavior of La0.5Nd0.2A0.3MnO3 manganites

In the present communication, structural, microstructural and electrical properties of La0.5Nd0.2Ca0.3MnO3 (LNCMO) and La0.5Nd0.2Sr0.3MnO3 (LNSMO) mixed valent manganite films, grown on single crystalline (0001) Al2O3 (ALO) substrates using pulsed laser deposition (PLD) technique, have been studied. After deposition, one set of LNCMO and LNSMO films were annealed in oxygen environment at 500 °C (hereafter referred as LNC–A & LNS–A, respectively) and compared their properties with other set of PLD as–grown LNCMO and LNSMO films (hereafter referred as LNC and LNS, respectively). Single crystalline growth of LNCMO films and polycrystalline growth of LNSMO films have been identified through X–ray diffraction (XRD) measurement. Atomic force microscopy (AFM) measurement for morphological studies indicates that divalent dopant and oxygen annealing process play an important role in governing the grain growth and surface nature of the studied films. A decrease in the value of resistance (R) and impedance (Z) with increase in frequency under the applied magnetic field (H = 1 T) has been observed for all the studied films. For LNC & LNC–A films, reactance is found to decrease monotonically with increase in frequency throughout its range studied while for LNS and LNS–A films, reactance initially increases followed by reduction in its value with increase in frequency under 0 and 1 T magnetic field has been observed. The variation in MR for LNS and LNS–A films has been discussed in the context of charge transport mechanism and Lorentz force. Further, to discuss the role of grain and grain boundaries, nyquist plots and their suitable fits under zero and 1 T external magnetic field have been investigated for all the studied mixed valent manganite films. Important role of phase separation scenario in studied manganite films has been discussed in detail based on magnetization measurement. Double magnetic phase transition and fluctuations in derivative of magnetization have been ascribed to the phase separation scenario of LNCMO / ALO and LNSMO / ALO mixed valent manganite thin films.

Gold-embellished mixed-valence manganite as a smart, self-regulating magnetoplasmonic nanomaterial

In the present study, lanthanum-strontium manganite (LSM) particles were embellished with gold nanoparticles to create a magnetoplasmonic composite core-shell material enabling bimodal heating regimens upon exposure to an alternate current (AC) magnetic field and/or near-infrared (NIR) radiation. The composite particles were prepared by combined solid-state synthesis, mechanochemical processing and wet chemistry. The core crystals were round but faceted, often growing into an equilibrium shape conforming to the Wulff construction and the orthorhombic Pbnm crystal symmetry. They were also multi-domain but nanocrystalline, exhibiting superparamagnetic behavior as the result of evening the local anisotropies by the exchange interaction. The deposition of the gold nanoparticles onto the manganite core resulted in the red shift of the localized surface plasmon resonance due to interfacial effects. This deposition also improved the conditions for colloidal stability in water by increasing the surface charge density in the electric double layer surrounding the particles. The hybrid magnetoplasmonic particles were evaluated for their potential for use in cancer ablation therapies by measuring their aqueous heating dynamics in an ultralow AC magnetic field mimicking the magnetic hyperthermia (MH) condition of deep-seated tumors and the NIR light mimicking the photothermal (PT) condition. Aggregation of spherical, 5–10 nm sized gold nanoparticles on the surface of LSM enhanced their NIR light absorption capacity. The heating of the particle suspensions was more intense in the PT mode than in the MH mode, but the latter mode allowed for the display of the smart, thermostatic behavior by the material. Namely, the stoichiometry of the manganite phase, La0·76Sr0·24MnO3, was adjusted to display the Curie temperature at 45.8 °C after decoration with gold and the thermostatic effect at 43–44 °C, with a potential to prevent the overheating of tissues during the MH treatments of cancer. The bimodal heating potential endows the particles with the ability to be used in personalized cancer therapies. These therapies would be able to adjust in situ the ratio between the two heating modes, one more suitable for superficial ablation (PT) and another one for the treatment of deeper tumors (MH).

Structural and Electrical Properties of La<sub>0.7</sub>Ca<sub>0.3</sub>MnO<sub>3</sub> /α-Fe<sub>2</sub>O<sub>3</sub> Composites

Colossal magnetoresistive (CMR) materials have huge potential in modern application and it has been widely used in magnetic sensing industry. From the literature, an incorporation of secondary insulating phase into mixed-valence manganites could improve its extrinsic effect especially low-field magnetoresistance (LFMR). However, nanoparticle addition could lead to substitution and diffusion with its parent compound. In this work, the structural and electrical properties of La0.7Ca0.3MnO3 (LCMO) were investigated by adding the α-Fe2O3 nanoparticle with ratio of 0.00, 0.05, 0.10, 0.15 and 0.20 as the artificial grain boundaries. The LCMO compound has been synthesised using sol-gel route. The samples were chosen to sinter at 800°C to obtain the pure LCMO phase by referring to the thermogravimetric analysis (TGA). The structural properties were investigated by an X-ray diffractometer (XRD) while electrical properties were measured by a four-point probe (4PP) system. XRD patterns showed the coexistence of two phases (LCMO & α-Fe2O3). LCMO crystallised in orthorhombic structure with space group Pnma while α-Fe2O3 exhibited in hexagonal form with space group R-3c. As the content of α-Fe2O3 increases, the resistivity of the samples increases drastically. Nevertheless, the addition of iron oxide has no significant effect on the metal-insulator transition temperature (T­MI). From the XRD and 4PP analysis, it can be deduced that the α-Fe2O3 nanoparticles do not react with LCMO compound and successfully formed the La0.7Ca0.3MnO3 /α-Fe2O3 composites. The resistivity increases when the nano-sized α-Fe2O3 is added into LCMO nanocomposites due to the insulator nature of α-Fe2O3.

Electron Doping Effect in the Resistive Switching Properties of Al/Gd1-xCaxMnO3/Au Memristor Devices.

We report on the resistive switching (RS) properties of Al/Gd1–xCaxMnO3 (GCMO)/Au thin-film memristors. The devices were studied over the whole calcium substitution range x as a function of electrical field and temperature. The RS properties were found to be highly dependent on the Ca substitution. The optimal concentration was determined to be near x = 0.9, which is higher than the values reported for other similar manganite-based devices. We utilize an equivalent circuit model which accounts for the obtained results and allows us to determine that the electrical conduction properties of the devices are dominated by the Poole–Frenkel conduction mechanism for all compositions. The model also shows that lower trap energy values are associated with better RS properties. Our results indicate that the main RS properties of Al/GCMO/Au devices are comparable to those of other similar manganite-based materials, but there are marked differences in the switching behavior, which encourage further exploration of mixed-valence perovskite manganites for RS applications.