Colossal Magnetoresistance Materials Research Articles

The First Molecular Ferroelectric Mott Insulator.

With the discovery of colossal magnetoresistance materials and high-temperature superconductors, Mott insulators can potentially undergo a transition from insulating state to metallic state. Here, in molecular ferroelectrics system, a Mott insulator of (C7H14N)3V12O30 has been first synthesized, which is a 2D organic-inorganic ferroelectric with composition of layered vanadium oxide and quinuclidine ring. Interestingly, accompanied by the ferroelectric phase transition, (C7H14N)3V12O30 changes sharply in conductivity. The occurrence of a Mott transition has been proven by electric transport measurements and theoretical calculations. This research has significantly expanded the applicative horizons of ferroelectric materials, and offering an ideal platform for the investigation of strongly correlated electron systems.

Control of chiral orbital currents in a colossal magnetoresistance material.

Colossal magnetoresistance (CMR) is an extraordinary enhancement of the electrical conductivity in the presence of a magnetic field. It is conventionally associated with a field-induced spin polarization that drastically reduces spin scattering and electric resistance. Ferrimagnetic Mn3Si2Te6 is an intriguing exception to this rule: it exhibits a seven-order-of-magnitude reduction in ab plane resistivity that occurs only when a magnetic polarization is avoided1,2. Here, we report an exotic quantum state that is driven by ab plane chiral orbital currents (COC) flowing along edges of MnTe6 octahedra. The c axis orbital moments of ab plane COC couple to the ferrimagnetic Mn spins to drastically increase the ab plane conductivity (CMR) when an external magnetic field is aligned along the magnetic hard c axis. Consequently, COC-driven CMR is highly susceptible to small direct currents exceeding a critical threshold, and can induce a time-dependent, bistable switching that mimics a first-order 'melting transition' that is a hallmark of the COC state. The demonstrated current-control of COC-enabled CMR offers a new paradigm for quantum technologies.

Colossal Magnetoresistance in Ti Lightly Doped Cr2Se3 Single Crystals with a Layered Structure.

Stoichiometric Cr2Se3 single crystals are particular layer-structured antiferromagnets, which possess a noncollinear spin configuration, weak ferromagnetic moments, moderate magnetoresistance (MR ∼14.3%), and poor metallic conductivity below the antiferromagnetic phase transition. Here, we report an interesting >16 000% colossal magnetoresistance (CMR) effect in Ti (1.5 atomic percent) lightly doped Cr2Se3 single crystals. Such a CMR is approximately 1143 times larger than that of the stoichiometric Cr2Se3 crystals and is rarely observed in layered antiferromagnets and is attributed to the frustrated spin configuration. Moreover, the Ti doping not only dramatically changes the electronic conductivity of the Cr2Se3 crystal from a bad metal to a semiconductor with a gap of ∼15 meV but also induces a change in the magnetic anisotropy of the Cr2Se3 crystal from strong out-of-plane to weak in-plane. Further, magnetotransport measurements reveal that the low-field MR scales with the square of the reduced magnetization, which is a signature of CMR materials. The layered Ti:Cr2Se3 with the CMR effect could be used as two-dimensional (2D) heterostructure building blocks to provide colossal negative MR in spintronic devices.

Real-space imaging of magnetic phase transformation in single crystalline Sm–Ca–Sr based manganite compound

Low-temperature-high-magnetic field magnetic force microscopy studies on colossal magnetoresistance material Sm0.5Ca0.25Sr0.25MnO3 have been carried out. These measurements provide real-space visualization of antiferromagnetic–ferromagnetic (AFM–FM) transition on sub-micron length scale and explain the presence of AFM–FM transition in the temperature-dependent magnetization measurements, but the absence of corresponding metal–insulator transition in temperature-dependent resistivity measurements at the low magnetic field. Distribution of transition temperature over the scanned area indicates towards the quench disorder broadening of the first-order magnetic phase transition. It shows that the length scale of chemical inhomogeneity extends over several micrometers.

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.

A highly accurate interatomic potential for LaMnO3 perovskites with temperature-dependence of structure and thermal properties

ABO3-type perovskites LaMnO3 is a colossal magnetoresistance material that undergoes an orthorhombic-rhombohedral phase transition as the temperature increases. Phase transition determines its structural properties and thermal conductivity that are important in industrial application. Structural transition is reflected mainly in Mn-O bond lengths and Mn-O-Mn angles between MnO6 octahedrons for Jahn-Teller effect during this progress and traditional born–mayer (BM) model has difficulty in description of structural distortion and thermal properties. In this paper, a new classic interatomic potential model for LaMnO3 was developed within the framework of bond valence (BV) theory. First-principles and intelligent optimization algorithm were used to fit the parameters, enabling the study of temperature-dependent structures by accurate large-scale molecular dynamics (MD) simulations. Calculated structural distortions by our model and thermal properties calculated with equilibrium molecular dynamics (EMD) method accorded with the experimental values within a reasonable error, and had higher accuracy than traditional born-mayer model. These results provided further insight about temperature-depended phase transition and further performance mining of LaMnO3. Most importantly, our potential model showed better accuracy than traditional potential model and is applicablefor all crystal materials with perovskite structures.

Structural and Elastic Behavior of Chromium Doped Pr<sub>0.5</sub>Sr<sub>0.5</sub>MnO<sub>3</sub> System

A series of colossal magneto resistance (CMR) materials with compositional formula Pr0.5Sr0.5Mn1-xCrxO3 (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by sol-gel technique using pure metal nitrates as the starting materials. These samples were characterized structurally by X-ray diffraction, FTIR and SEM. All the samples exhibit orthorhombic structure without any detectable impurities. The bulk densities for all the compositions were measured from the pellets. The Young’s and Rigidity moduli, Poisson’s ratio and Debye temperature of all the compositions were calculated with the experimentally measured ultrasonic longitudinal and shear velocities at room temperature using pulse transmission technique. As the materials are porous, zero porous elastic moduli have also been calculated using a well-known Hasselmann and Fulrath model. The observed variation of elastic moduli with varying chromium doping concentration has been studied qualitatively.

Structural, Magnetic and Magnetotransport Properties of Sol-Gel Synthesized La<sub>0.67</sub>Ca<sub>0.33</sub>MnO<sub>3</sub>:TiO<sub>2</sub> Nanocomposite

Colossal magnetoresistive (CMR) materials have been widely studied because of their huge potential in spintronic technology. An introduction of secondary phase to the manganite matrix is able to improve the low field magnetoresistance (LFMR). This method is favoured by recent research works as it requires a lower magnetic field compared to intrinsic magnetoresistance. Structural, magnetic properties and magnetotransport properties of polycrystalline (1-x) La0.67Ca0.33MnO3 (LCMO): x TiO2 composites where x = 0.00, 0.05, 0.10, 0.15 and 0.20 were investigated in this work. Polycrystalline La0.67Ca0.33MnO3 (LCMO) was synthesized via sol-gel method and pre-sintered at 800 °C before appending with nano-sized TiO2. All samples are in LCMO phase having an orthorhombic structure with space group Pnma. The crystal structural parameter is studied by using Rietveld refinement. As TiO2 content increases, the magnetization is getting higher as observed via vibrating sample magnetometer (VSM) analysis at room temperature. Magnetotransport properties of the pure LCMO sample have been studied from 80 – 220 K. The LFMR is enhanced as the temperature drops. The results have shown LCMO: TiO2 manganite composite is an excellent candidate for future magnetic sensors and memory devices.

Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10

Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.

Cation spin and superexchange interaction in oxide materials below and above spin crossover under high pressure

We derived simple rules for the sign of superexchange interaction based on the multielectron calculations of the superexchange interaction in the transition metal oxides that are valid both below and above spin crossover under high pressure. The superexchange interaction between two cations in d$^n$ configurations is given by a sum of individual contributions related to the electron-hole virtual excitations to the different states of the d$^{n + 1}$ and d$^{n - 1}$ configurations. Using these rules, we have analyzed the sign of the superexchange interaction of a number of oxides with magnetic cations in electron configurations from d$^2$ till d$^8$: the iron, cobalt, chromium, nickel, copper and manganese oxides with increasing pressure. The most interesting result concerns the magnetic state of cobalt and nickel oxides CoO, Ni$_2$O$_3$ and also La$_2$CoO$_4$, LaNiO$_3$ isostructural to well-known high-T$_C$ and colossal magnetoresistance materials. These oxides have a spin $\frac{1}{2}$ at the high pressure. Change of the interaction from antiferromagnetic below spin crossover to ferromagnetic above spin crossover is predicted for oxide materials with cations in d$^5$(FeBO$_3$) and d$^7$(CoO) configurations, while for materials with the other d$^n$ configurations spin crossover under high pressure does not change the sign of the superexchange interaction.

High magnetoresistance in La0.5Nd0.15Ca0.25A0.1MnO3 (A = Ca, Li, Na, K) CMR manganites: Correlation between their magnetic and electrical properties

The influence of A-site cation disorder of monovalent alkali ions on structural and magneto-transport properties of colossal magnetoresistive (CMR) materials has been reported. All the compounds crystallize in Pbnm space group with orthorhombic symmetry. At low temperature, the samples exhibit a paramagnetic (PM) to ferromagnetic (FM) transition and a semiconductor to metallic one. The variation in both Curie temperature (Tc) and metal-semiconductor transition temperature (TMS) can be explained by the change in Mn4+ content and average A-site cation radius. A decrease in resistivity in the vicinity of TMS has been observed with an increase in the applied magnetic field. The values of Tc are close to TMS, signifying strong correlations between the magnetic and electrical properties. The electrical resistivity data in paramagnetic-semiconducting region (T >TMS) may be described by the adiabatic small polaron hopping (ASPH) model. Maximum magnetoresistance (MR%) has been observed in the vicinity of TMS.

Room-Temperature Low-Field Colossal Magnetoresistance in Double-Perovskite Manganite.

We have discovered room-temperature low-field colossal magnetoresistance (CMR) in an A-site ordered NdBaMn_{2}O_{6} crystal. The resistance changes more than 2 orders of magnitude at a magnetic field lower than 2T near 300K. When the temperature and magnetic field sweep from an insulating (metallic) phase to a metallic (insulating) phase, the insulating (metallic) conduction changes to the metallic (insulating) conduction within 1K and 0.5T, respectively. The CMR is ascribed to the melting of the charge and orbital ordering. The entropy change which is estimated from the B-T phase diagram is smaller than what is expected for the charge and orbital ordering. The suppression of the entropy change is attributable to the loss of the short-range ferromagnetic fluctuation of Mn spin moments, which is an important key of the high temperature and low magnetic field CMR effect.

Structural and magneto-transport properties of Li-substituted La0.65Ca0.35-xLixMnO3 (0 ≤ x ≤ 0.15) CMR manganites

With an aspect to understand the influence of cation mismatch on magneto-transport properties of colossal magnetoresistive (CMR) materials, a systematic study of La0.65Ca0.35-xLixMnO3 (0 ≤ x ≤ 0.15) manganites has been undertaken. The materials were synthesized by sol-gel Pechini process and their structures were confirmed by the analysis of XRD data using Rietveld refinement technique. A paramagnetic to ferromagnetic transition and a semiconductor to metallic one have been observed at low temperature. Both Curie temperature (Tc) and metal-semiconductor transition temperature (TMS) in the vicinity of Tc first increases with Li doping and then show a decreasing trend, which is justified on the basis of a competition between double-exchange and super-exchange mechanisms. It has been concluded that the electrical resistivity data in high-temperature paramagnetic-semiconducting region (T > TMS) may be described by the small polaron hopping model. Also, a large magnetoresistance (MR%) has been observed in the vicinity of metal-semiconductor transition temperature TMS, which is comparable to the MR% in the low-temperature region.

La0·7Ca0.3−xSrxMnO3:Ag0.2 (0.0165 ≤ x ≤ 0.1) ceramics with large and stable TCR in different magnetic field environments

La0·7Ca0.3−xSrxMnO3:Ag0.2 (LCSMO:Ag) ceramics with varying strontium (Sr) content (x = 0.0165, 0.03, 0.05, 0.07, and 0.1) were synthesized using a combination of sol–gel and solid-state doping methods and studied as colossal magnetoresistance materials. Significant improvements in the peak temperature coefficient of resistivity (TCR) and resistivity (ρ) were observed across all ceramic samples when the mol% of Sr was adjusted. The low ρ and large TCR values resulted from a combination of two factors. First, the presence of Sr led to the increase in the grain size and decrease in the number of boundaries, which improved the electrical transport and reduced the boundary scattering effects. This resulted in additional improvement in the grain arrangement order. Second, the cell volume (Vcell) and octahedron volume (Voct) both decreased first and then increased. The MnO6 octahedron underwent gradual deformation into a flattened shape, which facilitated carrier migration and increased electrical sensitivity of the material. The ceramic with x = 0.03 maintained high and stable TCR values in different working environments, both without a magnetic field (TCRo, 54.76% K−1) and in the presence of a vertical field (TCR⊥, 33.33% K−1). Furthermore, the material exhibited a low resistivity and a higher insulator–metal transition temperature (Tp). Performance differences among the samples were comprehensively and systematically explained by conducting cross magnetic field tests with field directions parallel and perpendicular to the flat surface of the bulk material, respectively. Overall, the findings of this study indicate that these ceramics act as promising candidates for future applications in information storage devices and magnetic sensors.

Effect of colossal magnetoresistance material La0.67Ca0.33MnO3 on superconducting properties of YBa2Cu3O7-δ thin films

Abstract By specific design of the sample, in which SrTiO3 substrate is fully covered by a thin layer of the colossal magnetoresistive material La0.67Ca0.33MnO3 (LCMO) and the latter is partially covered by high-temperature superconductor YBa2Cu3O7-δ (YBCO), and by using multiple current and voltage contacts, direct evidence of the strong effect of LCMO on YBCO is obtained. It is found that LCMO strongly influences not only superconducting, but also normal state of YBCO, and it is argued that this is a consequence of the spin injection from the former to the latter. The effect of the deposition conditions and crystal orientation of YBCO layer on this effect is clarified. A surprising peak in the temperature dependence of resistance seen in ex-situ ab-plane oriented sample is explained as a combination of two effects: influence of spin-polarized electrons on superconductor below its critical temperature and the interface-controlled shift of Curie temperature of LCMO to low temperatures. Considering expected use of LCMO and YBCO in composite quantum computation circuits, their combination with another advanced quantum material, graphene, is explored.

Observation of Small Polaron and Acoustic Phonon Coupling in Ultrathin La0.7Sr0.3MnO3/SrTiO3 Structures

Understanding the underlying physics of interactions among various quasi‐particles is a fundamental issue for the application of spintronics and photonics. Here the observation of a coupling between the small polarons in the nanoscale ultrathin La0.7Sr0.3MnO3 (LSMO) films and the acoustic phonons in the SrTiO3 (STO) substrate using ultrafast pump–probe spectroscopy has been reported. According to the temperature‐ and wavelength‐dependent measurements, the amplitudes of the acoustic phonons are suppressed by tuning the small polarons absorption. This shows a coupled relationship between the acoustic phonons and the small polarons. At the probe photon energy of 1.55 eV where the polaron absorption is dominant, the acoustic phonons become unobservable. Furthermore, by performing the pump fluence dependent measurements on the LSMO films with different thicknesses, smaller acoustic phonon amplitudes are found in the thinner film with stronger small polaron binding energy. Such a coupled nature can be utilized to manipulate the small polarons using the acoustic phonons or vice versa, which is of great importance in device applications of colossal magnetoresistance materials.

Preparation and properties of La0.71Ca0.29Mn1−xCrxO3 polycrystalline composites

La0.71Ca0.29Mn1−xCrxO3 (x = 0, 0.01, 0.03, 0.05 and 0.10) polycrystalline ceramics were synthesized by sol–gel method with methanol as solvent. X-ray diffraction results assisted with Rietveld refinement analysis showed that all samples crystallized in the orthorhombic structure with Pnma space group. The Cr substitution modified not only the Mn–O bond length, but also the Mn–O–Mn bond angle. Scanning electron microscope (SEM) micrographs revealed that the samples had high density and dense grain boundary. Temperature dependence of resistivity (ρ–T) curves showed that all samples had metal–insulator transition (TMI) phenomenon. By the way, TMI decreased with the increase in Cr content. When the sample was applied with a magnetic field, the resistivity decreased and TMI shifted to the higher temperature region. The temperature coefficient of resistivity (TCR) of the sample reached the highest value of 38.9%·K−1 at x = 0.01, indicating that TCR can be subtly improved by slight substitution of Mn-site, thereby the colossal magneto-resistance materials have a huge application potential in bolometer/infrared detection.

Critical fluctuations upon photoinduced phase transition in manganite strips

Light, acting as an external stimulus to induce various intriguing phenomena ranging from photovoltaics to photoinduced catalysis, exerts prominent effects in strongly correlated systems. It would be of particular interest to investigate photon-induced emerging phenomena in spatially confined strongly correlated systems, which are important for applications of these materials in future electronic devices. Colossal magnetoresistive manganites materials offer an ideal platform for such study due to their sensitivity to photo-excitation. Here, we fabricated 900 nm wide La0.325Pr0.3Ca0.375MnO3 strips, whose width is comparable to the size of the electronic phase separation (EPS) domains in this system. We observed the photoinduced critical fluctuations in the strips, where abrupt resistivity jumps occurred upon photoinduced phase transition depending sensitively on the light intensity. Based on the microscopic views of the EPS domains under photoexcitation, we conclude that such photo-induced resistivity fluctuations originate from the photoinduced phase fluctuations of individual EPS domains when their size becomes comparable to the strip width.

Comparison of magnetisation and transport properties of processed La<SUB align="right">0.67Ca<SUB align="right">0.33MnO<SUB align="right">3 manganites for technological application

A lot of efforts are being made to look for suitable types of colossal magnetoresistance (CMR) materials with specific composition for technological applications. We have synthesised and performed the magnetisation and transport studies on La0.67Ca0.33MnO3 CMR polycrystalline materials prepared using melt and sintering technique. It is observed that the magnetoresistance is enhanced in partially melted sample with increase in insulator-to-metal transition (TMI) temperature enabling its possible applications at room temperature. This indicates the high degree of spin polarisation of the conduction electrons in partially melted sample. It has been observed that grain boundaries disrupt the local magnetic order and alter the electrical conduction of polarised electrons whereas the magnetisation behaviour is not significantly affected except the magnitude of the magnetisation. This suggests that the magnetic spin alignment in the weak link region of the sintered sample increases the absolute magnetisation. The melt processed method is the ideal method to achieve superior magnetic and transport properties for possible major technological application of colossal magnetoresistive devices.

A-site cationic disorder induced significantly large magnetoresistance in polycrystalline La0.2Gd0.5Ba0.3MnO3 compound

The observation of significantly large magnetoresistance at the liquid nitrogen temperature range in the polycrystalline La0.2Gd0.5Ba0.3MnO3 (LGBMO) compound has been addressed in the present manuscript. The motivation of considering LGBMO sample is the average ‘A’ site ionic radius 〈rA〉 and tolerance factor (t), almost same as that of La0.7Sr0.3MnO3 (LSMO), which is a well studied colossal magnetoresistive material. Magnetoresistance of the LGBMO compound has been compared with the LSMO as well as parent compound La0.7Ba0.3MnO3(LBMO) to show the enhancement of magnetoresistance in LGBMO compound. This observed nature has been elucidated considering the disorder induced short range magnetic interaction due to the enhance size disorder parameter (σ2). Our study revels that, size disorder parameter plays the crucial role for enhancing the colossal magnetoresistance.