Colossal Magnetoresistance Research Articles

Magnetic Doublon Bound States in the Kondo Lattice Model.

We present a novel pairing mechanism for electrons, mediated by magnons. These paired bound states are termed "magnetic doublons." Applying numerically exact techniques (full diagonalization and the density-matrix renormalization group, DMRG) to the Kondo lattice model at strong exchange coupling J for different fillings and magnetic configurations, we demonstrate that magnetic doublon excitations exist as composite objects with very weak dispersion. They are highly stable, support a novel "inverse" colossal magnetoresistance and potentially other effects.

Local structure of potassium doped nickel oxide: A combined experimental-theoretical study

The electronic structure of Mott and charge-transfer insulators can be tuned through charge doping to achieve a variety of fascinating physical properties, e.g., superconductivity, colossal magnetoresistance, and metal-to-insulator transitions. Strong correlations between $d$ electrons give rise to these properties but they are also the reason why they are inherently difficult to model. This holds true especially for the evolution of properties upon charge doping. Here, we hole-dope nickel oxide with potassium and elucidate the resulting structure by using a range of experimental and theoretical tools; potassium is twice as big as nickel and is expected to lead to distortions in its vicinity. Our measurements of the x-ray absorption fine structure (XAFS) show a significant distortion around the dopant and that the dopant is fully incorporated in the nickel oxide matrix. In parallel, the theoretical investigations include developing a Gaussian process for quantum Monte Carlo calculations to determine the lowest energy local structure around the potassium dopant. While the optimal structures determined from density functional theory and quantum Monte Carlo calculations agree very well, we find a large discrepancy between the experimentally determined structures and the theoretical doped structures. Further modeling indicates that the discrepancy is likely due to vacancy defects. Our work shows that potassium doping is a possible avenue to doping NiO, in spite of the size of the potassium dopant. In addition, the Gaussian process opens up a new route towards obtaining structure predictions outside of density functional theory.

Titanium effect on the structural, morphological and transport properties of La0.7Sr0.3Mn1-xTixO3 (x = 0.03, 0.06 and 0.12)

A formidable amount of work has been devoted aiming towards the understanding of the structure and properties of colossal magnetoresistance (CMR) manganites. Mixed valent manganites with the general formula R1-xAxMnO3 (R = trivalent rare earth ion and A = divalent ion like Sr+2, Ba+2, Pb+2, Ca+2 etc.) are found to exhibit variety of phenomena such as charge, spin and orbit ordering, electronic, magnetic and structural transitions depending on the type and amount of doping (x). In this paper, doping dependent modifications in the microstructure polycrystalline La0.7Sr0.3Mn1-xTixO3 (LSMTO) (x = 0.03, 0.06 and 0.12) compounds synthesized by hybrid method (sol gel method + solid state reaction method) have been discussed in detail. In the present work, the powder X-ray diffraction patterns reveals that the samples are single phase in nature and the samples show a rhombohedral structure in hexagonal lattice with the space group R3-c. The Rietveld refinement was performed using FullProf software for the detailed structural investigation. In this context, the lattice parameters, unit cell volume and χ2 were obtained for LSMTO samples. On the substitution of Mn+4 by Ti+4, the structure does not undergo any phase transformation, but lattice parameters steadily increase as Ti+4 doping concentration increases. It can be related to the larger ionic radius of Ti+4 (rTi+4 = 0.605 Å) ion compared to that of Mn+4 (rMn+4 = 0.540 Å) ion. FT-IR spectra for LSMTO of different concentration show the four peaks around 400–500 cm−1 (Mn-O-Mn bending mode), 610–670 cm−1 (Mn-O stretching mode), 2330–2400 cm−1 (moisture contain) and 540 cm−1 (Ti-O stretching modes). Weight loss % versus temperature plot (TGA) for the x = 0.06 sample confirms that the precursor shows three step weight loss profiles and does not melt up to the temperature of 1050 °C. A quantitative analysis of the energy dispersive spectroscopy (EDS) data indicates that the observed concentration of elements is very close to the calculated values from its chemical formula. Typical SEM micrographs for all the three samples show that each sample possesses fine and clear grain boundaries (GBs). From the R-T measurements, an increase in resistivity and a decrease in metal to insulator transition temperature (TMI) were observed by increasing doping level of Ti for Mn. All the LSMTO samples show the resistivity suppression under applied magnetic field. The maximum MR % (89%) was observed for x = 0.12 at 5 T compare to other doped samples. This result suggests that the magnetoresistance behavior is improved by addition of Ti at Mn. These results prove that concentration Ti substitution at Mn site enhances the various properties of this manganite system.

Fabrication of Stannate Perovskite Structure as Optoelectronics Material: An Overview

This paper presents a review of recent fabrication progress of perovskite-type material suited for the future optoelectronics applications. Wide varieties of optoelectronic devices include solar cell, liquid displays, transparent FETs, etc are becoming the mainstream for the future electronics global industry. In June 2015, the major breakthrough of perovskite structure in solar energy harvesting with PCE of 20.1% has achieved. Since then, numerous research has been conducted progressively to further enhance the performance of the perovskite structure as new alternative materials for optoelectronics applications. The perovskite-type oxide is having typical ABO3 crystallized structure. It is one of an important class of materials that have many exceptional physical properties such as superconductivity, colossal magnetoresistance, ferromagnetic, piezoelectric, high-transition-temperature superconductivity, ferroelectricity, piezoelectricity, and photoelectrochemical sensitivity. In this paper, we reviewed development progress one of the major classes of perovskite-type materials namely Stannate-based. Calculated data from simulation results such as DFT and first principle were excluded and only fabricated devices are covered in this paper.

Peculiarities of magneto-infrared reflectivity of nanostructured manganite films

The electrical resistance and reflectance of unpolarized infrared radiation have been studied in the La2/3Ba1/3MnO3 films with and without a variant structure. Applying of an external magnetic field leads to appearance of magnetoreflection and colossal magnetoresistance effects with maxima near the Curie temperature of the films. The high-angle boundaries markedly enhance the electric and magnetooptical properties of nanostructured epitaxial films with the variant structure. For example, an additional low-temperature contribution to the magnetoreflection has been revealed in La2/3Ba1/3MnO3/ZrO2(Y2O3) films. This peculiarity is associated with the change of the high-frequency conductivity of the films with the variant structure due to the processes of tunnelling of spin-polarized electrons across the boundaries of structural domains far below the Curie temperature of the sample. The detection of “tunnel magnetoreflection” in the doped lanthanum manganite films could promote extension of the scope of their application in the modern optoelectronics.

Probing Phase Separation and Local Lattice Distortions in Cuprates by Raman Spectroscopy

It is generally accepted that high temperature superconductors emerge when extra carriers are introduced in the parent state, which looks like a Mott insulator. Competition of the order parameters drives the system into a poorly defined pseudogap state before acquiring the normal Fermi liquid behavior with further doping. Within the low doping level, the system has the tendency for mesoscopic phase separation, which seems to be a general characteristic in all high Tc compounds, but also in the materials of colossal magnetoresistance or the relaxor ferroelectrics. In all these systems, metastable phases can be created by tuning physical variables, such as doping or pressure, and the competing order parameters can drive the compound to various states. Structural instabilities are expected at critical points and Raman spectroscopy is ideal for detecting them, since it is a very sensitive technique for detecting small lattice modifications and instabilities. In this article, phase separation and lattice distortions are examined on the most characteristic family of high temperature superconductors, the cuprates. The effect of doping or atomic substitutions on cuprates is examined concerning the induced phase separation and hydrostatic pressure for activating small local lattice distortions at the edge of lattice instability.

Defects induced huge magnetoresistance in epitaxial La1–xSrxMnO3 thin films deposited by magnetic sputtering

In this work, epitaxial La1–xSrxMnO3 (LSMO) films were fabricated on SrTiO3 substrates at temperatures (Ts) ranging from 550 to 750 °C by RF magnetron sputtering. Significant Ts-dependent structural, magnetic, and magnetotransport properties were observed. The LSMO (Ts = 750 °C) film exhibits the colossal magnetoresistance (CMR) of −47% under the magnetic field (H) of 5 T. In contrast, the LSMO (Ts = 650 °C) film demonstrates a huge magnetoresistance (MR) of −98% (H = 5 T) around the metal-insulator transition temperature and –59% at 5 K. The spin-glass-like behaviors indicate that the defects, particularly the oxygen vacancies, in the epitaxial LSMO (Ts = 650 °C) films destroy the double exchange. The huge MR is related to the defect modulated magnetic structures and spin-dependent magnetotransport properties. Our work helps to understand the physical mechanism of the CMR and provides a way for tuning the magnetotransport properties of the perovskite films.

Unusual electrical behaviour in sol–gel-synthesised PKMO nano-sized manganite

Mixed-valence manganites have gained tremendous attention in the scientific research for its colossal magnetoresistance phenomenon. Nevertheless, the study devoted to praseodymium-based manganites is still limited to date. The present work aims to investigate the grain size effect on sol–gel grown nano-sized Pr0.85K0.15MnO3 (PKMO). The grain size has been modified by heat treatment ranging from 600 °C to 1000 °C. PKMO samples have been studied in detail to elucidate the correlation of spin, charge, orbital and lattice degrees of freedom. The X-ray diffraction analysis revealed that all samples exhibit in single orthorhombic phase with the space group of Pnma (62). The obtained crystallite size and average grain size are in the range of 45–115 nm and 51–210 nm, respectively. The evolution of grains intensively affects the electrical and magneto-transport properties of PKMO. The temperature dependence of the resistivity has been fitted with theoretical expressions in different temperature regimes to investigate their conduction mechanisms. The resistivity exhibits an unusual trend when the grain size increases where a similar pattern also been observed in metal–insulator transition temperature (TMI). This behaviour can be ascribed to the grain size distribution, grain formation and also the occurrence of oxygen vacancies at the grain boundaries. Enhancement of high field magnetoresistance has been discovered below 180 K, whereas low field magnetoresistance is suppressed as the temperature increases and almost vanished at 300 K. The PKMO study demonstrated here is clearly dominated by extrinsic properties (grain evolution) from the evidence of electrical and magneto-transport measurements.

Spin-pair correlation driven the colossal magnetoresistance effect in multiferroics CdCr2S4

To understand the anomalous conductivity and colossal magnetoresistance effect of multiferroics CdCr2S4 around magnetic transition temperature TC, we propose the spin-pair correlation dependence of magnetic polarons model. In CdCr2S4, system shows the spontaneous magnetic order at TC and the magnetic order promotes the delocalization of magnetic polarons. According to the proposed model of a dual-conduction behavior, the normal and delocalized magnetic polarons coexist below TC due to the gradual delocalization process of magnetic polarons. Compared to the conductivity of normal magnetic polarons, the conductivity from the delocalized magnetic polarons is dominant. It is suggested that the spin-pair correlation modifies the hopping activation energy of delocalized polarons to realize the anomalous conductivity and colossal magnetoresistance effect. In addition, the applied magnetic field, which promotes the magnetic order and delocalization of magnetic polarons, also leads to the increase of conductivity via spin-pair correlation. It is found that the obtained conductivity and colossal magnetoresistance are in agreement with the experimental results.

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.

First-principles calculation of the effective on-site Coulomb interaction parameters for Sr2 AB O6(A=Cr,Mn,Fe,Co,Ni , and B=Mo,W) double perovskites

Double perovskites (DPs) are a large family of compounds that exhibit a wide range of properties of both fundamental and potential technological interest. Due to the presence of $3d,4d$, or $5d$ transition metal atoms with narrow ${t}_{2g}$ and ${e}_{g}$ bands in DPs, the correlation effects play an important role for the properties of these materials, leading to diverse physical phenomena, such as colossal magnetoresistance, ferroelectricity, magnetism, and superconductivity. By employing the constrained random-phase approximation within the full-potential linearized augmented-plane-wave method, we have calculated the effective on-site Coulomb interaction parameters between localized $d$ electrons in ${\mathrm{Sr}}_{2}AB{\mathrm{O}}_{6}\phantom{\rule{4pt}{0ex}}(A=\mathrm{Cr},\text{Mn},\text{Fe},\text{Co},\text{Ni}$, and $B=\mathrm{Mo},\text{W})$ DPs. We find that the correlated subspace can be defined to contain only the ${e}_{g}$ states in Ni-based compounds, leading to a simple two-band low-energy model, whereas at least an eight-orbital $(d+{t}_{2g})$ model is necessary for the other compounds. Except for Ni, the $U$ values for $A$ sites in Mo (W) based compounds are around 4 eV (4.5 eV), and they are almost independent of the $3d$ electron number, while the $U$ for Mo (W) ${t}_{2g}$ electrons slightly decreases with increasing $3d$ electron number, from 3 to 2.5 eV. Moreover, our calculations reveal that the contribution of the $3d\ensuremath{\rightarrow}3d$ channel to the total electronic screening is larger in DPs than the corresponding contribution in elementary transition metals.

Solid State and Materials Research in India

The current status of research in the area of solid state and materials chemistry in India have been presented and discussed. The article focuses on various materials that have been categorized as traditional solids (dense) and porous solids. While many of the material properties such as ferromagnetism, superconductivity, colossal magneto resistance, multiferroic behaviour, etc. have been explored mainly in dense solids, porous solids contribute towards many properties in the areas of catalysis, sorption, gas separation and hydrogen storage. Possible directions for future research, along with identifying and highlighting the strengths of the expertise available in this country, were also mentioned. © 2020 Indian National Science Academy. All rights reserved.

Electronic correlation determining correlated plasmons in Sb-doped Bi2Se3

Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magneto resistance and high temperature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy and Hall measurements all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas those in the bulk states increase, and that the newly-discovered correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining new correlated plasmons and opens a new path in engineering plasmonic-based topologically-insulating devices.

Microstructure and properties of superconducting, ferromagnetic and hybrid nanowire networks of La1.85Sr0.15CuO4 and La0.5Sr0.5MnO3

We have successfully fabricated La1-xSrxMnO3 (LSMO) nanowires with various x level, and La1.85Sr0.15CuO4 (LSCO) nanowires/nanoribbons via electrospinning. The colossal magnetoresistance (CMR) of the LSMO nanowire networks have been investigated. and the Tc of the LSCO nanowires and nanoribbons are around 19.2 K and 29.3 K respectively. Furthermore, we have established a LSCO/LSMO nanowire hybrid system. From obervation by scanning electron microscopy, the average diameter of the nanowires is around 220 nm and the average length can reach over 50 μmm. The randomly aligned LSCO and LSMO nanowires show numerous connections and form a complicated hybrid network system. The nanowires are polycrystalline with the grain size of ∼30 nm as confirmed by transmission electron microscopy and EBSD. According to four-probe electrical transportation measurements, the superconductivity of the hybrid sample is suppressed and an anti-magnetoresistance effect is observed. SQUID measurements of M(T) and M(H) were carried out as well, revealing the soft magnetic character of the nanowires.

Impact of Ag doping on the structural, surface morphologic and electrical properties of La0.625(Ca0.285Sr0.09)MnO3 polycrystalline ceramics

In this study, Ag-doped La0.625(Ca0.285Sr0.09)MnO3:Agz (LCSMO:Agz, z = 0, 0.1, 0.2, 0.3, and 0.4) polycrystalline ceramics were prepared by sol-gel method, then characterized by various techniques. X-ray diffraction (XRD) showed that all bulk prepared specimens crystalized in LCSMO phase with orthogonal cubic structure (space group of Pnma) and without impurities. Scanning electron microscopy (SEM) revealed good compactness with fewer holes. Average grain sizes and quantity of grain boundaries (GBs) correlated with Ag doping amounts. As z content rose, resistivity dropped, and both peaks temperature coefficient of resistance (TCR) and magnetoresistance (MR) first increased and then decreased. Several previous studies indicated that Ag doping amount could affect TCR and MR peaks. At z = 0.3, TCR peaks reached maxima of 10.27% (289.56 K), at z = 0.1, MR peaks reached maxima of 15.63% (293.08 K). As z increased from 0 to 0.4, the temperature of peak TCR (Tk) was adjusted to room temperature from 289.56 to 294.22 K. TCR peak of La0.625(Ca0.285Sr0.09)MnO3:Agz material was improved at room temperature when compared to values reported in the literature. This could be related to changes in tolerance factor (τ) and average A-site cationic radius (rA). Overall, LCSMO:Agz ceramics look promising for infrared detectors and colossal magnetoresistance (CMR) radiant heating instruments at room temperature.

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.

Physical and chemical strains co-tuned magnetic properties of double perovskite PrBaMn2O5.5+δ epitaxial films

Different layered perovskite-related oxides are known to exhibit important electronic, magnetic, and electrochemical properties including metal-insulator transition, colossal magnetoresistance, excellent mixed ionic/electronic conductivity, and especially their flexible tunability by external or internal stimuli. Here, we show that the microstructure and magnetic properties of double perovskite PrBaMn2O5.5+δ (PBMO) epitaxial films can be co-tuned by the physical strain via a proper choice of substrate and film thickness and the chemical strain from the concentration of oxygen vacancies. It is surprisingly found that the films with more oxygen vacancies reveal more Mn4+ formed along with Mn2+ under the influence of interface strain, and meanwhile, Mn4+ exhibits a thickness-dependent distribution with a high amount at the interface. Consequently, the increased proportion of Mn4+ diminishes the saturation magnetization and decreases the Curie temperature of PrBaMn2O5.5+δ epitaxial films, revealing the availability of physical and chemical strains tuning the properties of highly epitaxial double perovskite films.

Magnetic-Competition-Induced Colossal Magnetoresistance in n -Type HgCr2Se4 under High Pressure

The n-type HgCr_{2}Se_{4} exhibits a sharp semiconductor-to-metal transition (SMT) in resistivity accompanying the ferromagnetic order at T_{C}=106 K. Here, we investigate the effects of pressure and magnetic field on the concomitant SMT and ferromagnetic order by measuring resistivity, dc and ac magnetic susceptibility, as well as single-crystal neutron diffraction under various pressures up to 8GPa and magnetic fields up to 8T. Our results demonstrate that the ferromagnetic metallic ground state of n-type HgCr_{2}Se_{4} is destabilized and gradually replaced by an antiferromagnetic, most likely a spiral magnetic, and insulating ground state upon the application of high pressure. On the other hand, the application of external magnetic fields can restore the ferromagnetic metallic state again at high pressures, resulting in a colossal magnetoresistance (CMR) as high as ∼ 3×10^{11}% under 5T and 2K at 4GPa. The present study demonstrates that n-type HgCr_{2}Se_{4} is located at a peculiar critical point where the balance of competition between ferromagnetic and antiferromagnetic interactions can be easily tipped by external stimuli, providing a new platform for achieving CMR in a single-valent system.

Hand-Held Magnetic Field Meter Based on Colossal Magnetoresistance-B-Scalar Sensor

This paper describes a newly developed hand-held magnetic field meter, which enables the measurement of the magnitude of pulsed, alternating, or permanent magnetic flux density in a range from 0.06 up to 10 T. The device measures the magnitude of magnetic flux density independent of the field direction (B-scalar) and its measurement frequency ranges from dc to 100 kHz. This magnetometer consists of a hand-held magnetic field measurement meter and a pluggable probe whose main element is a spintronic sensor based on the colossal magnetoresistance (CMR) phenomenon. The new design of a probe containing a temperature sensor and a CMR-B-scalar sensor made from two manganite films with different resistivity versus temperature dependences ensures a reduced response signal dependence on temperature and thus increased the accuracy of the measurements. The hand-held meter has a graphical color touch screen for the display of the waveforms and provides measurements without the use of external oscilloscopes or other analog graphing devices. This magnetometer can be used for various industrial and scientific applications such as for the investigation of high-power electrical motors, fast high electrical current transients appearing in cases of the commutation of pulsed power circuits, for the measurement of pulsed magnetic fields in magnetic coils, or for measuring the dynamics of magnetic fields during electromagnetic metal forming and welding.