Showing papers on "Colossal magnetoresistance published in 2021"

Ultrafast dynamical Lifshitz transition

Abstract: Fermi surface is at the heart of our understanding of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials by equilibrium tuning of macroscopic parameters such as strain, doping, pressure, and temperature, a nonequilibrium dynamical route toward ultrafast modification of the Fermi surface topology has not been experimentally demonstrated. Combining time-resolved multidimensional photoemission spectroscopy with state-of-the-art TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal Td-MoTe2. We demonstrate that this nonequilibrium topological electronic transition finds its microscopic origin in the dynamical modification of the effective electronic correlations. These results shed light on a previously unexplored ultrafast scheme for controlling the Fermi surface topology in correlated quantum materials.

Bridging Structural Inhomogeneity to Functionality: Pair Distribution Function Methods for Functional Materials Development.

Abstract: The correlation between structure and function lies at the heart of materials science and engineering. Especially, modern functional materials usually contain inhomogeneities at an atomic level, endowing them with interesting properties regarding electrons, phonons, and magnetic moments. Over the past few decades, many of the key developments in functional materials have been driven by the rapid advances in short-range crystallographic techniques. Among them, pair distribution function (PDF) technique, capable of utilizing the entire Bragg and diffuse scattering signals, stands out as a powerful tool for detecting local structure away from average. With the advent of synchrotron X-rays, spallation neutrons, and advanced computing power, the PDF can quantitatively encode a local structure and in turn guide atomic-scale engineering in the functional materials. Here, the PDF investigations in a range of functional materials are reviewed, including ferroelectrics/thermoelectrics, colossal magnetoresistance (CMR) magnets, high-temperature superconductors (HTSC), quantum dots (QDs), nano-catalysts, and energy storage materials, where the links between functions and structural inhomogeneities are prominent. For each application, a brief description of the structure-function coupling will be given, followed by selected cases of PDF investigations. Before that, an overview of the theory, methodology, and unique power of the PDF method will be also presented.

Colossal magnetoresistance via avoiding fully polarized magnetization in the ferrimagnetic insulator Mn 3 Si 2 Te 6

Abstract: Colossal magnetoresistance is of great fundamental and technological significance and exists mostly in the manganites and a few other materials. Here we report colossal magnetoresistance that is starkly different from that in all other materials. The stoichiometric Mn3Si2Te6 is an insulator featuring a ferrimagnetic transition at 78 K. The resistivity drops by 7 orders of magnitude with an applied magnetic field above 9 Tesla, leading to an insulator-metal transition at up to 130 K. However, the colossal magnetoresistance occurs only when the magnetic field is applied along the magnetic hard axis and is surprisingly absent when the magnetic field is applied along the magnetic easy axis where magnetization is fully saturated. The anisotropy field separating the easy and hard axes is 13 Tesla, unexpected for the Mn ions with nominally negligible orbital momentum and spin-orbit interactions. Double exchange and Jahn-Teller distortions that drive the hole-doped manganites do not exist in Mn3Si2Te6. The phenomena fit no existing models, suggesting a unique, intriguing type of electrical transport.

Colossal Magnetoresistance without Mixed Valence in a Layered Phosphide Crystal.

Abstract: Materials with strong magnetoresistive responses are the backbone of spintronic technology, magnetic sensors, and hard drives. Among them, manganese oxides with a mixed valence and a cubic perovskite structure stand out due to their colossal magnetoresistance (CMR). A double exchange interaction underlies the CMR in manganates, whereby charge transport is enhanced when the spins on neighboring Mn3+ and Mn4+ ions are parallel. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here an enormous CMR at low temperatures in EuCd2 P2 without manganese, oxygen, mixed valence, or cubic perovskite structure is shown. EuCd2 P2 has a layered trigonal lattice and exhibits antiferromagnetic ordering at 11 K. The magnitude of CMR (104 %) in as-grown crystals of EuCd2 P2 rivals the magnitude in optimized thin films of manganates. The magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.

Room-temperature large magnetocaloric, electronic and magnetic properties in La0.75Sr0.25MnO3 manganite: Ab initio calculations and Monte Carlo simulations

Abstract: By using the Ab initio and Monte Carlo calculations, we have studied the magnetism of the crystalline La0.75Sr0.25MnO3 perovskite. The ferromagnetic phase of La0.75Sr0.25MnO3 is half-metallic, which is important in the relation to the colossal magnetoresistance properties of this compound. The total magnetic moment and the exchange couplings deduced from ab initio calculations lead, by using Monte Carlo simulations, to a quantitative agreement with the experimental transition temperatures. The maximum magnetic entropy change and the specific heat are found to be 9.23 J K−1 kg −1 and 191 J mol − 1 K−1, respectively for H = 6 T. Our results suggest that this material a promising candidate for magnetic refrigeration application near to room temperature at moderate fields.

The Physical Properties of Submicron and Nano-Grained La0.7Sr0.3MnO3 and Nd0.7Sr0.3MnO3 Synthesised by Sol–Gel and Solid-State Reaction Methods

Abstract: La0.7Sr0.3MnO3 (LSMO) and Nd0.7Sr0.3MnO3 (NSMO) possess excellent colossal magnetoresistance (CMR). However, research work on the neodymium-based system is limited to date. A comparative study between LSMO and NSMO prepared by sol–gel and solid-state reaction methods was undertaken to assess their structural, microstructural, magnetic, electrical, and magneto-transport properties. X-ray diffraction and structure refinement showed the formation of a single-phase composition. Sol–gel-synthesised NSMO was revealed to be a sample with single crystallite grains and exhibited intriguing magnetic and electrical transport behaviours. Magnetic characterisation highlighted that Curie temperature (TC) decreases with the grain size. Strong suppression of the metal–insulator transition temperature (TMI) was observed and attributed to the magnetically disordered grain surface and distortion of the MnO6 octahedra. The electrical resistivity in the metallic region was fitted with theoretical models, and the conduction mechanism could be explained by the grain/domain boundary, electron–electron, and electron–magnon scattering process. The increase in the scattering process was ascribed to the morphology changes. Enhancement of low-field magnetoresistance (LFMR) was observed in nano-grained samples. The obtained results show that the grain size and its distribution, as well as the crystallite formation, strongly affect the physical properties of hole-doped manganites.

Sudden Collapse of Magnetic Order in Oxygen-Deficient Nickelate Films.

Abstract: Antiferromagnetic order is a common and robust ground state in the parent (undoped) phase of several strongly correlated electron systems. The progressive weakening of antiferromagnetic correlations upon doping paves the way for a variety of emergent many-electron phenomena including unconventional superconductivity, colossal magnetoresistance, and collective charge-spin-orbital ordering. In this study, we explored the use of oxygen stoichiometry as an alternative pathway to modify the coupled magnetic and electronic ground state in the family of rare earth nickelates (RENiO_{3-x}). Using a combination of x-ray spectroscopy and resonant soft x-ray magnetic scattering, we find that, while oxygen vacancies rapidly alter the electronic configuration within the Ni and O orbital manifolds, antiferromagnetic order is remarkably robust to substantial levels of carrier doping, only to suddenly collapse beyond 0.21 e^{-}/Ni without an accompanying structural transition. Our work demonstrates that ordered magnetism in RENiO_{3-x} is mostly insensitive to carrier doping up to significant levels unseen in other transition-metal oxides. The sudden collapse of ordered magnetism upon oxygen removal may provide a new mechanism for solid-state magnetoionic switching and new applications in antiferromagnetic spintronics.

Dynamics of electronically phase-separated states in the double exchange model

Abstract: We present extensive large-scale dynamical simulations of phase-separated states in the double exchange model. These inhomogeneous electronic states that play a crucial role in the colossal magnetoresistance phenomenon are composed of ferromagnetic metallic clusters embedded in an antiferromagnetic insulating matrix. We compute the dynamical structure factor of these nanoscale textures using an efficient real-space formulation of coupled spin and electron dynamics. Dynamical signatures of the various underlying magnetic structures are identified. At small hole doping, the structure factor exhibits a dominating signal of magnons from the background N\'eel order and localized modes from magnetic polarons. A low-energy continuum due to large-size ferromagnetic clusters emerges at higher doping levels. Implications for experiments on magnetoresistive manganites are discussed.

Ta-doped SrTiO3 epitaxial thin film: A promising perovskite for optoelectronics

Abstract: SrTiO3 is a wide bandgap cubic perovskite oxide and displays many exotic properties, i.e., transparent conductivity, photocatalysis, metallicity, ferroelectricity, superconductivity, colossal magnetoresistance, two-dimensional electron gas, etc., due to the manipulations of defect chemistry and constituent elements via impurity doping. This paper reports on the intricacy of the structural and optoelectronic properties of the epitaxially stabilized 5 at. % Ta-doped SrTiO3 (001) thin films on LaAlO3 (001) substrates by systematically varying the growth temperature and oxygen partial pressure during the pulsed laser deposition process. The influences of Ta dopant and growth parameters on the epitaxial quality of these layers are understood by determining the dopant location and its concentration in the SrTiO3 lattice. The complex relationships of optical and electronic properties on growth parameters, dopant concentration, and single crystal quality of the films are demonstrated. The observed low resistivity (∼5 × 10−3 Ω cm) and high optical transparency (∼85%–90%) of optimized Ta-doped SrTiO3 films offer it as an exciting material for next generation transparent optoelectronics.

Arrested Phase Separation in Double-Exchange Models: Large-Scale Simulation Enabled by Machine Learning.

Abstract: We present large-scale dynamical simulations of electronic phase separation in the single-band double-exchange model based on deep-learning neural-network potentials trained from small-size exact diagonalization solutions. We uncover an intriguing correlation-induced freezing behavior as doped holes are segregated from half filled insulating background during equilibration. While the aggregation of holes is stabilized by the formation of ferromagnetic clusters through Hund's coupling between charge carriers and local magnetic moments, this stabilization also creates confining potentials for holes when antiferromagnetic spin-spin correlation is well developed in the background. The dramatically reduced mobility of the self-trapped holes prematurely disrupts further growth of the ferromagnetic clusters, leading to an arrested phase separation. Implications of our findings for phase separation dynamics in materials that exhibit colossal magnetoresistance effect are discussed.

Low field magnetotransport behavior of barium hexaferrite/ferromagnetic manganite bilayer

Abstract: Adding functionalities to existing ferroelectric/ferromagnetic materials showed promising results with exciting physical mechanisms. Pure and bilayer films of strong ferromagnetic oxides, viz, BaFe12O19 (BaM) and La0.67Sr0.33MnO3 (LSMO), were fabricated by pulsed laser deposition. Polycrystalline samples of dense structure, uniform thickness, and monodispersed grain distributions were used to form capacitor-like stack geometry for dielectric and magneto-dielectric (MD) measurements. High dielectric constants at moderately high frequencies with increased relaxation times were observed for the bilayer film and are attributed to the BaM/LSMO strained interface, while Maxwell–Wagner polarization plays an insignificant role. Modeling of dielectric loss tangents and AC conductivity revealed localized carrier hopping between Fe ions in the bilayer film. Pronounced hysteresis loops with a small coercive field and increased saturation magnetization values of BaM/LSMO bilayers, as compared with BaM/Pt, are demonstrated at 300 K; where the role of mixed valence Mn ions in +3 and +4 states at the bottom LSMO electrode is highlighted. MD measurements with varying magnetic fields showed magnetically tunable, large MD coupling values (∼287%) for BaM/LSMO/Pt. The phenomenally high MD values are discussed based on ionic polarization, colossal magnetoresistance of LSMO, and magnetostriction at the BaM/LSMO interface. Our findings propose significant applications of ferromagnetic oxide bilayers in the emerging field of magneto-dielectric coupling devices.

Metastable Materials Accessed under Moderate Pressure Conditions (P ≤ 3.5 GPa) in a Piston-Cylinder Press.

Abstract: In this review, we describe different families of metastable materials, some of them with relevant technological applications, which can be stabilized at moderate pressures 2–3.5 GPa in a piston-cylinder press. The synthesis of some of these systems had been previously reported under higher hydrostatic pressures (6–10 GPa), but can be accessed under milder conditions in combination with reactive precursors prepared by soft-chemistry techniques. These systems include perovskites with transition metals in unusual oxidation states (e.g., RNiO3 with Ni3+, R = rare earths); double perovskites such as RCu3Mn4O12 with Jahn–Teller Cu2+ ions at A sites, pyrochlores derived from Tl2Mn2O7 with colossal magnetoresistance, pnictide skutterudites MxCo4Sb12 (M = La, Yb, Ce, Sr, K) with thermoelectric properties, or metal hydrides Mg2MHx (M = Fe, Co, Ni) and AMgH3 (A: alkali metals) with applications in hydrogen storage. The availability of substantial amounts of sample (0.5–1.5 g) allows a complete characterization of the properties of interest, including magnetic, transport, thermoelectric properties and so on, and the structural characterization by neutron or synchrotron X-ray diffraction techniques.

Studies on Colossal Magnetoresistance Behaviour of Pr0.6Sr0.4MnO3/Pr0.5Ca0.5MnO3 Heterostructure Films

Abstract: Heterostructure bilayer thin films of Pr0.6Sr0.4MnO3/Pr0.5Ca0.5MnO3 were prepared by pulsed laser deposition, and the structural, magnetic and magnetotransport properties were studied, and the results were compared to that of Pr0.6Sr0.4MnO3 and Pr0.5Ca0.5MnO3 single-layer thin films. Structural analysis of the heterostructure reveals c-axis orientation for both the manganite layers. The heterostructure exhibits a higher metal-insulator transition temperature and a larger colossal magnetoresistance near room temperature than that of the single-layer counterparts. The electrical conduction above the metal-insulator transition could be explained within the realm of the adiabatic small polaron hopping mechanism, with lesser activation energies compared to the single-layer films. Magnetization measurements portray the presence of an unsaturated symmetric hysteresis loop for the heterostructure with the Curie temperature of ~270 K corresponding to the ferromagnetic ordering emanating from Pr0.6Sr0.4MnO3. These results could be qualitatively understood considering the combined effect of strain and ferromagnetic proximity of Pr0.6Sr0.4MnO3 with charge-ordered Pr0.5Ca0.5MnO3 manganite.

The rich physics of A-site-ordered quadruple perovskite manganites AMn7O12.

Abstract: Perovskite-structure AMnO3 manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron–phonon coupling, and Jahn–Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn7O12 were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn7O12 compounds host rich physics beyond the canonical AMnO3 materials.

Spin-Seebeck effect and thermal colossal magnetoresistance in the narrowest zigzag graphene nanoribbons.

Abstract: Device miniaturization and low-energy dissipation are two urgent requirements in future spintronics devices. The narrowest zigzag graphene nanoribbons (ZGNRs), which are composed of just two coupled carbon-atom chains connected with carbon tetragons, are promising candidates that meet both of the above requirements well. Using the first-principles calculations combined with non-equilibrium Green's function approach, thermal spin-dependent transport through this kind of narrow ZGNR is investigated, and several exotic thermal spin-resolved transport properties are uncovered: (i) when an external magnetic field is applied, the ZGNRs are transited from the intrinsic semiconducting to the metallic state, and the thermal colossal magnetoresistance effect occurs with order of magnitudes up to 104 at room temperature; (ii) the thermal spin-dependent currents display a thermal negative differential resistance effect, and a well-defined spin-Seebeck effect (SSE) together with a pure thermal spin current occurs; and (iii) under suitable device temperature settings, a nearly perfect spin-filtering effect occurs in these narrowest ZGNRs. The theoretical results not only uncover the narrowest nanoribbon structures to realize the SSE and other inspiring thermal spin transport features, but also push carbon-based material candidates towards thermoelectric conversion device applications.

Exotic Long-Range Surface Reconstruction on La0.7Sr0.3MnO3 Thin Films.

Abstract: Due to an extremely diverse phase space, La1-xSrxMnO3, as with other manganites, offers a wide range of tunability and applications including colossal magnetoresistance and use as spin-polarized electrodes Here, we study an unprecedented, exotic surface reconstruction (6 × 6) in La1-xSrxMnO3 (x = 03) observed via low-energy electron diffraction (LEED) Scanning tunneling microscopy (STM) shows the surface is relatively flat, with unit-cell step heights, and X-ray photoelectron spectroscopy (XPS) reveals a strong degree of Sr segregation at the surface By combining electron diffraction and first-principles computations, we propose that the long-range surface reconstruction consists of a Sr-segregated surface with La (6 × 6) ordering This study expands our understanding of manganite systems and underscores their ability to form interesting surface reconstructions, driven largely by cation segregation that can potentially be controlled for tuning surface ordering

Colossal electroresistance response accompanied by metal-insulator transition in a mixed-valent vanadate

Abstract: Colossal electroresistance (CER) in manganites, i.e., a large change in electrical resistance as a function of varying applied electric field or applied electric current, has often been described as complimentary to the colossal magnetoresistance (CMR) effect. Mixed valence vanadates with active ${t}_{2g}$ and empty ${e}_{g}$ orbitals, unlike manganites, have not naturally been discussed in this context, as double exchange based CMR is not realizable in them. However, presence of coupled spin and orbital degrees of freedom, metal-insulator transition (MIT) accompanied by orbital order-disorder transition, still make the vanadates important. Here we probe a Fe-doped hollandite lead vanadate $\mathrm{Pb}{\mathrm{Fe}}_{1.75}{\mathrm{V}}_{4.25}{\mathrm{O}}_{11}$ (PFVO), which exhibits a clear MIT as a function of temperature. Most importantly, a giant fall in the resistivity, indicative of a CER, as well as a systematic shift in the MIT towards higher temperatures are observed with increasing applied current. Detailed structural, magnetic, thermodynamic, and transport studies point towards a complex interplay between the structural distortion, orbital order/disorder effect, and the resultant MIT and magnetic ordering in this system.

Striping of orbital-order with charge-disorder in optimally doped manganites.

Abstract: The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the $${\frac{3}{8}}^{th}$$ doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn $${}_{3}^{A^{\prime} }$$ Mn $${}_{4}^{B}$$ O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state. Manganite perovskites display the intriguing property of colossal magnetoresistance (CMR), but only at very specific doping values. Now, a detailed crystallographic analysis of a prototype system reveals a novel type of orbital ordering that coexists with charge-disorder stripes, occurring precisely at the doping value where CMR is maximised.

Origin of magnetovolume effect in a cobaltite

Abstract: The layered perovskite ${\mathrm{PrBaCo}}_{2}{\mathrm{O}}_{5.5+x}$ demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we report with compelling evidence that the continuouslike MVE in ${\mathrm{PrBaCo}}_{2}{\mathrm{O}}_{5.5+x}$ is intrinsically of discontinuous character, originating from a magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic less-insulating small-volume (FLISV) phase. Furthermore, the magnetoelectric effect (ME) shows high sensitivity to multiple external stimuli such as temperature, carrier doping, hydrostatic pressure, magnetic field, etc. In contrast to the well-known ME such as colossal magnetoresistance and multiferroic effect which involve symmetry breaking of crystal structure, the ME in the cobaltite is purely isostructural. Our discovery provides a pathway to realizing the ME as well as the NTE, which may find applications in new techniques.

Electronic, Magnetic Properties and Magnetocaloric Effect of La2SrMn2O7 Bilayer Manganite: An Ab Initio calculations and Monte Carlo Study

Abstract: The structure of La2SrMn2O7 (LSMO) bilayer manganite has been investigated using the Monte Carlo simulations and ab initio calculations based on the full-potential linearized augmented plane-wave method. The ferromagnetic phase of LSMO is half-metallic with 100% spin polarization, which is important in relation to the colossal magnetoresistance properties of this compound. Thermal magnetization of LSMO bilayer manganite is obtained for several external magnetic fields h = 1, 3, 5 and 7 T. We have given the dM/dT as a function of temperatures for several external magnetic fields. The magnetic transition from ferromagnetic to paramagnetic is found. The second-order phase transition is found at the transition temperature. The temperature dependence of the magnetic entropy changes of temperatures of LSMO bilayer manganite for several external magnetic fields h = 1, 3, 5 and 7 T is also found. The field dependence of relative cooling power of LSMO bilayer manganite is given for several temperatures. Finally, the magnetic hysteresis cycle of LSMO bilayer manganite is obtained for several temperatures T = 160, 190, 213 and 220 K. The superparamagnetic behavior has been found around the transition temperature.

Spatially-resolved electronic structure of stripe domains in IrTe$_2$ through electronic structure microscopy.

Abstract: Phase separation in the nanometer- to micrometer-scale is characteristic for correlated materials, for example, high temperature superconductors, colossal magnetoresistance manganites, Mott insulators, etc. Resolving the electronic structure with spatially-resolved information is critical for revealing the fundamental physics of such inhomogeneous systems yet this is challenging experimentally. Here by using nanometer- and micrometer-spot angle-resolved photoemission spectroscopies (NanoARPES and MicroARPES), we reveal the spatially-resolved electronic structure in the stripe phase of IrTe$_2$. Each separated domain shows two-fold symmetric electronic structure with the mirror axis aligned along 3 equivalent directions, and 6$\times$1 replicas are clearly identified. Moreover, such electronic structure inhomogeneity disappears across the stripe phase transition, suggesting that electronic phase with broken symmetry induced by the 6$\times$1 modulation is directly related to the stripe phase transition of IrTe$_2$. Our work demonstrates the capability of NanoARPES and MicroARPES in elucidating the fundamental physics of phase-separated materials.

Total Scattering Study of Local Structural Changes in MnAs1–xPx Influenced by Magnetic Interactions

Abstract: The interplay between fluctuations of the local structure and magnetic interactions is of great importance for phenomena like superconductivity, colossal magnetoresistance, and frustrated magnetism...

Pressure-tuned colossal magnetoresistance effect in n-type CdCr2Se4

Abstract: The p-type CdCr2Se4 exhibits semiconducting conductivity while n-type CdCr2Se4 displays a semiconductor–metal transition and a concomitant colossal magnetoresistance near the Curie temperature. Here, we investigate the pressure effect on the conductivity for both types of compounds. We show that the resistance of p-type sample decreases continuously upon compression, while the semiconducting behavior dominates up to 27.9 GPa. For the n-type sample, the semiconductor-metal transition is suppressed gradually with the increase in pressure; meanwhile, the magnetoresistance becomes less and less pronounced and is negligible at 9.2 GPa. Combined with in situ high-pressure magnetization, x-ray diffraction, and Raman spectra investigations, a ferromagnetic ground state is deduced in the pressurized n-type CdCr2Se4 before the structural transition, which is in stark contrast to the pressure effect on n-type HgCr2Se4.