Showing papers on "Colossal magnetoresistance published in 2012"

Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

Abstract: The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe(2)As(2) following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin-phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

Spintronic oxides grown by laser-MBE

Abstract: The recent study of oxides led to the discovery of several new fascinating physical phenomena. High-temperature superconductivity, colossal magnetoresistance, dilute magnetic doping, or multiferroicity were discovered and investigated in transition-metal oxides, representing a prototype class of strongly correlated electronic systems. This development was accompanied by enormous progress regarding thin film fabrication. Within the past two decades, epitaxial thin films with crystalline quality approaching semiconductor standards became available using laser-molecular beam epitaxy. This evolution is reviewed, particularly with emphasis on transition-metal oxide thin films, their versatile physical properties, and their impact on the field of spintronics. First, the physics of ferromagnetic half-metallic oxides, such as the doped manganites, the double perovskites and magnetite is presented together with possible applications based on magnetic tunnel junctions. Second, the wide bandgap semiconductor zinc oxide is discussed particularly with regard to the controversy of dilute magnetic doping with transition-metal ions and the possibility of realizing p-type conductivity. Third, the field of oxide multiferroics is presented with the recent developments in single-phase multiferroic thin film perovskites as well as in composite multiferroic hybrids.

Ultrafast transient generation of spin-densitywave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

Abstract: The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2 following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin-phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

Gate-controlled linear magnetoresistance in thin Bi2Se3 sheets

Abstract: We explore the emergence of linear magnetoresistance in thin Bi2Se3 sheets upon tuning the carrier density using a back gate. With increasingly negative gate voltage, a pronounced magnetoresistance of ∼100% is observed, while the associated B-field dependence changes from quadratic to linear. Concomitantly, the resistance-versus-temperature curves evolve from metallic to semiconductor-like, and increasingly strong weak anti-localization behavior is manifested. Analysis of the magnetoresistance data reveals two contributions, namely from the bulk conduction band and from a state inside the bulk gap. The latter is responsible for the linear magnetoresistance and likely represents the topologically protected surface state.

Long-range transfer of electron–phonon coupling in oxide superlattices

Abstract: The interaction between electrons and phonons is important for many materials properties. The finding that phonon modes of a superconducting thin film can influence the properties of an adjacent normal conductor, even over comparatively long distances, suggests new ways of controlling electron–phonon interactions. The electron–phonon interaction is of central importance for the electrical and thermal properties of solids, and its influence on superconductivity, colossal magnetoresistance and other many-body phenomena in correlated-electron materials is the subject of intense research at present. However, the non-local nature of the interactions between valence electrons and lattice ions, often compounded by a plethora of vibrational modes, presents formidable challenges for attempts to experimentally control and theoretically describe the physical properties of complex materials. Here we report a Raman scattering study of the lattice dynamics in superlattices of the high-temperature superconductor YBa2Cu3O7 (YBCO) and the colossal-magnetoresistance compound La2/3Ca1/3MnO3 that suggests a new approach to this problem. We find that a rotational mode of the MnO6 octahedra in La2/3Ca1/3MnO3 experiences pronounced superconductivity-induced line-shape anomalies, which scale linearly with the thickness of the YBCO layers over a remarkably long range of several tens of nanometres. The transfer of the electron–phonon coupling between superlattice layers can be understood as a consequence of long-range Coulomb forces in conjunction with an orbital reconstruction at the interface. The superlattice geometry thus provides new opportunities for controlled modification of the electron–phonon interaction in complex materials.

Magneto-orbital helices as a route to coupling magnetism and ferroelectricity in multiferroic CaMn₇O₁₂.

Abstract: Orbital physics drives a rich phenomenology in transition-metal oxides, providing the microscopic underpinning for effects such as Colossal Magnetoresistance. In particular, magnetic and lattice degrees of freedom are coupled through orbital ordering, and it has long been hoped that this coupling could be exploited to create high-temperature multiferroics with large values of the electrical polarization. Here we report an unprecedented magneto-orbital texture in multiferroic CaMn7O12, found to give rise to the largest magnetically induced ferroelectric polarization measured to date. X-ray diffraction characterization of the structural modulation in these ‘magneto-orbital helices’, and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure, thus allowing for electric polarization. Additionally, the presence of a global structural rotation enables the coupling between this polarization and magnetic helicity required for multiferroicity. These novel principles open up the possibility of discovering new multiferroics with even larger polarization and higher transition temperatures. The coupling of magnetism and ferroelectricity is of relevance for applications such as sensing, but occurs only rarely in bulk materials. The large magnetically induced ferroelectric polarization observed here in CaMn7O12establishes a new approach to achieve a strong magnetoelectric coupling.

Giant magnetoresistance in layered manganese pnictide CaMnBi2

Abstract: We report the physical properties of a layered transition metal pnictide, CaMnBi2, which has a crystal structure similar to that of the superconducting iron pnictides. This compound is a bad metal with a long-range antiferromagnetic order at TN = 270 K. The linear temperature dependence of magnetic susceptibility above TN suggests that strong antiferromagnetic correlations exist in the paramagnetic state. A linear magnetic field dependence of the magnetoresistance implies the existence of the linear energy dispersion, which may result in the giant in-plane magnetoresistance (about 105% in 10 T at 2.5 K for H∥c). The results of de Haas-van Alphen effect are consistent with the presence of Dirac fermions.

Pressure-induced crystal structure and spin-state transitions in magnetite (Fe3O4).

Abstract: High pressure is an important dimension for the emergent phenomena in transition metal oxides, including high-temperature superconductivity, colossal magnetoresistance, and magnetoelectric coupling. In these multiply correlated systems, the interplay between lattice, charge, orbital, and spin is extremely susceptible to external pressure. Magnetite (Fe3O4) is one of the oldest known magnetic materials and magnetic minerals, yet its high pressure behaviors are still not clear. In particular, the crystal structure of the high-pressure phase has remained contentious. Here, we investigate the pressure-induced phase transitions in Fe3O4 from first-principles density-functional theory. It is revealed that the net magnetic moment, arising from two ferrimagnetically coupled sublattices in Fe3O4, shows an abrupt drop when entering into the high-pressure phase but recovers finite value when the pressure is beyond 65.1 GPa. The origin lies in the redistribution of Fe 3d orbital occupation with the change of crystal ...

Magnetoresistive effects in perpendicularly magnetized Tb-Co alloy based thin films and spin valves

Abstract: Tb-Co ferrimagnetic alloy thin films and spin valves have been grown to study their magnetoresistance response in various geometries. The studied Tb-Co alloys show strong perpendicular anisotropy and tunable magnetization by several orders of magnitude. Magnetoresistance signals such as giant magnetoresistance (GMR), anisotropic magnetoresistance (AMR), extraordinary Hall effect (EHE), and magnon magnetoresistance (MMR) have been studied. The angular dependence of those magnetoresistive effects is also investigated. Finally we demonstrate that by adjusting the Tb-Co layer composition in a spin valve structure, the sign and the amplitude of the GMR and EHE signal can be tuned.

STEM-EELS imaging of complex oxides and interfaces

Abstract: The success of the correction of spherical aberration in the electron microscope has revolutionized our view of oxides. This is a very important class of materials that is promising for future applications of some of the most intriguing phenomena in condensed matter physics: colossal magnetoresistance, colossal ionic conductivity, high Tc superconductivity, and ferroelectricity. Understanding the physics underlying such phenomena, especially in low dimensional systems (thin films, interfaces, nanowires, nanoparticles), relies on the availability of techniques capable of looking at these systems in real space and with atomic resolution and even beyond, with single atom sensitivity; in many cases, the system properties depend on minuscule amounts of point defects that alter the material’s properties dramatically. Atomic resolution spectroscopy in the aberration-corrected electron microscope is one of the most powerful techniques available to materials scientists today. This article will briefly review some state-of-the-art applications to oxide materials: from atomic resolution elemental mapping and single atom imaging to applications to real systems, including oxide interfaces and mapping of physical properties such as the spin state of magnetic atoms.

Colossal Magnetoresistance in the Mn2+ Oxypnictides NdMnAsO1-xFx

Abstract: Colossal magnetoresistance (CMR) is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could be the basis of the next generation of magnetic memory devices. Here we report CMR in the antiferromagnetic oxypnictide NdMnAsO1-xFx as a result of competition between an antiferromagnetic insulating phase with strong electron correlations and a paramagnetic semiconductor upon application of a magnetic field. The discovery of CMR in antiferromagnetic Mn2+ oxypnictide materials could open up an array of materials for further investigation and optimisation for technological applications.

Structural transitions and transport-half-metallic ferromagnetism in LaMnO3 at elevated pressure

Abstract: By means of hybrid density functional theory we investigate the evolution of the structural, electronic, and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO3 under pressure. We predict a transition from a low-pressure antiferromagnetic (AFM) insulator to a high-pressure ferromagnetic (FM) transport half metal (tHM), characterized by a large spin polarization (approximate to 80-90%). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the Pnma orthorhombic phase into a perfect cubic one (through a mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with a high-spin (S = 2, m(Mn) = 3.7 mu(B)) to low-spin (S = 1, m(Mn) = 1.7 mu(B)) magnetic moment collapse. These results interpret the progression of the experimentally observed non-Mott metalization process and open up the possibility of realizing CMR behaviors in a stoichiometric manganite.

Nanostructured thin manganite films in megagauss magnetic field

Abstract: We report on the use of the colossal magnetoresistance (CMR) effect in manganites for the measurement of pulsed magnetic fields up to the megagauss limit. To increase the application range in a magnetic field, we fabricated nanostructured La-Sr-Mn-O films consisting of nanocrystallites cummulated into clusters separated by highly amorphous inter-cluster boundaries. We demonstrate that the CMR effect does not saturate in these films at 77 K up to 91.4 T. Moreover, the magnetoresistance behavior at 290 K shows that nanostructured manganite films are promising candidates for the development of magnetic field scalar sensors operating in wide field and temperature ranges.

Unconventional Colossal Magnetoresistance in Sodium Chromium Oxide with a Mixed‐Valence State

Abstract: ions. This material demonstrates an unusual CMReffect that is closely related to its unconventional magneticstructure caused by spin frustration. The CMR of thiscompound is unique from several aspects. First, it is observedin a chromium oxide, not in a manganese oxide. Second, it isfound in a single-phase material having an insulating groundstate. Third, the CMR is not limited to the vicinity of themagnetic phase transition but becomes progressively moreprominent with decreasing temperature down to 0 K. Thediscovery of the NaCr

Colossal Magnetoresistance in Mn2+ Oxypnictides NdMnAsO1–xFx

Abstract: Colossal magnetoresistance is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could ...

Two-stage orbital order and dynamical spin frustration in KCuF 3

Abstract: Orbital order is important to many correlated electron phenomena, including colossal magnetoresistance and high-temperature superconductivity. A study of a previously unreported structure transition in KCuF3 suggests that direct interorbital exchange is important to understanding such order.

Two-stage orbital order and dynamical spin frustration in KCuF 3

Abstract: The orbital degree of freedom is integral to many exotic phenomena in condensed matter, including colossal magnetoresistance and unconventional superconductivity. The standard model of orbital physics is the Kugel‐Khomskii model 1 , which first explained the symmetry of orbital and magnetic order in KCuF3 and has since been applied to virtually all orbitally active materials 2 . Here we present Raman and X-ray scattering measurements showing that KCuF3 exhibits a previously unidentified structural phase transition at TD 50 K, involving rotations of the CuF6 octahedra. These rotations are quasi-ordered and exhibit glassy hysteresis, but serve to stabilize Neel spin order at TD 39 K. We propose an explanation for these effects by supplementing the Kugel‐Khomskii model with a direct, orbital exchange term that is driven by a combination of electron‐electron interactions and ligand distortions 3 . The effect of this term is to create a near degeneracy that dynamically frustrates the spin subsystem but is lifted at low temperature by subdominant, orbital‐lattice interactions. Our results suggest that direct orbital exchange may be crucial for the physics of many orbitally active materials, including manganites, ruthenates and the iron pnictides. It has long been believed that the prototypical orbital ordering material KCuF3 exhibits KugelKhomskii

Magnetic field mediated low-temperature resistivity upturn in electron- doped La1−xHfxMnO3 manganite oxides

Abstract: The low-temperature transport properties were systematically studied on the electron-doped polycrystalline La1−xHfxMnO3 (x = 02 and 03) compounds at the presence of external magnetic fields The resistivity of all samples exhibits a generally low-temperature resistance upturn behavior under zero magnetic field at the temperature of Tmin, which first shifts towards lower temperature at low magnetic field (H < 075 T) and then moves back to higher temperature as magnetic fields increase, which is greatly different with the previous results on the hole-doped manganites The best fitting of low-temperature resistivity could be made by considering both electron-electron (e-e) interactions in terms of T1/2 dependence and Kondo-like spin dependent scattering in terms of lnT dependence at all magnetic fields Our results will be meaningful to understand the underlying physical mechanism of low-temperature resistivity minimum behavior in the electron-doped manganites

Influence of Fe segregation at grain boundaries on the magnetoresistance of Sr2Fe1+δMoO6 polycrystals

Abstract: In this paper, we report the influence of Fe impurities on the magnetoresistance (MR) of double perovskite Sr2Fe1+δMoO6 (0.00 ≤ δ ≤ 0.20) system. The significant Fe impurities have been created by two different approaches: one by adding the extra amount of Fe2O3 at starting of the synthesis and other by sintering the pristine Sr2FeMoO6 sample at high temperature (1300 °C) in high reducing environment (∼8% H2 + 92% Ar). A remarkable 11% magnetoresistance at room temperature under the presence of low magnetic field (0.72 T) has been observed in the pristine sample. The achieved high low field magnetoresistance value in the sample may be due to the optimized synthesis conditions to get better inter-granular tunneling through grain boundaries. However, the presence of Fe impurity results into the sharp reduction in magnetoresistance because of reduced spin polarized tunneling. The loss of insulating nature of the grain boundaries and the inelastic scattering of the charge carriers through the metallic impurit...

Relationship between the magnetoresistance and the magnetocaloric effect in La 1 − x Ag x MnO 3 manganites

Abstract: A generalized expression relating the magnetoresistance of manganites La1 − xAgxMnO3 with the change in the magnetic entropy has been proposed. The correct inclusion of the acting mechanisms of appearance of the magnetoresistance is shown to lead to adequate agreement between the experimental and calculated values of ΔSM.

Time-integral type strongly correlated electronic thin-film laser energy meter

Abstract: Strongly correlated electronic (SCE) systems, such as high-temperature superconducting (HTSC) cuprate and colossal magnetoresistance (CMR) manganite thin films, exhibit giant laser-induced thermoelectric voltage (LITV) effect due to anisotropic Seebeck effect and have a great potential for laser detector applications. In this work, we develop a novel time-integral type LaSrMnO thin-film laser energy meter based on anisotropic Seebeck effect. An epitaxial LaSrMnO thin film prepared by means of the pulsed laser deposition method onto a vicinal cut LaAlO3 substrate is irradiated by a 1064-nm Q-switched Nd:YAG laser and its 2nd (532 nm), 3rd (355 nm), and 4th (266 nm) harmonics at room temperature. The time integral of the LITV signal shows a good linear relationship with the laser energy per pulse in the measured wavelength range, which not only confirms the theoretical analysis, but also provides the basis for designing time-integral type laser energy meter. The advantages over other conventional laser detectors, such as fast (nanosecond order) response, spectrally broadband (from infrared to ultraviolet) and flat response, exceptional chemical and thermal stability, real-time measurement, and energy savings, make the device a promising candidate for next-generation laser detectors and laser energy/power meters.

Magnetorefractive effect in manganites with a colossal magnetoresistance in the visible spectral region

Abstract: The magnetotransmission, magnetoreflection, and magnetoresistance of the La07Ca03MnO3 and La09Ag01MnO3 epitaxial films have been investigated It has been found that the films exhibit a significant magnetorefractive effect in the case of reflection and transmission of light in the fundamental absorption region both in the vicinity of the Curie temperature and at low temperatures It has been shown that the magnetorefractive effect in the infrared spectral region of the manganites is determined by a high-frequency response to magnetoresistance, whereas the magnetorefractive effect in the visible spectral region of these materials is associated with a change in the electronic structure in response to a magnetic field, which, in turn, leads to a change in the electron density of states, the probability of interband optical transitions, and the shift of light absorption bands The obtained values of the magnetotransmittance and magnetoreflectance in the visible spectral region are less than those observed in the infrared region of the spectrum, but they are several times greater than the linear magneto-optical effects As a result, the magnetorefractive effect, which is a nongyrotropic phenomenon, makes it possible to avoid the use of light analyzers and polarizers in optical circuits

Complex state found in the colossal magnetoresistance regime of models for manganites

Abstract: The colossal magnetoresistance (CMR) effect of manganites is widely believed to be caused by the competition between a ferromagnetic (FM) metallic state induced by the double-exchange mechanism and an insulator with complex spin, charge, and orbital order. Recent computational studies in small clusters have indeed reported a CMR precisely near the frontier between those two states at a realistic hole density x = 1/4. However, the detailed characteristics of the competing insulator were not fully understood in those previous investigations. This insulator is expected to display special properties that lead to the CMR; otherwise any competition between ferromagnetic and antiferromagnetic states would induce such an effect, which is not the case experimentally. In this report, the competing insulator at electronic density x = 1/4 and in the CMR regime is studied in detail using the double-exchange two-orbital model with Jahn-Teller lattice distortions on two-dimensional clusters, employing a careful large-scale cooling down process in the Monte Carlo simulations to avoid being trapped in metastable states. Our investigations show that this competing insulator has an unexpected complex structure, involving diagonal stripes with alternating regions displaying FM and CE-like order. The level of complexity of this new state even surpasses that of themore » recently unveiled spin-orthogonal-stripe states and their associated high degeneracy. This new state complements the long-standing scenario of phase separation, since the alternating FM-CE pattern appears even in the clean limit. The present and recent investigations are also in agreement with the many glassy characteristics of the CMR state found experimentally, due to the high degeneracy of the insulating states involved in the process. Results for the spin-structure factor of the new states are also here provided to facilitate the analysis of neutron scattering experiments for these materials.« less

Effects of Bi Substitution on Magnetic and Transport Properties of La 0.7−x Bi x Ag 0.3 MnO 3 Ceramics

Abstract: Colossal magnetoresistance, CMR ceramics with starting composition of La0.7−xBixAg0.3MnO3 (x=0–0.2) were synthesized using the conventional solid-state synthesis method to investigate the effects of Bi and Ag on their magnetic and electrical transport properties as well as their magnetoresistance behavior. Magnetic susceptibility measurements showed that the La0.7−xBixAg0.3MnO3 samples with x=0, 0.10 and 0.15 exhibit single paramagnetic to ferromagnetic transition at Curie temperature, TC, which was observed to decrease from 289.5 K (x=0) to 186.5 K (x=0.15) while the x=0.2 sample showed two magnetic transitions at TC1 (160.5 K) and TC2 (214.0 K). Electrical resistivity measurements showed metal–insulator transition behavior for all samples. Bi substitution caused resistivity to increase while metal–insulator transition temperature, TMI shifted to lower temperature from 252.7 K (x=0) to 136.3 K (x=0.20). The metallic region of the ρ(0,T) curve below TMI for all samples was well fitted to the equation ρ=ρo+ρ2T2+ρ4.5T4.5 indicating a combination of grain or domain boundary, electron–electron and electron–magnon scattering mechanism while the insulator region was governed by the Variable Range Hopping (VRH) model at TMI θD/2. The increase of hopping activation energy, Ea for the latter is suggested to be due to possible hybridization between Bi 6s2 lone pair and O orbital. Bi3+ substitution was also observed to enhance intrinsic MR at the vicinity of TMI due to increase in DE interaction when external magnetic field was applied. On the other hand, the substitution also caused reduction of extrinsic MR effect at low temperatures, which is suggested to be due to reduction of Mn spin disorder at grain boundaries as a result of the presence of small amount of Ag secondary phase.

Direct observation of magnetization reversal and low field magnetoresistance of epitaxial La0.7Sr0.3MnO3/SrTiO3 (001) thin films at room temperature

Abstract: We have observed the in-plane magnetic domain arrangement during magnetization reversal in a 40 nm thick La0.7Sr0.3MnO3/SrTiO3 (001) thin film patterned into 500 lm long microbridges of width 50 or 100 lm. Magneto-optical Kerr effect microscopy was used at room temperature and magnetic hysteresis loops were deduced from local averaging of intensity over the microbridge areas. Magnetization reversal proceeds by nucleation and propagation of 180 domain walls. When the magnetic field was applied parallel to the bridge, we observed the nucleation of only one or two domain walls and the reversal occurred by the propagation of them. When the magnetic field was applied perpendicular to the bridge, the reversal occurred mostly by the nucleation of several domain walls. The low field magnetoresistance (MR) and the low frequency noise at zero magnetic field were measured at room temperature. In addition to the linear and reversible colossal MR effect, hysteretic MR versus magnetic field curves could be observed, showing two maxima (minima) when the magnetic field is parallel (perpendicular) to the bridge length. The observed hysteretic MR behaviour is attributed to anisotropic MR inside the 180 Ne'el domain walls.

Positive colossal magnetoresistance observed in Co doped amorphous carbon/silicon heterostructures

Abstract: Heterostructures of Co-doped amorphous carbon (Co-C)/silicon were fabricated by growing Co-C films on n-type Si substrates using pulsed laser deposition. The heterostructures exhibited a positive colossal magnetoresistance (CMR) effect over a temperature range of 55-240 K. The magnetoresistance (MR) for the reverse bias voltage reached around 270% at 5 T, whereas the MR under a forward bias was 7% only. Besides, the transmission electron microscopy results demonstrate that Co atoms tended to be aggregated at Co-C/Si interface. The Co aggregation in the interface may be a possible origin of the positive CMR effect.

The x-ray magnetic circular dichroism spin sum rule for 3d4 systems: Mn3+ ions in colossal magnetoresistance manganites.

Abstract: The colossal magnetoresistance manganites La0.87±0.02Sr0.12±0.02MnO3+δ, La0.78±0.02Sr0.17±0.02MnO3+δ, and La0.66±0.02Sr0.36±0.02MnO3+δ (δ close to 0) were investigated by using soft x-ray magnetic circular dichroism (XMCD) and magnetometry. Very good agreement between the values for the average Mn magnetic moments determined with these two methods was achieved by correcting the XMCD spin sum rule results by means of charge transfer multiplet calculations, which also suggest a charge transfer of ~50% for Mn4+ and 30% for Mn3+. The magnetic moment was found to be localized at the Mn ions for x = 0.17 and 0.36 at 80 K and for x = 0.12 in the temperature range from 80 to 300 K. We discuss our findings in the light of previously published data, confirming the validity of our approach.

Tunable positive magnetoresistance effect of Co-doped amorphous carbon films

Abstract: Co-doped amorphous carbon (a-C:Co) films were deposited on n-type Si substrates by pulsed-laser deposition method. A positive magnetoresistance (PMR) effect has been observed after Co doped into a-C films. Such a PMR is tuned by the bias voltage and reaches a peak at a particular voltage, as observed from the Current-voltage relations of the a-C:Co/Si junctions at various magnetic fields. MR-H characteristics were further studied at the temperatures of 65 K, which showed that under the reverse electric field the a-C:Co/Si junctions had a colossal PMR (over 100%). Raman spectra results demonstrate that Co doping favors the formation of graphitic sp2 sites. The mechanism of the PMR effect is attributed to the interactions between the applied magnetic field and Co ions, which leads to the transition from sp2 sites to sp3 sites and increase the resistance.