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Magnetoresistance (MR) phenomenon couples the electron transport with magnetic field, which has been at the forefront of condensed matter physics and materials science. Large-MR behaviors are of particularly importance for magnetic sensor and information memory applications, and their scarcity has aroused intensive research. Moreover, due to the different physical origins, combination of large positive and negative MR (pMR and nMR) in one single compound has rarely been reported. In present work, we achieved a coexistence of large pMR and nMR in Cr2Si2Te6 ferromagnetic semiconductor single crystal with different field configurations. Specifically, a large nMR of about −60% was obtained under the in-plane field, while a large pMR higher than 1000% took over in the out-of-plane direction. We attribute this field direction-sensitive dualistic large MR behavior to the competition and cooperation effect from the ferromagnetic interaction, orbital scattering and electronic correlation that coexist in Cr2Si2Te6, which contribute to nMR, pMR, and nMR, respectively, in dominated temperature and field ranges, and show different weights under different field directions. The elucidated multiple MR mechanism in this ferromagnetic semiconductor will shed light on the pursuit of coexistence of large pMR and nMR for field-sensitive device applications.

Pursuing new ferromagnetic systems could bring about research breakthroughs for magnetism. Currently, studies of magnetic ordering are expanding from interatomic to intermolecular exchange interactions to produce ferromagnets in inorganic/organic crystals. To date, the crystalline framework used for stabilizing parallel spin alignment via an ordered lattice seems to be indispensable to ferromagnetism. In our case, we demonstrated room-temperature long-range ferromagnetic order in two-dimensional confined cobaltocene (Co(Cp)2) molecular monolayers. In a confined van der Waals (vdW) interlayer space, spontaneous uniform spin orientation of Co(Cp)2 can be settled. Ferromagnetic coupling is established by an intermolecular vibronic superexchange interaction, a new long-distance exchange interaction in a cooperative dynamic Jahn-Teller (J-T) Co(Cp)2 monolayer. As expected, the confined Co(Cp)2 monolayers exhibit a high ferromagnetic transition temperature (above room temperature) with a saturation magnetization up to 4 emu.g-1.

In this work, polycrystalline Pr1-xBaxMnO3 (0.25 ≤ x ≤ 0.36) ceramics were synthesized, and their magnetic and electrical transport properties were systematically studied. All samples show two metal-insulator transitions (MITs) corresponding to the high temperature TMI1 and low temperature TMI2, respectively, besides the non-Griffith phase above the ferromagnetic (FM) transition temperature TC. Combining the results of the transport and magnetic properties, it is found that the FM transition temperature TC coincides with the temperature TMI1, which is linearly related to the A-site ionic radius mismatch variance σ2, indicating the enhancement of FM interactions due to the increase of the degree of B-site ordering of Mn3+/Mn4+ ions. The positive correlation between ferromagnetic insulators (FMI) and magnetic interactions, including the FM and short-range antiferromagnetic (AFM) interactions, is confirmed. It is suggested that the first MIT at TMI1 is attributed to the Mn3+/Mn4+ double exchange interactions and the second MIT at TMI2 is closely related to the suppression of the AFM interactions under the internal FM field induced by the Mn3+/Mn4+ DE interactions. This work provides not only a theoretical understanding on the origin of MIT at low temperature, but also a new way for adjusting the FMI in perovskite manganese oxide Pr1-xBaxMnO3 for application.

The Sm-doping effect on the crystal structure, dielectric and magnetic properties of CeFeO3 ceramics are systemically investigated. A giant dielectric response with high dielectric constant and low dielectric loss at room temperature (RT) is observed in Ce0.5Sm0.5FeO3 compounds. The increase of the dielectric constant is attributed to the increase of the ratio of Fe2+/Fe3+ ionic contents due to the charge transfer between Ce3+ and Fe3+ ions induced by Sm doping. Besides, the starting temperature T1 of spin-reorientation transition from Γ4 (Gx, Ay, Fz) to Γ1 (Ax, Gy, Cz) increases with Sm doping, and even reaches to a higher one than RT. Meanwhile, composite spin configurations of Fe3+ ions containing Γ1 and/or Γ2 (Fx, Cy, Gz) are found at RT in 0.2≤x ≤ 0.5 samples (x is Sm doping level), besides Γ4 state, whose formation originates from the strengthening of the anisotropic exchange interactions between Fe3+ and Ce3+/Sm3+ spins and the coexistence of Ce- and Sm-rich regions caused by Sm doping. It is suggested that the combining and competing functions of the Fe2+-Fe3+ ferromagnetic double-exchange interaction related to the Fe2+ ionic content and the composite spin configurations of Fe3+ ions containing Γ1 and/or Γ2 dominate the dielectric loss of Ce1-xSmxFeO3 ceramics. The results provide a different view for the physical theory and adjustment of the dielectric properties for applications.

Quantum Griffiths singularity (QGS) is among the remarkable phenomena induced by disorder in quantum phase transitions (QPTs). In recent years, experimental progresses have been made in ultrathin s...

The cerium orthoferrite (CeFeO3) ceramics were synthesized successfully by solid-state method and the room-temperature multiferroic properties of CeFeO3 annealed under different atmospheres were investigated. A clear ferroelectric hysteresis loop with Pr = 0.8 μC/cm2 measured under 10 kV/cm is observed at room temperature in the CeFeO3 annealed under oxygen, implying the nature of ferroelectricity existing in the material which is ascribed to the crystal structure change and local lattice distortion closely related to the mixed valences of Ce and Fe ions, as revealed by X-ray diffraction, Raman spectra, and XPS analysis. Besides, Magnetic study reveals that CeFeO3 is a weak ferromagnetic material and the saturation magnetization increases with oxygen vacancies decreasing. The coexistence of the induced ferroelectricity and the improved ferromagnetism confirms that the better multiferroic property exists in CeFeO3 ceramics and the oxygen content (Ce valence) has a critical role in the ferroelectric property.

Double perovskites Dy2CoMnO6 (DCMO) was synthesized by a sol-gel method and the structure and magnetic properties were systematically investigated. Both X-ray diffraction and Raman spectrum indicate that double pervoskite DCMO has a monoclinic structure with a space group P21/n. A Griffiths phase is confirmed to be existing in DCMO by the magnetic behaviors, which is significantly different from the behaviors of the non-Griffiths phase of Gd2CoMnO6 (GCMO) recently reported in reference. It is suggested that the origins of the different short-range magnetic orders of them can be attributed to the different 3d-4f exchange interactions in DCMO and GCMO, besides the effect of the unavoidable anti-site disorder, interrupting the long-range ferromagnetic (FM) order of Co2+-O-Mn4+, resulting in the formation of short-range FM order. The effects of A-site rare-earth Re3+ ions on the magnetism of double perovskite Re2Co(Ni)MnO6 are dominated by the combining and competing functions of the radius and the moments of rare earth ions (or the 3d-4f exchange interactions). The FM/AFM coupling is not only related to the most outer 4f electrons of the rare earth ions, but also to the inner 5d/6s orbital electrons.

A series of Bi2MoO6 (BMO) nanoplates with a heterogeneous Bi2MoO6-x@Bi2MoO6 core/shell structure were successfully obtained by reduction with CaH2 at 140 °C and subsequent reoxidation in air under low temperatures (≤250 °C). It is found that the oxygen vacancies mainly exist in the core, while the shell is formed by surface disorder layer with few oxygen vacancies, which is almost stoichiometric. The controllable growth of the surface disorder layer can be realized by the reduction and/or reoxidation temperatures. The reoxidized BMO with the surface disorder layer exhibits about five times higher visible-light photocatalytic activity toward phenol degradation and an order of magnitude higher activity toward methylene blue (MB) degradation than those of the unmodified sample. The visible-light absorption is mainly dominated by the oxygen vacancies through the modulation of the band gap structure, which promote the solar light utilization and photogenerated charge carriers. Density functional theory calculation shows that BMO is a direct band gap semiconductor. It is suggested that the surface disorder layer could suppress the direct recombination of photogenerated carriers and improve surface reactivity, and results in the enhancement of the photocatalytic activity. In addition, the existence of the stoichiometric surface disorder can promote the catalyst stability. It is demonstrated that the reduction- reoxidation method applied here is simple, easily control, and beneficial for the design and realization of heterogeneous core-shell nanostructured photocatalysts to improve visible and infrared optical absorption by engineering the surface disorder and the oxygen vacancies inside core of Bi2MoO6 nanoplates and the related nanostructured photocatalysts.

The perovskite manganese oxides La 0.3 Ce 0.2 Ca x Sr 0.5-x MnO 3 (0 ≤ x ≤ 0.25) have been synthesized by a solid-state reaction, and their transport and magnetic properties have been systemically studied. It is found that the temperature dependent resistivity shows a maximum at T max below Curie temperature T c with temperature decreasing, which is correlated to the Ce 3+ spin-dependent scattering. Moreover, T max is almost unchanged for x ≤ 0.15, but decreases quickly and is closing on the temperature of T c while x > 0.15, which indicates the increase of Ce ionic valence. Meanwhile, the saturation magnetic moment Ms decreases with x increasing to 0.15, and increases anomaly when x > 0.15. XANES results reveal that when x ≤ 0.15, the valence of Mn ion is almost unchanged, but when x > 0.15, it obviously shifts from tetravalence to trivalence with Ca substitution. Meanwhile, XPS spectra results indicate the similar trend of Ce valence changing from trivalence to tetravalence. It is suggested that Ca substitution changes not only the chemical pressure, but also the valence of Ce ion. There is a threshold value for the chemical pressure corresponding to Ca- substitution content at around x = 0.15 (or A > tv = 1.2395 A), which is the starting point of charge transfer appearance in Ca-substituting La 0.3 Ce 0.2 Sr 0.5 MnO 3 compound. The transport and magnetic properties of the half-doped manganties La 0.3 Ce 0.2 Ca x Sr 0.5-x MnO 3 are dominated by the competition/cooperation of Ce 3+ spin-dependent scattering, interaction between Mn ions controlled by A-site ion-size, and the charge transfer from Ce ions to Mn ions.

Bi1−x(Sr1/2Pb1/2)xFeO3 (0.10 ≤ x ≤ 0.30) multiferroic compounds have been synthesized by a conventional solid-state reaction. The influences of A-site Sr/Pb co-doping on the structure, dielectric and magnetic properties of BiFeO3 are investigated systematically. X-ray diffraction reveals that the crystal structure of Bi1−x(Sr1/2Pb1/2)xFeO3 transforms from the rhombohedral symmetry (space group R3c) to the cubic symmetry (space group Pm 3 ¯ m) with Sr/Pb concentration increasing. Meanwhile, the intensities of Bi O bond vibrations continuously decrease and finally disappear. The RT polarization versus electric field (P-E) curves confim the ferroelectric nature for all the samples. Furthermore, it is found that the dielectric constant and dielectric loss tangent of the samples measured in the frequency range of 100–107 Hz decrease drastically at room temperature. A considerable reduction in the leakage current is observed for BFO with Sr/Pb doping content increasing. The ferromagnetic property enhances with the Sr/Pb substitution increasing, which is ascribed to the cooperation of the Fe2+ O Fe3+ double exchange interaction and the effective suppression of antiferromagnetic cycloidal spin structure caused by the crystalline structure transformation. All results indicate that the Sr and Pb co-doping can effectively improve the magnetic and high-frequency dielectric properties of the multiferroic BiFeO3 compounds.

The activity of electrocatalysts exhibits a strongly dependence on their electronic structures. Specifically, for perovskite oxides, Shao-Horn and co-workers have reported a correlation between the oxygen evolution reaction activity and the eg orbital occupation of transition-metal ions, which provides guidelines for the design of highly active catalysts. Here we demonstrate a facile method to engineer the eg filling of perovskite cobaltite LaCoO3 for improving the oxygen evolution reaction activity. By reducing the particle size to ∼80 nm, the eg filling of cobalt ions is successfully increased from unity to near the optimal configuration of 1.2 expected by Shao-Horn's principle. Consequently, the activity is significantly enhanced, comparable to those of recently reported cobalt oxides with eg∼1.2 configurations. This enhancement is ascribed to the emergence of spin-state transition from low-spin to high-spin states for cobalt ions at the surface of the nanoparticles, leading to more active sites with increased reactivity., The activity of electrocatalysts exhibits a strong dependence on their electronic structures. Here, the authors manipulate the eg filling of perovskite cobaltite LaCoO3 nanoparticles by changing particle size and show improved oxygen evolution activity with increased numbers of surface high-spin cobalt ions.

The magnetoresistance (MR) and magnetism of PrBaCo2O5+δ (PBCO) with different oxygen content are systematically investigated. It is found that with the temperature lowing, the MR effect changes from positive to negative for the sample annealed in Ar with low oxygen content, and is negative and increases in magnitude for the as-prepared and O2-annealed samples (AP-PBCO, O2–PBCO). A higher MR effect with ∼38% at around 70 K and 9 T for AP-PBCO is obtained. Besides, an abnormal decrease in magnitude for MR is observed for AP-PBCO and O2–PBCO. It is suggested that the spin disorder related polaron hopping energy WP is crucial for the MR effect in PBCO, besides the competition of AFM and FM interactions, which rests with the crystal structure dominated by the content and order of the oxygen vacancies. The observed abnormal MR can be attributed to the reduced polaron hopping energy WP related to the AFM order. The results not only experimentally confirm that the MR effect can be easily adjusted by the oxygen vacancies but also open up a new way for developing the high MR in layered double perovskites for application.

The epitaxial films of LaMnO3 were fabricated via a simple polymer-assisted deposition method. The effects of substrate orientation and annealing condition on the structure and properties of LaMnO3 films have been investigated. It is found by X-ray diffraction and Raman spectroscopy that increase in the oxygen content results in a decrease in unit cell volume along with a reduction in Jahn–Teller distortion. Besides, with increase in annealing temperature, the resistivity of the film decreases and the insulator–metal transition temperature TIM shifts to higher temperature. The maximum of the resistivity is highly substrate-orientation dependent in the ascending order of (1 0 0) ( 1 1 1). Furthermore, the TIM of LaMnO3 film increases with the substrate orientation changing from (1 1 1) to (1 0 0). It is shown that magnetic order correlates well with an insulator to metal behavior. All results reveal that the lattice distortion of MnO6 octahedron can be tuned by different annealing condition and the substrate orientation, which can be effective methods to adjust the structure, electrical and magnetic properties of LaMnO3 films.

Ferromagnetic two-dimensional (2D) ultrathin nanosheets hold great promise for next generation electronics. Ferromagnetic metal-free materials that usually possess only an s/p electronic configuration with weak spin–orbit coupling and a large spin relaxation time, would play an important role in constructing future spintronic devices. However, the absence of an intrinsic spin ordering structure in most metal-free materials greatly hampers the widening scope of ferromagnetic 2D nanostructures as well as in-depth understanding of their ferromagnetic nature. Herein, the induction of intrinsic ferromagnetism in 2D metal-free g-C3N4 ultrathin nanosheets has been achieved through a new effective strategy whereby hydrogen dangling bonds are introduced. In our case, g-C3N4 ultrathin nanosheets with hydrogen dangling bonds showed obvious room temperature ferromagnetic behavior that could even be tuned by the concentration of hydrogen. This work will pave a new pathway to engineer the properties of 2D nanomaterial systems.

LaCoO3 (LCO) epitaxial nano-thin films with different thickness (∼30 nm, 60 nm, and 80 nm) were grown on (1 0 0), (1 1 0), and (1 1 1) oriented LaAlO3 (LAO) substrates, respectively, by a simple polymer assisted deposition method. X-ray diffraction analyses confirm that all the LCO films are epitaxially grown in accordance with the orientation of LAO substrates, with biaxial compressive strain in the ab plane. Due to the different strain directions, the Jahn–Teller (JT)-like tetragonal distortion of CoO6 octahedron is dominant in LCO film on (1 0 0) LAO, but the rotation of CoO6 octahedron is dominant in LCO film on (1 1 1) LAO, while it is intervenient between them in LCO film on (1 1 0) LAO. For the same oriented films, as a result of the lattice relaxation effect, the strain is slightly relaxed as the film thickness increasing. An obvious ferromagnetic (FM) transition at TC ∼ 85 K is observed in all the (1 0 0) oriented LCO films, but a very weak FM signal in (1 1 0) oriented LCO films and a nonmagnet-like characteristic in (1 1 1) oriented LCO films below ∼85 K. Combined with the structural analyses, it is found that the change of the FM signal is closely related to the stain-induced JT tetragonal distortion of CoO6 octahedron which can stabilize Co3+ ions in the intermediate spin (IS) state. The higher degree of the JT tetragonal distortion of CoO6 octahedron corresponds to the higher field-cooled (FC) magnetization. However, the rotation mode of CoO6 octahedron will weaken the degree of JT tetragonal distortion and reduce the population of IS Co3+.

Electron-spin resonance (ESR) studies have been performed on Y x Ca 1− x MnO 3 (0.05 ⩽ x ⩽ 0.12) compounds in the temperature range from 140 K to 470 K. It is found that with the temperature increasing, the ESR linewidth increases to the maximum at T max (=200 K, 220 K, and 270 K for x = 0.08, 0.10, and 0.12, respectively), and then decreases for all the compounds except that with x = 0.05 which shows a continuous decrease in the whole temperature range. For x ⩾ 0.08, the change of the ESR linewidth can be described very well by the adiabatic small polaron hopping model in the temperature range from 140 K to T max . The ESR intensity decreases with temperature increasing for all the compounds between 140 K and 470 K except 300 K ⩽ T ⩽ 330 K in which it increases with temperature increasing. Meanwhile, the g -factor reaches the minimum at about T = 300 K. The characteristics of the ESR linewidth and intensity indicate the appearance of the short-range magnetic correlations and that the spin–lattice relaxation plays an important role for x ⩾ 0.08. In addition, the electronic transport properties of Y x Ca 1− x MnO 3 compounds reveal a metal–insulator transition at T MI which varies from 254 K to 293 K with Y content increasing. The resistivity versus temperature between 140 K and T MI can be fitted well by the adiabatic small polaron hopping model. It is suggested that the metal–insulator transition in Y x Ca 1− x MnO 3 compounds is correlated with the variation of the magnetism under high temperature.

Two dimensional (2D) nanosheets have shown great potential for their applications in next-generation nanoelectronic devices. However, developing 2D nanosheets for next-generation spintronics has been blocked due to their lack of robust intrinsic ferromagnetic behavior. Here, we highlight the robust ferromagnetic behavior in a 2D inorganic graphene-like structure. δ-FeOOH ultrathin nanosheets, as a new 2D material, exhibit an unprecedented high saturation magnetization value of 7.5 emu g−1 at room temperature, which is the highest value among graphene and graphene analogues. Atomic-scale topochemical transformation ensures the formation of 2D δ-FeOOH ultrathin nanosheets from intermediate Fe(OH)2 nanosheets. Moreover, the δ-FeOOH ultrathin nanosheet is found to be a semiconductor with a direct band gap of 2.2 eV. Owing to the advantages of robust ferromagnetism in semiconductors, δ-FeOOH ultrathin nanosheets are promising candidates for the construction of next-generation spintronics.

Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe2, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe2 by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb–Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm., Ferromagnetism and superconductivity possess inherently incompatible electronic spin ordering, and their coexistence requires elaborate engineering of material components. Here, the authors induce ferromagnetism in a two-dimensional superconducting crystal by the adsorption of hydrazine molecules.

Pressureless sintering in vacuum was applied to synthesize Ti 3 SiC 2 from elemental powders of Ti, Si and C. Based on the phase compositions and purities of the products obtained by X-ray diffraction, the elemental powders composition and sintering condition were optimized. It was found that the sample sintered at 1450 °C for 240 min from a mixture of 3Ti/1.75Si/2C (molar ratio) contained Ti 3 SiC 2 with the volume fraction as high as 93%. It was proposed that loss of Si through gaseous vaporization and contamination of C might be the main obstacles against obtaining high-purity material by this way.

Inducing ferromagnetism in two dimensional metal-free g-C3N4 nanosheets has been achieved through the introduction of hydrogen dangling bonds., Ferromagnetic two-dimensional (2D) ultrathin nanosheets hold great promise for next generation electronics. Ferromagnetic metal-free materials that usually possess only an s/p electronic configuration with weak spin–orbit coupling and a large spin relaxation time, would play an important role in constructing future spintronic devices. However, the absence of an intrinsic spin ordering structure in most metal-free materials greatly hampers the widening scope of ferromagnetic 2D nanostructures as well as in-depth understanding of their ferromagnetic nature. Herein, the induction of intrinsic ferromagnetism in 2D metal-free g-C3N4 ultrathin nanosheets has been achieved through a new effective strategy whereby hydrogen dangling bonds are introduced. In our case, g-C3N4 ultrathin nanosheets with hydrogen dangling bonds showed obvious room temperature ferromagnetic behavior that could even be tuned by the concentration of hydrogen. This work will pave a new pathway to engineer the properties of 2D nanomaterial systems.

Pr2CoMnO6 particles with different sizes (45 nm–1 μm) have been synthesized by a citrate-gel method with different annealing temperatures and investigated by x-ray diffraction, Raman spectroscopy, and magnetometry. It is found that at a lower annealing temperature related to sample synthesis, the samples with particle size D ≤ 200 nm show a higher ferromagnetic (FM) transition temperature Tc1, which can be attributed to the FM superexchange interaction of the ordered Co2+–O–Mn4+ pairs. As the annealing temperature increasing, a new FM transition originating from FM vibronic superexchange interaction of Co3+–O– Mn3+ pairs in the disordered phase appears at temperature Tc2 lower than Tc1. Raman spectra at room temperature show that both the antisymmetric stretching and symmetric stretching modes at around 490 and 637 cm−1, respectively, show a decrease in the linewidth and an increase in the intensity with the annealing temperature decreasing. All the results indicate that the ratio of the disordered phase to the ordered phase in the sample decreases as the annealing temperature decreasing. It is suggested that the annealing temperature dependent oxygen vacancies play a key role in the magnetic properties of the Pr2CoMnO6 particles. Furthermore, the temperature-dependent Raman spectrum for Pr2CoMnO6 particles reveals obviously softening of the phonon modes involving stretching vibrations of the (Co/Mn)O6 octahedra at Curie temperature, indicating a close correlation between magnetism and lattice in Pr2CoMnO6 particles. The Pr2CoMnO6 compound with smaller particle size retains a higher FM transition temperature and obvious spin–phonon coupling, which are of special interest for the application.

We studied using organic liquids (cyclohexane, n-butanol, and ethylene glycol) to modulate the transport properties at room temperature of an epitaxial VO2 film on a VO2/Al2O3 heterostructure. The resistance of the VO2 film increased when coated with cyclohexane or n-butanol, with maximum changes of 31% and 3.8%, respectively. In contrast, it decreased when coated with ethylene glycol, with a maximum change of −7.7%. In all cases, the resistance recovered to its original value after removing the organic liquid. This organic-liquid-induced reversible resistance switching suggests that VO2 films can be used as organic molecular sensors.