Layered BaV6O16·3H2O@GO as a high performance cathode material for calcium ion batteries.

The lack of suitable cathode materials has hampered the further development of calcium-ion batteries (CIBs). A novel composite cathode material, namely BaV6O16·3H2O@GO, was fabricated, which delivers a high specific capacity of 285.72 mA h g-1 at 50 mA g-1 after 50 cycles and a long cycle life, benefiting from a large layer spacing and robust structure. This study provides guidance for the development of vanadium-based cathode materials for CIBs.

Perpendicular magnetization anisotropy induced dynamical coherence reduction in stripe domain film.

We investigated the magnetization dynamics of the 350 nm permalloy film with in plane domain (IPD), stripe domain (SD), and labyrinth domain (LD) patterns. Experimental and micromagnetic simulation results showed that the change in magnetic domain structure from IPD to LD was due to the increasing perpendicular magnetic anisotropy (PMA) of the film. The magnetization dynamics indicated that the resonant modes of the film strongly depended on the magnetic domain structure. IPD films presented a uniform precession mode. The film with well-regular SD exhibited clear acoustic and optical resonance modes, and the formation of LD suppressed both resonance modes. Finally, the dynamics of magnetization dependent on the domain structure in these films were discussed by using the phenomenological resonance models.

Ultrahigh Frequency and Anti-Interference Optical-Mode Resonance with Biquadratic Coupled FeCoB/Ru/FeCoB Trilayers.

Microwave soft magnetic films (SMFs) are the key materials to effectively miniaturize and multifunctionalize the microwave electromagnetic components and devices. However, currently, single-layer SMFs encounter a frequency bottleneck at around 10 GHz. The ferromagnet/nonmagnetic spacer/ferromagnet sandwiched films with strong interlayer exchange coupling are possible solutions to break through that frequency limitation because they exhibit ultrahigh optical-mode (OM) resonance frequency frO up to 50 GHz, while the tiny permeability and the limited thickness are their own obstacles to overcome. In this study, biquadratic coupled FeCoB25nm/Ru0.25nm/FeCoB25nm sandwiched films with uniaxial magnetic anisotropy were deposited by a composition gradient sputtering method. Pure OM resonance with self-bias frO up to 18.21 GHz and a relative permeability µrO as high as 169 at the cut-off frequency was achieved. Moreover, both the frO and µrO remain unchanged in the magnetic field range of 0-80 Oe, indicating a strong anti-interference capability to small interference field. These results demonstrate that the biquadratic coupled OM resonance can solve the current frequency bottleneck of microwave SMFs by providing ultrahigh resonance frequency while maintaining considerable permeability, thus leading to potential applications of OM resonance in Ku-band microwave magnetic components.

Direct imaging of dopant sites in rare-earth element-doped permanent magnet and correlated magnetism origin.

Identifying the dopants and their occupation sites in rare-earth-doped permanent magnets is critical not only to understand the mechanism of tuning their magnetic properties, but also to develop guiding principles to further improve their performance. In this study, we present a direct observation of the preferred atomic sites of La atoms in La-doped M-type SrFe12O19 hexaferrite. Our data solidly clarified that only the Sr2+ cations were replaced by La3+ cations, and the La-doping caused the changes in the valence states of iron cations located at the 4f1 and 2a crystallographic sites. First principles calculations further revealed that after La-doping, the changes in the spin states of the Fe3+ cations located at the 4f1 tetrahedral sites resulted in magnetization enhancement and those of the 2a octahedral sites contributed to electrical neutrality, well matching the experimental atomic-column resolution EELS and magnetic measurement results.

Investigation on the structures and magnetic properties of carbon or nitrogen doped cobalt ferrite nanoparticles.

Carbon or nitrogen doped cobalt ferrite nanoparticles were synthesized in the air by a facile calcination process. X-ray diffraction, mapping, X-ray photoelectron spectroscopy, and mössbauer spectra results indicate that the nonmetal elements as the interstitial one are doped into cobalt ferrite nanoparticles. The morphologies of doped cobalt ferrite nanoparticles change from near-spherical to irregular cubelike shapes gradually with the increased carbon or nitrogen concentration, and their particles sizes also increase more than 200 nm. Furthermore, the saturation magnetization of carbon doped cobalt ferrite is improved. Although the saturation magnetization of N-doped cobalt ferrite is not enhanced obviously due to the involved hematite, they also do not drop drastically. The results reveal an approach to synthesize large scale ferrite nanoparticles, and improve the magnetic properties of ferrite nanoparticles, and also provide the potential candidates to synthesis co-doped functional magnetic materials.

Stress-Enhanced Interlayer Exchange Coupling and Optical-Mode FMR Frequency in Self-Bias FeCoB/Ru/FeCoB Trilayers.

Nowadays, the most popular method to increase ferromagnetic resonance (FMR) frequency ( fr) in self-bias soft magnetic films is to improve the anisotropy field HK. However, to push fr to higher frequencies only via raising HK becomes increasingly challenging because fr is already higher than 10 GHz by now. In this study, we fabricated a series of magnetically anisotropic FeCoB/Ru/FeCoB sandwich films possessing antiferromagnetic-like coupling and gradually increased uniaxial stress in the FeCoB sublayers from 52 to 110 MPa. It is quite remarkable that the acoustic mode of FMR gradually disappears, whereas the optical mode is enhanced in these structures. We observed simultaneous enhancement of HK and interlayer coupling field ( JIEC) with the uniaxial stress, which leads to a very pronounced optical-mode frequency increase from 8.67 to 11.62 GHz with a very sensitive stress response of 51 Hz/Pa. In contrast, the fr in a FeCoB single layer (acoustic mode) only varies from 3.47 to 5.05 GHz under similar stress. We believe that the strain-induced electron density variation of the Ru spacer's Fermi surface in the out-of-plane direction is responsible for the enhancement of JIEC. This study demonstrates that the antiferromagnetic coupling is a new route to achieve higher fr and provides the possibility of engineering and manipulating optical-mode resonance simply by controlling the interlayer coupling strength via stress.

A Nanocrystalline Fe2O3 Film Anode Prepared by Pulsed Laser Deposition for Lithium-Ion Batteries.

Nanocrystalline Fe2O3 thin films are deposited directly on the conduct substrates by pulsed laser deposition as anode materials for lithium-ion batteries. We demonstrate the well-designed Fe2O3 film electrodes are capable of excellent high-rate performance (510 mAh g- 1 at high current density of 15,000 mA g- 1) and superior cycling stability (905 mAh g- 1 at 100 mA g- 1 after 200 cycles), which are among the best reported state-of-the-art Fe2O3 anode materials. The outstanding lithium storage performances of the as-synthesized nanocrystalline Fe2O3 film are attributed to the advanced nanostructured architecture, which not only provides fast kinetics by the shortened lithium-ion diffusion lengths but also prolongs cycling life by preventing nanosized Fe2O3 particle agglomeration. The electrochemical performance results suggest that this novel Fe2O3 thin film is a promising anode material for all-solid-state thin film batteries.

High saturation magnetization of γ-Fe2O3 nano-particles by a facile one-step synthesis approach.

We have demonstrated the synthesis of γ-Fe2O3 nano-particles through a facile and novel calcination process in the air. There is no pH regulation, gas atmosphere, additive, centrifugation or other complicated procedures during the preparing process. A detailed formation process of the nano-particles is proposed, and DMF as a polar solvent may slower the reaction process of calcination. The structures, morphologies, and magnetic properties of γ-Fe2O3 nano-particles were investigated systematically, and the pure γ-Fe2O3 nano-particles obtained at 200 °C display uniform morphology good magnetic property. The saturation magnetization of obtained pure γ-Fe2O3 is about 74 emu/g, which is comparable with bulk material (76 emu/g) and larger than other results. In addition, the photocatalytic activity for degradation of methylene blue is also studied, which shows proper photocatalytic activity.

Hierarchical SrTiO3/NiFe2O4 composite nanostructures with excellent light response and magnetic performance synthesized toward enhanced photocatalytic activity.

Being capable of gathering advanced optical, electrical and magnetic properties originating from different components, multifunctional composite nanomaterials have been of concern increasingly. Herein, we have successfully demonstrated the preparation of SrTiO3/NiFe2O4 porous nanotubes (PNTs) and SrTiO3/NiFe2O4 particle-in-tubes (PITs) via a single-spinneret electrospinning and a side-by-side-spinneret electrospinning, respectively. The products were characterized by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-visible diffuse reflectance spectra and a vibrating sample magnetometer in detail. The results indicate that SrTiO3/NiFe2O4 PNTs are the heterojunction nanotubes by connecting perovskite SrTiO3 and spinel NiFe2O4 nanoparticles, but SrTiO3/NiFe2O4 PITs are the self-assembled core/shell structures by embedding SrTiO3 nanoparticles into NiFe2O4 nanotubes. Compared with pure SrTiO3 nanofibers, the two SrTiO3/NiFe2O4 composites exhibit a powerful light response and excellent room temperature ferromagnetism. The magnetic separations directly reveal that such amazing recycling efficiencies of about 95% for SrTiO3/NiFe2O4 PNTs and about 99.5% for SrTiO3/NiFe2O4 PITs are obtained. Furthermore, both the magnetic composites perform considerable photocatalytic activity in the degradation of rhodamine B. We propose that Kirkendall-diffusion and phase-separation are probably responsible for the formation of SrTiO3/NiFe2O4 PITs, and this work could provide a feasible way to assemble the core/shell structures of different materials.

A novel method to fabricate CoFe2O4/SrFe12O19 composite ferrite nanofibers with enhanced exchange coupling effect.

Nanocomposite of CoFe2O4/SrFe12O19 has been synthesized by the electrospinning and calcination process. A novel method that cobalt powder was used to replace traditional cobalt salt in the precursor sol-gel for electrospinning was proposed. The crystal structures, morphologies, and magnetic properties of these samples have been characterized in detail. Moreover, when the average crystallite size of the hard/soft phases reached up to an optimal value, the CoFe2O4 have an enhanced saturation magnetization of 62.8 emu/g and a coercivity of 2,290 Oe. Significantly, the hysteresis loops for the nanocomposites show a single-phase magnetization behavior, and it has been found that the exchange coupling interaction strongly exists in the CoFe2O4/SrFe12O19 magnetic nanocomposite nanofibers.

An induction method to calculate the complex permeability of soft magnetic films without a reference sample.

A new analytical method has been proposed by utilizing an electromagnetic induction principle with a short-circuited microstrip line jig and the complex permeability spectra can be calculated without a known reference sample. The new method using the short-circuited microstrip line can exhibit higher sensitivity and a wider frequency band than coplanar waveguide and pick-up coil. Two magnetic thin films having a good in-plane uniaxial anisotropy are measured by using the induction method. The results show typical complex permeability spectra in good agreement with the theoretical analytical results. The measured permeability values are verified by comparing with the initial susceptibility derived from the sweeping field results. The difference of measured permeability values is less than 5%.

Electrodeposition of FeCoCd films with in-plane uniaxial magnetic anisotropy for microwave applications.

FeCoCd thin films with 500 nm thickness are directly prepared through electrodeposition in the sulphate bath in which glycine and citric acid were added as complex agents. The composition, structure, and magnetic of FeCoCd films were investigated as a function of Cd2+ concentration, cathode current density, and deposition temperature. A wonderful soft magnetic FeCoCd film was prepared and its coercivity of easy axis and hard axis are 5 Oe and 4 Oe, respectively. The natural resonance frequency is about 3.0 GHz, which imply that the FeCoCd film is potential candidate for high frequency applications.