Zhang Jin-cang

Magnetoresistance of Multiwalled Carbon Nanotube Yarns

We measure zero-Geld resistivity and magnetoresistance of multiwalled carbon nanotube yarns (CNTYs). The CNTYs are drawn from superaligned multiwalled carbon nanotube arrays synthesized by the low-pressure chemical vapour deposition method. The zero-Geld resistivity shows a logarithmic decrease from 2K to 300 K. In the presence of a magnetic Geld applied perpendicular to the yarn axis, a pronounced negative magnetoresistance is observed. A magnetoresistance ratio of 22% is obtained. These behaviours can be explained by the weak localization effect.

High Magnetoelectric Coupling in Nano–Microscale Particulate Composites at Low Frequency

Nontoxic lead-free multiferroic magnetoelectric composites are successfully prepared by incorporating the dispersed Ni0.98Co0.02Fe2O4 (NCF) ferromagnetic nanoparticles into a (K0.5Na0.5)NbO3-LiSbO3 (KNN-LS) ferroelectric micromatrix. The dependence of the dielectric properties and dc magnetization on NCF phase content has been studied. Variation of dielectric constant and dielectric loss with frequency show dispersion in the low frequency range, and the dielectric constants decrease with the increase in ferrite NCF content. The magnetoelectric (ME) coupling effects including direct ME (DME) and converse ME (CME) effects are investigated in detail at room temperature. The results show that the NCF content significantly affects the ME effects. The CME and DME behaviors are strongly dependent on the driving field frequency and the bias magnetic field. High DME and CME coefficients are obtained at low frequency and at low magnetic bias field. The maximum value of DME and CME coefficients are 197.3 ps/m (12.2 mVcm−1Oe−1) and 314.7 ps/m, respectively.

Fabrication, Structural and Magnetic Properties for Aligned MnBi

MnBi compound is fabricated under a magnetic field of 1T, and the c-axis of hexagonal MnBi crystal is aligned parallel to the magnetic field direction. The saturation magnetization Ms decreases with the increase of temperature. At temperatures below 200 K, the coercive field Hc is about 150 Oe, while the coercive field Hc increases with temperature above 200K. From 200K to 300K, the remnant magnetization Mr and the Mr/Ms increase with the temperature. Below 200 K, Mr and Mr/Ms reach roughly a constant value. However, there is an abnormal increase at 100 K in Hc, Mr and Mr/Ms, which comes from a spin-reorientation in MnBi. Magnetization results show the spin-reorientation for MnBi at about 91 K due to the variations of the anisotropy constants.

Ferromagnetic resonance in Co/Pt multilayers

Out-of-plane angular dependence of ferromagnetic resonance spectra was measured in sputtered Co/Pt multilayers and analyzed with the Landau-Lifshitz-Gilbert equation. The effective demagnetizing field 4πMeff is found to increase with decreasing tPt and decreasing 1/tCo, which can be considered as a result of interplay between the interlayer coupling and a low-dimensional effect. The g factor increases with increasing tPt and decreasing tCo, indicating contribution of spin-polarization of Pt atoms and additional contribution of orbital moment of Co atoms. The in-plane resonance line-width increases with decreasing tCo and increases with increasing tPt.

Bronze and pyrochlore type iron fluorides as cathode materials for Li/Na batteries

Iron fluoride is the key candidate of cathode materials for high capacity Li/Na based batteries, owing to high ionicity of Fe–F and large theoretical capacity of fluoride. Exploring novel mineral phases of iron fluoride enables the improvement of intrinsic ion/electron conductivity, decreasing the use of electrochemical non-active components. It can lead to the activation of fluorides in terms of conversion reaction and large-sized cation (e.g., Na+) storage. This paper summarizes the recent progress on exploring novel structure prototypes of energy storage iron fluorides by module chemistry and open framework strategies, especially on hexagonal tungsten bronze (HTB), tetragonal tungsten bronze (TTB) and pyrochlore phases. These results address the important issues of fluorides on their poor conductivity, high carbon wire content, low Li-insertion power density and low Na-storage energy density. The HTB iron fluoride (FeF3 × 0.33H2O) is of tunnel structure, which is beneficial for the kinetic improvement and high rate Li (de)insertion. Complete dehydration of HTB phase is enabled by enhancing the crystallinity of HTB and removing its surface coating species. Pyrochlore iron fluoride of microporous framework (FeF3 × 0.5H2O) is endowed with interconnected three-dimensional (3D) open ion channels, enabling high reversible capacity of Na storage even under the involvement of conversion reaction. In contrast to HTB phase, dehydrating more flexible pyrochlore phase would cause serious amorphization (i.e., from topotactic densification of pyrochlore) however with the preservation of short-range ordering. The dehydration of open framework fluorides remarkably improves the conversion capacity and its retention, especially with good maintenance of higher voltage intercalation region. Ionic liquid based synthesis methods, e.g., dissolution-precipitation fluorination, ionothermal fluorination and solid-solid topotactic transformation in a top-down way, are explored to prepare the HTB and pyrochlore phases. The ionic liquid residual at fluoride grain or carbon wire surfaces allows an optimized ion/electron mixed conductive network based on the interaction with ionic liquid interlayer as binder. By using thermally more stable K-ion as channel filler instead of H2O molecule, more robust iron fluoride of TTB (K0.6FeF3) with coexistence of Fe2+ and Fe3+ is achieved by either conventional solid state reaction or mechanochemical method. TTB phase enables a near zero-strain reversible Na storage as cathode.

Influence of Magnetic Scattering and Interface Transparency on Superconductivity Based on a Ferromagnet/Superconductor Heterostructure

We investigate an antiferromagnet/ferromagnet/superconductor/ferromagnet (AF/F/S/F) spin-valve system with nanoscopic scale, described by Usadel equations in the dirty limit. The results show that the superconducting characteristics in the system strongly depend not only on the mutual orientation and thickness of two ferromagnetic layers, but also on the interface transparency and the magnetic scattering. The superconducting critical temperature can exhibit three types of characteristic behaviors with a variation of interface transparency. In particular, the reentrance phenomenon of the superconductivity is observed at the interface transparency ?B?n/?f = 7.1, while the reentrance phenomenon disappears in the presence of magnetic scattering. In addition, it is also found that the introduction of magnetic scattering results in the decrease of the spin-valve effect. The obtained results could provide some practical recommendations for the spin-valve effect in experimental observation.

Crystalline and Magnetic Enhancement of Nanocrystalline MnZn Ferrites Fabricated under a High Magnetic Field

Nanocrystalline Mn0.6Zn0.4Fe2O4 particles are synthesized under magnetic fields of 0 and 6 T, and their structural and magnetic properties are investigated. The magnetic field enhances the grain size and the lattice strain. Magnetic measurements show that the majority of the 6 T nanoparticles are superparamagnetic nearly from 40 to 300 K. It is interesting that the saturation magnetization of the 6 T sample is about 18% and 16% higher than that of the 0 T sample at 120 and 300 K, respectively.

Nano-sized Domain Wall Pinning Effects in Dilute Cu-Doped Perovskite LaMn1-xCuxO3 Manganites

Magnetic properties of Cu-doped LaMn1-xCuxO3 (x=0.05-0.30) systems are carefully studied in the temperature range of 2–300 K. A visible unexpected drop is observed in the ac susceptibility and the zero-field cooled dc magnetization curves for the dilute x ≤0.10 near 100K, which depends on the measuring frequency and magnetic field. Measurements on frequency dependence of ac susceptibility, observation of magnetic relaxation, and the existence of critical field indicate that the anomaly can be attributed to the domain wall pinning effects. This is directly proven by the results of ball milled nano-sized powder counterparts compared with the bulk materials.

In-Situ Alignment of MnBi Crystals Induced by High Magnetic Field above Curie Temperature

Above Curie temperature, MnBi crystals are aligned in situ along the c-axis in a Bi matrix by a high fabrication magnetic field Hf of 10 T. Magnetic testing shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from the alignment. The successful alignment may result from the fact that the easy magnetization direction is along the c-axis of MnBi and the high fabrication field of 10 T is large enough to rotate the MnBi crystal to this direction even though the temperature is above the Curie temperature.

Structure and magnetic properties of Fe1−x C x , solid solution prepared by mechanical alloying

Supersaturated solid solutions Fe1−xCx(0⩽x⩽0.9) of wide composition range have been prepared by mechanical alloying process. Nanocrystalline phase was formed for 0⩽x⩽0.67 and a large grain phase for 0.75⩽x⩽0.9. The large fraction of graphite volume puts off formation of nanocrystalline phase for high carbon content. In the large grain phase, magnetization follows simple magnetic dilution, and coercivity Hc is mainly due to dissolution of carbon at grain boundaries. In the nanocrystalline phase the alloying effect of carbon is revealed by a distinct reduction of average magnetic moment. The increasing lattice constant with increasing carbon content is observed for a;