Magnetization and phase diagram of a cubic nanowire in the presence of the crystal field and the transverse field

Magnetic properties of the cubic nanowire, which consists of a ferromagnetic core of spin-1 and a ferromagnetic shell of spin-3/2 with ferrimagnetic interface coupling, are investigated by the use of the effective-field theory with correlations (EFT). Two compensation points have been found for certain values of the system parameters in the nanowire. In particular, the effects of the transverse field and the crystal field on the magnetization and phase transition are described in detail.     

An oxide/silicon core/shell nanowire metal-oxide semiconductor field-effect transistor

This paper studies an oxide/silicon core/shell nanowire MOSFET(OS-CSNM).Through three-dimensional device simulations,we have demonstrated that the OS-CSNM has a lower leakage current and higher I on /I off ratio after introducing the oxide core into a traditional nanowire MOSFET(TNM).The oxide/silicon OS-CSNM structure suppresses threshold voltage roll-off,drain induced barrier lowering and subthreshold swing degradation.Smaller intrinsic device delay is also observed in OS-CSNM in comparison with that of TNM.     

表面效应对纳米线电极屈曲失稳的影响

纳米线电极在充/放电过程中引起电极的屈曲失稳行为可能会对结构造成力学损伤.本文针对纳米线电极结构,建立了包含锂扩散、应力、浓度影响弹性模量的多场耦合理论模型.基于构建的模型,研究了表面效应对纳米线电极屈曲失稳的影响.结果表明表面效应能够提高纳米线电极的抗屈曲性,延迟纳米线电极的临界屈曲时间.同时,表面效应的影响表现出半径尺寸和长细比的依赖性,即随着电极半径尺寸的增大而减小,而随着电极长细比的增大而增大.此外,模型还显示,在有表面效应的条件下,相对于弹性硬化属性的纳米线电极,具有弹性软化属性的电极因为具有更好的抗失稳性而更适宜作为电极材料.研究结果为纳米线电极的力学可靠性设计提供了一定的帮助.     

A quadrangular transverse Ising nanowire with an antiferromagnetic spin configuration

The phase diagrams and the temperature dependences of magnetizations in a transverse Ising nanowire with an antiferromagnetic spin configuration are investigated by the use of the effective-field theory with correlations (EFT) and the core–shell concept. Many characteristic and unexpected behaviors are found for them, especially for thermal variation of total magnetization m_T. The reentrant phenomenon induced by a transverse field in the core, the appearance of a compensation point, the non-monotonic variation with a compensation point, the reentrant phenomena with a compensation point and the existence of both a broad maximum and a compensation point have been found in the thermal variations of m_T.     

Influence of aspect ratio and anisotropy distribution in ordered CoNi nanowire arrays

The size effects on magnetic properties of nanowires arrays were studied varying the nanowires diameter and maintaining the same periodicity among them, for two different nominal compositions of Co and Ni in the alloy form. The competition among magnetocrystalline and shape anisotropies changes drastically from smallest to biggest diameters altering the easy axis direction. In the case of 75% of Co in alloy, experimental values of the effective anisotropy constant (K_eff) vary from positive to negative depending on the diameter, which means a reversal of the easy axis direction. For 50% of Co the shape anisotropy dominates over the magnetocrystalline for all studied diameters.     

The effect of substrate on magnetic properties of Co/Cu multilayer nanowire arrays

Ordered Co/Cu multilayer nanowire arrays have been fabricated into anodic aluminium oxide templates with Ag and Cu substrate by direct current electrodeposition. This paper studies the morphology, structure and magnetic properties by transmission electron microscopy, selective area electron diffraction, x-ray diffraction, and vibrating sample magnetometer. X-ray diffraction patterns reveal that both as-deposited nanowire arrays films exhibit face-centred cubic structure. Magnetic measurements indicate that the easy magnetization direction of Co/Cu multilayer nanowire arrays films on Ag substrate is perpendicular to the long axis of nanowire, whereas the easy magnetization direction of the sample with Cu substrate is parallel to the long axis of nanowire. The change of easy magnetization direction attributed to different substrates, and the magnetic properties of the nanowire arrays are discussed.     

The effects of surface dilution on magnetic properties in a transverse Ising nanowire

The phase diagrams (transition temperature and compensation temperature) and magnetizations of a cylindrical nanowire with a (positive or negative) core–shell interaction and a surface dilution, described by the transverse Ising model (TIM), are investigated by the use of the effective-field theory with correlations. The magnetic properties of the system are strongly affected by the surface dilution. In particular, the temperature dependences of magnetizations in the system have shown many characteristic phenomena, depending on the selected physical parameters.     

Ferrimagnetism in a decorated Ising nanowire

The magnetic properties (phase diagram and total magnetization) of a decorated Ising nanowire are examined by the use of the effective field theory with correlations. The system, consisting of spin-1/2 atoms and decorating spin-S  (S>1/2)$(S>1/2)$ atoms with a crystal-field interaction on the bonds, show ferrimagnetism, compensation point and reentrant behavior.     

Dynamic behaviors of the hexagonal Ising nanowire

By utilizing the effective-field theory based on the Glauber-type stochastic dynamics, the dynamic behaviors of the hexagonal Ising nanowire (HIN) system in the presence of a time dependent magnetic field are obtained. The time variations of average order parameters and the thermal behavior of the dynamic order parameters are studied to analyze the nature of transitions and to obtain the dynamic phase transition points. The dynamic phase diagrams are introduced in the plane of the reduced temperature versus magnetic field amplitude. The dynamic phase diagrams exhibit coexistence phase region, several ordered phases and critical point as well as a reentrant behavior.     

Shape and thickness effects on the magnetization reversal of Py/Cu/Co nanostructures

We present an experimental investigation of the magnetization reversal process in NiFe/Cu(10 nm)/Co circular and elliptical nano-elements with different thickness of the magnetic layers. The results obtained using element sensitive X-ray resonant magnetic scattering (XRMS) were compared with the previous measurements showing that the dipolar interlayer coupling favours the antiparallel alignment of the two magnetization layers at remanance. In the case of circular shape, the increased thickness of the ferromagnetic layers stabilizes the antiparallel alignment of the layers over a wider field range. A similar effect, accompanied by a delay in the onset of the antiparallel alignment, is observed in the case of elliptical nano-elements and applying the external field along the longer axis of the elements, due to the additional shape anisotropy.     

Magnetic properties in a transverse Ising nanowire with an antiferromagnetic spin configuration at the surface

The phase diagrams and temperature dependences of magnetizations in a transverse Ising nanowire with an antiferromagnetic spin configuration at the surface are studied by the use of the effective-field theory with correlations. A number of characteristic features, such as the possibility of reentrant phenomena being free from disorder-induced frustration, are obtained for the magnetic properties in the system.     

Phase diagrams of a transverse Ising nanowire

Phase diagrams of a cylindrical nanowire described by the transverse Ising model are investigated by the use of two theoretical frameworks, namely the mean field theory and the effective field theory with correlations. The phase diagram of the system is strongly affected by the surface situations. Some characteristic phenomena are found in the phase diagram, depending on the ratio of the physical parameters in the surface shell and the core.     

Ferrimagnetic behaviors in a double-wall cubic metal nanotube

A double-wall cubic metal nanotube consists of the ferromagnetic spin-1 inner shell and spin-3/2 surface shell. It is of the ferrimagnetic exchange coupling between two shells. Considering the single-ion anisotropy and transverse field exist together, the magnetization, the initial susceptibility, the internal energy and the specific heat have been investigated by using the effective-field theory with correlations. Some interesting phenomena have been found in the thermal variations of the system. Magnetization appears two or three compensation points in certain parameters. It is an unconventional ferrimagnetic behavior in the nanotube. The shapes of total magnetization and the initial susceptibility are great influenced by the surface exchange coupling, surface single-ion anisotropy and surface transverse field. Some results of nanotube may have potential applications in different research fields, such as electronics, optics, mechanics, and even biomedicine and molecular devices.     

Angular dependence of the coercivity and remanence of ordered arrays of Co nanowires

The angular dependence of the coercivity and remanence of ordered hexagonal arrays of Co nanowires prepared using anodic aluminum oxide templates was investigated. The experimental evolution of coercivity as a function of the angle, in which the external field is applied, is interpreted considering micromagnetic simulations. Depending on the angle between the axis of the wire and the applied magnetic field direction our results show that the magnetization reversal mode changes from vortex to a transverse domain wall. Besides, we observed that the dipolar interactions cause a reduction in coercive fields, mainly in the direction of easy magnetization of the nanowires. Good agreement between numerical and experimental data is obtained.     

Off-stoichiometry effects on BiFeO3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO₃ (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃ that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.     

Synthesis and ignition of energetic CuO/Al core/shell nanowires

Energetic thermites (mixtures of Al and metal oxides), due to their high energy densities, have broad applications in propulsion, thermal batteries, waste disposal, and power generation for micro systems. Reducing the sizes of Al and metal oxides down to the nanoscale has been shown to be effective in increasing their reaction rates and reducing their ignition delays. However, it remains a challenge to create mixtures of Al and metal oxides with nanoscale uniformity. Here we report synthesis and ignition studies on thermites with a new nanostructure, i.e., CuO/Al core/shell nanowires (NWs). The CuO NW cores were synthesized by the thermal annealing of copper films and served as templates for the deposition of Al shells by subsequent sputtering. The advantage of such a core/shell NW structure is that CuO and Al are uniformly mixed at the nanoscale. The onset temperatures of the exothermic reaction of the core/shell NWs were similar to those of nanoparticle (NP)-based thermites in terms of magnitude, insensitivity to equivalence ratios and sensitivity to heating rates. Moreover, the core/shell NW thermites, compared to NP-based thermites, exhibit greatly improved mixing uniformity and reduced activation energy for the thermite reaction.     

Facile synthesis and characterization of glass/cobalt core/shell composite spheres with tunable shell morphologies

Polyhedral cobalt microcrystals assembled on hollow glass spheres are successfully synthesized by a facile and easy-control hydrothermal reduction process, and thus hierarchical glass/cobalt core/shell composite hollow spheres are fabricated with low-density (0.96 g cm⁻³). By properly tuning the process conditions and the component of the reaction solution, a series of composite spheres with gradient in morphologies of the shell layer can be prepared. Based on a series of contrast experiments, the probable formation mechanism of the core/shell hierarchical structures is proposed. The magnetic properties of the products are studied and the results demonstrate that the composite spheres present ferromagnetic properties related to the special shell morphologies. The composite hollow spheres thus obtained may have some promising applications in the fields of low-density magnetic materials, conduction, and catalysis, etc. This work provides an additional strategy to prepared core/shell composite spheres with tailored shell morphology and magnetic properties.     

Reversible and irreversible magnetization components behavior in Ni nanowires

Polycrystalline Ni nanowires were electrodeposited in nanoporous anodized alumina membranes with mean diameter of approximately 42 nm. Their magnetic properties were studied at 300 K, by measurements of recoil curves from demagnetized state and also from saturated state. M_rev and M_irr components were obtained and M_rev(M_irr)_H curves were constructed from the experimental data. These curves showed a behavior that suggests a non-uniform reversal mode influenced by the presence of dipolar interactions in the system. A qualitative approach to this behavior is obtained using a Stoner–Wohlfarth model modified by a mean field term and local interaction fields.