Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si（111） substrate by a novel method： varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a Co FeB/MgO/CoFeB magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect（TRMOKE） for both the parallel state（P state） and the antiparallel state（AP state） of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.

First-principles study of electronic and magnetic properties of Fe atoms on Cu_(2)N/Cu(100)

First-principles calculations were conducted to investigate the structural,electronic,and magnetic properties of single Fe atoms and Fe dimers on Cu_(2)N/Cu(100).Upon adsorption of an Fe atom onto Cu_(2)N/Cu(100),robust Fe-N bonds form,resulting in the incorporation of both single Fe atoms and Fe dimers within the surface Cu_(2)N layer.The partial occupancy of Fe-3d orbitals lead to large spin moments on the Fe atoms.Interestingly,both single Fe atoms and Fe dimers exhibit in-plane magnetic anisotropy,with the magnetic anisotropy energy(MAE)of an Fe dimer exceeding twice that of a single Fe atom.This magnetic anisotropy can be attributed to the predominant contribution of the component along the x direction of the spin-orbital coupling Hamiltonian.Additionally,the formation of Fe-Cu dimers may further boost the magnetic anisotropy,as the energy levels of the Fe-3d orbitals are remarkably influenced by the presence of Cu atoms.Our study manifests the significance of uncovering the origin of magnetic anisotropy in engineering the magnetic properties of magnetic nanostructures.

Engineering magnetic nanostructures with inverse hysteresis loops

Top-down lithography techniques allow the fabrication of nanostructured elements with novel spin configurations, which provide a new route to engineer and manipulate the magnetic response of sensors and electronic devices and understand the role of fundamental interactions in materials science. In this study, shallow nanostructure-patterned thin films were designed to present inverse magnetization curves, i.e., an anomalous magnetic mechanism characterized by a negative coercivity and negative remanence. This procedure involved a method for manipulating the spin configuration that yielded a negative coercivity after the patterning of a single material layer. Patterned NiFe thin films with trench depths between 15%-25% of the total film thickness exhibited inverse hysteresis loops for a wide angular range of the applied field and the trench axis. A model based on two exchange-coupled subsystems accounts for the experimental results and thus predicts the conditions for the appearance of this magnetic behavior. The findings of the study not only advance our understanding of patterning effects and confined magnetic systems but also enable the local design and control of the magnetic response of thin materials with potential use in sensor engineering.

Real-space observation of individual skyrmions in helimagnetic nanostripes

Controllable formation and manipulation of domain walls in one-dimensional（1D） nanostripes underpins a promising type of emergent spintronic device. Magnetic skyrmion is topologically stable whirlpool-like spin texture and is expected to replace familiar domain wall phenomena to build such devices, owing to its prominent features including small size,topological stability and the small critical current required to move it. It is thus essential to understand skyrmions＇ properties in such a nanostructured element. In this paper, we mainly give fundamental insight into this issue. Experimental achievements in the formation and stability of individual skyrmions in the nanostripe are outlined in detail.

Wave-Vector and Temperature-Dependent Electron Transport in a Magnetic Nanostructure Modulated by Bias

We theoretically investigate the wave-vector and temperature-dependent electron transport in a magneticnanostructure modulated by an applied bias.The large spin-polarization can be achieved in such a device,and the degreeof spin-polarization strongly depends on the transverse wave-vector and the temperature.These interesting propertiesmay be helpful to spin-polarize electrons into semiconductors,and this device may be used as a spin filter.

Remanence Characteristic of Nanostructure of Hard／Soft Magnetic Multilayered Systems

The relation between microscopic properties (e.g., layer thickness, easy axis orientation) and the macroscopic magnetic properties such as remanent magnetization of the ferromagnetic multilayer system is investigated based on a simple micromagnet approach. We concentrate on a multilayer design with periodic boundary condition, where alternating soft/hard layers build a nanostructured multilayer. For any easy axis direction in the soft and hard layers a simple explicit expression of remanence of the system has been derived analytically. We find that the remanence clearly depends on the thickness of the soft magnetic layer and is nearly independent of the thickness of hard magnetic layer. On the other hand, the remanence increases upon reducing the angle enclosed by the saturation magnetization and the easy axis directions of soft magnetic layer. However, it is unsensitive to the easy axis direction of hard magnetic layer, but there exists a maximum remanence for a certain easy axis direction of hard magnetic layer.

The electron transport characters in a nanostructure with the periodic magnetic-electric barriers

This paper detailedly studies the transmission probability, the spin polarization and the conductance of the ballistic electron in a nanostrueture with the periodic magnetic-electric barriers These observable quantities are found to be strongly dependent not only on the magnetic configuration, the incident electron energy and the incident wave vector, but also on the number of the periodic magnetic-electric barriers The transmission coefficient and the spin polarization show a periodic pattern with the increase of the separation between two adjacent magnetic fields, and the resonance splitting increases as the number of periods increases. Surprisingly, it is found that a polarization can be achieved by spin-dependent resonant tunnelling in this structure, although the average magnetic field of the structure is zero.

Electronic Resonant Splitting in a Nanostructure with Periodic Magnetic-Electric Barriers

In this letter, the transmission probability and the conductance of the ballistic electron are studied in a nanostructure with the periodic magnetic-electric barriers. We find that the resonant splitting increases with the number of periods increasing, so the number of the resonant peaks increases and the peaks become sharper. For the m-th periodic magnetic-electric barriers tunneling the splitting is （m - 1）-fold.

Relaxation of Magnetic Nanoparticle Chain without Applied Field

The relaxation of a one-dimensional magnetic nanoparticle linear chain with lattice constant a is investigated in absence of applied field. There is an equilibrium state （or steady state） where a11 magnetic moments of particles lie along the chain （x-axis）, back to which the magnetic nanoparticle chain at other state will relax. It is found that the relaxation time T= is determined by Tx = 10β×α3. This relaxation is compared with that of single magnetic nanoparticle system.

Layer resolved magnetization reversal study in SmCo_5 /Fe nanocomposite bilayers

A hard/soft SmCo5/ Fe nanocomposite magnetic bilayer system is fabricated on x-ray transparent 100-200 nm thin SiaN4 films by magnetron sputtering. The microscopic magnetic domain pattern and its behaviours during magnetization reversal in the hard and the soft magnetic phases are studied separately by element specific magnetic soft x-ray microscopy at a spatial resolution of better than 25 Nm. We observe that the domain patterns for the soft and hard phases show coherent behaviours in varying magnetic fields. We derive local M（H） curves from the images of Fe and SmCo5 separately and find the switches for hard and soft phases to be the same.

Magnetic Fe_(2)P Nanowires and Fe_(2)P@C Core@Shell Nanocables

We report the synthesis of one-dimensional(1-D)magnetic Fe_(2)P nanowires and Fe_(2)P@C core@shell nanocables by the reactions of triphenylphosphine(PPh_(3))with Fe powder(particles)and ferrocene(Fe(C_(5)H_(5))_(2)),respectively,in vacuum-sealed ampoules at 380-400℃.The synthesis is based on chemical conversion of micrometer or nanometer sized Fe particles into Fe_(2)P via the extraction of phosphorus from liquid PPh_(3) at elevated temperatures.In order to control product diameters,a convenient sudden-temperature-rise strategy is employed,by means of which diameter-uniform Fe_(2)P@C nanocables are prepared from the molecular precursor Fe(C_(5)H_(5))2.In contrast,this strategy gives no obvious control over the diameters of the Fe_(2)P nanowires obtained using elemental Fe as iron precursor.The formation of 1-D Fe_(2)P nanostructures is ascribed to the cooperative effects of the kinetically induced anisotropic growth and the intrinsically anisotropic nature of hexagonal Fe_(2)P crystals.The resulting Fe_(2)P nanowires and Fe_(2)P@C nanocables display interesting ferromagnetic-paramagnetic transition behaviors with blocking temperatures of 230 and 268 K,respectively,significantly higher than the ferromagnetic transition temperature of bulk Fe_(2)P(TC=217 K).

Integrin-targeting with peptide-bioconjugated semiconductor-magnetic nanocrystalline heterostructures

Binary asymmetric nanocrystals （BNCs）, composed of a photoactive TiO2 nanorod joined with a superparamagnetic γ-Fe203 spherical domain, were embedded in polyethylene glycol modified phospholipid micelle and successfully bioconjugated to a suitably designed peptide containing an RGD motif. BNCs represent a relevant multifunctional nanomaterial, owing to the coexistence of two distinct domains in one particle, characterized by high photoactivity and magnetic properties, that is particularly suited for use as a phototherapy and hyperthermia agent as well as a magnetic probe in biological imaging. We selected the RGD motif in order to target integrin expressed on activated endothelial cells and several types of cancer cells. The prepared RGD-peptide/BNC conjugates, comprehensively monitored by using complementary optical and structural techniques, demon- strated a high stability and uniform dispersibility in biological media. The cytotoxicity of the RGD-peptide/BNC conjugates was studied in vitro. The cellular uptake of RGD-peptide conjugates in the cells, assessed by means of two distinct approaches, namely confocal microscopy analysis and emission spectroscopy determination in cell lysates, displayed selectivity of the RGD-peptide-BNC conjugate for the cw]33 integrin. These RGD-peptide-BNC conjugates have a high potential for theranostic treatment of cancer.

Enhancement of the Curie temperature of ferromagnetic semiconductor(Ga,Mn)As

In this review article,we review the progress made in the past several years mainly regarding the efforts devoted to increasing the Curie temperature(T C) of(Ga,Mn)As,which is most widely considered as the prototype ferromagnetic semiconductor.Heavy Mn doping,nanostructure engineering and post-growth annealing which increase T C are described in detail.

Giant magnetoresistance in a two-dimensional electron gas modulated by ferromagnetic and Schottky metal stripes

In this paper, by using the transfer matrix method, we theoretically investigate the magnetoresistance （MR） effect in a two-dimensional electron gas （2DEG） modulated by two Schottky metal （SM） stripes and two ferromagnetic （FM） stripes on the top and bottom of the 2DEG. From the numerical results, we find that a considerable MR effect can be achieved in this device due to the significant difference between electron transmissions through the parallel and antiparallel magnetization configurations. We also find that the MR ratio obviously depends on the magnetic strength and the electric-barrier height as well as the distance between the FM and SM stripes. These characters are very helpful for making the new type of MR devices according to their practical applications.

Recent developments of rare-earth-free hard-magnetic materials

Recent advances in rare-earth-free hard-magnetic materials including magnetic bulk, thin films, nanocomposites and nanostructures are introduced. Since the costs of the rare-earth metals boosts up the price of the high-performance rare-earth permanent magnets, there is a much revived interest in various types of hard-magnetic materials based on rare-earth-free compounds. The 3d transition metals and their alloys with large coercivity and high Curie temperatures(working temperatures) are expected to overcome the disadvantages of rare-earth magnets. Making rare-earth-free magnets with a large energy product to meet tomorrow's energy needs is still a challenge.

Air stable Fe nanostructures with high magnetization prepared by reductive annealing

Monodispersed Fe nanospindles and nanoparticles were successfully synthesized through environmentfriendly reductive annealing ？-Fe OOH nanorods. Effects of annealing temperature and reaction atmosphere on microstructure, phase, and magnetic property of Fe nanostructures were investigated.The as-obtained pure Fe nanoparticles with mean size of 45 nm had a high saturation magnetization up to 207 emu/g, close to that of bulk material（218 emu/g）, which exhibited high air stability. After exposing in air for 2 and 7 days, the as synthesized Fe nanoparticles still showed high magnetization of 182 and141 emu/g, respectively.

Fe–EBT Chelate Complex: A Novel Mean for Growth of α-FeOOH and γ-Fe_2O_3 Nanostructures

Acicular goethite（a-Fe OOH） and worm-like maghamite（γ-Fe2O3） nanostructures have been prepared adopting a novel route, using Na2[Fe（HL）2（H2O）2] chelate complex in alkaline medium. It is found that concentration of hydrated Fe（III） ions increased with increasing temperature, which later play a key role in generation of different phases of iron oxide. Phase and morphology of the products are investigated using XRD, FTIR, SEM, and TEM analysis. Using UV–Vis spectra, various electronic transitions of goethite and maghamite particles are examined. Maghamite nanostructures exhibit superparamagnetic property at room temperature. On the basis of experimental observations and analytical data, growth mechanism of the nanostructures is discussed.

Hydrothermal synthesis and characterization of NiS flower-like architectures

Under the influence of thiocyanate anions （SCN-） and cetyltrimethyl ammonium bromide （CTAB）, NiS flower-like architectures were successfully synthesized by a one-step hydrothermal method. The syn-thesized flower-like architectures, with a multilayered and highly ordered texture, have diameters of several micrometers. X-ray powder diffraction （XRD） shows that the NiS flower-like architectures are rhombohedral crystalline. On the basis of condition-dependent experiments, the diffusion-limited aggregation （DLA） model and cage effect were used to explain the growth process of rhombohedral crystalline NiS flower-like architectures. Magnetic measurements showed that the coercivity （He） of the as-obtained NiS flower-like architectures was 102.14 Oe.