Abnormal reduction of coercivity on magnetic cobalt–platinum alloy films

Ultrathin Co–Pt alloy films as substrate were studied by the surface magneto-optical Kerr effect. As the growth of Ni, the films show uniquely high polar Kerr responses without any in-plane signals. The coercivity decreased until the thickness of Ni film was higher than 5 ML. A new surface structure was discovered at 7–10 ML Ni/Co–Pt films by the low-energy electron diffraction. Interestingly, polar Kerr signal and coercivity of the 10 ML Ni/Co–Pt(1 1 1) template film reduced rapidly as Co films were further deposited onto only about 1–2 ML. Then the films show a canted magnetization with a rollback hysteresis in the polar configuration during the growth of Co. Coercivity of the 7 ML Co/Ni/Co–Pt film was found unusually down to almost 100 Oe.The corresponding magic number at around 7 ML of Co in the abnormal reduction of coercivity may be attributed to the cluster formations of Co.     

An effective method for improving diffraction performance of magnetostatic backward volume wave-based magneto-optic Bragg cells by using an appropriately tilted bias magnetic field in YIG film plane

The universal magneto-optic (MO) coupled-mode equations for magnetostatic waves (MSWs) and guided optical waves (GOWs) under arbitrarily tilted bias magnetic fields are presented for the first time and, as an example, applied to the noncollinear Stokes interaction between the incident TE₀-mode light and magnetostatic backward volume wave (MSBVW) excited by single-element microstrip line transducer in yttrium–iron–garnet (YIG) film. Our calculation indicates that, for the case of magnetization parallel to the MSBVW propagation direction, the diffraction efficiency (DE) is equal to the mode-conversion efficiency of the diffracted lights (MCDE) and the calculated curve of relative DE for the MSBVW-based MO Bragg cell in pure YIG waveguide is in good agreement with the experimental data. In contrast, the diffraction performance can be greatly improved by optimizing the bias magnetic field and the DE gain can be increased by 6.3 dB in the tangentially magnetized film. The angular dependences of the DE and the corresponding Bragg angle upon the magnetization direction are also discussed in the paper.     

High-gain composite microstrip patch antenna with the near-zero-refractive-index metamaterial

A high-gain rectangular microstrip patch antenna which is covered by a single layer metamaterial (MTM) superstrate with the near zero refractive index is proposed. The refraction of the metamaterial at frequency 3.51 GHz–3.57 GHz is very close to zero. The metamaterial with the near zero refractive index is placed 42 mm above an ordinary rectangular microstrip patch antenna. The effectively zero refractive index behavior of metamaterial superstrate can gather the wave emitted from the microstrip patch antenna and collimate it toward the normal direction of the antenna. The finite element method (FEM) and the finite difference time domain (FDTD) method are used to study the characteristics of this antenna. The results of the two methods indicate that the realized gain of the proposed antenna is increased by more than 6 dB, and the antenna has a flatness high gain in the predicted frequency band, where the proposed MTM is designed to have a near zero index of refraction. Therefore, the high-gain antenna is effectively enhanced based on the near-zero-refractive-index metamaterial.     

Magnetic,electrical transport and electron spin resonance studies of ferromagnetic insulating manganites Nd0.85Na0.15MnO3

We have investigated the magnetic, electrical transport and electron spin resonance (ESR) properties of polycrystalline Nd_0.85Na_0.15MnO₃ prepared by sol–gel method. A ferromagnetic–paramagnetic (FM–PM) transition is observed around 110 K, which is not accompanied by a metal–insulator transition. The sample displays the complete PM state associated with the ESR spectra fitted by single Lorentzian line shape above 130 K. Below 130 K, ESR spectra become distorted and then linewidth increases rapidly, where short-range magnetic order develops and coexists with PM phase due to the inhomogeneous magnetic state. In addition, the large difference between the activation energies obtained from the resistivity and ESR parameters (peak-to-peak linewidth and line intensity) at the frame of adiabatic small polaron hopping model is pointed out for Nd_0.85Na_0.15MnO₃.     

Preparation,surface modification and microwave characterization of magnetic iron fibers

In this paper, magnetic iron fibers of 3–10 μm diameter and an adjustable aspect ratio were synthesized successfully by a method involving pyrolysis of carbonyl under a magnetic field. A surface modification technology was also investigated. The electromagnetic parameters of the iron-fiber–wax composites were measured using the transmission/reflection coaxial line method in the microwave frequency range of 2–18 GHz. The results show that the prepared iron-fiber–wax composites exhibit high magnetic loss that can be further improved after phosphating. On the other hand, the complex permittivity was significantly decreased after phosphating. As a result, this kind of iron fiber may be useful for thin and lightweight radar-absorbing materials.     

Development of a new generation of high permeability non-oriented silicon steels

The paper describes a new generation of the high permeability fully processed silicon steel grades developed by Acesita, with core loss (W_1.5/60) in range of 3.10–4.20 W/kg and polarisation (J₅₀) from 1.71 to 1.75 T, respectively. The new grades have lower (Si+Al) content and a better crystallographic texture, with lower fractions of [1 1 1]||ND fibre and higher fractions of [0 0 1]||RD fibre. The new grades have better mechanical and magnetic properties than conventional grades.     

Characteristic and non-characteristic X-ray yields produced from thick Ti element by sub-relativistic electrons

Measurements are performed to study the electron impact energy dependence of doubly differential bremsstrahlung yields (DDBY) and of characteristic Ti K_α line yields produced from sub-relativistic electrons (10–25 keV) colliding with a thick Ti (Z = 22) target. The emitted radiation is detected by a Si-PIN photo-diode detector with energy resolution (FWHM) of 180 eV at 5.9 keV. The measured data of DDBY are compared with the results predicted by Monte-Carlo (MC) simulations using the general purpose PENELOPE code. A reasonable agreement is found between experimental and simulation results within the experimental uncertainty of measurements of 12%. Characteristic Ti K_α yields are obtained for the considered impact energy range and they are compared with the existing theoretical results. A good agreement is found between the present measurements and the theoretical calculations. Furthermore, data are presented for impact energy dependence of the ratio K_α/(K_α+ K_β) of a thick Ti target under impact of 10–25 keV electrons. The ratio shows a very weak dependence on impact energy in the studied range. The average value of the ratio is found to be 0.881 ± 0.003.     

Hierarchically porous nanoflowers from TiO2–B nanosheets with ultrahigh surface area for advanced lithium-ion batteries

In this work, hierarchically porous TiO₂–B nanoflowers have been successfully synthesized via a facile solvothermal method followed by calcination treatment. The TiO₂–B nanoflowers are constructed by thin nanosheets, presenting ultrahigh specific surface area, up to 214.6 m² g⁻¹. As anode materials for Li-ion batteries, the TiO₂–B sample shows high reversible capacity, excellent cycling performance and superior rate capability. The specific capacity of TiO₂–B could remain over 285 mA h g⁻¹ at 1 C and 181 mA h g⁻¹ at 10 C rate after 100 cycles. We believe that the pseudocapacitive mechanism, ultrahigh surface area and scrupulous nanoarchitecture of the TiO₂–B are responsible for the enhancement of electrochemical properties.     

Magnetic properties of spray-formed Fe–6.5%Si and Fe–6.5%Si–1.0%Al after rolling and heat treatment

The maximum silicon content in commercial Fe–Si steels is limited to about 3.5 wt%Si, since the ductility declines sharply as this maximum is exceeded, hindering the production of thin sheets by cold/hot rolling. However, the best magnetic properties are attained at about 6.5 wt%Si, a silicon content that renders magnetostriction practically null and minimizes magnetic losses. Using spray-forming, our research group has successfully produced this type of high silicon alloy in thin sheet form by carefully controlling the many variables of the process and subsequent rolling operations. In the present study, we investigated the magnetic properties and the microstructure of spray-formed Fe–6.5 wt%Si and Fe–6.5 wt%Si–1.0 wt%Al alloys after warm rolling and heat treatment. The main cause for the brittleness of Fe–6.5 wt%Si alloy has been attributed to the B2 phase long-range ordering, which leads to premature fractures. The presence of aluminum could avoid B2 formation and improve the alloy's ductility. The binary Fe–6.5 wt% Si alloy showed the best magnetic properties, which were ascribed to a recrystallized, coarse grain size (∼500 μm; and 340 μm for the Al-containing alloy). TEM analysis showed that a well-developed B2 domain structure (about 50–300 nm in size) was formed in the binary alloy when low cooling rates are prevailing after heat treatment. This structure contributed to improve additionally the magnetic properties, but its effect was not so strong as that of the grain size. The addition of Al to the binary alloy suppressed B2 formation, as indicated by Mossbauer spectroscopy, and apparently hindered excessive grain growth, which may explain the slightly poorer magnetic properties when compared with the binary alloy.     

Magnetic relaxation in pulse-magnetized high-temperature superconductors

A magnetic filed relaxation at the center of a pulse-magnetized single-domain Y–Ba–Cu–O superconductor at 78 K has been studied. In case of a weak magnetization, the magnetic flux density increases logarithmically and normalized relaxation rate defined as S = −d(lnB)/d(lnt) is negative (S = −0.037). When an external magnetic field magnitude increases, the relaxation rate first decreases in absolute value, then changes sign (becomes positive, S > 0) and after reaching some maximum finally reduces to a very small value. Non-monotonous dependence of S vs. H_a is explained by a non-homogeneous local temperature distribution during a pulse magnetization.     

Microfabrication and magnetoelectric properties of amorphous magnetic-tunnel-junctions with Co–Fe–B/Al–O/Co–Fe–B hardcore structure

Both single-barrier magnetic tunnel junctions (SBMTJs) and double-barrier magnetic tunnel junctions (DBMTJs) with an amorphous hardcore structure of Co₆₀Fe₂₀B₂₀/Al–O/Co₆₀Fe₂₀B₂₀ were microfabricated. A high TMR ratio of 102.2% at 4.2 K was observed in the SBMTJs after annealing at 265 °C for 1 h. High TMR ratio of 56.2%, low junction resistance-area product RS of 4.6 kΩ μm², small coercivity H_C=25 Oe, and relatively large bias-voltage-at-half-maximum TMR with the value V_1/2 greater than 500 mV at room temperature (RT) had been achieved in such Co–Fe–B SBMTJs. Whereas, high TMR ratio of 60% at RT and 89% at 30 K, low junction resistance-area product RS of 7.8 kΩ μm² at RT and 8.3 kΩ μm² at 30 K, low coercivity H_C=8.5 Oe at RT and H_C=14 Oe at 30 K, and relatively large bias-voltage-at-half-maximum TMR with the value V_1/2 greater than 1150 mV at RT had been achieved in the Co–Fe–B DBMTJs. Temperature dependence of the TMR ratio, resistance, and coercivity from 4.2 K to RT, and applied voltage dependence of the TMR ratio and resistance at RT for such amorphous MTJs were also investigated.     

Mutual phase locking in high-frequency microwave nano-oscillators as a function of field angle

We perform a qualitative analysis of phase locking in a double point-contact spin–valve system by solving the Landau–Lifshitz–Gilbert–Slonzewski equation using a hybrid-finite-element method. We show that the phase-locking behaviour depends on the applied field angle. Starting from a low field angle, the locking-current difference between the current through contact A and B increases with increasing angle up to a maximum of 14 mA at 30°, and it decreases thereafter until it reaches a minimum of 1 mA at 75°. The tunability of the phase-lock frequency with current decreases linearly with increasing out-of-plane angle from 45 to 21 MHz/mA.     

Magnetic properties of anisotropic Sm–Fe–N bulk magnets produced by spark plasma sintering method

Sm–Fe–N powders were consolidated into bulk materials by the spark plasma sintering (SPS) method. Although partial decomposition of the Sm₂Fe₁₇N₃ phase was noted in the magnets, the decomposition was reduced by the addition of a small amount of Zn powder to the Sm–Fe–N powder. The anisotropic Sm–Fe–N magnet obtained from a mixture of Sm–Fe–N and Zn powders exhibited a high remanence of 0.90 T with a coercivity of 0.54 MA m⁻¹.     

Magnetic properties of HITPERM-type alloys at high temperature

(Fe,Co)–Zr,Hf)–Cu–B (HITPERM-type) alloys with variable Hf, Zr and Co content were isothermally crystallised at 500–650 °C for 1 h, and the optimum nanocrystallisation temperature was selected on the basis of the minimum coercive field at room temperature. The quasistatic hysteresis loops were measured at temperature from 20 to 650 °C. Subsequently, the optimally annealed alloys were subjected to long-term annealing at 500, 550 and 600 °C. Working temperature of 600°C is too high for the investigated alloys to maintain stable magnetic properties. Temperature of 550 or 500 °C permits the material to be magnetically stable for a long period. The magnetic hysteresis loops recorded for the nanocrystalline alloys, where Fe:Co ratio is close to 1 and refractory metals content is 7 at.%, prove that coercive field increases slightly with temperature, but remains in the range of 20–40 A/m (depending on the alloy composition) from 20 to 550 °C. This proves that the investigated alloys, after optimisation of chemical composition, may be suitable for high temperature use.     

Structural,magnetic and electrical properties of Co1−xZnxFe2O4 synthesized by co-precipitation method

Nanoparticles of Co_1−xZn_xFe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.     

Exchange coupling and remanence enhancement in nanocomposite Nd–Fe–B/FeCo multilayer films

Nd–Fe–B-type hard phase single layer films and nanocomposite Nd₂₈Fe₆₆B₆/Fe₅₀Co₅₀ multilayer films with Mo underlayers and overlayers have been fabricated on Si substrates by rf sputtering. The hysteresis loops of all films indicated simple single loops for fixed Nd–Fe–B layer thickness (10 nm) and different FeCo layer thickness (d_FeCo=1–50 nm). The remanence of these films is found to increase with increasing d_FeCo and the coercivity decrease with increasing d_FeCo. It is shown that high remanence is achieved in the nanocomposite multilayer films consisting of the hard magnetic Nd–Fe–B-type phase and soft magnetic phase FeCo with 20 nm?d_FeCo?3 nm. The sample of maximum energy product is 27 MG Oe for d_FeCo=5 nm at room temperature. The enhancement of the remanence and energy products in nanocomposite multilayer films is attributed to the exchange coupling between the magnetically soft and hard phases.     

Superpositions of excited–deexcited coherent states and some of their non-classical properties

Applying the operator a + a⁺ to the superposed coherent states several times, superpositions of the excited–deexcited coherent states are obtained. Compared with the original superposed coherent states, these new states can have stronger squeezing and anti-bunching effects. The operation a + a⁺ can also induce squeezing or antibunching effect if the original states do not possess these properties. Calculations about the phase properties, the Q function and the Wigner function reflect the non-classical character of the excited–deexcited states from different aspects.     

The transport and magnetic properties of La–Pb–Mg–Mn–O manganites at low temperatures

The structure, transport properties and the magnetoresistance behavior in the temperature interval 77–400 K of the perovskite-like lanthanum manganites La_0.6Pb_0.4−xMg_x+yMnO₃ (x=0, 0.1, 0.2 and y=0, 0.2) were investigated. Polycrystalline bulk samples were prepared by sol–gel self-combustion and subsequent heat treatment at 1000 °C for different times, 40, 80, 160 and 320 min. All manganites exhibit a peak in the resistivity around 200–250 K, below the ferromagnetic ordering temperature (320–330 K). An isotropic and negative magnetoresistance has been observed in all compounds. Magnetoresistance MR exhibits a peak in the temperature range 130–150 K, below SC–metal transition temperature. Magnitude of MR at the peaks was nearly 27% in the magnetic field of 2 T. At room temperature, a magnetoresistance of 9.5% for La_0.6Pb_0.2Mg_0.2MnO₃ composition was obtained. Longer heat treatment time enhanced the magnetorezistive properties.     

Electrical and mechanical properties of graphene oxide on flexible substrate

Graphene oxide (GO) was deposited via the electrophoretic deposition (EPD) method to lower the oxygen concentration of graphene sheets for large-scale production. In addition, the direct synthesis of large-scale GO films using transfer processes on a polydimethylsiloxane (PDMS) substrate was conducted. The thickness of the GO films was controlled to adjust the optical, electrical, and mechanical properties. The Young's modulus values of films with thicknesses of 100–200 nm were 324–529 GPa. Moreover, the GO films exhibited excellent conductivity, with a sheet resistance of 276–2024 Ω/sq at 23–77% transparency. Experiments show that transfer processes for flexible substrates can produce high-quality cost-effective transparent conductive films.     

Cathodic and anodic pre-treated boron doped diamond with different sp content: Morphological,structural, and impedance spectroscopy characterizations

In this work, the influence of cathodic (Red) and anodic (Ox) pre-treatment on boron doped diamond (BDD) films grown with different sp²/sp³ ratios was systematically studied. The sp²/sp³ ratios were controlled by the addition of CH₄ of 1,3,5 and 7 sccm in the gas inlet during the growth process. The electrodes were treated in 0.5 mol L⁻¹ H₂SO₄ at −3 and 3 V vs Ag/AgCl, respectively, for 30 min. The electrochemical response of BDD films was investigated using electrochemical impedance spectroscopy (EIS) and Mott–Schottky Plot (MSP) measurements. Four film sample sets were produced in a hot filament chemical vapor deposition reactor. During the growth process, an additional H₂ line passing through a bubbler containing the B₂O₃ dissolved in methanol was used to carry the boron. The scanning electron microscopy morphology showed well faced films with a small decrease in their grain size as the CH₄ concentration increased. The Raman spectra depicted a pronounced sp² band, mainly for films with 5 and 7 sccm of CH₄. MSP showed a decrease in the acceptor concentration as the CH₄ increased indicating the CH₄ influence on the doping process for Red–BDD and Ox–BDD samples. Nonetheless, an apparent increase in the acceptor concentrations for both Ox–BDD samples was observed compared to that for Red–BDD samples, mainly attributed to the surface conductive layer (SCL) formation after this strong oxidation process. The EIS Nyquist plots for Red–BDD showed a capacitance increase for the films with higher sp² content (5 and 7 sccm). On the other hand, the Nyquist plots for Ox–BDD can be described as semicircles near the origin, at high frequencies, where their charge transfer resistance strongly varied with the sp² increase in such films.