Phase characterization in the Fe−Co−V and Fe−Co−Nb systems

Two distinct phases of the Fe?Co?Nb and three of the Fe?Co?V systems have been studied by means of X-ray diffraction and Mössbauer spectroscopy. The dependence of the lattice parameter of the alpha phase on the nominal solute content in equiatomic FeCo together with the alterations of the Mössbauer spectra have shown a very limited solubility of niobium in alpha FeCo. There are indications that the solubility of vanadium in alpha FeCo increases with increasing nominal content of solute in the alloy. A mechanism involving the withdrawal of cobalt from the alpha matrix to form a phase rich in Co and V (gamma) is proposed to explain such a varying solubility. The vanadium content of the sigma phase in equiatomic FeCo alloys with 22 wt% V is proposed to be less than 50 at%.     

High-temperature hysteresis and magnetization reversal in nanocomposite FeCo+MnO

The hysteretic behavior of mechanically alloyed nanocomposites FeCo+MnO was studied at high temperatures. These composites present an unusual high and thermally stable coercivity, compared to FeCo milled at equal conditions. Coercivity enhancement was observed in hysteresis loops obtained between room temperature and 750 K. It is attributed to the isolation of the FeCo ferromagnetic particles by the paramagnetic MnO (T_N=120 K). The M_rev(M_irr)_H curves are clearly linear for the composite, indicating that coherent rotation is the reversal mechanism in these materials.     

Temperature dependent properties and aggregation behaviour of FeCo nanoparticles produced sonoelectrochemically

The study describes synthesis of FeCo nanoparticles by using the pulsed sonoelectrochemical technique, a method which couples an electrochemical process with the employment of high power ultrasound. An ultrasonic horn is also used as the working electrode and is subjected to a pulsed galvanic current and pulsed out of phase ultrasound. Nanoparticles made of FeCo alloy were synthesized at different bath temperatures, in order to study and evaluate the influence of this parameter on process efficiency and nanoparticles’ features. Produced material was characterized by X-EDS, X-Ray diffraction, and finally by transmission electron microscopy. Moreover characterization of nanoparticles’ tendency to aggregation was performed with dynamic light scattering and by using a polyacrilate to stabilize the suspensions. Process efficiency was found to be strongly influenced by temperature, and from chemical analyses, a preferential deposition of iron was observed, due to the lower iron reduction overpotential. Structural characterization stated that FeCo nanoparticles showed a bcc structure and a mean grain size below 30 nm, which depended on synthesis temperature (T) and decreased with T to 5 nm. TEM characterization showed that nanoparticles exhibited the same mean dimensions like ones found from XRD analyses; this led to conclude that nanopowders are monocrystalline.     

Magnetic characterization of FeCo nanowire arrays by first-order reversal curves

FeCo nanowire arrays have been obtained by current pulse electrodeposition into nanoporous alumina templates. First-order reversal curve (FORC) diagrams have been used to investigate magnetostatic interaction and average coercivity of individual FeCo nanowires embedded in porous alumina templates. The FeCo nanowires with a wires length up to 3 μm and wires diameter ranging from 25 to 50 nm showed interacting single-domain behavior. Using FORC diagrams, the spread of coercivity distribution was seen to be almost independent of the wires diameter, but with increase in diameter the inter-wire magnetostatic interaction was increased. It was found that for arrays with higher diameter, the coercivity of the arrays is lower than the average coercivity of the individual wires. It was detected that an increase in wire diameter results in a considerable increase in the spread of the distribution in the H_u direction of FORC distribution. Curve fitting on the experimental data proved a relatively linear relation between interaction field and square diameter of the nanowires.     

Rare earth free exchange spring magnet FeCo/FePt(001): Giant magnetic anisotropy and energy product

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the thickness dependent magnetic properties of rare earth free exchange spring magnet FeCo/FePt(001). The FeCo adlayer thickness is increased from one monolayer (ML) to four ML coverage. It is observed that the FeCo adlayers and Fe atoms in FePt substrate show almost half metallic behavior, while an ordinary metallic feature is found in Pt atoms. The average magnetization increases with FeCo thickness and the estimated maximum energy product reaches 66 MGOe in FeCo(4 ML)/FePt(001). A giant perpendicular magnetocrystalline anisotropy (MCA) energy of 18.20 meV/cell is found in pure FePt(001) and it becomes 17.35 meV/cell even in FeCo(4 ML)/FePt(001). In addition, we find very large coercivity field in FeCo/FePt(001) systems. For instance, the calculated maximum coercivity field in FeCo(4 ML)/FePt(001) is about 188 kOe. Both energy product and coercivity field calculations may imply that the FeCo/FePt can be utilized for potential rare earth free exchange spring magnet material.     

Study of domain structures in FeCo/TiZr multilayers by means of 3D neutron depolarisation

Multilayers of composition 60(Co₆₈Fe₃₁V₁/Ti₇₅Zr₂₅) sputtered on Si wafers are studied by 3D neutron depolarisation analysis at glancing angles between −6.5° and 6.5°, both in zero and non-zero magnetic field. As the sample is rotated over a full turn in its plane, a 360° rather than 180° period is observed. This suggests that the magnetisation inside the CoFe sublayers is inclined by a few degrees out of the sample plane. Neighbouring FeCo sublayers cooperate by dipolar forces to form a structure of “inclined slab domains” with mutually antiparallel overall magnetisation. The thickness of these slabs includes 10–20 bilayers. In the hysteresis loop with the field parallel to the FeCo magnetisation a high remanence is found; when the field is (in-plane but) perpendicular to the FeCo magnetisation, the dipolar coupling is suppressed and a magnetisation loop with a lower remanence is observed.     

Tuning Carbon Content and Morphology of FeCo/Graphitic Carbon Core–Shell Nanoparticles using a Salt‐Matrix‐Assisted CVD Process

Large‐scale and tunable synthesis of FeCo/graphitic carbon (FeCo/GC) core–shell nanoparticles as a promising material for multipurpose biomedical applications is reported. The high‐quality graphitic structure of the carbon shells is demonstrated through high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), and Raman spectroscopy. A saturation magnetization of 80.2 emu g^?1 is reached for the pure FeCo/GC core–shell nanoparticles. A decrease in the saturation magnetization of the samples is observed with an increase in their carbon content with different carbon morphologies evolved in the process. It is also shown how hybrid nanostructures, including mixtures of the FeCo/GC nanoparticles and multi‐walled carbon nanotubes (MWNTs) or carbon nanorods (CNRs), can be obtained only by manipulation of the carbon‐bearing gas flow rate.     

Phase diagram of Fe1−xCox ultrathin film

Concentration-driven reorientation phase transitions in ultrathin magnetic films of FeCo alloy have been studied. It is established that, in addition to the easy-axis and easy-plane phases, a spatially inhomogeneous phase (domain structure), a canted phase, and also an “in-plane easy-axis” phase can exist in the system. The realization of the last phase is associated with the competition between the single-ion anisotropy and the magnetoelastic interaction. The critical values of Co concentration corresponding to the phase transitions are evaluated, the types of phase transitions are determined, and the phase diagrams are constructed.     

Effect of copper concentration on atomic site occupation by Fe ions and magnetic properties of (PrDy)–(FeCo)–B alloys

The effect of small copper additions (to ～6 at %) on the distribution of iron ions in six crystallographic sites of the unit cell of the main magnetic phase (PrDy)₂(FeCo)₁₄B has been detected. An increase in the copper concentration leads to a decrease in the 8j₁ site occupation by iron ions. Independently of the presence of copper, the temperature dependences of the saturation magnetization of all samples have a minimum which can correspond to the presence of a low-temperature phase or a compensation point in grain boundary regions of the (PrDy)₂(FeCo)₁₄B main magnetic phase. The alloys under study are not additive sets of all phases in their composition, but behave as new materials with the mutual influence of phases on each other.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Synthesis of FeCo nanoparticles by pulsed laser deposition in a diffusion cloud chamber

Magnetic FeCo nanoparticles were successfully synthesized in a diffusion cloud chamber setup within pulsed laser deposition (PLD) equipment. The variation of morphology and size of FeCo nanoparticles with the number of laser pulses, ambient gas pressure and temperature gradient was studied. It was observed that the morphology of the nanoparticles changes from “cloud-like” fractal clusters to particle chains; average particle size increased at higher argon gas pressure. Increasing the temperature gradient considerably reduced the agglomeration of the nanoparticles. Nanoparticles deposited using the diffusion cloud chamber are found to be crystalline.     

A study of the properties of core/shell/shell Ag/FeCo/Ag nanoparticles

The properties of heterophase core/shell/shell Ag/FeCo/Ag nanoparticles synthesized via a plasma method that are promising for biological applications are studied. As is established, the core/shell/shell Ag/FeCo/Ag nanoparticles exhibit a superparamagnetic state at room temperature that allows one to manage the hyperthermia process. The magnetic characteristics of core/shell/shell Ag/FeCo/Ag nanoparticles are interpreted by assuming partial oxidation of the surface layer of a ferromagnetic FeCo shell and formation of the antiferromagnetic Co_xFe_1–xО layer on the FeCo surface. The interaction between the surface antiferromagnetic Co_xFe_1–xО layer and the ferromagnetic FeCо shell causes the emergence of the exchange bias in Ag/FeCo/Ag nanoparticles.     

Elastic phase transitions in metals at high pressures

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-?to fourth-order elastic constants. The second-,third-?and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P?≈?69?GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.     

Structural phase transition of vanadium at 69 GPa

A phase transition was observed at 63-69 GPa and room temperature in vanadium with synchrotron x-ray diffraction. The transition is characterized as a rhombohedral lattice distortion of the body-centered-cubic vanadium without a discontinuity in the pressure-volume data, thus representing a novel type of transition that has never been observed in elements. Instead of driven by the conventional s-d electronic transition mechanism, the phase transition could be associated with the softening of C44 trigonal elasticity tensor that originates from the combination of Fermi surface nesting, band Jahn-Teller distortion, and electronic topological transition.     

多壁碳纳米管对p型Ba0.3FeCo3Sb12化合物热电性能的影响

用两步固相反应法合成了P型Ba填充方钴矿化合物Ba0.3FeCo3Sb12，并采用放电等离子烧结法(SPS)制备了Ba0.3FeCo3Sb12／多壁碳纳米管复合材料．研究了Ba0.3FeCo3Sb12／多壁碳纳米管复合材料的结构及多壁碳纳米管对其热电性能的影响规律：SEM分析表明多壁碳纳米管比较均匀地分布在Ba0.3FeCo3Sb12基体中；随着碳纳米管添加量的增加，Ba0.3FeCo3Sb12／多壁碳纳米管复合材料的电导率下降、塞贝克系数略微下降、晶格热导率大幅度降低，当碳纳米管含量为5％时其晶格热导率最小；当碳纳米管的含量为3％时，本研究得到的Ba0.3FeCo3Sb12／碳纳米管复合材料的最大ZT值达0．78．     

Specific features of hydrogenation of chromium-doped polycrystalline thin vanadium dioxide films

It has been found that hydrogen penetration into chromium-doped polycrystalline thin vanadium dioxide films occurs with a lower rate than in the case of pure vanadium dioxide films. It has been shown that hydrogenation of films with low chromium concentrations is accompanied by a decrease in the phase transition temperature below T _c = 340 K. However, at room temperature in these hydrogenated films, no traces of M1 monoclinic phase have been observed. As the chromium concentration increases, hydrogenation ceases to be accompanied by the decrease in the phase transition temperature.     

The high frequency magnetic properties of self assembled Fe–Co–Si–N nanogranular thin films

The effect of a variation in Si and N concentration on the microstructure, crystal structure and high-frequency magnetic properties of Fe–Co–Si–N nanogranular thin films was investigated. The films, prepared by rf magnetron sputtering, consisted of nanosized grains of FeCo as well as a Si and N rich intergranular amorphous phase. The Si concentration had a significant effect on the crystal structure of the FeCo phase. The resistivities of the Fe–Co–Si–N films were significantly enhanced by an increase in Si concentration. The resonant frequency of the Fe–Co–Si–N films could be tuned from 0.45 GHz to 2.1 GHz by controlling Si concentration. The N concentration greatly influenced magnetic properties and the variation in resonant frequency is in agreement with Kittel’s equation.     

Phase diagram of a thin film of the Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloy with the “bulk” or “planar” magnetoelastic interaction

Concentration-induced reorientation phase transitions in thin magnetic films of FeCo alloys have been investigated taking into account “planar” or “bulk” magnetoelastic interaction. The critical concentrations of Co corresponding to the phase transition points, as well as the types of the phase transitions, have been determined. The phase diagrams have been plotted.     

Cascade of phase transitions in the Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> monolayer

The reorientation phase transitions occurring in FeCo alloy monolayers have been investigated as a function of the cobalt concentration with inclusion of the magnetodipole and magnetoelastic interactions. The kinetic values of the cobalt concentration corresponding to the phase transition points have been determined and the phase diagram of the system under investigation has been constructed.     

Influence of hydrogenation on electrical conductivity of vanadium dioxide thin films

The influence of hydrogenation on electrical conductivity of vanadium dioxide thin films has been investigated. It has been shown using measurements of the electrical conductivity that the hydrogenation of vanadium dioxide thin films leads to a decrease in the temperature of the phase transition from the tetragonal phase (with “metallic” conductivity) to the semiconducting monoclinic phase. It has been found that, upon doping of vanadium dioxide with hydrogen, the electrical conductivity of the monoclinic phase can increase by several orders of magnitude. Nonetheless, the temperature dependence of the electrical conductivity of hydrogenated films exhibits a typical semiconducting behavior in the temperature range where the monoclinic phase is stable.