Z. W. Liu

Structural, electronic and magnetic properties of partially inverse spinel CoFe2O4: a first-principles study

Partially inverse spinel CoFe2O4, which may be prepared through various heat treatments, differs remarkably from the ideal inverse spinel in many properties. The structure of partially inverse spinel CoFe2O4 as well as its electronic and magnetic properties through a systemic theoretical calculation of (Co1−xFex)Tet (CoxFe2−x)OctO4 (x = 0, 0.25, 0.5, 0.75 and 1.0) have been investigated by the generalized gradient approximation (GGA) + U approach. It is found that the Co and Fe ions prefer their high spin configurations with higher spin moments at octahedral sites in all the studied cases, in line with experimental observations. The Co ions at the octahedral sites favour being far away from each other in the partial inverse spinels, which also show half metallicity at certain inversion degrees. (Some figures in this article are in colour only in the electronic version)

High frequency characteristics of FeCoN thin films fabricated by sputtering at various (Ar+N2) gas flow rates

FeCoN soft magnetic thin films with various nitrogen contents were fabricated by varying the gas flow rates of the argon and nitrogen gas mixture used during reactive rf magnetron sputtering. It was found that the resistivity of FeCoN film increased up to 211 μΩ cm with increasing nitrogen content. The film produced at the gas flow rate of 7 sccm exhibited the best soft magnetic properties with small coercivity and relatively large saturation magnetization. The microstructure of the films was examined by high-resolution transmission electron microscopy and α″-FeCo phase with very high saturation magnetization was observed in the films. The high-frequency properties of the FeCoN thin films were experimentally characterized and theoretically examined. The Hoffmann’s ripple theory was used to explain the permeability spectra for the films with strong magnetization dispersion. We found that the ripple effect is responsible for the large damping in the films, and those films with larger coercivities have stron...

FeCoSiN film with ordered FeCo nanoparticles embedded in a Si-rich matrix

FeCoSiN film consisting of ordered arrays of FeCo nanoparticles embedded in a Si-rich matrix was fabricated on silicon substrate by reactive rf magnetron sputtering. These nanoparticles have a face-centered cubic structure and a mean diameter of ∼7nm. The FeCoSiN film has a very high resistivity of up to 1000μΩcm due to the breakup of the metal continuum into isolated metal nanoparticles. The static and dynamic magnetic properties of the film were also investigated. A saturation magnetization of 1.0T, coercivity of less than 5Oe in both easy and hard axes, and anisotropy field of 27Oe were obtained. The microwave permeability measurement showed a permeability of around 400 at low frequency and a ferromagnetic resonance frequency of about 1.1GHz.

Microwave characteristics of low density hollow glass microspheres plated with Ni thin-film

Hollow glass microspheres coated with thin Ni films of various thicknesses (about 50–250nm) were obtained by electroless plating technique. The resultant magnetic microspheres had very low densities (∼0.39–1.28gcm−3) and low coercivities (<40Oe). As-plated films comprised of nanocrystallites embedded in amorphous matrix and postannealing treatment led to a submicrocrystalline structure and an increased saturation magnetization. The high frequency properties of the composites consisting of Ni-plated microspheres and polymer were investigated. The permittivity and permeability increased with increasing microsphere content in the composite and increasing Ni film thickness. These composites had ferromagnetic resonance peak in the range of 5–12GHz. Postannealing also had an effect on the microwave properties. The reflection loss of the composite was found dependent on the absorber material thickness, polymer:microsphere ratios, the Ni film thickness, as well as the heat treatment of the microspheres.

Microstructure and property evolution of isotropic and anisotropic NdFeB magnets fabricated from nanocrystalline ribbons by spark plasma sintering and hot deformation

Isotropic and anisotropic NdFeB magnets were synthesized by spark plasma sintering (SPS) and SPS+HD (hot deformation), respectively, using melt-spun ribbons as the starting materials. Spark plasma sintered magnets sintered at low temperatures (<700 °C) almost maintained the uniform fine grain structure inherited from rapid quenching. At higher temperatures, due to the local high-temperature field caused by the spark plasma discharge, the grain growth occurred at the initial particle surfaces and the coarse grain zones formed in the vicinity of the particle boundaries. Since the interior of the particles maintained the fine grain structure, a distinct two-zone structure was formed in the spark plasma sintered magnets. The SPS temperature and pressure have important effects on the widths of coarse and fine grain zones, as well as the grain sizes in two zones. The changes in grain structure led to variations in the magnetic properties. By employing low SPS temperature and high pressure, high-density magnets with negligible coarse grain zone and an excellent combination of magnetic properties can be obtained. An anisotropic magnet with a maximum energy product of ~30 MG Oe was produced by the SPS+HD process. HD at 750 °C did not lead to obvious grain growth and the two-zone structure still existed in the hot deformed magnets. Intergranular exchange coupling was demonstrated in the spark plasma sintered magnets and was enhanced by the HD process, which reduced the coercivity. Good temperature stability was manifested by low temperature coefficients of remanence and coercivity. The results indicated that nanocrystalline NdFeB magnets without significant grain growth and with excellent properties could be obtained by SPS and HD processes.

Intergranular exchange interaction in nanocrystalline hard magnetic rare earth―iron―boron―based melt-spun alloy ribbons

Rapidly solidified nanocrystalline RE(Fe/Co)B (RE = Nd,Pr) alloys with enhanced hard magnetic properties were obtained by melt spinning. The effects of processing, composition, phase constitution, microstructure and temperature on the exchange interaction between nanocrystallites were studied by analysing the magnetization and demagnetization curves, characteristic loops and Henkel plots. Uniform single phase behaviour was observed for single phase and nanocomposite alloys with fine nano-grains, produced by optimum melt spin processing. It was shown, by constructing Henkel plots, that decreasing the hard phase grain size enhances the exchange coupling. Also, introducing and increasing the volume fraction of soft phase crystallites within the 2/14/1 nanocrystalline matrix, by progressively reducing the RE:(Fe/Co) ratio, also increased the strength of the exchange coupling, which was manifested by increased height of the positive peak in the Henkel plot. At elevated temperatures, the exchange coupling was further enhanced due to an increased exchange length (Lex = (A/K)1/2), in spite of the fact that both the anisotropy constant K and the exchange constant A decrease with increasing temperature. Analysis of the microstructure parameters in the modified Brown equation, based on a nucleation mechanism of domain reversal and coercivity, indicate that Co substitution for Fe and introduction of soft α-(FeCo) nanocrystallites not only reduce the stray field effects associated with grain boundary roughness but also enhance the exchange coupling between neighbouring nanograins.

The magnetocaloric effect and critical behavior in amorphous Gd60Co40−xMnx alloys

The amorphous alloys Gd60Co40−xMnx (x = 0, 5, 10, 15) were prepared by melt spinning. The Curie temperature, Tc, increases monotonously with Mn addition, ranging from 198 K for x = 0 to 205 K for x = 15, while the maximum values of −ΔSM under the applied field change from 0 to 5 T are 7.7, 7.1, 6.2 and 5.4 J·kg−1·K−1 for x = 0, 5, 10, and 15, respectively. All samples undergo a second order ferri-paramagnetic phase transition. The critical behavior around the transition temperature is investigated in detail, using both the standard Kouvel-Fisher procedure as well as the study of the field dependence of the magnetocaloric effect. Results indicate that the obtained critical exponents are reliable, and that the present alloys exhibit local magnetic interaction.

Thickness-dependent properties of FeTaN thin films deposited on flexible substrate

The FeTaN thin films were deposited on the flexible plastic substrates by rf magnetron sputtering. Good soft magnetic properties with coercivity <10Oe have been obtained in the films with thickness <250nm. Relatively high values of the complex permeability and ferromagnetic resonance frequency were obtained and they can be adjusted by changing the film thickness. However, compared to the films on the rigid substrate, a decrease of the permeability level and a shift of the resonance peak in the permeability spectra have been noticed for the films on the flexible substrate. The electric properties, including sheet resistance, resistivity, and magnetoresistance for the films with various thicknesses, have also been investigated.

Microstructure and high frequency properties of nanogranular CoAlO thin films

Nanogranular CoAlO thin films were fabricated by rf magnetron sputtering using Co target partially covered by Al2O3 chips in argon atmosphere. The films consist of Co nanograins with a mean diameter of 5–30nm surrounded by an amorphous Al–O phase. It was found that Co grains of face-centered-cubic and hexagonal-close-packed structures coexist in the films with relatively low (Al,O) content, whereas Co grains of face-centered-cubic structure are preferentially formed in the films with higher (Al,O) content. Due to the microstructure changes with increasing (Al,O) content in the films, coercivity and magnetization decrease but the anisotropy field increases. The high frequency properties of the CoAlO films were experimentally characterized, and the effects of magnetization, anisotropy field, and damping factor on the permeability spectrum are also analyzed using the Landau-Lifshitz-Gilbert equation. The ferromagnetic resonance frequency was found to increase from 2.1to2.8GHz with the microstructure change i...

Diffusion of Nd-rich phase in the spark plasma sintered and hot deformed nanocrystalline NdFeB magnets

The role of the Nd-rich phase in developing microstructure and properties of isotropic and anisotropic NdFeB magnets has been investigated. Melt spun Nd-rich Nd13.5Fe73.5Co6.7Ga0.5B5.6 and Fe-rich Nd7.7Pr2.6Fe84.1B5.5 alloy powders were mechanically mixed with different ratios. The mixed powders were consolidated into isotropic magnets and anisotropic magnets by spark plasma sintering (SPS) and SPS followed by hot deformation, respectively. The composition and microstructure of diffusion area between Nd-rich and Fe-rich compositions for isotropic and anisotropic magnets were investigated. The gradient distribution of Nd content from Nd-rich to Fe-rich area due to the diffusion of liquid Nd-rich phase in the SPS and hot deformation was observed, which leads to gradually changes in grain structure. The remanent polarization of 1.29 T, coercivity of 995 kA/m, and maximum energy product of 293 kJ/m3 are obtained for an anisotropic magnet. The role of the Nd-rich phase has to be carefully considered in order to achieve deformation anisotropy and fine grains in the NdFeB magnets.The role of the Nd-rich phase in developing microstructure and properties of isotropic and anisotropic NdFeB magnets has been investigated. Melt spun Nd-rich Nd13.5Fe73.5Co6.7Ga0.5B5.6 and Fe-rich Nd7.7Pr2.6Fe84.1B5.5 alloy powders were mechanically mixed with different ratios. The mixed powders were consolidated into isotropic magnets and anisotropic magnets by spark plasma sintering (SPS) and SPS followed by hot deformation, respectively. The composition and microstructure of diffusion area between Nd-rich and Fe-rich compositions for isotropic and anisotropic magnets were investigated. The gradient distribution of Nd content from Nd-rich to Fe-rich area due to the diffusion of liquid Nd-rich phase in the SPS and hot deformation was observed, which leads to gradually changes in grain structure. The remanent polarization of 1.29 T, coercivity of 995 kA/m, and maximum energy product of 293 kJ/m3 are obtained for an anisotropic magnet. The role of the Nd-rich phase has to be carefully considered in order ...