Hongya Yu

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)

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.

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.

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 ...

Magnetic phase transitions and magnetocaloric properties of (Gd12-xTbx)Co7 alloys

The structure and magnetocaloric properties of polycrystalline (Gd12-xTbx)Co7 (x=0, 4, and 8) alloys with Ho12Co7-type monoclinic structure have been investigated. A spin reorientation transition occurs at about 123 K for the alloy with x=0. The peak values of magnetic entropy change −ΔSM under a magnetic field change ΔH of 5 T is 8.8 J·kg−1·K−1 at 160.8 K (Tc), 8.2 J·kg−1·K−1 at 140.8 K (Tc), and 7.1 J·kg−1·K−1 at 118.9 K (Tc) for the alloys with x=0, 4 and 8, respectively. The refrigerant capacities reach 478, 327, and 160 J·kg−1 for x=0, 4, and 8, respectively. The relatively large magnetic entropy change and reversible magnetization behavior make these alloys a good choice for magnetic refrigeration applications in the temperature range of 118–160 K.

Thermal Growth and Nanomagnetism of the Quasi-one Dimensional Iron Oxide

Quasi-one dimensional iron oxide nanowires with flat needle shape were synthesized on the iron powders by a rather simple catalyst-free thermal oxidation process in ambient atmosphere. The characterization by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman and high-resolution transmission electron microscopy (HRTEM) revealed that these nanos-tructures are single crystalline α-Fe 2 O 3 . The various dimensions with 40–170 nm in width and 1–8 μm in length were obtained by tuning the growth temperature from 280 to 480°C. A surface diffusion mechanism was proposed to account for the growth of quasi-one dimensional nanostructure. The typical α-Fe 2 O 3 nanowires synthesized at 430°C had a reduced Morin temperature T M of 131 K in comparison with their bulk counterpart. The coercivitis H c of these nanowires are 321 and 65 Oe at 5 and 300 K, respectively. The temperature of synthesis also has important effects on the magnetic properties of these nanowires.

NdFeB based magnets prepared from nanocrystalline powders with various compositions and particle sizes by spark plasma sintering

Abstract Isotropic NdFeB permanent magnets were prepared by spark plasma sintering. Melt spun NdFeB ribbons with two compositions and four particle size ranges, i.e. 200–400, 100–200, 45–100 and <45 μm, were employed as the starting materials. For the magnets with Nd rich compositions, high density (7·5 g cm−3) and good magnetic properties with remanent magnetic polarisation Jr>0·8 T and maximum energy product (BH)max>100 kJ m−3 were obtained. The influence of particle size on the magnetic properties is not very significant. For single phase NdFeB alloys with stoichiometric 2∶14∶1 composition, because of the deficiency of Nd rich phases, it is relatively difficult to consolidate microsized powders into high density bulk magnet, but generally, a larger particle size is beneficial to achieve better magnetic properties, including higher Jr and (BH)max.

Melt spun and suction cast Nd-Fe-Co-B-Nb hard magnets with high Nd contents

Nd-Fe-Co-B-Nb alloys with Nd contents of 9−9.5 at % were prepared by melt spinning and suction casting. It was found from the melt spun ribbon samples that Nb addition enhanced the glass forming ability and Co addition improved the thermal stability. Larger values of Jr and (BH)max were obtained for the ribbon samples than for the bulk ones due to the finer crystalline structure in the former. Nanocrystallite with amorphous structure was found in the suction cast rod samples. The as-cast Nd9Fe71.5B15.5Nb4 rod in a diameter of 2 mm exhibited the best hard magnetic behavior. A remanence of 0.59 T, a coercivity of 1154 kA/m, and a maximum energy product of 54.2 kJ/m3 have been obtained after heat treatment. The distribution of nonmagnetic FeNb phase plays a key role in the improvement of coercivity. Current work suggests that large size Nd2Fe14B/Fe3B nanocomposite magnets with high Nd contents and good magnetic properties can be obtained using a nanocrystalline precursor instead of bulk metallic glass.

High Coercivity FePtSiN Films With L1

FePtSiN films consisting of FePt nanoparticles embedded in Si-rich matrix were fabricated on silicon substrates by direct current (dc) reactive magnetron sputtering followed by vacuum annealing. The effects of Si-N additions and annealing temperature on the structure and magnetic properties were investigated. The as-deposited films had face-centered cubic (fcc) structure, which transforms into the face-centered tetragonal (fct) structure after thermal annealing at 600°C. The grain size of FePt increased with the annealing temperature but decreased with increasing Si-N content. Increasing Si content led to the formation of Si-N-rich amorphous phase distributed between the FePt nanograins, which reduced the lattice distortion and increased the coercivity. The fct-FePt films annealed at 700°C exhibited very high coercivity, up to 13.6 kOe at room temperature and about 17.5 kOe at 100 K. These FePtSiN films have shown promise for high-density magnetic recording medium.

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The electronic structures and magnetic properties of spinel Co1-x Zn (Cd)xFe2O4 (x = 0.0, 0.25, 0.5, 0.75, and 1.0) have been studied systematically by the first-principles calculation. The optimized structures show that with increasing Zn (Cd) content the crystal lattice parameter increases by 1.47% (5.13%) when the Co ions are completely replaced by Zn (Cd) ions. The results also indicate that the magnetic moment of Co/Fe ion increases and the total spin magnetic moment linearly increases with the Zn (Cd) doping from 3.00 μB for x = 0.0 up to 10.00 μB for x = 1.0 per formula unit. The electronic distribution in the Co spin-down subband is highly localized for the crystal field splitting between t2g and eg levels in the Zn (Cd) doped CoFe2O4 compounds and the energy band moves downwards while doped with Zn (Cd). Meanwhile, the bandgap decreases with the increase of Zn (Cd) except where x = 1 and the spinel structure is half-metal at some degree of doping.