D.C. Zeng

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.

Structure and size-dependent properties of NdFeB nanoparticles and textured nano-flakes prepared from nanocrystalline ribbons

A surfactant-assisted ball milling technique was employed to synthesize NdFeB nanoparticles and nanoflakes from melt-spun nanocrystalline powders. The microstructure evolution during ball milling and the particle size dependent magnetic properties were investigated. The formation mechanism of submicrometre flakes and textured nanoflakes were discussed. The surfactant and ball-to-powder weight ratio played a crucial role in the formation of nanoflakes. The coercivity of the particles decreased with decreasing particle size. Significant room temperature coercivity HC higher than 300 kA m−1 was obtained in the nanoparticles with a mean size of ~20 nm. The spin reorientation temperature was found to decrease with reducing particle size, which may be related to the change in magnetocrystalline anisotropy caused by the induced strain during ball milling.

Large magnetocaloric effect and refrigerant capacity in Gd–Co–Ni metallic glasses

The thermal stability, magnetocaloric effect, and refrigerant capacity (RC) of Gd–Co–Ni metallic glasses were investigated. These alloys possess high glass transition temperature and crystallization temperature as well as a relatively wide supercooled liquid region ΔTx(ΔTx = Tx − Tg) (40–55 K). With increasing the Co/Ni ratio, the Curie temperature TC of the amorphous Gd–Co–Ni increases from 140 K to 192 K. For a magnetic field change of 0–5 T, the maximum magnetic entropy change (−ΔSMmax) and RC values are in the range of 6.04–6.47 J kg−1 K−1 and 450–502 J kg−1, respectively. These values are comparable with that of La(Fe0.88Si0.12)13 and higher than those for the well known magnetic refrigerant Gd5Si2Ge1.9Fe0.1 alloy. The large magnetic entropy change and refrigerant capacity as well as high thermal stability make the alloys attractive candidates as magnetic refrigeration materials for service temperatures of 100–230 K.

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 ...

Exchange interaction in rapidly solidified nanocrystalline RE–(Fe/Co)–B hard magnetic alloys

The exchange interactions for rapidly solidified (Nd/Pr)(Fe/Co)B hard magnetic alloys with remanence enhancement were studied by analyzing characteristic magnetic curves and Henkel plots. The exchange coupling can be improved by increasing soft phase content within hard matrix. With increasing temperature, the exchange interaction is enhanced due to the increased exchange length. An increased recoil susceptibility was found for the alloys with improved exchange coupling. Analysis of the microstructure parameters indicated that Co substituting Fe and introducing soft α-(Fe,Co) phase not only reduce the stray field effects but also enhance the exchange interaction.

Interface Engineering of Domain Structures in BiFeO3 Thin Films

A wealth of fascinating phenomena have been discovered at the BiFeO3 domain walls, examples such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling. Thus, the ability to precisely control the domain structures and accurately study their switching behaviors is critical to realize the next generation of novel devices based on domain wall functionalities. In this work, the introduction of a dielectric layer leads to the tunability of the depolarization field both in the multilayers and superlattices, which provides a novel approach to control the domain patterns of BiFeO3 films. Moreover, we are able to study the switching behavior of the first time obtained periodic 109° stripe domains with a thick bottom electrode. Besides, the precise controlling of pure 71° and 109° periodic stripe domain walls enable us to make a clear demonstration that the exchange bias in the ferromagnet/BiFeO3 system originates from 109° domain walls. Our findings provide future directions to study the room temperature electric field control of exchange bias and open a new pathway to explore the room temperature multiferroic vortices in the BiFeO3 system.

Reducing Dy Content by Y Substitution in Nanocomposite NdFeB Alloys With Enhanced Magnetic Properties and Thermal Stability

Dy has been employed as an additive in NdFeB magnets to improve their coercivity and thermal stability, but the high cost of the Dy has become a major concern. In this work, the effects of Y substitution for Dy on the magnetic properties and thermal stability of melt-spun nanocomposite [Nd_0.8(Dy_1-xY_x)_0.2]₁₀Fe₈₄B₆ alloys have been systematically studied. The results show that Y substituting Dy can not only improve the remanence and energy product but also improve their thermal stability. [Nd_0.8(Dy_0.5Y_0.5)_0.2]₁₀Fe₈₄B₆ alloy with 50% Dy reduction exhibits a high maximum energy product of 139 kJ/m³ and low absolute values of temperature coefficients α and β, -0.090%/^°C and -0.394%/^°C, respectively. It is suggested that Y substitution be a feasible way to reduce the cost of the NdFeB magnets and maintain their high temperature performance.

Spin reorientations in RFe11-xCoxTi compounds (R=Tb, Er, Y)

Abstract We have studied the magnetic properties of several series of RFe 11− x Co x Ti compounds (R=Tb, Er and Y) by means of magnetic measurements and X-ray diffraction of magnetically aligned powder samples. The spin reorientations observed in several of these compounds were attributed to three different mechanisms. Spin-reorientation transitions observed at fairly low temperatures originate mainly from a competition between the various crystal-field terms associated with the crystal-field-induced R-sublattice anisotropy. At fairly high temperatures, spin reorientation transitions can arise as a consequence of a sign change of the 3d-sublattice anisotropy as a function of temperature. In the intermediate temperature range, spin-reorientation transitions were interpreted as resulting from a competition between the R-sublattice anisotropy and 3d-sublattice anisotropy.