Wenyong Zhang

Hf–Co and Zr–Co alloys for rare-earth-free permanent magnets

The structural and magnetic properties of nanostructured Co-rich transition-metal alloys, Co(100-x)TMx (TM = Hf, Zr and 10 ≤ x ≤ 18), were investigated. The alloys were prepared under non-equilibrium conditions using cluster-deposition and/or melt-spinning methods. The high-anisotropy HfCo7 and Zr2Co11 structures were formed for a rather broad composition region as compared to the equilibrium bulk phase diagrams, and exhibit high Curie temperatures of above 750 K. The composition, crystal structure, particle size, and easy-axis distribution were precisely controlled to achieve a substantial coercivity and magnetization in the nanostructured alloys. This translates into high energy products in the range of about 4.3-12.6 MGOe, which are comparable to those of alnico.

Magnetism of rapidly quenched rhombohedral Zr2Co11-based nanocomposites

The effect of quench rate and Zr content on nanostructure and magnetic properties of melt-spun ZrxCo100?x(x?=?16?21) is investigated. High quench rate favours the formation of rhombohedral Zr2Co11, which is the hard phase. The coercivity increases with an increase in quench rate. Zr addition in limited amounts decreases the grain size of magnetic phases, which may promote the effective exchange coupling of soft magnetic phases. Therefore, coercivity and maximum energy product of Zr2Co11-based materials are significantly enhanced. The best magnetic properties,iHc?=?3.0?kOe and (BH)max?=?4.6?MG?Oe, which are the highest reported values among Co?Zr binary alloys, are achieved for x?=?18. The temperature coefficients of coercivity and remanence between 100 and 380?K are ?0.05%?K?1, comparable to those of alnico magnet.

Structural and magnetic properties of Pr-alloyed MnBi nanostructures

The structural and magnetic properties of Pr-alloyed MnBi (short MnBi-Pr) nanostructures with a range of Pr concentrations are investigated. The nanostructures include thin films having Pr concentrations 0, 2, 3, 5 and 9 at.% and melt-spun ribbons having Pr concentrations 0%, 2%, 4% and 6%, respectively. Addition of Pr into the MnBi lattice has produced a significant change in the magnetic properties of these nanostructures including an increase in coercivity and structural phase transition temperature, and a decrease in saturation magnetization and anisotropy energy. The highest value of coercivity measured in the films is 23 kOe and in the ribbons is 5.6 kOe. The observed magnetic properties are explained as the consequences of competing ferromagnetic and antiferromagnetic interactions.

Coercivity Enhancement in

The Mo-content dependence of structure and magnetic properties of Zr₁₆Co_78-xMo_xSi₃B₃ ( x=0, 2, 3, 4, 5) nanocrystalline materials has been studied. The samples consist of hard-magnetic Zr₂Co₁₁ and soft-magnetic Co phases. The substitution of Mo for Co restrains the formation of Co, raises the content of Zr₂Co₁₁, and increases the mean grain size of Zr₂Co₁₁. Therefore, the coercive force of the sample increases with x. A coercive force of 7.9 kOe, which is a highest value reported among Zr-Co alloys, was achieved for x=5. The anisotropy field of Zr₂Co₁₁ remains almost unchanged with increasing Mo content.

{\rm Zr}_{2}{\rm Co}_{11}

Structural, electronic, and magnetic properties of a Heusler-type CoFeCrAl alloy have been investigated experimentally and by model calculations, with a focus on the alloys spin-gapless semiconductivity. The as-quenched samples are ferrimagnetic at room temperature with a Curie temperature of about 456 K, which increases to 540 K after vacuum annealing at 600 °C for 2 h. The saturation magnetizations of the as-quenched and 600 °C-annealed samples are 1.9 µB/f.u. and 2.1 µB/f.u., respectively, which are very close to the value predicted by the Slater–Pauling curve. The resistivity shows a nearly linear decrease with increasing temperature, from about 930 µΩ cm at 5 K to about 820 µΩ cm at 250 K, with dρ/dT of about −5 × 10−7 Ω cm K−1. We explain this high resistivity and its temperature dependence as imperfect spin-gapless semiconducting behavior, with a negative band-gap parameter of 0.2 eV.

-Based Nanocrystalline Materials Due to Mo Addition

The effect of Mo addition on phase composition and nanostructure of nanocrystalline Zr16Co84−xMox (x = 0–2.0) melt spun at 55 m/s has been investigated. All the ribbons consist mainly of a hard magnetic Zr2Co11 phase with rhombohedral crystal structure but also contain minor amounts of soft-magnetic phases. The increase in cell volume on alloying suggests that Mo mainly enters the rhombohedral Zr2Co11 structure and occupies the Co site. Mo addition promotes the formation of the hard magnetic phase and increases its volume fraction. The mean grain size of the hard magnetic phase remains almost unchanged with the increase of Mo content. But the average grain size of the soft magnetic phase decreases from about 200 nm to 50 nm. This promotes the exchange coupling of the hard and soft magnetic phases and thus leads to a significant increase in coercivity and isotropic energy product, from 0.6 kOe and 0.5 MGOe for x = 0 to 2.9 kOe and 4.2 MGOe for x = 1.5.

Magnetism, electron transport and effect of disorder in CoFeCrAl

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr18-xHfxCo82 ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr2Co11, orthorhombic Zr2Co11, hcp Co, and cubic Zr6Co23 phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr2 Co11 (rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (Hc) and the saturation magnetization are (Ms) found to linearly increases with x (x ≤ 2), then Hc slightly increases and Ms slightly decreases with increasing x. The maximum energy product (BH)max at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m3 and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials.

Phase composition and nanostructure of Zr2Co11-based alloys

The Co-substituted Mn2RuSn nanomaterials, namely, Mn2Ru0.5Co0.5Sn and Mn2Ru0.35Co0.65Sn have been synthesized and investigated. The presence of Co in the Mn2RuSn (a = 6.21 A) decreased the lattice parameter, where a = 6.14 A and 6.12 A for the as prepared Mn2Ru0.5Co0.5Sn and Mn2Ru0.35Co0.65Sn, respectively. The samples show a ferrimagnetic spin order with relatively small coercivities, similar to those of soft magnetic materials. There is a substantial increase in the Curie temperature (Tc = 448 K for Mn2Ru0.5Co0.5Sn and 506 K for Mn2Ru0.35Co0.65Sn) of Mn2RuSn (Tc = 272.1 K) due to Co substitution, which is a result of strengthening of the positive exchange interaction in this material. These materials are highly stable against heat treatment of up to 450 °C. The first-principles calculations are consistent with our experimentally observed structural and magnetic properties. They also provide insight on how the magnetic and electronic structures change when Ru is replaced with Co in Mn2RuSn.

Hf Doping Effect on Hard Magnetism of Nanocrystalline Zr

The addition of Molybdenum was used to modify the nanostructure and enhance coercivity of rare-earth-free Zr2Co11-based nanocrystalline permanent magnets. The effect of Mo addition on magnetic domain structures of melt spun nanocrystalline Zr16Co84-xMox (x = 0, 0.5, 1, 1.5, and 2.0) ribbons has been investigated. It was found that magnetic properties and local domain structures are strongly influenced by Mo doping. The coercivity of the samples increases with the increase in Mo content (x ≤ 1.5). The maximumenergy product (BH)max increases with increasing x from 0.5 MGOe for x = 0 to a maximum value of 4.2 MGOe for x = 1.5. The smallest domain size with a relatively short magnetic correlation length of 128 nm and largest root-mean-square phase shift Φrms value of 0.66° are observed for the x = 1.5. The optimal Mo addition promotes magnetic domain structure refinement and thus leads to a significant increase in coercivity and energy product in this sample.

_{18\hbox{-}{\rm x}}

Single-phase noncubic Mn-Ga films with a thickness of about 200 nm were fabricated by an in situ annealing of [Mn(x)/Ga(y)/Mn(x)]5 multilayers deposited by e-beam evaporation. Mn-Ga alloys prepared in three different compositions Mn2Ga5 and Mn2Ga were found to crystallize in the tetragonal tP14 and tP2 structures, respectively. Mn3Ga crystallizes in the hexagonal hp8 or tetragonal tI8 structures. All three alloys show substantial magnetocrystalline anisotropy between 7 and 10 Mergs/cm3. The samples show hard magnetic properties including coercivities of Mn2Ga5 and Mn2Ga about 12.0 kOe and of Mn3Ga about 13.4 kOe. The saturation magnetization and Curie temperature of Mn2Ga5, Mn2Ga, and Mn3Ga are 183 emu/cm3 and 435 K, 342 emu/cm3 and 697 K, and 151 emu/cm3 and 798 K, respectively. The samples show metallic electron transport up to room temperature.