Yinfeng Zhang

Coercivity enhancement in Dy-free Nd–Fe–B sintered magnets by using Pr-Cu alloy

The grain boundary phase of Dy-free sintered Nd-Fe-B magnets is modified by using Pr68Cu32 eutectic alloy. The coercivity of the modified magnets reaches 21 kOe, which is the highest value in Dy-free Nd-Fe-B sintered magnets. Microstructural investigations show that a smooth and thick grain boundary layer is formed, and the content of the ferromagnetic elements in the grain boundary layer decreases from 65 at. % to 9 at. %. In addition, the mean grain size (4.5 μm) in the doped sintered magnets is smaller than that (6.5 μm) in the original sintered magnets. The modification in grain boundary and grain size reduces the magnetic interactions among grains and hinders nucleation of reversed magnetic domains, resulting in a coercivity enhancement.

Nitrogenation effect of Sm2Fe17 alloys prepared by strip casting technique

Sm2Fe17 alloys prepared by strip casting technique can be directly nitrided to prepare interstitial Sm2Fe17NX compounds without pre-crushing. A phenomenon of spontaneous pulverization in the Sm2Fe17 strips induced by nitrogenation is found. This kind of pulverization may arise from the inter-granular fracture along the Sm-rich grain boundary phase as well as trans-granular fracture of Sm2Fe17 primary phase. Anisotropic Sm2Fe17N2.85 powders with a remanence (Br) of 14.1 kGs, a coercive force (iHc) of 12.1 kOe, and a maximum energy product ((BH)max) of 33 MGOe can be prepared using the nitrided strips.

One step preparation of pure 𝝉-MnAl phase with high magnetization using strip casting method

Ferromagnetic phase of Mn-Al exhibits great potential in the rare-earth free permanent magnetic materials due to its high magnetocrystalline anisotropy, high magnetization, high Curie temperature and low cost. In this work, the strip casting technique was applied to prepare MnAl magnetic phase. X-ray diffraction and energy dispersive X-ray analyses indicate that the as-prepared Mn54Al46 strip sample consists of pure τ-MnAl magnetic phase. It is found that the composition of Mn54Al46 is suitable to prepare τ-MnAl phase during the strip casting process. The Mn54Al46 strip sample synthesized through the strip casting exhibits a fairly high magnetization of 114 emu/g under a field of 5 T, while the coercivity of iHc = 2.8 kOe, magnetization of M5T = 63.9 emu/g at room temperature can be obtained for Mn54Al46 powder sample. This preparation method can produce a large amount of τ-phase MnAl alloy and promote mass industrialized production.

Tunable giant exchange bias in the single-phase rare-earth–transition-metal intermetallics YMn12−xFex with highly homogenous intersublattice exchange coupling

In this paper, we have found a family of intermetallic compounds YMn12-xFex (x = 6.6-8.8) showing a bulk form of tunable giant exchange bias effect which arises from global interactions among ferromagnetic (FM) and antiferromagnetic (AFM) sublattices but not the interfacial exchange coupling or inhomogeneous magnetic clusters. A giant exchange bias with a loop shift up to 6.1 kOe has been observed in YMn4.4Fe7.6 compound with the strongest competing magnetic interactions. In a narrow temperature range, the exchange bias field shows a sudden switching off whereas the coercivity shows a sudden switching on with increasing temperature. This unique feature indicates that the inter-sublattice exchange coupling is highly homogenous, which can be perfectly interperated by our theoretical calculations.

Synergetic crystallization in a Nd2Fe14B/α-Fe nanocomposite under electron beam exposure conditions.

Nd2Fe14B/α-Fe nanocomposite magnets are prepared through electron beam exposure with a greatly reduced annealing time of 0.1 s. This is by far the most effective approach due to the effect of an extremely high heating rate featuring a rapid thermal process. The impact that the rapid thermal process has on crystallization is expounded by the introduction of the Landau model and Langevin dynamical simulations. The change of crystallization sequence from the α-Fe phase preceding the Nd2Fe14B phase under conventional annealing conditions, to synergetic crystallization under electron beam conditions is investigated. Synergetic crystallization results in more intense interaction between the α-Fe phase and the Nd2Fe14B phase in order to refine the microstructure as the fraction of Fe increases within our addition range. Improved uniformity, and shifts in the microstructure and distribution of the α-Fe phase contribute to the improvement of the magnetic properties. Compared with conventional furnace annealing ones, the magnetic properties of samples under electron beam exposure conditions are improved. For the Nd10Fe83.3B6.2Nb0.2Ga0.3 alloy, coercivity is enhanced from 4.56 kOe to 6.73 kOe, remanence ratio increases from 0.75 to 0.79, and a superior squareness of the hysteresis loop is achieved.

Preparation of Highly Textured Hydrogenation–Disproportionation–Desorption–Recombination Powders for Nd–Fe–B Sintered Magnets

As-cast Nd-Fe-B strips without an annealing treatment were treated by an appropriate hydrogenation-disproportionation- desorption-recombination (HDDR) process, and highly textured HDDR strip powders with uniform Nd-rich grain boundary layers can be obtained. The HDDR powders with a particle size of 50 μm show a Br of 1.13 T and a i Hc of 10.5 kOe. The crystal texture in the HDDR powders may come from the memory for the own crystallographic orientation and the casting texture of Nd2Fe14B columnar grains in the as-cast strips. The highly textured HDDR powders with uniform Nd-rich grain boundary layers can be used to prepare the fine-grained sintered magnets by a conventional jet-milling technique and sintering process.

Magnetostrictions of Sm

The magnetostrictions of Sm2Fe17N3 and Sm2Fe17 were investigated using strain gauge rotating-sample methods. In a magnetic field of 8 T, the magnetostriction λt (λxt = λ// - λ⊥) of Sm2Fe17 is about -155 × 10-6 at 300 K. It becomes positive after nitrogenation [Sm2Fe17N3, λt is about + 44 × 10-6 at 300 K (H = 8 T)] due to the crystal field effect and the exchange striction. It was noticed that inverse magnetostriction effect can be used to increase the magnetic alignment and remanence of the bonded magnets. As a result, transverse die-pressing (H ⊥P) was very effective in increasing the degree of magnetic alignment of Sm2Fe17N3 powders, yielding remanence higher than that obtained by axial die-pressing (H//P). By increasing the degree of magnetic alignment of powders, the maximum energy product (BH)max of Sm2Fe17N3 bonded magnet was improved by 15%.