Net-shape and crack-free production of Nd–Fe–B magnets by hot deformation

In order to reduce the amount of material waste and therefore to use the precious rare earth element Nd in a more efficient way, routes for the production of crack-free hot-deformed nanocrystalline Nd–Fe–B magnets (using melt-spun ribbons as a precursor) have been investigated. In contrast to the conventional route in which material flows into a cavity, pressing tool has been used in order to exert a back pressure during backward extrusion, leading to crack-free and net-shape production of radially oriented ring magnets, without detrimental influence on magnetic properties. Micrographs demonstrate overall good alignment of elongated platelet shaped grains with radially oriented c-axis in most parts of the ring. A mean remanence Jr = 1.27 T and coercivity μ0iHc = 1.5 T has been obtained. Degree of texture reaches around 0.7. Furthermore, die-upsetting has been performed for different degrees of deformation to obtain crack-free, mechanically and magnetically homogenous, axially oriented tablet magnets

Grain size and coercivity tuning in Nd2Fe14B-based magnets prepared by high pressure hydrogen milling

Nd2Fe14B-based permanent magnets play important role in the emerging green energy technologies due to their superb energy product (BH)max. High coercivity at elevated operating temperatures is crucial for device performance and as an extrinsic property it is largely determined by the microstructure of the magnet. In this work, we use high pressure hydrogen milling to reduce the average grain size in Nd2Fe14B powders towards the critical single domain regime to study the influence on the resultant coercivity. In addition, Nd content has been varied in a broad range to investigate how it affects the coercivity. Milling in 100 bar hydrogen enables complete decomposition of the Nd2Fe14B phase into α-Fe, NdH2 and Fe2B at nominally room temperature. A subsequent hydrogen desorption heat treatment leads to the recombination to the parent phase, now with nearly two orders of magnitude reduction in the grain size. The results show that indeed Hc peaks around the critical single domain grain size of ≈200 nm of Nd2Fe14B. Higher contents of Nd-rich grain boundary phase lead to a continuous increase in coercivity up to μ0Hc = 1.5 T, likely due to the suppression of long-range magnetostatic interactions between the nanocrystalline Nd2Fe14B grains

Increased magnetic moment induced by lattice expansion from α-Fe to α' -Fe8N

Buffer-free and epitaxial α-Fe and α' -Fe8N x thin films have been grown by RF magnetron sputtering onto MgO (100) substrates. The film thicknesses were determined with high accuracy by evaluating the Kiessig fringes of X-ray reflectometry measurements allowing a precise volume estimation. A gradual increase of the nitrogen content in the plasma led to an expansion of the iron bcc unit cell along the [001] direction resulting finally in a tetragonal distortion of about 10% corresponding to the formation of α' -Fe8N. The α-Fe lattice expansion was accompanied by an increase in magnetic moment to 2.61 ± 0.06μ B per Fe atom and a considerable increase in anisotropy. These experiments show that—without requiring any additional ordering of the nitrogen atoms—the lattice expansion of α-Fe itself is the origin of the increased magnetic moment in α'-Fe8N

Electrical and magnetic properties of hot-deformed Nd-Fe-B magnets with different DyF3 additions

The effect of deformation and DyF3 additions on the electrical resistivity and the magnetic performance has been studied in hot-deformed Nd-Fe-B melt-spun ribbons and correlated with respective microstructures. Despite the nanocrystallinity of hot-compacted magnets, the specific electrical resistivity measured by four-point-method was shown to be comparable with that of sintered magnets. Die-upsetting reduces electrical resistivity within the magnetically hard plane because of an enhanced shape anisotropy of the grains. The addition of DyF3 overcompensates this reduction due to the presence of electrically insulating Dy-F rich inclusions and thus reduces eddy current losses within the magnet. Magnetic measurements reveal an increase in coercivity without a change in remanence for die-upset magnets with a total height reduction of 63% and 1.2 wt. % Dy (1.6 wt. %DyF3). Both properties, remanence and coercivity, demonstrate an effective reduction in heavy rare earth Dy for Nd-Fe-B magnets

Diffusion processes in hot-deformed Nd-Fe-B magnets with DyF3 additions

Nd-Fe-B melt spun ribbons have been hot-compacted and subsequently die-upset together with DyF3 in order to increase coercivity in nanocrystalline hot-deformed magnets. Magnetic measurements reveal enhanced coercivities for low and reduced coercivities for high Dy-fractions. This behaviour is due to a superposition of the formation of (Dy,Nd)(2)Fe14B and non-magnetic Dy and Nd fluoride and oxide phases. Energy dispersive and wavelength dispersive X-ray elemental maps verified this feature. Heat treatments at 600 degrees C induce a strong F diffusion along the flake boundaries without inducing grain growth. This diffusion is correlated with the changes in magnetic properties

Increased magnetic moment induced by lattice expansion from alpha-Fe to alpha-Fe8N

Here we provide experimental evidence by the careful preparation of thin films with controlled nitrogen content and induced tetragonality as a consequence. We have correlated the increase of the unit cell volume in expanded bcc Fe buffer-free and epitaxial thin films with their accurately measured magnetic moment, in-plane saturation field, magnetic domain patterns and change in electronic structure by systematically changing the amount of added nitrogen, allowing a comparison to the theoretical predictions

Growth, structure, and magnetic properties of gamma '-Fe4N thin films

We have grown phase-pure, buffer-free, and epitaxial alpha-Fe and gamma'-Fe4N thin films by RE. magnetron sputtering onto MgO (100) substrates. The film thicknesses and densities have been determined with high accuracy by evaluating the Kiessig fringes of the X-ray reflectometry measurements. We have determined the volume saturation magnetization of gamma'-Fe4N with corresponding accuracy to be M-s = 1556 +/- 62 emu/cm(3) at 10K or 2.30 +/- 0.09 mu(B), per Fe atom. The Curie-temperature, T-C, of the gamma'-Fe4N thin films was 716 K. The slightly increased average magnetic moment per Fe atom of gamma'-Fe4N as compared to alpha-Fe is in agreement with literature values and attributed to the volume expansion of the unit cell