We report on the magnetic properties of NdFeB powders produced by gas atomization, which is a powder manufacturing technology scarcely used in the past to produce such alloys. Using this technique, we have produced several ternary NdFeB alloys with Nd contents between 26.9 wt.% and 28.5 wt.%. The as-atomized powders were split into different size fractions by sieving. Subsequently, we measured the magnetic properties as a function of temperature, between 10 and 400 K, and particle size. The magnetic behavior depends strongly on the microstructure of the material, which in turn is determined by the particle size. It is reported a slope anomaly in the curve of magnetization as a function of temperature at around 150 K due to a spin-reorientation transition. Since gas-atomized powders are isotropic, this magnetic transition produces an increment of the magnetization below this temperature.

In this paper, we report the effect of thermal treatment at different temperatures (523, 623, and 723 K) on the magnetic properties of an amorphous powder with composition Fe72.5Si12.5B15 obtained by gas atomization. Differential scanning calorimetry curves indicate that a significant structural rearrangement takes place during annealing before crystallization. As expected, the intrinsic coercivity decreases with the annealing temperature, being the most significant drop from 3.26 to 0.44 Oe for the powder with a particle size <20 μm, non-annealed, and annealed at 723 K, respectively. An analysis of the effect of annealing on the anisotropy field distribution obtained directly from the magnetization curve is presented for both the non-annealed and the annealed sample with the lowest value of coercive field. The probability density function of the anisotropy field exhibits a lower applied field value at the maximum and is better fit by a Gaussian distribution after the heat treatment, which is a sign of better amorphicity after structural relaxation.

Present empirical correlations to predict the median particle size of water atomised powders have a validity restricted to a particular atomiser and alloy family. This work proposes a mathematical function that takes into account the influence of the heat transfer coefficient and, therefore, of the solidification time on the median particle size. This equation is applied in combination with previously proposed empirical correlations to extend their validity to a broader range of alloys. Experiments were conducted with alloys of different melting point (Fe base, Cu base and Sn). Quantitative measurements of the median particle size, tap density and several shape factors, and qualitative observations of the particle shape confirmed the importance of the heat transfer rate. It is shown that the inclusion of the solidification time effect results in a better agreement between calculated and experimental data when both low and high melting temperature alloys are taken together.