In this work, a bibliometric study was carried out to perform a scientific and technological analysis of exchange-spring magnets, an alternative permanent magnet synthesized by reducing or eliminating the use of critical raw materials, such as rare earths. The bibliometric analysis utilized the Scopus database, Orbit-Intellixir, VOSviewer, Orbit-Intelligence and Loglet Lab 4 software for maturity analysis, keyword network representations, charts and graphs for scientific articles and/or patents. A special analysis was performed on nanocomposite and thin-films systems based on Nd-Fe-B, SmCo₅ and Mn–Al–C alloys, either mixed or layered with a soft magnetic phase, where relevant information on their magnetic parameters was compilated in tables, highlighting the nanostructured systems that have been exhibited the best permanent magnet properties. The bibliometric analysis revealed that the primary production of scientific articles is concentrated in industrialized countries, and they are predominantly published in journals dedicated to magnetism. A patents analysis showed that Nissan motors is by far the main applicant, with most of its patents is focused on technological domains related to electrical machinery, apparatus, energy and metallurgy. On the other hand, the S-curve of maturity for scientific articles indicated that the study of exchange-spring magnets is entering a mature state. In contrast, patent production, following a bi-logistic model, is in a saturation stage for the second S-curve. Maturity analyses, employing S-curve, bi-logistic and multi-logistic models, were performed on nanocomposites and thin films based on Nd-Fe-B, SmCo₅ and Mn–Al–C alloys, respectively. We found that the investigation in Nd-Fe-B-based alloys is close to enter to a scientific saturation stage, while an average growth stage is observed for the SmCo₅ and Mn–Al–C-based alloys. This suggests that research on alternative magnets, capable of fulfilling technological applications where a Nd-Fe-B magnets are commonly used, is a topic of significant interest.

The rare-earth-free MnAlC alloy is currently considered a very promising candidate for permanent magnet applications due to its high anisotropy field and relatively high saturation magnetization and Curie temperature, besides being a low-cost material. In this work, we presented a simple fabrication route that allows for obtaining a magnetically enhanced bulk τ-MnAlC magnet. In the fabrication process, an electric arc-melting method was carried out to melt ingots of MnAlC alloys. A two-step solution treatment at 1200 °C and 1100 °C allowed us to synthesize a pure room-temperature ε-MnAlC ingot that completely transformed into τ-MnAlC alloy, free of secondary phases, after an annealing treatment at 550 °C for 30 min. The Rietveld refinements and magnetization measurements demonstrated that the quenched process produces a phase-segregated ε-MnAlC alloy that is formed by two types of ε-phases due to local fluctuation of the Mn. Room-temperature hysteresis loops showed that our improved τ-MnAlC alloy exhibited a remanent magnetization of 42 Am²/kg, a coercive field of 0.2 T and a maximum energy product, (BH)_max, of 6.07 kJ/m³, which is higher than those reported in previous works using a similar preparation route. Experimental evidence demonstrated that the synthesis of a pure room-temperature ε-MnAlC played an important role in the suppression of undesirable phases that deteriorate the permanent magnet properties of the τ-MnAlC. Finally, magnetic images recorded by Lorentz microscopy allowed us to observe the microstructure and magnetic domain walls of the optimized τ-MnAlC. The presence of magnetic contrasts in all the observed grains allowed us to confirm the high-quality ferromagnetic behavior of the system.

The effect of the nitrogenation process on intrinsic magnetic properties such as saturation magnetization, anisotropy field, magnetocrystalline anisotropy constant, and Curie temperature of the (Nd1-xCex)1.1Fe10CoTi (x = 0.0, 0.5 and 1.0) alloys system are analyzed. The anisotropy field increases from 1.8 to 2.8 T with the substitution of Nd by Ce. The nitrogenation process resulted in an increase of the saturation magnetization, anisotropy field, and Curie temperature. The best magnetic properties were found for the Nd1.1Fe10CoTiNy compound, for which it was estimated a saturation magnetization of 153.3 Am2kg-1, anisotropy field of 10.3 T, anisotropy constant of 5.98 MJm-3, and Curie temperature of 768.9 K. A slight reduction in mean particle size was obtained after the nitrogenation process (~5.4 to ~3.9μm).

In this work, the mechanochemical synthesis method was used for the first time to produce powders of the nanocrystalline Nd_1.1Fe₁₀CoTi compound from Nd₂O₃, Fe₂O₃, Co and TiO₂. High-energy-milled powders were heat treated at 1000 °C for 10 min to obtain the ThMn₁₂-type structure. Volume fraction of the 1:12 phase was found to be as high as 95.7% with 4.3% of a bcc phase also present. The nitrogenation process of the sample was carried out at 350 °C during 3, 6, 9 and 12 h using a static pressure of 80 kPa of N₂. The magnetic properties M_r, µ₀H_c, and (BH)_max were enhanced after nitrogenation, despite finding some residual nitrogen-free 1:12 phase. The magnetic values of a nitrogenated sample after 3 h were M_r = 75 Am² kg^–1, µ₀H_c = 0.500 T and (BH)_max = 58 kJ·m^–3. Samples were aligned under an applied field of 2 T after washing and were measured in a direction parallel to the applied field. The best value of (BH)_max ~ 114 kJ·m^–3 was obtained for 3 h and the highest µ₀H_c = 0.518 T for 6 h nitrogenation. SEM characterization revealed that the particles have a mean particle size around 360 nm and a rounded shape.

In this study, powders of the $\tau $ –Mn53.3Al45C1.7 hard magnetic phase were mixed and sintered with powders of the three soft magnetic phases, Fe58.5Co31.5X10 (X = Ti, Zr, and Ce), in a relation of 98/2 wt.%. The soft Fe58.5Co31.5X10 magnetic phases primarily exhibit an $\alpha $ –FeCo crystalline phase. The Fe58.5 Co31.5 Zr10 and Fe58.5Co31.5Ce10 samples contain the FeCoZr and Ce2 (Fe,Co)17 phases, respectively. Mössbauer spectroscopy (MS) analysis of the soft magnetic phases was carried out using sextets and quadrupole splitting distributions (QSDs). In addition, a small hyperfine field distribution (HFD) was performed only for Fe58.5Co31.5Zr10. The majority sextet with a mean hyperfine field of 36.4 T was associated with the $\alpha $ –FeCo phase, the most likely quadrupole splitting below 1.2 mm/s was associated with Fe $^{+3}$ , and those more significant than this value were associated with Fe $^{+2}$ . The hysteresis loops confirm the soft magnetic behavior, with saturation magnetization values of approximately 150 Am2kg-1. The $\tau $ –Mn53.3Al45C1.7 phase reveals the presence of the $\tau $ (84 wt.%) and $\beta $ (16 wt.%) crystalline phases, along with magnetic properties such as magnetization at 1.8 T of 38.2 Am2kg-1, coercive field of 0.377 T, and maximum energy product of 3.6 kJm-3. The mixtures of both phases exhibit the soft magnetic phase $\alpha $ –FeCo and $\tau $ and $\beta $ phases of the Mn-Al-C system. Among the three obtained mixed and sintered samples, the one mixed with Fe58.5Co31.5Zr10 demonstrates the best magnetic properties, with magnetization reaching 58.1 Am2kg-1, a coercive field of 0.361 T, and a maximum energy product of 7.4 kJm-3. All these magnetic properties surpass those of the $\tau $ –Mn53.3Al45C1.7 phase, except for the coercive field.