# Recycling of Nd-Fe-B sintered magnets into a single phase by solvothermal treatment and CaH2 reduction-diffusion

Abdelilah Chetouani, Fabienne Richomme, Jean-Marie Le Breton

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Abdelilah Chetouani, Fabienne Richomme, Jean-Marie Le Breton. Recycling of Nd-Fe-B sintered magnets into a single phase by solvothermal treatment and CaH2 reduction-diffusion. Materialia, 2024, 36, pp.102189. 10.1016/j.mtla.2024.102189. hal-04709802

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# Recycling of Nd-Fe-B sintered magnets into a single phase by solvothermal treatment and CaH₂ reduction-diffusion

A. Chetouani *, F. Richomme, J.M. Le Breton *

Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Normandie Univ, GPM UMR 6634, F-76000 Rouen, France

<figure><img src="image_2.png" /></figure>

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## ARTICLE INFO

### Keywords:

Recycling

Permanent magnets

Nd₂Fe₁₄B

## ABSTRACT

Recycling computer hard disk drives from Waste Electrical and Electronic Equipment (WEEE) has been achieved according to a two-step approach that combines solvothermal treatment and CaH₂ reduction-diffusion, resulting in the production of a single-phase material. The magnets underwent solvothermal treatment, followed by CaH₂ reduction at varying temperatures (200°C, 250°C and 300°C). We examined the impact of CaH₂ as a reducing agent on the final product, using X-ray diffraction, Mössbauer spectrometry and SQUID magnetometry. After the solvothermal treatment, the powder contains both Nd₂Fe₁₄BHₓ and Nd(OH)₃ phases. At 200°C, Nd(OH)₃ was reduced without degradation of the magnetic Nd₂Fe₁₄BHₓ phase, as confirmed by Mössbauer spectrometry. Furthermore, an analysis of the magnetic properties indicates an increase in magnetization after CaO removal, confirming its successful elimination.

## 1. Introduction

Recycling elements from end-of-life consumer goods has become a good strategy, absorbing material losses and providing secondary raw materials. Due to their excellent specific magnetic properties, rare earth permanent magnets (such as Nd-Fe-B) are used today in many industrial applications, such as electrical devices (electric motors, computer hard drives...) or even wind power [1]. They provide a compromise between the expected magnetic performance and their relative low cost compared to Sm-Co magnets. As these materials play an important role in improving the efficiency and performance of energy production devices, the demand for Nd-Fe-B magnets drastically increases. Consequently, huge amounts of rare earth elements are available in landfill waste. An environmentally friendly process is thus necessary to recycle these materials [2].

In the literature, numerous techniques, including hydrogen decrepitation [3], hydrometallurgy and pyrometallurgy [4], have been developed for recycling Nd-Fe-B magnets. Pyrometallurgical processes require high temperatures ranging from 700 to 1500°C. However, hydrometallurgical processes allow the recovery of rare earth elements at lower temperatures, the magnets being dissolved in strong mineral acids to precipitate rare earth elements as sulfates or oxalates. Hydrometallurgical processes offer significant advantages in the separation of rare

earth elements, particularly when it comes to recover individual elements. As compared to pyrometallurgy, hydrometallurgical processes are easier to implement and generate less gas pollution. Additionally, operating temperatures are typically below 110°C, thus reducing energy consumption. However, despite their advantages, hydrometallurgical processes also have certain limitations. They require the use of large amounts of chemicals, such as acids, to dissolve the Nd-Fe-B magnets. This high consumption of chemicals can lead to additional costs and environmental impacts. Another major disadvantage is the potential generation of large volumes of secondary waste during the dissolution process. These wastes must be treated to ensure appropriate residue management. This issue can lead to additional challenges in terms of costs, logistics and environmental impact [5]. Hydrogen decrepitation constitutes a promising method for the recycling of Nd-Fe-B magnets because it allows preserving the magnetic properties of the material with a low environmental impact. However, its implementation requires careful planning and appropriate management of risks associated with the use of hydrogen. In this study, we specifically focus on the solvothermal recycling method of Nd-Fe-B magnets. This approach has significant advantages, such as simplifying the process by reducing the number of necessary steps. In addition, since it operates at temperatures below 250°C, it is characterized by low energy consumption. Moreover, recycling by solvothermal treatment is applicable to all types of

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: abdelilah.chetouani@univ-rouen.fr (A. Chetouani), fabienne.richomme@univ-rouen.fr (F. Richomme), jean-marie.lebreton@univ-rouen.fr (J.M. Le Breton).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** XRD patterns of the powder from the initially fragmented HDD magnet (bottom) and of the powder treated in pure ethanol for 30 min (top). The unindexed peaks correspond to the Nd₂Fe₁₄BHₓ phase.</figcaption></figure>

magnets, including wind turbines and hybrid electric vehicles (HEV).

In this study, Nd-Fe-B magnets were submitted to a combination of a solvothermal treatment followed by a reduction diffusion (RD) process, with the aim to obtain a single-phase magnetic material. Subsequently, we applied water washing to the RD processed powder to remove CaO. We examined the influence of CaH₂ reduction-diffusion process on the microstructure of the final powder.

The preparation of recycled single phase magnetic powders could allow to obtain sintered magnetic materials free from non magnetic or soft magnetic phases. Additionally, single-phase magnetic powders could be used to make bonded magnets.

## 2. Experimental section

### 2.1. Preparation processes

The samples used for the recycling process are in the form of sintered arc segments, coated with a protective layer consisting of Ni-Cu, in order to protect them against oxidation. These magnets are extracted from computer hard disk drives (HDD) collected as WEEE.

The coating of the nickel magnets is removed using a scalpel. Then,

these magnets are fragmented in a crusher into pieces below 1 mm size. The fact of fragmenting the magnets beforehand makes it possible to accelerate the pulverization kinetics [6,7]. The powder is then put in a crystallization bomb which contains 30 ml of ethanol and heated at 225°C for 30 min. At the end of the solvothermal reaction, the precipitate is separated from the solution by centrifugation and the powder is finally dried at 50°C.

The powder resulting from the solvothermal treatment is mixed to an equal quantity of CaH₂ powder (purity 98 %). Then, it is subjected to a reduction diffusion (RD) reaction at different temperatures (200°C, 250°C and 300°C) for 1 h under argon atmosphere. CaO is then removed using water, and the remaining powder is dried at 50°C to obtain the final recycled powder.

### 2.2. Characterization techniques

X-ray powder diffraction (XRD) analysis of the samples was carried out on an Empyrean diffractometer equipped with Co Kα radiation (λKα₁ = 0.178897 nm). ⁵⁷Fe Mössbauer spectrometry analysis was carried out at room temperature using a ⁵⁷Co source in a Rh matrix to quantify the proportions of the Fe-containing phases. Magnetic measurements were performed at room temperature using a Quantum Design MPMS-XL5 SQUID magnetometer.

## 3. Results and discussion

### 3.1. Solvothermal treatment

The XRD patterns of the powder before the solvothermal treatment

<table><caption>**Table 1**
Lattice parameters of the Nd₂Fe₁₄BHₓ phase for initial and solvothermal treated powders.</caption><thead><tr><th></th><th>a (nm)</th><th>c (nm)</th><th>Unit cell Volume (nm³)</th><th>x</th></tr></thead><tbody><tr><td>Initial</td><td>0.8804</td><td>1.2207</td><td>0.946</td><td>0</td></tr><tr><td>Solvothermal</td><td>0.8835</td><td>1.2224</td><td>0.954</td><td>0.65</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 2. Room temperature Mössbauer spectra of the initial Nd-Fe-B powder (bottom) and of the powder treated in pure ethanol for 30 min (top).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Hysteresis loops at room temperature of the initial powder and of the powder treated in pure ethanol for 30 min.</figcaption></figure>

(denoted hereafter as the initial powder) and of the powder after the
solvothermal treatment are shown in Fig. 1. They reveal the predomi-
nant presence of the Nd₂Fe₁₄B peaks, accompanied by peaks associated
with Nd(OH)₃. The presence of the Nd(OH)₃ phase is due to the reaction
between the solvent and the Nd-rich phase. Structural parameters,
including lattice parameters and volume of the Nd₂Fe₁₄B structure, are
provided in Table 1. These results show that the lattice parameters of the
Nd₂Fe₁₄B phase increased after treatment. This increase is related to the
presence of hydrogen in the Nd₂Fe₁₄B phase. This is due to the hydro-
genation of the Nd₂Fe₁₄B magnetic phase upon solvothermal treatment,

which results in the insertion of H atoms in the lattice, thus leading to a
hydrogenated Nd₂Fe₁₄BHₓ phase [8]. The hydrogen content is estimated
using insights from our previous research [9], to approximately x =
0.65.

The Mössbauer spectra of the powder before and after the sol-
vothermal treatment are shown in Fig. 2. The two spectra are similar,
and were fitted with two contributions: the contribution of the magnetic
phase (Nd₂Fe₁₄B for the initial powder, and Nd₂Fe₁₄BHₓ for the sol-
vothermal treated powder) and the contribution of the Nd₁,₁Fe₄B₄
paramagnetic phase. The results of the analysis reveal for both spectra
<caption>**Table 2**</caption>

Magnetic properties of the initial powder and of the powder treated in pure ethanol for 30 min, deduced from the hysteresis loops.

<table><thead><tr><td>Parametre</td><td>Initial</td><td>Solvothermal</td></tr></thead><tbody><tr><td>M<sub>S</sub> (emu/g)</td><td>138.2</td><td>142.7</td></tr><tr><td>M<sub>R</sub> (emu/g)</td><td>58.2</td><td>15.4</td></tr><tr><td>H<sub>C</sub> (kOe)</td><td>0.4</td><td>0.31</td></tr></tbody></table>

that, (98 ± 1) % of the iron atoms are present in the magnetic phase, and (2 ± 1) % are present in the paramagnetic phase.

Mössbauer spectrometry allows to assess the potential decomposition of the magnetic phase into pure iron [10]. Here, the pure iron phase is not detected in the spectrum of the solvothermal treated powder, indicating that the proportion of iron derived from the degradation of the pure magnetic phase is very low, if not negligible, in the samples [9,

10].

The hyperfine field of the Nd₂Fe₁₄B contribution in the initial powder (28.7 ± 0.5 T) is in agreement with the literature [10,11]. The hyperfine field of the Nd₂Fe₁₄BHₓ contribution in the solvothermal treated powder (30.2 ± 0.5 T) is higher than that of Nd₂Fe₁₄B, in relation with the insertion of hydrogen atoms in the structure of the magnetic phase, in agreement with the results of the XRD analysis. This confirms that the solvothermal treatment leads to hydrogenation of the Nd₂Fe₁₄B magnetic phase.

The hysteresis loops at room temperature of both initial powder and powder treated by solvothermal route for 30 min are shown in Fig. 3. The magnetic properties, listed in Table 2, report low values of both remanent magnetisation M<sub>R</sub> and coercive field H<sub>C</sub>, due to the fact that the powders have not been aligned prior to the analysis. A slight increase in saturation magnetization M<sub>S</sub> is observed after solvothermal treatment. This is related to the hydrogenation of the Nd₂Fe₁₄B magnetic

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Pictures of the powders obtained after CaH₂ RD process and magnetic separation: dark magnetic Nd₂Fe₁₄BHₓ powder (left) and white non-magnetic Ca(OH)₂ powder (right).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** XRD pattern of the white powder obtained after magnetic separation. All the peaks correspond to the Ca(OH)₂ phase.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. XRD patterns of the recycled powder from a Nd-Fe-B magnet that underwent CaH₂ RD process at 200, 250 and 300°C followed by CaO removal and magnetic separation. The unindexed peaks correspond to the Nd₂Fe₁₄BHₓ phase.</figcaption></figure>

phase, in agreement with the increase of both lattice parameters and hyperfine field.

The XRD and Mössbauer analyses, and the magnetic measurements thus show that, after solvothermal treatment, the Nd₂Fe₁₄B magnetic phase is hydrogenated but not dissociated into pure iron, while the Nd-rich phase transforms into Nd(OH)₃. This result shows that the solvothermal treatment allows to obtain a powder containing one single magnetic phase: Nd₂Fe₁₄BHₓ (x = 0.65).

## 3.2. CaH₂ reduction-diffusion

The solvothermal treated powder was mixed with CaH₂ powder and subjected to a reduction by reaction diffusion (RD) process under a high-purity argon atmosphere for 1 h at different temperatures, 200, 250, and 300°C. The reduction reaction proceeds according to the following equation:

$$2\text{Nd(OH)}_3 + 6\text{CaH}_2 = 2\text{Nd} + 6\text{CaO} + 9\text{H}_2$$

The resulting powders were then thoroughly washed with water and subsequently with ethanol to remove any non-magnetic CaO residues. Following magnetic separation, as showed in Fig. 4, two distinct

powders were obtained: a dark magnetic powder containing the Nd₂Fe₁₄BHₓ magnetic phase and a white powder. The XRD analysis of the white powder (Fig. 5) shows that this powder consists in Ca(OH)₂. This observation further supports the hypothesis that the removal of CaO occurred in agreement with the following reaction:

$$\text{CaO} + \text{H}_2\text{O} = \text{Ca(OH)}_2$$

The XRD patterns of the solvothermal treated powder after RD process under a high-purity argon atmosphere for 1 h at 200, 250 and 300°C followed by CaO removal and magnetic separation from the Ca(OH)₂ phase are shown in Fig. 6. The peaks of the Nd(OH)₃ phase are not detected in these patterns, as shown by the comparison with the XRD pattern of the powder after solvothermal treatment. This indicates that the CaH₂ reduction-diffusion process at temperatures higher than 200°C leads to the elimination of Nd(OH)₃ from the solvothermal treated powder. The obtained powder is single phase and primarily composed of the magnetic phase.

In the sample treated at 200°C, the lattice parameters and unit cell volume of the Nd₂Fe₁₄B structure (a = 0.8837 nm, c = 1.2240 nm, V = 0.955 nm³) are the same as those measured in the solvothermal treated powder. This shows that the hydrogen content in the Nd₂Fe₁₄B phase in
<figure><img src="image_9.png" /><figcaption>Fig. 7. Room temperature Mössbauer spectra of the samples after CaH₂ RD process at 200, 250 and 300°C followed by CaO removal and magnetic separation.</figcaption></figure>

<caption>**Table 3**</caption>

Mössbauer relative intensities of the contributions used to fit the room temperature spectra of the samples after CaH₂ RD process at 200, 250 and 300°C followed by CaO removal and magnetic separation. The relative intensities of the contributions used to fit the room temperature spectra of the initial and solvothermal treated powders are given for comparison.

<table><thead><tr><th rowspan="3">Contribution</th><th colspan="4">Mössbauer relative intensities (%)</th></tr><tr><th rowspan="2">Initial (±1)</th><th rowspan="2">Solvothermal (±1)</th><th colspan="2">200°C (±1)</th><th rowspan="2">300°C (±1)</th></tr><tr><th>250°C (±1)</th><th>300°C (±1)</th></tr></thead><tbody><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>B</td><td>98</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>x</sub></td><td>—</td><td>98</td><td>98</td><td>70</td><td>48</td></tr><tr><td>Fe</td><td>—</td><td>—</td><td>—</td><td>28</td><td>50</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td></tr></tbody></table>

the sample treated at 200°C is the same as in the solvothermal treated powder. This result indicates that the CaH₂ reduction-diffusion process at 200°C does not lead to the dehydrogenation of the Nd₂Fe₁₄B phase. This is consistent with our previous investigations which showed that annealing under argon at temperatures below 300°C did not lead to complete dehydrogenation [12]. And this is consistent with the results of Williams et al. [13] and Harris et al. [14], who demonstrated the complete desorption of the magnetic phase under vacuum annealing for temperatures higher than 300°C.

To verify if the magnetic phase has undergone slight alterations, these powders were investigated by Mössbauer spectrometry. The obtained spectra are shown in Fig. 7. The Mössbauer spectrum of the sample annealed at 200°C is similar to the spectrum of the solvothermal treated powder shown in Fig. 2. This spectrum was fitted with the contribution of the Nd₂Fe₁₄BHₓ magnetic phase, and with the contribution of the Nd₁.₁Fe₄B₄ paramagnetic phase.

The Mössbauer spectra of the samples annealed at 250°C and 300°C appear different from that of the sample annealed at 200°C. They were

fitted with the contributions of the Nd₂Fe₁₄BHₓ and Nd₁.₁Fe₄B₄ phases, together with the contribution of the pure iron α-Fe phase. The results of the Mössbauer analysis are given in Table 3. They show that the proportion of the α-Fe phase in the annealed powders increase significantly with the temperature, in relation with an important degradation of the magnetic phase that dissociates into α-Fe. Thus, the formation of pure iron indicates that the heat treatment at temperatures higher than 200°C leads to the dissociation of the Nd₂Fe₁₄B phase [12,15,16,17] according to the following reaction mechanism:

$$Nd_2Fe_{14}BH_x = 2NdH_x + 12Fe + Fe_2B$$

Thus, the magnetic phase retains its integrity during CaH₂ RD process up to an annealing temperature of 200°C. The absence of a pure iron phase in this spectrum suggests that the amount of iron resulting from the degradation of the magnetic phase is below the detection threshold of Mössbauer spectrometry, corresponding to a relative intensity of 2%.

The values of the hyperfine field of the Nd₂Fe₁₄BHₓ contribution in the powders treated at 200°C, 250°C, and 300°C (29.1 ± 0.5 T, 29.4 ± 1.0 T and 29.7 ± 1.0 T, respectively) are higher than the value of the Nd₂Fe₁₄B contribution in the initial powder. This indicates that hydrogen is still present in the magnetic phase, in agreement with XRD analysis. Thus, the CaH₂ RD process and the subsequent CaO removal and magnetic separation do not affect the composition of the Nd₂Fe₁₄BHₓ hydried phase.

The hysteresis loops at room temperature of the powder annealed at 200°C before (powder containing CaO) and after CaO removal and magnetic separation (powder without CaO) are shown in Fig. 8. The results highlight an increase in saturation magnetization (M_s) after the washing process. This increase is consistent with the successful removal of non-magnetic CaO from the annealed powder. Indeed, the value of M_s increases from 79 emu/g to 142 emu/g. The mass fraction (%) of the Nd₂Fe₁₄BHₓ magnetic phase in the sample with CaO is estimated to 55.5%. Considering that the solvothermal treated powder and the CaH₂
<figure><img src="image_10.png" /><figcaption>Fig. 8. Hysteresis loops at room temperature of the sample after CaH₂ reduction-diffusion at 200°C before (with CaO) and after CaO removal and magnetic separation (without CaO).</figcaption></figure>

powder were mixed in equal proportions prior to the CaH₂ RD process, this result suggests that all Ca containing phases were removed from the mixture by magnetic separation.

## 4. Conclusions

This study achieved the successful recycling of Nd-Fe-B magnets from computer hard disk drives from WEEE through a two-step process that combines solvothermal treatment and CaH₂ reduction-diffusion, ultimately yielding a single-phase Nd₂Fe₁₄BHₓ (x = 0.65) material. Notably, the elimination of the Nd(OH)₃ phase at 200°C occurred without dissociation of the magnetic phase.

This research evidences an environmentally friendly and efficient approach to electronic waste recycling, preserving the integrity of the magnetic phase from valuable magnetic materials.

### CRediT authorship contribution statement

**A. Chetouani:** Writing – original draft, Visualization, Investigation, Formal analysis. **F. Richomme:** Validation, Formal analysis. **J.M. Le Breton:** Writing – review & editing, Supervision, Project administration, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors acknowledge the financial support from the French Agence Nationale de la Recherche and LabEx EMC3 through the project REFAIRE (Grant No. ANR-10-LABX-09-01)

### References

[1] C. Wang, Rare earth magnetic materials. Theory and Application of Rare Earth Materials, Springer, 2023, pp. 249–270.

[2] K. Binnemans, P. Jones, B. Blanpain, T. Van Gerven, Y. Yang, A. Walton, M. Buchert, Recycling of rare earths: a critical review, J. Clean. Prod. 51 (2013) 1–22.

[3] M. Zakotnik, I.R. Harris, A.J. Williams, Possible methods of recycling NdFeB-type sintered magnets using the HD/degassing process, J. Alloys Compd. 450 (2008) 525–531.

[4] M.A.R. Onal, C. Borra, M. Guo, B. Blanpain, T. Van Gerven, Hydrometallurgical recycling of NdFeB magnets: complete leaching, iron removal and electrolysis, J. Rare Earths 35 (2017) 574–584.

[5] M. Kaya, An overview of NdFeB magnets recycling technologies, Curr. Opin. Green Sustain. Chem. 46 (2024) 100884.

[6] M. Kchaw, Amélioration et développement d'un nouveau procédé de recyclage d'aimants permanents à base de terre rare, Université de Rouen Normandie, 2022. PhD thesis.

[7] M. Kchaw, V. Nachbaur, J.M. Le Breton, "A new hydro/solvothermal process for recycling Nd-Fe-B magnets", submitted for publication.

[8] J.M. Cadogan, J.M.D. Coey, Hydrogen absorption and desorption in Nd₂Fe₁₄B", Appl. Phys. Lett. 48 (1986) 442–444.

[9] J.M. Le Breton, F. Richomme, A. Chetouani, M. Kchaw, A Mössbauer investigation of the hydrogenation of the Nd₂Fe₁₄B phase during hydro/solvothermal recycling process applied to Nd-Fe-B magnets, submitted for publication.

[10] J.M. Le Breton, S. Steyaert, A Mössbauer investigation of the dissociation of the Nd₂Fe₁₄B phase, J. Phys.: Condens. Matter 11 (1999) 4941.

[11] F. Grandjean, G. Long, D. Tharp, O. Pringle, W. James, An analysis of the Mössbauer spectra of Nd₂Fe₁₄B in terms of a new model, J. Phys. Colloq. 49 (1988). C8-581-C8-582.

[12] A. Chetouani, F. Richomme, J.M. Le Breton, J.S. Thonikuzhiyil, P. Bernstein, J. Noudem, Investigation of the influence of the annealing atmosphere on the structure of powders after recycling Nd-Fe-B magnets by solvothermal process, J. Magn. Magn. Mater. (2024) 172213.

[13] A.J. Williams, P.J. McGuiness, I.R. Harris, Mass spectrometer hydrogen desorption studies on some hydried NdFeB-type alloys, J. Less Common Met. 171 (1991) 149–155.

[14] I.R. Harris, P.J. McGuiness, D.G.R. Jones, J.S. Abell, Nd-Fe-B permanent magnets: hydrogen absorption/desorption studies (HADS) on Nd₁₆Fe₇₆B₈ and Nd₂Fe₁₄B", Phys. Scr. 1987 (1987) 435.

[15] N. Maăt, V. Nachbaur, R. Lardé, J. Juraszek, J.M. Le Breton, An innovative process using only water and sodium chloride for recovering rare earth elements from
Nd-Fe-B permanent magnets found in the waste of electrical and electronic equipment", ACS Sustain. Chem. Eng. 4 (2016) 6455-6462.

[16] M. Zhang, W. Zhang, F. Chen, Y. Guo, F. Li, W. Liu, Effect of heat treatment on microstructure and magnetic properties of Ce-doped NdFeB ribbons, J. Supercond. Nov. Magn. 31 (2018) 2811-2816.

[17] P. l'Héritier, P. Chaudouet, R. Madar, A. Rouault, J.P. Sénaeur, R. Fruchart, Une nouvelle série d'hydrures métalliques ferromagnétiques de type Nd₂Fe₁₄BHₓ (0<x<5)", C.-R. séances de l'Acad. Sci. 299 (1984) 849-852.