BCMaterials Fortnightly Seminar #68: Andrés Martín & Cristina Echevarría

ANDRÉS MARTÍN

(BCMATERIALS)

Nitrogenation of (ZrNd)Fe₁₀Si₂ alloys with ThMn12 structure

Tetragonal R(Fe,M)₁₂ compounds (R = rare earth), with the ThMn₁₂ structure, have gained again interest as permanent magnet materials with reduced R content, because of the critical supply of the raw rare-earth materials occurred in recent years. However, some stability issues have to be overcome first. Addition of Zr in substitution of Nd helps to stabilize the ThMn₁₂ phase in the NdFe₁₀Si₂ alloy. While other stabilizing elements (Ti, V, Mo…) have a preference for the occupation of the 8i site in the structure, Si has a preference for the 8j and 8f sites, leaving the 8i site, which is the site with the largest moment for the ThMn₁₂ structure, occupied by Fe, so the RFe₁₀Si₂ shows higher saturation magnetization and Curie temperature than other substituted analogue. It is known that nitrogenation of ThMn₁₂ type of alloys, being N in the interstitial 2i sites of the structure) give rise to an enhancement of the Curie temperature and of the anisotropy field. Here we report for the first time the successful nitrogenation of a (ZrNd)Fe₁₀Si₂ alloy. The nitrogenation of a Si based alloy is a promising result for the development of bonded magnets due to the high saturation magnetisation of the parent alloy and the high Curie temperature and anisotropy field that can be reached after nitrogenation.

---

CRISTINA ECHEVARRÍA

(BCMATERIALS)

Synthesis of Fe-Sn metastable phases by non-standard techniques, for permanent magnets applications

The permanent magnets (PM) industry has an extreme dependence on rare earth (RE) elements that were identified by the latest European report as critical raw materials (CRM) [1]. Recently, interest has risen for the development of RE-free PM.  One of the materials considered is Fe₃Sn which has a hexagonal structure but with planar anisotropy which must be changed to uniaxial for the development of PM. This compound has absolutely no RE elements and its constituents are cheap and abundant. However, the hexagonal phase of Fe₃Sn (spatial group P6₃/mmc) is only stable in the temperature range 750-880ºC [2]. Thus, this metastable phase cannot be achieved by standard methods. Instead, it has been reported that such a compound can be obtained by solid state reaction (SSR) [3,4] or mechanical alloying (MA) [5], though in the latter case, a non-negligible percentage of Fe-bcc was still present after MA and the subsequent crystallization process, observed by Mössbauer spectroscopy [5]. Very few reports on the magnetic properties are available in literature [6]. Several Fe-Sn alloys have been synthesized by SSR, using the following procedure. First, stoichiometric amounts of the starting elements were hand-milled and then compacted into pellets using up to 10 tons at RT. Pellets were encapsulated in a quartz ampoule and heated to 800ºC for 48 hours in vacuum. This process was repeated twice in order to homogenize the composition in our sample. Structural characterization by means of XRD and SEM confirmed the purity and crystallinity of the sample, obtaining nearly a 100% of hexagonal Fe₃Sn. ⁵⁷ Fe Mössbauer spectra showed a single Fe-site of the hexagonal Fe₃Sn with a close to zero quadrupole splitting and hyperfine field B_hf = 25T, with no trace of bcc Fe (B_hf = 33T), confirming the purity of the sample. Magnetic characterization was completed by M(H) and M(T).  

1. EU 2014 Report on Critical raw materials.
2. K. von Goldbeck, IRON Binary Phase Diagrams (Springer), 139 (1982).
3. C. Sales et al., Sci. Rep. 4, 7024 (2014).
4. Giefers and M. Nicol, J. Alloys Compd. 422, 132 (2006).
5. Bansal et al., J. Appl. Phys. 76 (10), 5961 (1994).
6. Djéga-Mariadassou et al., Il Nuovo Cimento 46B, 35 (1966).