Rare-earth iron-based intermetallic compounds and their carbides: Structure and magnetic behaviors

doi:10.1016/j.jmmm.2009.08.047

Journal of Magnetism and Magnetic Materials

Volume 322, Issue 2, January 2010, Pages 224-229

https://doi.org/10.1016/j.jmmm.2009.08.047 Get rights and content

Abstract

The structural and magnetic properties of ${Sm}_{2} {Fe}_{17 - x} {Mo}_{x}$ , its out of equilibrium precursor ${SmFe}_{9 - y} {Mo}_{y}$ and their carbides are investigated by means of powder X-ray diffraction, magnetic measurements and high-resolution transmission electron microscopy. The structure of the nanocrystalline ${Sm}_{1 - s} (Fe, Mo)_{5 + 2 s}$ alloys are governed by the s Sm vacancy rate and the x amount of Mo. In this work, the Rietveld analysis shows that for $s = 0.33$ the structure is $R \bar{3} m$ of ${Th}_{2} {Zn}_{17}$ -type and $P 6 / mmm$ with the stoichiometry $1 / 9$ for $s = 0.36$ . Moreover, it points out a lattice expansion along the c-axis after Mo substitution for Fe and reveals a $6 c$ preferential occupation for Mo atoms in $R \bar{3} m$ structure, and $2 e$ for the out of equilibrium hexagonal precursor. Upon carbon insertion, the lattice expansion favors the Curie temperature increase up to 35% for ${Sm}_{2} {Fe}_{16.42} {Mo}_{0.58} C_{2}$ and 49% for the metastable ${SmFe}_{8.7} {Mo}_{0.3} C$ carbide. The Curie temperature and the coercive field of the $1 / 9$ carbides are higher than those of $2 / 17$ ones. This makes the out of equilibrium hexagonal precursor a competitive candidate for use as permanent magnet.

Introduction

With the advent of high-energy product Nd–Fe–B permanent magnets, research and development of new permanent magnetic materials have mainly focused on rare earth based alloys. A very great interest appeared towards the magnetic compounds of $R_{2} {Fe}_{17}$ insertion following the discovery by Coey of this new family [1].

Nevertheless, the rare-earth iron-rich $R_{2} {Fe}_{17}$ intermetallic compounds are not the most suitable for permanent magnets since they exhibit plane anisotropy and a low Curie temperature [2]. In order to make those series of samples more promising candidate for permanent magnets, partial substitutions by a non-magnetic element for Fe were achieved. As a result, the sublattice planar anisotropy of Fe is reduced in favor of the rare earth sublattice anisotropy, which is axial for Sm. Moreover, the magnetic properties of the compounds are improved drastically by interstitial insertion of light elements such as carbon [3] whose thermal stability is better than the nitrides [4], [5], leading to the resurgence of the interest in the investigation of $R_{2} {Fe}_{17}$ compounds and their substitutional $R_{2} {Fe}_{17 - x} M_{x}$ , as well as their metastable phases. The substitution of Fe by Al or Ga leads to a change of the system anisotropy from planar to uniaxial [6], [7], [8]. A Curie temperature enhancement was observed depending on both, M and x amount containing compounds [9] improved by nitrogen or carbon insertion [10].

In the present work, we will focus attention on the substitution effect of molybdenum (Mo) in ${Sm}_{2} {Fe}_{17}$ system. Such a substitution was theoretically predicted by atomistic simulation [11] but has never been obtained experimentally.

Herein, investigations on structural, intrinsic and extrinsic properties is presented. In the quest for more interesting properties [12], the out of equilibrium precursor phase is studied. Before this present work, no one has invoked the ${Sm}_{2} (Fe, Mo)_{17}$ nor $Sm (Fe, Mo)_{9}$ phase. Investigations are undertaken to check their optimal preparation condition as well as their carbides.

Section snippets

Experiment

Various alloys with nominal composition of ${Sm}_{2} {Fe}_{17 - x} {Mo}_{x}$ ( $x = 0.3$ , 0.38, 0.45 and 0.58) were elaborated by high-energy milling technique. A homogeneous ${Sm}_{2} {Fe}_{17}$ prealloy, samarium excess (99.9%) and pure Mo (99.9%) powders were used. The powders were subsequently annealed during 30 min from $T_{A} = 750^{\circ} C$ up to $1190^{\circ} C$ . Carbonation was achieved by reacting annealed samples [13] with appropriate amount of $C_{14} H_{10}$ powders at $420^{\circ} C$ .

X-ray diffraction was carried out with $CuK α$ radiation $(λ = 1.54178 \overset{˚}{A})$ on a Bruker

Structure analysis

Fig. 1(a) shows the X-ray diagrams of ${Sm}_{2} (Fe, Mo)_{17}$ sample. The results of the structure refinement performed for the $2 / 17$ alloys show the presence of the main phase (around 97.64%) with the rhombohedral $R \bar{3} m {Th}_{2} {Zn}_{17}$ -type structure. We note minor quantities of ${Sm}_{2} O_{3}$ (0.08%), ${SmO}_{6}$ (2.10%) and SmON (0.18%).

With increasing Mo content, the value of the unit-cell parameter c rises, reaching a certain limit, while the parameter a remains constant, leading to an expansion of the cell volume along the c

Conclusion

The present study illustrates, for the first time, that ${Sm}_{2} {Fe}_{17 - x} {Mo}_{x}$ alloys and the out of equilibrium hexagonal $P 6 / mmm$ phase $Sm (Fe, Mo)_{9}$ can be formed by mechanical alloying technique. The carbon transfer was performed successfully by means of a solid–solid reaction with $C_{14} H_{10}$ powders.

It was demonstrated that Fe substitution by Mo leads to an increase of the c unit cell parameter while a remains quite constant. So, an enlargement along c-axis of the unit cell volume occurs. The Rietveld

Acknowledgments

This work was main supported by the CNRS and the “Ministère de l’Enseignement Supérieur, de la Recherche Scientifique et de la Technologie” (LAB MA03) (Tunisia). The authors acknowledge the French-Tunisian Cooperation CMCU (project 06/S 1309).

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Rare-earth iron-based intermetallic compounds and their carbides: Structure and magnetic behaviors

Abstract

Introduction

Section snippets

Experiment

Structure analysis

Conclusion

Acknowledgments

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

J. Alloys Compd.

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

J. Alloys Compd.

J. Magn. Magn. Mater.

J. Alloys Compd.

J. Magn. Magn. Mater.

J. Alloys Compd.