Magnetic structures of RFexMn12 − x compounds (R = Tb and Y)

ELSEVIER

Journal of Magnetism and Magnetic Materials 196-197 (1999) 745-747

Magnetic structures of RFexMnl 2 (R = Tb and Y)

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~i~Journal of magnetism ~l~and magnetic materials

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E. Abad a, C. Piqu6 a, J.A. Blanco a'*, M. Artigas b, R. Burriel b, M.T.

Fernfindez-Diaz

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aDepartamento de Fisica, Universidad de Oviedo, 33007 Oviedo, Spain bICMA, CSIC - Universidad de Zaragoza, 50009 Zaragoza, Spain ¢Institut Laue-Langevin, 38042 Grenoble, France

Abstract

The magnetic ordering of RFexMn12-x compounds (R = Tb and Y) has been studied by powder neutron diffraction. The main coupling yields a non-collinear antiferromagnetic (AF) ordering of the transition metals (3d) sublattice, with the mean F e - M n magnetic moments much larger than in the A F RMna2 compounds. In samples with higher Fe content a 3d ferromagnetic (F) contribution is also observed, coexisting with the AF one, whose moments are significantly reduced. In TbFe6Mn6 a F component develops at low temperature on the Tb ions owing to the polarisation induced by the 3d sublattice. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Magnetic structures; 3d-4f magnetism; R(T, Mn)12-type compounds

Iron-rich ternary compounds containing rare earths (R) are of special interest because of their potential applications as permanent magnets. Among them, the family of R(T-M)i2 where T = Fe, Co; and M = Cr, Mn, Mo, Sc, Ti, V and W, has received considerable attention during the last years [1]. In particular, the study of RFexMnl z-x compounds is suitable to explore the interplay between 3d and 4f magnetism, where competing interactions (positive for Fe and negative for Mn) and magnetocrystalline anisotropy are present. Previous neutron diffraction experiments for R = Y determined the low-temperature antiferromagnetic structure and the relative occupation of the different lattice sites [2,3]. In the present paper, we extend this work to determine the temperature evolution of the magnetic moment for each site. A special interest has been paid to the effects caused on the 3d and 4f moments by increasing the F interactions with the Fe content. We have studied the temperature dependence of the neutron diffraction spectra in YFexMnl2-x (x = 4, 6 and 8) and TbFexMnl2_x (x = 4 *Corresponding author. Fax: +34-985103324; e-mail:jabr@ sauron.quimica.uniovi.es.

and 6) using the D1B diffractometer at the Institut LaueLangevin, Grenoble. Neutron diffraction data in the paramagnetic phase were obtained at room temperature showing peaks characteristic of the ThMna2-type structure (space group I4/mmm). The R ions lie at the 2a Wyckoff position and the transition metal at the 8i, 8j, and 8f positions. The Fe content and metal site occupancies, refined from the patterns recorded at room temperature, are given in Table 1. A clear preference for Fe to populate the 8f sites is observed along the series, whereas Mn shows a strong tendency towards the 8i sites. A similar metal distribution has been reported to exist in YFexMnx2-x compounds [3]. Within the accuracy of the experiment, the structural parameters are temperature independent except for the lattice constants which exhibit a non-linear thermal variation. The spectra recorded in the ordered phase show some additional reflections which can be indexed with a propagation vector k = (0, 0, 1), so that the magnetic moments related by the translation (2, i i2, 12) are antiparallel. The group theory analysis gives all the possible magnetic structures, compatible with the crystal symmetry. The

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Fig. I. Observed [points), calculated {solid linel and dil]Erence [at the bottom) neutron diffraction profiles for YFe~Mn,, and TbFe6Mn,,. The first, second, and third series of tick marks correspond to the positions of the allowed Bragg reflections: top. nuclear: middle, AF 3d component: bottom, F 3d and 4f components.

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best agreement between the observed and calculated intensities below TN (see Fig. 1} corresponds to the mode i - - A l x y + Asxy)8i -r- (A1~ - As.~.,,)si + (GI., Gs., A u . + As~.)sr, where Al.,y - S l . x - S 2 x - S3y 4- S 4 v, A and G being the usual symbols in Bertaut's notation. Therefore, the M n - F e magnetic moments lie in the basal plane of the tetragonal structure, leading to a non-collinear AF arrangement of the magnetic moments of the 3d metal sublattice. This ordering is similar to the one previously found in YFexMn12 ~ [2,3] and recently in ErFexMn12.-, [4] cornpounds. However, with increasing Fe concentration (x >~ 6), all F component in the 8j and 8f sites is necessary to be included in the refinements in order to account for the temperature variation of the spectra. This component reaches a value of 1.2 PB for YFesMn,~ and 0.6 PB for TbFe6Mn6 (see Fig. 2). Furthermore, for the latter compound, the Tb site develops a component in the basal plane (ej < 0) below 100 K, reaching 7.37 PB at 1.5 K (see Fig. 2, and Table 1) in agreement with the magnetisation measurements (see inset of Fig. 2). In TbFe4Mns this 4f component is reduced to 0.26 PB. The differences evidence the effects of the intersublattice coupling in these compounds. It is worth noting that the components of the magnetic moment in the basal plane of the tetragonal structure cannot be determined from powder diffraction due to symmetry considerations. Except for the YFesMn4 compound, the temperature dependence of the AF moments are quite similar for all

E. Abad et al. /Journal of Magnetism and Magnetic Materials 196-197 (1999) 745-747 i

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between the 3d and Tb ions are considerably stronger than those between T b - T b ions. This feature is responsible for a gradual polarisation of the Tb magnetic moments when the temperature is lowered, without any cooperative ordering, confirming our preliminary results of electrical resistivity and the heat capacity measurements [5]. The most striking behaviour is observed for YFesMn4, which presents a similar AF arrangement below TN, but the additional 3d F component is larger than those of the other compounds investigated. However, the heat capacity shows no indication of any magnetic phase transition below 350 K [5].

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We thank the Spanish CICYT for financial support (Research projects MAT96-1023-C03-03, MAT96-0448 and MAT97-0987). One of us (EA) is also grateful to F I C Y T for a graduate grant.

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Fig. 2. Temperature dependence of the ordered magnetic moments (a) AF and (b) F, for several crystallographic sites in YFe6Mn~, and TbFe6Mn6, respectively. Inset:temperature dependence of the magnetisation in TbFe6Mn6 under an applied magnetic field H = 1 kOe. the investigated compounds, showing clearly a cooperative behaviour, as observed from the heat capacity measurements [5]. In contrast, the thermal variation of the Tb magnetic moments, depicted in Fig. 2 for TbFe6Mn6, suggests that the magnetic interactions

References [1] H.S. Liu, J.M.D. Coey, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North Holland, Amsterdam, 1991, p. 1. [2] J. Deportes, D. Givord, Solid State Commun. 19 (1976) 845. [3] Y.C. Yang, B. Kebe, W.L James, J. Deportes, W. Yeon, J. Appl. Phys. 52 (1981) 2077. [4] M. Morales, M. Artigas, M. Bacmann, D. Fruchart, J.L. Soubeyroux, P. Wolfers, J. Alloys Compounds 262 & 263 (1997) 134. [5] E. Abad, C. Pique, J.A. Blanco, M. Artigas, R. Burriel, J. Magn. Magn. Mater. (1999), these Proceedings.