Single crystal magnetisation of UFe10Mo2

LETTER TO THE EDITOR

~14 journalof magnetism and magnetic materials

N ELSEVIER

Journal of Magnetism and Magnetic Materials 167 (1997) L185-L188

Letter to the Editor

Single crystal magnetisation of UFel0Mo 2 P. Estrela a, A.P. Gon~alves

b

M. Godinho a,*, M. Almeida

b,

J.C. Spirlet c

a Departamento de Fisica, Faculdade de Ci~ncias da Universidade de Lisboa, Campo Grande ed. C1, P-1700 Lisboa, Portugal b Departamento de Qulmica, Instituto Tecnoldgico e Nuclear, P-2686 Sacav£m Codex, Portugal European Commission, Joint Research Centre, Institute of Transuranium Elements, Pos(fach 2340, D-76125 Karlsruhe, Germany

Received 25 July 1996; revised 12 December 1996

Abstract

Magnetisation measurements have been performed for different directions on aligned UFemMo 2 single crystals. The results confirm a basal plane anisotropy and suggest an important magnetic contribution from the uranium sublattice. PACS: 75.50.Ww; 75.30.Gw Keywords: Magnetic anisotropy; Intermetallics; Magnetic structure

1. I n t r o d u c t i o n

UFel0MO 2 has a tetragonal ThMn12-type structure (space group I 4 / m m m ) [1] and presents a ferromagnetic-type transition at 198K, as recently reported by us in a study by single-crystal X-ray diffraction, 57Fe Mtissbauer spectroscopy and magnetisation in polycrystalline samples [2]. In this compound the Mo atoms were found randomly in the 8i positions and the magnetisation studies showed, at low temperatures, a spontaneous magnetisation for free powder samples of M 0 = 8.8 ~ B / f . u . , a value relatively low when compared with the isostructural compounds AFel0T 2 (A = actinide, lanthanide or Y; T = transition element). A basal plane type of magnetic anisotropy was derived from the comparison

between measurements on a powder free to rotate in the applied field and on a fixed powder. The low Curie temperature and magnetisation values were related to the fact that the Mo atoms occupy the 8i positions which are mainly responsible for the saturation magnetisation and uniaxial anisotropy of the iron sublattice. The role of the uranium atoms in the total magnetisation remained unclear. In this work we report a study of the magnetic anisotropy of UFel0Mo 2 single crystals.

2. E x p e r i m e n t a l details

UFel0Mo 2 was obtained as a polycrystalline ma~ 1200°C a polyphasic UFe9.sMOl. 5 sample, prepared by melting the pure elements under vacuum as described before [2]. Selected crystals typically with 1.1 × 0.4 × 0.3 m m 3 were removed from this polycrystalline material by annealing for 10 days at

* Corresponding author. Fax: m.godinho @cc.fc.ul.pt,

+351-1-757-3619;

email:

0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 6 ) 0 0 7 6 0 - 3

LETTER TO THE EDITOR

P. Estrela et aL / Journal of Magnetism and Magnetic Materials 167 (1997) L185-L188

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terial and used in magnetisation studies; a X-ray diffraction analysis confirmed the isolated grains as UFel0Mo 2 single crystals. Magnetisation measurements were performed on aligned single crystals in the temperature range 5 300K under magnetic fields up to 5.5T, using a SQUID magnetometer which allows the simultaneous detection of the longitudinal ([IH) and transverse ( _LH ) components of the total magnetisation.

the crystal around the field direction, showed a maximum in the direction corresponding to the baxis, confirming this axis as another easy direction of magnetisation. The magnetisation curves along the easy axis a show a ferromagnetic behaviour reaching saturation for relatively low fields, while the hard direction curves show no saturation for fields up to 5.5 T. The saturation magnetisation, M s, decreases from 9.51xB/f.u. at 5 K to 6.5 p~B/f.u, at 150K. At 5 K a small hysteretic behaviour is observed with a remanence of 2.2 O~B/f.u. and a coercive field of 280 Oe. At low fields and for the lowest temperatures, the easy direction M(H) curves present an irregular behaviour (Fig. 2a) which corresponds to a much lower initial susceptibility for temperatures T < 20 K. The change of regime for fields lower than 0.1 T is clearly seen from the derivative (dM/dH) of the magnetisation curves (Fig. 2b). This behaviour is most probably a consequence of the basal plane character of the anisotropy, enhanced by a small misalignment of the a-axis with respect to the ap-

3. Results and discussion

The single-crystal magnetisation results along several crystallographic axes confirmed the basal type of anisotropy. The a -axis is an easy direction of magnetisation, as can be seen from the comparison of the M(H) curves for H 1]a and H II c (Fig. 1). In the first case, the magnetisation values are higher for the whole range of temperatures and applied magnetic fields. For the magnetic field applied along the a direction, the transverse magnetisation ( M 5_ H ) measured along a complete rotation of

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LETTER TO THE EDITOR

P. Estrela et al. / Journal of Magnetism and Magnetic Materials 167 (1997) L185-L188

(a)

plied field H . In fact, considering the a- and b-axis as the only easy directions o f magnetisation, there is a critical value of the applied field for which the domains oriented along the perpendicular direction (b ± H ) will start to rotate towards the easy a-axis. This critical field corresponds to the field for which the m a x i m u m in the d M / d H curves (Fig. 2b) occurs and decays exponentially with the measurement temperature. F r o m the temperature dependence of the magnetisation after zero field cooling and field cooling (Fig. 3), a Curie temperature Tc = 2 0 0 ( 2 ) K was obtained in good agreement with the previous polycrystalline data [2]. For higher temperatures, a C u r i e - W e i s s behaviour is observed with a paramagnetic temperature 0 = 2 0 5 K and a Curie constant of 3.7 × 10 -~ e m u K / g O e for H = 50Oe, corresponding to an effective B o h r m a g n e t o n n u m b e r /z~ff = 17.1 ixB/f.u. F r o m our previous M/Sssbaner results on a polycrystalline sample [2] and assuming an average hyperfine field conversion factor of 1 1 . 9 T / t x B based on recent neutron and 57Fe M~Sssbauer results obtained for the UFel0Si 2 [3] compound with the same crystallographic struture and similar lattice constants, the iron contribution for the total magnetisation is

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T(K) Fig. 3. Temperature dependence of the magnetisation of UFel0MO2 after zero-field cooling (open symbols) and after field cooling (solid symbols) for a magnetic field of 0.1 T applied along the a- and c-axis.

LETTER TO THE EDITOR

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P. Estrela et aL / Journal of Magnetism and Magnetic Materials 167 (1997) L185-L188

estimated as 6.7 txs/f.u, at 80K. Compared to the reported M5ssbaner results for the isostructural compound YFel0Mo 2 ( T c = 360K) [4], in which the iron atoms have an average magnetic moment /.AFe 1.3 txB, UFel0MO 2 shows a drastic reduction of about 0.6 txB/Fe for the iron moments. This moment reduction could be responsible for weaker exchange interactions and therefore a lower Tc value. The magnetisation value of 6.7 p~B/f.u, obtained for the Fe sublattice from the M~Sssbauer results should be compared with the saturation value of 8.5 ~B/f.u. obtained at the same temperature ( T = 80K) from single-crystal magnetisation measurements. This comparison strongly suggests a ferromagnetic contribution of the uranium sublattice ( = 1.8 IxB/U) to the magnetic structure of the compound UFe~0Mo 2. In conclusion, these single-crystal measurements confirm the basal plane type of the anisotropy in UFeloMO2, and indicate an important ferromagnetic contribution of the U moments to the magnetic structure of this compound. =

Acknowledgements This work was partially suported by JNICT (Portugal) under contract PRAXIS/ 3 / 3 . 1 / F I S / 2 9 / 9 4 and by N A T O through Collaborative Research Grant No. 920996.

References [1] W. Suski, A. Baran and T. Mydlarz, Phys. Lett. A 136 (1989) 89. [2] A.P. Gonqalves, P. Estrela, J.C. Waerenborgh, M. Godinho, M. Almeida and J.C. Spirlet, J. Alloys Compounds 218 (1995) 183. [3] P. Estrela, M. Godinho, A.P. Gon~alves, M. Almeida and J.C. Spirlet, J. Alloys Compounds230 (1995) 35; J.C Waerenborg, J.A. Paix~o, M.S. Rogalski, A.P. Gonqalves and M. Almeida, to be published. [4] C. Christides, A. Kostikas, G. Zouganelis, V. Psycharis, X.C. Kou and R. GfiSssinger, Phys. Rev. B 47 (1993) 11220.