MAGNETIC BEHAVIOUR OF Y(Fe, Ir)2

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MAGNETIC BEHAVIOUR OF Y(Fe, Ir)2

A. van der Kraan, P. Gubbens, K. Buschow

To cite this version:

A. van der Kraan, P. Gubbens, K. Buschow. MAGNETIC BEHAVIOUR OF Y(Fe, Ir)2. Journal de

Physique Colloques, 1979, 40 (C2), pp.C2-190-C2-192. �10.1051/jphyscol:1979267�. �jpa-00218665�

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MAGNETIC BEHAVIOUR OF Y(Fe/Ir)2

A.M. van der Kraan, P.CM. Gubbens and K.H.J. Buschow Interuniversitair Reactor Instituut, Delft, the Netherlands

^Philips Research Laboratories, Eindhoven, the Netherlands

Abstract.- Magnetization as well as magnetic hyperfine field measurements of the intermetallic compounds Y(Fe,_xIr ) are reported. For x > 0.6 no magnetic moment has been observed while a hyperfine field at the Fe nuclei is still measured. This phenomenon can be explained when the magnetic moments are not oriented parallel because the anisotropy energy between the easy directions of magnetization has been increased by increasing Ir concentration.

The last years we have studied the possible origin of the magnetic behaviour of the cubic Laves phase compounds of stoichiometry RB2 formed between the rare earth or yttrium (R) and 3d-transition me- tals (B). Previously we have reported Mossbauer stu- dies of the pseudobinary systems Ho(Fe,Co)2, Y(Fe,Co)2 and Y(Fe,Al>2 /1-4/. Now we have studied the system Y(Fe, Ir)2- YIX2 is a paramagnet just as YC02 and YAI2, however Ir is a 5d-metal, Co a 3d-metal and Al has no d-electrons at all.

The intermetallic compounds were prepared by arc melting in an argon atmosphere. The samples were vacuum annealed for several weeks at 900°C in a sin- tered AI2O3 crucible.

Mossbauer experiments as well as magnetization measurements have been performed on eleven different samples of Y(Fe1_xIrx)2. The samples with x = 0.40, 0.65, 0.76 and 0.90 are enriched in 57Fe up to 12, 14, 21 and 95% respectively.

In figure 1 the mean magnetic moment per Fe atom, deduced from the magnetization measurements, supposing that Ir has no magnetic moment, is given as a function of the composition. In this figure it is shown that the mean magnetic moment at a Fe atom remains constant upon substitution of Fe by Ir up to 25% Ir. For higher Ir concentrations the mean magnetic moment is strongly decreasing so that at x ™ 0.6 it is nearly vanished. In figure 2 the Mossbauer spectra of some Y(Ie Irx)2 samples measured at T = 2 K are given. From the measured spectra it follows that both the width of the hyperfine lines and the effective hyperfine field slightly increase by increasing Ir concentrations up to 25% Ir and

that at higher concentrations the effective hyperfine field decreases. The determined mean effective field is plotted in figure 3 as a function of the Ir concent rat ion.

The spectrum of YFe2 consists of two six-line hyperfine patterns with an intensity ratio of 1:3 respectively, which is consistent with an easy direction of magnetization along one of the Jj 1 f^\

directions. Whenever Ir is substituted for Fe up to 50% essentially a one six-line pattern is found only, indicating that the easy direction of magnetization is along one of the [00TJ directions. The observed Mossbauer spectra for 0<xg0.50 can be explained by a model in which the moment of a Fe atom only depends upon its six nearest and twelve next nearest neighbours. It appears that the influence of a next nearest neighbour is approximately 1/3 of the influence of a nearest neighbour. This ratio has also been found in the Y(Fe,Al)2 system /3/. However Fig. 1 : The mean magnetic moment per Fe atom as a function of the Ir concentration in Y(Fej I rx)2. JOURNAL DE PHYSIQUE Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-190

Résumé.- Des mesures de l'aimantation ainsi que des champs hyperfins dans les composés intermétalli- ques Y(Fej_xIrx)2 sont présentées. Pour x > 0,6 on n'a pas observé de moment magnétique, bien qu'un champ hyperfin soit toujours mesuré aux noyaux du Fe. Ce phénomène s'explique quand les moments ma- gnétiques ne sont plus parallèles, par une augmentation de l'énergie d'anisotropie entre les directions faciles d'aimantation en fonction de la concentration en Ir.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979267

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the difference is that in the Y(Fe,Ir)2 system the effective hyperfine field at a Fe nucleus first in- creases up to 10% due to 7 112% per Ir in the nearest and 2 112% per Ir in the next nearest neighbour shell before it decreases by the same amount per Ir atom in the different neighbouring shells.

Fig. 2 : Mgssbauer spectra of some Y(Fel-,Irx)2 compounds at T = 2 K.

Fig. 3 : Variation of the iron hyperfine field He with : o Ir concentration in Y(Fel-xIrx)2 at T = 2K;

Co concentration in Y(Fel-xC~,), at T =

1.7 K taken from /2,4/.

In the Y(Fe,A1)2 system the effective hyperfine field only decreases upon the substitution of iron

by aluminium, while the influence of an A1 atom in one of the two neighbouring shells is twice as much as the influence of an Ir atom.

For x 30.60 the observed ~gssbauer spectra in- dicate (see Fig. 2) that the easy direction of magnetization is along one of the 1 1

]

directions as in YFe2. Furthermore for 0.50 < x < 0.60 a decrease of about 20 kOe in the mean effective hyperfine field has been found and a different concentration dependence of the field (see Fig. 3). It appears that for x r/ 0.60 the magnitude of the fields are lying on the straight line which connects the values of the iron hyperfine field in YFe2 and that in Y(Feo.o22Coo.s78), 141. As a Co atom in YCo2 has no magnetic moment, the straight line can be considered as the concentration dependence of the field due to the dilution of the Fe atoms only. From this consi- deration and the concentration dependence of the

field observed in the Y(Fel-xIrx)2 compounds, it follows that apparently for x < 0.60 an Ir atom gets a magnetic moment most likely induced by the surrounding iron moments. This picture is consistent with the change in the easy direction of magnetization with concentration. In the range where Ir has no induced moment this direction is determined by the Fe atoms only, giving rise to a

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easy axis of magnetization. The effect of an induced moment at the substituent due to the surrounding iron moments and a change of the easy direction of magnetization have also been found in the Y(Fel-xCox)2 system /2,4/.

In summary the observed behaviour of the hyperfine field in the Y(Fe,IrI2 system is like that in the Y (Fe,Co) system, although the influence of Co is more pronounced than that of Ir.

A comparison between the results of the magnetization measurements and the ~Gssbauer effect study as given in the figures 1 and 3 respectively, sug- gests that not only the magnetic moment at the Fe atoms is changing upon Ir substitution, but that the magnetic moments order differently also. The appa- rent discrepancy between the composition dependence of the mean magnetic moment per Fe atom observed by magnetization measurements and the mean effective

field determined by Gssbauer spectrometry might be attributed to an increasing magnetocrystalline anisotropy with increasing Ir concentration. In order to verify this suggestion, we have measured a Mijss- bauer spectrum of the sample Y(Feo.3sIro.6~)2 at T = 4.2 K in an external magnetic field of 50 kOe parallel to the y-ray beam. As no significant change in shape of the spectrum has been observed, the

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C2-192 ^JOURNAL^DE^PHYSIQUE

m a g n e t o c r y s t a l l i n e a n i s o t r o p y h a s t o b e much l a r g e r t h a n 5 0 kOe, w h i l e i n p u r e YFe2 t h e a n i s o t r o p y i s much l e s s t h a n 3 kOe. I n t h e s u b s t i t u t e d compound i t seems t h a t t h e magnetic moments a t t h e Fe atoms a r e n o t o r i e n t e d p a r a l l e l b e c a u s e m a g n e t i c a l l y t h e 8 p o s s i b l e

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d i r e c t i o n s have become d i f f e r e n t

.

Such a magnetic b e h a v i o u r have a l s o been found i n t h e Y(Fe,A1)2 compounds 131.

R e f e r e n c e s

/ I / Van d e r Kraan, A.M. and Gubbens, P.C.M., J . Phys.

C6 (1974) 469.

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/ 2 / ~ u i j p e n , M.G., Gubbens, P.C.M., Van d e r Kraan,

A.M., and Buschow, K . H . J . , P h y s i c a 86-88B (1977) 141.

/ 3 / Van d e r Kraan, A.M., Gubbens, P.C.M. and Buschow, K . H . J . , P r o c . I n t . Conf. M6ssb. S p e c t r . Bucharest, Romania (1977) 121.

/ 4 / Corson, M.R., Kolk, B . , Hoy, G . , Zimmerman, G.O., Van d e r Kraan, A.M. and Gubbens, P.C.M., Hyp.

I n t . (1978) 411.