EXPERIMENTAL EVIDENCES OF MAGNETIC CLUSTERS IN AN AMORPHOUS Fe0.34Sn0.66 ALLOYS

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EXPERIMENTAL EVIDENCES OF MAGNETIC CLUSTERS IN AN AMORPHOUS Fe0.34Sn0.66

ALLOYS

S. Nikolov, M. Piecuch, G. Marchal, Chr. Janot

To cite this version:

S. Nikolov, M. Piecuch, G. Marchal, Chr. Janot. EXPERIMENTAL EVIDENCES OF MAGNETIC CLUSTERS IN AN AMORPHOUS Fe0.34Sn0.66 ALLOYS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-666-C8-669. �10.1051/jphyscol:19808167�. �jpa-00220268�

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JOURNAL DE PHYSIQUE CoZZoque C8, supptdment au n08, Tome 41, aoiit 1980, page C8-666

EXPERIMENTAL EVIDENCES OF MAWETIC CLUSTERS IN AN AMCRPHCUS Fe0.34Sn066RLLOYS

S. nikolov, ?I. Piecuch, G . Marchal and Chr. Janot

Laboratoire de Physique du SoZide (LA 155), FacuZtS des Sciences - C.O. 140 54037 Vancy Cedex, France.

Abstract.- Using magnetization measurements and Mijssbauer spectroscopy i n v e s t i g a t i o n , i r o n - t i n amorphous f i l m s with t h e composition near t o FeSn2 have been s t u d i e d . Their magnetic behaviour, f a r from t h a t of a ferromagnet, has been i n t e r p r e t e d i n terms of s i z e d i s t r i b u t e d magnetic c l u s t e r s experiencing strong l o c a l a n i s o t r o p y which r e s u l t s i n spin-glass-like s t a t e s .

1 - INTRODUCTION.

Systematic studies of FexXl-, amorphous materials in which X is a IV group element have been camed out in the past few years [ I ] [2] [3]. They all exhibit a critical composition, near xcr = 0.40, for the appearance of a ferromagnetic order [2] despite of detail differences in their magnetic behaviour and in the magnetic properties of the corresponding equilibrium phases (FeSi does not show any magnetic order while FeSn and even FeSn2 are typical antiferromagnets). In the amorphous state the FexSnl-, alloys are mostly ferromagnetic down to the concentration x 1.0.50 131. However some local configurations including a relative large number of tin atoms might results in antiferromagnetic iron coupling through superexchange mecha- nism via tin atoms, as observed in some of the tin-iron equilibrium phases. Thus it is the purpose of this paper to report magnetic investigations in amorphous Fe0.34Sn0.66 films, near t o the composition of the antiferromagnetic FeSn2 equilibrium phase [4] and with x < xcr, having 'in mind that magnetic behaviour and chemical short range order may be strongly related.

2 - EXPERIMENTAL PROCEDURE.

The Fe0.34Sn0.66 amorphous fdms have been prepa- red using a vapour deposition technique as described else where in details [S].

Magnetization measurements have been performed with a vibrating Foner magnetometer up to 20 kOe at T = 4.2 K.

Typical data are shown in figure 1 in the form o f a f m t magnetization curve and an hysteresis loop. Obvious- ly saturation states have not been reached under 2 0 kOe and the maximum magnetization corresponds t o a mean magnetic moment of less than ji = 0.02 p g l i r o n atom.

On the other hand Mossbauer spectra have been recorded at temperatures ranging from 4.2 K to 20 K with an applied external magnetic field varylng from HeXt = 0 to 60 kOe. Typical zero field spectra are shown in figure 2 while the influence of an external field is illustrated in fi- gures 3 and 4. The main features as deduced from the analysis of these spectra are the following :

Fig. 1. - Typical magnetization data as measured on Fe0.34Sn0.66 amorphous alloys at T = 1.2 K

Fig. 2. - Typical Mossbauer spectra in the zero field condition recorded at different ternperarures.

f a l 4 . 2 . K f b l 6 K (cl 7 K f d l 1 0 K f e l 2 0 K

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808167

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Fig. 3.

-

M6ssbauer spectra a t T = 4.2 K with an applied external magnetic field.

(a) I 0 kOe f b ) 20 kOe f c ) 30 kOe ( d l 40 kOe f e ) j0 kOe f f ) 60 kOe (i) In the zero field condition (fig. 2) and T 2 7 K spectra are quadrupolar patterns typical of ccnon-rnapnetic,, atoms.

(ii) Again with Hext = 0 but T ,< 7 K, the quadrupolar have a noticeable broaden base due to unresolved magnetic contributions, When andysed in terms of conti- nuous distributions of randomly oriented hyperfine magnetic field P(H) and quadrupo1ar.effect P(EQ) the 4.2 K

spectrum is well fitted with the probability curves shown in figure 5.

According to the respective weights of P(EQ) and P(H) 70 % of the iron atoms would be ((magnetic)> and 30 % ctnon-magnetic, ar T = 4.2 K. The measured mean hyperfine field on magnetic iron atoms is about 131 kOe which gives H ⁼92 kOe if all the iron atoms are inclu- ded in the a v e r a b g procedure. Such a large hyperfme field corresponds r o u a y to magnetic moments jl'

---

^0.9^pB

/magnetic iron atom or jS"

--

^0.6^p~/ iron atom about 30 time larger than expected from magnetization data ! (iii) Looking at f i ~ r e 4(b), which shows a Mossbauer spectrum measured at T = 7-0 K, one can see an inner part corresponding to atoms that seem to experience a nearly zero total hyperfine field that is Hext -

Ynd

⁼0. Tnese atoms may correspond to paramagnetic iron atoms in an induced field of about 60 kQ.

I C I L O I

fig. 4. - Mijssbauer spectra at T = 20 K w'th Hext = 20 kOe ( a ) and 60 kOe ( b )

fig. 5. - Hyperjine field (a) and quadrupolar effecr ( b ) dism'butions corresponding to the ~MZissbauer spectrum ihown in figure Ifa).

(iv) At low temperature (T = 4.2 K) a 60 kOe external field has practically no influence on the ((magnetic atoms,) (figure 3 0 and again the ctnon magnetic)) contribution has to be splitted in paramagnetic atoms and a zero moment population.

T o illustrate these results the spectrum of f i g r e 3f has been compared t o calculated patterns as pictured in figure 6. The full line spectrum in figure 6a has been ob- tained by adding a magnetic contribution (with magnetic moments distributed at random in orientation, statistical weight unchinged with respect to zero field P(H)- given in

figure Sa but corresponding t o

Tftot

⁼-* ^Hi⁺g e x t ) and a t n o n magnetic)) contribution arising from zero moment atoms with the P(EQ) statistical weight (fig. 5b). In figure 6b the ctnon magnetic, contribution has been splitted in 75 % zero moment iron atoms experiencing Htot = HeXt

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c8-668 JOURNAL DE PHYSIQUE

and 25 % paramagnetic iron atoms experiencing Htotal = Qduced

-

^Hexternd⁼^0.None of these two fitring proce- dures can be considered as matching satisfactorily the txpe- nmental data.

ow ever

the m e r part of the spectrum seems to be more closely estimated in the second approach without any real desadvantages for the external part.

Fig. 6.

-

Comparison of Mossbauer spectroscopy dofa at T = 4.2 K and Hext = 60 kOe with cal- culated spectra (a) and ( b ) as explained in text.

3 - DISCUSSION A ~ D CONCLUSION.

The magnetic moments born by the iron magnetic asorns averagc macroscopically t o nearly zero but are almosr reaching 1 p ~magnetic iron atom locally. In addi- / tion an external field as strong as 60 kOe is unable to modify the random orientation of the magnetic moments.

Thus, these magnetic atoms are not ferromagnetically coupled in the long range since a typical ferromagnetic order would have resulted in the same values for magnetic moments as measured either by magnetization or by Moss- bauer spectroscopy and an even weak extemal field would have modify the magnetic part of the Mbssbauer spectra due t o alignment of the moments in an effective field

%t

-

Hext-

Long range antiferromagnetic coupling can be rejected as well. Again modifications in the hlossbauer spectra recorded under extemal field would have been observed due to : alignment of the moments parallel and antiparallel to the applied field and splitting o f the P(H) distribution into two peaks culrriinating respectively at (120 ^tHext) kOe or alternatively in the case of weak anisotropy and strong Hext, rotation of the magnetic moments at k of the

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applied field (spin- flop transition). Anyhow long r a n p anti- ferromagnetism is well known to be ruled out by the

amorphous character of the material.

Thus the {(magnetic atoms)) of the investigated alloy can only be considered as either single mametic atoms or magneric clusten under the influence of a very strong local magnetic anisotropy. Under an external field of 20 kOe (see fig. 1) only about 1 % of the magnetic atoms are able to align along the field.

In addition when observed in their paramagnetic state above the blocklng temperature T = 7 K, these magnetic atoms seem to be fairly unsensitive to temperature condition and the central parts of the Mossbauer spectra shown in figure 7 look quite similar. Thus the magnetic suscepti- bility of these atoms must be weakly temperature depen- dent in a first approximation at low temperature, and the local coupling must be strongly antiferromagnetic, with :

Fig. 7. - Influence 01' the temperature on the magnetic atom behaviour in their paramagnetic stare.

(0) T = 10 K Hat = 6 0 kOe f b ) T = 20 K H,,, = 6 0 kOe

t

qat ^'

130 kOe if estimated from the mean hyperfine field measured on magnetic iron atoms at T = 4.2 K (fig. Sa).

* under Hext ⁵60 kOe the total field measured in the paramagnetic state is nearly zero (fig. 7) resulting in Hhyp = Hinduced = 60 kOe.

* 8 must be larger than 20 K if one want .to explain the presence of paramagnetic atoms and the weak influence of the temperature on the induced hyperfine fieid between 10 and 20 K. With the proposed values of Hsat, Hext and Hhyp, the Brillouin function B j has to be of the order of 0.4 and J must be large enough t o write BJ

--

^,³ ^which

results in

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Even if one takes a relatively low estimation of 8, say T + 0

-

⁵⁰^K,the magnetic moment of the clusters has to be near 15 p ~ . For antiferromagnetic short range order

p* = JO b%pg with g = 2 and N is the number of iron atoms in the cluster. Thus the magnetic clusters would contain an average of about 50 atoms.

In conclusion the iron atoms in Fe0.34Sn0.66 m o r - phous alloy are mostly magnetic (70 % of the total iron content) below a blocking temperature of about 7 K. The- se magnetic atoms are antiferromapetically coupled proba- bly by superexchange via Sn atoms inside magnetic clusters whose size distribution would be partly responsible of the h y p e r f i e field distribution (fip. 5a). A strong local a n i s e tropy results in a noticeable unsensitiveness of the clusters against external field as large as 6 0 kOe. A similar behaviour had been previously observed in the crystallized FeSn2 phase [6].

Outside of the magnetrc clusters, some iron atoms are real non magnetic atoms in a zero moment state.

REFERENCES

[I] MARCHAL G., MANGIN Ph., PIECUCH M., JANOT Chr.

- J. Phys. 37 (1976) C6 763.

[2] MANGIN Ph., PIECUCH M.. MARCHAL G., JANOT Chr.

J. Phys. F 8 (1978) 2085

[3] RODMACQ B., PIECUCH M., MARCHAL G., MANGIN Ph., JANOT Chr.

Phys. Rev. B 2 (1980) 191 1-

[4] IYENGAR P.K., DASAUNACHARGA B.A., V Y AGARAGHA- VAN P.R., RAY A.P.

J. Phys. Soc. Japan 12 (1962) suppl. BIII, 41 151 MARCHAL G., MANGIN P h , JANOT Chr.

Phil. Mag. 32 (1975) 1007

[6] BECKMANN V., RITTER G., SPIERING H., WEGENER H.

Roc. Conference o n Application of the Massbauer effect Tihany, Hungary (1969) pp. 30-39.