MAGNETIC CLUSTERS IN Y-Ni AMORPHOUS ALLOYS NEAR THE CRITICAL CONCENTRATION

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MAGNETIC CLUSTERS IN Y-Ni AMORPHOUS ALLOYS NEAR THE CRITICAL CONCENTRATION

A. Liénard, J. Rebouillat, P. Garoche, J. Veyssié

To cite this version:

A. Liénard, J. Rebouillat, P. Garoche, J. Veyssié. MAGNETIC CLUSTERS IN Y-Ni AMORPHOUS

ALLOYS NEAR THE CRITICAL CONCENTRATION. Journal de Physique Colloques, 1980, 41

(C8), pp.C8-658-C8-661. �10.1051/jphyscol:19808165�. �jpa-00220266�

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JOURNAL DE PHYSIQUE CoZZoque C8, suppzdment au n08, Tome 41, aoiit 1980, pageC8-658

M A G N E T I C CLUSTERS I N Y - N i AMORi'HOUS A L L O Y S NEAR T H E C R I T I C A L CONCENTRATION

XX

A . ~ i ~ n a r d * , J.P. E e b o u i l l a t , P. Garoche and J . J . veyssi6'*

C . N . R.S. L d o r a t o i r e Louis NdeZ, a s s o c i i 2 I 'U.S.M. G., 166 X , 38042 GrenobZe-Cedex. France Xx C. E.A. - LETI/NCE-CENG, 8 5 X , 38041 GrenobZe Ceder, France.

Laboratoire de Physique des S o l i d e s , Universitd Paris-Sud, 91405 Orsay, France

A b s t r a c t .

-

F a g n e t i c measurements g i v e evidence f o r t h e e x i s t e n c e of i n t e r a c t i n g magnetic c l u s t e r s i n amorphous Y N i n e a r t h e c r i t i c a l c o n c e n t r a t i o n .

X 1-x

I

-

INTRODUCTION

Y x N i l - x amorphous a l l o y s have been shown t o behave a s very weak i t i n e r a n t ferromagnets a t low Y c o n c e n t r a t i o n ( I ) . High p r e s s u r e measurements

seem t o i n d i c a t e t h a t ferromagnetism d i s a p p e a r s i n a r a t h e r inhomogeneous'way ( 2 ) . The p r e s e n t paper r e p o r t s m a g n e t i z a t i o n r e s u l t s on two samples w i t h Y c o n c e n t r a t i o n s l i g h t l y h i g h e r t h a n t h e c r i t i c a l c o n c e n t r a t i o n X = 22 %. These r e s u l t s when com- bined with spec;f i c h e a t measurements ( 3 ) confirm

t h i s p o i n t of view a n d . g i v e evidence f o r t h e e x i s - tence of i n t e r a c t i n g N i magnetic c l u s t e r s .

The Y.245Ni.755 and Y . 2 3 7 N i . 7 6 3 amorphous samples were prepared u s i n g D.C. t r i o d e s p u t t e r i n g on A 1 s u b s t r a t e cooled t o l i q u i d n i t r o g e n tempera- t u r e . r l a g n e t i z a t i o n c u r v e s were determined u s i n g t h e high f i e l d f a c i l i t i e s a t t h e S e r v i c e National des Champs I n t e n s e s i n Grenoble.

I1

-

^RESULTS

F i g . 1 shows t h e i s o t h e r m a l m a g n e t i z a t i o n c u r v e s M(H,T) o b t a i n e d f o r t h i s a l l o y . Each i s o - therm tends t o be l i n e a r a t high f i e l d s and can be analysed a s t h e sum of two c o n t r i b u t i o n s :

Y(H,T) = Mc(H,T) +

X ~ H ^{( 1 )}

The f i r s t term I-! (H,T) i s due t o t h e magnetic n i c k e l atoms. To a f i r s t approximation, t h e second term o r i g i n a t e s from t h e paramagnetic P a u l i c o n t r i b u t i o n of t h e non magnetic atoms ; i t i s expected t o be only midly t e m p e r a t u r e dependent. Two d i f f e r e n t a r p r o a c h s a r e used t o d e t e r n i n e

X

and g i v e two

P

i d e n t i c a l v a l u e s ( t a b l e ) . The f i r s t one i s deduced from t h e high f i e l d s l o p e of t h e lowest temperature F(H) curve. The second one comes from t h e low f i e l d measurements. A f t e r s u b s t r a c t i n g a c o n s t a n t term t o t h e i n i t i a l s u s c e p t i b i l i t y :

d Y (H,T)

= I

^d^H

1

^E+ ^(J

xi(T)

- xp

⁼

^T

C ⁽²⁾

245Ni -755

---

a C u r i e law i s o b s e r v e d ( f i g . 2 ) w i t h Curie c o n s t a n t c . I

TEMPERATURE [KI H L k e l

F i g . 2. Thermal v a r i a t i o n of t h e i n v e r s e of t h e F i g . 1 . Magnetization isotherms M(H,T) v s f i e l d i n i t i a l c l u s t e r s u s c e p t i b i l i t y x i - X p

f o r t h e amorphous Y . 2 4 5 N i . 7 5 5 a l l o y . f o r t h e amorphous Y . 2 4 5 N i . 7 5 5 ~ ~ I O Y .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808165

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In a plot of the magnetic contribution Mc(H,T) versus the inverse applied field, in the high field limit, linear segments are observed, the

-

¹

extrapolation of which to H = 0 yields an esti- mate of the saturation magnetization PI (Table).

Otherwise Mc(H,T) seems to vary as a unique function of HIT (fig. 3) over the whole investigated field and temperature range, excepted for T= 1.45 K.

This behaviour suggests the presence of super- paramagnetic clusters.

I 1

Fig. 3. Cl'uster magnetization M

-X

H vs HIT for the Y 24 Ni 755 amorphous alloy P for diffe~ens thperature s.

The average cluster spin S and the number N of clusters per gram are calculated (Table) from the rela- tions :

2 2

Ng PB S( S+1 )

C = 3 k ; Ms = NgPBS (3) The closest Brillouin function B 7/2 is drawn on fig. 3. Experimental data are located below this curve, with a shift increasing when temperature decreases. These deviations reflect the existence of interactions between clusters as will be developed in the next section.

For this sample, the magnetization can be decomposed in the same way as for the previous alloy (eq. 1). A first determination of the paramagnetic Pauli contribution Xp(0) is taken from the high field susceptibility at the lowest temperature

studied (Table). In order to get a Curie Weiss temperature dependence for :

d Mc(H,T)

xi(T)

- x

(T) - -

-

P d T T-0 (4)

it is necessary to suppose a temperature variation for

x

(T) of the form :

P

x

(T) =

X

(0) (I-aT) with a = d (5)

P P

The paramagnetic Curie temperature 8 = 0.5 K is gi- ven by the 1/xi(T) vs T plot at very low tempera-

ture where the Pauli contribution is negligible (fig. 4).

Fig. 4. d.c. initial susceptibility and its inverse at very low temperature for the Y.237Ni.763 amorphous alloy.

The curve xi(T-8) vs (I-aT) (T-8) (fig. 5) allows the determination of the Curie constant C by its intercept with the vertical axis and a second determination of

x

(0) by its slope (Table). A sharp

P

maximum is observed in the low temperature d.c.

susceptibility at a characteristic temperature Tf = 0.45 K (fig. 4). Below Tf a time dependent remanent magnetization is observed when measlred after cooling in a small field ( 18 Oe).This suggests a clust&r glass behaviour with a freezing temperature at T

f'

Fig. 5. Variation of xi(T-8) vs (I-aT).(T-8) for the Y+237Ni.763 amorphous alloy.

xi is the initial susceptibility ; a= 11790 K-1 and 0 = 0.5 K.

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C8-660 JOURNAL DE PHYSIQUE

As for the previous sample, the magnetic contribution Mc(H,T) may be shown to vary as 1/H at high field and low temperature (fig. 6). The saturation magnetization value M is extrapolated for

-

1

H = 0 at the lowest temperature (Table).

.g. 6. Variation of the cluster magnetization M

-

xpH with ~ - 1 for the amorphous Y.237Ni.763

Using eq (3) with the C and Ms values, the average cluster spin S. and their concentration N, are calculated for this sample and listed in the table.

The closest Brillouin function B 9/2 is drawn on fig.7 as a function of H / (T -€I), together with the experimental data. The shift already.observed on the previous Y.245Ni.755 alloy (fig.3) is much cLearer for the present sample.

Fig. 7. Cluster magnetization M-X H vs H/(T-8) for the Y.237Ni.763',$m~rphous alloy with P Xp=Xp(0)(l-aT)

?$&iss=

^.0.5 ^K.

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-

DISCUSSION

For these two alloys with compositions near the critical concentration an ideal superparamagne- tic giant moments interpretation is not acceptable for the two following reasons :

-

deviations from a unique function Wc vs H/T are observed at low temperatures for the both alloys

-

coupling interactions between the clusters are proved by the existence of i) a freezing temperature Tf = 0.45 K for the Y,237Ni.763 alloy ii) a maximum of the magnetic specific heat at T = 0.65 K for the Y.241Ni.755 alloy ( 3 ) . It

m

should be noticed that these experiments are com- plementary so that when one result is observed, the other one is expected.

As a consequence, we consider that these magnetic properties nay be described as those of a cluster glass. In a first step, let us try to understand them using the same approach as for spin glasses. A quantitative way to determine the interactions involved has been developed by Larkin and Khmel' nitskii (4)

,

then by Matho and Nunez-Regueiro ( ). The coupling via the conduction electrons is RKKY type, due to Friedel oscillations. Using the formalism of (5), a linear variation in H-' is expected over a limited range of fields for the approach to the saturation magnetization, according to the law :

The magnitude of the RKKY interaction W I can be obtained from the thermal variation of the pheno- menological field HO(T), according to :

where As and Bs are coefficients depending on the 'spin S and c is the cluster concentration per atom.

Fig. 6, as an example, shows for each temperature the expected linear segments from which the HO(T) parameter can be deduced. For each sample

,

the therml variation of H (T) is shown to follow

0

eq (7) (fig. 8) and the corresponding RKKY interaction W I is deduced (Table). The T temperature (5)

defined as Ts

=m)

cW is also listed -- 1

in the table. k~

This last formalism does not have to be seen as the only possible explanation for the observed interactions. The cluster coupling could also arise from volume anisotropy of dipolar origin,as sugges- ted by the very small freezing temperature measured.

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CONCLUSION

The results show that in the YxNi,-x amorphous alloys, the onset of a weak homogeneous ferromagnetism order is preceded at lower Ni concentration by ',

a local environment magnetism. Magnetic polariza-

.4Q- tion clouds nucleate at locally nickel rich rggions

where the Stoner criterion is satisfied. This .is 1

' the consequence of the sensitivity of the nickel '~37~~763- - 2 0 moment to its neighbouring(6). A more extended con-

centration range would be necessary to see the gra- 0 dual evolution toward the homogeneous magnetism.Yet

0 lo T l K l ²⁰ the nickel behaviour here observed presents evident

similarities with other disordered Ni based alloys Fig. 8. Thermal variation of the parameter Ho for near their critical concentration, for example

the two amorphous alloys. (10)

Ni~h(~) Ni~t(~) ~i~u(~)and amorphous Nil-x

.

Acknowledgements 245Ni. 755

'

.

~ 3 7 ~ ~ . 763

We are grateful to D.Givord and R. Lemaire for helpful discussions and J.L. Tholence for his contribution in the very low temperature measurements.

Tf K

-

.45

-

REFERENCES

T~

K

.66 .73

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J.J. Vevssi6.J.Phvs.F: Metal Phvs.9, ^{N O}6

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xp(o),o

1 0 - ~ e m u / ~ 2.43 3.33

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i

^129: 8. J.C. Ododo and W. Howarth, Solid State Comm.

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JETP

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phys.,

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(1978) 845.

~,(O)~+ab 1 0 - ~ e m u / ~

2.43 3.48

S UB 3.8 4.33 0

K 0 0.5 Ms

emuIg .787 .838

N c

W i l k ~

I0 cllg l~-~cl/at K 19

1.12 1.23 94 1.04

C

1 0 - ~ e m u / ~ 169 200