MICROMAGNETIC PROPERTIES OF AMORPHOUS CoxY1-x SPUTTERED FILMS WITH Mo OVERLAYERS

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MICROMAGNETIC PROPERTIES OF AMORPHOUS

CoxY1-x SPUTTERED FILMS WITH Mo

OVERLAYERS

J. Alameda, M. Contreras, D. Givord, A. Liénard

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplkment au no 12, Tome 49, dkcembre 1988

MICROMAGNETIC PROPERTIES

OF

AMORPHOUS

C o x Y l - , SPUTTERED

FILMS

WITH M o O V E R L A Y E R S

J. M. Alameda (I), M. C. Contreras (I), D. Givord ( 2 ) , and A. Lienard (2) ( I ) Dpto. de Fisica, Universidad de Oviedo, Oviedo 3307, Spain

(') Lab. L. Ne'el, CNRS, 166X, 38042 Grenoble Cedez, France

Abstract. - Magnetic and magneto-optical properties of amorphous CoxY1-, ( z = 0.80, 0.35) films (thickness range: 50- 1 000 %I) with Mo overlayers are studied. Transverse biased initial susceptibility measurements are reported. The structure constant S increases as the thickness decreases, i.e.: S = 0.04 and 0.15 erg/cm2 for 1 000

A

and 50

A

respectively. Surface effects on Coo.35Yo.65 are discussed.

Surface effects on magnetic and magneto-optical properties of amorphous Co,Y1-, films were reported in a previous paper [I]. In particular, evidence of a magnetically ordered phase at room temperature for films where x

5

0.60, was observed. This is in di- sameement with the well stablished behaviour in bulk

_-

amorphous samples [2,3]. In reference [l] we proposed a simple explanation for this effect, i.e.: Y segregates to the top of the surface and reacts with oxygen, for-

ming oxide patches across the film surface (selective oxidation is a part of the driving force for Y segre- gation). Consequently, localized Co-enriched regions may exist near the film- air interface. The anomalous magnetic and magneto-optical properties observed at room temperature for films with nominal composition x

<

0.60, may arise f r ~ m Co-enriched regions. Here we present new experimental evidence which seems t o confirm this hypothesis.

Amorphous Co,Yl-, films were obtained by d.c. triode sputtering with single cathode configuration. Sputtering targets (obtained by induction-levitation melting) of nominal compositions x = 0.80 and x =

0.35 were used. Glass substrates, fixed on t o a rota- ting sample holder, were held a t 77 K during deposi- tion. Film thickness was varied between 50 t o 1 000

A.

Some films were in situ protected after deposition by Mo overlayers (thickness: 1 000

A),

transverse biased initial susceptibility (TBIS) measurements were car- ried out using a magneto-optical set-up already descri- bed [I]. Details about the determination of transverse Kerr effect parameter SK = A R /

R,

where R is the reflectivity and AR the magneto-optical modulation, are given elsewhere [I]. In Mo-protected films, only 6~ from the glass-film interface can be measured. Howe- ver, for unprotected films, SK from both glass-film and film-air interfaces may be analysed.

Ftegarding SK values, the main experimental results may be summarized as follows:

Within the limits of experimental errors (SK

<

,

no magneto-optical signal was found for Mo-protected C00.35Yo.65 films. This was not the case for unprotec- ted films, where 6~ = -1.5

x

lo-* was observed from film-air interface. (Negative means that Kerr polarity is opposite t o the observed in Co-rich amorphous films (See Ref. 111). This later value is of the same order of

magnitude as that previously reported for

x

5

0.60 [I]. Furthermore, the same magnitude for 6~ but with op- posite (positive) polarity was detected from the glass- film interface in unprotected films whose thickness was lower than 300

A.

For higher thickness (1 000 A), no magneto-optical signal was detected from the glass- film interface.

From the above results, we conclude that the magneto-optical signal found in unprotected C00.35Y0.6~ films arises from the film-air interface. Furthermore, taking into account that the change in the polarity for 6~ is related to optical effects in the topmost oxide layer, the above results are justi- fied if the magneto-optical signal arises from discrete Co-enriched sub-surface layers localized below oxide patches which occur across the film surface. Scanning Auger deep-profiles analysis, now in progress, seems to confirm these hypothesis.

In C O O . ~ ~ Y ~ . ~ O films protected with Mo overlayers SK = 4.2

x

This value is slightly higher than that observed in a previous work Il] where 6~ = 3.4 x

for unprotected Co0.79Y0.21. Taking into account that in the latter case 6~ was measured from the air-film interface, the above results are in agreement with the present hypothesis, if we suppose that oxide patches extend over 10 % of the film surface.

C8 - 1734 JOURNAL DE PHYSIQUE

Otherwise, in reference [I] we suggest that magne- tization ripple deduced from TBIS measurements for films where x

>

0.70, may arise, not only from the in- trinsic contribution of randomly distributed local ani-

sotropies inherent to amorphous alloys [6], but an ad-

ditional contribution from magnetic inhomogeneities localized in the film-air interface could be expected. To elucidate this point, TBIS measurements were car- ried out in Coo.soY0.20 films protected with Mo over- layers. Similar values for the structure constant S [4,7]

t o those already reported for unprotected film [I] were found. In both cases, S = 0.04 erg/cm2 when film thickness is close t o 1 000

A.

(It is noteworthy that they are also similar t o S values recently reported for CoZr amorphous films [5]). Furthermore, S increases

as the thickness decreases. This is evidenced in fi- gure 1. It must be remembered that S is defined as:

where K,,, is the local anisotropy constant related to a random distribution of local easy axes and V is the volume where local anisotropies are correlated. from figure 1, it follows that the diameter of such regions is a t least lower than 100

A.

Furthermore, K I , , . v ~ / ~ =

1.2 x erg/cm-3/2 is obtained from

s2

VS. 1

/

t

curves if film thickness is within 100 to 1 000

A.

For film thickness lower than 100 A, perpendicular anisotropy effects on the macroscopic magnetic beha- viour (stripe domains) [6], makes ripple characteriza- tion from TBIS measurements ambigous. This is the main reason of the observed scatter in S values. A full

analysis of these results will be given elsewhere. here conclude that S values found here and in reference [I],

are related to the intrinsic magnetic properties of this;

/

I

I

I

I

I

I

/

I

I

I

200 LW 6W thickness (A)

Fig. 1. - Structure constant (S) vs. t for Co0.s0Y0.20 films with Mo overlayer. Inset:

s2

vs. 1

/

t ( 0 ) Experimental

points.

[I] Alameda, J. M., Contreras, M. C., Lagunas,

A. R., J . Magn. Magn. Mater. 72 (1988) 279. [2] Gignoux, D., Givord, D., Lienard, A., J. Appl.

Phys. 53 (1982) 2321.

[3] Buschow, K. H. J., J. Appl. Phys. 53 (1982)

7713.

[4] Hoffmann, H., Phys. Status Solidi 33 (1969) 175. [5] Hoffmann, H., Heller, G., Kammerer, K. H., IEEE

Trans. Magn. MAG-23 (1988) 2731.

[6] Alameda, J. M., Contreras, M. C., Rubio, H.,

Phys. Status Solidi A 85 (1984) 511.

[7] Hoffmann, H., IEEE Trans. Magn. MAG-4