THICKNESS DEPENDENCE OF MAGNETIZATION AND MAGNETOSTRICTION OF NiFe AND NiFeRh FILMS

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THICKNESS DEPENDENCE OF MAGNETIZATION

AND MAGNETOSTRICTION OF NiFe AND NiFeRh

FILMS

K. Ounadjela, H. Lefakis, V. Speriosu, C. Hwang, P. Alexopoulos

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, SupplGment au no 12, Tome 49, dbcembre 1988

THICKNESS DEPENDENCE OF MAGNETIZATION AND MAGNETOSTRICTION

OF NiFe AND NiFeRh FILMS

K. ~unadjela*, H. ~ e f a k i s l , V. S. Speriosul, C.

wan^^

and P. S. Alexopoulosl

IBM, Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099, U.S.A.

Abstract. - The saturation magnetization, 4rMs, and the magnetostriction constant, A,, of NislFelg, NislFelg

/

Ta and Ni72Fe17Rhll

/

Ta thin films were studied as a function of film thickness before and after annealing. For films of thickness t

<

200

A,

4rM, and A, were found to be strongly dependent on film thickness with even larger variation after annealing. Auger depth profiles have shown the existence of inhomogeneous interfacial layer at the film surface, Ta/film and film/substrate interfaces. The presence of such layers of different composition and magnetic properties from the film

bulk, can explain the observed behavior of 4rMS and A,.

Introduction

The magnetic and physical properties of NiFe and NiFe-base alloys, being of significant technological im- portance in magnetic recording, have been the sub- ject of numerous studies. The thickness dependence of

magnetic properties however, has not been adequately investigated. While the behavior of saturation mag- netization 4nMs has been described [l] there are no reportes on the saturation magnetostriction tonstant As.

In this work, the behavior of 4 r M S and A. as a func- tion of thickness has been investigated ill. sputtered NiFe, NiFe/Ta and NiFeRh/Ta films before and after easy axis annealing. The Rh-alloy was chosen because it was found to enhance permalloy corrosion resistance

[2] and Ta capping in order to examine the effect of surface oxidation on NiFe. By correlating the mag- netic data with composition depth-profiles determined by Auger Electron Spectroscopy (AES), we obtain a clearer understanding of the behavior of 4nMs and A, for very thin films.

Experimental

The films were deposited by diode r.f. sputtering on glass substrates in a uniform magnetic field of 30 Oe and were of NislFe~g and Ni72Fe17Rhll nominal com- position. The film thickness was controlled by the de- position time. The Ta-cap was 100 A-thick and de- posited on half of the NiFe films after a vacuum break and on all NiFeRh films before vacuum break. The samples were annealed in vacuum at 250 OC for 2 hours with a magnetic field applied along the easy axis.

Results and discussion

thin films is shown in figure 1. In all cases, 4aMs decreases from the respective bulk values attained for thickness

t

2

400

A.

The decrease is gradual down to thickness t li200

A,

below which it becomes very rapid. Annealing intensifies this effect, with the Iargest change in magnetization for uncapped films. At 400

A,

the magnetization of the NiFeRh alloy reaches its bulk value, 8.4 kG, which is lower than that of permalloy due to Rh dilution. l ~ l s l n l I q S i As- deposited Annealed 6 1 ' 1 m 1 ' I ' I ' I 0 200 400 600 800 1000

NiFe Thickness (A)

Fig. 1. - 4rMs as a function of film thickness for NiFe, NiFe/Ta, NiFeRh/Ta before and after easy &xis annealing. Figure 2 shows the corresponding behavior of A,. For as-deposited NiFe, A,

< 0, and becomes more neg-

ative with decreasing film thickness. For the NiFe/Ta films, the change of A, at low thickness is less pro- nounced. However, the behavior of A, for as-deposited NiFeRh/Ta films is very different. For these films As

> 0 and increases with decreasing film thickness.

The difference in sign of A, between NiFe and NiFeRh The variation of 4nM, as a function of thickness films thick enough to be considered possessing bulk

for the NislFelg, NislFelg/Ta, and N i 7 2 F e 1 7 ~ l l / ~ a properties (e.g.

t

= 400

A)

is due t0 the dilution effects of Fth in permalloy which, according to split band theory, gives a positive contribution to A, when ' ~ a ~ n e t i c Recording Institute. the Fermi energy passes between the d bands of two

2~~~ General Products Division, San Jose, Ca, U.S.A. magnetic transition elements [3]. Annealing results in

C8 - 1710 _{JOURNAL DE PHYSIQUE} As-

'

I ' deposited Annealed NiFe/Air

-

0

-..-

O d ' ' I ' I

1

0 200 400 600 800 1000 Thickness (A)

Fig. 2. - A, as a function of film thickness for NiFe, NiFe/Ta, NiFeRh/Ta before and after easy axis annealing.

quite different effects for the NiFe versus NiFe/Ta and

NiFeRh/Ta films. For all NiFe samples, A, becomes more negative, while for NiFe/Ta A, moves towards positive values, changing sign for films below 100 k. Similarly, for NiFeFth/Ta A, increases after annealing.

For t

>

500

A,

A, becomes slightly more negative for both as-deposited and annealed samples. We attribute this to the change in crystallographic orientation as the film thickness increases. X-ray diffraction analysis of 600

A

and 1000 k NiFe films showed an increased (111) texture in the thicker film. For permalloy, in the neighborhood of 81/19 composition, Aloe

>

0, while

A l l 1

< 0

[4]. Thus a preferred (111) orientation will result in a more negative value of A,.

The behavior of ~ T M , and A, described above can be explained by the presence of two surface layers of dif- ferent composition and magnetic properties from the film bulk, as revealed by AES analysis [5]. Auger depth profiles of NiFe films before and after annealing clearly showed the existence of three distinct regions of dif- ferent composition: top surface oxide, film bulk and film/substrate interface. The composition of the sur- face oxide layer was a mixture of Ni and Fe oxides and was shown to be Ni poor and Fe-oxide rich before and after annealing respectively in agreement with previ- ous literature reports [6, 71. The film/substrate inter- facial region is similarly composed of Ni and Fe oxides. The composition of the surface oxide layer was not uni- form throughout but varied with (oxide) thickness and contained, towards the oxide-layerlfilm-bulk interface, some metallic Ni [7].

The surface oxide thickness of an air-exposed 210 k- thick NiFe film was determined to be N 20

k

before

annealing and roughly thrice that after annealing. The thickness of the film/substrate interfacial region re- mained the same (- 60

11)

in both cases. The Auger data showed that both surface layers contain a ma- trix rich in Fe oxide with some metallic and oxidized Ni. These layers have lower saturation magnetization and larger negative magnetostriction than permalloy.

For bulk Ni, 47rMS and A, are 6 kG and -4 x respectively. The Fe-rich oxide layer implies a partial depletion of Fe in the film bulk. A 1 % decrease in bulk Fe concentration would not greatly reduce 4aM, in the film bulk, but it would double the magnetostricti'on 141. This effect is dramatically enhanced by annealing because of the increased thickness of the top surface oxide layer.

In the case of the as-deposited NiFe/Ta films, mag- netostrietion behaves in a way similar to uncapped NiFe films. We attribute this to the presence of an oxide layer between the NiFe and Ta, since the former was exposed to air during vacuum break. The slightly more positive A, values for as-deposited NiFe/Ta com- pared to those for NiFe are likely related to some in- terdiffusion between Ta and this oxide layer during de- position. The magnetostriction increases as film thick- ness decreases also because of intermixing in the top Ta/film interface region. The origin of the positive contribution of Ta t o A, is the same as that of Rh, as has already been discussed. After annealing, A,

for both NiFe/Ta and NiFeRh/Ta films, moves toward positive values, changing sign for the thinnest film in

the case of NiFe/Ta. The extent of intermixing at the Ta/NiFeFth interface after annealing must, however, be rather limited, that is below the depth and con- centration detection limit of AES since Auger depth profiles of these films before and after annealing were very similar. For such intermixing, the electronic ef- fect between the Ta and NiFeRh would be enhanced, explaining the increase of A, for very thin films.

Acknowledgments

We are happy to acknowledge Grace Gorman for X-ray diffraction measurements. KO wishes to thank IBM for its support during his stay at Almaden Re- search Center.

[I] Bajorek, C. H. and Wiltz, C. H., J. Appl. Phys. 43 (1972) 3538.

[2] Klockholm, E. and Aboaf, J. A., J. Appl. Phys. 52 (1981) 2474.

[3] Berger, L., AIP Conf. Proc. 34 (1976) 355; Physica

30 (1977) 1141.

[4] Bozorth, R. M. and Walker, J. G., Phys. Rev. 89 (1951) 624.

[5] Ounadjela, K., Lafakis, H., Speriosu, V., Hwang, C. and Alexopoulos, P., to be published.

[6] Pollak, R. A. and Bajorek, C. H., J. Appl. Phys. 46 (1975) 1382;

Lee, W., Scherer, G. and Guarnieri, C. R., J. Electr.

SOC. 126 (1979) 1533.

[7] Brundle, C. R., Silverman, E. and Madix, R. J., J.