Ferromagnetic resonance studies in Fe/Ti multilayers

Journal of Magnetism and Magnetic Materials 272–276 (2004) e973–e974

Ferromagnetic resonance studies in Fe/Ti multilayers

H. Lassri ^a , H. Ouahmane ^b , A. Berrada ^c, *, A. Kaabouchi ^a , A. Dinia ^d , R. Krishnan ^e

a

LPMM, Facult e des Sciences, Universit

e Hassan II Ain Chock, B.P. 5366 M

aarif, Casablanca, Morocco

b

Universit e moulay Isma

ıl, Facult e des Sciences et Technique d’Errachidia, D

epartement de Physique, Errachidia, Morocco

c

Laboratoire de Physique des Mat eriaux, Facult

e des Sciences, avenue Ibn Batouta, B.P. 1014, Rabat 10000, Morocco

d

IPCMS-GEMM (7504, CNRS) ULP-ECPM, 23 rue Loess, Strasbourg 67037, France

e

Laboratoire de Magn etisme et d’Optique, URA 1531, 45 Avenue des Etats Unis, Versailles, Cedex 78035, France

Abstract

The magnetic properties of sputtered Fe/Ti multilayers are presented. The magnetization results reveal an interdiffusion at the Fe/Ti interfaces. The FMR spectra were obtained as a function of orientation of the applied magnetic field from in-plane to out-of-plane and were fitted theoretically to determine the magnetic anisotropy. We also observed one or two weak peaks at the high-field-side of the uniform resonance mode, which we attributed to a surface mode. Spin-waves resonance modes were observed in the Fe/Ti multilayers (t

¼ 40 and 60 AÞ ( and the effective exchange constants were determined.

r 2003 Elsevier B.V. All rights reserved.

PACS: 75.30.Et; 75.70.Cn; 76.50.þg

Keywords: Fe/Ti multilayer; Ferromagnetic resonance; Anisotropy; Spin-wave

The interlayer coupling between ferromagnetic layers in multilayered and layered films containing transition metals has been shown to have an important influence on the magnetic and the electronic properties in these systems [1,2]. Ferromagnetic resonance (FMR) and spin wave resonance (SWR) have been used to study the effective parameters of multilayers [3], such as the effective exchange constant (A

), the effective magne- tization (4pM

), etc. In this work, we used the FMR method to determine the interface anisotropy and the effective exchange constant.

The multilayers have been prepared by DC triode sputtering. The chamber was first evacuated to a pressure of 10

Torr: Argon of 5 N purity was used as the sputter gas and its pressure was kept constant at 7 10

Torr: The rates of deposition were 17 A=min ( for Ti and 20 A=min for Fe. The thickness of the Fe ( layer was varied in the range 20–60 A and that of Ti ( layer was in the range 10–20 A: ( The number (N ) of bilayers were in the range 10–30.

Low angle X-ray diffraction studies were made to check the periodic structure. The saturation magnetiza- tion (M

) was measured using a vibrating sample magnetometer (VSM). The FMR spectra, at 9:8 GH z and room temperature, are obtained as a function of the orientation of the applied magnetic field from in-plane to out-of-plane.

Our results would indicate that the interface is diffuse due to the interdiffusion and the structural imperfection effects.

In perpendicular geometry, the resonance spectra are composed of more than one mode and the more-intense central peak correspond to the principal mode, the lower- field one to a volume spin wave and the higher-field one may be identified as a surface mode, as described theoretically by Puszkarski [4]. A typical FMR spectrum in perpendicular configuration for ðFe

=Ti

Þ

multilayer is shown in Fig. 1.

The angular dependence of the resonance field (Fig. 2) fitted well to the following expressions of uniform mode in a single-layer film:

H sinðy y

Þ ¼ 4pM

siny cosy ð1Þ

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*Corresponding author. Tel./fax: +212-37671118.

E-mail address:

[email protected] (A. Berrada).

0304-8853/$ - see front matter

2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmmm.2003.12.1155

and

ðo=gÞ

¼ ½H cosðy y

Þ 4pM

cosð2yÞ

½H cosðy y

Þ 4pM

cos

y; ð2Þ where o is the microwave frequency, g the gyromagnetic ratio, y

and y are the angles of H and M

with respect to the film normal.

By fitting the data from the angle-dependent FMR measurements, the information about the perpendicular anisotropy can be extracted. Then the interface aniso- tropy constant K

is deduced to be K

¼ 0:9 erg=cm

: Here K

is a positive value, which means that the interface anisotropy confines the magnetization along the film normal.

For the surface mode (k ¼ ik

) observed in the Fe layer (with t

¼ 40 A), the field splitting with respect to ( the uniform mode is described by

H

H

¼ 2Ak

=M

: ð3Þ If the presence of such a mode were to be interpreted solely in terms of a surface anisotropy, taking [5,6]

k

E K

=A; one would deduce at 300 K AK

¼ 0:9 erg=cm

for the Fe/Ti interface, if we take for our Fe layer M

¼ 1700 emu=cm

and A ¼ 2 10

erg=cm;

in agreement with that obtained from the FMR study.

If the multilayers were assumed to be coupled by interlayer exchange interaction into a single magnetic system and the spin-waves were sustained by the whole multilayer, the resonance field of multiple peaks could be fitted to the resonance conditions similar to the single-layer films as follows:

o=g ¼ H

4pM

þ 2A

k

=M

; ð4Þ where k

is the spin-wave number and k

¼ np=L: n is the volume spin-wave mode number, H

the correspond- ing field, and L the total thickness of the film. Spin- waves resonance modes were observed in some Fe/Ti multilayers and the relation of resonant field H

with the mode number n obeys the so-called n

law (Fig. 3). Both odd- and even-numbered modes are observed and this fact indicates that the pinning in this film is asymme- trical [4] at the two surfaces of the Fe layer. This is consistent with the presence of the surface mode.

From the slope of the straight line, we obtained the effective exchange constants A

¼ 0:97 10

and 0:48 10

erg=cm for t

¼ 40 and 60 A; ( respectively.

These values are less than that of the Fe single-layer thin film made under the same conditions as that of the Fe layers in multilayers, A ¼ 2 10

erg=cm; by more than one order of magnitude, which shows the weak interlayer coupling between Fe layers in Fe/Ti multi- layers.

References

[1] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304.

[2] L. Tong, M. Pan, J. Du, M. Lu, H. Zhai, J. Magn. Magn.

Mater. 198 (1999) 437.

[3] Z.J Wang, S. Mitsudo, K. Watanabe, S. Awaji, K. Saito, H.

Fujimori, M. Motokawa, J. Magn. Magn. Mater. 176 (1997) 127.

[4] H. Puszkarski, Prog. Surf. Sci. 9 (1979) 191.

[5] L.J. Maksymowicz, H. Jankowski, J. Magn. Magn. Mater.

109 (1992) 341.

[6] H. Lassri, H. Ouahmane, H. El Fanity, M. Bouanani, F.

Cherkaoui, A. Berrada, Thin Solid Films 389 (2001) 245.

ARTICLE IN PRESS

Fig. 1. FMR spectrum for

multilayer for perpendicular geometry at 300 K:

Fig. 2. Angular dependence of uniform resonance field for

multilayer.

0 10 20 30 40

13 14 15 16 17

(Fe40 Å / Ti10 Å)₃₀ (Fe60 Å / Ti20 Å)₁₀

Hr (kOe)

n²

Fig. 3. Resonance field H

in perpendicular orientation versus n

H. Lassri et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e973–e974

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