Magnetic studies of Fe/Pt multilayers

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ELSEVIER

Journal of Magnetism and Magnetic Materials 156 (1996) 41-42

~ Journal of magnetism and magnetic

~ i materials

MiSssbauer study of Fe/Pt multilayers

A. Fnidiki ~, J.S. Douheret a, j. T e i l l e t a,* , H. Lassri b, R. Krishnan b

a URA CNRS 808, Facult~ des Sciences de Rouen. 76821 Mont-Saint-Aignan Cedex. France b Laboratoire de Magndtisme et Mat~riaux Magndtiques, CNRS, 92125 Meudon Bellevue Cedex, France Abstract

F e / P t multilayers with constant ept = 1.5 nm and varied eft in the range 0 . 2 - 5 . 4 nm are studied by MOssbauer spectrometry and X-ray diffraction. For eFe < 2.0 rim, the 57Fe MiSssbauer spectrum is typical of a disordered magnetic interface phase. For higher thicknesses, in addition to this phase, a bcc iron phase is also observed, giving evidence that the thickness of the iron layer is thick enough to allow its centre to be constituted of pure iron. For magnetic spectra, the magnetic anisotropy was always found in the plane of layers.

1. Introduction

Recently, a lot of studies have been devoted to multilayers with Fe or Co transition m e t a l / P t or Pd noble metal to elucidate their magnetic and structural properties (see for example Refs. [1,2]). Here, we present a study of F e / P t multilayers by room-temperature conversion elec- tron M/Sssbauer spectrometry (CEMS) of 57Fe in order to obtain local information on the magnetic and structural state of iron sublattice. The global structure of the film is also determined by X-ray diffraction with Co Kc~ radia- tion.

2. Experimental procedure

F e / P t multilayers were evaporated at room temperature by cathodic if-sputtering on a glass substrate covered with a 10 nm thick Pt buffer layer. The thickness of the Pt layer was kept constant (ept = 1.5 nm) while the thickness of the iron layer (doped 57

• in - Fe) varied in the range of 0 . 2 - 5 . 4 nm ( e v e = 0.2, 1.2, 1.5, 2.0, 3.6, 5.4 nm). The number of periods varies from 22 to 5 in order to have a similar total thickness (35 to 50 nm) for all samples. CEMS room-temperature spectra were recorded using a conventional spec- trometer equipped with a home-made helium proportional counter and fitted with a least-squares technique.

3. Results and discussion

The quality of F e / P t multilayers was characterized by small-angle X-ray diffraction, where the presence of Bragg

Corresponding author. Fax: +33-3514-6652: email:

jacques.teillet @univ-rouen.fr.

peaks and Kissing fringes is an indication of the multilayer structure. High-angle X-ray diffraction patterns show broadened lines typical of crystallized compounds (Fig. 1).

For eFe < 2 nm, the pattern is in agreement with fcc PI and FePt alloy, but the mean composition of this alloy cannot be determined because the position of the lines is only a little dependent on the composition of the alloy. The broadening of the lines could come from a modulation of the alloy composition, but it could also be a signature of a

a

!i

10 20 30 40 50 O(deg)

¢a

,'r ¢~:

xleq,

10 20 30 40 50 O(deg)

Fig. 1. X-ray diffraction patterns of Fe/Pt multilayers (ep~ = 1.5 nm). The peaks marked • correspond to the disordered interface phase and the peaks marked $ correspond to (~-iron. (a) e~,, - 1.2 nm: (b) e w, = 2.0 rim.

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42 A. Fnidiki et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 41-42

Velocity ( m m / s )

-2 0 +2

i - - - -

1.00

- l o o +1o

o - - - = "

1,00 ' ~ H ( T )

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!

15

0 = 1 { 1 1 ~ l l l

o io 2o 30 H(T)

Fig. 2. CEMS spectra at room temperature and hyperfine field distributions of some Fe/Pt multilayers (ept = 1.5 nm). (a) eF~ = 0.2 nm; (b) eFe ⁼ 1.2 nm; (c) eFe = 3.6 nm.

disordered F e - P t interface with a varied composition, and X-ray diffraction does not allow us to conclude on the structure of the interface. For eFo > 2 rim, additional peaks characteristic of a bcc a-iron phase appear, indicating pure iron in the centre of the iron layers. The relative intensity of these peaks increases with eFe.

CEMS spectra of F e / P t multilayers at room temperature are reported in Fig. 2.

For low values of iron thickness (eFe = 0.2 nm), the spectrum is a slightly asymmetrical paramagnetic doublet with broad lines (line width F = 0.71 m m s 1, quadrupole splitting QS = 0.5 m m s - I , isomer shift 6 = 0.28 mm s ~), characteristic of a distribution of quadrupole interac- tions. For this iron thickness, which is less than an iron monolayer, the magnetic order cannot be set up due to the large number of platinum neighbours which prevent magnetic percolation of iron atoms at room temperature. For larger thicknesses, the slightly asymmetrical magnetic

5O 4O 3O

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o 2 4

I r o n t h i c k n e s s ( n m )

Fig. 3. The relative amount of a-Fe as a function of iron thickness eFe (ept = 1.5 rim).

34 E

3 2

._~

~ 30

g 28 26 0

bulkiron

..J

F i

f i i

2 4

iron t h i c k n e s s ( n m ]

Fig. 4. Average hyperfine field in Fe/Pt multilayers as a function of iron thickness eFe at 300 K (ept = 1.5 nm).

spectra with broad lines are related to a distribution of Pt and Fe environments leading to correlated distributions of both the isomer shift and hyperfine field. Using a linear correlation, the spectra were fitted with the histogram method, constraining the width of each elementary spectrum to be the same. For eFe = 1.2 or 1.5 nm, the continu- ous hyperfine field distribution (Fig. 2b) may be attributed, in agreement with X-rays, to a magnetic interface (Fe, Pt) phase, which is probably compositionally modulated in the layer. For eve = 2.0, 3.6 and 5.4 nm, the hyperfine field distribution (Fig. 2c) is the sum of a distribution analogous to the previous distribution, characteristic of the disordered interface phase and of a very narrow contribution, centred on 33 T, characteristic of the body-centred cubic a-iron.

This result agrees with our X-ray diffraction results and indicates that the thickness of the iron layer is then enough for the centre of the iron layer to be constituted of pure iron. The relative intensity of the oMron contribution increases with eFe (17% for eve = 2 nm, 26% for eF~ = 3.6 rim, 37% for eft = 5.4 nm) (Fig. 3) leading to an increase of the mean hyperfine field with eve (Fig. 4).

At room temperature, the magnetic anisotropy can be deduced from the relative intensities of the CEMS lines.

For epe >_ 1.2 nm, the iron moments lie in the plane of the film. This behaviour is due to the dominant effect of shape anisotropy related to the demagnetizing field.

These results are consistent with previous results on F e - P t multilayers. The appearance of a-iron from eF~ = 2 nm was already observed in Ref. [3]. On the other hand, the nature of the interface remains controversial. It could be a compositionally modulated, ordered or disordered F e - P t phase. Nevertheless, the regular shape of the hyperfine field distribution pledges for a disordered phase.

R e f e r e n c e s

[1] E. Devlin, V. Psycharis, A. Kostikas, A. Simopoulos, D.

Niarchos, A. Jankowski, T. Tsakalakos, H. Wan and G.

Hadjipanayis, J. Magn. Magn. Mater. 120 (1993) 236.

[2] R.A. Brand, Th. von Schwartzenberg, O. Bohn~ and W.

Keune, J. Magn. Magn. Mater. 126 (1993) 248.

[3] B.M. Lairson, M.R. Visokay, R. Sinclair and B.M. Clemens, J. Magn. Magn. Mater. 126 (1993) 577.