Journal of Magnetism and Magnetic Materials 126 (1993) 261-264 North-Holland

M6ssbauer and magnetic studies of Fe/Co and Fe/Cu multilayers

C).F. Bakkaloglu a M.F. Thomas a R.J. Pollard b and P.J. Grundy a Department of Physics, University of Liverpool, Liverpool L69 3BX, UK b Department of Physics, University of Salford, Salford M5 4WT, UK

Magnetometry and M6ssbauer spectroscopy investigations of series of Fe/Co and Fe/Cu multilayers show that in some structures a perpendicular component of magnetisation can occur, in contrast to the in-plane magnetisation of others, and that a mixed, interfacial layer probably exists in these sputter-deposited films. The magnetic anisotropy of the multilayers is obtained from magnetisation and applied field M6ssbauer measurements.

1. Introduction

Magnetic multilayer films (MLs) are of great inter- est because of their many unusual fundamental and potentially applicable properties. There are, however, few reports of the hyperfine parameters within such structures. This paper presents results of a study of the hyperfine parameters and easy directions of magnetisa- tion within Fe layers in Fe /Co and Fe /Cu systems obtained by means of M6ssbauer spectroscopy, and also the results of complementary investigations of magnetisation and hysteresis in these multilayers by magnetometry.

The Fe /Co system incorporates two ferromagnetic and mutually soluble elements, whereas in F e / C u MLs the spacer layer is nonmagnetic and the two metals have very limited solubility in equilibrium at and below room temperature [1]. Consequently, one might expect contrasting magnetic behaviour between these two sys- tems arising from differing interlayer coupling and interracial mixing effects.

2. Experiment

The MLs were deposited in a UHV-compatible and computer controlled multi-source sputtering system onto iron-free polyimide substrates in 3 × 10 -3 mbar of argon. The layer thicknesses in the MLs were in the range 10-150 .A for Fe /Co and were 20 and 30 .~ for Fe /Cu. In each ML the Fe and Co or Cu layers were of equal thickness. In order to maximise the M6ssbauer signal the number of periods in the MLs was adjusted with layer thickness so that each ML contained at least 0.2 Ixm of Fe. The deposition rates, layer thicknesses and microstructures of the MLs were determined by

Correspondence to: Professor P.J. Grundy, Department of Physics, University of Salford, Salford M5 4WT, UK. Fax: +44 (0) 61 745 5903.

calibration measurements using X-ray microanalysis and diffraction techniques and by cross-sectional elec- tron microscopy and electron diffraction. The magne- tometry was carried out at room temperature using a VSM and a sensitive AGFM (alternating gradient force magnetometer).

Previous Mfssbauer studies have been made at room temperature on both Fe /Co [2] and F e / C u [3] MLs. In this study Mfssbauer transmission spectroscopy is used to complement the conversion electron M6ssbauer spectroscopy of these earlier investigations. The pre- sent investigations have been carried out at 290, 77 and 4.2 K. The 57Fe transmission M6ssbauer spectra were recorded with a conventional double ramp constant acceleration spectrometer using a SVCo in Rh source. Fields up to 10 T could be applied from superconduct- ing Helmoltz coils.

3. Results and discussion

Cross-sectional electron microscopy of ultra-micro- tomed or ion-milled sections of the MLs clearly showed the well defined layered structure of the samples. Mi- crographs reveal polycrystalline films with a grain size varying between 50 and 150 A depending on the layer thicknesses. Plan-view and cross-sectional electron diffraction patterns from the MLs show the Fe, Co and Cu to be in bcc, hcp and fcc phase, respectively. In general, the patterns show no sign of a preferred orientation in the MLs.

The magnetisation measurements were carried out at room temperature with an in-plane applied field. The results on the 30 and 20 A. F e / C u MLs show that they have low coercivities and low saturation fields with their easy direction in the sample plane (0, the angle between the normal to the film and the magneti- sation = 90°). However, the 75, 50 and 20 ,~ F e / Co MLs exhibit hysteresis loops which have a characteris- tic shoulder between the irreversible loop and the

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262 O.F. Bakkaloglu et al. / M6ssbauer studies of Fe / Co and Fe / Cu

approach to saturation (fig. 1). The loops suggest that 0 4= 90 °, and that as the layer thickness decreases and the number of interfaces increases a perpendicular component of anisotropy develops which leads to an increasing in-plane saturation field. It may be signifi- cant that the 10 ,~ Fe /Co ML with the thinnest layers shows a simple, easy in-plane loop.

The experimental values of saturation magnetisa- tion, M s, agree well with calculated values for a ferro- magnetic coupling between the layers. However, im- proved agreement (within 2%) for the Fe /Co MLs is obtained if an interface layer with an average equi- atomic composition FeCo, mean width 9 ,~ and mean moment of 2.38/x B (Mve = 2.2/x 8, Mco= 1.72/z B) is invoked [4]. This approach is consistent with the hyper- fine fields studies described below and the magnetisa- tion measurements on the 10 A Fe /Co ML described above. A similar calculation for the F e / C u MLs is not possible because of a lack of information on the Fe moment in mixed Fe /Cu structures. However, hyper- fine field measurements [3,4] do show that the Fe moment is reduced below that of pure Fe.

M6ssbauer spectra for all the samples were mag- netic sextets (fig. 2). These spectra provide information on magnetic hyperfine fields from the line positions and on the direction of magnetisation relative to the M6ssbauer ~,-ray beam through the relative line inten- sities. The narrow linewidths obtained from all the MLs, except the 10 * F e / C u structure (not shown in fig. 2), is evidence that the iron sites enjoy a well defined crystalline environment. The 10 ,~ Fe /Cu sam- ple showed heavily broadened lines at 290, 77 and 4.2 K, suggesting a noncrystalline structure. The spectra were firstly analysed with a fit to a single sextet compo- nent to establish the mean hyperfine field ] Bhf]. t Bhfl increases with decreasing layer thickness in Fe /Co MLs, but the trend is reversed in the F e / C u system. Calculations of A I Bhf] = I Bhf] --Bhf (bulk iron) for each temperature are consistent with the view that these Fe /Co ML systems (AlBhf] ~ constant) have Curie temperatures similar to pure iron but Fe /Cu MLs have reduced Curie temperatures, as also re- ported by van Noort [3].

Hyperfine fields Bhf(1) and Bhf(2) have been ob- tained from a two-sextet fit to the spectra. The mag- netic hyperfine field at Fe sites in FexCo]_ x alloys is found [5] to be greater than that for bulk Fe. Accord- ingly, in our Fe /Co samples we identify the compo- nent with larger hyperfine field, Bhf(1), with Fe in a

Fig. 1. AGFM hysteresis loops of 10, 20, 50 and 75 ,~ Fe/Co multilayers.

+2E-2

- 2E -2 -5E+3

10 A Fe/Co ' I • I • I ' i •

M (emu) H C = 2 1 . 9 0 e

Mr - 1 0 . 2 m e m u

- M S = 1 3 . 7 m e r n u

, I , I , I , I ,

+2E-2 i M (emu) H c = 1 0 3 O e

M r = 4 . 1 5 r n e m u

Ms = 1 2 . 3 m e m u

0

-2E -2 , I , -3E+3

+2E-2 i M (emu) H C = 5 7 . 1 0 e

Mr = 4 . 5 5 r n e m u

Ms = 1 2 . 3 m e m u

0

- 2 E - 2 i -3E+3

i

M (emu) H c = 2 9 . 8 0 e

- M r = 7 . 2 3 rnemu

M S = 1 4 . 3 m e m u

+2E-2

- 2E -2 -3E+3

) J =

0

5oA [

0

H (Oe) , I , I , i , I ,

0 +5E+3

20A I I

S

H (Oe) I , I i

+3E+3

I I

y-

) I I I

0

75A

i

H (Oe)

+3E+3

H (Oe) I J I i

+3E+3

O.F. Bakkaloglu et al. / M6ssbauer studies of Fe /Co and Fe / Cu 263

bulk Fe 0.00 - -

7.50

150 A 0 .00 - - ~

0.75

75 A 0.00

.=_o 0.75 ~- 5o A o o . o o - - ~ ~ ~ ~ f . .Q

0.20 3o A ~. o.oo - - ~

0.50

2O A 0 . 0 0 ~ ~ ~ - - -

0.40

IoA 0 . 0 0 ~ ~ v ~ - -

0.75 I , l i I I I , l l

-10.0-8.0 -6.0 -4.0-2.0 0.0 2.0 4.0 6.0 8.0 10.0

Velocity (mm/sec)

Fig. 2. Best fits to the M6ssbauer spectra of bulk Fe and Fe /Co multilayers with layer thicknesses from 150 to 10 ,~.

Experimental points are omitted for clarity.

mixed interface layer and the smaller component , Bhf(2) , with the remaining iron layer. The values of chemical shift for these two components confirm this scheme. The thickness of the interface layer can be calculated [4], assuming the same Debye -Wal l e r factor for the two components, from:

11 Xp X

I z 2Yq Y '

where 11/12 is the ratio of absorption areas of sextets assigned to the pure iron layer of thickness X and the interface layer of thickness Y and p and q are the respective mean densities of Fe in these layers. For sample symmetry p / q = 2 and X + Y is the ML layer thickness. Individual MLs show [4] substantial varia- tions on a mean value of interface layer thickness I YI = 9 ,~ calculated from the above equation. In a

similar manner a mean value for Y of 8.6 A is obtained for the two F e / C u MLs. If the interface layer does exist its presence or thickness, which appears to vary between 14 and 6 ,~ for the F e / C o systems, does not appear to depend on the miscibility of the component metals but is most likely determined by the sputtering conditions.

Appl ica t ion of a magnet ic field during the M6ssbauer measurements enabled an investigation of the orientat ion of the magnetisation vector of the Fe atoms [6]. In these measurements the ~-ray beam was

0.50

.~ 0.00 g

0.75

1~ 0.00 ~o

o. 0.50

0.00

0.75

0.00

0.00

0.75

0.00

0.75

No field

0.8 T

1.3T

1.7T

2.2T

2.5 T

, I I I -10.0-8.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0

o..

0.00

0.25

0.00

0.40

0.00

0.50

0.00

0.50

0.00

0.75

0.00

0.50

fiek:l

0.05 T

0.10T -W sw- 0.12T

0.18T

- l v . 0 - 8 0 ~ -6 .0 -4 .0 -2 .0 0.0 2.0 4.0 6.0 8.0 101.0

Velocity (mm/sec) Velocity (mrn/sec)

Fig. 3. Best fits to the MSssbauer spectra of the (a) 20 ,~ Fe /Co and (b) 10 ,~ Fe /Co multilayers taken at 4.2 K.

264 O.F. Bakkaloglu et al. / M6ssbauer studies o fF e / C o and Fe / Cu

directed normal ly to the film, and in MLs with mag- net isa t ion in the p lane of the film, the field was ap- plied along the normal to the layers (case a) and in MLs with a normal c o m p o n e n t of magne t i sa t ion the field was appl ied in-plane (case b). The evolut ion of the M6ssbaue r spectra of the 10 ,~ F e / C o ML (case a) is shown in fig. 3(a). The intensi t ies of the second and fifth lines decrease with increasing field s t rength. At 2.5 T the intensity rat io becomes 3 : 0 : 1, indicat ing that the magnet i sa t ion vector is along the 7-ray beam. The equivalent spectra for the 20 A F e / C o M L (case b) are shown in fig. 3(b). In cont ras t to the 10 A M L the second and fifth lines now increase with field until at 0.18 T the rat io is 3 : 4 : 1, indicat ing tha t that angle be tween the Fe magne t i sa t ion and the ~,-ray beam is 90 ° .

Values of anisotropy can be ob ta ined from such measurements , with the total energy of the system in the field B wri t ten as E = Kp s in2d~-p~B cos 0 for

Table 1 Values of anisotropy constant Kp and K n in J atom evaluated from the applied field M6ssbauer measurements at 4.2 K.

Thickness Kp (xl0 -24) K n (×10 -24) (A)

Fe/Co 75 0.38 +_ 0.03 0.26 +_ 0.02 50 0.92_+0.19 1.02_+0.21 20 1.07+0.16 1.21_+0.18

Fe /Cu 30 ~ 4.9 20 ~ 5.0

the in-plane magnet i sed MLs and E = K n sin40 + Kp sin2~b - / ~ B sin 0 for the MLs with a normal com- ponen t of magnet isat ion. The magnet i sa t ion vector makes an angle ~b with the sample p lane and 0 with the applied field and Kp and K n are in-plane and normal anisotropy constants . Mean values of Kp and Kn, ob ta ined using the values of 0 measu red as the applied field is varied and assumed values of mean momen t s for the two types of ML, are given in table 1. Because of the uncer ta in ty in the demagnet i s ing term, a compar ison of the K v values is only reasonable for samples with in-plane magnet isa t ion.

Acknowledgements: The authors would like 1o thank Dr K. O 'Grady for the use of the A G F M facility at U C N W and the S E R C for suppor t th rough grants G R / F 1 9 0 9 8 and F40870.

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