Structural and magnetic properties of Fe/Ni and Fe/Co multilayers

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Structural and magnetic properties of Fe/Ni and Fe/Co multilayers

R. Krishnan, H. O. Gupta, H. Lassri, C. Sella, and M. Kaabouchi

Citation: Journal of Applied Physics 70, 6421 (1991); doi: 10.1063/1.349916 View online: http://dx.doi.org/10.1063/1.349916

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/70/10?ver=pdfcov Published by the AIP Publishing

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Structural and magnetic properties of ‘fe/Ni and Fe/Co multilayers

R. Krishnan, H. 0. Gupta, and H. Lassri,

Laboratoire de MagnCtisme et Mat&iaux Magnhtiques, CNRS, 92195 Meuden, France

C. Sella and M. Kaabouchi

Laboratoire de Physique des Matgriaux, CNRS, 92195 Meudon, France

Fe/l% and Fe/Co multilayers (ML) have been deposited by dc triode sputtering, with a layer thickness (t) in the range l-10 mu. For t> 4 nm both metals are in crystalline form (bee Fe, fee Ni, hcp Co). For t < 2 nm one has a single-phase structure (fee in Fe/Ni ML,

bee in Fe/Co ML) which can be explained by assuming the presence of a l-rim-thick mixed layer at the interfaces. This interfacial mixing affects the magnetic properties. The basic difference between the two systems is the fact that while Fe-Ni inter-facial alloys are magnetically soft those of Fe-Co are hard which leads to different magnetic behavior.

Furthermore, the Fe layers are better crystallized in our samples than those prepared by rf magnetron or ion-beam sputtering.

I. INTRODUCTION

There is a great demand for high density magnetic recording and work is going on in a vigourous manner in several laboratories in search of new materials. To realize magnetic heads suitable for this technology, one has to look for materials with relatively high saturation magnetization, low coercivity, near zero magnetostriction, and a high permeability at frequencies as high as 30 MHz and even above. One of the solutions lies in multilayers based on Fe, such as Fe/N&’ Fe/Co.2-4 It is well known that the magnetic properties of multilayer films are very sensitive to the nature of the interfaces between constituent layers and may be affected by the film preparation method. Recently Fe/Ni and Fe/Co multilayers have been deposited by ion- beam sputtering,‘>2 by r-f magnetron sputtering,” and by a two-facing target type dc sputtering.3 -In order to clarify whether the structural and magnetic properties of these types of multilayers depend on the deposition method, we have prepared Fe/Co and Fe/Ni multilayer films by dc triode sputtering and describe here some of the results ob- tained.

II. EXPERIMENT

Fe/Co and Fe/Ni multilayem were deposited on wa- ter-cooled glass and silicon substrates using a dc low energy (300-700 V) triode sputtering unit equipped with an accurate thickness monitoring system based on the dependence of the deposition rate on the,target current.’ Thick- ness can be controlled with an accuracy better than 1 A, therefore a high stack regularity can be achieved. The base pressure was 10 -’ Torr and the sputtering pressure 7X 10vJ Torr.

The substrate to target distance was kept at 17 cm enabling us to minimize the interaction of the plasma with the deposition surface. Under these conditions (distance, pressure, voltage) the energy of the atoms arriving at the surface is sufficiently low, favoring sharp interfaces, and the effect of the impact of backscattered Ar neutrals, pro- ducing a constant mixing of arriving atoms, is reduced.

The rate of deposition for Fe, Co, and Ni were 20, 20, and

27 A per minute, respectively. In all Fe/Ni and Fe/Co multilayers we have prepared, the metal layer has equal thickness. The number of bilayers was in the range 5-8.

The period varied from 2 to 20 nm.

The magnetization was measured using a vibration sample magnetometer (VSM) and the B-H in-plane loops were taken with an inductive type B-H looper where the applied field could be rotated in the film plane. Ferromag- netic resonance was observed at 9 GHz with external fields applied both in the film plane ( HiI) and perpendicular to it

(HL ). Low-angle x-ray diffraction was carried out to check the periodicity, Structural studies were made by transmission electron microscopy and electron diffraction.

The number of interfaces are multiplied in the multilayers which increases the accuracy of measurements by classical techniques.

Ill. RESULTS AND DISCUSSION A. Structural properties

In Fe/Ni multilayers, when the metal thickness is above 4 nm, electron diffraction patterns show that the crystal structure is a mixture of fee Ni and bee Fe (Fig. 1) . The various rings are labeled in Fig. 1. Both Fe and Ni are in crystalline form with a grain size of about 10 nm. For thinner metal layers (rNi = tFe = 2 nm), the diffraction rings of the bee Fe structure become larger and weaker than those of fee Ni. For tNi = t& = 1 nm, the bee-Fe rings disappear and a fee Ni single phase is observed (Fig. 1).

The Ni layers appear to constrain the Fe into a fee phase.

The formation of this fee single phase is attributed to interdiffusion or mixing at the interfaces. Low-angle x-ray diffraction peaks indicate that all Fe/Ni films are modulated, even the films with fee single phase. Thus these films are composed of a modulated Ni-rich fee phase and Fe-rich fee phase.

In Fe/Co multilayers, when the metal thickness is above 4 nm both bee Fe and hcp Co are in crystalline form with a grain size of about 10-20 nm [Fig. 2(a)]. For thinner metal layers (t ne = to, = l-2 nm) the diffraction rings of hcp Co disappear and only those of bee Fe are observed

6421 J. Appl. Phys. 70 (lo), 15 November 1991 0021-8979/91 /106421-03$03.00 0 1991 American Institute of Physics 6421 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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FIG. 1. Electron diffraction patterns of Fe/Ni multilayers showing a mixture of fee Ni and bee Fe for (a) ire = &?Ni = 10 nm, N=5 bilayers and a single fee phase for (b) $+ = fNi = 1 nm, N=S bilayers.

Fig. 2(b)]. In these bee single-phase films, the broken character of the diffraction rings and the micrographs indicate a recrystallization with larger grains (up to 20 nm).

The coherent grain size becomes larger with decreasing layer period. Thus these single-phase films, in which Fe

t =G, =6nm

NF: 5 bilayers

t = Go = 1 nm NF2 8 bilayers

bee Fe + hcp Co single bee phass (Fe, CO) FIG. 2. Electron diffraction pattern and electron micrograph of Fe/Co multilayer showing (a) a mixture of bee Fe and hcp Co for TFe

= rFIi = 6 nm, N=5 bilayers and (b) a single bee phase, with larger coherent grains, for it+ = tc,, = 1 nm, N=8 bilayers.

MODULATION A nm

FIG. 3. The variation of saturation magnetization and coercivity as a function of the modulation in Fe/Ni multilayers.

layers constrain the Co into a bee phase, are composed of a modulated Fe-rich bee phase and Co-rich bee phase pro- duced by an interdiffusion or mixing mechanism. The various rings are indexed. The texture that we observe is sim- ilar to that observed by Haga et aL3 who had used dc sputtering. On the contrary, Fe layers showed poor crystallinity when prepared by ion beam sputtering.2

B. Magnetic properties of Fe/Ni

Figure 3 shows the variation of the saturation magnetization (477-M) as a function of the modulation A

= (tN + tFe). This result is in agreement with that from Ref. 1. For A)8 nm, 4?rM is independent of A and is close to the calculated value of 13.8 kG (assuming 47N of Fe and Ni to be 21.45 and 6.095 kG, respectively). It is seen clearly that for A < 8 nm, 47&Z decreases to reach a value of 10.5kO.5 kG. This indicates the presence of an alloy layer at the interface. Since we know neither the alloy layer thickness nor its 4&f, it is difficult to find a unique solu- tion. However, FMR spectra of the multilayers could also be explained by the characteristics of the alloy layer, We have carried out a calculation and found that an interfacial layer of 1.5 nm thick with an average 471&4= 12 kG gave the best fit for the absorption peak position in field and intensity.b So we used the above parameters and calculated

4?rM for A= 4 and 2 nm and the results agree within f. 10% with the experimental value. Considering the various parameters involved it is not possible to refine this aspect any further.

In-plane B-H loops are rectangular and no significant anisotropy was observed in all the samples except for those with A<4 mu. Figure 4 shows both the easy and hard axes in-plane loops for the sample with A = 4 nm where the anisotropy was about 4 Oe. The presence of uniaxial anisotropy has been observed in many multilayers by many authors.7 However the origin of this is not yet well under- stood. The modulation dependence of the coercivity is shown in Fig. 3. There is a peak near A = 8 nm and H,

decreases to 1.5 Oe for A<4 nm. The lowest value of Hc for A = 4 nm can be attributed to near zero magnetostriction for this modulation as reported in Ref. 1. Also Motomura et al8 have reported that for a given Fe layer thickness

8422 J. Appl. Phys., Vol. 70, No. 10, 15 November 1991 Krishnan et a/. 6422

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‘= 4.6 Oe!

-

4c=1.5 Oe

FIG. 4. In plane (a) easy and (b) hard axes loops in for A=4nm.

Fe/Ni multilayers

H, increases with decreasing Ni layer thickness. So the ratios of thickness can also play a role here.

Annealing was carried out in a vacuum of 10 - ’ Torr at 180,250, and 320 “C for a duration of 2 h. There was no significant change in the magnetization after annealing at T<250 “C. However, after annealing at 320 ‘C, the magnetization showed a decrease of about 10-12% in all the cases indicating that further interdiffusion has occurred at the interface.

C. Magnetic properties of Fe/Co

Unlike in the case of Fe/Ni multilayers, in Fe/Co the magnetization values are lower than the calculated values

(19.6 kG). Figure 5 shows the results. It is known that a Fe-rich Fe-Co alloy has a 47rA4 higher than that of even Fe but our results do not indicate this.

Although F M R study is discussed elsewhere6 it is in- teresting to present here the results concerning the linewidth (n). It is well known that the linewidth increases due to several factors, such as, the magnetic and chemical inhomogeneities and anisotropy. Figure 6 shows the A de-

t 0 F IO- i3 I=

s!

5?

i 1 1

5 10 1’5 2’0

MODULATION A nm

FIG. 5. The variation of saturation magnetization as a function of the modulation in Fe/Co multilayers (-calculated value).

i 200- 0 .f

% 2

c IOO- z LL! 2 i

I

5 IO 15 20

MODULATION /2nm

FIG. 6. The variation of the linewidth as a function of the modulation in Fe/Ni and Fe/Co multilayers.

pendence of the linewidth measured with the field applied in the film plane (AH,,) in both Fe/Ni and Fe/Co multilayers. The sharp increase in the linewidth for A < 2 nm, in the case of Fe/Co could be attributed to the formation of magnetically hard Fe-Co alloys which are known to have a large anisotropy. In the case of Fe/Ni, however, the increase is relatively smaller and this could be attributed to the inhomogeneous broadening.

In conclusion, the magnetic properties of Fe/Ni and Fe/Co multilayers are shown to be related to the structure of the layer and the nature of the interfaces. The basic difference between the two systems is the fact that while Fe/Ni alloys are magnetically soft those of Fe/Co are hard. Also it is found that in our samples Fe layers show a better crystallinity as compared to those prepared by ion beam or rf magnetron sputtering.

ACKNOWLEDGMENTS

The partial support of this work from BRITE EURAM contract No. 0-153C is gratefully acknowledged.

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*R. Krishnan, C. Sella, M. Kaabouchi, B. Ramamurty Acharya, Shiva Prasad, and N. ‘Venkatramani, paper to be presented in ICM’ 1991, Edinburgh.

6423 J. Appl. Phys., Vol. 70, No. 10, 15 November 1991 Krishnan et al. 6423

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