Ž .Thin Solid Films 389 2001 245�249

Ferromagnetic resonance studies of electrodeposited Ni�Cumultilayers

H. Lassria,�, H. Ouahmaneb,c, H. El Fanity d, M. Bouananie, F. Cherkaouie, A. Berradad

aLaboratoire de Physique des Materiaux, Departement de Physique, Faculte des Sciences Ain Chok, B.P. 5366, Casablanca, Morocco´ ´ ´bLaboratoire de Physique de Solide, Departement de Physique, Faculte des Sciences, B.P. 4010, Meknes, Morocco´ ´ ´

cDepartement de Physique, Faculte des Sciences et Techniques d’ Errachidia, B.P. 509, Errachidia, Morocco´ ´dLaboratoire de Physique des Materiaux, Departement de Physique, Faculte des Sciences, B.P. 1014, Rabat, Morocco´ ´ ´

eLaboratoire d’Electrochimie et de Chimie Analytique, Departement de Chimie, Faculte des Sciences, B.P. 1014, Rabat, Morocco´ ´

Received 23 February 2000; received in revised form 17 August 2000; accepted 4 December 2000

Abstract

The magnetic properties of Ni�Cu multilayers, prepared by the electrodeposition technique, have been systematically studiedŽ .by magnetic measurements and ferromagnetic resonance FMR . The Ni thickness dependence of the magnetization and

magnetic anisotropy is discussed. The FMR spectra were obtained with an applied magnetic field parallel and perpendicular tothe film plane at 300 K. The FMR linewidth of the granular multilayer is rather broad, which is attributed to some roughness.From FMR, a positive value for Ni�Cu interface anisotropy has been obtained. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: Electrodeposited Ni�Cu multilayer; Magnetization; Ferromagnetic resonance; Magnetic anisotropy

1. Introduction

The properties of magnetic compositionally modu-lated structures are of considerable interest because ofthe insight they provide into the fundamental nature ofmagnetism. Owing to its technological importance, in-herent two-dimensional nature and lack of clear-cuttheoretical understanding, the surface anisotropy in

� �these materials is of particular interest 1 . In general,we assume that the interface anisotropy energy con-stant K could be treated as originating from severalseffects, which alter the surface spins at the interfaces,

� �such as misfit strain anisotropy 2,3 , surface roughness� � � �4,5 and Neel anisotropy 6 . When there is some´interdiffusion between the layers, roughness effectsmay greatly alter the magnetic interface anisotropy.For ultrathin films, an in-plane easy axis of magnetiza-

� Corresponding author.

tion M is expected owing to the demagnetizing energy.In the literature, there are exceptions: a change from aperpendicular orientation of the easy axis of M for filmthickness t below a few monolayers to an in-planeorientation for larger t has been reported for several

� �multilayers or superlattices 7�9 . The interpretation isstraightforward: the surface anisotropy can overcomethe shape anisotropy 4�M for few monolayers, yieldinga magnetization perpendicular to the film if the con-tribution of the surface is positive. With increasingthickness, the effective surface contribution decreases,leading to a switching of M to in-plane orientation.Baberschke et al. have been reporting the reverse case,namely, a reorientation of M from in plan to out plan

Ž . � �with increasing thickness for Ni�Cu 001 films 10 . Inthe search for new materials with uniaxial anisotropy,we investigated multilayers in which Cu was the non-magnetic layer. In this paper, we have described ourstudies on Ni�Cu multilayers prepared by the elec-trodeposition technique.

0040-6090�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 1 9 2 0 - 9

( )H. Lassri et al. � Thin Solid Films 389 2001 245�249246

2. Experimental

The multilayers were electrodeposited by the dual� �bath technique 11 . The Ni layer was deposited for 1s

in the electrolyte bath which contained NiSO , 7H O,4 2Ž .varying between 30 and 200 g�l; NiCl : 40 g�l ;2

Ž .H BO : 35 g�l at 300 K, and the current density was3 3fixed at 2 A�dm2. The Cu layer was deposited for 1s inthe electrolyte bath which contained CuSO , 7H O4 2

Ž .varying between 150 and 200 g�l; H SO : 40 g�l at2 4300 K and the current density was fixed at 2 A�dm2.We varied the concentration of NiSO and CuSO in4 4

Ž .order to have the thickness of the magnetic layers tNiŽ .equal to the thickness of copper layers t . TheCu

samples have been deposited onto copper substrates atroom temperature. The Cu substrate was dipped forapproximately 1 min into a 10% H SO solution in2 4order to remove the surface oxide and then carefullyrinsed in H O. The thickness of the magnetic layers2

˚was varied in the range 100�1820 A. The number ofbilayers was in the range 3�5. The saturation magneti-

Ž .zation 4�M was measured using a vibrating sampleSŽ .magnetometer VSM . The size of the VSM samples

was 4�4 mm2. To reduce the measuring error, equalspecimens were put together in one measurement toobtain the hysteresis loops and then an average mag-netic moment per unit Ni volume was calculated. Fer-

Ž .romagnetic resonance FMR studies were carried outat 9.8 GHz with the static field applied both perpendic-

Ž . Ž .ular H and parallel H to the film plane. We� �

calculated the effective magnetization 4�M �4�Meff S�H where H is the perpendicular anisotropy field,K Kg the spectroscopic splitting factor, and the linewidth.

3. Results and discussion

The magnetization measurements were performedusing a VSM. Hysteresis loops were measured with anapplied field both parallel and perpendicular to the filmplane at 300 K. Two typical results are shown in Fig. 1.All the samples are more easily magnetized with thefield parallel to the film plane.

The VSM values for the saturation magnetizationwere found by taking the total moment of each sampleand dividing by Ni volume. Many of the films show atotal moment somewhat less than would be expected

Ž .from bulk fcc Ni approx. 4�M �6082 G . AlthoughS

Ž . Ž .Fig. 1. Reduced magnetic hysteresis loops for a Ni �Cu˚ ˚120 A 120 A 3Ž . Ž .and b Ni �Cu multilayers with applied magnetic field˚ ˚1170 A 1170 A 5

Ž . Ž .parallel H and perpendicular H to the film plane at 300 K.� �

other authors have reported a somewhat reduced mo-ment in the thickness regime we have explored, we donot see a systematic decrease with layer thickness for

˚ ˚300 A� t �1820 A. The scatter in values of 4�MNi Sfrom VSM is much less and is probably caused byerrors in the Ni amount. For samples with a Ni layer

˚thickness less than 300 A, the magnetization decreasesŽ .as t is decreased Table 1 . Our results would indi-Ni

cate that the interface diffuses due to Cu diffusion intothe Ni layer, which is not surprising considering thelarge interfacial roughness.

In the FMR spectra, the absorption line with maxi-mum intensity is the uniform mode, corresponding tothe regular in-phase excitation of the magnetization inNi�Cu multilayers. Fig. 2 shows two typical examples

˚when t �1170 and 105 A. The FMR spectrum ofNi˚Ni�Cu multilayer with t �300 A is similar to that ofNi

˚t �1170 A, as shown in Fig. 2.Ni˚ Ž .For t �105 A, besides the uniform mode H ,Ni 0

there are some absorption lines in the FMR spectra.The weak resonance mode above the uniform mode is

Ž .identified as a surface mode H , as described theo-surf� �retically by Puszkarski 12 . On such a film, the Cu

layers may not be ideally continuous films under our

Table 1Summary of magnetic parameters on Ni�Cu multilayers

˚Ž . Ž . Ž . Ž . Ž .t A 4�M G 4�M G H Oe � H OeNi S eff K

105 3768 1793 1975 2168120 3957 2239 1718 �

300 5100 4390 710 16001170 5025 4903 125 14931820 5025 5000 25 �

( )H. Lassri et al. � Thin Solid Films 389 2001 245�249 247

Ž . Ž . Ž . ŽFig. 2. FMR spectra for a Ni �Cu and b Ni˚ ˚105 A 105 A 5 1170. Ž�.�Cu multilayers. Applied dc field along the film plane˚ ˚A 1170 A 5

Ž .and � applied dc field perpendicular to the film plane at 300 K.

deposition conditions; some ‘pinholes’ might exist inthe Cu layers, leading to the partial contact of the

Žsuccessive Ni layers. The effective field including de-.magnetizing field and anisotropy fields , because of the

proximity effect and rough interfaces in Ni�Cu multi-layer samples, is not completely homogeneous through-out the entire Ni layers; therefore, the absorptionmode below the uniform mode is observed in the FMRspectrum. We have considered only the strongest modeto calculate 4�M and the g factor. The g factor foreffall the samples lies in the range 2.0�2.2, in agreementwith the classical value of 2.2 for Ni. This justifies thechoice of the absorption mode for our calculation. The4�M and the line width � H in the parallel geome-eff �

try of Ni�Cu multilayers change correlatively with vary-Ž .ing Ni layer thickness t � t . With increasing t ,Ni Cu Ni

Žthe effective magnetization increases from a value of

˚ ˚.1790 G for t �105 A to 4903 G for t �1170 A andNi NiŽthe linewidth decreases from a value of 2168 G for

˚ ˚.t �105 A to 1493 G for t �1170 A , respectivelyNi NiŽ .Table 1 . The FMR linewidths of our earliest samplesare rather broad, which is an indication of some mi-croscopic inhomogeneity of the interfacial alloy and theinterfacial roughness.

The effective anisotropy was deduced using the fol-lowing expressions where the resonance field H cor-resresponds to the main mode:

For the perpendicular geometry we have:

Ž . Ž .��� �H �4�M . 1� � eff

For parallel geometry we get:

2Ž . Ž . Ž .��� �H H �4�M . 2 eff

� �According to previous studies 1,13 , an interface mag-netic anisotropy of multilayers can be deduced throughthe dependence of the perpendicular anisotropy on thethickness of the magnetic layer, if the interfaceanisotropy essentially enhances the first-orderanisotropy energy K . Then the perpendicular1

Ž .anisotropy field excluding the demagnetization termcan be written as:

Ž .H �H �2 H �t , 3K U S Ni

i.e.

Ž .H �4�M �4�M �H �2 H �t . 4K S eff U S Ni

The perpendicular anisotropy field H �2 K �M in-K 1 Scludes two terms, a volume anisotropy H �2 K �MU U SŽwhere K is the sum of magnetoelastique anisotropyU

.energy K and a magnetocrystalline energy KME MCand a surface-induced one H �2 K �M .S S S

When the demagnetization energy is included, Eq.Ž .3 can be written as:

Ž .K �K �2 K �t , 5eff V S Ni

where

2 2 Ž .K �K �2�M , K �K �2�M . 6V U S eff 1 S

K and K are the effective anisotropy energy andeff VŽeffective volume anisotropy energy, respectively in-

.cluding the demagnetizing energy . K was also de-effduced from VSM measurements as the area betweenthe perpendicular and the parallel magnetizationcurves, in order to get rid of the exchange coupling. InFig. 3, we have reported the variation of H obtainedKfrom both FMR and VSM measurements as a functionof 1�t for the multilayers with different Ni layerNi

( )H. Lassri et al. � Thin Solid Films 389 2001 245�249248

Fig. 3. Variation of H vs. 1�t for Ni�Cu multilayers at 300 K.K Ni

thicknesses. A linear variation of H vs. 1�t isK Nishown. From the slope of the straight line, the value ofthe interface anisotropy constant K is deduced to beSK 0.21 erg�cm2, where M �400 G is used. Also byS Sextrapolating the straight line to 1�t �0, we obtainedNi

Ž 4 3.H ��56 Oe K ��1.1�10 erg�cm . Here, KU U Sis a positive value, which means that the interfaceanisotropy confines the magnetization to the film nor-mal. The small volume anisotropy K value for Ni�CuUmultilayers may arise from the large interfacial rough-ness. A very large difference in K has been foundUbetween our Ni�Cu multilayers and Ni magnetic films

Ž . Ž . � �grown on Cu 100 �Si 100 14 , displaying the impor-tant role of the epitaxial growth of magnetic metals onthe magnetic properties of film.

Ž .For the surface mode k� ik observed in the NiS˚Ž .layer with t �105 A , the field splitting with respectNi

to the uniform mode is described by:

Ž . 2 Ž .H �H � 2 A�M k 7surf 0 S S

where A is the exchange stiffness constant obtained byŽthe conventional spin-wave theory the saturation mag-

3�2 .netization obeys the T law . The exchange constantat 300 K is found to be 1.2�10�7 erg�cm, which is

Ž �6 .lower than that of the nickel bulk A�10 erg�cm ,which is not surprising considering the large roughinterface.

If the presence of such a mode were to be inter-preted solely in terms of a surface anisotropy, taking� �15,16 k �K �A, one would deduce at 300 K aS Svalue K 0.23 erg�cm2 characterizing the Ni�Cu in-S

terface, in agreement with that obtained from the FMRstudy.

The demagnetization field and magnetostatic energyof a thin film with surface roughness has been calcu-

� �lated by Bruno 4 . It is shown that the surface rough-ness gives rise to an effective perpendicular anisotropywhose order of magnitude is evaluated as a function ofthe parameters characterizing the roughness. Realsamples always present some roughness, and it is im-portant to take this into account in the theoreticaltreatment. Bruno et al. characterized it by the rough-

Žness � the average deviation from the ideally flat. Žsurface and the correlation length � the average

.lateral size of flat areas on the surface .In order to study the effect of roughness on dipolar

� �anisotropy, Bruno et al. have calculated 4 analyticallythe magnetostatic energy of a film with periodic rough-ness, within the continuous medium approximation.

The surface anisotropy density is given by:

2 Ž . � Ž .� Ž .K �2�M 3�4 � 1� f 2���� 8S S

where

�� ��3 4Ž . Ž .f x � 4 �x� Ý Ý

n�0 m�0

�1�22 2ŽŽ . Ž . .1�exp �x 2n�1 � 2m�1ž /

2 2Ž . Ž .� � 2n�1 2m�1

1�22 2ŽŽ . Ž . . Ž .� 2n�1 � 2m�1 9

This surface anisotropy is purely dipolar and hasnothing to do with the magnetocrystalline surface

� �anisotropy pointed out by Neel 6 . We can evaluate the´˚order of magnitude of K by choosing �38 A andS

˚ 2�140 A; we obtain K 0.21 erg�cm .S

4. Conclusion

We have presented the FMR results for Ni�Cumultilayers prepared by electrodeposition in a dualbath, and we have reached the conclusion that we canprepare multilayers with different thicknesses of Ni andCu. The magnetic anisotropy of the Ni�Cu multilayershas been investigated by FMR and VSM measure-ments. A positive interface anisotropy constant fa-voring a perpendicular easy axis was obtained. Theinterface roughness gave rise to an effective perpendic-ular anisotropy.

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