0 Elsevier, Paris
Electrodeposited Ni/Cu multilayers Ann. Chim. Sci, Mat, 1999,24, pp. 505-508
PROPERTIES OF ELECTRODEPOSITED NXu MULTILAYER STRUCTURES
H. EL FANITyd, M. BOUANANIb, H. I.&SRIC, F. CHERKAOUIb, H. OUHAMANEd, A. DINIAe, A. BERRADAa
a Laboratoire de Physique des MatBriaux, Dopartement de Physique, Faculti des Sciences, BP. 1014, Rabat, Maroc.
b Laboratoire d’Elactrochimie et de Chimie Analytique, Dcpartement de Chimie, Faculti des Sciences, BP. 1014, Rabat, Maroc.
’ Dopartement de Physique, Faculti des Sciences Ain Choc, BP. 5366, Casablanca, Maroc.
d DBpartement de Physique, Facult6 des Sciences, BP. 4010 Meknes, Maroc.
e IPCMS-GEMM, CNRS, 23 rue du Loess, 67037 Strasbourg, France.
Abstract - The interface structure and magnetic properties of electrodeposited NiiCu multilayers have been investigated. The layer thickness of both Cu and Ni range from 200 to 6OOOA. The Ni and the Cu layers are polycrystalline with a dominant (111) tibre texture. The magnetization and M-H loops of the samples were determined using a vibrating sample magnetometer (VSM). Ferromagnetic resonance (FMR) was observed at 9.8 GHz. The linewidth of the granular multilayer is attributed either to some roughness or to small fluctuations of magnetization and is about 1.5 kOe when the applied magnetic field is in the plane of the film.
R&urn& - PropriMs structurales des multicouches Ni/Cu ClectrodCposCes. Nous Ctudions la structure aux interfaces et les proprittes mag&iques des multicouches Ni/Cu Blectrodeposees. Les epaisseurs des couches de Cu et de Ni sont comprises entre 200 et 6OOOA. Le Ni et le Cu sont polycristallins avec une texture dominante dans la direction (111). Les courbes M-H sont dt%ermin& a l’aide d’un magnetometre ii Cchantillon vibrant (VW). La resonance ferromagnetique (FMR) est observee a 9,8 GHz. La largeur de la raie de la multicouche granulaire est attribuee soit a la rugosite, soit a de faibles fluctuations de l’aimantation; cette largeur est de l’ordre de 1.5 kOe quand le champ magnetique est applique dans le plan de la multicouche.
1. INTRODUCTION
The magnetization of ultrathin layers has attracted extensive interest in recent years [l]. The two-dimensional, coupling between neighbouring layers, and the deformation of the lattices have all been explored for their contributions to the observed magnetic properties.
Owing to the ferromagnetic nature of Ni, a study of the magnetic properties of this system can reveal additional information on the state of the interfaces. Therefore, we have focused our attention on some magnetic properties such as, magnetization, anisotropy properties; structural studies have been analysed using X-ray diffraction using Cu Ka radiation, h =15406A.
Reprints: H. El Fan&y, Laboratoire de Physique des Materiaux, Departement de Physique, Faculte des Sciences, Rabat, Maroc.
506 H. El Fanity et al.
2. EXPERIMENTAL DETAILS
NtiCu multilayers were electrodeposited by the dual bath technique [S]. The Ni-layer is deposited during 1s in the electrolyte bath which contains NiS0,,7H,O: (varying between 30 and 200 g/l); NiCI,: (4Og/l); H3B03: (35g/l). The Cu-layer is deposited during 1s in the electrolyte bath which contains CuS0,,7H,O: (varying between lSOg/l and 2OOg/l); H,S04: (4Og/l). Both the Cu and Ni layers are deposited at 25°C and the current density is fixed at 2A/dm*. We have varied the concentration of NiSO., and CuS04 in order to have ANi=ACu (range from 200 to 6000A), thus we do not mention t-, for each multilayer.Tin doped indium oxide was used as substrate and platinum counter electrode. The structures studied were analysed by x-ray diffraction measurements. Magnetization (M) and M-H loops were measured with a vibrating sample magnetometer. The M values were derived i?om the data by dividing the total moment of each sample by the volume corresponding to the amount of Ni (derived from the plating current 2A/dm’). RFM is observed at 9.8 GHz with the dc applied field both in the film plane and perpendicular to it.
3. RESULTS AND DISCUSSION 3.1. Structures
The structures were analysed by X-ray diffraction. The X-ray diffraction patterns of Ni/Cu (tNi=lOOOA) show that both the Ni and Cu metal layers are polycrystalline with a dominant (111) tibre texture figure I). The lattice parameters are aNi=3.5268A and ac”=3.6135A which are close to the values of the corresponding bulk structure. The difference between the experimental and the bulk values Aa/a=0.08%, generally attributed to compressive stresses in sublayers parallel to the Ni/Cu interfaces [9], seems weak in our case. For the sample with tui=1500A, the central and satellite peaks are too close to be resolved, and only the envelope function can be seen.
I I
40 60
If0 100 120
2 k+%+ 140
Figurel. X-ray diffraction pattern of Ni/Cu structure with the intereticular d,,tl (A) values deduced from dsine=nh (tNi = 1000 A)
3.2. Magnetization behaviour
Figure 2 shows both the parallel and the perpendicular M-H magnetization curves for the samples with tNi=l5OOA at 300K. Magnetization with H/l can be easily saturated while a much larger field is required in the case of HI. The error in the determined magnetization is about 10 %, resulting primarily from errors in sample measurement and, to a lesser extent, from errors in thickness control and VSM magnetometer. All the samples show quasi rectangular in-plane M-H loops indicating the easy plane magnetization. The spontaneous magnetization depends on l/tNi. It is known [3] that this variation can be described phenomeno- logically by the relation:
M= MO( 1 -2tO/tui) (1)
Electrodeposited Ni/Cu multilayers 507
where, M and MO stand respectively for the spontaneous magnetization of the multilayer and of bulk Ni, and TV is the dead layer at each interface. Figure 3 shows a plot of M.tNi as a function of tNi at 300K. It is seen that the experimental points follow a straight line. The intercept on the abscissa gives the value of 2t0 which is found to be lOA. The slope of the straight line yields MO= 480 emu/cm3 at 300K.
-;400 H// HI
0
‘;
E s
300 LTII
200 I 100
Oo 2 4 10 12 14
Figure 2. Magnetization curves at 300K in the parallel and the perpendicular geometries (111)
Ni/Cu multilayers with tNi’l5OOA.
Figure 3. The variation of the product M.tNi as a function of tNi
3.3. Anisotropy
In order to obtain the parallel anisotropy K,ff and the parallel anisotropy field Ha, the torque was measured in the plane. Figure 4 shows the field dependence of torque curve measured in the film plane for (Ni/Cu)o films where tNi=lOOOA and H=15 kOe. At H=15 kOe, we have reach saturation state and the fitting with sinusoidal fonction allows to deduce K,ff=1.2 lo6 erg/cm3. The amplitude of the torque curves for Ni/Cu films increases with the field until the saturation, for which the torque curve becomes a regular sinusoid (seefigure 4).
Torque studies yield the effective anisotropy KS. The tNi dependence of Keff could be analysed on the basis of the well known phenomenological model which predicts the following relation:
&e Kv+2Ks/tNi (2)
where the volume anisotropy K,= K,-.yst-2nMs2 (GIyst being the crystalline anisotropy) and KS is the surface anisotropy arising from the surface Ni atoms. The following on a straight line of I(tff-tni with tni allows to deduce K, by extrapolation at tNi=O. In our case &=O.123 erg/cm’ is relatively weak and this is in agreement with results on NiiTi [4] and that on Ni/Ag [S].
8;
0 100 200 300
0 (degree)
Figure 4. The torque curve for Ni/Cu samples with tni= 1OOOA
3.4. Ferromagnetic resonance
The condition for resonance depends on the shape of the sample because of the demagnetizing field. In the present experiment the samples are thin tilms 200-1500A thick and several square millimetres in area. For these samples, the external field can be placed in the plane of the sample
H. El Fanity et al.
(parallel resonance) or can be placed out of the plane and perpendicular to it (perpendicular resonance).
The Kittel equations for parallel and perpendicular resonance are, respectively:
f/y (H) - (H+4rtMeff)“2 (3)
f/y H-4nM,ff (4)
where y is the gyromagnetic ratio of the electron spins, which is 2.8 GHz/kOe for free spin.
We have obtained the ferromagnetic resonance spectrum of the Ni/Cu granular multilayer sample at 9.8 GHz with the magnetic field both parallel and perpendicular to the film plane. Room- temperature FMR spectra are shown infrgure 5. The parallel resonance is centred at 1.33 kOe and has a linewidth AH/l= 1.5 kOe, while the perpendicular resonance is centred at 7.8 kOe with a linewidth AHL= 2 kOe (AH1 is much larger than AH//). Using the Kittel equations for the Ni/Cu sample, we obtain g= 2.3 and 4rrM,ft= 4.9 kOe. The FMR linewidths of our earliest samples are rather broad, which is an indication of some microscopic inhomogeneities of the films. Other granular multilayer systems [7] are found to have flat, islande like precipitates. We subsequently found that even the polished IT0 substrates used gave the same order of magnitude broad lines when coated by evaporation or electrodeposition, indicating that some roughness was replicated from the substrates [6].
I2669
I8786 H (Oe)
Figure 5. FMR spectrum of Ni (5OOA)/Cu with external field applied parallel and perpendicular to the plane of the film (9,8GHz)
In conclusion, we have presented magnetic and structural results for Ni/Cu multilayers prepared by electrodeposition in a dual bath. We reach the conclusion thatit is possible to prepare multilayers with different thicknesses of Ni and Cu and good crystal quality by this method.
4. REFERENCES
[1] Chin-An Chang, J.Appl. Phys., 68 (1990) 4873.
[2] Mark A. Hollanders, Barend J. Thijsse, Eric J. Mittemeijer, Physical Review, B 42 (9) (1990) 5481.
[3] Grang Xiao and C.L. Chien, J.Appl. Phys., 61 (1987) 4061.
[4] M. Porte, H. Lassri, R. Krishnan, M. K&tbouchi, M. Mti and C. Sella, Appl. Surf. Sci., 65-66 (1993) 131.
[5] R. Krishnan, M. Porte, M. Tessier, H. Szymczak and R Zuberek, Proceedings of the fifth International Conference on Physics of Magnetic Materials, edited by W. Gorzkowski, M. Gutowski, H. H. Lachowicz and H. Symczak, World Scientific, Singapore, New Jersey, London, Hong Kong, (1990) 294.
[6] M. Dariel, L. H. Bennett, D. S. Lashmor, P. Lubitz, M. Rubinstein, W. L. and M. Z. Harford, J.
Appl. Phys., 61 (1987) 4067.
[7] T.L. Hylton, K.R. Coffey, M.A. Parker and J. K. Howard, Science, 26 1 (1993) 102 1.
[8] J. P. Celes, A. Haseeb and J. R. Roos, Trans. Inst. Metal Finish, 73 (3) (1992) 123.
(Article requ le 15/07/98, sous forrne definitive le 10/12/98.)
