Ferromagnetic resonance investigation of electrolytically deposited Co films on Au(1 1 1)

Surface Science 482±485 (2001) 1035±1039

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Ferromagnetic resonance investigation of electrolytically deposited Co ®lms on Au(1 1 1) J. P¯aum a,*, D. Spoddig a, J. Pelzl a, J.L. Bubendor€ b, J.P. Bucher b a b

Institut f ur Experimentalphysik, Fakultat fur Physik und Astronomie, Gebaude NB/3, AG Festkorperspekroskopie, Ruhr-Universitat-Bochum, 44780 Bochum, Germany Institut de Physique et Chemie des Mat eriaux de Strasbourg (IPCMS), Universit e Louis Pasteur, 23 Rue du Loess, F-67037 Strasbourg Cedex, France

Abstract The magnetic properties of cobalt ®lms deposited electrolytically on Au(1 1 1) have been studied by ferromagnetic resonance. A sixfold contribution of the magnetic anisotropy, much smaller than in bulk cobalt, is detected. The sixfold magnetic anisotropy of hcp(0 0 0 1) and fcc(1 1 1) Co decreases with increasing ®lm thickness due to increasing misalignment of the preferred orientation of the Co crystallites. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical methods; Magnetic measurements; Crystallization; Cobalt; Magnetic ®lms

1. Introduction Electrodeposition is frequently used in the industry for the preparation of homogeneous laterally extended magnetic metallic ®lms. These ®lms are commonly polycrystalline with thickness of the order of microns. As compared to UHV techniques like evaporation or sputtering the electrodeposition o€ers the unique means that the driving forces of the coating process can be controlled. Despite this advantage, the electrodeposition method has been applied only recently to the epitaxial growth of thin magnetic ®lms. The major goal of the recent research is devoted to monitor the layer by layer growth by thin layer sensitive

* Corresponding author. Tel.: +49-234-7002411; fax: +49234-7094172. E-mail address: p¯[email protected] (J. P¯aum).

techniques such as scanning tunnelling microscopy (STM), magnetic optical Kerr e€ect (MOKE), nuclear magnetic resonance (NMR) and ferromagnetic resonance (FMR). The growth and the structure and related magnetic properties of electrolytical Ni ®lms on Au(1 1 1) on mica substrates have been studied by in situ STM and by FMR and MOKE experiments [1,2]. The experimental ®ndings show that the structure and the correlated magnetic anisotropies are strongly a€ected by the choice of the overpotential and the pH value of the solution. Electrolytically deposited Co ®lms on Cu(1 0 0) investigated by in situ MOKE and STM demonstrate the high local quality of the ®lms in the monolayer and submonolayer range [3,4]. The magnetization was found to vanish below 1.5 monolayers as in the case of MBE grown ®lms. However, in contrast to these MBE ®lms the coercivity of the electrodeposited ®lms which increased roughly linearly with the layer thickness

0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 7 4 2 - 7

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J. P¯aum et al. / Surface Science 482±485 (2001) 1035±1039

was one order of magnitude larger. A detailed study of Co deposited on Au(1 1 1) using in situ STM and ex situ MOKE and NMR show that the structure and to some extend also the magnetic properties can be adjusted by the choice of the overpotential and the pH value [5±7]. At higher pH values and high overpotentials the hcp structure dominates in the early stage of the growth process for Co-layer thicknesses below 2 nm. It was possible to stabilize the perpendicular orientation of the magnetization for thicknesses between 2 and 7 monolayer [5]. Increasing the Co-layer thickness the fcc phase is progressively formed. For lower pH values the proportion of the fcc becomes lower and more faulty. The present work is a continuation of these studies. We report on a detailed investigation of the magnetic properties with the particular emphasis to the dependence of the anisotropy and of the lateral correlations on the preparation parameters during the electrodeposition.

was measured in the dissolution step just before the last deposition process. To protect the Co sample for the subsequent ex situ studies the ®nal layer was covered by a 6 nm thick Cu ®lm which was deposited at the end of the process by adding CuSO4 (10 2 M) to the solution. From SQUID magnetometry characterization, the 100 nm thick cobalt ®lms, are found to have saturation ®elds of 10 kOe and coercive ®elds of 200 Oe. The experimental set-up used for the FMR measurements consists of a conventional X-band spectrometer. The experimental conditions were: microwave frequency f ˆ 9:33 GHz microwave power P ˆ 0:2 mW. The samples were positioned in a TE102 rectangular cavity on a goniometer head which allowed rotation of the ®lm in plane and out of plane with respect to the magnetic ®lm. All measurements reported here were performed at room temperature.

3. Results and discussion 2. Experimental The preparation of the electrodeposited Co ®lms was the same as already described in detail in Refs. [5,6]. Therefore, we shall only present here the main points that are important for the discussion of the present results. The Au substrates were prepared by vacuum evaporation onto cleaved mica substrates. To obtain (1 1 1) textured ®lms with terraces the 100 nm thick gold ®lms were ¯ame annealed. Subsequent electrodeposition of Co has only been carried out on those substrates p which displayed the characteristic 3  22 reconstruction and deconstruction in the cyclic voltamogram. The electrodeposition was performed by a three-electrode arrangement under potentiostatic control. The electrolyte solution of CoSO4 , CoCl2 , H3 BO3 , by distilled water was contained in a Pyrex cell of about 50 ml volume. The overpotential g during deposition corresponds to the departure from the Nernst potential of the Co/Co2‡ couple. The Co deposition starts at g ˆ 0:18 V. Films treated in this report have been grown at the overpotential g ˆ 0:68 V. The amount of deposited Co, up to an equivalent thickness of 100 nm,

The theoretical model for the evaluation of the magnetic parameters is based on the equation of motion for the precession of the magnetization M around an e€ective internal ®eld Beff . This problem has to be solved for each magnetic layer of the speci®c sample. This leads to a FMR-dispersion relation expressed in terms of the partial derivatives of the free energy density F per surface unit [7]. The resonance frequency xres at the in-plane con®guration where the external ®eld is varied by the angle / is then given by the relation …xres =c†2 ˆ fB0 cos …/

/0 † ‡ l0 Meff

nBn0 cos …n/†g  fB0 cos …/

/0 †

n2 Bn0 cos …n/†g …1†

where Bn are the e€ective anisotropy ®elds de®ned by the ratio of the anisotropy constant Kn and the saturation magnetization Ms : Bn0 ˆ Kn0 =Ms . The e€ective magnetization also contains the contribution of the interface anisotropies Ks which with respect to the volume terms scale with 1=d where d is the thickness of the magnetic ®lm. n0 is di€erent

J. P¯aum et al. / Surface Science 482±485 (2001) 1035±1039

Fig. 1. FMR signal of an electrodeposited 100 nm thick Co ®lm on Au(1 1 1) as a function of the external magnetic ®eld which was aligned in ®lm plane. The spectrum was recorded at room temperature at a microwave frequency of 9.33 GHz.

to n due to historical reasons. For cobalt the hexagonal symmetry is of importance with n ˆ 6 and n0 ˆ 4. Fig. 1 shows a typical FMR spectrum of an electrodeposited Co ®lm of thickness 100 nm. The trace corresponds to the ®eld derivative of the imaginary part of the magnetic high frequency susceptibility. Information about the magnetic parameters of the sample under investigation are provided by di€erent characteristics of the FMR resonance line. Provided that the sample magnetization is collinear with the external ®eld the area under the FMR absorption curve is proportional to the magnetic moment of the sample. However, to get absolute values this method requires a complicate normalization procedure that takes into account e€ects like skin depth for thicker samples. The equal peak intensities of the resonance signal in Fig. 1 indicate that the magnetization of the sample is collinear to the applied magnetic ®eld. Additionally, the shape of the FMR line is observed to be very symmetric around the resonance position Bres , which also shows that the direction of external magnetic ®eld and magnetization are parallel over the region of resonant absorption. The peak-to-peak resonance line width (DBpp in Fig. 1) represents a material speci®c property of each ferromagnet and is composed of a homogeneous intrinsic contribution that is frequency de-

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pendent and an inhomogeneous frequency independent contribution. From both parts one gets insight in the phenomenon of damping and in the properties in¯uencing this damping process. The strong spin±orbit coupling of Co, which is two times larger for Co than for Fe, provides a strong damping of the magnetization expressed in the broad line width DBpp which is observed in MBE grown ®lms [8]. The peak to peak halfwidth DBpp is about 45 mT at a microwave frequency of 9.33 GHz. Comparison with the value measured in epitaxially MBE grown Co ®lms with DBpp  25 mT indicates that the intrinsic strong spin±orbit coupling cannot be the only mechanism responsible for such a large broadening of the FMR lines. Of course, due to the loss of coherence within the ferromagnetic layers on mica substrates in comparison to ®lms prepared by epitaxially growth techniques one can assume that the frequency independent part as well as the intrinsic damping part of the FMR line width should be increased for the electrolytically grown samples, e.g. by grain boundaries and local anisotropies. Measurements performed at higher microwave frequencies will allow the separation of these two contributions. From the position of the resonance line, Bres , and its angular dependence one can deduce several important magnetic parameters such as the e€ective magnetization Meff and the anisotropy constants of di€erent orders Kn0 . In addition, one can obtain information about the in¯uence of the surface or interface on the magnetic behaviour of the volume via Ks , and information about changes of local intrinsic properties by the spectroscopic splitting factor g which is a measure of the ratio of the spin and the orbital contribution to the local magnetic moment in the magnetic compound. From the position of the FMR line, indicated by Bres in Fig. 1 the magnetization of the samples as well as possible anisotropy contributions were deduced. For the estimation of these quantities we performed in-plane FMR measurements where the magnetic ®eld and magnetization were lying in the ®lm plane and the sample was rotated with respect to the applied magnetic ®eld. For the interpretation of the data a spectroscopic splitting factor g factor of g ˆ 2:19 was adopted which corresponds to the value of the hcp

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J. P¯aum et al. / Surface Science 482±485 (2001) 1035±1039

bulk system. The angle dependences of the resonance ®elds are displayed in Figs. 2 and 3 for two samples with di€erent Co thicknesses tCo ˆ 80 and 100 nm, respectively. As could be clearly seen by the ®gures both samples show a variation of the resonance ®eld with a period of 60° and a decreasing amplitude with increasing ®lm thickness. The ®tted line results from a minimization of the total free energy density with respect to the angular dependence of the magnetization process.

Fig. 2. Variation of the resonance frequency Bres of the FMR signal of an electrodeposited 80 nm thick Co ®lm on Au(1 1 1) as a function of the orientation of the external magnetic ®eld which was aligned in ®lm plane. The spectrum was recorded at room temperature at a microwave frequency of 9.33 GHz.

Fig. 3. Variation of the resonance frequency Bres of the FMR signal of an electrodeposited 100 nm thick Co ®lm on Au(1 1 1) as a function of the orientation of the external magnetic ®eld which was aligned in ®lm plane. The spectrum was recorded at room temperature at a microwave frequency of 9.33 GHz.

According to relation (1) the FMR is sensitive to the angular deviation of Ms which yields for the di€erence of the easy axis and the hard axis of magnetization Bhard Beasy res res  72K4 =Ms . The prefactor is a typical feature of the dynamical response detected by the FMR spectroscopy (see relation 1) which makes this technique extremely sensitive for higher order anisotropy investigations as compared to other static methods. The sixfold anisotropy, K4 =Ms , is caused by the hexagonal crystallographic structure of the ®lms, namely hcp (0 0 0 1) with the c-axis along the plane normal, fcc(1 1 1) or a mixture of both. Indeed, by NMR investigations on the hyper®ne ®eld of the samples a coexistence of the hcp and fcc phase was found and, moreover, for the pH value of 4 and an overpotential of g ˆ 0:68 V at which the samples were prepared, a relative increase of the fcc phase in comparison to the hcp phase up to an thickness of around 100 nm was established [9]. Above this thickness both phases reach an equilibrium state of 60% fcc and 40% hcp. Assuming that the statistical distribution of the preferred directions of the crystallites becomes much more irregular with increasing ®lm thickness permits an explanation for the decrease of K4 =M from tCo ˆ 80±100 nm. Of course, one has to notice that due to the loss of preferred orientation of the crystallites the values of the sixfold anisotropy only amount to 0.05 mT and, therefore, are much smaller than the value obtained for epitaxially grown hcp Co of K4 =M ˆ 4 mT [10]. An additional hint to changes of the structural composition is given by the value of the e€ective magnetization, l0 Meff . For both ®lms the values of the magnetization, l0 Meff ˆ 2:34 T for tCo ˆ 80 nm and l0 Meff ˆ 2:22 T for tCo ˆ 100 nm, are remarkably enhanced by 25% to the bulk value of l0 Meff ˆ 1:82 T. Probably this is related to the orientation of the hcp c-axis (the easy axis of magnetization) and the fcc [1 1 1]-axis along the surface normal as found by electron di€raction or by localized chemical e€ects. In addition, we were able to resolve an in-plane uniaxial anisotropy contribution which is estimated to 0.5 mT for both samples. From former studies on electrodeposited ferromagnetic ®lms it is known that this anisotropy contribution is

J. P¯aum et al. / Surface Science 482±485 (2001) 1035±1039 Table 1 Magnetic parameters of the electrodeposited Co ®lms at room temperature Anisotropy ®elds

100 nm Co ®lm

80 nm Co ®lm

Bulk hcp Co

l0 Meff K4 =Ms Ku =Ms

2.22 T 0.04 mT 0.5 mT

2.34 T 0.07 mT 0.5 mT

1.82 T 4.0 mT ±

strongly a€ected by growth conditions, like deposition rate, substrate, etc. and therefore, we are able to get some helpful insight in the mechanism of the growth process, vice versa [1,2]. A summary of FMR parameters is given in Table 1.

4. Conclusions Using FMR technique we have shown the possibility of characterizing the quality of an electrodeposited ®lm by inspection of its magnetic properties. The results of the present work clearly indicate a strong in¯uence of the preparation condition on these properties. Investigation of a sample series, prepared at di€erent pH values and at various other overpotentials should allow one to establish a relation between deposition parameters and structural as well as magnetic properties providing the possibility of tailoring magnetic samples

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for application, such as devices using giant-magneto resistance (GMR). Acknowledgements The work was performed in the frame of a PROCOPE project. Partial support by the SFB 166 is acknowledged. References [1] J.L. Bubendor€, J. P¯aum, E. Huebner, Dr. Raiser, J.P. Bucher, J. Pelzl, J. Magn. Mag. Mat. 165 (1997) 199. [2] J. P¯aum, E. H ubner, Th. Zeidler, T. Schmitte, J. Pelzl, Thin Solid Films 318 (1998) 186. [3] W. Schindler, J. Kirschner, Rev. Sci. Instrum. 67 (1996) 3578. [4] W. Schindler, J. Kirschner, Phys. Rev. B 55 (1997) R1989. [5] J.L. Bubendor€, E. Beaurepaire, C. Meny, P. Panissod, J.P. Bucher, Phys. Rev. B 56 (1997) R7120. [6] J.L. Bubendor€, L. Cagnon, V. Costa-Kieling, P. Allongue, J.P. Bucher, Surf. Sci. 384 (1997) L836. [7] J.L. Bubendor€, E. Beaurepaire, C. Meny, J.P. Bucher, J. Appl. Phys. 83 (1998) 7043. [8] G.V. Skrotskii, L.V. Kurbatov, in: S.V. Vonsovskii (Ed.), Ferromagnetic Resonance, Pergamon, Oxford, 1966, p. 12±77. [9] J.L. Bubendor€, C. Meny, E. Beaurepaire, P. Panissod, J.P. Bucher, Eur. J. Phys B 17 (2000) 635. [10] F. Schreiber, A. Soliman, P. B odeker, R. Meckenstock, K. Br ohl, J. Pelzl, I.A. Garifullin, J. Appl. Phys. 75 (8) (1994) 7004.