www.elsevier.com/locate/susc

Surface Science 600 (2006) 4195–4199

Magnetic field effects on surface morphology and magnetic propertiesof Co–Ni–P films prepared by electrodeposition

Violeta Georgescu a,*, Mihaela Daub b

a Faculty of Physics, Al. I. Cuza University, Carol I Blvd. No. 11, Iasi – 700506, Romaniab Max-Planck-Institute for Microstructure Physics, Halle, Germany

Available online 19 May 2006

Abstract

In this paper we investigated the effect of the direction of an external magnetic field, applied during electrodeposition, on the magneticproperties and morphology of Co–Ni–P layers. The films (400 nm thickness) were electroplated on Cu plane substrates using the sameelectrodeposition parameters (temperature, pH and current density) for all experiments, with a magnetic field of 120 kA m�1 applied sothat the magnetic flux lines were horizontally parallel or perpendicular to the cathode surface. The films were compared with similarsamples obtained in the absence of magnetic field.

We observed that an induced anisotropy appeared in the Co–Ni–P films due to the preferential orientation of the easy axis of mag-netization in the magnetic field direction. These films presented also changes of the surface topography and phase images of the surface.The magneto-induced modifications in the Co–Ni–P morphology can be explained by the specific local convection of ions at the interfacecathode–electrolyte, which promotes changes both in the electrical charges of the double layer and in the thickness of the diffusion layer.� 2006 Elsevier B.V. All rights reserved.

Keywords: Magnetic films; Metallic films; Atomic force microscopy; Magnetic measurements; Electrochemical methods; Surface structure, Morphology,Roughness, and Topography; Magnetic field effect

1. Introduction

The magnetic field effects in electrochemical deposition(magnetoelectrolysis) have been intensively studied in sev-eral publications in the recent years [1–5]. The applied mag-netic field influences the ion transport and the diffusionprocess in the electrolyte. At the same time, an inducedmagnetic anisotropy and changes in the morphology ap-pear in the magnetic films prepared by this method [6–13]. The method of superimposing an external magneticfield during the electrodeposition process offers the possi-bility to tailor the microstructure and magnetic propertiesof the films mainly by the magnetohydrodynamic effects[14–17].

0039-6028/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2005.12.079

* Corresponding author. Fax: +40 232 201150.E-mail address: vgeor@uaic.ro (V. Georgescu).

We selected Co–Ni–P as an alloy of interest for thisstudy because of its interesting magnetic properties, sinceCo and Ni are ferromagnetic elements and P presents dia-magnetic behaviour. Therefore, we could expect that thedifferent ion species located in the regions close to the cath-ode surface could be differently deviated in the magneticfield. The Co–Ni–P has already been studied [18] becauseof potential applications in high density recording ormicroelectromechanical systems (MEMS), but, to ourknowledge, there has been no published study until now,which focuses on the influence of the magnetic field appliedduring electrochemical deposition on the structural or mag-netic properties of this interesting alloy.

In this work, Co–Ni–P thin films with modulated mag-netic anisotropy (in plane/perpendicular to plane) havebeen obtained by magnetoelectrolysis and their propertieshave been investigated. We intend to demonstrate thatmagnetoelectrolysis is a convenient low-cost method for

4196 V. Georgescu, M. Daub / Surface Science 600 (2006) 4195–4199

tailoring the structure and the magnetic properties of mag-netic thin layers.

2. Experimental

The Co–Ni–P layers were obtained by electrodepositionfrom an aqueous solution containing 60 g l�1 CoSO4 Æ7H2O, 40 g l�1 NiSO4 Æ 7H2O, 70 g l�1 NH4Cl, 2 g l�1

NaH2PO2 Æ H2O and 0.5 g l�1 C12H25NaO4S. All experi-ments were performed at temperature 55 �C, pH 3.0 andcurrent density 154.4 A m�2. A nickel electrode (a disk of2.3 cm in diameter) was used as an anode. The cathodewas a disk of Cu (also 2.3 cm diameter) mounted parallelto the anode plane. During deposition the bath was un-stirred. Prior to electroplating, the substrates were mechan-ically polished with silicon carbide emery paper, degreasedin a 1 M NaOH solution at 80 �C and then in 20% HCl.

The electrochemical cell was inserted between the polesof a Weiss electromagnet [19], so that the magnetic fluxlines run horizontally parallel or perpendicular to the cath-ode surface. The magnetic field applied during experimentswas Hd = 120 kA m�1 for a 7.0 cm gap. This value was se-lected according to our previous experiments on magneto-electrolysis of alloys [20,21].

Surface analyses were performed using an atomic forcemicroscope (AFM) in tapping mode, at room temperatureand with a commercial Si3N4 tip of radius 10 nm. The mag-netic properties were investigated at room temperaturewith a torsion magnetometer in fields up to 300 kA m�1,and with an induction method [19] in a maximum field of57.3 kA m�1 using a digital oscilloscope interfaced to acomputer. The excitation field was applied parallel to thefilm plane in the induction set-up. The winding of thepick-up coil is inversely connected in series with the dummypick-up coil located at the center of a long solenoid, whichis parallel and identical with the main solenoid for samplemagnetization. The curves representing the derivative ofmagnetization with respect to time (dM/dt) vs. the ac mag-netic field intensity (H) with sinusoidal-field excitations (ata frequency of 50 Hz) were by this method recorded.

3. Results

In order to obtain thin layers with customized anisot-ropy, the substrate was arranged either parallel or perpen-dicular to the direction of magnetic field (Hd). Thecorresponding samples were labeled (1) for in plane fieldand (2) for Hd perpendicular to plane. Electrodepositionin the absence of an externally applied magnetic field wasalso investigated and the layers labeled (0) were comparedwith the ones obtained in a magnetic field. Thus, threemain categories of Co–Ni–P layers were investigated: thesamples (0), deposited without an externally applied mag-netic field, the samples (1), obtained in the presence of anexternal field Hd = 120 kA m�1 (1508 Oe) parallel to thesubstrate plane, and the samples (2), prepared in a perpen-dicular field of the same value as before. According to our

preliminary studies [20], the magnetic field was largeenough to saturate the magnetic films.

All experiments were carried out under galvanostaticconditions, until the amount of electrical charge reached4.632 C cm�2. Therefore, the film thickness was consideredthe same, of about 400 nm, as measured for samples elec-trodeposited without an applied magnetic field. The com-position of these samples, with no applied magnetic field,was 85 wt.% Co, 14 wt.% Ni and 1 wt.% P, as obtainedfrom EDAX measurements. The superimposed magneticfield could modify the current efficiency for different ions;nevertheless we neglected this influence on the compositionand thickness of the films, because it does not affect, in afirst approximation, the properties investigated in this pa-per. The saturation magnetization (which is an intrinsicmagnetic parameter) was of about 1.45 T for all our sam-ples, which means that the overall composition of the filmswas very similar.

We have found that a magnetic field applied during thepreparation of Co–Ni–P films influences the precipitationmode and the film morphology [20,21]. In Fig. 1 we presentthe AFM images of three typical samples deposited with-out an externally imposed magnetic field (Fig. 1a) and withthe magnetic field applied parallel (Fig. 1a) or perpendicu-lar (Fig. 1c) to the film plane. The surface morphology(shape and size of the crystallites) is different in these threetypical samples. In other words, the film’s nucleation andgrowth processes at cathode/electrolyte interface are modi-fied as the result of applying a magnetic field. The meansquare roughness of the three samples (calculated fromthe 6 lm · 6 lm AFM topography scan) has the followingvalues: (0) 54.7 nm, (1) 6.6 nm and (2) 7.3 nm. The size andshape of the cystallites are different in these samples, beinglarger and irregular in the case (0), smaller with acicularshape in the case (1) and rounded, more regular, in the case(2).

The cross-section topographic profiles corresponding tothe three images from Fig. 1 (along a horizontal selectedline 2 lm long) are presented in Fig. 2. During the AFMmeasurement, the phase information was also collectedby monitoring the phase lag between the oscillating drivesignal used to resonate the cantilever and the oscillatingdetection signal. This signal indicates differences in adhe-sion across the imaged area arising from different phasesin the film and by different crystal faces being exposed,which exhibit different frictional properties. Fig. 3(a–c)shows the profiles taken along the same selected line onthe surface as in the topographic profiles of the imagesfrom Fig. 1(a–c), respectively. The largest profile differencewas found for the case (0), where the height difference be-tween crystallites is also maximal. From this point of view,the samples deposited in perpendicular magnetic field arethe most homogeneous (Fig. 3c).

The XRD investigations on Co–Ni–P films were carriedout using Co Ka radiation with a DRON-3 installation.The lattice parameter a of the fcc phase in Co–Ni solidsolution was calculated according to the position of the

Fig. 1. Topographical AFM images of the Co–Ni–P samples electrode-posited in the case of: (a) Hd = 0, (b) Hd parallel to the plane and (c) Hd

perpendicular to the film plane. The size of each AFM scan is6 lm · 6 lm.

Fig. 2. Cross-sectional height profiles (along a central horizontal line 2 lmlong) corresponding to the three images from Fig. 1, for films deposited in:(a) Hd = 0, (b) Hd parallel and (c) Hd perpendicular to the film plane.

V. Georgescu, M. Daub / Surface Science 600 (2006) 4195–4199 4197

maximum of XRD peaks (111) and (200), as it follows: (0)a = 3.740 A, (1) a = 3.676 A, and (2) a = 3.692 A. Thefilms obtained on Cu polycrystalline substrates exhibited(111) preferred orientation. Samples (0) and (1) exhibiteda weaker (11 1) preferred orientation in comparison with

samples (2). As a result of the influence of Hd, the latticeconstant of the Co–Ni–P films is decreasing in the sameorder (0), (2), (1) as the crystalline grain size.

We also measured the magnetic susceptibility and hys-teresis curves of various Co–Ni–P films at room tempera-ture and found that the shape of these curves is a verysensitive function of the type of induced anisotropy. Asan example, the susceptibility (v) vs. H curves are shownin Fig. 4, for a measuring magnetic field of 57.3 kA m�1.The susceptibility curves in Fig. 4 were recorded for foursamples prepared (a) in the absence of an external imposedmagnetic field, (b) with magnetic field parallel to the planeof the film, and (c) with magnetic field perpendicular to thefilm plane. In Fig. 4d we present the case of a sample pre-pared when alternating the direction of Hd between theperpendicular and the parallel direction for four timesduring the film electrodeposition (starting with Hd perpen-dicular to the plane). By this procedure, a film with modu-lated anisotropy composed from four layers of the type[(2)/(1)/(2)/(1)] was obtained, each layer having an esti-mated thickness of 100 nm.

The differences in shape and value of the susceptibilitycurves for different films reflect the magnetic anisotropychanges, which resulted under the influence of a magneticfield applied during electroplating. The peak in the v(H)curves appears at a field Hc having the following values

Fig. 3. Cross-sectional phase profiles along the same central horizontalline as in Fig. 2, for films deposited in: (a) Hd = 0, (b) Hd parallel and (c)Hd perpendicular to the film plane.

Fig. 4. Magnetic susceptibility curves for electrodeposited samples: (a)Hd = 0, (b) Hd parallel to the film plane, (c) Hd perpendicular to the filmplane and (d) when Hd is changing its direction in a sequence of the type[(2)/(1)/(2)/(1)]. Scale: OY = arbitrary units, OX = 5.7 kA m�1/div(72 Oe/div); the arrows indicate the Barkhausen jumps.

4198 V. Georgescu, M. Daub / Surface Science 600 (2006) 4195–4199

for the samples represented in Fig. 4: (0) 9.7 kA m�1, (1)1.0 kA m�1, (2) 10.0 kA m�1 and for the film with modu-lated anisotropy 0.8 kA m�1 and 7.5 kA m�1. The filmsdeposited in parallel Hd are the softest magnetically(Hc ffi 13 Oe). The peak in the v vs. H curves appears at afield Hc close to the coercive field. The samples (2), depos-ited in a magnetic field perpendicular to the film plane, arethe hardest magnetically from the studied samples. From

torsion magnetometry measurements we observed thatthe easy magnetization axis of the series of samples (2) isperpendicular to film plane. This behaviour is in accor-dance with XRD experiments, taking into account thath1 11i is the easy axis of magnetization for cfc Co–Nistructures.

4. Discussion

It is already known that, microscopically, the migrationof the charged species in an unstirred solution can be influ-enced, among other processes, by ion movement in electricfield, hydrogen evolution on the cathode surface, gravita-tional field, magnetic field, molecular and ion thermal agi-tation. These effects are mediated in the bulk of thesolution, but in the electrochemical double layer, a typicalstructure of the cathode potential distribution can befound, with high electric fields in some regions of the sur-face of the cathode. The local current distribution dependsstrongly on the surface roughness and is changing its pro-file during the nucleation and growth of the electrodepos-ited film.

The main force operating on ions in magnetoelectrolysis(with a velocity v imposed by electric field and not by themagnetic field itself) is FL = q(E + v · B) where q is theelectrical charge of a particle, E is the electrical field vectorand B is the magnetic induction vector.

The magnetic field acts on the magnetic ions to create alocal density of energy E = �cvmB2/2lo, where c is the mo-lar concentration, vm is the molar susceptibility of the ions,and lo is the permeability of free space. It results in a force

Fm ¼ vm

B2

2lorcþ vmc

BrBlo

;

which causes diamagnetic and paramagnetic ions to movein opposite directions [2,4,5]. The magnetic field leads tomass transport changes of ionic species. The magnetic fieldacts effectively only in the diffuse double layer near the elec-trode (cases 1 and 2), by different modifications in both theelectrical charge of the double layer and in the thickness ofthe diffusion layer. The electrodeposition process in mag-netic field is influenced by additional convection and thegrain growth is favourable in the direction of the easiestmagnetisation. When the magnetic field is applied perpen-dicular to the current in the diffuse double layer (case 1),the magneto-hydrodynamic effect gives rise to a non-elec-trostatic field parallel to the cathode. The effect of this tan-gential field on the highly charged diffuse layer is to inducemotion near the interface, which creates arrangements inrows of the grains on the surface (the case b in the AFMimages from Fig. 1). The rate of nucleation is enhancedand small grains can uniformly grow since the convectionnear the interface provides sufficient magnetic ions to eachgrain. The film is in this case soft magnetically, and themovements of the domain walls are the main processesoccurring in the magnetization of films (Fig. 4b).

V. Georgescu, M. Daub / Surface Science 600 (2006) 4195–4199 4199

When the magnetic field is applied along the mean direc-tion of ion motion (case 2), the path of the ions is helical.The helical motion of ions near the cathode has an influ-ence on the nucleation of grains (columnar-shaped, withthe column axis h111i perpendicular to the substrate plane,as in Fig. 1c and XRD measurements). The phase contentis more uniform distributed in the film (Fig. 3c) and theeasy axis of magnetization in this case is perpendicular tothe film plane. The magnetization occurs mainly by mag-netic moment rotation, without wall displacements orBarkhausen jumps, as indicated by susceptibility curvesin Fig. 4c. The non-linear behaviour in susceptibility curveswith respect to the magnetic field amplitude is determinedby the interaction between the moving domain walls andthe local potential well formed by dynamic defects gener-ated at the interfaces between grains, depending on thecrystallite shapes and effective magnetic anisotropy.

5. Conclusions

A systematic investigation of the magnetic field effectson the electrochemical deposition of a Co–Ni–P alloy usingmicrostructural and magnetic methods has been under-taken. It has be shown that the morphology and the mag-netic properties (susceptibility and anisotropy) change as aresult of an externally applied magnetic field oriented par-allel or perpendicular to the cathode surface, in comparisonwith the deposition without an applied magnetic field. Thechanges in the shape of v(H) curves reflect changes in mag-netization processes and in switching properties of the pro-duced films.

The essential result of this research is a direct experimen-tal demonstration of the combined effect of the imposedmagnetic and electric fields on magnetic properties andstructure of Co–Ni–P films. The magnetic characteristics,the crystallographic structure and the morphology of elec-trodeposited films were found to depend on the direction ofthe applied magnetic field (in the same experimental condi-tions). The ability to align the easy axis in an alloy by mag-netic field imposed during electrodeposition provides a

practical means for constructing magnetic media with dualdirections of magnetization.

References

[1] T.Z. Fahidy, Prog. Surf. Sci. 68 (2001) 155.[2] J.M.D. Coey, G. Hinds, J. Alloys Compd. 326 (2001) 238.[3] S. Legeai, M. Chatelut, O. Vittori, J.-P. Chopart, O. Aaboubi,

Electrochim. Acta 50 (2004) 51.[4] O. Devos, O. Aaboubi, J.-P. Chopart, A. Olivier, C. Gabrielli, B.

Tribollet, J. Phys. Chem. A 104 (2000) 1544.[5] G. Hinds, J.M.D. Coey, M.E.G. Lyons, Electrochem. Commun. 3

(2001) 213.[6] O. Devos, A. Olivier, J. Chopart, O. Aaboudi, G. Maurin, J.

Electrochem. Soc. 145 (1998) 401.[7] R.A. Tacken, L.J.J. Janssen, J. Appl. Electrochem. 25 (1995) 1.[8] R.N. O’Brien, K.S.V. Santhanam, J. Appl. Electrochem. 27 (1997)

573.[9] A. Bund, S. Koehler, H.H. Kuehnlein, W. Plieth, Electrochim. Acta

49 (2003) 147.[10] S.R. Ragsdale, K.M. Grant, H. White, J. Am. Chem. Soc. 120 (1998)

13461.[11] S. Bodea, L. Vignon, R. Ballou, P. Molho, Phys. Rev. Lett. 83 (1999)

2612.[12] A. Krause, C. Hamann, M. Uhlemann, A. Gebert, L. Schultz, J.

Magn. Magn. Mater. 290–291 (2005) 261.[13] M. Uhlemann, A. Gebert, M. Herrich, A. Krause, A. Cziraki, L.

Schultz, Electrochim. Acta 48 (2003) 3005.[14] K. Msellak, J.-P. Chopart, O. Jbara, O. Aaboubi, J. Amblard, J.

Magn. Magn. Mater. 281 (2004) 295.[15] I. Tabakovic, S. Riemer, V. Vas’ko, V. Sapozhnikov, M. Kief, J.

Electrochem. Soc. 150 (2003) C635.[16] H. Matsushima, T. Nohira, I. Mogi, Y. Ito, Surf. Coat. Technol. 179

(2004) 245.[17] V. Georgescu, M. Georgescu, Phys. Stat. Sol. (a) 189 (2002) 1051.[18] D.-Y. Park, N.V. Myung, M. Schwartz, K. Nobe, Electrochim. Acta

47 (2002) 2893.[19] H. Zijlstra, Experimental Methods in Magnetism – 1. Generation and

Computation of Magnetic Fields, North-Holland Publishing Com-pany, Amsterdam, 1967.

[20] V. Georgescu, M. Georgescu, A. Mandreci, N. Apetroaei, in: Proc.203rd Meeting of the Electrochemical Society, Paris, France, April27– May 2, 2003, H3/657.

[21] V. Georgescu, A. Mandreci, Analele Stiintifice ale Universitatii ‘‘Al. I.Cuza’’, Iasi, S. 1b, Fizica T. XLV-XLVI, 1999–2000, pp. 7–14.