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Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

Effect of Phosphor Incorporation on Magnetic PropertiesofElectrodeposited CoPtW Thin Films

WEI Guo-ying, GE Hong-liang, WU Qiong, ZHOU Qiao-ying, WANG Xin-yan(Magnetism Key Laboratory of Zhejiang Province, College of Science, China Jiliang University, Hangzhou, China)

Abstract: The electrochemical behaviors of CoPtW(P) electrodeposition have been analyzed by means of cyclicvoltarnrnetry. Effect of hypophosphite concentration in the electrolyte on the alloy composition, crystalline structure andmagnetic properties has been investigated. It is turned out that the intensity of [00.1] P.D in the XRD pattern ofCoPtW(P)has been strengthened when the concentration of hypophosphite increased from 0 to 0.1 M. Parallel and perpendicularcoercivity of electrodeposited CoPtW(P) reached a maximum when the hypophosphite concentration was 0.06 M, but theperpendicular magnetic anisotropy increased gradually with the increase of hypophosphite concentration. Moreover,saturation magnetization of the electrodeposited CoPtW(P) with more P content decreased when the films were exposed tothe air. This can be explained by the fact that the hydrogen evolution would be favored thanks to an increase ofhypophosphite concentration, which led to the acceleration of corruption speed.Keywords: CoPtW(P) thin film; magnetic property; electrodeposition

1 IntroductionFabrication of cobalt and its alloys has beenwidely used in the production of new magneticmaterials utilized as microactuators, sensors, andmagnetic recording heads, etc[I-41. Electrodepositionprocesses possess several advantages over conventional casting techniques, enabling very close tailoringof the microstructure and magnetic properties of thefilms by better control of the growth. At the same time,this technique entails a purely additive step into themicrofabrication process, thus limiting the generationof defects due to etching prccesses'i",

In recent years particular interest is growingtowards the electrodeposited CoPt alloys with perpendicular anisotropy, a very interesting material not onlyfor perpendicular high-density magnetic recordingmedia, but also as magnet for small electronic devices.Recent discoveries have shown that by adding a thirdalloying element, W, the magnetic properties of theCoPt alloys at high thickness can be increased, andanother advantage is the decreasing Pt content to lessthan half while maintaining high coercivity [7-9].Co-deposition of phosphor compounds (phosphor wasadded in the electrolyte as NaH2P02) can increase the

coercivity as a result of decoupling the magnetic grains.This work discusses the effect of hypophosphite concentration in the electrolyte on the alloy composition,crystalline structure and magnetic properties.

2 Experiments2.1 Preparation of electrodeposited CoPt.W(P)

thin filmsThe alloy was prepared in a three-electrode cell by

means of electrodeposition. A copper plate was used asthe working electrode with the surface area of 0.5 em".The surface was polished to a mirror finish withdifferent alumina grades down to 0.05 urn, and cleanedultrasonically before the experiments. The platinumplate anode was about 10 times larger.

All chemicals were of analytical grade andprepared by ultra-pure millipore water. The electrolytehad the following composition: 0.1 M CO(NH2S03h,0.01 M (NHthPtCI4, 0.27 M Na2W042H20, 0.2 M( N H t h C 6 ~ 0 7 and x(x=O-O.1) M NaH2P022H20.Thesolutions were adjusted to pH 8.50.1 with suitableadditions of H2S04 or NaOH solutions. All investigations were conducted at a temperature of 60 'C and thecurrent of 12 rnA.

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3 Results and DiscussionCyclic voltammetry has been used to determine

the electrochemical details of CoPtW(P) electrodeposition. Fig.l shows typical cyclic voltammograms fordeposition of CoPtW and CoPtWP at 5 mV SI.

It can be seen that CoPtWP alloy depositionappeared in the more negative potential than CoPtWalloy deposition and the cathode current of electrodeposited CoPtW was larger than that of electrodeposited CoPtWP, which implied that the electro-

2.2 CharacterizationThe cyclic voltammetry study was performed in a

conventional three-electrode system. GC electrodewas used as the working electrode. Cyclic voltammetry sweeps were recorded as a function of scan rate(5 mV/s).

X-ray diffraction (XRD) patterns of the sampleswere recorded on an X-ray diffractometry (X' PertMPD Philips PW 1830) with Cu Ka radiation. Themagnetic properties of CoPtW(P) thin films weremeasured by vibrating sample magnetometer (VSM)(LakeShore 7407) in magnetic field both perpendicularand parallel to the substrate. The surface morphologywas assessed by SEM (Cambridge Stereoscan 360).

0,02

0.Q1

0,00

-0,01

-0,02

- O,06MNaH,PO,- withoutNaH,PO,

deposition of CoPtWP was more difficult than that ofCoPtW. Moreover, a crossover of the cathodic andanodic branches in the CVs was observed, whichmeans that electrodeposition of both CoPtW andCoPtWP involved the formation of a new phase; duringthe positive scan, one can see the potential of CoPtWalloy oxidation peak was less positive than that ofCoPtWP alloy oxidation peak. From the CVs' results.we can conclude that the incorporation of P canincrease the corruption resistance of alloys.

Fig.2 indicates the XRD patterns detected inelectrodeposited CoPtW(P) alloys that resulted fromchanges of the concentration of hypophosphite. XRDanalysis showed that Co-Pt-W(P) films exhibited astrong dependence on the phosphor content of thedeposited alloy. At a high hypophosphite concentrationof 0.1 M, CoPtWP developed a microstructureconsisting of hexagonal-closed-packed (hcp) matrixwith [00.1] preferential orientation. This means thatCoPtWP films have the c-axis oriented perpendicularto the film plane. At a relative low phosphor content of1.41 a1.% and 3.96 a1.%, however, CoPtWP filmsexhibited two mixed phases (fcc-hcp) and the PO of[00.1] became weaker. From the above results, it implies that as the hypophosphite concentration increasesthe growth direction of CoPtWP films changes fromparallel to perpendicular orientation with respect to thec-axis of the hexagonal structure.

The transition of PO in electrodeposited CoPtWPfilms is closely related to magnetic properties. Fig.3shows the variation of both parallel coercivity and per-

-0,03

-0,04-1,4 -1.2 -1.0 -0,8 -0,6 -0.4 -0,2 0,0 0,2 0.4

EN

Potential scan started at 01 V toward negative direction and thenreversed at -1.15 V.at a rate of 5 mV!s; Experiments were madeunder stirring condition

Fig.l Typical CVcurve obtained in solutions: (1)CoS040.1 M, Na2W040.27 M, ( N H . . h P t C ~ 0.2 M and(NH..)2C611ti07 0.2 M (pH=8.0); (2) CoS04 0.1 M,Na2W04 0.27 M, (NH..hPtCI40.2 M, (NH..hC6I1ti070.2 M and NaH2P02 0.06M (pH=8.0)

ilrp(001)..e. . .fnnr A h ~ ( O O 1 )

I f -----f- t-' ~ l O ) ~ , ~ - - - ' "'-'------ ---- -

a-O.02 M; b-0.04 M; c-O.l M; s- SubstrateFig. 2 XRD patterns of CoPtWPthin films electro

deposited from different hypophosphiteconcentrations

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Table1 Saturation magnetization (Ms) of CoPtWP thinfilms exposed to ai r for different days

M,ofCoPtWP M,o fCoPtWP M,ofCoPtWPConcentration of explosed for 0 explosed for 30 explosed for 60hypophosphiteIM day/temu-g" ) days/temu-g") days/(emu'g- l )

0 6.8384 6.68 6.460.02 9.9347 9.699 9.3230.04 13.858 13.422 130.06 22.1695 21.685 21.1260.08 16.164 15.15 13.8940.1 9.6598 8.2127 7.1235

0.10.08.06.04.02Concentration ofNaH,PO, in the solutionlM

/ " O , ~, ,, ,, ,, '_- 0i ' / \--

0.00

20002200

18001600

o1 14001200

U 1000800600400200 + - - , - ~ - - r - ~ - - , - ~ - , - ~ - - r - ~ - - , -

Fig.3 Variation of parallel and perpendicular coercivityof electrodeposited CoPtWP

pendicular coercivity with increasing the hypophosphite concentration. It can be seen that the coercivities of CoPtWP films reached the maximum in theparallel and perpendicular direction when the concentration of H2P02- was 0.06 M. Moreover, the perpendicular magnetic anisotropy has been strengthenedwith the concentration of H2P02- (Fig.3), which can beexplained by the fact that the high perpendicularmagnetic anisotropy (PMA) originates from the highmagnetocrystalline anisotropy of the hcp phase ofCoPtWP with c-axis perpendicular to the film[71.

The saturation magnetization (M s) of the CoPtWPthin films exposed to the air for different days has beentested. It was observed that M, of the electrodepositedCoPtW(P) with more P content decreased withexposing days in the airs (Table 1). This could beattributed to the fact that the hydrogen evolution wouldbe favored thanks to an increase of hypophosphiteconcentration (FigA), which led to the acceleration ofcorruption speed.

(a) 0.38at%; (b) 0.49at%Fig. 4 Surface morphology of eIectrodeposited CoPtWP

thin films with different phosphor atomic percentage4 Conclusions

CoPtWP thin films have been prepared byelectrodeposition. The electrochemical essential ofCoPtW(P) electrodeposition was discussed by meansof cyclic voltammetry method. Itwas found out that theelectrodeposition of CoPtWP was more difficult thanthat of CoPtW. And the incorporation of an appropriateamount of P can increase the corruption resistance ofalloys. XRD results shows that the intensity of [00.1]P.O of CoPtW(P) has been strengthened when the con-

centration of hypophosphite increased from 0 to 0.1 M.Parallel and perpendicular coercivity of electrodeposited CoPtW(P) reached a .maximum when thehypophosphite concentration was 0.06 M, but theperpendicular magnetic anisotropy increased graduallywith the increase of hypophosphite concentration.Moreover, saturation magnetization of the electrodeposited CoPtW(P) with more P content decreased whenthe films were exposed to the air.

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