Effect of the preparation conditions on the properties of Fe-Pt nanoparticles produced by sonoelectrodeposition on the properties of Fe3 Ơ4 nanoparticles

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\ Nl’ Jouiiiul u f Science, NĩailicihalÌLồ - Pliy.'iics 25 (2009)

1-Effect o f the preparation conditions on the properties o f Fe-Pt

nanoparticles produced by sonoelectrodeposition

Nguyen Hoang Luong* , Nguyen Hoang Hai, Nguyen Dang Phu

C e n te r f o r M a te r ia ls S cien ce. F a cu lty o f P h ysics, C o lle g e o f S c ien ce , VNU

3 3 4 N iịuyen Trai, H anoi. V ietnam

R cccivcd 10 Macrh 2009

A b s tr a c t Fc-Pl materials have been w idely prepared by vacuum e v a p o r a tio n technique. Rcccntly, 
chcm ical and physicochcm ica! mcthixJs have been successfully used to make Fc-Pt nanoparticlcs, 
Ihin films. This paper reported another physicochcm icaỉ method, nam ely sonoclcctrodcposition, to 
produce Fe-Pt nanoparliclcs. In the sonoclcctrodcposilion, the elcctrodcposition proccss was 
assisted with a sonicator. The Ti horn o f the sonicator played a roic as the calhodc on which Fc-Pt 
nanoparticlcs were deposited. After a ccrtain lim e o f deposition, a sonic pulse was applied to 
rciiiovc the particles frt)m the Ti catỉuxic. The com position o f Fe-Pt parlicics can he controlled by 
chanuing th e c o n c e n tr a tio n o f F'e a n d Pt io n s in the c lc c tr o ly tc a n d th e d e p o s it io n v o lta g e . 1Ì1C

parliclc size can be adjusted by the lim e o f deposition. The as-dcpositcd Fc-Pt nanopariiclcs were 
fcrromagnctic at room lempcraturc. upon annealing at 7(X)°C for I h under ÌI2 atmosphere, ihc 
saturation magnetization and the cocrcivity o f the nanoparliclci: were im p r o v e d siunificantly. 
SoiKKlcclrcxlcposilion is a promising technique to make large quantity o f Fc-Pt nanopurticlcs.

Kcy^vonl\: I c T l , L1(J N l i u c l u i c , hu>clccHc>dcj>o.NÌliuii, l u a g i i c l i c u a i K t p u l l i c ĩ c h , l i a i d U i a ^ i i c l i c

nuilcrials..

1. Introduction

FcPi alloy can be in either a disordered face-centered cubic (fee) phase in which the statistical 
distribution of ihe Fe and Pt atoms is substitutionally random, or in a partially or complcteiy ordered 
face-ccnlcred tetragonal (fct) phase in which Fe and Pt atoms occupy specific sites. The ordered phase 
can change to the disordered phase and vice versa at elevate temperatures [1]. The fct FePt alloy 
possesses excellent hard magnetic properties with the saturation magnetization, PiMs. of 1.4 T, the 
CuiTÌe temperature, Tc, o f 750 K, and the crystalline anisotropy Ku of 7 MJ/m^ [2]. Despite the high 
cost of Pt, FePt thin films or particles have been paid much attention for their use as ultrahigh density 
magnetic storage media and microcleclronic mechanical system (MEMS) due to the mechanical and 
chcmical stability of the ordered fct Llo structure. Beside the equiatomic composition FePt, the Fc3Pt 
and FePt3 with L I2 structure and less preferred magnetic properties can exist. The ordered FePt 
materials arc normally obtained from the disordered materials via the order-disordcr transition. But

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N.ỊỈ, Liumg et ai. / VNU Jourfuil o f Science, M athematics - Physics 25 Ị2009) ĩ -7

sometimes the ordered phase can be obtained from pure Fe and Pt layers via the diiiusion of the twvo 
materials at high temperatures [3].

There are several ways to make FePt nanostructured materials including physical techniques sLic:h

as mechanical deformation [3], arc-melting [4], vacuum evaporation (sputtering, thermal evaporatiorn) 
[5, 6], laser ablation pulse [7], chemical methods [8-10], and physicochemical method such aas 
electrodeposition [11. 12]. Up to now, the vacuum evaporation is the most used methocd 
Electrodeposition is a promising way to obtain FcPt thin films because it is less expensive thain 
physical methods, less complicated than chemical methods. But by this technique, it is difficult to gcet 
nanoparticles with large quantity. Sonoelectrochemistry was developed to make nanoparticles [13]. Ii 
combined the advantages o f sonochemistry and electrodeposilion. Sonochemistry is a very usefiul 
synthetic method which was discovered as early as 1934 that the application of ultrasonic energ;>^ 
could increase the rate o f electrolytic water cleavage. The effects of ultrasonic radiation on chcinicral 
reactions are due to the very high temperatures and pressures, which develop in and around ihie

collapsing bubble [14]. Sonoelectrochemislry has the potential benefit o f combining sonochcmistirv 
with electrochemistry. Som e o f these beneficial effects include acceleration of mass transporrt,

cleaning and degassing o f the electrode surface, and an increased reaction rate [15]. In this paper, wve 
report the use o f the sonoelectrochcmical method for the preparation of FePt nanopaniclcs.

2. Experimental

The sonoelectrochemical device employed is similar to that described in ref. [16]. A titanium horrn 
with diameter o f 1.3 cm acted as both the cathode and ultrasound emitter (Sonics v e x 750). Tine 
electroactive part of the sonoelectrode was the planar circular surface at the bottom of the Ti horn. Avn 
isolating plastic jacket covered the immersed cylindrical part. This sonoelectrode produced a soniic 
pulse that immediately followed a current pulse. One pulse driver was used to control a galvanostiai 
and the ultrasonic processor, which was adapted to work in the pulse mode. A home-made galvanostiat 
(without using a reference electrode) was used to control the constant current regime. A platinum plaite

wivh a sq u are o f I cm^ w a s u se d as a co u n ter e le c tr o d e T h e current p u ls e wn*; ch n n g eil froiTi 1 5 to 110

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N .ỉ ỉ L u o n g e t a l / VNƯ Jou rnal o f Science, M athem atics - P h ysics 25 (2009) / - 7

3. lỉLesults an d d iscu ssio n

TT’he chem ical com position o f the Fe-Pi nanoparticles was controlled by adjusting the current 
derusỉitty (coư esp on din g to the applied voltage). W hen the current density o f 15 - 20 m A /cm ^ the 
conr^-osition o f nanoparticles was close to the expected fequiatomic c o n ^ o sitio n (Table 1). At higher 
curriemt densities, the atom ic percent o f Fe was higher because the standard elecừ od e potential Fe^VFe 
(-0.4^4 V [ 17]) is more negative than that o f (0.742 V f 18]).

Tab'he 1. Chemical composition, magnetic properties o f the samples. J is ứie currait density, to n is Ulc deposition 
linvc o f a pulse; Fe, Pt are ứie atomic pCTcent o f Fe and Pt in the samples, respectively; //c is Uie coercivity; Ms is

ứic nnaigneti/ation at 13.5 kOe); MJMs is tíie magnetic squareness

Naine J

(mA/cm’)
/on
(s)

Fe
fat. %)

Pt
(at. %)

//c
(kOe)

M,
(emu/g)

MJM,

SI 30 0.5 69 31 5.0 40 0.26

S2 25 0.5 61 39 6.0 45 0.36

S3 20 0.5 52 48 6.1 49 0.41

S4 15 0.5 45 55 8.5 23 0.70

S5 15 0.6 - - 9.1 24 0.72

S6 15 0.7 - - 8.5 30 0.4

S7 15 0.8 - - 8.7 43 0.4

As-prepared Annealed

%

ìữom

Fig. 1. TEM images o f tlic as-prcparcd (left) and annealed (right) FePt 
nanoparticles

(700°c/l

h).

F ig u re 1 is the TEM im ages o f typical as-prepared and annealed sa n p les. Particle size o f the as- 
prqỉ.ared FePt sam ple w as 5 - 1 0 nm. After annealing the particle s ừ e increased to 1 0 - 2 5 nm due to 
the aggregation and particle growth. In addition, the size distribution o f the annealed particles was 
larger than that o f the as-prq)ared samples.

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N.H. Luong et oL / VNU Jounuil o f Science, Maíhemaĩics - Physics 25 (2009) 1-7

atomic weight is much less than that of Pt which is similar to the XRD result o f FePt foils prepared boy 
cold deformation [20]. The Pt peaks in the as-prepared samples are broad due to the small size of thhe 
particles. Using the Scherrer formula with the full width at half maximum of the strongest peak (11 1Í), 
the mean particle size of Pt particles was deduced to be 5.2 nm which is much smaller than the particlle 
size obtained from the TEM image. The particles were not disordered FePt but they can be formed boy 
many small domains o f pure Fe and Pt. The formation of FePt by electrodeposition was not occurrecd 
and may be ascribed to the large difference in the standard electrode potential of the annd
Pt'^^/Pt. Upon annealing, the formation o f the ordered Llo fct phase happened by the diffusion processs

between Fe and Pt domains.

Annealed

As- prepared

20 3 0 4 0 5 0
20 (degree)

Fig. 2. X R D patterns (Cu K a radiation) o f the as-prepared (bcỉttom) and annealed (top) 
nanoparliclcs compared to those o f the intensities for L lo FcPt (pdf file 4 3 1 3 5 9 ) and for 
Pt (marked by the astcricks, pdf file 04-0802). The fundamental peaks o f FcPl structure

w ere denoted b y ‘T ’ and the supcrlattice peaks were denoted by “s” .

T (*C )

Fig. 3. D SC ưacc for the FcPt nanoparticlcs (heating rate 10°c/m in ).

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N.H. L u o n g e t al. / VNU Journal o f Science, M athem atics - P h ysics 25 (2009) 1-7

via th ie diffusion process o f Fe and Pt domains because the diffusion occurred at any teưỊìerature. Both 
fundlaimental and superlattice reflections o f the L l o phase were presented in Fig. 2. There was a small 
peak: next to the (1 1 1 ) m ain peak o f the L l o FePt w hich can b e assigned to the Pt-rich FePt phase - 
FePtta.. The presence o f this phase can be explained b y the incorrplete diffusion betw een Fe and Pt 
dorruaiins.

Fig. 4. Magnetic curves o f sample S4. The maximum 
applied field was 13.5 kOe.

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N.H. L u o n g e t al. / VNƯ Jou rnal o f Science, M athem atics - P h ysics 25 (2009) Ỉ -7

the magnetic squareness MJM^ reduced with J because o f the presence o f the FejPt. O f cou rsee, the 
kink in the m agnetic curves w as more evident in the sam ples with high Fe content.

t » 0 5 s

J {m A ^ fn *)

J = 1 5 m A/cm *

Fig. 5. Dependaice o f magnetic squaraiess and coercivity on cuưent 
density y (top) and deposition lime /on (bottom).

In another series o f s a n p le s, w e fixed the current density and change the d q )osition tim e ion.,. We 
thought that by changing tau the size o f particles w ould change and therefore the niagnctic prupccrtici) 
o f the particles w ould b e affected. After annealing, He did not change with as significantly a as A/f 
d id The coercivity o f S4 - S7 was in the range from 8.5 kOe to 9 .1 kOe (T able 1 and Fig. 5). Thisis can 
be explained by the fact that the chem ical c o n p o sitio n mainly depended on the current dennsity

M agnetic properties o f the s a n p le s S4 and S5 were alm ost the same. Sam ple S5 sh ow ed the 1 m ost 
preferred hard m agnetic properties. The m agnetic squareness sh ow ed a sudden drop and A/s showAfcd a 
sfrong change when the d q josition tim e was longer than 0.6 s as sh ow n in sam ple S 6 and S7 wA'hich 
presented sứ on g ratio o f a soft m agnetic phase. In our experim ent, w e only used 0.08 mM 
(coưesponding to 1 m M /l) in a bath o f 80 ml w hich was enough to m ake about 20 mg FePt

nanoparticles. For long deposirion time, the concentration o f in the electrolyte reddueed

significantly with time. Therefore, there were more ions d q )osited on the Ti hom than 1 ions.

A s the result, there can b e m ore Fe in sam ples S 6 and S7 w hich caused their lo w m agnetic squareeness 
and high

A/s-4. C onclusion

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N .ỉỉ. Liumịị eĩ ciL / VNU J ou rm l o f Science, M aílicmaíics - Physics 25 (2009) 1-7

^showed a high coercivity of 9.1 kOe at room temperature. This method possesses some advantages 
ccompared to common methods such as simple preparation, low-cost equipment and easy scale-up.

A cknow ledgm ents. This work is supported by the Key Project QGTD.08.05 of Vietnam National 
I University, Hanoi and the European Commission Project Selectnano-TTC (Contract No. 516922). The 
; authors would like to thank Prof. Le Van Vu for XRD measurements and Mr. Pham Van Dinh for 
texperimental assistance.

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