After annealing at various temperatures from 450°C to 700°C, the nanoparticles were found to have an ordered L10 structure and to show hard magnetic properties.. We have previously repor
Trang 1Magnetic Properties of FePd Nanoparticles Prepared
by Sonoelectrodeposition
TRAN PHUONG LOAN,1LUU MANH KIEN,2TRAN THI HONG,1
1.—Hanoi University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam 2.—Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan 3.—e-mail: luongnh@hus.edu.vn
Fe60Pd40 nanoparticles were prepared by sonoelectrodeposition After annealing at various temperatures from 450°C to 700°C, the nanoparticles were found to have an ordered L10 structure and to show hard magnetic properties Among the samples investigated, the nanoparticles annealed at 600°C exhibited the highest coercivity which amounts to 2.31 kOe at 2 K and 1.83 kOe at 300 K
Key words: FePd, L10structure, sonoelectrodeposition, magnetic
nanoparticles, hard magnetic materials
INTRODUCTION FePd nanoparticles have attracted interest for
their potential applications in ultrahigh-density
magnetic recording media due to the large uniaxial
magnetocrystalline anisotropy of Ku 1.8 9 107
erg cm 3 of the L10 ordered structure.1 11 Ordered
face-centered tetragonal (fct) L10 FePd materials
are normally obtained from disordered
face-cen-tered cubic (fcc) materials via the order–disorder
transition Several approaches to the preparation of
FePd nanoparticles have been reported including
epitaxial growth by electron beam deposition,4 6
chemical synthesis7,8,11 (which is modified from the
FePt nanoparticles synthesis method by Sun
et al.12), modified polyol process,9 and microwave
irradiation2 As pointed out by Watanabe et al.,9
FePd nanoparticles synthesized by the modified
polyol process including thermal decomposition do
not exclusively show the ordered L10 phase
transi-tion similar to L10-type materials such as FePt and
CoPt Especially, the FePd nanoparticles prepared
by Chen and Nikles11 did not transform to the L10
phase after annealing at a sufficiently high
temper-ature of 700°C
We have previously reported the hard magnetic properties of FePd nanoparticles synthesized by sonochemistry.13In this paper, we report the use of the sonoelectrodeposition method for the prepara-tion of FePd nanoparticles To our knowledge, FePd nanoparticles have never been fabricated by sono-electrodeposition, which is a technique combining the advantages of electrodeposition and the mechanical waves of ultrasound to produce metallic nanoparticles.14 Recently, Co–Pt nanoparticles encapsulated in carbon cages prepared by sonoelec-trodeposition have been reported by Luong et al.15 Magnetic properties of FePt nanoparticles also prepared by sonoelectrodeposition have been reported by Nam et al.16
EXPERIMENTAL The experimental setup employed by us is similar
to that described in Ref 17 A titanium horn of diameter of 1.3 cm acted as both the cathode and ultrasound emitter (Sonics VCX 750) The electroac-tive part of the sonoelectrode was the planar circular surface at the bottom of the Ti horn, while
an isolating plastic jacket covered the immersed cylindrical part This sonoelectrode produced a sonic pulse that immediately followed a current pulse A home-made galvanostat was used to control the constant current regime (without using a reference (Received October 12, 2015; accepted April 21, 2016)
Ó2016 The Minerals, Metals & Materials Society
Trang 2electrode) A platinum plate of 1 cm2was used as a
counter electrode The density of the current pulse
was 15 mA/cm2 The duration, ton, of the current
pulse was 0.5 s, then the current was turned off for
a duration, toff, of 0.8 s During ton, FePd
nanopar-ticles were deposited on the surface of the electrode
When the current was switched off, a 0.2-s
ultra-sound pulse of power density 100 W/cm2 was
acti-vated to remove the nanoparticles from the
electrode
The volume of the electrolysis cell was 100 ml
containing 0.15 M iron(II) acetate [Fe(C2H3O2)2],
0.1 M palladium(II) acetate [Pd(C2H3O2)2], and
0.5 M Na2SO4, which were mixed under
(Ar + 5%H2) atmosphere After deposition, the FePd
nanoparticles were washed and separated from the
solution by using a centrifuge (Hettich Universal
320) at 5000 rpm for 30 min Nanoparticles were
dried in air at 70°C for 30 min The as-prepared
samples were then annealed at various
(Ar + 5%H2) atmosphere The structure of the
nanoparticles was characterized by an x-ray diffrac-tometer (XRD; D5005, Bruker) The average crys-tallite size, d, was calculated from the line broadening using Scherrer’s formula, d = 0.9k/ (Bcosh), where k is the wavelength of x-rays and B
is the half-maximum line width The particle mor-phology was examined by a transmission electron microscope (TEM; JEM1010, JEOL) The chemical composition of our sample was Fe60Pd40as revealed from energy dispersion spectroscopy (EDS; OXFORD-ISIS 300) measurements Magnetic prop-erties of samples were studied by using Quantum Design’s superconducting quantum interference device (SQUID) with a magnetic field up to 50 kOe
in the temperature range from 2 K to 300 K
RESULTS AND DISCUSSION Figures1 and 2 show the TEM images and size distributions of the as-prepared and Fe60Pd40 nanoparticles annealed at 600°C, respectively Par-ticle size of the as-prepared Fe60Pd40 sample was
Fig 1 TEM image and size distribution of the as-prepared Fe 60 Pd 40 nanoparticles.
Fig 2 TEM image and size distribution of the annealed Fe 60 Pd 40 nanoparticles (600°C/1 h).
Luong, Trung, Loan, Kien, Hong, and Nam
Trang 3about 7–10 nm After annealing, the particle size
increased to about 15–20 nm, showing that the
particles were agglomerated
The XRD patterns of the as-prepared and
Fe60Pd40 nanoparticles annealed at 600°C are
shown in Fig.3 Before annealing, the XRD results
showed the reflections of a pure Pd structure, as
observed in Ref.13in FePd nanoparticles prepared
by sonochemistry The reflections from Fe are very
weak due to the fact that their atomic weight is
much less than that of Pd, which is similar to the
XRD result of FePt foils prepared by cold
deforma-tion18 and of FePt nanoparticles prepared by
sono-electrodeposition.16 The average crystallite size
calculated by using Scherrer’s formula was found
to be 10 nm, in agreement with the particle size
obtained from the TEM image Upon annealing, the
formation of the ordered L10 fct phase occurred
Samples showed the tetragonal order phase of FePd
alloy with diffraction peaks at 24°, 33°, 41°, 47°, 49°,
53.5°, 60.5°, 69° which can be assigned to (001),
(110), (111), (200), (002), (201), (112), (220)
reflec-tions, respectively The diffraction peak at 44.5° can
be due to the formation of the a-Fe phase in the
sample By using Scherrer’s formula, the average
crystallite size was estimated to be 20.1 nm for the
sample annealed at 600°C, in agreement with that
obtained from the TEM image
Magnetic measurements of the as-prepared
sam-ple (data not shown) exhibited low saturation
magnetization, MS, and coercivity, HC After
annealing, the hard magnetic FePd phase was
formed Figure4 presents the magnetic curves of
the Fe60Pd40 nanoparticles annealed at 600°C for
1 h at different temperatures The curves show
typical hard magnetic hysteresis loops, indicating
the effect of annealing The temperature
depen-dence of the coercivity of Fe60Pd40 nanoparticles
annealed at various temperatures from 450°C to
700°C is shown in Fig.5, from which it can be
clearly seen that the Fe60Pd40 nanoparticles annealed at 600°C exhibit the highest coercivity For this sample, the coercivity was 2.31 kOe at 2 K and slightly decreases with increasing temperature
to the value of 1.83 kOe at 300 K Watanabe et al.9 prepared Fe49.2Pd50.8nanoparticles by the modified polyol process, i.e simultaneous reduction of palla-dium acetylacetonate (Pd(acac)2) and thermal decomposition of iron pentacarbonyl (Fe(CO5)) in a solvent These authors reported the value of 2.04 kOe at 5 K for the coercivity of Fe49.2Pd50.8samples annealed at 600°C for 1 h We note that the
Fe49.2Pd50.8 nanoparticles in Ref 9 have been annealed at only one temperature (600°C) Gajbhiye
et al.19also prepared Fe Pd nanoparticles by the
Fig 3 XRD patterns of the as-prepared and annealed Fe 60 Pd 40 nanoparticles (600°C/1 h).
Fig 4 Magnetic curves of Fe 60 Pd 40 nanoparticles annealed at 600°C for 1 h at different temperatures.
Trang 4modified polyol process and annealed the samples at
450°C, 550°C and 600°C for 1 h They obtained
HC= 1.18 kOe at 300 K for the Fe43Pd57 sample
annealed at 550°C For the sample annealed at
600°C, HC= 1.3 kOe, larger than that for the
sample annealed at 550°C These authors noted,
however, that the annealing at 600°C will lead to
severe agglomeration, which is detrimental for
technical applications Hou et al.8 synthesized
Fe48Pd52 nanoparticles by a chemical method and
annealed the samples at 550°C, 600°C and 700°C for
30 min They reported that the coercivity of the
samples increases with the annealing temperature
up to 600°C, reaching a value of HC 2 kOe at room
temperature These authors alsso noted that further
increase of the annealing temperature decreases the
coercivity, suggesting the formation of a new soft
phase Fe3Pd at higher temperature
Figure6 shows the annealing-temperature dependence of the coercivity of Fe60Pd40 nanoparti-cles measured at different temperatures As can be seen from this figure, the coercivity of the Fe60Pd40 nanoparticles increases with the annealing temper-ature up to 600°C due to a better atomic ordering of the fct phase Further increase of the annealing temperature decreases the coercivity, suggesting that a soft phase Fe3Pd exists in the sample, as supported by our XRD results for the samples annealed at 650°C and 700°C (data not shown)
CONCLUSIONS
Fe60Pd40 nanoparticles have been prepared by sonoelectrodeposition After annealing at various temperatures from 450°C to 700°C, the nanoparti-cles were found to have an ordered L10phase, with good coercivity up to 2.31 kOe at 2 K and 1.83 kOe
at room temperature Sonoelectrodeposition is a
nanoparticles
ACKNOWLEDGEMENTS This research is funded by Vietnam National Foundation for Science and Technology Develop-ment (NAFOSTED) under Grant Number ‘‘103.02-2013.61’’ The authors would like to thank Prof Y Nozue of Osaka University, Japan, for providing SQUID
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