Magnetism and magneto-transport studies in sonochemically prepared amorphous Co100-xPtx nano alloys

Article abstract of DOI:10.1016/j.ssc.2003.12.036

Bulk amorphous Co_(100-x)Pt_x (0≤x≤50) nano-alloys have been synthesized using high frequency ultrasound, displaying single domain (4-5 nm) behavior wherein weakly exchange-coupled particles lead to a field-dependent resistivity behavior. Magneto-resistivity is correlated to the order-disorder parameter in these powder compacts. The plot of Δρ/ρ₀ as a function of reduced magnetization indicates that all the systems are weakly exchange coupled.

Full text of DOI:10.1016/j.ssc.2003.12.036

Solid State Communications 129 (2004) 781–784
www.elsevier.com/locate/ssc
Magnetism and magneto-transport studies in sonochemically
prepared amorphous Co1002xPtx nano alloys
*
Manju Lata Rao, S. Sundar Manoharan
Materials Chemistry Laboratory, Indian Institute of Technology Kanpur, Kanpur 208016, India
Received 4 November 2003; accepted 15 December 2003 by C.N.R. Rao
Abstract
Bulk amorphous Co(1002x)Pt
single domain (4–5 nm) behavior wherein weakly exchange-coupled particles lead to a field-dependent resistivity behavior.
x
ð0 # x # 50Þ nano-alloys have been synthesized using high frequency ultrasound, displaying
Magneto-resistivity is correlated to the order–disorder parameter in these powder compacts. The plot of Dr=r as a function of
0
reduced magnetization indicates that all the systems are weakly exchange coupled.
q 2004 Published by Elsevier Ltd.
PACS: 61.43. 2 j; 61.43.Er; 61.46. þ w; 67.57.Hi
Keywords: A. Metal alloys; A. Nanostructures; B. Chemical synthesis; C. Order–disorder effects; D. Electronic transport
1
. Introduction
localized transient hot spot with an effective temperature of
000 K and a sub-microsecond collapse time. The rapid
5
1
cavitational cooling rate (.10 K s ) is much higher than
0
21
Amorphous systems have provided a rich field to study
magnetism in disordered systems. Amorphous and more
recently nano-crystalline materials have been investigated
for applications in magnetic devices requiring magnetically
soft materials [1–3]. Specifically, the Co–Pt alloy is one of
the candidates for ultrahigh density magnetic recording
media because of its high magnetic anisotropy and good
chemical stability upon corrosion [4–6]. The demands on
bulk soft magnetic materials include higher combined
induction and permeability. To achieve such a goal, the
alloy chemistry, structure, and ability to tailor micro-
structural features become important. Sonochemistry offers
a viable route to achieve such features. Sonochemical
decomposition of corresponding metal-carbonyl precursors
that obtained using conventional melt-spinning techniques
5
6
21
(10 to 10 K s ) used to prepare amorphous metals. There
are important differences in the electrical conduction in
amorphous and crystalline alloys. These include enormous
differences in the conduction electrons mean free path and
magnitude of potential fluctuations. Unlike the bulk
magnetic measurements, magneto-resistive effect is less
sensitive to domain effects if their size is greater than the
mean free path [9]. In this report, we demonstrate the
synthesis of Co–Pt nanoalloys, using sonochemistry and
show the magnetic and magneto-transport features, peculiar
of these amorphous Co–Pt nanoalloys.
yields Co(1002x)Pt
x
ð0 # x # 50Þ nano alloys, where the
chemical effects of ultrasound does not originate from the
direct coupling with molecular vibrations, but from a non-
linear phenomenon called sonochemical cavitation: the
nucleation, growth and subsequent implosive collapse of a
bubble in a liquid [7,8]. Acoustic cavitation generates a
2
. Experimental
In a typical reaction, nanosized Co1002xPt
x
ð0 # x # 50Þ
alloys were prepared by irradiating stoichiometric quantities
of Co (CO) and PtCl in decalin with a high intensity
2
8
2
2
2
*
Corresponding author. Fax: þ91-512-2597436.
ultrasonic probe, 20 KHz, 100 W cm in Ar atmosphere
for 3 h. 1-Propanol was injected into the reaction mixture for
E-mail address: ssundar@iitk.ac.in (S. Sundar Manoharan).
0038-1098/$ - see front matter q 2004 Published by Elsevier Ltd.
doi:10.1016/j.ssc.2003.12.036

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M. Lata Rao, S. Sundar Manoharan / Solid State Communications 129 (2004) 781–784
reduction of PtCl . The product was centrifuged, washed
2
a self-assembly of Co–Pt chains with dimensions of
,200 nm. Fig. 1(b) shows monodispersed, nearly spheri-
cally particles with an average diameter of 4–5 nm.
Electron micro diffraction shows that the Co–Pt alloys are
amorphous as initially formed. (Inset to Fig. 1(b), bottom-
right). By irradiating the particles with a high-intensity
electron beam in the TEM chamber, crystallization was
induced presumably from the local heating in the electron
beam. In situ crystallization results in the formation of ring
pattern, as shown in inset to Fig. 1(b) (top-left).
with dry pentane in an inert glove bag and dried in vacuum.
The structural properties were characterized by transmission
electron microscopy (TEM) and X-ray diffraction (XRD),
using Co Ka radiation. Magnetization as a function of field
was measured using an Oxford Mag VSM, in the
temperature range of 5–300 K up to 30 KOe, for samples
heated at 300 8C. Resistance as a function of temperature
was carried out employing the four-probe method. Magneto-
resistance of the samples was performed at 4.2 and 300 K up
to a field of 8 T.
The magnetic size of the particles, dmax estimated from
the slope of the magnetization curve employing Eq. (1)
ranges from 1.5–2.5 nm [11]. The magnetic dimensions of
the nano–alloys are much smaller than the electron mean
˚
free path, l (which is about hundreds of A), allowing domain
effects to contribute to the magneto resistive effects.
3
. Results and discussion
Sonochemically prepared Co–Pt nano alloys are dis-
tinctly different from nano-sized Co–Pt alloys reported in
the literature using other wet chemical routes [10], wherein
alloys prepared by sonochemistry are realized due to high-
energy cavitation phenomena. In wet chemical processes,
alloy formation occurs from a post-annealing process of
intimately mixed metals. TEM images of the ‘as-prepared’
Co–Pt amorphous alloy are shown in Fig. 1(a) forming
ꢀ
ꢁ
1=3
18kTðdM=dHÞ
H¼0
dmax
¼
ð1Þ
2
s
prM
Differential scanning calormetric (DSC) curve shows a glass
transition at T ; and a weak crystallization peak at T2
1
concurrent with the powder diffraction pattern. The ‘as-
synthesized’ powders are X-ray amorphous. On thermal
annealing at 300 8C, the system shows that the hcp phase is
stabilized for x ¼ 0 and 10 compositions. For x . 10; the
alloys still show amorphous nature as shown as Fig. 2(b).
Although it is understood that conventional X-ray diffrac-
tion is not a particularly efficient technique in the study of
fine nano-structures such as those produced in early stages
of crystallization, a clear signature of local ordering is
available only for pure Co, heat-treated at 300 8C. Co–Pt
Fig. 1. (a) TEM picture showing self-assembly of the ‘as prepared’
Co–Pt nano alloys with dimensions ,200 nm length. (b) TEM
picture of the dispersed Co–Pt nano-alloys showing nearly
spherical, monodispered particles. The inset on bottom-right
shows the selected area electron diffraction of the alloys as initially
formed. Prolonged exposure to the electron beam induces crystal-
lization as shown in the inset (top-left).
Fig. 2. (a) Shows the DSC curve. T1 corresponds to the glass
transition temperature, T2 to the crystallization temperature, and
T3 2 T4 the reversible phase transformation temperature. Inset (b)
shows the X-ray diffraction patterns of the nano alloys heat-treated
at 300 8C. Inset (c) shows the demagnetization curves for the
amorphous nano alloys exhibiting a typical ferromagnetic behavior.

M. Lata Rao, S. Sundar Manoharan / Solid State Communications 129 (2004) 781–784
783
undergoes a reversible phase transformation from an fcc to
fct phase at T and T in the DSC curve.
3
4
In Fig. 3, we show the plot of resistance as a function of
field for the Co–Pt nanoalloys, annealed at 300 8C. All the
alloy compositions show a negative magneto-resistance
ratio ,2% at 4.2 K and 4 T applied magnetic field. We
further show the influence of disorder parameter on the
electronic transport. According to Nordheim’s theory, [12]
for the disordered alloys, the residual resistivity r ; which is
0
a good estimate of disorder in the systems depends on the
composition x; given by the formula r ¼ const: xð1 2 xÞ: In
0
Fig. 4, we show the variation of r as function of x: (r
0
0
increases with Pt concentration showing an oscillatory
maximum for the x ¼ 30 composition. Another unusual
feature in the resistivity curve of amorphous magnetic alloys
is the existence of the low temperature resistance minimum,
Tmin that reflects the local magnetic order in the alloys [13].
A plot of Tmin observed in the resistivity curve at low
temperature, shows a similar behavior as that of r0-
dependence of Pt concentration. The maxima in both Tmin
and r0 match well with the oscillatory dependence of
negative magneto-resistance. Although the MR ratio is
fairly low for all the compositions, the maxima for x ¼
Fig. 4. Plot of remenance magnetization, B ; negative magneto-
r
resistance, MR%; Tmin and residual resistivity, r as a function of Pt
0
content showing an oscillatory dependence due to structural
disorder effects and weak-magnetic correlation.
moments. Magneto-resistance studies made at 300 K,
however, show a positive MR of ,3% for x ¼ 30
composition only, while all other compositions show a
negative MR ,1%. This is attributed to the thermal
scattering at room temperature besides the short-range
disorder effects.
3
0 composition (having lowest (–)% MR) can be argued
based on the structural and magnetic disorder suggesting
that the magneto-resistance behavior in amorphous nano
alloys depends both on the structural disorder and the
weak exchange coupling. The plot of Dr=r0 curves of
Co–Pt as a function of m ¼ M=Ms shows a flat-top
parabola, [14] which suggests weakly correlated magnetic
We finally show the remenance ratio M =M as a function
r
s
of x with values ,0.5, suggesting the existence of weak
exchange coupling. We observe a similar trend showing a
monotonic increase with increasing Pt concentration, with a
very sharp increase for x ¼ 30 composition. The evolution
of H and M =M with x is the result of the transition from an
c
r
s
assembly of almost independent particles to a network of
connected magnetic particles. The increasing coalescence of
magnetic particles could also contribute to the scattering
effect due to random ordering. A corresponding decrease in
H coinciding with the peak in MR% is also seen, because
c
both GMR and H are sensitive to multi domain or cluster
c
formation.
4
. Conclusions
In summary, we have studied the structural, magnetic
and magneto-transport properties of sonochemically pre-
pared Co1002xPt nano alloys. The weak oscillatory
x
dependence of MR seems to agree well with the structural
disorder and weak exchange coupling. The study of
amorphous alloys leads to striking and yet fairly understood
differences from the better-known crystalline systems.
x
These results suggest that Co1002xPt nano-alloys could
Fig. 3. Resistance as a function of field up to 8 T measured at 4.2 K
prove a model system for understanding the fundamental
relation between magnetism and magneto-transport in nano
particles.
for Co1002xPt
x
nano alloys heat-treated at 300 8C showing a
pn ee rg ca et ni vt e. magneto-resistance ratio of , 2% is noted up to x ¼ 40

7
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M. Lata Rao, S. Sundar Manoharan / Solid State Communications 129 (2004) 781–784
[6] S.H. Liou, S. Huang, E. Klimek, R.D. Kirby, Y.D. Yao,
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was supported by Department of Atomic Energy (BRNS)
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