Fabrication of ordered array of nanowires of La
0.67
Ca
0.33
MnO
3
„
x
Ä 0.33… in alumina templates with enhanced ferromagnetic
transition temperature
K. Shantha Shankar, Sohini Kar, and A. K. Raychaudhuri
a)
Department of Physics, Indian Institute of Science, Bangalore-12, India
G. N. Subbanna
b)
Materials Research Centre, Indian Institute of Science, Bangalore-12, India
We report fabrication of array of crystalline nanowires 共average diameter of 65 nm兲 of colossal
magnetoresistive oxide La
0.67
Ca
0.33
MnO
3
共LCMO, x⫽ 0.33) within anodized alumina templates by
filling the pores with a sol that allows formation of LCMO phase at the relatively low temperature
of 600°C. The crystalline nanowires with correct stoichiometry stabilize in the orthorhombic phase
at room temperature. The nanowires are ferromagnetic at room temperature and exhibit enhanced
ferromagnetic transition temperature well in excess of 300 K, which is substantially higher than that
of single crystalline LCMO. This enhancement we attribute to the size induced lattice contraction in
thenanowires.
Synthesis of one-dimensional nanomaterials with exotic
functional properties is a field of intense activity. In addition
to the well-known example of carbon nanotubes, nanowires
of a number of functional materials such as GaN, GaP, TiO
2
,
BaTiO
3
, and ZnO have been synthesized.
1–3
One of the im-
portant motivations in synthesizing nanowires is the distinc-
tive physical and chemical properties that are different from
those of conventional bulk materials. In this letter, we report
the fabrication of arrays of nanowires of colossal magnetore-
sistive oxides like lanthanum calcium manganese oxide
(La
0.67
Ca
0.33
MnO
3
) 共LCMO兲, using porous alumina tem-
plates. The nanowires 共approximate diameter 60–70 nm兲
were found to be crystalline and most interestingly they have
a ferromagnetic Curie temperature that is significantly en-
hanced (T
C
⬇315 K) compared to that of even bulk single
crystal (T
C
⬇235– 260 K).
Rare-earth manganites of Perovskite structure with the
general formula R
1⫺ x
A
x
MnO
3
共where R and A are rare and
alkaline earth ions, respectively兲 have attracted considerable
attention because of their unusual magnetic and electronic
properties.
4,5
The main attraction of these materials is the
large change in their electrical resistance on application of a
magnetic field. An extremely important criterion for the se-
lection of these materials for application in magnetoelec-
tronic devices is the ferromagnetic transition temperature
T
C
, as magnetoresistance is prominent near T
C
. Manganites
with T
C
close to or in excess of 300 K are most desirable for
applications in devices operating at room temperature. Thus
the fabrication of nanowires of LCMO with a transition tem-
perature ⬎300 K is an important achievement. These oxides
have been prepared in various physical forms like single
crystals, polycrystalline pellets, epitaxial and polycrystalline
films and nanopowders. However, there are very few reports
on the nanowire preparation of these materials.
6
The nanowires were synthesized by template aided sol-
gel route
7,8
using anodic aluminum oxide 共AAO兲 with pores
of nominal size of 100 nm and average pore density 10
9
/cm
2
as templates. We have used a sol-gel based polymer precur-
sor route that yields highly homogeneous and stoichiometric
complex oxides at moderate temperatures.
9,10
It is also pos-
sible to control the viscosity and stability of the sol easily in
polymer precursor route, which are crucial for template aided
synthesis. We have synthesized nanowires using templates
with diameter of 20 and 100 nm with varying chemical com-
positions in both Ca and Sr substituted lanthanum mangan-
ites. However, to be specific in this report we discuss the
results obtained on a particular composition, namely
La
0.67
Ca
0.33
MnO
3
共LCMO, x⫽ 0.33).
The sol used in the fabrication of LCMO nanowires was
prepared by dissolving stoichiometric ratio of lanthanum,
calcium, and manganese nitrates in required amount of wa-
ter. Ethylene glycol of nearly equal volume was added and
heated on a hot plate until a sol of desired viscosity forms
共⬇1Pas兲. The AAO membranes were dipped in the sol for
30–60 min and then subsequently heated to higher tempera-
tures, after cleaning the surfaces. Heating the membranes at
600 °C was sufficient to get the desired phase.
The LCMO nanowires thus prepared were studied for
structural and magnetic properties. Scanning electron mi-
croscopy was done along the cross section and compared
with that of the bulk. It was evident that the pores of the
membrane were almost completely filled to form uniform
nanowires. Energy dispersive x-ray analysis was done to
confirm the formation of nanowires of desired composition.
These nanowires could not be checked for oxygen stoichi-
ometry, however we predict that they are oxygen stoichio-
metric, as the nanopowders prepared starting from the same
sol and prepared under similar conditions were found to be
oxygen stoichiometric by redox titration analysis.
11
Transmission electron micrograph of an individual
a兲
Author to whom correspondence should be addressed; electronic mail:
arup@physics.iisc.ernet.in
b兲
Deceased.
LCMO nanowire removed from the supporting template is
shown in Fig. 1. Most of the nanowires are around 60–70 nm
in diameter and tens of microns in length. This shrinkage is
expected as the nanowires are prepared by heat treating the
sol-filled membranes. We have carried out detailed selected
area electron diffraction 共SAED兲 studies on the nanowires
and found that they crystallize in the orthorhombic structure
关consistent with the x-ray diffraction 共XRD兲 results兴. The
rings corresponding to 共002兲 and 共020兲 planes are marked in
the SAED image shown in the inset.
XRD data were recorded on the nanowires freed from
the membrane by dissolving in dilute NaOH 关shown in Fig.
2共a兲兴. In the same graph we show the XRD pattern of a
conventional LCMO (x⫽0.33) sample prepared by solid
state reaction routes 关Fig. 2共c兲兴. The pattern for the nanowire
could be indexed to an orthorhombic cell with lattice param-
eters of a⫽ 5.435(0.04) Å, b⫽ 7.699(0.03) Å, and c
⫽ 5.450(0.09) Å and V⫽ 228 Å
3
, unit cell anisotropy de-
fined as
␦
⬅(b/(a
2
⫹ c
2
)
0.5
)⫺ 1 is equal to 0.000 27 (
␦
⫽ 0
for cubic cell兲. The lattice parameters and the unit cell an-
isotropy parameter are smaller compared to those of bulk
microcrystalline powder
关
a⫽ 5.4687(0.002) Å, b
⫽ 7.775(0.001) Å, and c⫽ 5.484(0.0019) Å, with the cell
volume of 233.18 Å
3
and
␦
⫽ 0.00391.] Such lattice volume
contraction on particle size reduction is reported on other
nanomaterials.
12
In nanocrystalline LCMO with particle size
of 30 nm 关XRD data given in Fig. 2共b兲兴, prepared by sol-gel
method, we find similar lattice contraction and reduction in
␦
.
11
Compared to the nanopowders, the
␦
has reduced further
in nanowires implying that the structure of the nanowire
sample is more symmetric.
Figure 3 shows the plot of magnetic susceptibility versus
temperature of the nanowire sample freed form the mem-
brane along with that of a single crystal of LCMO 共of same
composition兲 prepared by float zone technique. The inset
shows the Curie–Weiss plot. The ferromagnetic-
paramagnetic transition is at 315 K for the nanowire sample
and is 235 K for the single crystalline sample. The substan-
tial shift in T
C
to higher temperatures 共by ⬃80 K兲, in the
nanowire samples, is definitely the most noteworthy effect of
size reduction.
InviewoftheenhancementofT
C
in both the nanowires
and nanocrystalline powders
11
of LCMO, we would like to
conclude that this is a general consequence of the size reduc-
tion. We propose that the T
C
enhancement arises mainly
from the hardening of the Jahn–Teller 共JT兲 phonon mode
⍀
ph
as the size is reduced. This is the stretching mode of the
MnO
6
octahedra and in LCMO (x⫽ 0.33) ⍀
ph
⬇78 meV. In
the presence of the JT distortion, T
C
⬇⌬ exp(⫺
␥
E
JT
/⍀
ph
),
13
where ⌬⫽ bandwidth 共2eV兲,
␥
⫽ dimensionless coupling
constant 共0.3–0.4兲, E
JT
⫽ JT energy 共1eV兲. The numbers in
brackets are the values for a typical manganite like LCMO
(x⫽ 0.33) with T
C
⬃250– 300 K. ⍀
ph
increases with appli-
cation of pressure
14
and has a mode Gruniesen parameter ⌫
⬅⫺
⍀
ph
/
V⬇2. The cause of T
C
enhancement on size re-
duction can thus be traced to the sensitivity of the phonon
mode ⍀
ph
. Taking the reduction in cell volume of nearly
2.6% 共in the nanowire sample兲 and ⌫⬇2, we get
FIG. 1. Transmission electron microscopy image on a LCMO nanowire
freed from the template and the inset shows the SAED image recorded on
the nanowire.
FIG. 2. XRD pattern of 共a兲 LCMO nanowire, 共b兲 LCMO nanopowder 共av-
erage particle size of 30 nm兲,and共c兲 microcrystalline powder prepared by
conventional solid state reaction.
FIG. 3. Plot of magnetic susceptibility vs temperature of LCMO nanowires
and bulk single crystal. Inset gives the Curie–Weiss plot.
␦
⍀
ph
/⍀
ph
⬇5.2% and
␦
T
C
⬇60 K assuming that other pa-
rameters remain unchanged. (
␦
T
C
⬇80 K needs only a
␦
⍀
ph
/⍀
ph
⬇6.5%). An additional enhancement
␦
T
C
⬇10– 15 K is expected from a 5% increase in ⌬ which arises
due to the decrease in both the unit cell anisotropy
␦
and the
unit cell volume. The observed enhancement is thus likely to
be caused by both enhancement of ⌬ and predominantly an
increase in ⍀
ph
. The explanation given above has a close
parallel to the pressure induced T
C
enhancement in manga-
nites, wherein dT
C
/dP⬇12–15 K/GPa for P⬍ 3 GPa for
LCMO 共0.33兲.
15
However, the pressure induced enhance-
ment saturates at
␦
T
C
⬇30 K– 40 K. We believe that this dif-
ference is mainly due to the difference between the exter-
nally applied hydrostatic pressure and the surface pressure,
16
caused by the size reduction. The surface pressure is ex-
pected to have a significant non-hydrostatic component par-
ticularly in long nanowires.
To conclude, we have prepared nanowires of LCMO by
a template-aided sol-gel process. This method can be ex-
tended to the nanowire fabrication of many multicomponent
materials. The T
C
of the nanowires was significantly en-
hanced above that of the single crystals.
One of the authors 共K.S.S.兲 acknowledges CSIR for re-
search associateship and A.K.R. thanks the department of
Science and Technology, Government of India for a spon-
sored project.
1
C. R. Martin, Science 266, 196 共1994兲; C. J. Brumlik, V. P. Menon, and C.
R. Martin, J. Mater. Res. 9,1174共1994兲.
2
Y. Yang, H. Chen, Y. Mei, J. Chen, X. Wu, and X. Bao, Acta Materialia 50,
5090 共2002兲.
3
M. H. Hwang, Y. Y. Hu, H. Feick, N. Tran, E. Weber, and P. D. Yang, Adv.
Mater. 共Weinheim, Ger.兲 13,113共2001兲.
4
R. Mahindiran, S. K. Tiwary, A. K. Raychaudhuri, and T. V. Ramakrish-
nan, Phys. Rev. B 53, 3348 共1996兲.
5
Colossal Magnetoresistance, Charge Ordering and Related Properties of
Manganese Oxides, edited by C. N. R. Rao and R. Raveau 共World Scien-
tific, Singapore, 1998兲.
6
X. Y. Ma, H. Zhang, J. Xu, J. Niu, Q. Yang, J. Sha, and D. Yang, Chem.
Phys. Lett. 363, 579 共2002兲.
7
J. C. Hulteen and C. R. Martin, J. Mater. Chem. 7, 1075 共1997兲; Y. Zhou
and H. Li, ibid. 12, 681 共2002兲.
8
O. V. Melezhyk, S. V. Pradius, and V. V. Brei, Microporous and Mesopo-
rous Materials 49,39共2001兲.
9
N. G. Eeror and H. U. Anderson, in Better ceramics through chemistry II,
Materials Research Society Symposia Proceedings Vol. 73, edited by C. J.
Brinker, D. E. Clark, and D. R. Ulrich 共MRS, Pittsburgh, PA, 1986兲,pp.
571–577.
10
E. C. Paris, E. R. Leite, E. Longo, and J. A. Varela, Mater. Lett. 37,1
共1998兲.
11
K. Shantha Shankar, S. Kar, G. N. Subbanna, and A. K. Raychaudhuri,
Solid State Commun. 129, 479 共2004兲.
12
G. Apai, J. F. Hamilton, J. Stohr, and A. Thompson, Phys. Rev. Lett. 43,
165 共1979兲.
13
G. Zhao, K. Conder, H. Keller, and K. A. Muller, Nature 共London兲 381,
676 共1996兲.
14
A. Congeduti, P. Postorino, E. Caramagno, M. Nardone, A. Kumar, and D.
D. Sarma, Phys. Rev. Lett. 86, 1251 共2001兲.
15
J. J. Neuneier, M. F. Hudley, J. D. Thompson, and R. H. Heffer, Phys. Rev.
B 52, R7006 共1995兲.
16
J. Weissmuller and J. W. Cahn, Acta Mater. 45,1899共1997兲.
