Vol.3, No.6, 496-501 (2011) Natural Science
Copyright © 2011 SciRes. OPEN ACCESS
Low temperature magnetoresistive effects and coulomb
blockade in La0.7Ca0.3MnO3 nanoparticles synthesis by
auto-Ignition method
Aamir Minhas Khan1, Arif Mumtaz2, Syed Khurshid Hasanain2, Anwar Ul Haq1
1Department of Physics, Air University, Islamabad, Pakistan; aamir.minhas@mail.au.edu.pk
2Department of Physics, Quaid-I-Azam University, Islamabad, Pakistan; minhas.qau@gmail.com
Received 23 September 2010; revised 14 October 2010; accepted 30 October 2010.
Electrical transport properties of the La0.7Ca0.3
MnO3 nanoparticles have been investigated in
the temperature range 300 to 9 K as a function
of magnetic field. Samples were prepared by
auto-ignition method. In low temperature regime
from 40 to 9 K, an increase in the resistivity has
been observed. This effect is found to decrease
as magnetic field is increased. It is assumed
that these effects are due to the magnetic con-
tacts between the nanoparticles.
Keywords: CMR (Colossal Magnetoresistance) CB
(Coulomb Blockade)
Magnetoresistance (MR) phenomenon, in which the
electrical transport properties are strongly affected by
applied magnetic fields, was first reported in 1988 [1] in
metallic multilayers. Similar results were followed by
granular metallic systems [2,3] and in permalloy/Al2O3/
CoFe junctions [4]. All of these records show that new
tools are now reachable to obtain artificial MR by ma-
nipulating the micro/nanostructure of metallic com-
pounds. Thus Magnetic nanoparticles exhibit interesting
electronic and magnetic properties arising due to the
structural and magnetic disorders in their surfaces espe-
cially in nano-sized perovskites [5,6]. In early 1990s, in
mixed-valence manganese oxides (hereafter referred to
as manganites) a new kind of MR was rediscovered [7].
Under a field of several Tesla it was possible to achieve
MR at temperatures relatively close to transition tem-
perature, leading to the name of colossal MR (CMR).
This class of oxides was theoretically modeled in 1950s
[8,9], when the first studies were carried out on the
crystallographic structure, and found to have “perovskite
“structure. CMR properties of these materials make them
technologically important for applications in magnetore-
sistive devices [5,10], principally in magnetic recording
or magnetic data storage devices. Intrinsic phase separa-
tion in (CMR) material is also an important phenomenon
leading to the new applications of spintronics [11,12]
Within the past decade, one-dimensional nanowires and
nanowire arrays have captured the interests of many
groups in a wide variety of fields, mainly due to their
unusual properties and potential for integration into and
miniaturization of current technologies.
Therefore, hole-doped manganese perovskites
L1–xAxMnO3, where L and A are trivalent lanthanide and
divalent alkaline earth ions respectively, have been stud-
ied extensively [13,14]. By changing values of x in
La1–xCaxMnO3 we may get a variety of magnetic and
transport states in the material, ranging from different
antiferromagnetic insulators to ferromagnetic metals.
Here we focus on x = 3, because it lies in ferromagnetic
/metallic phase in the phase diagram [15].
The resistivity of the nano sized manganite below 50
K under zero magnetic field is reported by many work-
ers. D. Niebieskikwiat [16] synthesized magnatites by
three different methods namely Gel Combustion (GC),
Urea Sol-Gel (USG) and Liquid Mix (LM). The particle
size in the samples was 30 nm and ρ of the samples fol-
low the order ρGC > ρUSG >ρLM.. The low temperature
increase of ρ was not observed in sample of 95 nm parti-
cle size. MA L´opez-Quintela [17] also observed the
maximum upturn in very low temperature for 60 nm as
compared to 500 nm. Y. G. Zhao [18] observed the same
trend in thin film, i.e. maximum upturn for 36 nm as
compared to 108 nm. M. García-Hernández [6,19] also
observed the maximum upturn for 20 nm as compared to
80 nm. In all of these papers explanation is followed by
T–1/2 curve fitting. The T–1/2 model accounts for the low-
temperature resistance of granular metals embedded
within an insulating matrix, and it is only valid in the
tunneling regime, where typical intergrain resistances are
A. M. Khan et al. / Natural Science 3 (2011) 496-501
Copyright © 2011 SciRes. OPEN ACCESS
much larger than h/e2 ~12×103 . It also assumes that
the energy barriers are inversely proportional to the grain
radius. M. García-Hernández [6,19] also observed the
effect of very high magnetic field of 9 T on the low
temperature upturn and explained on the basis of refor-
mulation of the T–1/2 model.
Nanoparticles of La0.7Ca0.3MnO3 were synthesized by
citrate auto-ignition method [20]. La2O3, CaCO3,
Mn(CH3CO O)2.4H2O and C6H8O7 were used in sto-
chiometric amounts as starting materials. First La2O3 and
CaCO3 were dissolved into HNO3 to convert them into
their corresponding water soluble nitrates.
Mn(CH3CO O)2.4H2O and citric acid were dissolved in
water separately. All the solutions were mixed and
stirred for 4 - 5 minutes to make a homogeneous solution
and then citric acid was added to it as a chelating agent.
To avoid precipitation, pH value of the solution was ad-
justed to a neutral value of 7 using the aqueous NH3 so-
lution. That solution was slowly evaporated at 80˚C -
90˚C for 60 to 90 minutes, which resulted brown colored
viscous gel. This was further heated in a box furnace at
~250˚C. After about 30 minutes, the gel was foamed;
swelled and large volume of gases was evolved leading
to an automatic ignition with glowing flints. This pro-
duced highly porous black fluffy material with fine
powder of about 20 nm (called a precursor). The precur-
sor was annealed at 1000˚C for 10 hours. Before pelleti-
zation, structure of the powder was studied using powder
X-ray diffraction (XRD). Figure 1 shows the XRD pat-
tern. All peaks could be indexed for an orthorhombic
unit cell of a = 5.4412 Å b = 7.6637 Å c = 5.4479 Å. The
phase defection limit of XRD is 5%. If it is less than 5%
then, no XRD peak will be defected. As we assume that
our sample has, if at all, less than 5% such phase. The
average particle size as estimated by Scherrer’s formula
was 40 nm.
For the resistivity measurements rectangular pellet of
the samples (length = 4.7 mm, width = 3.4 mm, thick-
ness = 1.6 mm) were prepared. Powdered sample was
first mixed with few drops of liquid binder Poly Vinyl
Alcohol (PVA), compressed under pressure of 7 - 8 ton
and then heated at 650˚C for 5 hours to get good com-
pact discs. The four probe configuration was used to
measure resistivity values. For the resistivity (T) meas-
urement under magnetic field, a Hall probe was used in
which, D.C magnetic field was applied perpendicular to
the flow of current.
The electrical transport property constitutes the most
Figure 1. Evolution of the XRD patterns.
attractive physical property of the manganites due to
their high CMR values [17]. Figure 2(a) shows the re-
sistivity of the sample as a function of temperature and
magnetic field (H = 200 Oe, H = 1 kOe and H= 4 kOe).
Resistivity without magnetic field, increases with the
decrease of temperature and exhibits a pronounced peak
at metal-insulator transition TMI 256 K due to para-
magnetic to ferromagnetic transition [21]. The suscepti-
bility measurements also show a peak in d//dT versus
temperature curve conforming paramagnetic ferromag-
netic transition at 256 K, as shown in Figures 2(b) and
3 and their trend is shown in Figure 4. Resistivity be-
tween temperature range of 300 K to 256 K shows nega-
tive temperature coefficient of resistivity (i.e., dρ/dT < 0)
indicating an insulating nature. Whereas the positive
temperature coefficient (i.e., dρ/dT > 0) between 256 K
to 40 K, i.e., below TMI displays a metallic behavior of
the sample, together with a paramagnetic to ferromag-
netic transition in the close vicinity [22].
050100 150 200 250 300
Temperature (K)
Resistivity (Ohm-mm)
1k Oe
4k Oe
Figure 2. (a) Resistivity versus temperature curves under dif-
ferent magnetic field. (b) Susceptibity derivative versus tem-
perature curve.
4k Oe1k Oe
A. M. Khan et al. / Natural Science 3 (2011) 496-501
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050100 150 200 250 300
Susceptibility (emu/g-Tesla)
Temperature T(K)
Figure 3. Susceptibility versus Temperature plot.
100 150 200 250 300
Magnetization M (emu/g)
Temperature (K)
Figure 4. Magnetization versus Temperature Plot.
In the temperature range T < 40 K, resistivity values
without magnetic field exhibits a minima in (T) and an
increase in resistivity values observed below 35 K. Here
dρ/dT values again indicate a negative slope leading to
an insulating behavior in ferromagnetic phase. In the
temperature range from 35 K to 9 K, the increase in re-
sistivity values is of the order of 8.049 ohm-mm to 8.829
ohm-mm, and dρ/dT is –0.0236 (ohm-mm/K) at 9 K.
This effect has not been reported in high-quality single
crystals [17]. Since our material is polycrystalline and
consists of nanoparticles as such it may due to an elec-
trostatic blockade of carriers between grains [17] and
will be discussed later on in detail.
It is interesting to note that TMI values are changing
due to application of magnetic field but no field de-
pendence is observed. Values of TM-I are 256 K, 259 K
and 258 K for the magnetic fields of 200 Oe, 1 kOe and
4 kOe field respectively. However, dρ/dT is negative in
the temperature range of 300 K to 256 K and is positive
below TM-I up to 40 K, showing no significant difference
as compare to zero field values. It is worth mentioning
that the values of the resistivity at TM-I are independent
of the magnetic field and are 34.911 ohm-mm, 36.227
ohm- mm, 36.382 ohm-mm and 36.271 ohm-mm for
magnetic fields of zero, 200 Oe, 1 kOe and 4 kOe, re-
spectively as shown in Table 1. Room temperature nor-
malized resistivities are also field independent (Figure
5). This is in contradiction to the work of P K Siwach, M.
Garcı´a- Herna´ndez, Ning Zhang and B. Roy
[6,15,22,23], where resistivity at TM-I decreases with the
increase of magnetic field, further investigation is in
hand to explain the reasons. Value of the dρ/dT at 9 K
increases with the increase of the magnetic field, and is
of the order of –0.012 ohm-mm/K, –0.007 ohm-mm/K
and 0 ohm-mm/ K for the magnetic fields of 200 Oe, 1
kOe and 4 kOe magnetic field respectively. Also below
35 K the increase of resistivity is 0.244 ohm-mm, 0.11
ohm-mm, 0.086 ohm-mm and 0 ohm-mm at zero, 200Oe,
1kOe and 4 kOe, respectively as shown in Table 1. This
variation in the dρ/dT is also reported in thin films of
manganite [6,19] which was more prominent in the
smaller grain sized particles [17]. But to best of our
knowledge this zero slope behavior of the resistivity has
not been reported earlier in polycrystalline nanoparticles.
To understand the low temperature resistivity (T)
behavior of the nanoparticles, we may use core-shell
model as proposed by Zhang et al. [22]. This model as-
sumes that the core of nanoparticles has the properties of
the bulk material and low freezing temperature for spins
in the outer surface layer of the particles [22]. We may
analyze the resistivity curves as the sum of these two
contributions, i.e. ,
ρ(T ) =A ρ0+B exp(D/T)1/2
where, ρ0 stands for the resistivity of the bulk material
without increase in resistivity below 40 K. The second
term describes the Coulomb Blockade CB effect, i.e.
50100 150 200 250 300
1k Oe
200 Oe
4k Oe
/ 3 0 0
Temperature (K)
Figure 5. Room temperature normalized resistivity.
A. M. Khan et al. / Natural Science 3 (2011) 496-501
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Table 1. Effects of magnetic field on resistivity parameters.
Magnetic Field (k Oe) TMI (K) Resistivity ρ at TMI (ohm-mm) Increase of resistivity ρ
(ohm-mm). (below 35 K)
dρ/dT at 9 K
Slope D calculated
from Eq.1.
Zero 256 34.911 0.82 –0.023 0.095
0.2 256 36.227 0.1109 –0.012 0.047
1 259 36.382 0.0867 –0.007 0.032
4 258 36.271 0 0 0
increase in resistivity below 40 K [6,19]. Where A and B
are constants.
ρ(T ) exp(D/T)1/2 (1)
The plot of log ρ vs T shows a best fit for log ρ vs T–1/2
instead of the T–1 postulated for a pure CB effect [19].
T–1/2 model assumes the charging energy EC to be particle
size dependent [6,24]. The slopes of the lines (D) shown
in Figure 6, are proportional to the electrostatic charging
energy EC [25,26] and are magnetic field dependent also
Figure 7 shows the magnifying slopes. This variation in
charging energies EC with the applied magnetic field may
not be explained by a standard CB [6] model.
This effect may be explained if we take into account
the transportation of electrons from one Mn site to the
other by double exchange DE. The basic idea of double
exchange is that the initial and final states are degenerate
states, leading to a delocalization of the hole on the Mn4+
site or electron on the Mn3+ site. Thus the transfer of an
electron occurs simultaneously from Mn3+ to O2 and
from O2 to Mn4+; this process is a real charge transfer
process and involves an overlap integral between Mn 3d
and O 2p orbitals [27]. In 1955 Anderson and Hasegawa
introduced modified model, in which they treat spin
magnetic moments of each Mn ion classically and the
mobile electron quantum mechanically. They showed
that electron transfer probability teff between neighboring
Mn ions depends on the angle θ between their magnetic
moments as teff = t cos(θ/2). Which varies from 1 for θ =
0 to zero for θ = 180˚ [28] (i-e transfer of electron be-
tween neighboring Mn ions is favored when angle be-
tween their spin magnetic moments is zero and this
transfer become more difficult when orthognality in-
Large number of dangling bonds or existence of the
noncoordination atoms in the surfaces, structure sensi-
tivity of the material and defects all together effect the
double-exchange DE interaction in the surface. The in-
sulating property exhibited by the (T) curve, below 35
K in the ferromagnetic phase, may be due to the break-
down of the DE mechanism caused by the broken
Mn–O–Mn bonds at the surface of the nanoparticles and
the translational symmetry breaking of the lattice [23].
Taking into account the structure sensitivity of the mag-
netic configuration of such material [22] we can suppose
that with the applied magnetic field spin magnetic mo-
0.18 0.21 0.24 0.27 0.30 0.33
1. 85
1. 90
1. 95
2. 00
2. 05
2. 10
4k Oe
1K Oe
200 Oe
ln (Ohm-mm)
T -1/2 (K-1/2)
Figure 6. Natural Log of Resistivity as a function of T-1/2.
0.18 0.24 0.30
ln (Ohm-mm)
T -1/2 (K -1/2)
Figure 7. Magnifying slope of the curve immerging from the
same point.
ments of two neighboring Mn ions, through O ion be-
tween them, at the neighboring nanoparticles get aligned
[6,19]. Which may give rise to the establishment of a
good magnetic contact between the neighboring nano-
particles as suggested by M. Garcı´a-Herna´ndez [6]?
This provides new productive conduction channels for
the electrons across the particle boundaries as claimed in
the literature [29]. These magnetic contacts help carriers
to flow from particle to particle. If N is number of chan-
nels at the contact then conductance must be of the order
A. M. Khan et al. / Natural Science 3 (2011) 496-501
Copyright © 2011 SciRes. OPEN ACCESS
of Ne2/h [6]. Furthermore, the capacitances of individual
particles are renormalized by coupling to the other parti-
cles and, therefore, are no longer determined solely by
the particle radius as suggested EC = e2/4πε0εd. The fol-
lowing hypothesis makes the main difference with re-
spect to the standard CB model.
Particle capacitances mainly depend on the quality of
the contacts established with the neighboring particles
and therefore on the connectivity of the system and par-
ticles with good contacts show no CB effects, even if
their radii are small [6], as shown in the Figure 8 for
resistivity at 4 kOe (i.e. there is no rise in temperature
below 0 K for 4 K Oe magnetic field). In this context,
connectivity may also be understood in a broad sense.
Realizations of such magnetic contacts microscopic
weak links are misaligned Mn spins at the surface
[29-32], distortions of the Mn-O-Mn angles due to
structurally unbalanced environments at the grain sur-
face and impurities or defects [6]. The blocked spins in
the surface can be aligned by external magnetic field,
just like in crystals [6]. The average relative angle of the
local spins, in surface Δfs is larger than that in body
phase Δfb at a given temperature below TC, i.e. Δfs > Δfb
when T < TC. Two neighboring nanoparticles can be
electrically connected only when atoms of both sides, on
the edge of nanoparticle, overlap each other partly and
form a Mn-O-Mn. This demands small enough interpar-
ticles distance i.e. half the Mn-O-Mn bond length [22],
which might be achieved by pallet preparation of the
sample under high enough pressure (i.e. under 7 ton) and
heating (heating at 650˚C for 5 hours). Also a Mn ion
and an O–2 ion, respectively, sit at the two sides of a
connective point of neighboring nanoparticles. As the
field increases blocked Mn spins at the surface of a na-
noparticle get align and probability of such a DE phe-
nomenon at the surface increases, which depends upon
10 15 20 25 30 35
6. 5
7. 0
7. 5
8. 0
( d )
Resistivity (Ohm-mm)
4 k Oe
1k Oe
200 Oe
T e m e r a t u r e ( K )
Figure 8. Resistivity curves below 35 K.
the surface spin population [22]. Magnetic field stabi-
lizes the contact points which tend to strengthen cou-
pling of the nanoparticles. Thus in this way coupling
between neighboring nanoparticles enhances with the
application of magnetic field, leading to delocalization
of the charges to neighboring particles. As a result, a
decrease of the resistivity is observed experimentally,
upon application of a magnetic field below 35 K.
Nanoparticles of La0.7Ca0.3MnO3 have been synthe-
sized by citrate auto-ignition method and their electrical
properties (resistivity) investigated. Below 35 K, in-
crease in the resistivity values with the decrease in tem-
perature is observed without magnetic field. Below 35 K
charging energy was found to be sensitive to the applica-
tion of a magnetic field, which could not be explained by
the pure CB model. Charging effect dependence upon
magnetic field was explained assuming that there exist
good contacts between the neighboring nanoparticles
due to alignment of the magnetic Mn spins at the sur-
faces of the neighboring nanoparticles with the magnetic
field. These contacts delocalize the charges to neighbor-
ing nanoparticles.
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