Low temperature magnetoresistive effects and coulomb blockade in La0.7Ca0.3MnO3 nanoparticles synthesis by auto-Ignition method

doi:10.4236/ns.2011.36069

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Vol.3, No.6, 496-501 (2011) Natural Science

http://dx.doi.org/10.4236/ns.2011.36069

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; [email protected]

2Department of Physics, Quaid-I-Azam University, Islamabad, Pakistan; [email protected]

Received 23 September 2010; revised 14 October 2010; accepted 30 October 2010.

ABSTRACT

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)

1. INTRODUCTION

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

497

much larger than h/e2 ~12×103 Ω. 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.

2. EXPERIMENTAL PROCEDURES

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.

3. RESULTS AND DISCUSSION

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)

(b)

(a)

1k Oe

4k Oe

ZFC

Figure 2. (a) Resistivity versus temperature curves under dif-

ferent magnetic field. (b) Susceptibity derivative versus tem-

perature curve.

4k Oe1k Oe

ZFC

A. M. Khan et al. / Natural Science 3 (2011) 496-501

498

050100 150 200 250 300

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

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

0.4

0.8

1.2

1.6

ZFC

(c)

1k Oe

200 Oe

4k Oe

/  3 0 0

Temperature (K)

Figure 5. Room temperature normalized resistivity.

(c)

A. M. Khan et al. / Natural Science 3 (2011) 496-501

499

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

(ohm-mm/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-

creases).

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

ZFC

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

1.000

1.005

1.010

1.015

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

500

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

ZFC

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.

4. CONCLUSIONS

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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