J. Electromagnetic Analysis & Applications, 2011, 3, 27-32
doi:10.4236/jemaa.2011.31005 Published Online January 2011 (http://www.SciRP.org/journal/jemaa)
Copyright © 2011 SciRes. JEMAA
Correlation of Magnetoresistance and
Thermoelectric Power in La1-xLixMnOy
Ahmed Mohamed Ahmed1*, Mahrous Rashad Ahmed1,2, Saad Abed El Rahman Ahmed1
1Physics Department, Faculty of Science, Sohag University, Sohag, Egypt; 2Physics Department, University College, Um-Elqura
University, Makkah, KSA.
Email: *Fikry_99@yahoo.com
Received November 25th, 2010; revised December 1st, 2010; accepted December 20th, 2010.
The temperature dependences of the thermoelectric power, TEP, (S) and magnetoresistance (MR) effect of La1-xLixMnOy
(x = 0.05, 0.1, 0.15, 0.2 & 0.25 at %) fixed valence doped compounds were studied between 80K and 320K. X-ray
powder diffraction (XRD) showed that the samples are single phase. We found the correlation of structures to the mag-
netoresistance (MR) and the thermoelectric power (S) which we assigned to change the specific branches of the magnon
/phonon spectra with x. We also observed a splitting of the Curie and the metal insulating temperature TC > Tms.
Keywords: Thermoelectric Power, Magnetoresistance, the Curie Temperature
1. Introduction In general, the thermal variation of the electrical resis-
tivity for both divalent and monovalent lanthanum man-
ganites are known to be dominated by polaronic transport
for T > Tc [15,20], while below Tc, electron–electron and
electron–magnon interactions are usually believed to do-
minate the conductivity [21-23]. Seebeck coefficient has
also been subject of several studies [15,21,24-26], par-
ticularly in divalent-doped manganites. In the high tem-
perature (T > TC), most of the reports on thermoelectric
power suggested a small polaron hopping [27,28], while,
below TC in the ferromagnetic region obtained from the
TEP is governed by the coexistence of both phonon drag
and magnon drag effects [26].
Recently, the rediscovery of colossal magnetoresistance
(CMR) in perovskite manganese oxides R1-xAxMnO3
(where R is a trivalent rare-earth element and A is a di-
valent element such as Ca, Sr, Ba, or Pb) has generated a
considerable interest because of their various electronic,
magnetic and structural properties and potential applica-
tions [1-4]. It has long been thought that the magnetic
structure and electronic transport properties of
R1-xAxMnO3 are correlated via the double-exchange (DE)
mechanism [5], i.e. the hopping of eg electrons between
Mn3+ and Mn4+ ions mediated by oxygen anions. However,
recent studies have proved that the strong Jahn–Teller
type electron–phonon coupling [6], orbital degree of
freedom [7,8], cation size mismatch [9-11], oxygen
non-stoichiometry [12-14], etc. can strongly influence the
behavior of these types of systems under the external
conditions such as temperature, magnetic field, electric
fields …. etc. In case of doping the compound La +3
x MnO3 by monovalent alkali (A) atoms like Li1+,
Na1+, Rb1+ because of larger valence difference between
La+3 and alkali-metal ions, fewer alkali-metal ions are
required to obtain a specific carrier concentration com-
pared to divalent doping and a consequent increase in
conductivity is achieved with less inhomogeneity
2. Experimental
The polycrystalline La1-xLixMnOy samples (x = 0.05, 0.1,
0.15, 0.2 & 0.25 at %) have been prepared by standard
solid state reaction method. Stoichiometric amounts of
La2O3, Li2CO3 and MnCO3 powders (all having 99.99%
purity) were thoroughly mixed and ground. After grind-
ing, the powders were pressed into pellets with a pressure
of 2 tones cm-2 and calcined at 1273K for 15 h in air.
Followed by cooling to room temperature, they were re-
ground and again pressed into pellets with a pressure of 7
tones cm-2 and subsequently annealed at 1373K for 10 h.
The samples were examined by X-ray powder diffraction
Correlation of Magnetoresistance and Thermoelectric Power in LaLi MnOy Compounds
28 1-x x
analysis which indicated the presence of single phase
with perovskite-type structure. The XRD analysis was
performed using Brucker (Axs-D8Advance) diffractome-
ter at room temperature with CuKα radiation ( =
1.5406Ǻ). Resistivity measurements were performed in a
variable temperature liquid nitrogen cryostat. The elec-
trical resistivity was measured by using the standard
four-probe technique using an air drying conducting sil-
ver paste. The sample temperature was monitored by ca-
librated Pt-100 thermocouple in the range 80–320 K. The
temperature accuracy was 0.5 K. A constant current in
the range of 10 mA–100 mA was supplied by the current
source [Delta G/electronic 0-20, 0-100 mA] and voltage
across the sample was measured with a digital voltmeter.
The magnetoresistance (MR) ratio is defined by MR =
 
 , where
and o
are resis-
tivities with and without vertical applied magnetic field
(0.5 T) respectively using [GMW magnet system model
347-70 Gl electromagnet 0.6 T]. The thermoelectric
power measurements were carried out using the sample
two-heater method with copper electrodes [29-31]. The
difference in temperature between the two opposite sur-
faces of the sample was adjusted to be equal to 3K during
the entire measurement. The temperature accuracy of the
measurements is described above. The thermoelectric
voltage was measured by a digital voltmeter with sensi-
tivity 1 mV as described in Reference [29-31].
3. Results and Discussion
Figure 1 shows powder X-ray diffraction patterns of the
polycrystalline La1-xLixMnOy samples (x = 0.05, 0.1, 0.15,
0.2 & 0.25 at %). The XRD patterns of samples reveals
that the prepared samples have a single-phase rhombo-
hedral structure with a space group R3-c. A remarkable
fact from the comparison of these patterns is that the sys-
tematic substitution of La by Li does not produce relevant
effect on them. In general, all the peaks of five samples
satisfy the La-Li-Mn-O phase. The structural parameters
are refined by the standard Rietveld technique [32]. The
lattice volume, lattice distortion and the bond lengths
(Mn-O) decrease with increasing Li substitution [33]. It is
well known that one of the possible origins of the lattice
distortion of perovskite- based structures is the deforma-
tion of the MnO6 octahedra originating from Jahn-Teller
(JT) effect that is inherent to the high- spin (S = 2) Mn3+
ions with double degeneracy of the eg orbital. Obviously,
this kind of distortion is directly related to the concentra-
tion of Mn3+ ions.
Another possible origin of the lattice distortion is the
average ionic of the A-site element, which is governed by
the tolerance factor
Ao Bo
 
, where rA, rB
and rO radius of A cation, B cation and oxygen element.
As t is close to 1, the cubic perovskite structure is real-
ized. As rA decreases, so does t, and the lattice structure
transforms to the rhombohedral (0.96 t 1) and then to
the orthorhombic structure (t 0.96), in which the bend-
ing of B-O-B bond increase and the bond angle deviates
from 180˚. For LaLiMnO3 samples, we think that the
room temperature structural transformation originates
mainly from the variation of the tolerance factor t in-
duced by the partial substitution of smaller Li1+(r = 0.76
Ǻ) ions for large La3+ ions (r =1.032 Ǻ). The reverse lat-
tice distortion from orthorhombic to rhombohedral sym-
metry due to larger Sr3+ ions partially substituting for
La3+ ions has been observed La1-xSrxMnO3 compounds
The resistivity versus temperature shows a semicon-
ducting behavior above the metal- semiconductor transi-
tion temperature (Tms) for all cases as shown in Figure 2.
An applied magnetic field of 0.5 T suppresses the resis-
tivity, giving a large negative magnetoresistance. It is
observed that above Tms the resistivity is much dependent
of T and it changes from fraction of -cm to 500 -cm
for samples rich with Li content. Therefore, it is observed
that the values of MR are values close along the range
temperature. In other side, the widening of transition
peak narrow with increasing Li content [36].
The negative magnetoresistance MR% defined as: MR
 was calculated and plotted versus am-
bient temperature as in Figure 3. In general, we observed
that the value of MR increase with decreasing Li content.
For x = 0.15 sample, a MR ratio as high as fifty percent-
age is obtained around Tc in magnetic field of 0.5 T.
The magneto-resistance MR of ceramic La1-xLixMnOy
can be viewed as a superposition of a continuously de-
creasing function which is connected with spin-depend-
ent transfer at grain boundaries and an intrinsic MR-peak
Figure 1. X-ray diffraction patterns of La1-xLixMnOy (x =
0.05, 0.1, 0.15, 0.2 & 0.25 at %). All patterns of samples
reveals that the samples have a single-phase rhombhedral
structure with a space group R3-c.
Copyright © 2011 SciRes. JEMAA
Correlation of Magnetoresistance and Thermoelectric Power in La1-xLixMnOy Compounds
Copyright © 2011 SciRes. JEMAA
(a) (b)
(c) (d)
Figure 2. The variation of resistivity in zero magnetic field and H = 0.5 T with temperature, T(K), for Li-content of
La1-xLixMnOy = where a – x = 0.05. b – x = 0.1, c – x = 0.15, d – x = 0.2 and e – x = 0.25 at %. The resistivity shows a semi-
conducting behaviour above the metal–semiconductor transition temperature (Tm) for all cases. The semiconductor–metal
transition (Tm), moves to a lower temperatures with the increment of the Li concentration.
Correlation of Magnetoresistance and Thermoelectric Power in LaLi MnOy Compounds
30 1-x x
at Tc [37]. The superposition was observed for all the five
composition (i-MR) as in Figure 3. That means that there
are additional structures in the i-MR. This superposition
corresponds to the first peak of S(T) in Figures 3,4. In
other side we observe that while the values of S(T) in-
crease with decreasing Li-content the value of MR de-
crease. As shown in Figure 4 there are two transitions for
each composition. The first peak has a value of TC corre-
sponds to that value obtained from the magnetoresistance
(T) curve [38] can be identified with the (highest)
peak of the (bulk quantity) i-MR (at TC). The second
peak of S(T) corresponds to the m-s transition which de-
termined of o
as shown in from Figure 2.This obser-
vation shows that there are a good correlation between
MR and TEP.
However, both the value of Seebeck coefficient (S) and
the value of resistivity (ρ) increase with increasing dop-
ing of Lithium as in Figures 2,4. We expect that when
the Li-content increase not only the La-content decreases
but also the charge carrier density does [38]. Therefore,
the La/Li mixture ions play a prominent role in control-
ling the resistivity.
However, the Li-doping can also have beneficial ef-
fects in regard to the magnetoresistance: 1) we find that
MR increase with decreasing Li content and 2) the low
Li-substitution tends to broaden the magnetoresistive
peak around Tc (Figure 2); both effects are helpful if one
thinks about the robust magnetoresistive sensors. In an
effort to understand this behavior we found an unex-
pected correlation of structures in MR and the thermoe-
lectric power TEP results; which gives us an important
clue to the origin of this broaden magnetoresistive and
the fine structure which agrees with it.
The values of metal-semiconductor transition tem-
perature (Tm-S) and the ferromagnetic transition tempera-
ture (Tc) tabulated in Table 1. Generally the value of
both Tc and Tms which are determined from Seebeck or
resistivity measurements decreases with increasing Li
content. In addition, the values of Tc (S) are higher than
Tc(MR) and also the values of Tms(S) are equal or higher
than Tms(ρ). We believe that the lithium alters the
Mn4+/Mn3+ ratio which is one of the factors which deter-
mine the transport and magnetic properties of samples.
The decrease in Tms with increasing of Li-content can be
interpreted due to an increasing strength of the Mn–O–
Mn bond with decreasing the average radius average rA
of ions of A-sites because of the partial substitution of
smaller Li1+ ions for larger La3+ ions. This substitution
causes a narrowing of the bandwidth, thus, decreasing of,
eg electrons which, in turn, results in a weakening of the
double exchange interaction magnetism [9].
Figure 3. The variation of magnetoresistance with tem-
perature, T(K), for Li-content of La1-xLixMnOy. the value
of MR increase with decreasing Li content. The MR
ratio of the La1-xLixMnOy reaches to 55% under a low
field (H = 0.5 Tesla) for low dopin g Li-c ontent.
Figure 4. The variation of Seebeck coefficient with tem-
perature, T(K), for Li-content of La1-xLixMnOy. The values
of S (T) increase with increasing Li concentration. Tm1 cor-
responds to the Curie temperature but Tm2 corresponds to
m-s transition temperature.
Table 1. Curie and M-S transition temperature as function
of doping, x.
c (M)(K) Tc (S) Tc (MR) Tms(S) Tms(ρ)
x = 0.05 - 203 223 123 123
x = 0.1 215 203 213 113 113
x = 0.15 211 173 203 113 103
x = 0.2 150 173 193 103 83
x = 0.25 163 183 93 83
Copyright © 2011 SciRes. JEMAA
Correlation of Magnetoresistance and Thermoelectric Power in LaLi MnOy Compounds31
1-x x
4. Conclusions
The thermoelectric power and magnetoresistance of mo-
novalent alkali metal (Li) substituted LaMnO3 polycrys-
talline pellets prepared by solid-state reaction procedure
has been studied between 80 and 320K. X-ray diffraction
patterns showed a single-phase rhombohedral structure of
all samples. The M–S transition temperature decreases
with increasing Li content. This is due to the partial sub-
stitution of smaller Li1+ ions for larger La3+ions or due to
Zener bond blocking. The MR ratio of the La1-xLixMnOy
is about 55% under a low field (H = 0.5 Tesla) for low
doping Li-content. This result opens new opportunities to
improve performance of colossal magnetoresistive de-
vices. The S(T) curves show that Seebeck coefficient is
positive sign. In addition, there are two transition the first
we point it correspond to Tc which determine from MR
measurements, and the second peak correspond to Tms(ρ).
The two corresponds of transition temperature confirm
that there are a correlation of structures in MR and the
thermoelectric power TEP; which gives us an important
clue as to the origin of this broaden magneto resistive and
the fine structure which goes with it.
5. Acknowledgements
The authors would like to thank H. F. Mohamed for his
continuous help.
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