The effect of grain size on electrical transport and magnetic properties of La< sub> 0.9</sub> Te< sub> 0.1</sub> MnO< sub> 3</sub>

Abstract

The effect of grain size on structural, magnetic and transport properties in electron-doped manganites La 0.9 Te 0.1 MnO 3 has been investigated. All samples show a rhombohedral structure with the space group C R − 3 at room temperature. It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of grain size. All samples undergo paramagnetic (PM)-ferromagnetic (FM) phase transition and an interesting phenomenon that both magnetization and the Curie temperature C T decrease with increasing grain size is observed, which is suggested to mainly originate from the increase of the Mn-O bond length O Mn d − . Additionally, ρ obviously increases with decreasing grain size due to the increase of both the height and width of tunneling barriers with decreasing the grain size. The results indicate that both the intrinsic colossal magnetoresistance (CMR) and the extrinsic the extrinsic interfacial magnetoresistance (IMR) can be effectively tuned in La 0.9 Te 0.1 MnO 3 by changing grain size. PACS: 75.30.Kz; 75.47.Lx; 75.47.Gk (Y. P. Sun).

Full text

Page 1

1
The effect of grain size on electrical transport and magnetic
properties of La0.9Te0.1MnO3
J.Yanga, B. C. Zhaoa, R. L. Zhanga, Y. Q. Maa, Z.G. Shenga, W. H. Songa,
Y. P. Suna,b,*
aKey Laboratory of Materials Physics, Institute of Solid State Physics, Chinese
Academy of Sciences, Hefei, 230031, P. R. China
bNational Laboratory of Solid State Microstructures, Nanjing University, Nanjing
210008, P. R. China
Abstract
The effect of grain size on structural, magnetic and transport properties in
electron-doped manganites La0.9Te0.1MnO3 has been investigated. All samples show a
rhombohedral structure with the space groupCR
−
3 at room temperature. It shows that
the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the
increase of grain size. All samples undergo paramagnetic (PM)-ferromagnetic (FM)
phase transition and an interesting phenomenon that both magnetization and the Curie
temperature
C
T decrease with increasing grain size is observed, which is suggested
to mainly originate from the increase of the Mn-O bond length
O Mn
d
−. Additionally,
ρ obviously increases with decreasing grain size due to the increase of both the
height and width of tunneling barriers with decreasing the grain size. The results
indicate that both the intrinsic colossal magnetoresistance (CMR) and the extrinsic the
extrinsic interfacial magnetoresistance (IMR) can be effectively tuned in
La0.9Te0.1MnO3 by changing grain size.


PACS: 75.30.Kz; 75.47.Lx; 75.47.Gk
Keywords: A. Colossal magnetoresistance; B. Chemical synthesis; D. electrical
transport
* Corresponding author. E-mail address: ypsun@issp.ac.cn (Y. P. Sun).

Page 2

2
Mixed-valent manganites perovskites have attracted considerable attention in recent
years because of the observation of CMR and more generally due to the unusually
strong coupling between their lattice, spin, and charge degrees of freedom. Although
the focus of interest has primarily rested with the hole-doped manganites
Ln1-xAxMnO3 (Ln = La-Tb, and A = Ca, Sr, Ba, Pb, etc.) due to their potential
applications such as magnetic reading heads, field sensors and memories [1], naturally
many researches have placed emphasis on electron-doped compounds such as
La1-xCexMnO3 [2-5], La1-xZrxMnO3 [6], La2.3-xYxCa0.7Mn2O7 [7], and La1-xTexMnO3
[8-10] due to the potential applications in spintronics. These investigations also
suggest that the CMR behavior probably occur in the mixed-valent state of
Mn2+/Mn3+. The basic physics in terms of Hund's rule coupling between
g e electrons
and
g
t2core electrons and Jahn-Teller (JT) effect due to Mn3+ JT ions can operate in
the electron-doped manganites as well.
From the viewpoint of future applications, the manganites with smaller grain size
will be required. For this class of materials, lower sintered temperatures (Ts) will be
necessary as the grain size increases with Ts. The effect of grain size on structural,
magnetic and transport properties in hole-doped manganites has been extensively
investigated [11-17]. All these researches indeed suggest that the magnetic properties
are markedly affected by grain size. So it is of vital interest to investigate the effect of
the grain size on the magnetoresistance and related transport properties because of the
electrical transport properties of the manganites are closely linked to their magnetic
properties. However, many controversial results have been reported concerning the
influence of grain size on the magnetic properties of polycrystalline manganites.
Sánchez et al. studied the effect of grain size in La0.67Ca0.33MnO3 and found that both
magnetization and the Curie temperature (
C
T ) decreased with decreasing grain size
[11]. Hueso et al. also observed the similar rule and they proposed that it was mainly
attributed to the presence of a nonmagnetic surface layer created by noncrystalline
material that was important as the particle size decreases [12]. Zhang et al.
investigated a structure-dependent change of magnetization in the manganite oxide

Page 3

3
La0.85Sr0.15MnOz and found that the contrary statements, i.e., both magnetization and
C
T decreases with increasing grain size [13]. They supposed that the contradiction
could originate from the different doping level because the structure or magnetism
was sensitive to the doping level as presented in [18]. To understand the essence of
this issue, in this article, we will report our investigation of the effect of grain size on
structural, magnetic and electrical transport property in the electron-doped manganites
La0.9Te0.1MnO3. We claim that our material is an electron-doped manganite based on
simple valence arguments. In fact, X-ray photoemission spectroscopy (XPS)
measurement revealed that the Te ions were in the tetravalent state and the manganese
ions could be considered as in a mixture state of Mn2+ and Mn3+ for the compound
La1-xTexMnO3 [8-10].
Nanometer samples of La0.9Te0.1MnO3 were prepared by a citrate gel technique
[19]. Stoichiometric amounts of high-purity La2O3, TeO2, and Mn metal powders
were dissolved in diluted nitric acid in which an excess of citric acid and ethylene
glycol was added to make a metal complex. After all the reactants had completely
dissolved, the solution was mixed and heated on a hot plate resulting in the formation
of a gel. The gel was dried at 250 °C, and then preheated to 600 °C to remove the
remaining organic and to decompose the nitrates of the gel. The resulting powder was
pressed into pellets, each of which was sintered at a different temperature (Ts = 800
°C, 900 °C and 1000 °C) in air. In order to obtain a polycrystalline reference pattern,
ceramic samples of La0.9Te0.1MnO3 were prepared by the conventional solid-state
reaction method in air. Appropriate proportions of high-purity La2O3, TeO2, and
MnO2 powders were thoroughly mixed according to the desired stoichiometry, and
then calcined at 700 °C for 24 h. The powders obtained were ground, pelletized, and
sintered at 1050 °C for 96 h with three intermediate grindings, and finally, the furnace
was cooled down to room temperature. For the sake of description, the samples are
labeled by sample A, B, C and D corresponding to their sintering temperature, i.e.,
800 °C, 900 °C, 1000 °C and 1050 °C, respectively.
The crystal structures were examined by x-ray diffractometer using Cu

Page 4

4
α
K radiation at room temperature. The magnetic measurements were carried out with
a Quantum Design superconducting quantum interference device (SQUID) MPMS
system (2 ≤T≤400 K, 0 ≤ H ≤5 T). Both zero-field-cooling (ZFC) and field-cooling
(FC) data were recorded. The resistance was measured by the standard four-probe
method from 25 to 300 K.
Fig.1 shows the x-ray diffraction (XRD) pattern of La0.9Te0.1MnO3 samples of (A)
800 °C, (B) 900 °C, (C) 1000 °C-sintered by a citrate-gel technique and (D) obtained
through the conventional solid-state reaction method. The powder x-ray diffraction at
room temperature shows that all samples are single phase with no detectable
secondary phases and the samples have a rhombohedral structure with the space
groupCR
−
3 . The average grain size
hkl
D is estimated through the classical Scherrer
formula [20] θBλ kDhkl
cos
=
, where
hkl
D is the grain size derived from the (0 2 4)
peak of the XRD profiles, k is a constant (shape factor ~ 0.9), θ is the angle of the
diffraction, B is the difference of the full width at half-maximum (FWHM) of the
peak between the sample and the standard SiO2 used to calibrate the intrinsic width
associated to the instruments, and λ is the wavelength of X-ray. The grain size
obtained from the Scherrer formula is thus estimated as 40 nm, 53 nm, 82 nm, and
120 nm for samples A, B, C, and D, respectively. As it can be clearly seen from the
inset of Fig.1, the grain size increases with increasing Ts and the diffraction peak (0 2
4) shifts towards the lower angle from sample A to D, which is consistent with many
results reported elsewhere [11,16]. The structural parameters of the samples are
refined by the standard Rietveld technique [21] and the fitting between the
experimental spectra and the calculated values is quite well based on the consideration
of lower RP values as shown in Table 1. It shows that the lattice parameters of
La0.9Te0.1MnO3 samples vary monotonously with increasing the sintered temperature
Ts. The Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the
increase of the particle size.
Fig.2 shows the temperature dependence of magnetization M of La0.9Te0.1MnO3
under both zero-field-cooled (ZFC) and field-cooled (FC) modes at H = 0.1 T for all

Page 5

5
samples. The Curie temperature
C
T (defined as the one corresponding to the peak of
dT dM in the M vs. T curve) is 268, 253, 240 and 239K for samples A, B, C and D,
respectively. Obviously, the Curie temperature decreases with the increase of Ts from
sample A to C.
C
T of sample D is almost same as that of sample C. It demonstrates
that there may exist a critical value of the grain size. Below this critical value, the
Curie temperature of the sample may be affected strongly by the grain size. Whereas
beyond this critical value, the grain size does not almost affects
C
T . Here, for the
studied sample La0.9Te0.1MnO3, the critical value of the grain size may be about 80
nm. We suggest that the
C
T reduction should be attributed to the weakening of
double exchange (DE) interaction because of the decrease of the bandwidth and the
mobility of
g e electrons due to the increase of Mn-O bond length and the decrease of
Mn-O-Mn bond angle. Additionally, it is surprising that the magnetization M
decreases at low temperatures with the increase of grain size, which is exact contrary
to those obtained by Sánchez et al. [11], in which both magnetization and
C
T
increase with increasing grain size of La0.67Ca0.33MnO3.
The magnetization as a function of the applied magnetic field at 5 K is shown in
Fig.3. It shows that, for all samples, the magnetization reaches saturation at about 1T
and keeps constant up to 5T, which was considered as a result of the rotation of the
magnetic domain under the action of applied magnetic field. Additionally, similar to
the result shown in Fig.2, the magnetization M decreases at low temperatures with
increase grain size. According to DE theory, the magnetization M is given by [22]
2
4SJxbM ∝
, where b is the transfer integral between neighboring Mn ions, S the
ionic spin, J the intra-atomic exchange integral, and x the doping level of the
compound. In the present case, two possible factors influencing M are x and b. The
oxygen content of the samples was determined by a redox (oxidation reduction)
titration. The detailed method to determine the oxygen content of samples will be
reported in elsewhere [23]. Only slightly surplus oxygen has been observed for our