Insulator-metal transition and the magnetic phase diagram of La1-xTexMnO3

Abstract

The structural, electrical transport and magnetic properties of perovskite oxides La1-xTexMnO3 have been investigated and thus the magnetic phase diagram of La1-xTexMnO3 compounds as a function of temperature and the doping level x has been obtained. All samples have rhombohedral structure and undergo paramagnetic-ferromagnetic (PM-FM) transition accompanied with metal-insulator transition (MIT). Whereas a charge ordering (CO) transition begins to appear at for the sample with x=0.60. Moreover, the variation of the Curie temperature and the MIT temperature is quite complex and the results are discussed in terms of three factors including the average A-site cation radius , the size mismatch and the Te content. In addition, there has an evident magnetoresistance (MR) at low temperatures for all samples.

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Insulator-metal transition and the magnetic phase diagram of
La1-xTexMnO3() 6 . 01 . 0
≤≤ x

J.Yang 1, W. H. Song1, Y. Q. Ma1, R. L. Zhang1, B. C. Zhao1, Z.G. Sheng1, G. H.
Zheng1, J. M. Dai1, and Y. P. Sun*1 2
1Key Laboratory of Internal Friction and Defects in Solids, Institute of Solid State
Physics, Chinese Academy of Sciences, Hefei, 230031, P. R. China
2Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210008, P. R.
China
Abstract
The structural, electrical transport and magnetic properties of perovskite oxides
La1-xTexMnO3 () 6 . 01 . 0
≤≤ x have been investigated and thus the magnetic phase
diagram of La1-xTexMnO3 () 6 . 01 . 0
≤≤ xcompounds as a function of temperature and
the doping level x has been obtained. All samples have rhombohedral structure and
undergo paramagnetic-ferromagnetic (PM-FM) transition accompanied with
metal-insulator transition (MIT). Whereas a charge ordering (CO) transition begins to
appear at KTCO
250~for the sample with x=0.60. Moreover, the variation of the
Curie temperature
C
T and the MIT temperature
P
T is quite complex and the results
are discussed in terms of three factors including the average A-site cation radius <rA>,
the size mismatch
2
σ and the Te content. In addition, there has an evident
magnetoresistance (MR) at low temperatures for all samples.
PACS: 75.30.Kz 75.47.Lx 71.30.+h
Keywords: insulator-metal transition; the magnetic diagram; a charge ordering
transition; the size mismatch; magnetoresistance


Corresponding author. Phone: 86-551-5592757; Fax: 86-551-5591434; email:
ypsun@issp.ac.cn

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1. Introduction
Recently, doped manganese perovskites have attracted much renewed attention due
to the discovery of colossal magnetoresistance (CMR) [1]. In the systems of
Ln1-xAxMnO3 (Ln=La-Tb, and A=Ca, Sr, Ba, Pb, etc.), the existence of the dopant
leads to mixed valent of the Mn ions (Mn3+/Mn4+). Many theories have been proposed
to explain the mechanism about CMR such as double exchange [2], polaronic effects
[3], phase separation combined with percolation [4,5] and Griffiths singularity [6].
Their studies indeed suggested that the mixed valent of Mn3+/Mn4+ is a key
component for understanding the CMR effect and the transition from the
ferromagnetic metal to paramagnetic semiconductor. Note that there exists an inherent
symmetry between Mn2+ and Mn4+ as both are non Jahn-Teller ions whereas Mn3+ is a
Jahn-Teller ion. So electron doping is expected to drive the manganese into a mixture
of Mn2+and Mn3+ when using the tetravalent element to dope the parent compound
LnMnO3. Thus, the basic physics in terms of Hund's rule coupling and Jahn-Teller
effect could operate in the electron doped phase as well. Recently, many researches
have placed emphasis on electron-doped compound such as La1-xCexMnO3 [7-10],
La1-xZrxMnO3 [11] and La2.3-xYxCa0.7Mn2O7 [12]. Their studies suggested that the
CMR behavior probably occurred in the mixed-valent state of Mn2+/Mn3+.
In the following we report Te doping manganites La1-xTexMnO3() 6 . 0 1 . 0
≤≤ x.
This material has been reported by Tan et al. in the doping range of 0.04-0.2 [13-15].
Their studies demonstrated that the Curie temperature
C
T and the MIT temperature
P
T monotonously increase with increasing Te content. However, our experimental
data indeed shows that the variation of
C
T and
P
T is not monotonously in the broad
range of 0.1-0.6. Moreover, the magnetic phase diagram of La1-xTexMnO3
(
1 . 0
) 6 . 0
≤≤ x as a function of temperature and the doping level x was obtained from
the magnetic and resistivity measurements. In order to understand the change of
C
T
and
P
T clearly, we combine the structure and electronic transport properties with the

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A-site cation mean radius <rA>, the size mismatch
2
σ
and the characteristic
temperature
0 T .
2. Experiment
A series of ceramic samples of La1-xTexMnO3() 6 . 0 1 . 0
≤≤ xwere synthesized by a
conventional solid-state reaction method in air. The powders mixed in stoichiometric
compositions of high-purity La2O3, TeO2 and MnO2 were ground, then fired in air at
700 °C for 24h. The powders obtained were ground, palletized, and sintered at 1030
°C for 24h with three intermediate grinding, and finally, the furnace was cooled down
to room temperature. The structure and lattice constant were determined by powder
x-ray diffraction (XRD) using
α
CuK radiation at room temperature. The resistance as
a function of temperature was measured by the standard four-probe method from 25
to 300K. The magnetic measurements were performed on a Quantum Design
superconducting quantum interference device (SQUID) MPMS system (2 ≤ T ≤400 K,
0 ≤ H ≤5 T).
3. Results and discussion
The magnetic phase diagram of La1-xTexMnO3() 6 . 01 . 0
≤≤ x as a function of
temperature and the doping level x is shown in Fig.1. The curved line is drawn as a
guide to the eye approximately on the boundary between the spin-disordered
paramagnetic state and the spin-ordered state. The structural, magnetic and transport
properties of the manganites La1-xTexMnO3 in the various doping regimes are
discussed below.
3.1. Structural characterizations
X-ray powder diffraction (XRD) at room temperature shows that the samples are
single phase below x=0.2 and the second phase begin to exist beyond x=0.2,which is
consistent with many electron-doped manganites [8,11]. But the amount of second
phase is below 5%, we can consider that the samples are almost single phase. XRD
patterns of all samples have a rhombohedral lattice with the space groupCR
−
3 . The

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structural parameters are refined by the standard Rietveld technique [16], and the
experimental spectra and calculated XRD pattern for x=0.1 are shown in Fig.2. The
structural parameters obtained are listed in Table I. As we can see, the Mn-O-Mn
bond angle increases initially, and then decreases with increasing Te content.
Especially there exist a minimum in Mn-O-Mn bond angle for x=0.25. In succession,
the bond angle increases again and stabilizes a definite value for higher Te content.
Whereas the Mn-O bonds length displays the inverse correlation to the variation in the
Mn-O-Mn bond angle.
3.2. Magnetic properties
The Magnetization of La1-xTexMnO3() 6 . 01 . 0
≤≤ x
under 0.1T magnetic field
cooling (FC) is measured. Fig.3 (a) shows the dependence of M on temperature T.
The
C
T (defined as the one corresponding to the peak of dTdM in the M vs. T
curve) values are obtained as 235, 236, 188, 184, 185, 233, 177, 181, 182 and 186K
for x=0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 and 0.60, respectively. The
Curie temperature
C
T increases slightly when the x value changes from x=0.1 to
x=0.15, decreases abruptly from x=0.15 to x=0.30 but stabilizes for 30 . 0 20. 0
≤≤ x ,
then
C
T increases rapidly when x value changes from x=0.30 to x=0.35 and
decreases sharply but varies steadily for higher values. The saturation of
C
T may be
attributed to a competition between the double exchange and the core spin interaction,
which leaded to the canting of the core spins as the doped level increases [11]. As for
the variation of
C
T , we will discuss in the following. Moreover, there exist a little
anomaly seen at a temperature of about 40K in the M-T curve. In the previous study
[14], similar phenomenon appears in the M-T curve, but the authors neglect this and
ascribed the character of bifurcation at low temperature in the FC and ZFC curves to
the appearance of the spin-glass. In order to clarify the magnetic structure of the
samples, the frequency dependence of the ac magnetic susceptibility (χ) for the
samples with x=0.20 and 0.40 was performed under the frequency of 188, 1172, and

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1876 Hz, respectively. As shown in Fig.3 (b), upon cooling from room temperature
the in –phase χ′(T) curves display abrupt upturns at about 200K and 180K for the
samples with x=0.20 and 0.40, respectively, exhibiting the PM-FM transition. Upon
further decreasing the temperature, the χ′(T) curves display a cusp at about 40K for
the two samples. Moreover, the peak temperature of the cusp does not shift towards
higher temperature with increasing frequency. These features are characteristic of
phase transition. To our best knowledge, the Curie temperature of Mn3O4 is about
41K [17,18].So we suggest that the second phase Mn3O4 provides a small contribution
to the magnetization of the samples at about 40K and the amount of impurity
increases with increasing the doping level corresponding with the distinctness of the
anomaly for higher doping level.
3.3. Transport properties
The temperature dependence of the resistivity of La1-xTexMnO3() 5 . 0 1 . 0
≤≤ x is
shown in Fig.4 (a). The experimental data were obtained in 0 and 0.5T magnetic
fields for all the samples. As we can see, there is an insulator-metal transition caused
by paramagnetic-ferromagnetic phase transition under zero and 0.5T magnetic fields
for all the samples. Moreover, when external magnetic field is applied, the resistivity
is suppressed significantly and the temperature of resistivity peak shifts slightly to a
higher value. This suggests that the external magnetic field facilitates the hopping of
g e between the neighboring Mn ions, which agrees with the DE model. At the same
time, the magnitude of the resistivity vary several orders with the amount of dopant
Te from x=0.10 to x=0.60. Only one resistivity peak is observed for the samples with
20 . 0
≥
x, whereas for the samples with x=0.10 and x=0.15, an additional shoulder can
be observed. The resistivity peak at the high temperature corresponding to the
metal-insulator transition is defined as
1P
T and the temperature of the
low-temperature transition peak is denoted by
2P
T (221K for x=0.10 and 222K for
x=0.15). The values of
1P
T for the studied samples obtained from theρ (T) curve are
243, 246, 224, 136, 204, 216, 211, 185 and 221K for x=0.10, 0.15, 0.20, 0.25, 0.30,