Abstract: The magnetic and transport properties of (La1-xPrx)2/3Ba1/3MnO3 (x = 0, 0.1677, 0.333, 0.500, 0.677, 0.833, 1.000) compounds, prepared by the solid state reaction have been investigated. The metal-insulator transition (Tp) was determined by using the standard four-point probe resistivity measurement between of 30 and 300 K. With increasing Pr doping, Tp shifted to lower temperatures, which are greater than 300, 270, 250, 226, 202, 186 and 158 K for x = 0, 0.1677, 0.333, 0.500, 0.677, 0.833 and 1.000, respectively. By analyzing the data using several theoretical models, it was concluded that the metallic (ferromagnetic) part of the resistivity (ρ), (below TP), fits well with the equation ρ = ρ0 + ρ2T2. This indicates that ρ0 is due to the grain/domain boundary effects. The second term ~ρ2T2 appears to be attributed to electron-electron scattering. In the high temperature range (T>TP) (the paramagnetic insulating regime) adiabatic small polaron models fit well in different temperature regions, thereby indicating that polaron hopping might be responsible for the conduction mechanism. The activation energy (Ea) also increases as the doping concentration increases.
INTRODUCTION
Transition metal based on mixed oxides exhibit a very rich spectrum of remarkable
electric, magnetic and optical properties, tunable by the composition in broad
limits. The variety of properties are often due to the different behavior of
the 3d electrons, which may be more or less localized, giving rise to intra-atomic
correlation effects of different strengths. Rare-earth manganese oxides of the
form Ln1-xAxMnO3 (Ln = rare-earth element,
A = alkaline-earth elements) have attracted considerable scientific and technological
interest due to their rich physical properties and potential applications. The
parent compound of these materials, LaMnO3
MATERIALS AND METHODS
This study was conducted in Universiti Putra Malaysia (UPM), Malaysia in 2005. Nominal stoichiometric (La1-xPrx)2/3Ba1/3MnO3 (x = 0, 0.1677, 0.333, 0.500, 0.677, 0.833 and 1.000) polycrystalline samples were synthesized by the conventional solid-state reaction method. The starting materials were high purity (99.9%) lanthanum oxide (La2O3), praseodymium oxide (Pr6O11), barium carbonate (BaCO3) and manganese carbonate (MnCO3). All materials were mixed and then heated at 900°C in air for 12 h. After calcinations, the resultant black powders mixture were reground, pelletized and sintered in air at 1300°C for 24 h. Four probe dc electrical resistance measurement were carried out in a closed cycle helium refrigerator in the temperature between of 30 and 300 K. The microstructure was investigated by using a Scanning Electron Microscope from (Leo 1455 VP sem) was used to investigate the electrical properties.
RESULTS AND DISCUSSION
All the (La1-xPrx)2/3Ba1/3MnO3 samples show a metal to insulator transition temperature (Tp) below 300 K except for samples x = 0.0. The phase transition (Tp) becomes lower due as Pr constant was increased. A higher Tp for pure La0.67Ba0.33MnO3 obtained at 330 K as reported by Ju et al. (1995). Figure 1a-c shows the temperature dependence of resistance for increasing Pr content from x = 0.0 and x = 1. The sample showed semiconducting transport behaviour above Tp and metallic behavior below Tp. The Tp of (La1-xPrx)2/3Ba1/3MnO3 decrease linearly with Pr concentration. With increase the Pr doping, Tp shifted to lower temperatures, which are greater than 300, 270, 250, 226, 202, 186 and 158 K for x = 0.000, 0.167, 0.330, 0.500, 0.670, 0.833 and 1.000, respectively (Fig. 1d).
When La was successively substituted by Pr, the nominal ratio of Mn3+/Mn4+ and hence the hole density in the system remains unchanged due to the same valence of La and Pr.
| Fig. 1: | (a-c): The temperature variation resistance of (La1-xPrx)2/3Ba1/3MnO3 samples and (d): Tp of (La1-xPrx)2/3Ba1/3MnO3 systems as function of Pr concentration |
Due to the smaller size of the Pr ions, the electron bandwidth (bp) increases and hence the hopping amplitude for the electrons in the eg band becomes smaller causing the increment in the resistivity of the samples with the increase in x.
Low-temperature (T<Tp) regime: Figure 2 shows the adjusted resistivity versus temperature plot. Below Tp, the electronic conduction mechanism in the ferromagnetic metallic phase is generally understood according to double exchange theory. The Mn3+-O-Mn4+ coupling allows conduction through charge transfer from half-filled to empty eg orbital. In this regime, the metallic behavior of the samples can be explained in terms of electron-magnon scattering of the carriers. In this temperature regime, the resistivity data fit quite well with the following expression:
ρ = ρ0 + ρ2T2 |
(1) |
where, the first term ρ0 corresponds to the resistivity arising due to domain, grain boundary and other temperature independent scattering mechanism. Electron is difficult to cross the boundary due to the disorder of domain and grain boundary. The second term ρ2T2 appears as a result of electron - magnon scattering. Thus, the spin scattering cannot be neglected in this regime as the measured data can be best explained by the electron-magnon scattering.
It is noted that the values of both ρ0 and ρ2 increase with the increase of x. However, the decrease of temperature independent of ρ0 is more significant with x compared to that of ρ2 Table 1. As the doping increases, the size of the domain boundary decreases and ρ0 becomes larger. The increase of ρ2 with x is due to the suppression of spin fluctuation.
The microstructure of (La1-xPrx)2/3Ba1/3MnO3 with x = 0.000, 0.167, 0.330, 0.500, 0.670, 0.833 and 1.000 are shown in Fig. 3a-g. The micrographs were recorded on the fracture surface of sintered samples. The grain size is in the range of 1 to 3 μm. The grains of samples of x = 0.000, 0.167, 0.330 and 0.500 were observed in round shape, while the others are mixture of round and square shapes. The grain for sample x = 0 is more compact comparedto others with average size of ~3 μm and clear grain boundaries. The x = 0.167 to x = 1.000 samples have no clear grain boundary as the grains were too fine. The thickness of grain boundary can be explained by the increment of resistivity. The large pores can be seen in x = 0.500 and 0.833, but the grain size decreases to ~0.8 and ~1 μm, respectively. The grain boundary is clear to see, it was almost 0.33 μm. Figure 3h shows the graph of sample grain size in diameter against the concentration of Pr. The grain size decreases exponentially as the concentration increases. The increment of x from 0.500 to 1.000 show that the change in grain size is notsignificant. The ionic radius sizeplays the main role, where the Pr3+ ion is smaller than La3+. This indicates that more Pr3+ took the place of La3+.In (La1-xPrx)1/2Ba1/2MnO3 system, substitution of Pr3+ ion for the La3+ site in LaBaMnO3 led to grain growth inhibition, lanthanum segregation and second phase formation. Similar observation have been reported by Brzozowski et al. (2005), for Nb2O5 substituted BaTiO3. The reduction of the size and connectivity between the particles is clearly with different doping concentrations.
High-temperature (T > Tp) regime: To explain the electrical conduction just above TP, i.e., (Tp<T<θD/2), (θD is the Debye temperature) adiabatic Small Polaron Hopping (SPH) model of Mott has been suggested. This Mott model (Woo-Hwan, 2001) was based on a strong electron-phonon coupling approximation and has been extensively used for explaining the conductivity data of many transition metal oxides. If the charge carriers responsible for conduction are small polarons, the temperature dependence of the DC resisitivity due to hopping process of small polarons predicted theoretically takes the form:
ρ = ρα T exp(Eα/kBT) |
(2) |
where, Ea is the activation energy and ρα is the residual resistivity given by ρα = 2kB/3ne2α2v where, kB is Boltzman½s constant, e is electron charge, n is number density of charge carriers, a is site-to-site hopping distance and v is longitudinal optical phono frequency (estimated from the relation hv = kBθD, where θD is the Debye temperature). It has been reported by Hoo-Hwan (2001), the electrical resistivity data , in the temperature region T >θD/2 were fitted to the adiabatic small polaron-hopping model represented by the equation, ρ = ραTexp (Eα/kBT) and from the best fits, the activation energy values were calculated. Further, θD values were also estimated from the plots of ln (ρ/T) versus (1/T) (Fig. 4). It is also clear from Table 2 that the activation energy increases with increase in Pr concentration. This is due to the hole doping in the eg band, causing the localization of charge carrier to occur and hence increased the energy required to liberate a carrier. Furthermore, Ea values also increases with decrease in the grain size. This behavior may be explained as follows. It is known that with decreasing grain size, the interconnectivity between grains increases, which reduces the possibility of conduction electron to hop to the neighboring site, thereby increasing Ea.
| Fig. 2: | Adjusted resistivity data showing T2 dependence for (La1-xPrx)2/3Ba1/3MnO3
system below the respective Tp for (a) x = 0, (b) x = 0.167,
(c) x = 0.33, (d) x = 0.5, (e) x = 0, (f) x = 0.833 and (g) x = 1.0. Solid
lines are the best fit to the equation ρ = ρ0 + ρ2T2 |
| Fig. 3: | (a-g) SEM images of the fracture surface of (La1-xPrx)2/3Ba1/3MnO3 and (h) grain size of the (La1-xPrx)2/3Ba1/3MnO3 system |
| Fig. 4: | Plot of ln ρ/T vs. 1/T for (La1-xPrx)2/3Ba1/3MnO3 |
| Table 1: | The plots of ρ0 and ρ2 as function of x |
| Table 2: | Activation energy, Ea, θD/2 and phonon frequency, v (Hz) from resistivity data |
Table 2 also shows the phonon frequency against Pr content. It indicates that the frequency of lattice vibration decreases with increasing Pr content. As the concentration of Pr increases, the energy of phonon decreases. This also shows the correlation between Tp and the activation energy (Ea), for conduction in the paramagnetic (PM) phase, where Tp is proportional to the polaron binding energy (Ea). The lattice distortions around the defects favor the localization of the polarons (Ea increases). This becomes apparent in the paramagnetic (PM) phase as an increase of Ea and a decrease of the temperature at which metallic behavior is observed (Tp). We can conclude that in samples without grain boundaries, the Tp is related to the electron-lattice interaction, which is increased by the introduction of chemical defects.
CONCLUSION
The low temperature resistivity signifies the importance of electron-magnon scattering whereas the high temperature transport properties are mainly governed by the small polaron hopping mechanism.
In low temperature resistivity range (T<Tp), it can be concluded that the size of the domain boundary decreases and ρ0 becomes larger, as the doping of Pr increases. The increase of ρ2 with x is suggested to be due to the suppression of the spin fluctuation. In the high-temperature (T>Tp) regime, it is found that the activation energy (Ea) increases with decreasing grain size, as well as increasing Pr doping concentration. This is due to the hole doping in the eg band, causing localization of charge carriers to occur and hence increases the energy required to liberate a carrier.
ACKNOWLEDGMENTS
The Ministry of Science, Technology and Innovation of Malaysia is gratefully acknowledged for the grant under IRPA vote: 03-02-04-0374-SR0003/07-07 (Fabrication of Magnetic Sensors Head based on Magnetoresistive and Magnetostrictive Thin Films Circuits for Devices Applications) and by UKM-OUP-NBT-26-117/2008 (Group of Nanomaterials Engineering).