Various fitting parameters for (La/Pr)2/3Ba1/3(Mn1-xSbx)O3 (0≤ x≤ 0.03) systems using equation (Eq. 3).
Magneto-resistance (MR) is generally defined as the relative change in the electrical resistivity of a material upon the application of magnetic field. Mathematically, MR can be defined by the following equation,
where ρH and ρ0 are the electrical resistivities with and without magnetic field respectively. Electrical resistivity can increase or decrease upon the application of magnetic field and accordingly MR is defined as positive or negative respectively. By using the above definition observed MR could be ~106% or even higher, but if we define MR by the following relation,
then maximum MR will be ~100%.
W. Thomson (Lord Kelvin) in 1857, for the first time, discovered magneto-resistance1. All metals exhibit MR due to Lorentz force but the value of MR is only few percent even after the application of higher magnetic field. For example, Cu metal exhibits 1% MR at room temperature under 7 Tesla applied magnetic field. In the case of ferromagnetic metals like Fe and Co, MR is more (~ 15%) as compared to that of nonmagnetic metals such as gold (Au). Semimetal bismuth (Bi) shows about 18% MR in a transverse magnetic field of 0.6T and 250% MR at room temperature under the application of 5 Tesla magnetic field.
In search of higher MR values near room temperatures, the last two decades have witnessed the discovery of an enormous suppression in electrical resistivity upon the application of magnetic field in certain class of materials called manganites. The observed change was so large that a special name “colossal magneto-resistance (CMR)” had to be coined for this phenomenon to distinguish it from the already existing giant magneto-resistance (GMR) in the magnetic multilayers. For example, Chahara et al., R. von Helmolt et al., Jin et al., Xiong et al. have reported ~ 106 % MR under the application of 6T magnetic field. Searle and Wang et al.,  were the first to report MR studies in La1-xPbxMnO3 single crystal manganites. The renewed surge of interest in manganites in the 1990s started with the experimental observation of large magneto-resistance (MR) studies in Nd0.5Pb0.5MnO3 by Kusters et al. and in La2/3Ba1/3MnO3 by R. von Helmolt et al.. MR effect was found to be as high as 60% (using Eq. 2) in thin films at room temperature, and it was exciting to observe that this value was higher than found in artificial magnetic/nonmagnetic multilayers i.e. Giant magneto-resistive materials (GMR), allowing for potential applications in magnetic recording. Thereafter Chahara et al.[ 3] on La3/4Ca1/4MnO3 and Ju et al. on La1-xSrxMnO3 films produced similar results. Jin et al.[ 5] observed 127,000% around 77K using Eq. (1) for defining MR in thin films of La0.67Ca0.33MnO3 and Xiong et al.[ 6] 100,000% in Nd0.7Sr0.3MnO3 film at 60K.
1.1. Crystal structure
Rare-earth manganites possess a perovskite crystal structure (Fig. 1a) with the general formula R1-xAxMnO3, where R is any trivalent rare-earth ion like La+3, Pr+3, Nd+3, Eu+3 etc. and A is divalent alkaline-earth ion (Ca+2, Sr+2, Ba+2 etc.) whereas x represents the amount of doping of such divalent ions at the rare-earth site and equal number of Mn+3-ions are converted into Mn+4- ions. The mixed valency of manganese ions can also be controlled by varying the oxygen content,. In the perovskite structure each manganese (Mn) ion is surrounded by six oxygen ions forming an octahedron (Fig. 1a).
In MnO6 octahedron overlapping of manganese 3d orbitals with the 2p orbital of oxygen splits the five-fold degeneracy of the manganese 3d orbitals due to their different shapes (Fig. 1b). This is called crystal field splitting (CFS). Thus in an MnO6 octahedron three degenerate orbitals (dxy, dyz, dzx) with lower energy (because their lobes are oriented between the O-2 ions) are called t2g levels and two degenerate orbitals (dx2- y2, d3z2- r2) with higher energy (their lobes are towards O-2 ions and hence large Coulomb repulsion) called eg levels.Due to crystal field splitting the energy difference between t2g and eg levels is ~ 1.5 eV. All electrons in these energy levels get aligned parallel to each other due to strong on-site Hund′s coupling, leading to a total spin (S) of 2 for Mn+3 and S = 3/2 for Mn+4 ions respectively. The three electrons of Mn+4 - ion occupy t2g states whereas fourth electron of Mn+3 goes to the eg state. The eg electrons are more itinerant than the t2g electrons and can hop from one Mn-site to the other. However, according to Jahn-Teller (JT) theorem this configuration is not stable and the degeneracy of the eg levels are further removed. The oxygen ions surrounding the Mn+3 -ion slightly readjust their positions, creating an asymmetry in different directions which ultimately breaks the degeneracy. The breaking of degeneracy due to orbital- lattice interaction is called Jahn-Teller splitting (Fig. 2). Only those ions like Mn+3 which have odd number of electrons in the eg levels can undergo Jahn-Teller distortion. Therefore Mn+3 is a Jahn-Teller ion whereas Mn+4 is not. In the case of manganites there are 21 degrees of freedom (modes of vibration) for the movement of oxygen and Mn ion. Out of these only two types of distortion are relevant for the splitting of eg levels i.e. Q2 and Q3 (Fig. 3) . Q2 is a basal plane distortion (defined as Q2 mode) in which one diagonally O pair is displaced inwards whereas the other pair is displaced outwards. On the other hand, Q3 is a tetragonal distortion which results in elongation or distortion of MnO6 octahedron. JT distortion can be static or dynamic.
Perovskite manganites not only exhibit the colossal magnetoresistance (CMR) effect but also other important phenomena like charge ordering, phase separation at nanoscale owing to the subtle balance between charge, spin, lattice and orbital degrees of freedom. As mentioned earlier, electrical resistivity of these manganites can change dramatically in response to an applied magnetic field. The occurrence of magneto-resistance can be explained by Zener’s theory of double exchange along-with the formation of Jahn-Teller polarons. In order to understand magnetoresistance behavior and other peculiar properties
exhibited by these materials, substitution engineering is carried out either at the rare-earth site or at the manganese site. However, out of the two, the last is more appropriate as it directly affects the mechanism happening among different MnO6 octahedra. In this regard, several authors have substituted different transition metal ions like Fe+3, Al+3, Ga+3, Zn+2, Cu+3, Ni+2 at the Mn-site and have shown that Mn-site substitution severely affects the conduction mechanism as well as the magnetic properties-[ 28]. However, fewer results exist for higher valent ion substitution at Mn-site to see its effect on the transport properties-. Substitutions at Mn-sites in manganites, irrespective of their electronic and magnetic nature, lower the ferromagnetic transition temperature but to different extents except in the case of Ru-doped Pr0.5Sr0.5MnO3 where it is found to increase with Ru. In this work, we have attempted to substitute Sb+5 ion on the Mn-site in two different manganite materials
2. Experimental procedure
Bulk polycrystalline manganite samples of La2/3Ba1/3(Mn1−xSbx)O3 and Pr2/3Ba1/3(Mn1−xSbx)O3 (0 ≤ x ≤ 0.03) [referred hereafter as (La, Pr)2/3Ba1/3(Mn1−xSbx)O3] were synthesized following the conventional solid-state reaction method. Powders of La2O3, Pr6O11, BaCO3, Sb2O5 and MnO2 were mixed in nominal stoichiometric ratios, grounded properly and calcined several times between 900-1100 ˚C for 15 h with intermediate grindings. Finally, the pellets were made from the calcined powders and sintered at1260 ˚C for 20 h. X-ray diffraction (XRD) was carried out with Cu
3. Results and discussion
3.1. Structural studies 40,
Fig. 4 shows the x-ray diffractograms of La2/3Ba1/3(Mn1−xSbx)O3 and Pr2/3Ba1/3(Mn1−xSbx)O3 (0 ≤ x ≤ 0.03) series. All samples are single phase in nature. It is also clear that Sb+5 ion occupies the Mn-site in LBMO and PBMO. Substitutional criteria of valence, ionic size and the coordination assured the preferential occupation of Mn+4 ion site by Sb+5 ion. The parent compound LBMO possesses the cubic structure [lattice parameter = 3.9094( 0.09%)
The lattice parameters were calculated using the powder X software. Similar results were also observed in case of Sb-substituted PBMO samples except that the crystal structure here is orthorhombic.
3.2. Morphological studies
The SEM micrographs of bulk LBMO, 3Sb-LBMO, PBMO, 3Sb-PBMO are shown in Fig. 5(a-d). The SEM micrographs clearly show the grain and grain boundaries. The grains show good connectivity among each other and the grain size is found to increase with doping in both cases. It is assumed that Sb-doping helps in promoting the grain growth.
3.3. Electrical resistivity measurement
Figure 6 shows the electrical resistivity variation with temperature (
With Sb-doping at the Mn-site the two transitions, in both the cases, shift to lower temperatures successively with the overall increase in the resistivity. However, T
3.4. Electrical resistivity at low temperature (< 50K)
At low temperatures, transport mechanisms such as weak localization,, electron-electron interaction,, Kondo effect,, etc. can cause an upturn in the resistivity with decreasing temperature, whereas scattering mechanisms such as electron-electron scattering,, electron-phonon scattering,, electron-magnon scattering,, magnon-magnon scattering increase resistivity with increasing temperature in metals and alloys. Figure 7 shows the temperature dependence of resistivity of Sb-doped LBMO and PBMO systems in the temperature range 4K ≤ T ≤ 50K. It is clear that electrical resistivity exhibits an increasing trend with decreasing temperature reminiscent of semiconducting behavior for all compositions. There is a resistivity minimum which is lowest for the undoped samples and shift to high temperatures with antimony substitution. In order to analyze the electrical resistivity behavior at low temperatures we tried to fit the data using the following equation:
where ρ0=1/a and ρ1= b/a2 are constants. ‘a’ is temperature independent residual conductivity; ‘b’ is the diffusion constant and is due to weak localization effect46. The other two terms viz. ρ2T2 and ρ5T5 arise due to the electron-electron and electron-phonon scattering53. The fitting parameters ρ0, ρ1, ρ2, ρ5 for the two systems are shown in Table 1.
|La2/3Ba1/3(Mn1-xSbx)O3 (0≤ x≤ 0.03)|
|x = 0||0.01436||0.00023||1.8885×10-7||6.89×10-13|
|x = 0.01||0.01723||0.00033||3.3297×10-7||6.9613×10-13|
|x = 0.02||0.02282||0.00093||1.269×10-6||3.184×10-13|
|x = 0.03||0.03529||0.00081||9.8659×10-7||1.8152×10-12|
|Pr2/3Ba1/3(Mn1-xSbx)O3 (0≤ x≤ 0.03)|
|x = 0||0.2175||0.0082||8.9x10-6||4.83x10-11|
|x = 0.01||3.0015||0.1922||1.7x10-4||5.26x10-10|
|x = 0.02||19.044||2.4068||2.53x10-3||7.18x10-10|
|x = 0.03||362.885||61.8983||7.69x10-2||1.09x10-7|
It is observed that the value of fitting parameters ρ0, ρ1, ρ2, ρ5 increases with doping. This indicates that the weak localization, electron-electron and electron-phonon scattering increases with doping and making pristine material more disordered. This is also obvious from the fact that random distribution of ions La+3/Pr+3/Ba+2/Mn+3/Mn+4/Sb+5 leads to random variation in the potential experienced by the electrons so the electrons get trapped due to less kinetic energy at low temperatures. The other two terms i.e. electron-electron and electron-phonon scattering will increase because of the difference in ionic sizes. It is worth mentioning here that in addition to these effects the Coulomb blockade (CB) effect could also contribute to the observed electrical resistivity upturn as the large ionic size mismatch between La+3/Pr+3 and Ba+2 ions leads to the strong disorder at the grain boundaries and eventually increases the Coulomb charging energy EC leading to increase in resistivity. This would get further enhanced for grains with smaller sizes (EC ∝ 1/d). In this case grain size d, however, has increased with Sb-doping (figure 5), which implies that EC should decrease and hence the resistivity upturn should also decrease but here a reverse situation is observed. The CB effect, therefore, cannot account for the resistivity upturn for the Sb-doped samples, even though it might be one of the possible sources of localization in the pristine sample case.
3.5. Transport mechanism in the metallic region
We used the following Eq. to fit the metallic region of the temperature dependent electrical resistivity data
here ρ0 is the temperature independent residual resistivity. ρ2.5T2.5 represents the electrical resistivity due to electron-magnon scattering processes in the ferromagnetic phase. We found that in the metallic regime electrical resistivity data for Sb doped samples best fit the Eq. (4). The quality of these fittings, in general, is evaluated by comparing the square of the linear correlation coefficient (R2) obtained for each equation. The values of R2 were found as high as 99.9% for Eqn. (4) which confirms the applicability of electron-magnon scattering process. The obtained fitted parameters are given in Table 2. It is observed that value of both ρ0 and ρ2.5 increases with doping. This reflects the dominating nature of the electron-magnon scattering mechanism with increasing Sb content. Similar results were observed for the Sb-doped PBMO manganites.
|x = 0||0.11296||8.1315x10-6||0.99994|
|x = 0.01||0.31814||0.00005||0.99905|
|x = 0.02||3.18173||0.00015||0.99962|
|x = 0.03||33.90211||0.00103||0.99952|
3.6. Transport mechanism above T
Conduction in manganites at higher temperature (T ≥ T
|x = 0||91.2||21.693||-0.8491||69.507||139.014|
|x = 0.01||98.02||14.016||-0.6444||84.004||168.008|
|x = 0.02||112.92||7.903||-0.457||105.001||210.002|
|x = 0.03||126.15||5.869||-0.3941||120.281||240.562|
3.7. Magnetoresistance studies
Generally, application of the external magnetic field enhances spin ordering, promotes the charge transfer and thus suppresses the electrical resistivity. Alignment of the spins of the neighboring Mn-sites favors the electronic motion. Figure 8 (a) & (b)shows the temperature dependent magnetoresistance behavior of all the samples from 77K to 300K, for an applied magnetic field of 0.6T. For Sb-doped LBMO samples a sharp peak is observed in all the samples near T
3.8. Thermoelectric power
Thermoelectric Power (TEP) is especially suited to explore the carrier dynamics because it is expected to be very sensitive to the local moments63 and thus it monitors the ferromagnetic transition and provide new insight into the dynamics of manganite systems. S(T) is less affected by the presence of grain boundaries and hence the grain boundary effects that are observed in the resistivity temperature measurements, can be masked. The thermoelectric power data for the two series is shown in Figure 10. In the case of Sb-doped LBMO series, a crossover of thermopower is observed from positive to negative at temperature T
case of Sb-LBMO, insulator-metal transition T
where e is the electronic charge; k
Similar to ρ(T) data, several factors, such as impurity, complicated band structure, electron-electron, electron-magnon scattering etc also affect S(T) data in the FM metallic regime. S(T) data in this intermediate temperature FM metallic region has been analyzed by the following relation68,69:
here S0, S3/2, and S4 are fitting parameters. S0 has no physical origin but inserted to account for the low temperature data, S3/2T 3/2 is attributed to single magnon scattering process,. The term S4T 4 dominant in high temperature region near TC, is thought to arise from spin wave fluctuation in the FM phase,. Figure 12 shows the fitting of Eq. (6) in the metallic region of the Sb-doped LBMO samples and the fitting parameters are given in Table 4. From Table 4, it is clear that S3/2 is nearly five orders of magnitude larger than that of S4, implying that the second term i.e. electron-magnon scattering in Eq. (6) dominates the transport mechanism in the FM metallic regime below T
|x = 0||4.5365± 0.03756||-0.00036± 0.00002||- 6.3773x10-10 ± 9.951x10-12|
|x = 0.01||4.7905± 0.06958||-0.00009± 0.00004||- 1.0287x10-9± 2.2475x10-11|
|x = 0.02||4.43458± 0.08659||0.00003± 0.00006||- 1.212x10-9± 3.5867x10-11|
|x = 0.03||3.69632± 0.0923||0.00028± 0.00006||- 1.4882x10-9± 3.8002x10-11|
3.9. Thermal conductivity
Figure 13 shows the temperature dependent thermal conductivity (κ) measurements of Sb-doped manganites. The magnitude of κ(T), typically of non-crystalline materials (bad metals), lies in the range of 5-70 mW/cm K. For a crystalline solid, such a low value of thermal conductivity in manganites can be attributed to the disorder due to strong Jahn-Teller effect. Magnitude of κ decreases with decreasing temperature down to T
At high temperatures thermal conductivity of the crystalline solids is expected to follow the temperature dependency according to the formula κ aMC
|x = 0||30.48||0.201||0.0917||12.065|
|x = 0.01||45.95||0.1333||0.194||17.45|
|x = 0.02||64.2||0.0954||0.1265||12.71|
|x = 0.03||119.97||0.0511||0.1231||10.116|
3.10. Specific heat
The observed specific heat measurements [C
corresponding I-M transition temperature T
In order to separate the lattice contribution and to estimate the excess specific heat (∆C
Substitution effect of Sb5+ ion on the structural, morphological, magneto-transport and thermal properties (including thermoelectric power, specific heat and thermal conductivity) of La2/3Ba1/3MnO3 (LBMO) and Pr2/3Ba1/3MnO3 (PBMO) manganites have been carried out. Structural measurements on polycrystalline samples synthesized showed that lattice parameters increase with Sb-doping. The grain size is also found to increase with Sb. Electrical resistivity variation with temperature of these managnites showed two insulator-metal like transitions (T
The authors are thankful to Prof. Y. K. Kuo (National Dong Hwa University Taiwan), Prof. G. L. Bhalla (University of Delhi, India), Prof. Ashok Rao (Manipal University, India) and Prof. D. K. Pandya (I.I.T. Delhi, India) for scientific discussions. One of the authors (SKA) would like to thank the Council of Scientific and Industrial Research (CSIR, India) for providing the financial grants under the Emeritus Scientist Scheme.