8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Nano-size effect on structural, transport and magnetic properties of
Pr1-xSrxMnO3 (x = 0.2, 0.3 and 0.4) manganites
S. Mondal, A. Taraphder and T. K. Nath*
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur
Kharagpur 721302, India
*E-mail: tnath@phy.iitkgp.ernet.in
Abstract
The effect of Sr doping for Pr and their grain size variation on the structural, electronic-, magneto-transport and magnetic properties have been investigated in details in a series of chemically synthesized nanometric Pr1-xSrxMnO3 samples with x = 0.2, 0.3 and 0.4. No structural change is observed with the doping concentration (x) but their electronic-, magneto-transport and magnetic properties are found to have strong dependence on the doping concentration (x) as well as on particle size (Ø). The metal-insulator transition temperature (Tp) and the magnetic transition temperature (Tc) are found to be different in these nanometric particles in sharp contrast to their bulk counterpart. The subtle play of spin, charge, lattice and orbital coupled effects of eg and teg band electrons are found to have major role in the observed unusual behavior. The dominant contribution in the LFMR at low temperature is found to be mainly due to the spin polarized tunneling MR of the conduction eg electrons between the adjacent ferromagnetic grains.
1. Introduction
The perovskite manganites with generic formula RE1−xAExMnO3 (RE = trivalent rare earth cation, AE = Ca, Sr, Ba and Pb, i.e., divalent alkaline earth cation) have drawn considerable attention, especially following the discovery of their negative colossal magneto-resistance [1-4] (CMR) effect. In these materials subtle balance between charge, spin, lattice and orbital degrees of freedom leads to a variety of phases with fascinating properties like charge ordering (CO) and orbital ordering (OO) [5-7], insulator–metal (I–M) transition [8,9], ferromagnetic insulator (FMI), canted anti-ferromagnetic insulator (CAFI), colossal magneto-resistance (CMR) [1-4,10,11] etc. The occurrence of negative magnetoresistance (NMR) in manganites is believed to be due to the double exchange (DE) [12] interaction mediated by hopping of eg electrons, between Mn3+ and Mn4+, thereby facilitating both the electrical conductance and the ferromagnetism in the ferromagnetic metallic (FMM) phase. On the other
hand the ferromagnetic insulator phase is due to super exchange (SE) interaction between localized electrons. The doping concentration and nanometric dimension modifies their negative magneto-resistance behavior. To investigate the nanosize effect on their different magnetic and transport properties we have taken materials of Pr1-xSrxMnO3 with x = 2.0, 3.0 and 4.0. A series of nanometric Pr1-xSrxMnO3 (PSMO) were synthesized by chemical pyrophoric reaction process. The effect of nanometric particle size variations on electronic-, magneto-transport and magnetic properties of these materials have been thoroughly investigated in this work. The low-field magneto-resistance (LFMR) of these materials has also been thoroughly explored in this investigation in the temperature range of 77 – 300 K.
2. Experimental details
A series of nanometric Pr1-xSrxMnO3 (x = 0.2, 0.3 and 0.4) were synthesized through chemical pyrophoric reaction process [13] by making solution of Pr6O11, SrCO3 and Mn(CH3COO)2 in their stoichiometric ratios. The mixed solution was burnt after adding little amount of tri-ethyl amine (TEA). The burnt ash like powder was calcined at 850°C for 5 hours with intermediate grinding. The calcined powders were pressed to make a circular pellet at a pressure of 6 Tons and finally the pellet was sintered at the same temperature for 2 hours. To investigate the particle size variation effect the sample with x = 0.3 was sintered at two more different temperatures (950°C and 1050°C). XRD (Philips PW1710) study was carried out with Cu-Kα radiation (λ = 1.542 Å) to investigate the phase of the sample. Field-emission scanning electron micrograph (FESEM) of the samples was recorded to estimate the particle size and the surface morphology of the samples. Temperature dependent Ac-susceptibility of all the samples was measured by a home made ac-susceptibility measurement set up which was calibrated with a standard magnetic sample. The DC resistivity of the samples was measured down to liquid nitrogen temperature (77 K) by a standard four probe method. The high-resolution low-field magneto-resistance (LFMR) measurements of all the samples were also carried out up to a field of 5000 Oe at several temperatures down to 77 K by the same four probe technique.
3. Results and discussion
The XRD patterns (Fig. 1) confirm the single phase of the materials of orthorhombic structure with Pbnm space group. Using the Debye-Scherrer formula for the most intense peak we have estimated the crystallite size of the samples. The average crystallite sizes of the samples
are found to be in the range of 20-50 nm. Average particle size of the samples as observed from FESEM micrographs (Fig. 2) also corroborates our XRD results. It is clear from the FESEM micrographs that agglomeration of the sample is higher for the low doped samples (x = 0.2).
Temperature dependent Ac-susceptibility measurements (Fig. 3) of the samples show double magnetic transitions in the case of sample with x = 0.2 whereas for the other two samples a single magnetic transition is observed. The higher transition temperature of all samples corresponds to the paramagnetic to ferromagnetic transition whereas the lower transition temperature of the sample for x = 0.2 is most likely due to the superparamagnetic or cluster-glass kind of ordering. From the earlier investigation by N. Rama et al. [14] on similar composition, the low temperature magnetic transition in our x = 0.2 sample may be attributed to the cluster-glass type of ordering. From Fig. 3 it can be clearly observed that the paramagnetic to ferromagnetic transition temperature (Tc) varies with the doping concentration (x). With the increase of hole doping the Curie temperature (Tc) is observed to be enhanced. For the low doped sample (x = 0.2), Tc is ~ 190 K for the average particle size of 40 nm whereas for the similar average particle size the Tc s are observed to be ~ 278 K and 290 K for x = 0.3 and 0.4, respectively. At slightly higher doping concentration the Mn4+ and Mn3+ ratio is modified. As a result the ferromagnetism originated from double exchange (DE) mediated by the itinerant electrons in the eg band remains dominant till higher temperatures. The tolerance factor might also be partly responsible for this shift in Tc with the increase of x [15,16]. With the variation of average particle size for a particular hole doping concentration (x) the change in Tc is found to be almost negligible as shown in Fig. 4.
Fig. 5 shows the change in normalized resistivity with temperature for all the samples. The absolute value of the conductivity is found to be maximum for the optimum doping concentration (x = 0.4) as described by the Zener double exchange model. From Fig. 6 it can be observed that the metal - insulator transition temperature (Tp) shifts with the particle size for same composition (x = 0.3) of the materials. So the metal-insulator transition temperature (Tp) is found to shift both with variation of the sample composition (x) for same average particle size as well as with the variation of average particle size for same sample composition (x). Generally the Curie temperature (Tc) and the metal-insulator transition temperature (Tp) are found to be same for single crystalline or bulk polycrystalline PSMO samples [15]. For nano dimensional PSMO system the non-identical Tp and Tc is mainly due to the surface effect as the surface to volume
ratio is very high. The magnetism in these nanometric materials is mainly dictated by ordering of core-spins in the core-shell structure whereas the electronic transport is dictated by both kind of electrons in the eg band in the core as well as the misaligned electrons at the enhanced grain surface (shell) [13]. Moreover, the lattice strain generated by the mismatch of Pr3+ and Sr2+ is enhanced at the grain surface boundary. This may results in canting of the Mn spin at the grain boundary thus hampering the electron transfer at the grain boundary [17, 18]. Such strain affects the grain boundary electron transfer rendering the Mn spin canting thus lowering the transition temperature. The increase in grain size leads to the decrease in grain boundary effect and as a result Tp goes towards the Tc.
Fig. 7a shows the variation of low field MR with the applied magnetic field in the range of 0 – 5000 Oe at 77 K for the samples with x = 0.2, 0.3 and 0.4 of same average particle size. From the figure it can be observed that the absolute value of LFMR at highest field does not change much with the change in doping concentration. However, the nature of MR curves (field dependence) is distinctly different. For the higher doping concentration the spin-polarized tunneling contribution in MR is observed to be dominant whereas the intrinsic MR is dominant for the lowest doping material. As a result total MR is found to be not changed much with the doping concentration (x). The highest LFMR is found to be ~ 20% at the 5000 Oe at 77 K. Fig.7b shows that the field dependence of MR at several temperatures for x = 0.3. At lower temperature the MR of the Pr0.7Sr0.3MnO3 sample is higher. This is mainly due to the reduction in spin fluctuation of this ferromagnetic material. The eg electrons spin are more ordered at lower temperatures. These half-metallic manganites are highly spin-polarized below their respective Tc s [13]. As a result at low temperature the spin-polarized tunneling of conduction electrons across the enhanced grain surface is more pronounced with the application of magnetic field showing the large value of spin-polarized tunneling MR.
4. Conclusions
In summary, we have investigated in details the structural, electronic-, magneto-transport and magnetic properties of a series of chemically synthesized nanometric Pr1-xSrxMnO3 samples with x = 0.2, 0.3 and 0.4. No structural change is observed with the doping concentration (x) but the electronic-, magneto-transport and magnetic properties of the samples are found to have strong dependence on the doping concentration (x) as well as on particle size (Ø). The metal-
insulator transition temperature (Tp) and the magnetic transition temperature (Tc) is found to be different in these nanometric PSMO particles in sharp contrast to their bulk counterpart. The dominant contribution in the LFMR is found to be mainly due to the spin polarized tunneling MR of the conduction eg electrons between the adjacent ferromagnetic grains and it is enhanced with the decrease in particle size (Ø). We believe that the subtle play of the spin, charge, lattice and orbital coupled effects of eg and teg band electrons have major role in their observed unusual behavior.
Acknowledgments
The authors are thankful to the persons of FE-SEM laboratory, IIT Kharagpur for their help in recording FESEM micrographs. Authors are also thankful to the DAE (BRNS) for the financial support through project (code: PNE ).
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