Study of 50MeVLi3+ irradiation induced changes in La0.7Pb0.3MnO3

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Abstract

A typical giant magnetoresistive (GMR) sample viz. La0.7Pb0.3MnO3 has been irradiated by 50MeVLi3+ ion beam. Irradiated samples have been studied from the measurement of resistivity down to 77K in presence of 1.5T magnetic field. Although magnetoresistance is not maximum at Tim as the samples are polycrystalline, however, a small peak like structure has been found around Tim. The peak is most prominent for maximum fluence.

Introduction

Over the last five decades hole doped rare-earth perovskite manganites R1−xAxMnO3 (R is La or a rare-earth element such as Sm, Nd, Pr, etc. and A is a bivalent alkaline cation such as Ca, Sr, Pb, Ba, etc.) have elucidated scientists due to their various interesting properties (Fontcuberta et al., 1996; Rao and Raveau, 1998). These materials, depending upon doping concentration show a metal–insulator transition (MIT) at a temperature Tim close to the Curie temperature Tc. Among several theoretical models Zener's (1951) double exchange (DE) model is the most basic one. It states that the electrical conduction as well as ferromagnetic coupling in these compounds originate from the transfer of electrons through DE (Mn3+–O–Mn4+) mechanism. The interesting properties of such materials particularly, a large negative magnetoresistance (MR) near Tim (or Tc), open a scope for vast technological application (McCormack et al., 1994). However a large drop of resistance due to magnetic field generally occurs at high magnetic fields and at very low temperatures. Enthusiastic efforts have been made to find a system with large MR at relatively lower field and nearly room temperature. As a major achievement Li et al. (1997) pointed out that magnetoresistance value of a GMR system is much larger in polycrystalline samples compared to that in high quality single crystals of the same composition. It is well known that natural grain boundaries act as strong spin scattering centers and give rise to a contribution to the resistance in these sintered materials. Suppression of magnetic inhomogeneities at the grain boundary as well as inside the grain due to magnetic field could be a major clue in search of low field magnetoresistance effect. Recently, perovskite manganite system La1−xPbxMnO3 (LPMO) with their high magnetic ordering temperature (near room temperature), are being studied elaborately (Mahendiran et al., 1995), because of their Tim values near room temperature. In the present effort it has been attempted to observe the behavior of polycrystalline La0.7Pb0.3MnO3 system due to the presence of natural as well as artificially created defect centers.

Ion beam irradiation has been found to be an important way to induce non-equilibrium states in matter (Averback and Ghaly, 1997), which in general cannot be created by substitution or heat treatment. In the last few years materials modification using ion beam irradiation has been increased a lot. Recently it has been found that ion-beam irradiation (∼ MeV–GeV) induced transient heating, point defects, local structural transformations, columnar defects, sputtering, etc. can be useful to control properties of a GMR material needed for specific application (Chen et al., 1996). Kumar et al. (1998) have observed a systematic variation of Tc or resistivity peak Tp after 90MeV16O and 250MeV107Ag beam irradiation on epitaxial thin film of La0.67Ca0.33MnO3 (LCMO). Browning et al. (1998) have identified a spin disorder in the LCMO thin film irradiated by 10MeV127I ion. Ogale et al. (1998) reported a gradual change in structural, electrical and magnetic properties of epitaxial LCMO thin film irradiated by 90MeV16O beam up to a dose of 1013ions/cm2 followed by a drastic change at a dose of 1014ions/cm2. They found the presence of two different phases in the sample irradiated at that dose. In the present work we have investigated the effect on transport properties of La0.7Pb0.3MnO3 sample before and after 50MeVLi3+ ion beam irradiation of different fluences.

Section snippets

Experimental

La0.7Pb0.3MnO3 sample has been prepared by usual solid state reaction technique. Samples have been irradiated with three different fluences (6.5×1013,1.5×1014 and 6.5×1014ions/cm2) by 50MeVLi3+ beam at 15UD pelletron accelerator of Nuclear Science Centre (NSC), New Delhi. Detailed SRIM (Ziegler et al., 1985) calculation has been done to optimize beam energy and the thickness of the sample (250μm) so that implantation free radiation damage can take place.

The resistivity of all the samples (one

Results and discussion

The rate of energy deposition and defect production by energetic ion beam in GMR system is very much similar to HTSC oxides (Kumar et al., 1998a). For heavy ions at energies of a few MeV or above electronic energy loss (dE/dx)e is greater than nuclear energy loss (dE/dx)n by 2-3 order of magnitude as estimated by Rutherford scattering law. The stopping range of 50MeVLi3+ ions in the present system (LPMO) is greater than the sample thickness as calculated from SRIM 2000 program. Also (dE/dx)e

Conclusion

Study of 50 MeV Li+3 beam irradiation on La0.7Pb0.3MnO3 system has been done from resistivity measurements down to 80K in presence of magnetic field. The peak resistivity has been found to be decreased for the initial dose and then increased for subsequent doses. Highest dose of irradiation has enhanced the MR% near MIT. It appears that irradiation induced defects play a significant role in the generation or release of lattice strain in such rare earth manganite materials. The properties of a

Acknowledgements

The author S.C. gratefully acknowledges the financial help extended by CSIR, Govt. of India for carrying out the work. A.B. and B.K.C. are grateful to DST, Govt. of India for financial support.

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    Therefore at a fluence of 1014 ions/cm2, as estimated, one point defect must be produced per several hundreds of unit cells. In case of La0.7Pb0.3MnO3, Tim has been found [21] to decrease monotonically with increasing fluence of Li beam. However, the peak resistivity (around Tim) and also the resistivity values (within temperature 80–350 K) have been found to be lowered by the initial fluence of 6.5 × 1013 ions/cm2 and then increased for the subsequent higher fluence.

1

Present address: Physical Metallurgy Section, Material Characterization Group, Indira Gandhi Center for Atomic Research, Kalpakkam, India.

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