Nonlinear conductivity and magnetoresistance in perovskite and spinel manganites

Abstract

The thesis describes the nonlinear current-voltage characteristics in perovskite manganites and its grain boundary origin. Chapter 1 gives an introduction to the various electronic and magnetic properties exhibited by manganites and the present understanding of the phenomenon of colossal magnetoresistance. A brief outline on the transport properties of manganite spinels is also presented.

Chapter 2 describes the various experimental techniques involved in characterizing manganites, including electrical measurements in the presence of an external magnetic field. Chapter 3 describes a new wet chemical method involving the redox reactions for the preparation of manganites. High-quality specimens are obtained with large concentrations of Mn(IV).

Chapter 4 describes the effect of diamagnetic ions substituted in the Mn-sublattice on the electronic transport and magnetoresistance of LaMnO?. Detailed analysis is carried out to understand the variation of electrical resistivity, thermopower, and magnetoresistance. Weak ferromagnetism is introduced in LaMnO? on substitution of diamagnetic ions. The superexchange interaction between Mn(III) and Mn(IV) ions leads to antiferromagnetism in LaMnO? due to dz² orbital ordering. Substitution of diamagnetic ions disturbs this ordering, and the exchange interactions lead to ferromagnetic coupling. The large magnetoresistance observed is explained on the basis of increased hole concentration and enhanced ferromagnetic interactions.

Magnetically tunable current-voltage characteristics with high nonlinearity are observed in substituted manganite perovskites. The results presented in Chapter 5 show that the nonlinear behavior is a grain boundary phenomenon and that they arise from multichannel inelastic tunneling occurring below T_c. Such contributions are negligible above T_c, so that linear relations prevail. The external magnetic field modifies the J-E relations so that larger currents flow in the nonlinear region, indicating that the inelastic tunneling is spin-assisted. Further, the nonlinear characteristics are modifiable with the grain sizes, as shown in Chapter 6 for the lanthanum strontium manganites with large differences in microstructure. High-temperature annealing in different partial pressures of oxygen showed high nonlinearity coefficients in smaller grain-sized specimens.

Inelastic tunneling not only involves the individual point defects but also the defect complexes. The nonlinear conduction in manganites substituted with other transition metal ions showed field-assisted enhancement in the mobility of charge carriers.

Chapter 7 deals with the electrical field effects in the inelastic tunneling processes. The effect of valence fluctuations in the A-sublattice on the magnetic and electrical properties are discussed in Chapter 8. The substitution of Ce(IV) or Pr(IV) in LaMnO? leads to increased electron density in the B-sites. The metal-insulator transitions and the ferromagnetic coupling in these materials are presented.

Chapter 9 describes the electronic and magnetoresistive properties of manganite spinels. The magnetoelectric coupling in spinels is explained on the basis of spin-splitting of the conduction band as well as the donor levels below T_c, thereby altering the mobility of charge carriers under the applied electric field.