Colossal magnetoresistance and related properties of mixed-valent manganites : Studies on polycrystalline solids, thin films and superlattices

Abstract

A new series La Cd MnO (x = 0.15 and 0.33) has been synthesized by the ceramic method, and their electrical, magnetic, and magnetoresistance properties were studied. Epitaxial thin films of La . Cd . MnO were fabricated by the pulsed laser deposition technique. The colossal magnetoresistance (CMR) properties of the thin films were studied and compared with those of polycrystalline La . Cd . MnO and La . A . MnO (A = Ca, Sr, Ba, or Pb).

The following are the main conclusions drawn from this work:

a. La . Cd . MnO crystallizes in an orthorhombic structure, whereas La . Cd . MnO crystallizes in a rhombohedral structure. b. La . Cd . MnO is a ferromagnetic insulator with a Curie temperature of ~155 K, while La . Cd . MnO is ferromagnetic and metallic below 250 K. c. A low-temperature magnetoresistance (MR) of 45% and 35% is observed (at 4.2 K under a 6 T magnetic field) in La . Cd . MnO and La . Cd . MnO , respectively, due to spin-dependent scattering of charge carriers at the grain boundaries. d. The magnetoresistance observed at the respective Curie temperatures of La . Cd . MnO and La . Cd . MnO is 55% and 70%, respectively. e. The epitaxial thin film did not show significant MR at low temperature. Near the transition temperature ( Tc), it showed about 80% magnetoresistance. f. The magnetoresistance observed in La . Cd . MnO is as high as that observed in La . Ca . MnO thin films, because both Cd and Ca have comparable ionic radii and introduce similar distortions in the lattice.

Magnetic structure, magnetic, and magnetoresistance properties were also studied on monovalent ion-doped lanthanum manganites. Thin films were fabricated by pulsed laser deposition, and their magnetoresistance was investigated. The conclusions of this chapter are summarized below:

(a) Both LNMO and LKMO polycrystalline samples are ferromagnetic below room temperature. The ferromagnetic transition temperature for LNMO is ~325 K, and for LKMO it is ~300 K. (b) The saturation magnetic moment obtained from bulk magnetization measurements is higher than that obtained from magnetic structure refinement. Further, no component is observed in the ab-plane. These factors suggest that the magnetic moment is randomly canted along the [001] crystallographic axis. (c) Both LNMO and LKMO polycrystalline samples show an insulator-to-metal transition. A magnetoresistance of ~35% was observed near the corresponding transition temperature. Low-temperature magnetoresistance was observed due to scattering of charge carriers from domain walls at the grain boundaries. (d) Thin films of LNMO and LKMO showed an insulator-to-metal transition at ~290 K and 275 K, respectively. The MR values at the transition temperature were 71% and 69% for LNMO and LKMO, respectively. No significant magnetoresistance was observed at low temperatures.