Microstructure, magnetoresistance, and magnetic properties of pulsed-laser-deposited external, internal, and mixed-doped lanthanum manganite films

Abstract

In this article we report our studies on the microstructure, magnetoresistance (MR) behavior, and magnetic properties of the La1−x−yCaxMnO3 system in thin-film form. By varying the values of x and y in La1−x−yCaxMnO3, we have synthesized an external- (x=0.3, y=0), an internal- (x=0, y=0.3), and a mixed-doped (x=0.2, y=0.1) system with the same Mn3+/Mn4+ ratio. Thin films of these materials have been grown in situ on (100) LaAlO3 substrates using a pulsed-laser-deposition technique. Atomic force microscopy, x-ray diffraction and high-resolution transmission electron microscopy measurements carried out on these films have shown that the films are smooth, highly crystalline, and epitaxial on the (100) LaAlO3 substrates. Electrical resistance and magnetoresistance have been measured in the 10–300 K range in magnetic fields up to 5 T using a superconducting quantum interference device magnetometer. The MR ratios (calculated using the expression, [R0−RH]/RH, where R0 and RH are resistances in zero and applied fields) of the La0.7Ca0.3MnO3 (x=0.3, y=0), La0.7MnO3 (x=0, y=0.3), and La0.7Ca0.2MnO3 (x=0.2, y=0.1) films are found to be 825%, 700%, and 750% at 200, 240, and 220 K, respectively. The MR ratios of these films, calculated using the expression, [R0−RH]/RH, are 91%, 87%, and 88%, respectively, at the same temperatures. The variation in the insulator-to-metal transition and the MR ratio is attributed to internal chemical pressure and vacancy localization effects. Below Tc/2 (Tc is paramagnetic-to-ferromagnetic transition temperature), resistance increases as T2 for La0.7Ca0.3MnO3 and La0.7Ca0.2MnO3 while it increases as T5/2 for La0.7MnO3. The T2 and T5/2 dependence of resistance suggests that the transport is predominantly governed by an electron–electron scattering and a combination of electron–electron, electron–phonon, and electron–magnon scattering, respectively. High-temperature resistance has been observed to be consistent with small polaron hopping conductivity for all three systems. Magnetization measurements carried out on the films show that the films have reasonably square hysteresis loops with sharp Tc’s. Below Tc/2, the magnetization decreases as T2 for La0.7Ca0.2MnO3 and La0.7MnO3, suggesting single-particle excitations in them, while it decreases as T3/2 for La0.7Ca0.3MnO3, representing collective oscillations in this system.