On the properties of La0.7Sr0.3MnO3 thin films on (001) SrTiO 3 substrates

Abstract

The performance of spintronic devices are heavily dependent on both the static and dynamic magnetic properties of both their functional interfaces and bulk properties. In this thesis work we study the stoichiometry, magnetotransport, dynamic and static magnetic properties of the ferromagnetic perovskite La0.7Sr0.3MnO3 grown by pulsed laser deposition.

Surface stoichiometry of the films was examined using soft synchrotron and hard X-ray radiation, following exposure to air and subsequent in vacuo annealing. Organic contaminants were gradually removed upon incrementally increased temperatures, and found reduced to ∼ 0.4% of the topmost crystalline unit cell at 270 ◦C. After all heating steps a Mn depleted and Sr rich surface oxide layer were observed, which were of reduced crystalline quality and was charge depleted. This surface is consistent with observed magnetically inactive layers in such films.

Using variable temperature ferromagnetic resonance spectroscopy and magnetometry, the magnetic properties of films between 4 and 150 unit cells thick were studied. We found that ∼ 4 unit cells are magnetically inactive, but all thicker films display constant volume magnetization. The reduced ordering temperature for thinner films were well described by a mean field model, and explains the seemingly thickness dependent magnetocrystalline anisotropies. Critical exponent analysis indicateed a 3D to 2D transition of magnetic order with reduced thickness, and the 6 unit cell film displayed severely altered dynamic properties owing to possible polaronic confinement. We found that the line-width broadening were dominated by two-magnon scattering in the high temperature phase for thinner films, and given by magnetic anisotropy spread at lower temperatures and for thicker films. For films of more than 40 unit cells, extra resonance lines appeared in the high temperature phase, correlated with a gradually contracted unit cell in the c-direction.

Magnetotransport of as grown and nano-patterned (La0.7 Sr0.3MnO3)40/(LaMnO3)n/(La0.7 Sr0.3MnO3)40 heterostructures (0 ≤ n ≤ 8) on Nb doped SrTiO3 substrates were investigated using scanning tunneling microscope based point contacts. Two types of low field magnetoresistance were identified; one originating from domain walls/grain boundaries in the La0.7Sr0.3MnO3 films, and the other from the Schottky junction forming at the substrate-film interface. LaMnO3 inter-layers were found to reduce the rectifying properties of the junctions, and nano-patterning by electron beam lithography enhances them, in accordance with a proposed transport model based on the locality of the injected current. The results illustrate the sensitivity of local probing and and the existence of a junction-field dependent magnetoresistance.