Dissertation

A UHV compatible SQUID magnetometer system for investigations in surface, interface, and thin film magnetism

Spagna, Stefano

1995

Overview

The magnetic properties of Co thin films on silicon and magnetite single crystal substrates grown by electron beam evaporation were studied in situ in UHV using a superconducting quantum interference device magnetometer (UHVSQM). In this system, freshly deposited thin film samples from a separate molecular beam epitaxy (MBE) facility, were transported into the UHVSQM for magnetic characterization without breaking vacuum. The combination of the extremely high sensitivity of the SQUID for magnetic measurements with UHV surface analysis techniques offers the unique capability for films to be prepared, processed, and characterized repeatedly without substantial contamination. Magnetization isotherms as function of oxygen exposure on polycrystalline thin ferromagnetic (FM) Co layers indicate that the magnetic properties of these materials are profoundly altered as a result of oxidation and atmospheric contamination. In addition, field-cooled hysteresis loops performed at low temperatures indicate that the antiferromagnetic (AF) Co-oxide layer that result from the oxidation process is responsible for an induced exchange anisotropy in the FM Co films. The data indicates that interfacial roughness and non-uniform coupling across the interface may cause the AF to break up into a domain structure analogous to the one found in bulk FM materials. The resulting AF magnetic structure is apparently dependent on the competition between destabilizing forces at the interface when the FM layer moment's direction is reversed and stabilizing coercive forces within the AF domains. During the course of this research, we also investigated the effect of clean and oxidized Co layers on the magnetic properties of oriented single crystal$\\rm Fe\\sb3O\\sb4$films. These experiments were aimed in gaining an understanding of the underlying mechanism responsible for the large increase in coercivity$\\rm (H\\sb{c}),$and induced uniaxial anisotropy exhibited by this system as a result of the Co coverage. A systematic study suggests that a thin layer of Co $\\sp{2+}$ions is apparently responsible for an initial increase in$\\rm H\\sb{c}.$However, a second and much larger enhancement in$\\rm H\\sb{c}$occurs at low temperatures when these films evolve from a clean to an oxidized state. These observations constitute the first direct evidence for two separate mechanism, which strongly affect the magnetic properties of the$\\rm Fe\\sb3O\\sb4$films, and could be useful in deriving a model that satisfactorily describes the behavior of this system.

• Publisher: ProQuest Dissertations & Theses
• Subject: Condensation / Condensed matter physics
• ISBN: 9798208379813