Nanostructured systems composed of two or more technologically important materials are useful for device applications and intriguing for the new fundamental physics they may display. Magnetism at the nanoscale is dominated by size and surface effects which combined with other media lead to new spin dynamics and interfacial coupling phenomena. These new properties may prove to be useful for optimizing sensors and devices, increasing storage density for magnetic media, as well as for biomedical applications such as drug delivery, MRI contrast enhancement, and hyperthermia treatment for cancer. In this project we have examined the surface and interface magnetism of composite nanoparticles and multilayer thin films by using conventional DC magnetization and AC susceptibility as well as transverse susceptibility, a method for directly probing the magnetic anisotropy of materials. Au and Fe3O4 synthesized together into three different nanoparticle configurations and ranging in size for 60 nm down to 9nm are used to study how the size, shape, and interfaces affect the most fundamental properties of magnetism in the Au-Fe3O4 system. The findings have revealed ways in which the magnetic properties can be enhanced by tuning these parameters. We have shown that by changing the configurations of the Au and Fe3O4 particles, exotic behavior can be observed such as a large increase in anisotropy field (H[subscript]K ranging from 435 Oe to 1650 Oe) and the presence of exchange bias. Multilayer thin films have been studied as well which combine the important classes of ferromagnetic and ferroelectric materials. In one case, barium hexaferrite/barium strontium titanate thin films, the anisotropic behavior of the ferromagnet is shown to change due to the introduction of the secondary material. In the other example, CrO2/Cr2O3 bilayers, exchange coupling is observed as Cr2O3 is an antiferromagnet as well as a ferroelectric. This coupling is manifest as a uniaxial anisotropy rather than the unidirectional anisotropy associated with exchange biased bilayers. Not only will such multifunctional structures will be useful for technological applications, but the materials properties and configurations can be chosen and tuned to further enhance the desired functional properties.

Nanoparticle assemblies are of current interest as they are used in a wide variety of industrial and biomedical applications. This work presents two studies aimed at understanding the magnetization dynamics and interparticle interactions in nanoparticle assemblies and various types of ferrofluids. First, we studied the influence of varying strengths of dipolar interaction on the static and dynamic magnetic properties of surfactant-coated monodispersed manganese-zinc ferrite nanoparticles using reversible transverse susceptibility. We tracked the evolution of the anisotropy peaks with varying magnetic field, temperature, and interaction strength. The anisotropy peaks of weakly interacting particles appears as non-symmetric peaks and at lower fields in a unipolar transverse susceptibility scan. On the other hand, a strongly interacting particle system exhibits symmetric anisotropy peaks situated at higher field values. In the second study, we successfully synthesized stable ferrofluids out of high quality Fe 3O4 and CoFe2O4 nanoparticles. Such ferrofluids are excellent systems for the investigation of physics of relaxation phenomena in magnetic nanoparticles. Motivated by the need to understand their peculiar magnetic response, a comparative study on Fe 3O4 - and CoFe 2O4 -based ferrofluids was performed. We investigated cases in which particle blocking and carrier fluid freezing temperatures were close and far apart from each other. Our experimental results reveal the true origin of the glass-like relaxation peaks that have been widely observed in ferrofluids by many groups but remained largely unexplained. Contrary to the speculation of previous literature, we argue that the formation of the magnetic anomaly is due not only to the particle blocking but also to its correlation with the the carrier fluid freezing effects. It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher scaling law clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time.

<p>The study of ultrathin magnetic films offers novel magnetic phenomena due to the reduced symmetry of these 2D systems. The magnetic anisotropy differentiates behaviour in ultrathin films from the bulk environment, as additional anisotropies emerge from the ultrathin film thickness and the inherent strain of ultrathin films. In this work, the in-plane magnetic anisotropy of strained ferromagnetic (FM) ultrathin Co(0001) films grown on a W(110) substrate is measured over a range of temperatures (150-320 K). Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to determine the film structure and thickness. The anisotropy is derived from the quotient of the saturation magnetization and the transverse susceptibility, which are measured using the surface magneto-optic Kerr effect (SMOKE).</p> <p>This work’s second objective is to study the Néel transition in antiferromagnetic (AFM) ultrathin films. The zero net magnetization of AFM materials and the minute sample size of ultrathin films make magnetic measurements impossible with conventional methods. An alternative approach is to study a single AFM ultrathin film that is coupled by the interfacial exchange interaction to a FM ultrathin film. The upper layers of ultrathin Co/W(110) films were oxidized to produce ultrathin CoO/Co/W(110) films, creating an AFM/FM bilayer system. SMOKE measurement of the transverse magnetic susceptibility of the FM Co layer reveal the Néel transition of the AFM layer indirectly through the interfacial exchange interaction.</p> / Master of Science (MSc)