<p> Magnetic nanostructures have widespread applications in many areas of physics and engineering, and nanosphere lithography has recently emerged as promising tool for the fabrication of such nanostructures. The goal of this research is to explore the magnetic properties of a thin film of ferromagnetic material deposited onto a hexagonally close-packed monolayer array of polystyrene nanospheres, and how they differ from the magnetic properties of a typical flat thin film. The first portion of this research focuses on determining the optimum conditions for depositing a monolayer of nanospheres onto chemically pretreated silicon substrates (via drop-coating) and the subsequent characterization of the deposited nanosphere layer with scanning electron microscopy. Single layers of permalloy (Ni80Fe20) are then deposited on top of the nanosphere array via DC magnetron sputtering, resulting in a thin film array of magnetic nanocaps. The coercivities of the thin films are measured using a home-built magneto-optical Kerr effect (MOKE) system in longitudinal arrangement. MOKE measurements show that for a single layer of permalloy (Py), the coercivity of a thin film deposited onto an array of nanospheres increases compared to that of a flat thin film. In addition, the coercivity increases as the nanosphere size decreases for the same deposited layer. It is postulated that magnetic exchange decoupling between neighboring nanocaps suppresses the propagation of magnetic domain walls, and this pinning of the domain walls is thought to be the primary source of the increase in coercivity.</p>

<p> Collective magnetic behavior in nanostructures is a phenomenon commonly observed in various magnetic systems. It arises due to competing inter/intra&ndashparticle; interactions and size distribution and can manifest in phenomena like magnetic freezing, magnetic aging, and exchange bias (EB) effect. In order to probe these rather complex phenomena, conventional DC and AC magnetic measurements have been performed along with radio&ndashfrequency; transverse susceptibility (TS) measurements. We also demonstrate the magnetic entropy change as a parameter sensitive to subtle changes in the magnetization dynamics of nanostructures. The focus of this dissertation is to study the collective magnetic behavior in core-shell nanostructures of Fe/&gamma;&ndashFe;₂O₃ and Co/CoO, La_0.5Sr_0.5MnO₃ nanowires, and LaMnO₃ nanoparticles.</p><p> In the case of core/shell Fe/&gamma;&ndashFe;₂O₃, we found the particles to critically slow down below the glass transition temperature, below which they exhibit aging effects associated with a superspin glass (SSG) state. We demonstrate that it is possible to identify individual magnetic responses of the Fe core and the &gamma;&ndashFe;₂O₃ shell. Consistently, a systematic study of the magnetocaloric effect (MCE) in the Fe/&gamma;&ndashFe;₂O₃ system reveals the development of inverse MCE with peaks associated with the individual magnetic freezing of the core and the shell. From these obtained results, we establish a general criterion for EB to develop in core/shell nanostructures, that is when the core is in the frozen state and the magnetic moments in the shell begin to block. This criterion is shown to be valid for both ferromagnetic/ferrimagnetic (FM/FIM) Fe/&gamma;&ndashFe;₂O₃ and ferromagnetic/antiferromagnetic (FM/AFM) Co/CoO core&ndashshell; nanostructures. We also elucidate the physical origin of the occurrence of asymmetry in field-cooled hysteresis loops and its dependence on magnetic anisotropy in the Co/CoO system by performing a detailed TS study.</p><p> We have performed a detailed magnetic study on hydrothermally synthesized single crystalline La_0.5Sr_0.5MnO₃ nanowires. The temperature and field dependent evolution of the different magnetic phases leading to development of the inverse MCE and EB in the nanowires is discussed. Finally, we have studied the collective magnetic behavior of LaMnO₃ nanoparticles synthesized by the sol&ndashgel; technique. The nanoparticle ensemble shows the unusual co&ndashexistence; of super-ferromagnetism (SFM), as well as the SSG state, which we term the &lsquoferromagnetic; superglass&rsquo; (FSG) state. The existence of FSG and the characteristics of its magnetic ground state are discussed.</p>