Joseph Sandstrom

Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

An unanticipated superparamagnetic response has been observed in cobalt ferrite materials after thermal treatment under inert atmosphere. Cobalt ferrite particles were prepared via normal micelle precipitation that typically yields CoxFe3−xO4 nanoparticles (x=0.6−1.0). While samples thermally treated under oxygen show majority spinel phase formation, annealing in nitrogen gas yields materials consisting of Co-Fe alloy, FeS, and CoFe2O4 spinel. After thermal treatment, thermomagnetic studies reveal composition-insensitive, but highly treatment-sensitive, saturation magnetization, coercivity, blocking temperature, and Verwey transition temperature dependence. Extremely high saturation magnetization (159 emu/g) with low coercivity (31 Oe) was observed for one of the treated compositions, which drastically deviates from prototypical cobalt ferrite with large magnetocrystalline anisotropy. We attribute such unique magnetic response to Co-Fe alloy coexisting with FeS and CoFe2O4 spinel where the diameter of the...

Comparative study of low-temperature PECVD Of amorphous silicon using mono-, di-, trisilane and cyclohexasilane

The hydrogenated amorphous silicon a-Si∶H films were grown by plasma-enhanced chemical vapor deposition (PECVD) using liquid cyclohexasilane Si<inf>6</inf>H<inf>12</inf> (CHS). The growth rate of a-Si∶H was studied as a function of substrate temperatures in the range of 30 °C≪T≪450 °C using deposition conditions that were optimized for monosilane SiH<inf>4</inf>. The same parameters were used for a- Si∶H films grown using disilane (Si<inf>2</inf>H<inf>6</inf>) and trisilane (Si<inf>3</inf>H<inf>8</inf>) precursors. It was found that the a-Si∶H film growth rate of CHS is lower with respect to those of mono-, di- and trisilane in an Ar plasma. Addition of ∼10% of H<inf>2</inf> dramatically increases the deposition rate for CHS-based films about 700% to 8 Å/sec. The as-deposited films were characterized by FTIR and Raman spectroscopy to probe the hydrogen content and local bonding environment. It was found that the films grown using Ar/H<inf>2</inf> mixtures as carrier gas have a reduced hydrogen content relative to polysilane fragments indicating higher quality amorphous silicon.

High-Permeability Particles for Magnetic Composites

Electromagnetic shields and flux concentrators for magnetic sensors could utilize flexible and insulating composites applied using simple thin film deposition methods such as dipcoating, spin-coating, spraying, etc. As the fi rst step towards development of composites with superior performance, efforts focu sed on isolating nanoparticles with large magnetizations under low fields. In this paper, we provide the results of proof-of-concept studies for two systems: metal-functionalized silicone-based materials (metal-silicone); and, Co-ferrite (Co 2+ 1-xFe 2+ xFe 3+ 2O4) nanoparticles. The metal-silicone materials studied included a polysiloxane that containe d a pendant ferrocene where an optimum saturization magnetization of 5.9 emu/g (coe rcivity = 11 Oe) was observed. Co-ferrite nanoparticle samples prepared in this study showed unprecendented saturation magnetization (i.e., Ms > 150 emu/g) with low coercivity (H c ~ 10 Oe) at room temperature and offer potential appl ication as flux concentrators.