Madhur Sachan

Direct visualization of dipolar ferromagnetic domain structures in Co nanoparticle monolayers by electron holography

Electron holography has revealed dipolar ferromagnetic domain structures in ordered monolayer arrays of Co nanoparticles. In zero-field-cooling experiments, we demonstrate the formation of micron-sized magnetically ordered regions with transverse domain walls, and a spatially varying moment order parameter. Truncated vortex structures that can be modified by an applied field are observed near the sample edges. The collectively magnetized state shows soft magnetic properties and long-range order that is stable over times over 1h at 108K.

Combustion-driven compaction of nanostructured Sm-Co and Fe mixtures

Sm-Co and Fe mixture nanostructured magnets were prepared by compaction of powder precursors. We describe an inert atmosphere process using combustion-driven compaction which significantly reduces the problem of loss in coercivity during compaction. It is possible to retain some degree of crystallographic alignment with this compaction process.

Self-assembled nanoparticle arrays as nanomasks for pattern transfer

Argon ion milling was used to transfer the pattern of sparse 12 nm iron oxide nanoparticles into underlying thin films of Pt and magnetic tunnel junction stacks and quantify their etching rates and morphological evolution. Under typical milling conditions, Pt milled at 10 nm min−1, while the isolated particles of iron oxide used for the mask milled at 5 nm min−1. Dilute dispersions of nanoparticles were used to produce the sparse nanomasks, and high resolution scanning electron microscopy (SEM) and atomic force microscopy were used to monitor the evolution of etched structures as a function of milling time. SEM measurements indicate an apparent 20% increase in feature diameter before the features began to diminish under additional milling, suggesting redeposition as a limiting feature in the milling of dense arrays. Simulations of the milling process in nanoparticle arrays that include redeposition are consistent with this observation. These simulations predict that an edge-to-edge spacing of 3 nm in a dense array is feasible, but that redeposition reduces the final structure aspect ratio from that of the masking array by as much as a factor of two.

Iron nanoparticle assemblies: structures and magnetic behavior

Self-assembly of spherical, surfactant-coated nanoparticles is discussed, an examples are presented to demonstrate the variety of structures that can be formed, and the conditions that lead to them. The effect of the concentration on the magnetic properties is then examined for 8.5 nm Fe nanoparticles. Dilute dispersions, arrays formed by evaporation of the dispersions, and nanoparticle crystals grown by slow diffusion of a poorly coordinating solvent were characterized by zero field-cooled magnetization, remanent hysteresis loop, and magnetic relaxation measurements. The average spacing between the particles was determined from a combination of transmission electron microscopy and small angle x-ray scattering. In the arrays the spacing was 2.5 nm between the edges of the particle cores, while in the nanoparticle crystals the particles were more tightly packed, with a separation of 1.1 nm. The reduced separation increased the magnetostatic interaction strength in the nanoparticle crystals, which showed distinctly different behavior in the rate of approach to saturation in the remanent hysteresis loops, and in the faster rate of time-dependent magnetic relaxation.

Field evolution of magnetic correlation lengths in ϵ-Co nanoparticle assemblies

Small-angle neutron scattering measurements of Co nanoparticle assemblies reveal three characteristic length scales associated with the interparticle and intraparticle magnetic orders. The first length scale stemming from particle size and separation does not vary with applied field. In contrast, the magnetic correlation length increases from 71±9nm in zero field at 5K to greater than 1000nm in fields larger than 0.2T. The random-field length scale decreases from 37±8nm when H=0to9.1±0.3nm in H=0.2T, and the contribution of this term is less significant in large fields.

Length Scales of Magnetic Correlations in √e¬μ-Co Nanoparticle Assemblies using Small Angle Neutron Scattering

This article presents experiments on the determination of the length scales of magnetic correlation in ε-Co nanoparticle assemblies using SANS (small angle neutron scattering). In this experiment a 0.5 nm neutron beam was passed through a sealed sample containing a dense assembly of the nanoparticles, and the scattered intensity was collected with two-dimensional detector. Data were collected with magnetic fields ranging from 0 to 5 T, which were applied perpendicular to the neutron beam. The temperature varied between 5 and 275 K.

Compaction of nano-structured SmCo/Fe magnets

Nanostructured SmCo/Fe magnets are prepared by cold isostatic pressing method at 35 MPa followed by combustion driven compaction at 2 GPa at room temperature. The direction of compaction parallel and perpendicular to c-axis alignment of the green compact is investigated using X-ray pole figures. For the sample compacted perpendicular to the c-axis, an elongation of the intensity pattern is observed for the [002] pole indicating a partial orientation in that direction. On the other hand, no preferential orientation is observed for the sample compacted parallel to the alignment axis. Texture analysis is then used to quantify the degree of alignment.

Langmuir Layers of Magnetic Nanoparticles

Methods to form magnetic nanoparticle monolayers using non-aqueous Langmuir layers are reported. Following a discussion of the driving forces in various self-assembly techniques, we describe how aqueous Langmuir layers can be modified for use in conjunction with oxidationsensitive nanoparticles. Monolayers are formed using Fe and–Co nanoparticles, and transferred to carbon-coated transmission electron microscopy grids using the Langmuir-Schaefer method.