Ryan Booth

Functional magnetic nanoparticle assemblies: formation, collective behavior, and future directions.

This Perspective describes recent progress in the development of functional magnetic nanoparticle assemblies. After describing the formation of two- and three-dimensional particle arrays in terms of the size-dependent driving forces, we focus on magnetic nanoparticle arrays. We discuss how the self-organized structure can modify the magnetic behavior, relative to that of isolated particles. We highlight an important development, described in this issue of ACS Nano by Kostiainen and co-workers, who have demonstrated not only the novel aqueous self-assembly of magnetic particles but also controlled and reversible disassembly. Finally, we explore two inter-related future directions for self-assembly of magnetic nanoparticles: the formation of more complex, hierarchical structures and the integration of self-assembly with fabrication techniques for electronic devices.

Ten-nanometer dense hole arrays generated by nanoparticle lithography.

Large area dense hole arrays with a feature size of ~10 nm were generated using self-assembled monolayers of nanoparticles as etch masks. To fabricate the hole arrays, monolayers of nanoparticles were irradiated by electron beam to turn surfactants into amorphous carbon, treated by acid to remove the nanoparticle cores, and then etched by CF(4) to deepen the holes. Evaporated gold films preferentially diffuse into the holes to generate gold nanoparticle arrays. However no obvious diffusion into holes was observed for a sputtered iron platinum film.

Crystallographic orientation and the magnetocaloric effect in MnP

Here we present results for single crystals of manganese phosphide (MnP), a material exhibiting a first-order ferromagnetic to paramagnetic transition at a Curie temperature of 290 K. MnP has a saturation field of about 7.5 kOe along the c-axis. Along this easy magnetic axis, ΔS was measured to be 2.2, 3.3, and 6.0 J/kg K in applied fields of 10, 20, and 50 kOe, respectively. The density of MnP is 5.49 g/cm3. No hysteresis or irreversibility was measured over a range of temperatures from 10 to 300 K.

The magnetocaloric effect in thermally cycled polycrystalline Ni-Mn-Ga

The effect of repeated thermal cycling on the magnetocaloric effect in Ni54.3Mn20.1Ga25.6 was investigated. During the virgin cycle, the magnetic entropy change, ΔS, was −7.3 J/kg K and −6.9 J/kg K and the refrigeration capacities were 17 J/kg and 19 J/kg under an applied field of 20 kOe for heating and cooling, respectively. The heating and cooling transition temperatures were 340 K and 331 K, respectively. Over the course of 37 thermal cycles through the martensitic transition the moment of the sample decreased monotonically. We attribute this feature to small volumes of the sample remaining in the non-magnetic austenite phase after repeated cycling. The heating Curie temperature changed stochastically by 1 K throughout the cycling process due to the presence of strain or its release by cracking, while the cooling Curie temperature remained constant. On the final cycle the magnetic entropy change was −10.9 J/kg K and −9.4 J/kg K and the refrigeration capacities were both 17 J/kg under an applied field o...

Polarization-analyzed small-angle neutron scattering. II. Mathematical angular analysis

Polarization-analyzed small-angle neutron scattering (SANS) is a powerful tool for the study of magnetic morphology with directional sensitivity. Building upon polarized scattering theory, this article presents simplified procedures for the reduction of longitudinally polarized SANS into terms of the three mutually orthogonal magnetic scattering contributions plus a structural contribution. Special emphasis is given to the treatment of anisotropic systems. The meaning and significance of scattering interferences between nuclear and magnetic scattering and between the scattering from magnetic moments projected onto distinct orthogonal axes are discussed in detail. Concise tables summarize the algorithms derived for the most commonly encountered conditions. These tables are designed to be used as a reference in the challenging task of extracting the full wealth of information available from polarization-analyzed SANS.

Origin of reduced magnetization and domain formation in small magnetite nanoparticles

The structural, chemical, and magnetic properties of magnetite nanoparticles are compared. Aberration corrected scanning transmission electron microscopy reveals the prevalence of antiphase boundaries in nanoparticles that have significantly reduced magnetization, relative to the bulk. Atomistic magnetic modelling of nanoparticles with and without these defects reveals the origin of the reduced moment. Strong antiferromagnetic interactions across antiphase boundaries support multiple magnetic domains even in particles as small as 12–14 nm.

Internal magnetic structure of magnetite nanoparticles at low temperature

Small-angle neutron scattering with polarization analysis reveals that Fe3O4 nanoparticles with 90 A diameters have ferrimagnetic moments significantly reduced from that of bulk Fe3O4 at 10 K, nominal saturation. Combined with previous results for an equivalent applied field at 200 K, a core-disordered shell picture of a spatially reduced ferrimagnetic core emerges, even well below the bulk blocking temperature. Zero-field cooling suggests that this magnetic morphology may be intrinsic to the nanoparticle, rather than field induced, at 10 K.

Particle moment canting in CoFe 2 O 4 nanoparticles

Polarization-analyzed small-angle neutron scattering methods are used to determine the spin morphology in high crystalline anisotropy, 11 nm diameter CoFe2O4 nanoparticle assemblies with randomly oriented easy axes. In moderate to high magnetic fields, the nanoparticles adopt a uniformly canted structure, rather than forming domains, shells, or other arrangements. The observed canting angles agree quantitatively with those predicted from an energy model dominated by Zeeman and anisotropy competition, with implications for the technological use of such nanoparticles.

High-frequency permeability of Ni and Co particle assemblies

A coaxial transmission line was constructed, characterized, and calibrated for the frequency dependent measurement of complex relative permeability (μr) and complex permittivity (er). The permeability of Ni powder with a grain size of < 1 μm was measured as a function of packing density to verify the system performance. 8–10 nm diameter Co nanoparticles were synthesized, dried to a powder, and measured. The real part of the permeability for the Co nanoparticles decreased over time as a result of oxidation, and decreased the overall magnetic volume due to the formation of an antiferromagnetic CoO shell. Similarly, the imaginary part of the permeability decreased as a function of oxidation. This was attributed to the insulating CoO shell reducing eddy current losses in the nanoparticle composite.