Magnetic correlations of iron oxide nanoparticles as probed by polarized SANS in stretched magnetic nanoparticle-elastomer composites.

We have investigated the magnetic correlations among 7 nm iron oxide nanoparticles embedded in stretched silicone elastomers using polarized Small Angle Neutron Scattering (SANS). The magnetic nanoparticle (MNP)-elastomer composite can be stretched during experiments, and macroscopic deformations cause rearrangement of the iron oxide particles on the nanoscale. Polarized neutrons can be used to nondestructively probe the arrangement of magnetic nanoparticles before and after stretching, so that the relationship between applied stress and nanoscale magnetization can be interrogated. We find that stretching the MNP-elastomer composite past a certain threshold dramatically changes the structural and magnetic morphology of the system. The unstretched sample is modeled well by ~40 nm clusters of ~7 nm particles arranged in a hard sphere packing with a "volume fraction" parameter of 0.3. After the sample is stretched 3× its original size, however, the scattering data can be modeled by smaller, 16 nm clusters with a higher volume fraction of 0.4. We suggest that the effect can be understood by considering a stretching transformation on FCC-like crystallites of iron oxide nanoparticles embedded in an elastomeric medium.

Exchange Bias in Bulk α - Fe / γ - Fe 70 Mn 30 Nanocomposites for Permanent Magnet Applications.

Here we report on the microstructural factors influencing the formation of the interfacial exchange bias effect in three-dimensional transition-metal-based nanocomposite systems, with relevance to permanent magnet applications. Bulk phase-separated nanocomposites consisting of the ferromagnetic α -Fe and metastable antiferromagnetic γ - Fe 70 Mn 30 phases exhibit a notable low-temperature exchange bias and substantial coercivity ( H ex = 24.6 kA / m , H C = 95.7 kA / m ) as well as a near room-temperature blocking temperature. Structural investigation by synchrotron X-ray diffraction, neutron scattering, and transmission electron microscopy confirm that the ferromagnetic α -Fe phase nucleates as small precipitates ( d ≈ 50 nm ) at the grain boundaries of the antiferromagnetic γ - Fe 70 Mn 30 grains ( d = 360 - 740 nm ) and grows anisotropically upon heat treatment, resulting in an elliptical geometry. These results indicate that optimization of the exchange bias effect in bulk nanocomposite systems may be achieved through maximizing the surface-to-volume ratio of ferromagnetic precipitates in an antiferromagnetic matrix, enhancing the magnetocrystalline anisotropy of the antiferromagnetic phase to facilitate interfacial pinning and ensuring a balanced distribution of the ferromagnetic and antiferromagnetic phases. This work further clarifies critical factors influencing the formation of an exchange bias in an inexpensive transition-metal-based bulk nanocomposite system with potential for scalable production.

Origin of surface canting within Fe3O4 nanoparticles.

The nature of near-surface spin canting within Fe3O4 nanoparticles is highly debated. Here we develop a neutron scattering asymmetry analysis which quantifies the canting angle to between 23° and 42° at 1.2 T. Simultaneously, an energy-balance model is presented which reproduces the experimentally observed evolution of shell thickness and canting angle between 10 and 300 K. The model is based on the concept of Td site reorientation and indicates that surface canting involves competition between magnetocrystalline, dipolar, exchange, and Zeeman energies.

Core-shell magnetic morphology of structurally uniform magnetite nanoparticles.

A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.