Nanoparticles

Role of the nanocrystallinity on the chemical ordering of Co(x)Pt(100-x) nanocrystals synthesized by wet chemistry

Co(x)Pt(100-x) nanoalloys have been synthesized by two different chemical processes either at high or at low temperature. Their physical properties and the order/disorder phase transition induced by annealing have been investigated depending on the route of synthesis. It is demonstrated that the chemical synthesis at high temperature allows stabilization of the fcc structure of the native nanoalloys while the soft chemical approach yields mainly poly or non crystalline structure. As a result the approach of the order/disorder phase transition is strongly modified as observed by high-resolution transmission electron microscopy (HR-TEM) studies performed during in situ annealing of the different nanoalloys. The control of the nanocrystallinity leads to significant decrease in the chemical ordering temperature as the ordered structure is observed at temperatures as low as 420 °C. This in turn preserves the individual nanocrystals and prevents their coalescence usually observed during the annealing necessary for the transition to an ordered phase.

Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit

To study the magnetic dynamics of superparamagnetic nanoparticles, we use scanning probe relaxometry and dephasing of the nitrogen vacancy (NV) center in diamond, characterizing the spin noise of a single 10 nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivity's dependence on the applied decoherence measurement method. By comparing the change in relaxation (T1) and dephasing (T2) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticle's diameter and distance from the NV center using an Ornstein-Uhlenbeck model for the nanoparticle's fluctuations. This scanning probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior.

Uniform excitations in magnetic nanoparticles

We present a short review of the magnetic excitations in nanoparticles below the superparamagnetic blocking temperature. In this temperature regime, the magnetic dynamics in nanoparticles is dominated by uniform excitations, and this leads to a linear temperature dependence of the magnetization and the magnetic hyperfine field, in contrast to the Bloch T(3/2) law in bulk materials. The temperature dependence of the average magnetization is conveniently studied by Mössbauer spectroscopy. The energy of the uniform excitations of magnetic nanoparticles can be studied by inelastic neutron scattering.

Magnetic fluctuations in nanosized goethite (α-FeOOH) grains

Mössbauer spectra of antiferromagnetic goethite (α-FeOOH) particles usually show an asymmetric line broadening, which increases with increasing temperature, although the magnetic anisotropy is expected to be so large that magnetic relaxation effects should be negligible. By use of high resolution transmission electron microscopy we have studied a sample of goethite particles and have found that the particles contain many defects such as low angle grain boundaries, in accordance with previous studies of other samples of goethite particles. Such defects can result in a magnetic mismatch at the grain boundaries between nanometer-sized grains, leading to a weakened magnetic coupling between the grains. We show that the Mössbauer data of goethite can be explained by fluctuations of the sublattice magnetization directions in such weakly coupled grains. It is likely that the influence of defects such as low angle grain boundaries also plays a role with regards to the magnetic properties in other antiferromagnetic nanograin systems. We discuss the results in relation to Mössbauer studies of α-Fe(2)O(3) and α-Fe(2)O(3)/NiO nanoparticles.

Magnetic properties of cobalt ferrite-silica nanocomposites prepared by a sol-gel autocombustion technique

The magnetic properties of cobalt ferrite-silica nanocomposites with different concentrations (15, 30, and 50 wt %) and sizes (7, 16, and 28 nm) of ferrite particles have been studied by static magnetization measurements and Mossbauer spectroscopy. The results indicate a superparamagnetic behavior of the nanoparticles, with weak interactions slightly increasing with the cobalt ferrite content and with the particle size. From high-field Mossbauer spectra at low temperatures, the cationic distribution and the degree of spin canting have been estimated and both parameters are only slightly dependent on the particle size. The magnetic anisotropy constant increases with decreasing particle size, but in contrast to many other systems, the cobalt ferrite nanoparticles are found to have an anisotropy constant that is smaller than the bulk value. This can be explained by the distribution of the cations. The weak dependence of spin canting degree on particle size indicates that the spin canting is not simply a surface phenomenon but also occurs in the interiors of the particles.

First synthesis by liquid-liquid phase transfer of magnetic CoxPt100-x nanoalloys

Controlled synthesis of magnetic nanoparticles with a well-known size and composition is always a challenge. A soft chemical synthesis was developed to obtain magnetic alloy nanocrystals with a high ability to control composition, size, and polydispersity. Cobalt-platinum alloy nanocrystals were synthesized using a colloidal approach by the liquid-liquid phase transfer method. Structural characterization using HRTEM and XRD was carried out on nanocrystals in the range of 25-75% cobalt composition, which indicated the formation of nanoalloy bimetallic CoxPt100-x. Adjusting the alkylamine capping agent and the kinetics of the reduction process allowed tuning of the size in the range of 1.8-4 nm while keeping an equiatomic composition. The narrow size distribution led to the possibility of inducing nanoparticle self-organization over a long range. The magnetic properties of the Co50Pt50 nanoalloy in the disordered face-centered cubic phase A1 were studied for different nanoparticle sizes.

Seven-Nanometer Hexagonal Close Packed Cobalt Nanocrystals for High-Temperature Magnetic Applications through a Novel Annealing Process

Seven-nanometer cobalt nanocrystals are synthesized by colloidal chemistry. Gentle annealing induces a direct structural transition from a low crystalline state to the hexagonal close packed (hcp) phase without changing the size, size distribution, and the lauric acid passivating layer. The hcp structured nanocrystals can be easily redispersed in solvent for further application and processing. We found that the magnetization at saturation and the magnetic anisotropy are strongly modified through the annealing process. Monolayer self-assembly of the hcp cobalt nanocrystals is obtained, and due to the dipolar interaction, ferromagnetic behavior close to room temperature has been observed. This work demonstrates a novel approach for obtaining small size hcp structured cobalt magnetic nanocrystals for many technological applications.

Spin rotation in alpha-Fe2O3 nanoparticles by interparticle interactions

Nanoparticles of alpha-Fe2O3 (hematite) typically have the sublattice magnetization directions in the hexagonal (001) plane below the Ne el temperature. By use of Mo ssbauer spectroscopy we have found that for agglomerated particles the sublattice magnetization may be rotated of the order of 15 degrees out of plane, depending on the particle size. The spin rotation can be explained by exchange interaction between neighboring particles with nonparallel (001) planes. The results imply that interparticle interactions can lead to spin directions deviating from the easy axis defined by the magnetic anisotropy.