Daniel Esmarch Madsen

Experimental and theoretical studies of nanoparticles of antiferromagnetic materials

The magnetic properties of nanoparticles of antiferromagnetic materials are reviewed. The magnetic structure is often similar to the bulk structure, but there are several examples of size-dependent magnetic structures. Owing to the small magnetic moments of antiferromagnetic nanoparticles, the commonly used analysis of magnetization curves above the superparamagnetic blocking temperature may give erroneous results, because the distribution in magnetic moments and the magnetic anisotropy are not taken into account. We discuss how the magnetic dynamics can be studied by use of magnetization measurements, Mossbauer spectroscopy and neutron scattering. Below the blocking temperature, the magnetic dynamics in nanoparticles is dominated by thermal excitations of the uniform mode. In antiferromagnetic nanoparticles, the frequency of this mode is much higher than in ferromagnetic and ferrimagnetic nanoparticles, but it depends crucially on the size of the uncompensated moment. Excitation of the uniform mode results in a so-called thermoinduced moment, because the two sublattices are not strictly antiparallel when this mode is excited. The magnetic dipole interaction between antiferromagnetic nanoparticles is usually negligible, and therefore such particles present a unique possibility to study exchange interactions between magnetic particles. The interactions can have a significant influence on both the magnetic dynamics and the magnetic structure. Nanoparticles can be attached with a common crystallographic orientation such that both the crystallographic and the magnetic order continue across the interfaces.

The correlation between superparamagnetic blocking temperatures and peak temperatures obtained from ac magnetization measurements

We study the correlation between the superparamagnetic blocking temperature TB and the peak positions Tp observed in ac magnetization measurements for nanoparticles of different classes of magnetic materials. In general, Tp = α + βTB. The parameters α and β are different for the in-phase (χ � ) and out-of-phase (χ �� ) components and depend on the width σV of the log-normal volume distribution and the class of magnetic material (ferromagnetic/antiferromagnetic). Consequently, knowledge of both α and β is required if the anisotropy energy barrier KV and the attempt time τ0 are to be reliably obtained from an analysis based solely on the peak positions.

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.

Synthesis of oxacyclic scaffolds via dual ruthenium hydride/Brønsted acid-catalyzed isomerization/cyclization of allylic ethers.

A ruthenium hydride/Brønsted acid-catalyzed tandem sequence is reported for the synthesis of 1,3,4,9-tetrahydropyrano[3,4-b]indoles (THPIs) and related oxacyclic scaffolds. The process was designed on the premise that readily available allylic ethers would undergo sequential isomerization, first to enol ethers (Ru catalysis), then to oxocarbenium ions (Brønsted acid catalysis) amenable to endo cyclization with tethered nucleophiles. This methodology provides not only an attractive alternative to the traditional oxa-Pictet-Spengler reaction for the synthesis of THPIs, but also convenient access to THPI congeners and other important oxacycles such as acetals.

On the analysis of magnetization and Mössbauer data for ferritin

Experimental data for antiferromagnetic nanoparticles are often analyzed as if the particles were ferromagnetic. However, due to the volume dependence of the magnetization resulting from uncompensated spins, such analysis will yield erroneous results. This is demonstrated as we analyze ac and dc magnetization data as well as Mössbauer spectra obtained for ferritin. The values of the median energy barrier obtained from the different data are in very close agreement when a distribution of volumes and a volume dependence of the magnetization are taken into account. However, when the volume dependence of the magnetization is neglected, erroneous values of the anisotropy energy barrier and the attempt time τ(0) are obtained.

Interactions between goethite particles subjected to heat treatment

We have studied the effect of heating on the magnetic properties of particles of nanocrystalline goethite by use of Mossbauer spectroscopy. Heating at 150 °C for 24 h leads to a change in the quadrupole shift in the low-temperature spectra, indicating a rotation of the sublattice magnetization directions. Fitting of quantiles, derived from the asymmetrically broadened spectra between 80 and 300 K, to the superferromagnetism model indicates that this change is due to a stronger magnetic coupling between the particles.

Neutron study of magnetic excitations in 8-nm {alpha}-Fe{sub 2}O{sub 3} nanoparticles

By use of inelastic neutron scattering we have studied magnetic fluctuations in 8-nm particles of antiferromagnetic {alpha}-Fe{sub 2}O{sub 3} (hematite) as a function of temperature and applied magnetic fields. The fluctuations are dominated by uniform excitations. Studies have been performed on both coated (noninteracting) and uncoated (interacting) particles. We have estimated the magnetic anisotropy energy and found that the data are in good agreement with the value obtained from Moessbauer spectroscopy. The energy {epsilon}{sub 0} of the uniform excitations depends strongly on the uncompensated moment, which is caused by finite-size effects, and we have estimated the size of this moment from the experimental neutron data. The field dependence of {epsilon}{sub 0} for the interacting nanoparticles differs strongly from that of the noninteracting nanoparticles, and this is a result of the influence of exchange interaction between the particles.