144 research outputs found
Measuring magnetic correlations in nanoparticle assemblies
We illustrate how to extract correlations between magnetic moments in assemblies of nanoparticles from, e. g., electron holography data providing the combined knowledge of particle size distribution, inter-particle distances, and magnitude and orientation of each magnetic moment within a nanoparticle superstructure, We show, based on simulated data, how to build a radial/angular pair distribution function f(r, theta) encoding the spatial and angular difference between every pair of magnetic moments. A scatter-plot of f(r, theta) reveals the degree of structural and magnetic order present, and hence provides a measure of the strength and range of magnetic correlations
Anomalous Particle Size Dependence of Magnetic Relaxation Phenomena in Goethite Nanoparticles
By use of Mössbauer spectroscopy we have studied the magnetic properties of samples of goethite nanoparticles with different particle size. The spectra are influenced by fluctuations of the magnetization directions, but the size dependence is not in accordance with the Néel-Brown expression for superparamagnetic relaxation of the magnetization vectors of the particles as a whole. The data suggest that the magnetic fluctuations can be explained by fluctuations of the magnetization directions of small interacting grains within the particles
Spin Structures in Magnetic Nanoparticles
Spin structures in nanoparticles of ferrimagnetic materials may deviate locally in a nontrivial way from ideal collinear spin structures. For instance, magnetic frustration due to the reduced numbers of magnetic neighbors at the particle surface or around defects in the interior can lead to spin canting and hence a reduced magnetization. Moreover, relaxation between almost degenerate canted spin states can lead to anomalous temperature dependences of the magnetization at low temperatures. In ensembles of nanoparticles, interparticle exchange interactions can also result in spin reorientation. Here, we give a short review of anomalous spin structures in nanoparticles
High-susceptibility nanoparticles for micro-inductor core materials
According to the laws of magnetism, the shape of magnetically soft objects
limits the effective susceptibility. For example, spherical soft magnets cannot
display an effective susceptibility larger than 3. Although true for
macroscopic multi-domain magnetic materials, we show that magnetic
nanoparticles in a single-domain state do not suffer from this limitation. This
is a consequence of the particle moment being forced to saturation by the
predominance of exchange forces, and only allowed to rotate coherently in
response to thermal and/or applied fields. We apply statistical mechanics to
determine the static and dynamic susceptibility of single-domain particles as a
function of size, temperature and material parameters. Our calculations reveal
that spherical single-domain particles with large saturation magnetisation and
small magneto-crystalline anisotropy, e.g. FeNi particles, can have very a
large susceptibility of 200 or more. We further show that susceptibility and
losses can be tuned by particle easy axis alignment with the applied field in
case of uniaxial anisotropy, but not for particles with cubic anisotropy. Our
model is validated experimentally by comparison with measurements on
nanocomposites containing spherical 113 nm -FeO particles
up to 45 vol% finely dispersed in a polymer matrix. In agreement with the
calculations for this specific material, the measured susceptibility of the
composites is up to 17 (3) and depends linearly on the volume fraction of
particles. Based on these results, we predict that nanocomposites of 30 vol% of
superparamagnetic FeNi particles in an insulating non-magnetic matrix can have
a sufficiently large susceptibility to be used as micro-inductor core materials
in the MHz frequency range, while maintaining losses below state-of-the-art
ferrites.Comment: 10 pages, 5 figures, 1 table, 20 numbered equation
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