61 research outputs found
Observation of superspin glass state in magnetically textured ferrofluid (gamma-Fe2O3)
Magnetic properties in a magnetically textured ferrofluid made out of
interacting maghemite (gamma-Fe2O3) nanoparticles suspended in glycerin have
been investigated. Despite the loss of uniform distribution of anisotropy axes,
a superspin glass state exists at low temperature in a concentrated, textured
ferrofluid as in the case of its non-textured counterpart. The onset of
superspin glass state was verified from the sample's AC susceptibility. The
influence of the anisotropy axis orientation on the aging behavior in the
glassy states is also discussed
Noise Measurement of Interacting Ferromagnetic Particles with High Resolution Hall Microprobes
We present our first experimental determination of the magnetic noise of a
superspinglass made of < 1 pico-liter frozen ferrofluid. The measurements were
performed with a local magnetic field sensor based on Hall microprobes operated
with the spinning current technique. The results obtained, though preliminary,
qualitatively agree with the theoretical predictions of Fluctuation-Dissipation
theorem (FDT) violation [1].Comment: 4pages, 2 figure
Superspin glass aging behavior in textured and nontextured frozen ferrofluid
The effect of magnetic anisotropy-axis alignment of individual nanoparticles
on the collective aging behavior in the superspin glass state of a frozen
ferrofluid has been investigated. The ferrofluid studied here consists of
maghemite nanoparticles (\gamma-Fe2O3, mean diameter = 8.6 nm) dispersed in
glycerin at a volume fraction of ~15%. The low temperature aging behavior has
been explored through 'zero-field cooled magnetization' (ZFCM) relaxation
measurements using SQUID magnetometry. The ZFCM response functions were found
to scale with effective age of the system in both textured and non-textured
superspin glass states, but with markedly different scaling exponents, \mu. The
value of {\mu} was found to shift from ~0.9 in non-textured case to ~ 0.6 in
the textured case, despite the identical cooling protocol used in both
experiments
Growth of a dynamical correlation length in an aging superspin glass
We report on zero field cooled magnetization relaxation experiments on a
concen- trated frozen ferrofluid exhibiting a low temperature superspin glass
transition. With a method initially developed for spin glasses, we investigate
the field dependence of the relaxations that take place after different aging
times. We extract the typical number of correlated spins involved in the aging
dynamics. This brings important insights into the dynamical correlation length
and its time growth. Our results, consistent with expressions obtained for spin
glasses, extend the generality of these behaviours to the class of superspin
glasses. Since the typical flipping time is much larger for superspins than for
atomic spins, our experiments probe a time regime much closer to that of
numerical simulations
Anisotropy-axis orientation effect on the magnetization of {\gamma}-Fe2O3 frozen ferrofluid
The effect of magnetic anisotropy-axis alignment on the superparamagnetic
(SPM) and superspin glass (SSG) states in a frozen ferrofluid has been
investigated. The ferrofluid studied here consists of maghemite nanoparticles
(\gamma-Fe2O3, mean diameter = 8.6 nm) dispersed in glycerine at a volume
fraction of ~15%. In the high temperature SPM state, the magnetization of
aligned ferrofluid increased by a factor varying between 2 and 4 with respect
to that in the randomly oriented state. The negative interaction energy
obtained from the Curie-Weiss fit to the high temperature susceptibility in the
SPM states as well as the SSG phase onset temperature determined from the
linear magnetization curves were found to be rather insensitive to the
anisotropy axis alignment. The low temperature aging behaviour, explored via
"zero-field cooled magnetization" (ZFCM) relaxation measurements, however, show
distinct difference in the aging dynamics in the anisotropy-axis aligned and
randomly oriented SSG states.Comment: to appear in Journal of Physics D: Applied Physic
Exchange-bias and magnetic anisotropy fields in core–shell ferrite nanoparticles
Exchange bias properties of MnFe2O4@γ–Fe2O3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type. © 2021, The Author(s)
Forming of bioabsorbable clips using magnesium alloy strips with enhanced characteristics
The paper shows the feasibility of manufacturing surgical clips using bioabsorbable AZ31B magnesium alloy strips characterized by enhanced forming characteristics, and, at the same time, improved corrosion resistance to human fluids that make them suitable for temporary applications. The novel process chain used for the clip manufacturing includes an extrusion-cutting step, to fabricate strips with a peculiar shear texture and extremely refined microstructure, and a dedicated bending operation to achieve the clip final shape. The strip forming operation setup was numerically designed by a multi-level finite element approach to provide the clip required shape without fracture occurrence
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