80 research outputs found
Direct visualization of dynamic magnetic coupling in a Co/Py bilayer with picosecond and nanometer resolution
We present a combination of ferromagnetic resonance (FMR) with spatially and
time-resolved X-ray absorption spectroscopy in a scanning transmission X-ray
microscope (STXM-FMR). The transverse high frequency component of the
resonantly excited magnetization is measured with element-specifity in a
Permalloy (Py) disk - Cobalt (Co) stripe bilayer microstructure. STXM-FMR
mappings are snapshots of the local magnetization-precession with nm spatial
resolution and ps temporal resolution. We directly observe the transfer of
angular momentum from Py to Co and vice versa at their respective
element-specific resonances. A third resonance could be observed in our
experiments, which is identified as a coupled resonance of Py and Co.Comment: Version submitted to Physical Review Applied with updated author list
and supplemental information (Ancillary file
Unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling
We report strong unidirectional anisotropy in bulk polycrystalline B20 FeGe
measured by ferromagnetic resonance spectroscopy. Bulk and micron-sized samples
were produced and analytically characterized. FeGe is a B20 compound with
inherent Dzyaloshinskii-Moriya interaction. Lorenz microscopy confirms a
skyrmion lattice at in a magnetic field of 150 mT.
Ferromagnetic resonance was measured at ,
near the Curie temperature. Two resonance modes were observed, both exhibit a
unidirectional anisotropy of in
the primary, and in the secondary
mode, previously unknown in bulk ferromagnets. Additionally, about 25 standing
spin wave modes are observed inside a micron-sized FeGe wedge, measured at room
temperature ( K). These modes also exhibit unidirectional
anisotropy
Inertial effects in ultrafast spin dynamics
The dynamics of magnetic moments consist of a precession around the magnetic
field direction and a relaxation towards the field to minimize the energy.
While the magnetic moment and the angular momentum are conventionally assumed
to be parallel to each other, at ultrafast time scales their directions become
separated due to inertial effects. The inertial dynamics give rise to
additional high-frequency modes in the excitation spectrum of magnetic
materials. Here, we review the recent theoretical and experimental advances in
this emerging topic and discuss the open challenges and opportunities in the
detection and the potential applications of inertial spin dynamics.Comment: 11 pages, 8 figure
Enhanced biomedical heat-triggered carriers via nanomagnetism tuning in ferrite-based nanoparticles
Biomedical nanomagnetic carriers are getting a higher impact in therapy and
diagnosis schemes while their constraints and prerequisites are more and more
successfully confronted. Such particles should possess a well-defined size
with minimum agglomeration and they should be synthesized in a facile and
reproducible high-yield way together with a controllable response to an
applied static or dynamic field tailored for the specific application. Here,
we attempt to enhance the heating efficiency in magnetic particle hyperthermia
treatment through the proper adjustment of the core–shell morphology in
ferrite particles, by controlling exchange and dipolar magnetic interactions
at the nanoscale. Thus, core–shell nanoparticles with mutual coupling of
magnetically hard (CoFe2O4) and soft (MnFe2O4) components are synthesized with
facile synthetic controls resulting in uniform size and shell thickness as
evidenced by high resolution transmission electron microscopy imaging,
excellent crystallinity and size monodispersity. Such a magnetic coupling
enables the fine tuning of magnetic anisotropy and magnetic interactions
without sparing the good structural, chemical and colloidal stability.
Consequently, the magnetic heating efficiency of CoFe2O4 and MnFe2O4
core–shell nanoparticles is distinctively different from that of their
counterparts, even though all these nanocrystals were synthesized under
similar conditions. For better understanding of the AC magnetic hyperthermia
response and its correlation with magnetic-origin features we study the effect
of the volume ratio of magnetic hard and soft phases in the bimagnetic
core−shell nanocrystals. Eventually, such particles may be considered as novel
heating carriers that under further biomedical functionalization may become
adaptable multifunctional heat-triggered nanoplatforms
FePt icosahedra with magnetic cores and catalytic shells
Surprisingly oxidation resistant icosahedral FePt nanoparticles showing hard-magnetic properties have been fabricated by an inert-gas condensation method with in-flight annealing. High-resolution transmission electron microscopy (HRTEM) images with sub-Angstrom resolution of the nanoparticle have been obtained with focal series reconstruction, revealing noncrystalline nature of the nanoparticle. Digital dark-field method combined with structure reconstruction as well as HRTEM simulations reveal that these nanoparticles have icosahedral structure with shell periodicity. Localized lattice relaxations have been studied by extracting the position of individual atomic columns with a precision of about (0.002 nm. The lattice spacings of (111) planes from the surface region to the center of the icosahedra are found to decrease exponentially with shell numbers. Computational studies and energy-filtered transmission electron microscopy analyses suggest that a Pt-enriched surface layer is energetically favored and that site-specific vacancies are formed at the edges of facettes, which was experimentally observed. The presence of the Pt-enriched shell around an Fe/Pt core explains the environmental stability of the magnetic icosahedra and strongly reduces the exchange coupling between neighboring particles, thereby possibly providing the highest packing density for future magnetic storage media based on FePt nanoparticles
Effect of High-Pressure Torsion on the Microstructure and Magnetic Properties of Nanocrystalline CoCrFeNiGax (x = 0.5, 1.0) High Entropy Alloys
In our search for an optimum soft magnet with excellent mechanical properties which can be used in applications centered around “electro mobility”, nanocrystalline CoCrFeNiGax (x = 0.5, 1.0) bulk high entropy alloys (HEA) were successfully produced by spark plasma sintering (SPS) at 1073 K of HEA powders produced by high energy ball milling (HEBM). SPS of non-equiatomic CoCrFeNiGa₀.₅ particles results in the formation of a single-phase fcc bulk HEA, while for the equiatomic CoCrFeNiGa composition a mixture of bcc and fcc phases was found. For both compositions SEM/EDX analysis showed a predominant uniform distribution of the elements with only a small number of Cr-rich precipitates. High pressure torsion (HPT) of the bulk samples led to an increased homogeneity and a grain refinement: i.e., the crystallite size of the single fcc phase of CoCrFeNiGa₀.₅ decreased by a factor of 3; the crystallite size of the bcc and fcc phases of CoCrFeNiGa—by a factor of 4 and 10, respectively. The lattice strains substantially increased by nearly the same extent. After HPT the saturation magnetization (Ms) of the fcc phase of CoCrFeNiGa₀.₅ and its Curie temperature increased by 17% (up to 35 Am²/kg) and 31.5% (from 95 K to 125 K), respectively, whereas the coercivity decreased by a factor of 6. The overall Ms of the equiatomic CoCrFeNiGa decreased by 34% and 55% at 10 K and 300 K, respectively. At the same time the coercivity of CoCrFeNiGa increased by 50%. The HPT treatment of SPS-consolidated HEAs increased the Vickers hardness (Hv) by a factor of two (up to 5.632 ± 0.188) only for the non-equiatomic CoCrFeNiGa₀.₅, while for the equiatomic composition, the Hv remained unchanged (6.343–6.425 GPa)
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