Here, we report an experimental study on the rotational dynamics of hard magnetic hexaferrite nanoparticles in water. A stable aqueous colloid of SrFe12O19 was synthesized by the borate glass-ceramic dissolution technique and studied by TEM, small-angle X-ray scattering (SAXS), magnetometry, and optical transmission methods in applied DC and AC magnetic fields. The particles represent nanoplates with a mean diameter of 50 nm and a mean thickness of 5 nm having a coercive force of 4700 Oe and a saturation magnetization of 51.5 emu/g. According to magnetic field dependent SAXS data, a probability orientation function was suggested considering the colloidal particle rotation in the applied field as an activation-free process. The magnetization dynamics of the colloidal rotators was described by an interplay of magnetic torque and drag force in the frame of a non-interacting particle model. At frequencies below 100 Hz, the particles are able to fully rotate after the magnetic field. At higher frequencies, the complete following becomes impossible due to the energy dissipation and the particle movement changes to oscillations around randomly oriented axes. These vibrational axes can be aligned by a strong external permanent magnetic field, causing the coherent particle oscillations and correspondingly the rise of the high-frequency response of the colloid optical transmission. As a result, the efficient magneto-optical light modulation has been achieved at frequencies exceeding 5 kHz, revealing the fastest response rates among known colloidal magneto-optical media.
Magnetically hard ferrites attract considerable interest due to their ability to maintain a high coercivity of nanosized particles and therefore show promising applications as nanomagnets ranging from magnetic recording to biomedicine. Herein, we report an approach to prepare nonsintered single-domain nanoparticles of chromium-substituted hexaferrite via crystallization of glass in the system SrO–Fe2O3–Cr2O3–B2O3. We have observed a formation of plate-like hexaferrite nanoparticles with diameters changing from 20 to 190 nm depending on the annealing temperature. We demonstrated that chromium substitution led to a significant improvement of the coercivity, which varied from 334 to 732 kA m−1 for the smallest and the largest particles, respectively. The results provide a new strategy for producing high-coercivity ferrite nanomagnets.
Spray deposition is a scalable and
cost-effective technique for
the fabrication of magnetic hybrid films containing diblock copolymers
(DBCs) and magnetic nanoparticles. However, it is challenging to obtain
spray-deposited anisotropic magnetic hybrid films without using external
magnetic fields. In the present work, spray deposition is applied
to prepare perpendicular anisotropic magnetic hybrid films by controlling
the orientation of strontium hexaferrite nanoplatelets inside ultra-high-molecular-weight
DBC polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films. During spray deposition, the evolution of
DBC morphology and the orientation of magnetic nanoplatelets are monitored
with in situ grazing-incidence small-angle X-ray scattering (GISAXS).
For reference, a pure DBC film without nanoplatelets is deposited
with the same conditions. Solvent-controlled magnetic properties of
the hybrid film are proven with solvent vapor annealing (SVA) applied
to the final deposited magnetic films. Obvious changes in the DBC
morphology and nanoplatelet localization are observed during SVA.
The superconducting quantum interference device data show that ferromagnetic
hybrid polymer films with high coercivity can be achieved via spray
deposition. The hybrid films show a perpendicular magnetic anisotropy
before SVA, which is strongly weakened after SVA. The spray-deposited
hybrid films appear highly promising for potential applications in
magnetic data storage and sensors.
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