Rotational dynamics of colloidal hexaferrite nanoplates

Abstract

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.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…The phase behavior of anisotropic colloids is very sensitive to applied fields such as electric or magnetic fields [53][54][55]. A field-induced isotropic to nematic transition can be observed in these suspensions.…”

“…With increasing elasticity of the gel, the transition to the nematic order occurred at progressively large value of the magnetic field. TR-SAXS was used to follow the rotational dynamics of anisotropic magnetic particles (e.g., hexaferrite platelets) in an alternating magnetic field [54] and explore the magneto-optical properties of the system.…”

Glass-Ceramic Synthesis of Cr-Substituted Strontium Hexaferrite Nanoparticles with Enhanced Coercivity

“…Thus, the hexaferrite nanoparticles are a reasonable material when nanomagnets are required for diverse purposes. For example, hexaferrites are attractive for high-density magnetic recording tape media [ 1 , 4 , 5 ] and low-frequency magnetic hyperthermia [ 6 ], as well as for magnetic colloids of hard magnetic particles, which are hugely different from traditional magnetite-based ferrofluids and demonstrate several unique properties [ 7 , 8 , 9 ]. Certainly, the colloidal hexafeFrrite nanoparticles are building blocks for the creation of magnetic nanostructures [ 10 , 11 ], nanocomposites [ 12 , 13 ], and various nanomaterials [ 14 , 15 , 16 , 17 , 18 ].…”

“…The most convenient are glasses in the SrO–Fe 2 O 3 –B 2 O 3 system, since the matrix is formed by borates, which are easily soluble in weakly acidic solutions. Consequently, the pure hexaferrite phase can be extracted in the form of fine powders or stable colloids [ 8 , 10 ].…”

In situ magnetorheological SANS setup at Institut Laue-Langevin

“…The rotational friction of magnetic nanorods is sensitive to the stiffness of hydrogels [21] or the viscosity of liquid samples [22]. Recording the concomitant orientational redistribution with AC susceptibility, optical transmission spectroscopy [23], depolarized dynamic light scattering [24], SAXS [25], or SANS [26] enables to extract hydrodynamic properties of the carrier medium.…”