The dynamic magnetization of immobilized spherical single-domain magnetic nanoparticles (MNPs) with uniaxial or cubic magnetocrystalline anisotropy was studied computationally by executing simulations based on the Landau–Lifshitz–Gilbert (LLG) equation. For situations when a static magnetic field was suddenly applied and then removed, the effects of particle diameter and anisotropy (considering both type of symmetry and characteristic energy) on the characteristic magnetic relaxation time were studied parametrically. The results, for both anisotropy symmetries, show that when a static magnetic field is suddenly turned on or off the MNPs undergo a successive two-step or combined one-step relaxation. Whether a MNP relaxes with one or two steps when the field is turned on is determined by the competition between the energy of the applied magnetic field, the magnetic anisotropy energy, and thermal energy. When the applied magnetic field is suddenly turned off, our results show good agreement with theoretical predictions for the cases of and , where represents the magnetic anisotropy energy barrier, is the Boltzmann constant and represents the absolute temperature. For the case of an applied alternating magnetic field (AMF) that is typical of magnetic particle imaging (MPI) applications, the effects of particle diameter and anisotropy symmetry were studied in terms of time-domain magnetization dynamics, dynamic hysteresis loops, harmonic spectra, and x-space point spread functions (PSFs). Results illustrate that the type of magnetocrystalline anisotropy (uniaxial versus cubic) has a significant effect on the MPI performance of the nanoparticles. These computational studies provide insight into the role of particle diameter and magnetic anisotropy on the performance of MNPs for applications in magnetorelaxometry and MPI.

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Morphology and culture studies on germlings of Sargassum thunbergii (Mertens et Roth) Kuntze were carried out under controlled laboratory conditions. Growth characteristics of these germlings grown under different temperatures (from 10 to 25°C), irradiances (from 9 to 88 μmol photons m −2 s −1 ), and under blue and white light conditions are described. The development of embryonic germlings follows the classic "8 nuclei 1 egg" type described for Sargassaceae. Fertilized eggs spent 5-6 h developing into multicellular germlings with abundant rhizoids after fertilization. Under conditions of 20°C, 44 μmol photons m −2 s −1 and photoperiod of 12 h, young germlings with one or two leaflets reached 2-3 mm in length after 8 weeks. Temperature variations (10, 15, 20, 25°C) under 88 μmol photons m −2 s −1 significantly influenced the growth rate within the first week, although this effect became less obvious after 8 weeks, especially at 15 and 20°C. Variation in germling growth was highly significant under different irradiances (9, 18, 44, 88 μmol photons m −2 s −1 ) at 25°C. Low temperature (10°C) reduced germling growth. Growth of germlings cultured under blue light was lower than in white light. Optimal growth of these germlings occurred at 25°C and 44 μmol photons m −2 s −1 .

The effect of interparticle interactions on the magnetization dynamics and energy dissipation rates of spherical single-domain magnetically blocked nanoparticles in static and alternating magnetic fields (AMFs) was studied using Brownian dynamics simulations. For the case of an applied static magnetic field, simulation results suggest that the effective magnetic diameter of interacting nanoparticles determined by fitting the equilibrium magnetization of the particles to the Langevin function differs from the actual magnetic diameter used in the simulations. Parametrically, magnetorelaxometry was studied in simulations where a static magnetic field was suddenly applied or suppressed for various strengths of magnetic interactions. The results show that strong magnetic interactions result in longer chain-like particle aggregates and eventually longer characteristic relaxation time of the particles. For the case of applied AMF with and without a static bias magnetic field, the magnetic response of interacting nanoparticles was analyzed in terms of the harmonic spectrum of particle magnetization and dynamic hysteresis loops, whereas the energy dissipation of the particles was studied in terms of the calculated specific absorption rate (SAR). Results suggest that the effect of magnetic interactions on the SAR varies significantly depending on the amplitude and frequency of the AMF and the intensity of the bias field. These computational studies provide insight into the role of particle–particle interactions on the performance of magnetic nanoparticles for applications in magnetic hyperthermia and magnetic particle imaging.

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The behavior of spherical single-domain magnetic nanoparticles in strong inhomogeneous magnetic fields is investigated through Brownian dynamics simulations, taking into account magnetic dipole–dipole interactions, repulsive hard-core Yukawa potential, hydrodynamic particle-wall interactions, and the mechanism of magnetic dipole rotation in the presence of a magnetic field. The magnetic capture process of nanoparticles in prototypical magnetic field gradients generated by a sudden reversal in perpendicular magnetization of a flat substrate (defining a “capture line”) is studied as a function of strength of the magnetic field and volume fraction of the magnetic nanoparticles. Capture curves show a regime where capture follows a power law model and suggest that particles with the Brownian relaxation mechanism are captured at a slightly faster rate than particles with the Néel relaxation mechanism under similar conditions of the field gradient. Additionally, evaluation of the shape of the aggregates of captured particles suggests that greater dipole–dipole interactions result in aggregate structures that are flatter/wider than in the case of negligible dipole–dipole interactions. These results can help guide the design of systems for magnetically directed assembly of nanoparticles into complex shapes at a substrate.