arXiv subjects

Maria del Puerto Morales

Publications and source records attributed to Maria del Puerto Morales.

4 recordsLinked to original sources

arXiv2025

Magnetic Dissipation in Ferrofluids

Ferrofluids, composed of magnetic nanoparticles suspended in a non-magnetic carrier liquid, have attracted considerable attention since their discovery in the 1960s. Their combination of liquid and magnetic properties gives rise to complex behaviors and unique functionalities, enabling a wide range of technological applications. Among these is the ability of the magnetic material to be moved by and to absorb heat when exposed to an external magnetic field -- a process that can occur through various dissipation mechanisms depending on the system. A detailed understanding of these mechanisms is crucial for tailoring materials to specific applications. We provide a comprehensive overview of the theoretical principles underlying different energy dissipation processes and propose a coherent framework for their interpretation. Particular attention is devoted to describing the frequency-dependent susceptibility, which is the key parameter to describe dissipation. We demonstrate that dissipation, predicted from magnetometry-based studies, matches well with direct, frequency-dependent calorimetric results, expanding the available frequency range of the characterization. The demonstrating measurements were carried out with a dilute ferrofluid containing magnetite nanoparticles of a mean diameter of 10.6 nm.

cond-mat.mtrl-sci↗

arXiv2024

Incidence of the Brownian relaxation process on the magnetic properties of ferrofluids

Ferrofluids containing magnetic nanoparticles represent a special class of magnetic materials due to the added freedom of particle tumbling in the fluids. We studied this process, known as Brownian relaxation, and its effect on the magnetic properties of ferrofluids with controlled magnetite nanoparticle sizes. For small nanoparticles (below 10 nm diameter) the Néel process is expected to dominate the magnetic response, whereas for larger particles, Brownian relaxation becomes important. Temperature- and magnetic field-dependent magnetization studies, differential scanning calorimetry, and AC susceptibility measurements were carried out for 6, 8, 10.6, and 13.5 nm diameter magnetite nanoparticles suspended in water. We identify clear fingerprints of the Brownian relaxation for the sample of the large diameter nanoparticles as both magnetic and thermal hysteresis develop at the water freezing temperature, whereas the samples of small diameter nanoparticles remain hysteresis-free down to the magnetic blocking temperature. This is supported by the temperature-dependent AC susceptibility measurements: above 273 K, the data show a low-frequency Debye peak, which is characteristic of the Brownian relaxation. This peak vanishes below 273 K.

cond-mat.mtrl-sci↗

arXiv2009

Gold Nanoparticles Generated in Ethosome Bilayers, As Revealed by Cryo-Electron-Tomography

Gold nanoparticles have been synthesized inside ethosomes, vesicles composed of phospholipid, ethanol and water, which could be very efficient not only in delivery probes to the skin but also as diagnostic and therapeutic multimodal agents. High efficiency encapsulation of gold nanoparticles is achieved by a simple strategy: the nanoparticles synthesis occurs simultaneously with the ethosomes formation, in the absence of any undesirable reducing agents. A three-dimensional reconstruction of a gold-embedded ethosome generated by cryoelectron tomography reveals that the gold particle is localized inside the lipid bilayer, leaving the ethosome surface and core free for further functionalization. The resulting gold nanoparticles are homogeneous in size and shape and, depending on synthesis temperature, the size ranges from 10 to 20 nm, as revealed by TEM. The ethosome-nanoparticles hybrids size has been investigated by means of dynamic light scattering and has been found to vary with temperature and gold salt concentration from 700 to 400 nm. Gold nanoparticles encapsulated ethosomes offer a versatile platform for the enhancement of pharmacological efficacy in transdermal and dermal delivery systems.

physics.chem-ph↗

arXiv2008

Surface anisotropy broadening of the energy barrier distribution in magnetic nanoparticles

The effect of surface anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the $T\ln(t/τ_0)$ scaling approach. The comparison between the distributions of the anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core anisotropy, and of the total anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable $T\ln(t/τ_0)$, enables the determination of the surface anisotropy as a function of the particle size. We show that the contribution of the particle surface to the total anisotropy energy can be well described by a size--independent value of the surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe$_{3-x}$O$_{4}$ particles of 4.9 nm in average size and $x$ about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective surface anisotropy constant $K_{s}=2.9\times 10^{-2}$ erg cm$^{-2}$ gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.

cond-mat.mtrl-sci↗