Enhancement of Neel Relaxation at Magnetic Heating Performance of Iron Oxide Nanoparticles

The study is based on understand the titanium (Ti) doping effect to enhance the Neel relaxation at magnetic heating performance of magnetite (Fe3O4). Ti doped magnetite ((Fe1-x,Tix)3O4; x= 0.02, 0.03 and 0.05) superparamagnetic nanoparticles were synthesized via sol-gel technique. The analyses were performed for (Fe1-x,Tix)3O4 and core-shell (SiO2 coated (Fe1-x,Tix)3O4) nanoparticles in order to understand the influence of silica coating on the magnetic properties of nanoparticles. The target of study to enhance the Neel relaxation mechanism on magnetic heating. The interparticle spacing and Ti amount were two parameters that we focused on the study. The results provided that coating with SiO2 has no specific effect on heating performance of (Fe1-x,Tix)3O4 nanoparticles. While the increase in temperature (ΔT) under 150 kHz RF signal reached up to 22oC in 10 minutes for SiO2 coated (Fe0.97,Ti0.03)3O4 nanoparticles, which was very close value of uncoated Fe3O4 nanoparticles.

Difference between Brownian and Néel relaxation mechanisms in a magnetizing field

Measurements of the dynamic susceptibility of a magnetic fluid based on cobalt ferrite particles stabilized in water by a double surfactant layer have been carried out. Cobalt ferrite, in comparison with magnetite, has a significantly higher energy of magnetic anisotropy. Therefore, for particles of cobalt ferrite, the Brownian mechanism of relaxation of magnetic moments is characteristic. The Debye (with a finite relaxation time) contribution to the dynamic susceptibility and the high-frequency (dispersionless) contribution are distinguished by constructing Cole-Cole diagrams. It was found that with an increase in the magnetizing field, the Debye contribution to the dynamic susceptibility decreases, while the high-frequency one (having a zero relaxation time) remains unchanged. The indicated property of the dynamic susceptibility of a fluid with a Brownian relaxation mechanism is radically different from the properties of the susceptibility of a fluid with Néel particles. Previously, measurements were made of the susceptibility of a fluid based on magnetite particles stabilized with oleic acid in kerosene. The magnetite particles have significantly lower anisotropy energy and are characterized by the predominance of the Néel relaxation mechanism. Turning on the magnetizing field caused a decrease in both the Debye part of the susceptibility and the high-frequency part of the susceptibility of magnetite particles.

Quantitative Analysis of the Specific Absorption Rate Dependence on the Magnetic Field Strength in ZnxFe3−xO4 Nanoparticles

Superparamagnetic ZnxFe3−xO4 magnetic nanoparticles (0 ≤ x < 0.5) with spherical shapes of 16 nm average diameter and different zinc doping level have been successfully synthesized by co-precipitation method. The homogeneous zinc substitution of iron cations into the magnetite crystalline structure has led to an increase in the saturation magnetization of nanoparticles up to 120 Am2/kg for x ~ 0.3. The specific absorption rate (SAR) values increased considerably when x is varied between 0 and 0.3 and then decreased for x ~ 0.5. The SAR values are reduced upon the immobilization of the nanoparticles in a solid matrix being significantly increased by a pre-alignment step in a uniform static magnetic field before immobilization. The SAR values displayed a quadratic dependence on the alternating magnetic field amplitude (H) up to 35 kA/m. Above this value, a clear saturation effect of SAR was observed that was successfully described qualitatively and quantitatively by considering the non-linear field’s effects and the magnetic field dependence of both Brown and Neel relaxation times. The Neel relaxation time depends more steeply on H as compared with the Brown relaxation time, and the magnetization relaxation might be dominated by the Neel mechanism, even for nanoparticles with large diameter.

Dynamics of interacting magnetic nanoparticles: effective behavior from competition between Brownian and Néel relaxation

We identify the influence of dipolar and steric interactions on the Brownian and Néel contributions to the magnetization dynamics of magnetic nanoparticles from extensive computer simulations using a combined Brownian dynamics/Monte-Carlo method.

Anisotropic self-assemblies of magnetic nanoparticles: experimental evidence of low-field deviation from the linear response theory and empirical model

Anisotropic assemblies of magnetic nanoparticles with a collective-interactive behavior that can be tuned by an alternating magnetic field amplitude display a cascade of unexpected physical effects and allow reformulation of Néel relaxation times.

Experimental Evaluation on the Heating Efficiency of Magnetoferritin Nanoparticles in an Alternating Magnetic Field

The superparamagnetic substance magnetoferritin is a potential bio-nanomaterial for tumor magnetic hyperthermia because of its active tumor-targeting outer protein shell, uniform and tunable nanosized inner mineral core, monodispersity and good biocompatibility. Here, we evaluated the heating efficiency of magnetoferritin nanoparticles in an alternating magnetic field (AMF). The effects of core-size, Fe concentration, viscosity, and field frequency and amplitude were investigated. Under 805.5 kHz and 19.5 kA/m, temperature rise (ΔT) and specific loss power (SLP) measured on magnetoferritin nanoparticles with core size of 4.8 nm at 5 mg/mL were 14.2 °C (at 6 min) and 68.6 W/g, respectively. The SLP increased with core-size, Fe concentration, AMF frequency, and amplitude. Given that: (1) the SLP was insensitive to viscosity of glycerol-water solutions and (2) both the calculated effective relaxation time and the fitted relaxation time were closer to Néel relaxation time, we propose that the heating generation mechanism of magnetoferritin nanoparticles is dominated by the Néel relaxation. This work provides new insights into the heating efficiency of magnetoferritin and potential future applications for tumor magnetic hyperthermia treatment and heat-triggered drug release.