Role of Dipolar Interactions on the Determination of the Effective Magnetic Anisotropy in Iron Oxide Nanoparticles

Abstract

Challenging magnetic hyperthermia (MH) applications of immobilized magnetic nanoparticles require detailed knowledge of the effective anisotropy constant (K eff ) to maximize heat release. Designing optimal MH experiments entails the precise determination of magnetic properties, which are, however, affected by the unavoidable concurrence of magnetic interactions in common experimental conditions. In this work, a mean-field energy barrier model (𝚫E), accounting for anisotropy (E A ) and magnetic dipolar (E D ) energy, is proposed and used in combination with AC measurements to a specifically developed model system of spherical magnetic nanoparticles with well-controlled silica shells, acting as a spacer between the magnetic cores. This approach makes it possible to experimentally demonstrate the mean field dipolar interaction energy prediction with the interparticle distance, d ij , E D ≈ 1/d ij 3 and obtain the E A as the asymptotic limit for very large d ij . In doing so, K eff uncoupled from interaction contributions is obtained for the model system (iron oxide cores with average sizes of 8.1, 10.2, and 15.3 nm) revealing to be 48, 23, and 11 kJ m −3 , respectively, close to bulk magnetite/maghemite values and independent from the specific spacing shell thicknesses selected for the study.

Influence of Controlled Dipolar Interaction for Polymer-Coated Gd-Doped Magnetite Nanoparticles toward Magnetic Hyperthermia Application

“…The interactions can be enhanced or weakened by aligning the moments in a specific direction depending on the desired outcome. Henceforth, understanding the role of controlling dipole–dipole interactions is essential for maximizing the SAR for MH application. − …”

Insights into the Magnetic Properties of Single-Core and Multicore Magnetite and Manganese-Doped Magnetite Nanoparticles

“…When comparing the curve measured at 5 and 250 K, high magnetization values are still observable at 250 K but with an almost negligible coercive field, which is indicative of the superparamagnetic behavior for such temperatures for all of the samples. The magnetization to saturation (M S ) and anisotropy constant (K) have been estimated for all of the samples from their curves measured at 5 K by fitting the data of the curves to the expression (5) where a is a constant, b = 8K 2 /105M S 2 , and c corresponds to the susceptibility at high applied magnetic field values. 39 The M S and K values are collected in Table 2 together with the coercive fields (H c ) for a measuring temperature of 5 K and the anisotropy field (H K ) that can be calculated for crystals with cubic symmetry through the expression,…”

“…Dipolar interactions occurring between moments of different particles favor the magnetic order, which usually results in an increase of the magnetic response and a decrease of the anisotropy and coercive field of the system . However, theoretical and experimental works have been also reported indicating an opposite effect of dipolar interactions on the energy barrier. , On the other hand, at low-temperature values, interactions occurring between canted spins on the surface of contiguous particles are more important and the material presents harder magnetic behavior. The magnetic behavior of interacting particles can be considered as a collective behavior for which the effective magnetic volume is a consequence of the different particle interactions and leads to a certain energy barrier distribution for magnetization. − …”

Multiwalled Carbon Nanotubes Decorated with Mn0.5Zn0.5Fe2O4 Nanoparticles for Magneto-Photothermal Cancer Therapy

“…Two types of hyperthermia depending on the stimulus used as a trigger, i.e ., optical or magnetic properties activated nanoheating agents. Magnetic hyperthermia (MHT) utilizes heat dissipation by MNPs to kill tumor cells when under an alternating magnetic field (AMF), − while photothermal therapy (PTT) employs photothermal agents that are activated by near-infrared laser (NIR) irradiation. − However, both magnetic hyperthermia and photothermal therapy have their limitations. , For instance, highly concentrated intratumoral injections of magnetic nanoparticles are required to compensate the low specific loss power (SLP) under low magnetic field intensity for the MHT, the real-time temperature monitoring is invasive thermometry, and the nanoparticles distribute non-uniformly within the tumor . Therefore, enhancing SLP value through modifications in the material properties, size, shape, and aggregation state of magnetic nanoparticles emerges as the pivotal factor for augmenting the effectiveness of magnetic hyperthermia. − In PTT, high-power laser exposure can damage surrounding healthy tissue, and some nanoparticles (gold and silver) used are not FDA-approved materials and may not meet biosafety requirements .…”