Control of Anisotropy and Magnetic Hyperthermia Effect by Addition of Cobalt on Magnetite Nanoparticles.

Magnetic hyperthermia (MH) has emerged as a promising technology with diverse applications in medical and technological fields, leveraging the remote induction of temperature elevation through an alternating magnetic field. While Fe3O4 nanoparticles with an average size around 12-25 nm are commonly employed in MH systems, this study introduces a strategy to produce smaller particles (less than or equal to 10 nm) with enhanced heating efficiency, as measured by specific power absorption (SPA). We conducted an exhaustive and detailed investigation into the morphological and magnetic properties of CoxFe3-xO4 nanoparticles, aiming to optimize their MH response. By varying the Co content, we successfully tuned the effective magnetic anisotropy while maintaining saturation magnetization nearly constant. The MH analysis indicates that these nanoparticles predominantly heat through the Néel mechanism, demonstrating robust reproducibility across different concentrations, viscosity mediums, and ac field conditions. Notably, we identified an optimal anisotropy or Co concentration that maximizes SPA, crucial for developing magnetic systems requiring particles with specific sizes. This work contributes to advancing the understanding and application of MH, particularly in tailoring nanoparticle properties for targeted and efficient heat generation in various contexts.

Annealing effects on the magnetic and magnetotransport properties of iron oxide nanoparticles self-assemblies.

In magnetic tunnel junctions based on iron oxide nanoparticles the disorder and the oxidation state of the surface spin as well as the nanoparticles functionalization play a crucial role in the magnetotransport properties. In this work, we report a systematic study of the effects of vacuum annealing on the structural, magnetic and transport properties of self-assembled ∼10 nm Fe3O4nanoparticles. The high temperature treatment (from 573 to 873 K) decomposes the organic coating into amorphous carbon, reducing the electrical resistivity of the assemblies by 4 orders of magnitude. At the same time, the 3.Fe2+/(Fe3++Fe2+) ratio is reduced from 1.11 to 0.13 when the annealing temperature of the sample increases from 573 to 873 K, indicating an important surface oxidation. Although the 2 nm physical gap remains unchanged with the thermal treatment, a monotonous decrease of tunnel barrier width was obtained from the electron transport measurements when the annealing temperature increases, indicating an increment in the number of defects and hot-spots in the gap between the nanoparticles. This is reflected in the reduction of the spin dependent tunneling, which reduces the interparticle magnetoresistance. This work shows new insights about influence of the nanoparticle interfacial composition, as well their the spatial arrangement, on the tunnel transport of self-assemblies, and evidence the importance of optimizing the nanostructure fabrication for increasing the tunneling current without degrading the spin polarized current.

Manganese silicide nanowires via metallic flux nanonucleation: growth mechanism and temperature-dependent resistivity.

Mn5Si3nanowires are believed to be the building blocks of the newest trends of flexible and stretchable devices in nanoelectronics. In this context , growing Mn5Si3nanowires, as well as characterizing their electronic transport properties provide insight into their phenomenology. In this work, we report on the growth mechanism of Mn5Si3nanowires produced by the metallic flux nanonucleation method, as well as the resistivity measurements of these nanostructures. Our calculation allows us, by using the Washburn equation for pore infiltration, to give a guess on why we obtain Mn-rich nanowires. In addition, some morphological aspects of the diameter-modulated Mn5Si3nanowires were discussed based on the classical nucleation theory. From the resistivity measurements for the smallest diameter among the nanowires, we observed a significant reduction of around 37% of the phonons characteristic temperature by fitting the Bloch-Grünesein formula with other sources of scattering. Our results lead to a better understanding on the recent metallic flux nanonucleation growth method, as well as going a step further into the electronic transport properties of the Mn5Si3nanowires.

Adjusting the Néel relaxation time of Fe3O4/Zn x Co1-x Fe2O4 core/shell nanoparticles for optimal heat generation in magnetic hyperthermia.

In this work it is shown a precise way to optimize the heat generation in high viscosity magnetic colloids, by adjusting the Néel relaxation time in core/shell bimagnetic nanoparticles, for magnetic fluid hyperthermia (MFH) applications. To pursue this goal, Fe3O4/Zn x Co1-x Fe2O4 core/shell nanoparticles were synthesized with 8.5 nm mean core diameter, encapsulated in a shell of ∼1.1 nm of thickness, where the Zn atomic ratio (Zn/(Zn + Co) at%) changes from 33 to 68 at%. The magnetic measurements are consistent with a rigid interface coupling between the core and shell phases, where the effective magnetic anisotropy systematically decreases when the Zn concentration increases, without a significant change of the saturation magnetization. Experiments of MFH of 0.1 wt% of these particles dispersed in water, in Dulbecco modified Eagles minimal essential medium, and a high viscosity butter oil, result in a large specific loss power (SLP), up to 150 W g-1, when the experiments are performed at 571 kHz and 200 Oe. The SLP was optimized adjusting the shell composition, showing a maximum for intermediate Zn concentration. This study shows a way to maximize the heat generation in viscous media like cytosol, for those biomedical applications that require smaller particle sizes.

Adjusting the Néel relaxation time of Fe3O4/ZnxCo1-xFe2O4 core/shell nanoparticles for optimal heat generation in magnetic hyperthermia.

In this work it is shown a precise way to optimize the heat generation in high viscosity magnetic colloids, by adjusting the Néel relaxation time in core/shell bimagnetic nanoparticles, for Magnetic Fluid Hyperthermia applications. To pursue this goal, Fe3O4/ZnxCo1-xFe2O4 core/shell nanoparticles were synthesized with 8.5 nm mean core diameter, encapsulated in a shell of ~1.1 nm of thickness, where the Zn atomic ratio (Zn/(Zn+Co) at%) changes from 33 at% to 68 at%. The magnetic measurements are consistent with a rigid interface coupling between the core and shell phases, where the effective magnetic anisotropy systematically decreases when the Zn concentration increases, without a significant change of the saturation magnetization. Experiments of magnetic fluid hyperthermia of 0.1 wt% of these particles dispersed in water, DMEM (Dulbecco modified Eagles minimal essential medium) and a high viscosity butter oil, result in a large specific loss power (SLP), up to 150 W/g, when the experiments are performed at 571 kHz and 200 Oe. The SLP was optimized adjusting the shell composition, showing a maximum for intermediate Zn concentration. This study shows a way to maximize the heat generation in viscous media like cytosol, for those biomedical applications that requiere smaller particle sizes .

Controlling the dominant magnetic relaxation mechanisms for magnetic hyperthermia in bimagnetic core-shell nanoparticles.

We report a simple and effective way to control the heat generation of a magnetic colloid under alternate magnetic fields by changing the shell composition of bimagnetic core-shell Fe3O4/ZnxCo1-xFe2O4 nanoparticles. The core-shell structure constitutes a magnetically-coupled biphase system, with an effective anisotropy that can be tuned by the substitution of Co2+ by Zn2+ ions in the shell. Magnetic hyperthermia experiments of nanoparticles dispersed in hexane and butter oil showed that the magnetic relaxation is dominated by Brown relaxation mechanism in samples with higher anisotropy (i.e., larger concentration of Co within the shell) yielding high specific power absorption values in low viscosity media as hexane. Increasing the Zn concentration of the shell, diminishes the magnetic anisotropy, which results in a change to a Néel relaxation that dominates the process when the nanoparticles are dispersed in a high-viscosity medium. We demonstrate that tuning the Zn contents at the shell of these exchange-coupled core/shell nanoparticles provides a way to control the magnetic anisotropy without loss of saturation magnetization. This ability is an essential prerequisite for most biomedical applications, where high viscosities and capturing mechanisms are present.

A noção de tarefa, pré-tarefa e trabalho na teoria de E. Pichon-Rivière / Les notions de tâche, pré-tâche et travail dans la théorie d'E. Pichon-Rivière / The concepts of task, pretask and work in the theory of E. Pichon-Rivière

Segundo a perspectiva de Pichon-Rivière, não é fácil distinguir "tarefa" de "trabalho", já que essas duas noções ocupam um lugar fundamental nesse quadro conceitual. A tarefa é o encaminhamento de um grupo ou de um sujeito em direção a seus objetivos; ela é, ao mesmo tempo, uma práxis e uma trajetória de caráter multidimensional e dialética. O trabalho, assim como a tarefa, é também uma atividade que permite organizar as subjetividades individuais e coletivas, mas as duas noções comportam entre si diferenças importantes. A pré-tarefa é um modo de subjetividade (pensar, sentir e agir) oposto ao da tarefa: ela exprime uma resistência à mudança. Se o trabalho, muitas vezes, é alienante e reprodutor da desigualdade social, ele é também, de forma paradoxal, um espaço de fortalecimento da identidade e de satisfação das necessidades de reconhecimento e de realização pessoal e social, o que o aproxima da tarefa, atividade que é, por definição, saudável.(AU)

Il n'est pas évident de distinguer la "tâche" du "travail" en suivant la perspective de Pichon-Rivière, car ces deux notions prennent, dans ce cadre conceptuel, une place fondamentale. La tâche est l'acheminement d'un groupe ou d'un individu vers ses objectifs; c'est à la fois une praxis et une trajectoire à caractère multidimensionnel et dialectique. Le travail, de même que la tâche, est aussi une activité permettant d'organiser les subjectivités individuelles et collectives, mais ces deux notions comportent également des différences importantes. La pré-tâche est une forme de subjectivité (penser, sentir et agir) opposée à celle de la tâche: elle exprime une résistance au changement. Alors que travail, tout en étant souvent aliénant et reproducteur de l'inégalité sociale, constitue fréquemment et de manière paradoxale, un espace de renforcement de l'identité et de satisfaction des besoins de reconnaissance et de réalisation personnelle et sociale, ce qui le rapproche de la tâche, activité, par définition, salutaire.(AU)

It is not easy to differentiate "task" and "work" from the perspective of Pichon-Rivière's conceptual framework, because both concepts have a fundamental place within his theory. The task is the march of a group or one subject towards their goal; it is a praxis or trajectory that can be characterized as multidimensional and dialectical. The work, like the task, is an activity that organizes individual and collective subjectivities, but there are important differences between them. The pre-task is a form of subjectivity (thinking, feeling and acting) opposed to the task: it expresses a resistance to change. Although frequently alienated and replicator of social inequality, work often generates - paradoxically - a strengthening of identity, satisfaction of acknowledgment needs and personal and social development. For these reason, work can be assimilated to a task, activity that is, by definition, healthy.(AU)

A noção de tarefa, pré-tarefa e trabalho na teoria de E. Pichon-Rivière / Les notions de tâche, pré-tâche et travail dans la théorie d'E. Pichon-Rivière / The concepts of task, pretask and work in the theory of E. Pichon-Rivière

Segundo a perspectiva de Pichon-Rivière, não é fácil distinguir "tarefa" de "trabalho", já que essas duas noções ocupam um lugar fundamental nesse quadro conceitual. A tarefa é o encaminhamento de um grupo ou de um sujeito em direção a seus objetivos; ela é, ao mesmo tempo, uma práxis e uma trajetória de caráter multidimensional e dialética. O trabalho, assim como a tarefa, é também uma atividade que permite organizar as subjetividades individuais e coletivas, mas as duas noções comportam entre si diferenças importantes. A pré-tarefa é um modo de subjetividade (pensar, sentir e agir) oposto ao da tarefa: ela exprime uma resistência à mudança. Se o trabalho, muitas vezes, é alienante e reprodutor da desigualdade social, ele é também, de forma paradoxal, um espaço de fortalecimento da identidade e de satisfação das necessidades de reconhecimento e de realização pessoal e social, o que o aproxima da tarefa, atividade que é, por definição, saudável...

Il n'est pas évident de distinguer la "tâche" du "travail" en suivant la perspective de Pichon-Rivière, car ces deux notions prennent, dans ce cadre conceptuel, une place fondamentale. La tâche est l'acheminement d'un groupe ou d'un individu vers ses objectifs; c'est à la fois une praxis et une trajectoire à caractère multidimensionnel et dialectique. Le travail, de même que la tâche, est aussi une activité permettant d'organiser les subjectivités individuelles et collectives, mais ces deux notions comportent également des différences importantes. La pré-tâche est une forme de subjectivité (penser, sentir et agir) opposée à celle de la tâche: elle exprime une résistance au changement. Alors que travail, tout en étant souvent aliénant et reproducteur de l'inégalité sociale, constitue fréquemment et de manière paradoxale, un espace de renforcement de l'identité et de satisfaction des besoins de reconnaissance et de réalisation personnelle et sociale, ce qui le rapproche de la tâche, activité, par définition, salutaire...

It is not easy to differentiate "task" and "work" from the perspective of Pichon-Rivière's conceptual framework, because both concepts have a fundamental place within his theory. The task is the march of a group or one subject towards their goal; it is a praxis or trajectory that can be characterized as multidimensional and dialectical. The work, like the task, is an activity that organizes individual and collective subjectivities, but there are important differences between them. The pre-task is a form of subjectivity (thinking, feeling and acting) opposed to the task: it expresses a resistance to change. Although frequently alienated and replicator of social inequality, work often generates - paradoxically - a strengthening of identity, satisfaction of acknowledgment needs and personal and social development. For these reason, work can be assimilated to a task, activity that is, by definition, healthy...