Magnetochemistry

A significant growth of research on digital microfluidics has been achieved over the past several decades, and the field is still attracting increasing attention for fulfilling relevant mechanisms and potential applications. Numerous studies have been devoted to actively manipulating droplets in a variety of fundamental and applicational scenarios. In this work, the deformation of ferromagnetic fluid droplets is studied under an external uniform magnetic field. The droplets are precisely dispersed on the bottom surface of a container assembled with polymer methacrylate (PMMA) plates. Mineral oil is applied instead of air as the surrounding medium for easy stretching and preventing water solvent evaporation in ferrofluid. The design and processing of the container are firstly carried out to observe the shape and characterize the wettability of the droplets in the immiscible mineral oil medium. Furthermore, the droplets’ deformation and the working mechanism are given under the action of the horizontal uniform magnetic field. At different magnetic field intensities, the droplet is stretched in the horizontal direction parallel to the applied field. Due to volume conservation, the dimension in the height reduces correspondingly. With the coupling effect of magnetic force, viscous force and interfacial tension, the contact angle first increases with the magnetic field and then basically remains unchanged upon magnetization saturation. Consistent with the experimental results, the numerical method clearly reveals the field coupling mechanism and the nonlinear deformation of the sessile droplet. Full article

Magnetic tunnel junction (MTJ) sensors have been one of the excellent candidates for magnetic field detection due to their high sensitivity and compact size. In this paper, we design a magnetometer with in situ magnetic feedback consisting of an MTJ sensor. To analyze and evaluate the detectivity of the MTJ magnetometer, a noise model of the MTJ sensor in the magnetometer without magnetic feedback is first developed. Then, the noise model of the MTJ magnetometer with in situ magnetic feedback is also established, including the noises of the MTJ sensor and the signal conditioning circuit, as well as the feedback circuit. The equivalent noise model of the MTJ magnetometer with in situ magnetic feedback is evaluated through nonlinear fitting for the noise voltage spectrum. Although the noise generated by the MTJ sensor is much greater than that of the signal conditioning circuit, the noise introduced by the feedback coils into the MTJ sensor is slightly more than twice that generated by the MTJ sensor itself. The measurement results show that the detectivity of the MTJ magnetometer with in situ magnetic feedback reaches 526 pT/Hz^1/2 at 10 Hz. The equivalent noise analysis method presented in this paper is suitable for the detectivity analysis of magnetometers with magnetic feedback. Full article

Bimetallic FeCo and FeNi nanoparticles attract much attention due to their promising magnetic properties and a wide range of practical applications as recording and storage media, catalytic systems in fuel cells, supercapacitors, lithium batteries, etc. In this paper, we propose an original approach to the preparation of FeCo- and FeNi/N-doped carbon nanocomposites by means of a coupled process of frontal polymerization and thermolysis of molecular co-crystallized acrylamide complexes. The phase composition, structure, and microstructure of the resulting nanocomposites are studied using XRD, IR spectroscopy, elemental and thermal analysis, and electron microscopy data. The main magnetic characteristics of the synthesized nanocomposites, including the field dependences and the ZFC-FC curves peculiarities, are studied. It is shown that the obtained FeCo/N-C nanocomposites exhibit exchange bias behavior at low temperatures. In turn, FeNi/N-C nanocomposites are ferromagnetically ordered. Full article

To study the influence of sinter basicity on the microstructure, phase composition, and physicochemical and metallurgical properties, samples of agglomerates with different basicities were sintered and investigated. A comprehensive study of the structure, composition, chemical, and metallurgical properties of the sinter was conducted, and the optimum values for these properties were determined. The results of the mineralogical transformations that occurred during the sintering process are also presented. The magnetite contained in the concentrate partially dissolves in the silicate component and flux during agglomeration, forming a complex silicate SFCA with the general formula M₁₄O₂₀ (M–Ca, Si, Al, and Mg), which is the binder of the ore phases of the agglomerate. The proportion of ferrosilicates of calcium and aluminum in the sinter depends on the basicity of the sinter charge, and the morphology of the SFCA phase depends on the cooling rate of the sinter. The more CaO in the sinter charge, the more SFCA phase is formed in the sinter, and slow cooling results in the growth of large lamellar and dendritic SFCA phases. Full article

As novel methylene blue adsorbents, polyvinyl alcohol and activated charcoal were used to modify the surface of superparamagnetic iron oxide nanoparticles. The adsorption capacity after 69 h was 26.50 ± 0.99–40.21 ± 1.30 mg/g, depending on the temperature (333.15, 310.15, and 298.15 K) and the initial concentration of methylene blue, which was between 0.017 and 0.020 mg/mL. Based on thermodynamics parameters, the adsorption process can be considered to be spontaneous endothermic physisorption. Kinetics studies show that the pseudo-second-order model was the best-fitted model. Adsorption isotherm studies show that the best-fitted models were the Langmuir, Langmuir, and Temkin and Pyzhev isotherm models when adsorbing MB at 333.15, 310.15, and 298.15 K, respectively. Full article

We investigated the magnetic properties of the antiferromagnetic (AFM) topological insulator MnBi${}_2$Te${}_4$ with a partial substitution of Mn atoms by non-magnetic elements (A${}^{\mathrm{IV}}$ = Ge, Pb, Sn). Samples with various element concentrations (10–80%) were studied using SQUID magnetometry. The results demonstrate that, for all substitutes the type of magnetic ordering remains AFM, while the Néel temperature (T${}_{\mathrm{N}}$) and spin-flop transition field (H${}_{\mathrm{SF}}$) decrease with an increasing A${}^{\mathrm{IV}}$ = Ge, Pb, Sn concentration. The rate of decrease varies among the elements, being highest for Pb, followed by Sn and Ge. This behavior is attributed to the combined effects of the magnetic dilution and lattice parameter increase on magnetic properties, most prominent in (Mn${}_{1-x}$Pb${}_x$)Bi${}_2$Te${}_4$. Besides this, the linear approximation of the experimental data of T${}_{\mathrm{N}}$ and H${}_{\mathrm{SF}}$ suggests higher magnetic parameters for pure MnBi${}_2$Te${}_4$ than observed experimentally, indicating the possibility of their non-monotonic variation at low concentrations and the potential for enhancing magnetic properties through doping MnBi${}_2$Te${}_4$ with small amounts of nonmagnetic impurities. Notably, the (Mn${}_{1-x}$Pb${}_x$)Bi${}_2$Te${}_4$ sample with 10% Pb substitution indeed exhibits increased magnetic parameters, which is also validated by local-probe analyses using ARPES. Our findings shed light on tailoring the magnetic behavior of MnBi${}_2$Te${}_4$-based materials, offering insights into the potential applications in device technologies. Full article

Bioglasses have been used throughout the past century as a biomaterial in the bone regeneration field. However, recent studies have attempted to use them as a therapeutic material as well, mainly in the treatment of osteosarcomas. The most widely recognized bioglass is the 45S5 Bioglass, invented by Larry Hench et al., which presents higher bioactivity. A possible application of this bioglass in the treatment of osteosarcomas can be accomplished by adding specific ions, such as iron, that will allow the use of magnetic hyperthermia and Fenton reaction as therapeutic mechanisms. In this study, a 45S5 Bioglass containing 10%mol of Fe₂O₃ was produced using the melt-quenching method. A group of samples was prepared by changing the overall ball milling time, from 1 h up to 48 h, to analyze the effects of iron in the bioactive glass matrix and evaluate the influence of particle size on their physical and biological properties. The studied bioglasses showed no evidence of changes in the amorphous structural nature compared to the 45S5 Bioglass. The data of the impedance spectroscopy study revealed that the addition of Fe₂O₃ can increase the standard rate constant of the Electro-Fenton reaction, with the sample milled for 12 h showing the most promising results. The reduction in the particle size influenced the cytotoxicity and the bioactivity. The samples with lower particle sizes showed a higher level of cytotoxicity. Full article

Soft magnetic metallic absorbents suffer from severe oxidation, reduction in permeability and deterioration in microwave absorption when exposed to high temperatures. In this study, we prepared flaky 304 stainless-steel powders as new microwave absorbents via deformation-induced ferromagnetism. The 304 stainless-steel powders showed significant increases in saturation magnetization (Ms) from 1.03 to 82.46 emu/g when their shape was changed from spheroids to flakes; the Ms further increased to 92.29 emu/g after heat treatment at 500 °C in air. The permeability of 304 alloy powders also showed an obvious increase after ball milling and remained roughly stable after heat treatment at 500 °C in air. Moreover, the permittivity exhibited a sharp decrease after heat treatment, enabling the improvement of impedance matching and microwave absorption. After heat treatment at 500 °C in air for 100 h, the simulated reflection loss of 304 stainless-steel powders with wax still showed attractive levels, giving a minimum value of −22 dB and remaining below −6 dB over 8.5–16.5 GHz at a thickness of 2 mm. Our work can help to include paramagnetic alloy systems as new microwave absorbents for working in harsh environments. Full article

Electrophoretic nanostructuring is a promising approach for the creation of functional surfaces and active layers. The potency of this approach may be further enhanced by additional factors of various natures, such as magnetic fields. In this work, we have studied the process of electrophoresis in thin layers of water- and kerosene-based magnetic liquids and the effect of additional magnetic fields on the occurring processes. It was found that the electrophoresis process can be significantly affected by inhomogeneous magnetic fields. The possibility of compensating electrophoresis processes in such systems by means of inhomogeneous magnetic field influence was shown. Structural changes in magnetic colloids on hydrocarbon bases under the influence of an electric field have been studied. The role of electrohydrodynamic flows arising in this process is considered, and the influence of the magnetic field on the configuration of the formed labyrinth structure is studied. The dependence of the threshold value of the electric field strength corresponding to the emergence of the structure on the temperature and additionally applied magnetic field has been established. The obtained results could contribute to the development of an original method for determining the charge and magnetic moment of a single nanoparticle. Full article

A dire need for real-time detection of toxic chemical compounds exists in both civilian and military spheres. In this paper, we demonstrate that inexpensive, commercially available Fe₂O₃ nanoparticles are capable of selective sensing of chemical warfare agents (CWAs) using frequency-dependent impedance spectroscopy, with additional potential as an orthogonal magnetic sensor. X-ray magnetic circular dichroism analysis shows that Fe₂O₃ nanoparticles possess moderately lowered moment upon exposure to 2-chloroethyl ethyl sulfide (2-CEES) and diisopropyl methylphosphonate (DIMP) and significantly lowered moment upon exposure to dimethyl methylphosphonate (DMMP) and dimethyl chlorophosphate (DMCP). Associated X-ray absorption spectra confirm a redox reaction in the Fe₂O₃ nanoparticles due to CWA structural analog exposure, with differentiable energy-dependent features that suggest selective sensing is possible, given the correct method. Impedance spectroscopy performed on samples dosed with DMMP, DMCP, and tabun (GA, chemical warfare nerve agent) showed strong, differentiable, frequency-dependent responses. The frequency profiles provide unique “shift fingerprints” with which high specificity can be determined, even amongst similar analytes. The results suggest that frequency-dependent impedance fingerprinting using commercially available Fe₂O₃ nanoparticles as a sensor material is a feasible route to selective detection. Full article

In this paper, we study the effects of viscosity on the evolution of the double tearing mode (DTM) in a pair of adjacent Harris sheets based on the resistive MHD model in the NIMROD code. Similar to the tearing mode in the conventional single Harris sheet, a transition is observed in the generation of both normal and monster plasmoids at Prandtl number $P_r=1$. In the $P_r<1$ regime of the DTM, normal plasmoids (small plasmoids) are generated along with monster plasmoid, whereas in the single tearing mode (STM) cases, such a generation is not observed. When $P_r$ is above the critical value, the generation of monster plasmoid is halted. Correspondingly, in the $P_r<1$ regime, a quadrupolar flow advects along the poloidal direction, but in the $P_r>1$ regime this flow advection is inhibited. Full article

Traditional printed products have to some extent affected the development of smart structures and their application in multiple fields, especially in harsh environments, due to their complex mechanisms and control principles. The 4D printing technology based on magnetically controlled smart materials exploits the advantages of magnetically controlled smart materials with good operability and security, and its printed smart structures can be obtained under magnetic field drive for unfettered remote manipulation and wireless motion control, which expands the application of printed products in complex environments, such as sealed and narrow, and has broad development prospects. At present, magnetically controlled smart material 4D printing technology is still in its infancy, and its theory and application need further in–depth study. To this end, this paper introduces the current status of research on magnetically controlled smart material 4D printing, discusses the printing process, and provides an outlook on its application prospects. Full article

As an abundant potentially dangerous waste, red mud (RM) requires a straightforward method of resource management. In this paper, an RM catalyst loaded with cobalt (Co-RM) was prepared by the coprecipitation method for the efficient activation of persulfate (PS). Its degradation performance and mechanism of ofloxacin (OFL) were investigated. The characterization results of scanning electron microscopy, X-ray diffractometer, and energy dispersive spectrometer showed cobalt was successfully loaded onto the surface of RM, and the catalyst produced could effectively activate PS. Under the conditions of 15 mg/L OFL, 0.4 g/L Co-RM, 4 g/L PDS, 3.0 pH, and 40 °C temperature, the maximum removal rate of OFL by the Co-RM/PDS system was 80.06%. Free radical scavenging experiments confirmed sulfate radicals were the main active substances in the reaction system. The intermediates in OFL degradation were further identified by gas chromatography-mass spectrometry, and a possible degradation pathway was proposed. Finally, the relationship between defluorination rate and time in the Co-RM/PDS degradation OFL system was described by the first-order kinetic equation. This work reports an economical, environmental solution to the use of waste RM and provides a research basis for the further exploration of RM-based catalysts. Full article

Magnetite/gold core-shell nanoparticles (magnetite/gold NPs) have important optical and magnetic properties that provide potential for applications, especially biomedical ones. However, their preparation is not exempt from difficulties that might lead to unexpected or undesired structures. This work reports the synthesis and characterization of magnetite/gold NPs using tetramethylammonium hydroxide (TMAH) to promote the formation of a continuous interface between the magnetite core and the thin gold shell. The synthesized magnetite/gold NPs were characterized using transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), field emission scanning electron microscope (FE-SEM), ζ-potential, vibrating sample magnetometer (VSM), selected area electron diffraction (SAED), UV-Visible spectroscopy, and dynamic light scattering (DLS), confirming the core-shell structure of the NPs with narrow size distribution while evidencing its plasmonic and superparamagnetic properties as well. Further, the magnetite/gold NPs were associated and stabilized with a β-cyclodextrin nanosponge (β-CDNSs), obtaining a versatile magneto-plasmonic system for potential applications in the encapsulation and controlled release of drugs. Full article

In this study, Ni-Fe alloy nanoparticles were prepared using the proteic sol–gel method, followed by a reduction in H₂ at 500 and 700 °C, namely hereafter as NiFe-500 and NiFe-700, respectively. The morphological, structural, and magnetic properties were tuned via the thermal treatment in H₂. The samples were studied using XPS, TEM, Mössbauer spectroscopy, DC magnetic measurements, and electrochemical measurements. Ritveld refinements showed that the sample NiFe-500 has FCC (face-centered cubic) and BCC (body-centered cubic) NiFe alloys, while the sample NiFe-700 has only FCC NiFe alloy. For both samples, magnetization measurements in the range of 300–900 K showed the presence of the Griffiths phase, indicating the formation of clusters of either Fe or Ni-Fe alloys rich in Fe. The sample NiFe-500 presented ferromagnetic (FM) transitions at 533, 700, and 834 K, assigned to the alloys Ni₃₇Fe₆₃-FCC, Ni₄₆Fe₅₄-FCC, and Ni₅₅Fe₄₅-FCC, respectively. In contrast, we could not observe the FM transition of the BCC Ni-Fe alloy because of limitations in our experimental setup (T ≤ 900 K). Meanwhile, three FM transitions were observed for the sample NiFe-700 at 480, 655, and 825 K, attributed to the alloys Ni₃₄Fe₆₆-FCC, Ni₄₃Fe₅₇-FCC, and Ni₅₄Fe₄₆-FCC, respectively. At 5 K, the samples NiFe-500 and NiFe-700 have saturation magnetizations of 164.2 and 173.6 emu g⁻¹, respectively. For application in Oxygen Evolution Reaction catalysis, the samples NiFe-500 and NiFe-700 showed different overpotentials of 319 and 307 mV at 10 mA cm⁻². These low overpotential values indicate a higher electrochemical activity of the FCC Ni-Fe alloy and, for both samples, a superior electrocatalytic activity in comparison to RuO₂ e IrO₂ conventional catalysts. Furthermore, the samples showed high electrochemical stability in chrono potentiometric studies for up to 15 h. This current work highlights that the Ni-Fe alloys produced via the proteic sol–gel and with a reduction in H₂ methods can be promising for OER systems due to their good performance and low costs. Full article

This study describes the synthesis of an innovative nanomaterial (patent application number BR 1020210000317) composed of cobalt ferrite functionalized in niobium pentoxide CoFe₂O₄@Nb₅O₂ (CFNb), synthesized via green synthesis using tangerine peel extract. The material emphasizes the combination of a magnetic material (which allows for easy recovery after application) with niobium pentoxide (a metal which is abundant in Brazil). CFNb was applied as a catalyst for the paracetamol (PCT) degradation by photocatalysis. The new materials were characterized through surface and pore analysis (S_BET, S_EXT, S_mic, V_mic, and V_TOTAL), photoacoustic spectroscopy (PAS), zero charge point (pH_PZC, scanning electron microscopy (SEM/EDS), and X-ray diffraction (XRD). The reaction parameters studied included pH and catalyst concentration. The results indicated that the CFNb nanocatalysts were efficient in the paracetamol degradation, presenting better results in conditions of low pH (close to 2) and low catalyst concentration under irradiation of the 250 W mercury vapor lamp (greater than 28 mW·cm⁻²) at 60 min of reaction. Full article

We propose an experimental method for the unidirectional excitation of spin waves. By structuring Au nanowire arrays within a coplanar waveguide onto a thin yttrium iron garnet (YIG) film, we observe a chiral coupling between the excitation field geometry of the nanowire grating and several well-resolved propagating magnon modes. We report a propagating spin wave spectroscopy study with unprecedented spectral definition, wavelengths down to 130 nm and attenuation lengths well above 100 μm over the 20 GHz frequency band. The proposed experiment paves the way for future non-reciprocal magnonic devices. Full article

In this paper, the electron and magnetic state of iron placed either on the surface or in the core of TiO₂ nanoparticles were investigated using magnetometric methods, electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. It was demonstrated that the EPR spectra of TiO₂ samples with iron atoms localized both on the surface and in the core of specific features depending on the composition and size of the nanoparticles. Theoretical calculations using the density functional theory (DFT) method demonstrated that the localization of Fe atoms on the surface is characterized by a considerably larger set of atomic configurations as compared to that in the core of TiO₂ nanoparticles. Mössbauer spectra of the samples doped with Fe atoms both on the surface and in the core can be described quite satisfactorily using two and three doublets with different quadrupole splitting, respectively. This probably demonstrates that the Fe atoms on particle surface and in the bulk are in different unlike local surroundings. All iron ions, both on the surface and in the core, were found to be in the Fe³⁺ high-spin state. Full article

The microfluidics of magnetic fluids is gaining popularity due to the possibility of the non-contact control of liquid composite systems using a magnetic field. The dynamics of non-magnetic droplets and gas bubbles in magnetic fluids were investigated for various configurations of magnetic fields, coatings, and channel geometries, as well as the rate of component supply and their physical properties. Optimal regimes for forming droplet and bubble flows were determined. The mechanism for non-contact control of the size of droplets and bubbles using a magnetic field is proposed in this article. The dependences of the sizes of non-magnetic inclusions in magnetic liquids on the continuous phase flow rate and the displacement of magnets were obtained. The obtained dependences of the volume of non-magnetic inclusions on the flow rate of the continuous phase follow the classic dependences. Changing the size of air bubbles can be achieved by shifting the magnet from −5 mm to +2 mm. The ratio of the maximum and minimum breakaway inclusion varies from 5 to 2 depending on the flow rates of the continuous phase. The range of changing the size of oil droplets with the displacement of magnets is from 1.1 to 1.51. These studies show how, with the help of various mechanisms of influence on microfluidic flows, it is possible to control the size of bubbles and droplets forming in microchannels. The obtained data can be applied for controlled microfluidic dosing and counting devices. Full article