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Magnetic iron nanoparticles (MNPs) encapsulated in a carbon shells and containing sulfo groups on the surface (Fe@C-SO3H) were synthesized. The aggregative stability of aqueous suspensions of the Fe@C-SO3H nanoparticles was studied in solutions of bovine serum albumin and calcium chloride and in phosphate-buffered saline, which simulate blood plasma. It is shown that the Fe@C-SO3H particles practically do not aggregate in the phosphate buffer saline for a long time (several days). On the contrary, MNPs encapsulated in a protein shells using ultrasonic treatment form aggregates up to 110–120 nm in size under these conditions. Suspensions of the Fe@C-SO3H particles are stable in aqueous solutions of calcium chloride in the range of the solid phase concentration between 0.05 and 0.10 g L−1. The results obtained indicate the possibility of using the functionalized Fe@C-SO3H nanoparticles in vitro experiments in biological media.

A method is proposed for measuring the coercive force using a U-shaped electromagnet with a gap between the electromagnet poles and the test sample. It is demonstrated that the device can operate in a dynamic mode under pulsed magnetization reversal. Numerical simulation and experimental results are compared.

The possibility of using the transverse relaxation time T2 of protons in aqueous media for quantitative measurement of the capture of magnetic nanoparticles by cells has been studied and demonstrated. The measurement of T2 was performed on a portable original NMR relaxometer with a measuring cell for a standard well of a biological plate. The novelty of the approach is that quantitative measurements of the capture kinetics were carried out using measurements of the proton relaxation time of the nutrient medium, which is determined by the remaining number of magnetic particles (not captured by the cells) in the medium. To study the kinetics of capture, two types of magnetic nanoparticles were synthesized: magnetite particles Fe3O4 and composite particles Fe@C with an iron-carbon shell structure. The surface of the particles was functionalized with amine-and carboxyl groups. The capture of aminated particles of Fe@C cells is established by microscopy and NMR-relaxometry by measuring the time T2. It is shown that the proposed method makes it possible to register very small concentrations of trapped magnetic nanoparticles equal to tens of pg/cell.

A new approach to remove TiO2 nanoparticles from water is studied. Composite Fe3O4-SiO2 and Fe-C-SO3H nanoparticles with magnetic cores (5 nm) were designed to be used as flocculants for magnetic sedimentation of nonmagnetic TiO2 nanoparticles in water. The sedimentation dynamics of heterogeneous water suspensions (starting solid phase concentration of 100 mg/l) of (TiO2)1−x(Fe3O4-SiO2)x (x = 0.15–0.55) and (TiO2)1−x(Fe-C-SO3H)x (x = 0.01–0.17) was studied in a gradient magnetic field (B = 0.3 T, dB/dz

The dependence of the transverse relaxation time T2 on the liquid, flow velocity in a porous medium (for the water and water suspension of magnetic nanoparticles (MNPs)) is studied. The T2 value allows measuring the filtration velocity of a liquid on the order of 0.05 mm/s. MNPs in a small concentration decrease T2 but at the same time the dependence of T2 on the liquid’s velocity becomes more pronounced.

Nanoparticles with surface-modified shells are used for selective sorption of different impurities from water. Subsequent removal of nanoparticles from water can be strongly enhanced by using particles with magnetic core, providing their separation by sedimentation or filtration in magnetic field. The velocity of the magnetic sedimentation is strongly controlled by the particles aggregation which can be efficiently changed by interparticles interaction, in particular, by mixing the positively charged particles with negatively charged particles. The aggregation and sedimentation dynamics of the mixtures of the complex nanoparticles having the magnetite or the iron core, e.g., Fe 3 O 4 /SiO 2 , Fe 3 O 4 /SiO 2 NH 2 and Fe/C NH 2 in water were studied in a vertical gradient magnetic field B ≤ 0.3 T, dB/dz ≤ 0.13 T/cm. The sedimentation time t s was measured using two independent methods: by optically registered turbidity and from residual particle concentration in water by Nuclear Magnetic Resonance Relaxometry (NMRR) method. The t s reaches a minimum (7–10 min) for the (1:1) mixtures of the target Fe 3 O 4 /SiO 2 and the seed Fe 3 O 4 /SiO 2 NH 2 nanoparticles. Adding of the seed Fe/C NH 2 nanoparticles to the target Fe 3 O 4 /SiO 2 water suspension is more effective due to smaller sizes and to larger magnetic moments of the Fe-based particles. The sedimentation time of about (6−10) minutes is observed for rather small fraction (6, 15 mass%) of the added Fe/C NH 2 particles, which is important for application to reduce the residual sediment volume significantly. Preliminary exposure of the mixtures in the absence of magnetic field favors to more complete nanoparticles aggregation and, as a result, more rapid sedimentation of the nanoparticles in a vertical gradient magnetic field. The data obtained are useful for development of the methods aimed at magnetic separation of magnetic and nonmagnetic nanoparticles in water purification technologies.

A method has been proposed for determining the coercive force in local domains of bulk samples magnetized normally to the surface. The method is based on local measurement of the normal magnetic-field component at the sample surface, in the gap between the sample and the pole of a magnetizing device. The coercive force is determined by the magnetic field for which local magnetization in the measured domain becomes zero. Conditions for the correctness of coercive-force measurements have been established.

The problem of determining saturation magnetization in a local area of a bulky ferromagnet is considered. A mockup of the device is presented. Experimental data are provided for local measurements of the saturation magnetization of a ferromagnetic sample, based on the stray field at the boundary of the magnetized area.

The sedimentation dynamics of Fe3O4@SiO2 nanoparticles (20/30 nm) in water with additions of Na2SO4, CaCl2, NaH2PO4 salts with concentration of (5–100) mM were studied in a vertical gradient magnetic field (B1 ⩽ 0.3 T, dB/dz ⩽ 0.13 T/cm). The presence of magnetic field leads to nanoparticle sedimentation time decreasing from 103 h to 101 h and to 10−2 h when the salts are added. The results are interpreted on the basis of aggregate formation caused by electrostatic, steric and magnetic inter-particle interactions in water medium. While the stability of Fe3O4@SiO2 nanoparticles in pure water is dependent on steric inter-particle interactions involving polymeric Si–O groups on the surface of the composite particles, the presence of Na+ and Ca2+ cations in water leads to a decrease of electrostatic repulsion because the charge on the nanoparticle surfaces decreases. Magnetic moments, induced in the magnetic field, contribute to the enlargement of the aggregates and, in interacting with the magnetic field gradient, facilitates a quicker sedimentation of the suspension. The data obtained are useful for the development of methods aimed at the magnetic separation of magnetic nanosorbents in water purification technology.

Sedimentation dynamics of magnetite (γ-Fe3O4) nanopowders (10 - 20 nm) in water in the presence of a gradient magnetic field was studied by optical and Nuclear Magnetic Resonance (NMR) relaxometry methods. The magnetic field B ≤ 0.3 T, dB/dz ≤ 0.13 T/cm was produced by the system of permanent strip magnets. The initial sedimentation rate of the nanoparticles in water and under magnetic fields is higher for less concentrated suspensions (c0 = 0.1 g/l) than for more concentrated ones (c0 = 1 g/l). This might be connected with the formation of gel structures due to strong magnetic attraction between ferromagnetic nanoparticles. In the gravitation field, the suspensions of the particles (10 - 20 nm) remain stable for over 20 hours. The sedimentation process can be greatly accelerated by the action of a vertical gradient magnetic field, reducing the sedimentation time down to several minutes. In a gradient magnetic field enhanced by a steel grid, sedimentation of the nanopowder (c0 = 0.1 g/l) for 180 minutes resulted in reduction of the iron concentration in water down to 0.4 mg/l. In flowing water regime, the residual iron concentration in water 0.3 mg/l is reached after 80 minutes.

The use of the effect of a decrease in the transverse relaxation time T 2 of the NMR signal of water protons in the presence of magnetic particles has been suggested for the quantitative estimation of the concentration of magnetite (Fe3O4) nanopowder in water. A calibration dependence of the relaxivity T 2 −1 on the iron concentration has been obtained for model suspensions of magnetite nanoparticles with sizes of approximately 20 nm in the concentration range of 0.15–70 mg/L. For comparison, the concentration dependences of T 2 −1 for aqueous solutions of Fe(NO3)3 · 9H2O and FeSO4 · 7H2O and paramagnetic salts Ni(NO3)2 · 9H2O, Cr(NO3)3 · 9H2O, and CuSO4 · 5H2O have been studied to show that they correlate with their paramagnetic susceptibilities.

H1 NMR relaxometry is a method that is extremely sensitive to the presence of magnetic nanoparticles, which significantly affect the transverse relaxation time of the water proton. Accordingly, the use of magnetic nanoparticles as labels allows detection of even extremely small amounts of the test substance. This paper analyzes the prospects for applying the method of solid-phase NMR-relaxometric determination of biologically active molecules. The nitrocellulose membranes are chosen as a solid phase and nanoparticles based on iron core with a carbon shell are used as magnetic labels. The possibility of detecting small concentrations of magnetic particles in porous medium is demonstrated. Finally, the ability to detect extremely low concentrations of an analyte, in this case, streptavidin protein (0.5 ng/ml to 100 ng/ml), which is actively used in various fields of biology and medicine, is demonstrated. © Published under licence by IOP Publishing Ltd. Russian Science Foundation, RSF: 17-15-01116 The work was carried out within the Russian Science Foundation project 17-15-01116. equipment of the Ural Center for Shared Use Modern nanotechnology UrFU was used.

The dynamics of a system of domain walls has been considered in finite samples of a rectangular or a trapezoid (wedge) shape. Based on the solution to the set of equations of motion for the system of domain walls, it has been shown that the equilibrium positions and amplitudes of vibrations of domain walls are determined by the magnetostatic field caused by the sample shape. The role of possible physical mechanisms controlling the translational motion of the domain structure is discussed.

Sedimentation dynamics of magnetite (γ-Fe3O4) nanopowder (10-20 nm) in water in a gradient magnetic field Bmax=0.3 T, (dB/dz)max=0.13 T/cm was studied for different water flow speeds and starting particle concentrations (0.1 and 1.0 g/l). The aggregates formation in water was monitored under the same conditions. In cyclical water flow, the velocity of particle sedimentation increases significantly in comparison to its rate in still water, which corresponds to the intensified aggregate formation. However, at a water flow speed more than 0.1 cm/s sedimentation velocity slows down, which might be connected to aggregate destruction in a faster water flow. Correlation between sedimentation time and the nanoparticle concentration in water does not follow the trend expected for spherical superparamagnetic particles. In our case sedimentation time is shorter for c=0.1 g/l in comparison with that for c=1 g/l. We submit that such a feature is caused by particle self-organization in water into complex structures of fractal type. This effect is unexplained in the framework of existing theoretical models of colloids systems, so far. Provisional recommendations are suggested for the design of a magnetic separator on the permanent magnets base. The main device parameters are magnetic field intensity B≥0.1 T, magnetic field gradient (dB/dz)max≈(0.1-0.2) T/cm, and water flow speed V0.15 cm/s. For particle concentration c=1 g/l, purification of water from magnetite down to ecological and hygienic standards is reached in 80 min, for c=0.1 g/l the time is reduced down to 50 min.

Calculations of the compensating field as a function of operating parameters of a compensating element based on axially symmetrical models of surface and volume distributions of magnetic charges are discussed. The efficiency of compensation for a spurious output due to both the difference between transfer factors of magnetically sensitive elements of a magnetic-modulation transducer (ferromagnetic probe) and longitudinal component of the terrestrial field depends essentially on the optimal selection of the compensating element length, its diameter, shape, separation from one of the magnetically sensitive elements, and other factors.

Calculations of the compensating field as functions of operating parameters of a compensating element based on the model of axially symmetrical distribution of surface magnetic charges are discussed. The efficiency of the compensation for spurious signals due to the misalignment of magnetic axes of a differential magnetic-modulation transducer and the transverse component of the terrestrial magnetic field is strongly affected by the selection of the optimal length of the compensating element, its diameter, shape, separation from the magnetically sensitive element, and other factors.

Magnetite (γ-Fe3O4) nanoparticles are promising effective sorbents for water cleaning of heavy metal, radionuclides, organic and biological materials. A good sorption capacity can be achieved due to their high specific surface area. Application of gradient magnetic fields helps to separate the magnetic nanoparticles from the water suspension, which is rather hard to do using the conventional mechanical filtration and sedimentation methods without coagulants. The sedimentation dynamics of magnetite nanoparticles with sizes of 10–20 nm in aqueous media in the presence of a gradient magnetic field was studied by optical and NMR relaxometry methods. The gradient magnetic field was produced by a series of strip permanent magnets with B ≤ 0.5 T, dB/dz ≤ 0.13 T/cm and in some cases enhanced by a steel grid with sharp edges (dB/dz ≤ 5 T/cm). Dynamic Light Scattering in the water suspension with different nanoparticle concentrations (c 0 = 0.1–1 g/l) revealed the characteristic features in the aggregate formation, which is reflected in the sedimentation behavior. The sedimentation rate of the nanoparticles in water and in magnetic fields is higher for less concentrated suspensions (c 0 = 0.1 g/l) than for more concentrated ones (c 0 = 1 g/l), which might be connected with the formation of a gel structures due to a strong magnetic attraction between ferromagnetic nanoparticles. In 180 min this resulted in the reduction of the iron concentration in water down to 0.4 mg/l, which is close to hygienic and environmental norms for drinking water and fishery.

The sedimentation dynamic of magnetite (Fe3O4) nanoparticles in water was investigated, both in the presence of a vertical gradient magnetic field and in the gravitational field only. The nanopowders (four samples with average particle diameter ranging from 16 to 84 nm) were prepared by a gas-condensation synthesis method. The sedimentation was monitored by measuring the light transmission coefficient k of the suspension as a function of time. The sedimentation process is of rather complex character for both the large and the small particles. Specifically, the light transmission reflects the different stages of the particle aggregation. Magnetite nanoparticles tend to aggregate into micron-sized aggregates which sediment rather rapidly in high concentrated suspensions (for example 5 g/L), even in the absence of a magnetic field. Gradient magnetic fields (for example H = 6 kOe, dH/dz = 1.6 kOe/cm) help to increase the sedimentation rate tremendously and reduce the total sedimentation time from several days up to several minutes—here for an average particle size of 16 nm. An effective removal of heavy metal pollutants (Cr, Cu, etc.) from water can be achieved using the optimal combination of the magnetite particle size, particle concentration in water suspension, and magnetic field strength and gradient.

A review is presented of theoretical investigations into the influence of domain structure and dynamic behaviour of domain walls on some low- and high-frequency properties of ferromagnetic materials, The effect of dynamic bowing of domain walls on electromagnetic losses in electrical steel is analyzed in detail. The theories of Pry, Bean and Bishop are generalized to explain experimentally observed nonlinear dependence of eddy-current power losses on frequency and wall spacing. A study is made of the influence of domain structure on the parametric excitation of spin waves with account being taken of domain wall structure. It is shown that the lowest parametric excitation threshold corresponds to spin waves within domain walls and that the parametric excitation effect is rather sensitive to domain wall structure. The influence of a characteristic domain structure arising on the basis of a twinning structure of magnetically ordering alloys on the Mossbauer spectra is analyzed. It is shown that the observed special features of these spectra may be explained on the basis of this domain structure.