Magnetic phase in the near-surface region of an FeBO3 single crystal

An FeBO3 single crystal was studied from 291 K up to the Neel temperature T(N) = 348.35 K by depth selective conversion electron Mossbauer spectroscopy in ultrahigh vacuum (10(-9) mbar). A new magnetic near-surface phase was found. Its thickness D diverges on approaching T(N) and gives a critical exponent for the correlation length of nu = 0.59(4). The phase boundary between the bulk and near-surface phase could be identified.     

Physicotechnological principles of the development of biocompatible magnetic nanoparticles for medicine

Nanodispersive powder of a zinc-substituted magnetite was developed. Functional characteristics (biocompatibility, dispersity, magnetic state) allow us to recommend it for approbation in medical and biological applications. The nature of the investigated field dependencies of magnetization indicates that for particles of 3–13 nm, a superparamagnetic state is realized at room temperature, reflecting the specificity of the small particles’ magnetism.     

The properties of Mn–CuFe2O4 spinel ferrite nanoparticles under various synthesis conditions

The structural, morphological, magnetic, dielectric, and gas analyzing properties are studied in CuFe₂O₄(Mn–CuFe₂O₄) substituted spinel ferrite nanoparticles synthesized via evaporation and automatic combustion. The obtained nanoparticles are established to possess a spherical shape. The smallest size of Mn–CuFe₂O₄ (~9 nm) nanoparticles is achieved at using automatic combustion. X-ray diffraction and Mössbauer spectroscopy reveal that the crystal lattice constant and the Mn–CuFe₂O₄ nanoparticle size are larger at augmenting the annealing temperature from 600 to 900°С. The dielectric permeability and losses of Mn–CuFe₂O₄ nanoparticles are studied at various synthesis conditions and temperatures of annealing. Various aspects of gas sensibility of synthesized Mn–CuFe₂O₄ nanoparticles are tested, as well. The maximum response to the presence of liquefied petroleum gas is 0.28 at the optimum working temperature of 300°C for Mn–CuFe₂O₄ nanoparticles obtained via automatic combustion and it is 0.23 at 250°C for deposited nanoparticles.

     

Simultaneous gamma, x-ray, and electron Mössbauer spectroscopy of the volume and surface magnetic structure of hexagonal M-ferrites

Direct comparative studies are made between the magnetic structures of a surface layer of thickness ～40 nm and the bulk magnetic structure of ferromagnetic single crystals of hexagonal M ferrites (BaFe₁₂O₁₉, SrFe₁₂O₁₉, PbFe₁₂O₁₉) with a magneto-plumbite structure. Measurements are made by simultaneous gamma, x-ray, and electron Mössbauer spectroscopy in order to investigate the properties of the surface layer and the bulk crystal simultaneously. Experimental data obtained with a depth resolution of ～ 10 nm show that the orientation of the magnetic moments of the iron ions (along the crystallographic c axis) does not change on approaching the surface from the crystal volume. Thus, to within an experimental error of ～ 10 nm, single crystals of the hexagonal ferrites BaFe₁₂O₁₉, SrFe₁₂O₁₉, and PbFe₁₂O₁₉ with a ferromagnetic structure do not have a “ transition” surface layer whose magnetic structure differs from that of the bulk crystal such as that which exists, with a depth of several hundred nm, in antiferromagnetic materials with weak ferromagnetism.     

Mössbauer Studies of Core-Shell FeO/Fe3O4 Nanoparticles

Physics of the Solid State - FeO/Fe3O4 nanoparticles were synthesized by thermal decomposition. Electron microscopy revealed that these nanoparticles were of the core-shell type and had a spherical...     

Magnetic Core/Shell Nanocomposites MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> for Biomedical Application: Synthesis and Properties

Magnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe₂O₄) is a core, and an amorphous silicon dioxide (silica SiO₂) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mössbauer spectroscopy. It is found that the MgFe₂O₄/SiO₂ MNC has the core/shell structure formed by the Fe?O–Si chemical bond. After coating with silica, the MgFe₂O₄/SiO₂ MNC saturation magnetization significantly decreases in comparison with MgFe₂O₄ particles without a SiO₂ shell. Spherical particles agglomerated from MgFe₂O₄ nanocrystallites ～9.6 and ～11.5 nm in size function as cores coated with SiO₂ shells ～30 and ～50 nm thick, respectively. The total size of obtained CS MNCs is ～200 and 300 nm, respectively. Synthesized CS MgFe₂O₄/SiO₂ MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.     

Magnetic states of the surface and bulk of ferrites near a phase transition at the Curie temperature

Investigations of the magnetic state of a surface layer ~200 nm thick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe₁₂O₁₉) are performed in the phase transition region around the Curie temperature (T _c). The method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy, which made it possible to compare directly the phase states of the surface and bulk of the sample, is used for the measurements. It is observed experimentally that in BaFe₁₂O₁₉ the transition of a surface layer ~200 nm thick to the paramagnetic state occurs at temperatures below T _c. It is established that the transition temperature T _c(L) of a thin layer localized at depth L from the surface of the crystal increases with distance from the surface and reaches the value T _c at the lower boundary of the “critical” surface layer. Therefore, near T _c a nonuniform state in which the crystal is magnetically ordered in the bulk but disordered at the surface is observed. A phase diagram of the states of the surface and of the bulk of macroscopic magnets near the Curie (or Néel) point is proposed on the basis of all the experimental results obtained in the present work as well as previously published results.     

Surface magnetism of Sc-substituted Ba-M hexaferrites

A technique of simultaneous gamma-ray, x-ray, and electron Mössbauer spectroscopy is used to study the magnetic structure of the surface layer with direct comparison to the magnetic structure inside single crystal samples of hexagonal Ba-M ferrites, in which part of the iron ions have been replaced by diamagnetic Sc ions (chemical formula BaFe_12?δ Sc_δO₉). It is found that when the diamagnetic Sc ions are introduced into the crystal lattice of BaFe_12?δ Sc_δO₁₉ at concentrations (x=0.4 and 0.6) far below the level at which the collinear magnetic structure inside the sample is destroyed, a macroscopic layer of thickness ～300 nm develops on the surface, in which the magnetic moments of the iron ions are oriented noncollinearly with respect to the moments inside the sample. The deviation 〈θ〉 of the magnetic moments in BaFe_11.6Sc_0.4O₁₉ was 10° ± 62° for x=0.4, and when the Sc concentration was raised to 0.6, the angle 〈θ〉 increased to 17° ± 62°. The noncollinear magnetic structure in the surface layer in these crystals develops because of further reduction in the energy of the exchange interactions owing to the presence of a “defect,” such as the surface. For the first time, therefore, an anisotropic surface layer whose magnetic properties differ from those in the interior of a sample has been observed experimentally in ferromagnetic crystals, as predicted by Néel [L. Néel, Phys. Radium. 15, 225 (1954)].     

Magnetic properties of nanodispersed ferrite powders with cryochemical prehistory

Calcium hexagonal ferrite in the form of a system of nanocrystals has been synthesizes using elements of cryochemical technology for the first time. The obtained ensemble of particles corresponds to the model Stoner-Wohlfarth system according to the following characteristics: the phase composition, the shape of the basic magnetization curve, and the coercive force. The temperature dependences of the magnetization in the range 300–700 K at fixed values of the magnetic field indicate the presence of a transition to the super-paramagnetic (SPM) state. The boundary temperatures T _BH ⁽¹⁾ and T _BH ² of the range of the SPM transition have been determined, and the role of the external magnetic field, which stimulates the transition in this process in accordance with the theory, has been confirmed.     

Dependence of the structure and nagnetic properties of FeTaN films on nitrogen concentration

The structural morphology and magnetic properties of thin FeTaN films with a high Ta content (10 wt %) prepared by annealing compounds deposited by reactive rf magnetron sputtering in an Ar + N gas mixture are studied. The dependence of the properties of FeTaN films on their nitrogen content and annealing temperature were established. The deposition and thermal treatment regimes favoring the preparation of thin nanostructural FeTaN films with high soft magnetic characteristics [B _s = 1.6 T, H _c = 0.2 Oe, and μ₁ (1 MHz) = 3400] were determined.