J. Vejpravová

Surface spin effects in La-doped CoFe2O4 nanoparticles prepared by microemulsion route

A comparative study of pure CoFe2O4 nanoparticles and La-doped CoFe2O4 nanoparticles, prepared by microemulsion route has been performed. The samples were characterized using x-ray diffraction and transmission electron microscopy in order to obtain average particle size. The doping of small amount of La3+ ions (up to 3 molar %) causes significant reduction of the particle size using the identical preparation route. The samples were investigated by magnetization measurements, which revealed the coercivity values strongly dependent on particle size, but not significantly on level of La3+ doping. Detailed in-field Mossbauer spectroscopy studies were performed in order to determine spin canting angles and cation distribution within the spinel network. The non-negligible canting angles up to 40° in the La-doped samples were observed. The presence of the spin surface effects was also supported by magnetic measurement as the magnetization did not saturate even in considerably high magnetic fields (7 T). Moreover...

Magnetism of sol-gel fabricated CoFe2O4∕SiO2 nanocomposites

Details of synthesis and characterization of sol-gel-produced CoFe2O4 nanoparticles embedded in the amorphous SiO2 matrix are presented together with results of an extended magnetization study of these materials. The particle size was found to increase from 6to15nm by varying the temperature of a subsequent annealing from 800to1100°C. All samples exhibited superparamagnetic behavior with values of the blocking temperature TB increasing with the particle size. At temperatures above TB the magnetization curves follow the expected Langevin scaling of M vs H∕T, which is consistent with the formation of the superparamagnetic state. For T<TB, the coercivity field Hc was found to be proportional to T and the frequency-dependent ac susceptibility was found to obey the Neel–Arrhenius law. Both observations are compatible with a model of noninteracting randomly oriented single-domain particles.

The analysis of the specific heat of RFe2Si2 compounds

The series of isostructural intermetallics RFe 2 Si 2 (R = La, Pr, Nd) crystallizes in the tetragonal ThCr 2 Si 2 -type structure with the space group I4/mmm. The specific heat of high-quality single crystals PrFe 2 Si 2 and NdFe 2 Si 2 was studied in comparison with nonmagnetic polycrystalline LaFe 2 Si 2 in the 1.5-300K temperature region. The detailed analysis of the specific heat yielded the magnetic entropy and the crystal field level scheme of both crystals.

The effect of a thin gold layer on graphene: a Raman spectroscopy study

An understanding of interactions between graphene and its surroundings is crucial for application of graphene in electronic devices. Raman spectroscopy is a convenient and efficient tool to provide information about the doping, stress and defects in graphene; however application of this method is limited to the Si/SiO2 substrate which provides interference enhancement. Here we present a comprehensive Raman study of the single-layer graphene – sapphire – gold system. Due to plasmons generated in the gold-layer the Raman signal of graphene is significantly enhanced. We study the influence of the gold layer thickness and gold particle size on the enhancement. The analysis of the Raman maps showed that graphene on sapphire is only slightly doped and the spatial distribution of doping is quite homogenous. Also no significant strain was generated in graphene sandwiched by sapphire and gold.

Graphene wrinkling induced by monodisperse nanoparticles: facile control and quantification.

Controlled wrinkling of single-layer graphene (1-LG) at nanometer scale was achieved by introducing monodisperse nanoparticles (NPs), with size comparable to the strain coherence length, underneath the 1-LG. Typical fingerprint of the delaminated fraction is identified as substantial contribution to the principal Raman modes of the 1-LG (G and G’). Correlation analysis of the Raman shift of the G and G’ modes clearly resolved the 1-LG in contact and delaminated from the substrate, respectively. Intensity of Raman features of the delaminated 1-LG increases linearly with the amount of the wrinkles, as determined by advanced processing of atomic force microscopy data. Our study thus offers universal approach for both fine tuning and facile quantification of the graphene topography up to ~60% of wrinkling.

Methyl groups control coordination number, stoichiometry, network and magnetism in a Cu(II)-azide-pyrazine (6,3) 2D net

The methyl groups of [Cu2(2,5-dimethylpyrazine)(N3)4]n, 1, force penta-coordination on the Cu(II) ions, which, together with the di-µ-1,1azido zigzag chains, result in a (6,3) 2D net with dominant ferromagnetic interactions.

Nanocomposite of superparamagnetic maghemite nanoparticles and ferroelectric liquid crystal

In this study we present a novel type of hybrid nanocomposite prepared by mixing maghemite nanoparticles and a chiral liquid crystalline compound. The hybrid system exhibits ferroelectric as well superparamagnetic properties. The impact of nanoparticles and the effect of an applied magnetic field on the ferroelectric liquid crystalline properties were established. The ferroelectric properties are conserved up to a concentration of 5.6% of maghemite. The magnetic behavior of the composites is typical for a system of superparamagnetic nanoparticles with inter-particle dipolar interactions and surface spin disorder.

Highly efficient mesenchymal stem cell proliferation on poly-ε-caprolactone nanofibers with embedded magnetic nanoparticles

In this study, we have developed a combined approach to accelerate the proliferation of mesenchymal stem cells (MSCs) in vitro, using a new nanofibrous scaffold made by needleless electrospinning from a mixture of poly-ε-caprolactone and magnetic particles. The biological characteristics of porcine MSCs were investigated while cultured in vitro on composite scaffold enriched with magnetic nanoparticles. Our data indicate that due to the synergic effect of the poly-ε-caprolactone nanofibers and magnetic particles, cellular adhesion and proliferation of MSCs is enhanced and osteogenic differentiation is supported. The cellular and physical attributes make this new scaffold very promising for the acceleration of efficient MSC proliferation and regeneration of hard tissues.

Nanocomposites of monodisperse nanoparticles embedded in high-K oxide matrices – a general preparation strategy

We present a general approach, which enables preparation of multifunctional nanocomposites of monodisperse nanoparticles embedded in oxide matrices. The two-step route has been successfully applied to nanocomposites composed of CoFe2O4 nanoparticles embedded in high-K oxide matrices (ZrO2, Al2O3 and TiO2). First, hydrophobic CoFe2O4 nanoparticles were produced by hydrothermal synthesis and then their incorporation in the oxide matrix was completed by the sol–gel method using the corresponding alcoxides. The as-prepared samples were subjected to annealing at temperatures ranging from 200 to 700 °C, and characterized in detail by the Powder X-Ray Diffraction (PXRD), Energy Dispersive Analysis (EDX), Mossbauer Spectroscopy (MS) and magnetic measurements. The particle size does not change with the annealing temperature, while the amorphous matrices crystallize at temperatures above 400 °C. At much higher annealing temperatures, partial decomposition of the CoFe2O4 occurs accompanied by formation of additional phases. The magnetic measurements also confirmed presence and stability of the uniform CoFe2O4 nanoparticles in the matrices. Thus the proposed method allows preparation of new types of nanocomposites constituted of uniform nanoparticles of the desired type (magnetic, luminescent etc.) embedded in the favored oxide matrix.

Understanding particle size and distance driven competition of interparticle interactions and effective single-particle anisotropy

Magnetic response of single-domain nanoparticles (NPs) in concentrated systems is strongly affected by mutual interparticle interactions. However, particle proximity significantly influences single-particle effective anisotropy. To solve which of these two phenomena plays a dominant role in the magnetic response of real NP systems, systematic study on samples with well-defined parameters is required. In our work, we prepared a series of nanocomposites constituted of highly-crystalline and well-isolated CoFe2O4 NPs embedded in an amorphous SiO2 matrix using a single-molecule precursor method. This preparation method enabled us to reach a wide interval of particle size and concentration. We observed that the characteristic parameters of the single-domain state (coercivity, blocking temperature) and dipole-dipole interaction energy ([Formula: see text]) scaled with each other and increased with increasing [Formula: see text], where d XRD was the NP diameter and r was the interparticle distance. Our results are in excellent agreement with Monte-Carlo simulations of the particle growth. Moreover, we demonstrated that the contribution of [Formula: see text] acting as an additional energetic barrier to the superspin reversal or as an average static field did not sufficiently explain how the concentrated NP systems responded to an external magnetic field. Alternations in the blocking temperature and coercivity of our NP systems accounted for reformed relaxations of the NP superspins and modified effective anisotropy energy of the interacting NPs. Therefore, the concept of modified NP effective anisotropy explains the magnetic response of our concentrated NP systems better than the concept of the energy barrier influenced by interparticle interactions.