The internal structure of magnetic nanoparticles determines the magnetic response.

This work aims to emphasize that the magnetic response of single-domain magnetic nanoparticles (NPs) is driven by the NPs' internal structure, and the NP size dependencies of magnetic properties are overestimated. The relationship between the degree of the NPs' crystallinity and magnetic response is unambiguously demonstrated in eight samples of uniform maghemite/magnetite NPs and corroborated with the results obtained for about 20 samples of spinel ferrite NPs with different degrees of crystallinity. The NP samples were prepared by the thermal decomposition of an organic iron precursor subjected to varying reaction conditions, yielding variations in the NP size, shape and relative crystallinity. We characterized the samples by using several complementary methods, such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), high resolution TEM (HR-TEM) and Mössbauer spectroscopy (MS). We evaluated the NPs' relative crystallinity by comparing the NP sizes determined from TEM and PXRD and further inspecting the NPs' internal structure and relative crystallinity by using HR-TEM. The results of the structural characterization were put in the context of the NPs' magnetic response. In this work, the highest saturation magnetization (Ms) was measured for the smallest but well-crystalline NPs, while the larger NPs exhibiting worse crystallinity revealed a lower Ms. Our results clearly demonstrate that the NP crystallinity level that is mirrored in the internal spin order drives the specific magnetic response of the single-domain NPs.

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.

Magnetic impurities in single-walled carbon nanotubes and graphene: a review.

Control over magnetism in single-walled carbon nanotubes (SWCNTs) and graphene is of fundamental importance. Creation and manipulation using the unpaired spins without the need for archetypal magnetic elements results in sp(2)-hybridised nanocarbons being at the forefront of applications in both spintronics and nanoelectronics. The crucial limitation for the experimental observation of the intrinsic carbon magnetism stems from the presence of magnetic impurities, from which a magnetic response usually dominates. Thus, the rigorous identification of such magnetic impurities and their efficient removal is of enormous importance. The present review reports on the current state-of-the-art methodology for the detection and quantification of magnetic impurities in SWCNTs and graphene, reflecting both the preparation and subsequent purification procedures. First, the most common techniques for the preparation of SWCNTs (i.e., arc discharge, laser ablation and chemical vapour deposition) and the corresponding magnetic impurities are reviewed. Then, the available volume, surface and local probes for the identification and quantification of the impurities are discussed, and their efficiency and limitations are evaluated for the given cases. A summary of the current understanding of graphene-related magnetism in the context of the identified impurities is also given. Finally, the key knowledge is reviewed with respect to future prospects in the field.

Addressing asymmetry of the charge and strain in a two-dimensional fullerene peapod.

We prepared a two-dimensional C70 fullerene peapod by the sequential assembly of (12)C graphene, C70 fullerenes and (13)C graphene. The local changes in the strain and doping were correlated with local roughness revealing asymmetry in the strain and doping with respect to the top and bottom graphene layers of the peapod.

Platinum-doped CeO2 thin film catalysts prepared by magnetron sputtering.

The interaction of Pt with CeO(2) layers was investigated by using photoelectron spectroscopy. The 30 nm thick Pt doped CeO(2) layers were deposited simultaneously by rf-magnetron sputtering on a Si(001) substrate, multiwall carbon nanotubes (CNTs) supported by a carbon diffusion layer of a polymer membrane fuel cell and on CNTs grown on the silicon wafer by the CVD technique. The synchrotron radiation X-ray photoelectron spectra showed the formation of cerium oxide with completely ionized Pt(2+,4+) species, and with the Pt(2+)/Pt(4+) ratio strongly dependent on the substrate. The TEM and XRD study showed the Pt(2+)/Pt(4+) ratio is dependent on the film structure.