A. L. Danilyuk

Manifestation of coherent magnetic anisotropy in a carbon nanotube matrix with low ferromagnetic nanoparticle content

The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromagnetic NP content. Samples were synthesized by floating catalyst chemical vapor deposition. We found that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, the extended magnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions reflect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies.

Interaction of electromagnetic radiation in the 20–200 GHz frequency range with arrays of carbon nanotubes with ferromagnetic nanoparticles

Summary The interaction of electromagnetic radiation with a magnetic nanocomposite based on carbon nanotubes (CNT) is considered within the model of distributed random nanoparticles with a core–shell morphology. The approach is based on a system composed of a CNT conducting resistive matrix, ferromagnetic inductive nanoparticles and the capacitive interface between the CNT matrix and the nanoparticles, which form resonance resistive–inductive–capacitive circuits. It is shown that the influence of the resonant circuits leads to the emergence of specific resonances, namely peaks and valleys in the frequency dependence of the permeability of the nanocomposite, and in the frequency dependence of the reflection and transmission of electromagnetic radiation.

Spin-dependent transport of electrons through ferromagnetic/insulator/semiconductor nanostructures

The model of spin-dependent electron transport through ferromagnetic/insulator/semiconductor nanostructures was developed on the basis of the transport equation accounting for carrier scattering and the image forces at the interfaces. Modeling was performed for Co/Al2O3/p-Si and CoFe/MgO/n-Si nanostructures. Tunneling magnetoresistance was modeled to be 7-13 % in Co/Al2O3/p-Si nanostructures biased in range from 0.7 to 2.0 V. A scattering well in the collector region was shown to increase the tunneling magnetoresistance by 4-5 %. In CoFe/MgO/n-Si nanostructures the tunneling magnetoresistance varivaries from 5 to 50% when the external bias is ranged from 0.1 to 2 V.

Impact of CNT medium on the interaction between ferromagnetic nanoparticles

We present results on low-temperature magnetization approaching the saturation law in aligned bundles of CNTs with ferromagnetic nanoparticles embedded inside inner channels of nanotubes for two directions of the magnetic field, parallel and perpendicular to the CNT axes. Elaborating experimental data, we were able to extract the explicit form of the correlation functions describing the orientation of the magnetic anisotropy axes in real space. In the parallel field the long-range coherence in the magnetic anisotropy axes is the characteristic feature. In the perpendicular direction the peculiar feature is the 2D exchange coupling. The nature of the exchange interaction and the role of the CNT medium in it is also discussed.

Synthesis and Properties of the Arrays of Magnetically Functionalized Carbon Nanotubes

Vladimir Labunov1, Alena Prudnikava1, Kazimir Yanushkevich2, Aleksander Basaev3, Alexander Danilyuk1, Yulia Fedotova4 and Boris Shulitskii1 1Belarussian State University of Informatics and Radioelectronics, 2Institute of Solid-State and Semiconductor Physics NASB, 3Scientific and Manufacturing Complex “Technological Centre,”Moscow Institute of Electronic Technology, 4National Scientific and Educational Centre of Particle and High-Energy Physics BSU, 1,2,4Belarus, 3Russia

Magnetic properties of CNT arrays synthesized by the injection CVD method at various catalyst concentrations

The arrays of multi-wall carbon nanotubes (CNTs) filled with ferromagnetic nanoparticles (MFCNTs) have been obtained by the high temperature pyrolysis of fluid hydrocarbon (o-xylene) in a mixture with the volatile source of catalyst (ferrocene) using Ar as the gas-carrier. The influence of the catalyst concentration cx (0.5%, 5%, and 10%) in the feeding solution on the composition, crystalline structure, morphology and, accordingly, magnetic properties of MFCNT arrays in a wide temperature range was investigated. The X-ray diffraction, SEM and TEM methods revealed that CNT arrays are filled by Fe3C and Fe phases and that the higher is the catalyst concentration in the feeding solution, the higher is Fe3C and Fe content in CNT arrays. Temperature dependence of the specific magnetization σ(T) shows that σ increases with the increasing of ferrocene concentration in a whole temperature range under investigation (78 ≤T < 600 K). It is shown that σ(T) follows the Bloch law in the temperature range 80-300 K with Bloch constant B=1.65.10-5 K-3/2 and the Stoner law at 300-480 K for samples with cx=10% and, correspondingly, 80-450 K with Bloch constant B=6.1.10-5 K-3/2 and 450-480 K for samples with cx=5%. The lower value of Bloch constant, which characterizes the exchange interaction, in the case of cx=10% might be attributed both to dimensional effects and the decrease of the effective magnetic momentum of Fe phases atoms. The hysteresis loops demonstrate that coercivity Hc(cx=5%) decreases, but Hc(cx=10%) is constant or even slightly increases with increasing the temperature. This phenomenon is explained by the increase both the saturation magnetization and shape anisotropy.

Simulation of mechanical properties and residual stress of nanostructural coatings based on transition metals nitrides

Physical properties of novel nanostructural coatings, formed by ion-plasmous flux from solid solutions of transition and refractory metals (Ti, Zr, Cr) have been intensively studied to enhance characteristics of tool materials. We have developed the modeling technique for effective predictions of internal stresses and calculation of elastic properties of nanostructural coatings composed of metal nitrides. Quantum-mechanical modeling of microstructure, elastic constants, bulk modulus and residual stress for binary and ternary metal nitride clusters have been performed. The dependences of these characteristics on the crystal structure deformations have been investigated. The essential modification of elastic constants and bulk moduli with changes in lattice constants and stoichiometric composition has been observed. The influence of elastically stressed state of sample on X-ray diffraction intensity has been examined by using the exponential model. The model of residual stress distribution identifying in depth of wear-resistant nanostructural coating from the data of diffraction experiments has been developed.