E. Lähderanta

Orientational mobility and relaxation spectra of dendrimers: Theory and computer simulation

The developed theory of the orientational mobility of individual segments of a perfectly branched dendrimer is used to calculate the relaxation spectrum of a dendrimer. Frequency dependences of NMR relaxation 1/T(1) and of the nuclear Overhauser effect have been theoretically calculated from the Brownian dynamics simulation data. The dendrimer segmental orientational mobility is governed by three main relaxation processes: (i) the rotation of the dendrimer as a whole, (ii) the rotation of the dendrimers branch originated from a given segment, and (iii) the local reorientation of the segment. The internal orientational mobility of an individual dendrimer segment depends only on the topological distance between this segment and the terminal shell of the dendrimer. Characteristic relaxation times of all processes and their contributions to the segmental mobility have been calculated. The influence of the number of generations and the number of the generation shell on the relaxation times has been studied. The correlation between the characteristic times and the calculated relaxation spectrum of the dendrimer has been established.

A spin-wave logic gate based on a width-modulated dynamic magnonic crystal

An electric current controlled spin-wave logic gate based on a width-modulated dynamic magnonic crystal is realized. The device utilizes a spin-wave waveguide fabricated from a single-crystal Yttrium Iron Garnet film and two conducting wires attached to the film surface. Application of electric currents to the wires provides a means for dynamic control of the effective geometry of waveguide and results in a suppression of the magnonic band gap. The performance of the magnonic crystal as an AND logic gate is demonstrated.

NMR Studies of Carbosilane Dendrimer with Terminal Mesogenic Groups

The 4-generation carbosilane dendrimer with terminal cyanobiphenyl mesogenic groups in dilute solution of CDCl(3) was investigated using (1)H NMR technique. The spectrum was obtained and the relaxation time, T(1), was measured in the temperature range 320-225 K. For the first time, the extrema of T(1) values were achieved for majority of the dendrimer functional groups. The values of activation energies of the dendrimer functional groups were obtained. The relaxation data for outer and inner methyl groups show that the dendrimer investigated has dense corona and hollow core. This structure is formed because the mesogenic groups do not allow terminal segments to penetrate into the dendrimer, that is, the backfolding effect is absent. The NMR spectral and relaxation data give evidence for changing conformation of the dendrimer internal segments with decreasing temperature. This reorganization is most likely connected with a change of dendrimer size. We suppose that our experimental results will provide additional information for understanding principles of dendrimer nanocontainer operation. NMR can possibly be a tool for indicating the encapsulation effect as well as the dendrimer effective size.

Ferromagnetism of low-dimensional Mn-doped III-V semiconductor structures in the vicinity of the insulator-metal transition

The structural and transport properties of GaAs/Mn/GaAs/InxGa1−xAs/GaAs quantum wells (x≈0.2) with Mn δ-layer (4–10 at. %), separated from the well by a GaAs spacer, have been studied. The hole mobility in the investigated structures has exceeded the values known for magnetic III-V heterostructures by two orders of magnitude. For structures with the conductivity of the metal type, we have succeeded to observe at low temperatures Shubnikov–de Haas oscillations just confirming the two dimensionality (2D) of the hole energy spectrum. Exactly those 2D holes promote the ferromagnetic ordering of the Mn layer. That has been proven by (i) observing maxima (at 25–40 K) in temperature dependencies of the resistance, which positions agree with calculated values of Curie temperatures (for structures with the indirect interaction of Mn atoms via 2D holes), and (ii) revealing the negative spin-dependent magnetoresistance (NMR) as well as the anomalous Hall effect (AHE), which values are also in good agreement with cal...

Photoluminescence properties of Eu3+ ions in yttrium oxide nanoparticles: defect vs. normal sites

The position of activator ions in the lattice has a fundamental effect on the luminescent properties of phosphors. In this paper, we describe the normal and defect sites of Eu3+ ions in a Y2O3 crystal lattice, their interaction and difference in physical properties. Analytically detectable amounts of defect sites reached in Y2O3:Eu3+ nanopowders with an average size of 40–50 nm were synthesized by a novel combined Pechini-foaming method. The luminescence properties of Eu3+ ions in defect sites are most pronounced in highly doped nanocrystalline powders (32 and 40 at%). To explain this phenomenon we suggest the mechanism of irreversible energy transfer from normal to defect sites of Eu3+ ions.

Edge state magnetism in zigzag-interfaced graphene via spin susceptibility measurements

Development of graphene spintronic devices relies on transforming it into a material with a spin order. Attempts to make graphene magnetic by introducing zigzag edge states have failed due to energetically unstable structure of torn zigzag edges. Here, we report on the formation of nanoridges, i.e., stable crystallographically oriented fluorine monoatomic chains, and provide experimental evidence for strongly coupled magnetic states at the graphene-fluorographene interfaces. From the first principle calculations, the spins at the localized edge states are ferromagnetically ordered within each of the zigzag interface whereas the spin interaction across a nanoridge is antiferromagnetic. Magnetic susceptibility data agree with this physical picture and exhibit behaviour typical of quantum spin-ladder system with ferromagnetic legs and antiferromagnetic rungs. The exchange coupling constant along the rungs is measured to be 450 K. The coupling is strong enough to consider graphene with fluorine nanoridges as a candidate for a room temperature spintronics material.

Synthesis and magnetic properties of the InSb-MnSb eutectic

It has been demonstrated by a combination of physical and chemical methods that InSb and MnSb form an eutectic (6.5 mol % MnSb, Tm = 513°C). A composition consisting of a [110] oriented InSb single crystal matrix and faceted single-crystal MnSb inclusions has been grown by the Bridgman method. The composition shows pronounced electrical conductivity anisotropy. The electrical conductivity along needle-like MnSb inclusions is 5 times higher than the electrical conductivity in the perpendicular direction. The composition possesses magnetic properties and has a Curie temperature of ∼600 K. The material is of interest as a substrate for ferromagnet/semiconductor heterostructures.

Influence of scattering and interference effects on the low-temperature magnetotransport of Cu2ZnSnS4 single crystals

Activated resistivity, ρ(T), and positive magnetoresistance (MR) are observed in Cu2ZnSnS4 single crystals within the temperature interval between T ∼ 2 and 300 K in pulsed magnetic fields of B up to 20 T. Between T ∼ 50 and 150 K, the charge transfer is described by the Mott variable-range hopping (VRH) transport over localized states of the defect acceptor band with width W ∼ 12–25 meV. Up to the highest applied fields, MR follows the law ln ρ(B) ∝ B2 pertinent to the VRH conduction at λ ≫ a, where λ is the magnetic length and a is the localization radius. The joint analysis of the MR and ρ(T) data yielded a series of microscopic parameters, including the values of a ≈ 22–45 A, depending on the proximity of a sample to the metal-insulator transition. However, below T ∼ 3–4 K the Shklovskii-Efros VRH conduction is observed. Here, the behavior of MR changes drastically, exhibiting a sharp contraction of the quadratic MR region and transformation of the MR law into those of ln ρ (B) ∝ B2/3 or ∝ B3/4 when B...

Average relaxation time of internal spectrum for carbosilane dendrimers: Nuclear magnetic resonance studies

A new theoretical description of the interior mobility of carbosilane dendrimers has been tested. Experiments were conducted using measurements of the (1)H NMR spin-lattice relaxation time, T(1H), of two-, three- and four-generation carbosilane dendrimers with three different types of terminal groups in dilute chloroform solutions. Temperature dependences of the NMR relaxation rate, 1/T(1H), were obtained for the internal CH(2)-groups of the dendrimers in the range of 1/T(1H) maximum, allowing us to directly evaluate the average time of the internal spectrum for each dendrimer. It was found that the temperature of 1/T(1H) maximum is practically independent of the number of generations, G; therefore, the theoretical prediction was confirmed experimentally. In addition, the average time of the internal spectrum of carbosilane dendrimers was found to be near 0.2 ns at room temperature, and this value correlates well with the values previously obtained for other dendrimer structures using other experimental techniques.

Kelvin probe force gradient microscopy of charge dissipation in nano thin dielectric layers

Application of Kelvin probe force gradient microscopy (KPFGM) to visualize the local charge dissipation in thin dielectric layers is considered. By this method, the local charge behavior in nano thin SiO2, Si3N4, and LaScO3 dielectric layers has been studied. Local charging of the layers has been performed at the point contact with a conductive probe. KPFGM potential images reveal variations of the surface potential in the locally charged areas, which makes it possible to detect the injected charge and to study its behavior. Special experiments on the SiO2 layers with embedded Si-nanocrystals, when lateral spreading of injected charge had been suppressed, permitted to demonstrate high (better than 20 nm) lateral resolution of KPFGM observations. A simple electrostatic model has been developed to estimate the total amount of injected charge. The obtained estimations made it possible to control charge retention in the dielectric layer and possible leaks into the substrate. The studied dielectric layers demo...