Dmitri Litvinov

Tuning the Magnetic Properties of Nanoparticles

The tremendous interest in magnetic nanoparticles (MNPs) is reflected in published research that ranges from novel methods of synthesis of unique nanoparticle shapes and composite structures to a large number of MNP characterization techniques, and finally to their use in many biomedical and nanotechnology-based applications. The knowledge gained from this vast body of research can be made more useful if we organize the associated results to correlate key magnetic properties with the parameters that influence them. Tuning these properties of MNPs will allow us to tailor nanoparticles for specific applications, thus increasing their effectiveness. The complex magnetic behavior exhibited by MNPs is governed by many factors; these factors can either improve or adversely affect the desired magnetic properties. In this report, we have outlined a matrix of parameters that can be varied to tune the magnetic properties of nanoparticles. For practical utility, this review focuses on the effect of size, shape, composition, and shell-core structure on saturation magnetization, coercivity, blocking temperature, and relaxation time.

Gold Nanoparticle Effects in Polymerase Chain Reaction: Favoring of Smaller Products by Polymerase Adsorption

Gold nanoparticles were recently reported to reduce the formation of nonspecific products in polymerase chain reaction (PCR) at remarkably low temperatures, with hypothesized mechanisms including adsorption of DNA and heat-transfer enhancement. In contrast to these reports, we report that gold nanoparticles do not enhance the specificity of PCR but rather suppress the amplification of longer products while favoring amplification of shorter products, independent of specificity. Gold nanoparticles bearing a self-assembled monolayer of hexadecanethiol did not affect PCR, suggesting that surface interactions play an essential role. This role was further confirmed by experiments in which a similar effect on PCR was observed for the same total surface area of particles over a 100-fold range of per-particle surface area. The effect was seen with Taq and Tfl polymerases but not with Vent polymerase, and the effects of nanoparticles can be reversed by increasing the polymerase concentration or by adding bovine serum albumin (BSA). Transient high-temperature nanoparticle pre-exposure of PCR mix containing polymerase but not template or primers, followed by nanoparticle removal, modified subsequent nanoparticle-free PCR. Interaction between polymerase and gold nanoparticles was confirmed by changes in nanoparticle absorption spectrum and electrophoretic mobility in the presence of polymerase. Taken together, these results suggest that the nanoparticles nonspecifically adsorb polymerase, thus effectively reducing polymerase concentration.

Recording physics of perpendicular media: soft underlayers

The results of both theoretical and experimental studies of some of the key aspects of the recording physics specific to perpendicular media with a soft underlayer are presented. Some of the discussed issues are the material requirements for the preferred soft underlayer design such as the role of magnetic anisotropy, the proper choice of the magnetic moment, and the control of the soft underlayer noise.

Semiconducting cubic boron nitride

Abstract We present new developments in the preparation of semiconducting cubic boron nitride. The thin films were grown on (100) silicon substrates using electron cyclotron resonance (ECR) ion-assisted magnetron sputtering with the kinetic energy of the incident nitrogen ions controlled by a dc substrate bias. Using this technique we have been able to grow thick (up to 2 μm) boron nitride films containing 100% of the cubic phase. We have found that the relatively high nitrogen ion energy (∼ 100 cV), required to nucleate the cubic phase, can be reduced substantially (to ∼ 60 eV) once the cubic phase is formed, leading to reduced film stress, larger grain size (∼ 1000 A) and improved adhesion. The films have p -type conductivity. The carrier activation energy is 60 meV and we have observed Hall mobilities of 500 cm 2 V −1 s −1 at n a ≈ (5 × 10 18 cm −3 ).

Microstructural Origin of Switching Field Distribution in Patterned Co/Pd Multilayer Nanodots

We have identified an important microstructural origin of the switching field distribution (SFD) in patterned Co∕Pd multilayer nanodots. In this study, we patterned a marked array of 115nm Co∕Pd nanodots on 50-nm-thick Si3N4 substrate. We identified the dots with unusually small and large (>2 standard deviations of the mean) switching fields with magnetic force microscopy, followed by microstructural characterization of the same dots with transmission electron microscopy (TEM). From electron diffraction, we found that most nanodots with small switching fields have strong (200) spots, whereas those with large switching fields lack these spots. While bright-field TEM images reveal an average grain size of 7nm, dark-field images of the (200) spots reveal on average, a single grain of >10nm in lateral dimensions. Since we observed a direct correlation between strong (200) reflections and small reversal fields, we conclude that the largest grain in each nanodot, with an in-plane [001] is the likely cause for p...

Reduced bias growth of pure-phase cubic boron nitride

We report results on an improved growth process for cubic boron nitride (c-BN) films. The films are deposited on a dc-biased silicon substrate using ion-assisted sputtering. First, we grow a BN template layer at a bias voltage which maximizes the sp3 content. After this template layer attains a thickness of ∼500 A, corresponding to the coalescence of the mosaiclike grain structure, we find that we can reduce the substrate bias to about 50% of its initial value while sustaining pure phase c-BN growth. The reduction in nitrogen ion energy results in a dramatic increase in the growth rate as well as significantly improved film quality.

Perpendicular magnetic recording: Writing process

In this article, a detailed overview of the methodology to design a write transducer for recording onto perpendicular media at areal densities beyond 1 Tbit/in.2 is presented. The two basic modes of perpendicular recording, single-layer recording media in combination with a ring type head and double-layer recording media with a soft underlayer in combination with a single pole head, are compared with each other theoretically and experimentally. Moreover, perpendicular recording is compared to longitudinal recording from the perspective of the writing process. The system efficiency is redefined for perpendicular recording to take into account the critical role of the soft underlayer. The effects of using “soft” magnetic shields around the trailing pole are analyzed. It is shown that at least a factor of 2 increase in the field can be obtained at areal densities beyond 500 Gbit/in.2 if shields are used. Such an open issue as the skew angle sensitivity in perpendicular recording is analyzed. It is shown that...

In situ texture monitoring for growth of oriented cubic boron nitride films

We report evidence for oriented growth of pure-phase cubic boron nitride on silicon (100) substrates. The films are deposited at high temperatures (up to 1200 °C) by reduced-bias ion-assisted sputtering. The growth technique produces highly textured c-BN films with relatively large grain size (∼1000 A) and reduced residual stress as the bias voltage is decreased. We have been able to grow thick (up to 2 μm) cubic boron nitride films containing 100% of the cubic phase with the (001) crystallographic axis of c-BN oriented perpendicular to the surface of the film. We show how reflection high-energy electron diffraction applied to texture monitoring in polycrystalline films can be used as an in situ process control technique that allows texture identification and quantitative characterization of its angular spread.

Effect of nitrophenyl functionalization on the magnetic properties of epitaxial graphene.

Graphene displays unprecedented electronic properties including room-temperature ballistic transport and quantum conductance, and because of its small spin-orbit interaction, graphene has the potential to function as the building block of future spintronic devices. Theoretical calculations indicate that a defective graphene sheet will be simultaneously semiconducting and magnetic; thus it would act as a room-temperature magnetic semiconductor. Recently, ferromagnetic ordering at room temperature has been observed by magnetometry measurements on bulk samples of reduced graphene oxide.

Magnetic force microscopy study of magnetic stripe domains in sputter deposited Permalloy thin films

A magnetic force microscopy based study on the formation of stripe domains in Permalloy (Ni80Fe20) thin films is presented. Our results show that the critical thickness for stripe domain initiation depended on the sputtering rate, the substrate temperature, and the film thickness. Beyond the stripe domain formation, an increase of the period of a highly ordered array of stripe domains was evident with increasing film thickness. Thin films sputtered at room temperature with thickness variation between ∼80 and ∼350nm exhibited square-root growth dependency on stripe domains periodicity from ∼150to∼380nm, respectively. Above a certain thickness, the domain period decreased and the periodicity deteriorated with the array becoming more random, which is a strong indicator of relatively high structural perpendicular anisotropy. To illustrate, Permalloy sputtered at 100°C initially showed linear dependence in stripe domain periodicity growth up until ∼650nm thick films. The magnetic stripe domain structure began ...