Nanodot Systems Research Articles

Magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM) studies on Cr/Ni nanodot arrays deposited using innovative nano-stencil method

AbstractNanodot arrays of Nickel (Ni) on Chromium (Cr), [Cr in two forms i. e. (Cr dot) and Cr (layer)] were fabricated by electron beam evaporation technique using nano-stencil. The deposition of a periodic dot pattern was confirmed through atomic force microscopy (AFM) imaging. The nano magnetic properties of the Cr/Ni nanodot arrays were probed using both Magnetic Force Microscopy (MFM) and Magneto-Optical Kerr Effect (MOKE) magnetometer. MOKE measurements unveiled saturated hysteresis with the exchange bias effect for the in-plane configuration and a minor hysteresis loop for the out-of-plane configuration in the Cr/Ni nanodot system. The experimental results were found to be in good agreement with the theoretical Stoner-Wohlfarth (SW) model. Additionally, the exchange bias effect was also observed in the Cr (thin film)/Ni (nanodot) system for both the in-plane and out-of-plane configurations, showing the dominating behavior of the Cr layer over the Ni dots due to its continuous layer and higher thickness. The observation of exchange bias properties was also verified through MFM measurements conducted with both positive and negative in-plane magnetic fields. Hence, this straightforward nano-stencil method holds significant promise for the streamlined production of patterned AFM/FM arrays enabling the comprehensive study of exchange bias phenomena. The magnetic properties of the Cr/Ni structure can be manipulated by depositing Cr either in nanodot form or in continuous layer form.

Strain-mediated 180° switching in CoFeB and Terfenol-D nanodots with perpendicular magnetic anisotropy

A micromagnetic and elastodynamic finite element model is used to compare the 180° out-of-plane magnetic switching behavior of CoFeB and Terfenol-D nanodots with perpendicular magnetic easy axes. The systems simulated here consist of 50 nm diameter nanodots on top of a 100 nm-thick PZT (Pby[ZrxTi1-x]O3) thin film, which is attached to a Si substrate. This allows voltage pulses to induce strain-mediated magnetic switching in a magnetic field free environment. Coherent and incoherent switching behaviors are observed in both CoFeB and Terfenol nanodots, with incoherent flipping associated with larger or faster applied switching voltages. The energy to flip a Terfenol-D memory element is an ultralow 22 aJ, which is 3–4 orders more efficient than spin-transfer-torque. Consecutive switching is also demonstrated by applying sequential 2.8 V voltage pulses to a CoFeB nanodot system with switching times as low as 0.2 ns.

Tuning the magnetic phase of a graphene nanodot using its dielectric environment

The magnetic properties of a hexagonal graphene nanodot are investigated using a configuration-interaction approach. The phase diagram of the nanodot system is obtained for a wide range of Coulomb interaction strengths and dielectric environments, and shows that the magnetic phase transition can be effectively tailored by changing the dielectric environment. Selective magnetic phase between non-magnetic and ferromagnetic could therefore be achieved simply by changing to an appropriate substrate for the graphene nanodot.

TH-C-201B-07: How Accurate Is Estimating CT Skin Dose Based on CTDI?

Purpose: To explore alternative techniques for the measurement of CTskin dose and compare with the CTDI method. Method and Materials: Using a Farmer chamber point doses were measured under various scan conditions on the surface of anthropomorphic phantoms as well as at the top‐hole position of CTDI phantoms on two CTscanners of different manufacturers. Also at the top‐hole position of CTDI head and body phantoms CTDI100 values were measured using a 100mm pencil ion chamber. These CTDI100 (top) values were then normalized by the pitch factors. The ratio of CTDI100 (top)/pitch over the point dose was obtained under each scan condition and evaluated for quantitatively determining if the CTDI method underestimates or overestimates skin dose. Size of the Farmer chamber (length 18.8mm) prompted the use of nanoDOT dosimeters (4mm diameter Landauer) for additional point dose measurements under the same conditions as the Farmer chamber. The nano DOTs were calibrated using the same kVp as the CT scans;calibration was conducted separately for each ∼20–65%. Results: For cine scans CTDI100 (top)/pitch overestimated skin dose by‐20–65%. For helical scans CTDI100 (top)/pitch overestimated skin dose by ∼20–50%. The variations mainly depend on the size and shape of the phantom and on the type of scanner. The nanoDOT system produced results in general agreement (within 15%) of those obtained using a Farmer chamber. Conclusion: Alternative techniques using Farmer chamber and nanoDOTs for determination of CTskin dose were evaluated and compared to CTDI results. Farmer chamber produces readily available dose reading vs the nanoDOTs which must be processed using a separate readout system. On the other hand nanoDOTs can be attached to a patient's skin for monitoring skin dose during a clinical exam and do not cause artifacts in images. Both techniques show promise for clinically practical skin dose CTdosimetry.

Fe nanodot system fabricated by non-lithographic method and its structural properties

In this work, we study the magnetic structure and morphology of the Fe nanodot system fabricated by the non-lithographic method, using anodic aluminum oxide (AAO) membrane as a template. By the two-steps aluminum anodization, the AAO patterns with the hexagonal pore arrangement have been achieved. Using AAO pattern as a template, under suitable conditions we successfully deposited the iron metal in the pores by the thermal vacuum evaporation. By the exposure of the deposited system from the bottom of the AAO membrane, the hexagonal ordered Fe nanodot system has been obtained. The morphologies of the nanodot system were imaged by the Atomic Force Microscopy (AFM) and Field Emission Scanning Microscopy (FESEM) methods. The magnetic structures were investigated by the Energy Dispersive X-Ray Fluorescence Spectroscopy (EDS) and Magnetic Force Microscopy (MFM) methods. Experimental results confirmed that the MFM image of the fabricated Fe nanodot system is similar to their AFM image.

Epitaxial FePt thin films and nanodots integrated with Si (1 0 0)

We present findings on the synthesis and processing of various FePt nanostructures using pulsed laser deposition. FePt was deposited as (a) continuous thin films, (b) single layered nanodots and (c) multi-layered nanodots. Integration of these FePt nanostructures with Si (1 0 0) was achieved by using epitaxial TiN as a template buffer layer. Epitaxy and L10 order in the FePt structures were investigated in detail by x-ray diffraction and transmission electron microscopy techniques. Remarkably, we achieved ⟨0 0 1⟩ c-axis growth orientation and L10 ordering at a low processing temperature of 500 °C. The following epitaxial relationship was obtained in the FePt structures: FePt (0 0 1) ⟨0 0 1⟩ ∥ TiN (1 0 0) ⟨0 0 1⟩ ∥ Si (1 0 0) ⟨0 0 1⟩. The 9.5% misfit strain between FePt and TiN is relaxed by domain matching epitaxy where 10/19 and 11/10 domains alternate across the interface. The composition of the nanostructures was determined by Rutherford backscattering and was found to be Fe41Pt59. The magnetic properties of each of the three nanostructured systems were studied individually and then compared to evaluate them as potential information storage media. Room temperature perpendicular coercivity of 3200 Oe for the single layered FePt nanodot sample was much improved when compared with the continuous epitaxial FePt film sample showing a coercivity of 2250 Oe. In the FePt nanodot system, each nanodot acts as a single domain and assuming one bit of information is stored in each nanodot, magnetic storage density can exceed one terabit per square inch, corresponding to 250 million pages of information per chip.

Prediction of size disorder induced resonant tunneling in large scale nanodot arrays

AbstractWe have successfully investigated the effect of size fluctuation on the resonant tunneling characteristics of nanodots arrays. The approach was realized with taking into account the induced electron energy level disorder under the framework of the generalized Breit–Wigner formula. It is found that the dot size fluctuation has played an important role in both the tunneling current profile and number of the resonant peaks. By the aid of the statistical concept to characterize the size fluctuation in large scale nanodots systems, we have shown a clear picture for the dependence of the resonant peak number and peak to valley ratio on the degree of disorder and spatial configuration in the nanodots. The whole investigation will give a general prediction of the origin of resonant tunneling phenomena observed in large scale nanodot systems. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Forces that Drive the Self-assembly of Metallic Dots on Semiconductor Substrates

ABSTRACTThe scaling and ordering of metallic nanoclusters on semiconductor substrates has been explored computationally. Numerical techniques were implemented to calculate the total electrostatic and van der Waals energies for systems containing multiple disks. Observations show that interactions in charge clouds beneath the metallic disks led to an electrostatic repulsive force. Attraction was observed due to the van der Waals energy. An energy barrier exists for disk coalescence. This work suggests the potential of double layer charges in the development of large-scale nanodot systems that require precise control over both dot location and size.

Local self-organization of islands in embedded nanodot systems

We show that strain distribution on the surface of an isotropic spacer layer induced by an embedded island of large base-to-height aspect ratio deviates significantly from the description of the point force dipole model in the regime of small spacer layer thickness. In this regime, the strain profile displays several local maxima above the embedded island. The regions with local strain maxima serve as nucleation centers for growth of surface islands under appropriate growth conditions, resulting in locally well-organized surface islands above the embedded island. Our theoretical results are in excellent agreement with recent experiments for Ge islands embedded in Si.