Magnetic Nanodevices Research Articles

Fabrication of dense non-circular nanomagnetic device arrays using self-limiting low-energy glow-discharge processing.

We describe a low-energy glow-discharge process using reactive ion etching system that enables non-circular device patterns, such as squares or hexagons, to be formed from a precursor array of uniform circular openings in polymethyl methacrylate, PMMA, defined by electron beam lithography. This technique is of a particular interest for bit-patterned magnetic recording medium fabrication, where close packed square magnetic bits may improve its recording performance. The process and results of generating close packed square patterns by self-limiting low-energy glow-discharge are investigated. Dense magnetic arrays formed by electrochemical deposition of nickel over self-limiting formed molds are demonstrated.

Water-Soluble and Biocompatible MnO@PVP Nanoparticles for MR Imaging <I>In Vitro</I> and <I>In Vivo</I>

The uniform-sized manganese oxide nanoparticles (the oleic-capped MnO NPs) were synthesized by the thermal decomposition of Mn-oleate complex and were transferred into water with the help of cationic surfactant of cetyltrimethyl ammonium bromide (CTAB), then the poly(vinylpyrrolidone) (PVP) membrane was further coated on to them with the aid of anionic dispersant of poly(styrenesulfonate) (PSS) by layer-by-layer electrostatic assembly to render them water soluble and biocompatible. They were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and MTT assay. In vitro cellular uptake test revealed the MnO@PVP NPs were low cytotoxic, biocompatible and could be used as a T,-positive contrast agent for passive targeting magnetic resonance imaging (MRI). Interestingly, signal enhancement in cerebral spinal fluid (CSF) spaces in vivo experiment suggested that the MnO@PVP NPs can pass through the blood brain barrier (BBB). These results show that MnO@PVP NPs are good candidates as MRI contrast agents with the lack of cytotoxicity and have great potential applications in magnetic nano-device and biomagnetic field.

Sub-10-nm-resolution electron-beam lithography toward very-high-density multilevel 3D nano-magnetic information devices

We report a study on the optimization of ultra-high-resolution electron-beam lithography for nanoscale patterning with two separate lift-off processes using positive and negative resists; the optimized method is suitable for the emerging area of nano-magnetoelectronics. If used together, these high-aspect-ratio processes can achieve information cells with a diameter of 9 nm, a square pitch of 26 nm, and an etch depth of at least 50 nm, as required for recording densities greater than 3 Tbit/in2, provided that 3D integration includes between 2 and 8 independent magnetic bits. Such effective patterning can be used to further develop magnetic bits packed for ultra-high-density disk recording and the emerging field of magnetic tunneling junctions for logic and memory applications.

Characterization of Fe3O4/SiO2 composite core-shell nanoparticles synthesized in isopropanol medium

Based on a solvothermal method, Fe3O4 was modified by inorganic particles in isopropanol solution to synthesize core-shell structure Fe3O4/SiO2 complex under mechanical agitation. The product was characterized by the X-ray powder diffraction (XRD), the Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), and scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), etc. The synthesis conditions of the product were studied and the results showed the morphology and properties of the products were affected by the amount of ammonia, silicon source quantity, and the amount of Fe3O4 particles. Magnetic testing results show that the synthesized Fe3O4 particles showed excellent magnetic property, which made them have potential application in magnetic nanodevices and biomedicine.

Spin wave-assisted reduction in switching field of highly coercive iron-platinum magnets

Recent rapid progress in spintronic and magnetic storage nanodevices has required nanomagnets to balance competing goals for high coercive field and low switching field. However, a decisive route for highly efficient magnetization switching has not been established yet. Here we propose a novel switching method using a spin wave of magnetic structures twisted in a nanometre scale. We have experimentally demonstrated extremely low field-magnetization switching in a highly coercive FePt by using a spin wave excited in a soft magnetic permalloy (Ni81Fe19), where permalloy is exchange-coupled to FePt through the interface. We can tune the switching field by varying the magnitude and frequency of the radio frequency magnetic field, and a significant decrease in switching field by one order of magnitude is achieved under the optimum conditions. The spin wave-assisted magnetization switching is a promising technique for ultralow-energy magnetization manipulation.

Information transport in field-coupled nanomagnetic logic devices

The information transport in field-coupled nanomagnetic logic (NML) systems is demonstrated by investigating signal propagation in a circular chain of magnets. Design criteria for the magnet layout, signal injection timing, and the required clocking field are presented. The strong interaction between the magnets is estimated by simulations and verified by hysteresis curve measurements. Signal transmission in the magnetic wire is confirmed by magnetic force microscopy measurements, especially the propagation of a metastable pair of magnets with parallel magnetization. For the first time, a field-coupled magnetic logic device is successfully operated for hundreds of clocking cycles. Extensive studies verify the reliability and robustness of information transport in field-coupled NML systems from perpendicular magnetic media.

First-Principles Study on Electronic Structure and Magnetic Properties of Freestanding Ni Nanobelts

Calculations have been performed for the relaxed structures and electronic properties of Ni nanobelts with the cross-section 3×3, 3×5, 3×7 and 3×9 atomic layers, using the first-principles projector-augmented wave (PAW) potential within density functional theory (DFT) framework. For all of the four size Ni nanobelts, the most atoms relax inward and the magnetic moment decrease as the belt width increase. Compared with bulk Ni, the freestanding Ni nanobelt offer strong spin polarization at the Fermi level as well as considerable magnetic moment. Thus such structures can be potentially utilized to design magnetic nano-devices.

Spin precession modulation in a magnetic bilayer

We report on modulation of the spin precession in a Co/garnet bilayer by femtosecond laser excitation using time-resolved magneto-optical tools. Damped oscillations in the Faraday rotation transients representing precessional motion of the magnetization vector are observed in both the 2 nm Co layer and 1.8 μm garnet of the bilayer with distinct frequencies differing by about a factor of two. The excitation efficiency of these precessions strongly depends on the out-of-plane magnetic field. The modulation effect with the coupling in a magnetic bilayer can be useful for non-thermally controlling the magnetization of nanomagnets and ultrafast switching in magnetic nanodevices.

Physico-chemical studies on synthesis, characterization, and magnetic properties of Li–Ca–Zn nanoferrites

The synthesis of Li0.25Ca0.5–X Zn X Fe2.25O4 (LCZ) spinel nanoferrites by the solution combustion method using oxalyl-dihydrazide as a fuel leads to the formation of fine nanoparticles. Structural studies such as X-ray diffraction and transmission electron microscope confirm the formation of single phase spinel with particle size in the range 26–43 nm while Mossbauer data under the shadow of magnetic studies attribute to the superparamagnetic nature of the ferrites obtained, resulting from the transition of ferrimagnetic to superparamagnetic phase. In addition, the magnetic studies reveal an initial increase in the saturation magnetization (M S) from 33.5 emu/g for X = 0–41.4 emu/g (X = 0.3) followed by a gradual decrease to 32.8 emu/g (X = 0.5) as the concentration of zinc increases while Curie temperature lies in the range 312–527 °C for the same series. Keeping in view the various properties of the nanoferrites formed, variation of dielectric constant has also been carried out with both frequency (20 Hz–1 MHz) and temperature (up to 500 °C). We believe that the synthesized LCZ nanoferrites can be suitable for their application in nanomagnetic and nanoelectronic devices for drug delivery and bioencapsulating systems.

Effects of spin-polarized current on pulse field-induced precessional magnetization reversal

We investigate effects of a small DC spin-polarized current on the pulse field-induced precessional magnetization reversal in a thin elliptic magnetic element by micromagnetic simulations. We find that the spin-polarized current not only broadens the time window of the pulse duration, in which a successful precessional reversal is achievable, but also significantly suppresses the magnetization ringing after the reversal. The pulse time window as well as the decay rate of the ringing increase with increasing the current density. When a spin-polarized current with 5 MA/cm2 is applied, the time window increases from 80 ps to 112 ps, and the relaxation time of the ringing decreases from 1.1 ns to 0.32 ns. Our results provide useful information to achieve magnetic nanodevices based on precessional switching.

Magnetic nano-oscillator driven by pure spin current

With the advent of pure-spin-current sources, spin-based electronic (spintronic) devices no longer require electrical charge transfer, opening new possibilities for both conducting and insulating spintronic systems. Pure spin currents have been used to suppress noise caused by thermal fluctuations in magnetic nanodevices, amplify propagating magnetization waves, and to reduce the dynamic damping in magnetic films. However, generation of coherent auto-oscillations by pure spin currents has not been achieved so far. Here we demonstrate the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses. Counterintuitively, radiation losses enable excitation of auto-oscillation, suppressing the nonlinear processes that prevent auto-oscillation by redistributing the energy between different modes. Our devices exhibit auto-oscillations at moderate current densities, at a microwave frequency tunable over a wide range. These findings suggest a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics.

Stability criterion for critical points of a model in micromagnetics

A recent modification of a classic Landau-Lifshitz equation that includes the socalled spin-transfer torque is widely recognized in physics community as a model of magnetization dynamics in certain nanodevices. Motivated by some experimental evidence, we introduce a generalization of this model, coupled Landau-Lifshitz equations with spin-transfer torque terms, and analyze it from dynamical systems standpoint. An explicit stability criterion for the critical points in terms of all parameters of the system is derived and illustrated with stability diagrams. Our analysis provides certain guidelines for the design of magnetic nanodevices with optimized response to control parameters.

Spectroscopic defect imaging in magnetic nanostructure arrays

We introduce a method for imaging defective structures in an array of magnetic nanodevices using ferromagnetic resonance force microscopy with contrast between normal and defective devices provided through differences in resonance condition. In a demonstration of this technique, two dimensional scans of an array resolve not only intentional differences in resonant field between 200 nm circular dots and an intentional oval “defect,” but also smaller differences between the nominally identical circular dots in the array.

Development of CAD tools for nanomagnetic logic devices

ABSTRACTSingle‐domain models are widely used for the approximate description of nanoscale magnetic switching. The goal of this article is to demonstrate them as a computationally efficient and physically established tool for the modeling and design of nanomagnetic logic (NML) devices. We introduce a systematic method for deriving single‐domain parameters from numerical magnetic field calculations and investigate the validity of the single‐domain description for lithographically fabricated nanomagnets in the sub‐100‐nm size range. We illustrate how a library of NML building blocks can be constructed and aid the design of complex circuitry. It will be demonstrated how single‐domain models can be used to understand error formation in NML structures. The methods described here serve as a theoretical framework for a Simulation Program with Integrated Circuit Emphasis (SPICE)‐like modeling tool for NML devices. Copyright © 2012 John Wiley & Sons, Ltd.

Electrical input structures for nanomagnetic logic devices

Co/Pt nanomagnets close to a current driven wire are proposed as an electrical input interface for nanomagnetic logic. Their switching field shows a significant dependence of the external magnetic field sweep rate and the temperature, which can be modeled by the calibrated Sharrock formula, realizing coercivity extrapolations in the µs time scale. The influence of the current pulse in the wire is modeled as a generated Oersted field and an additional temperature rise in the structure. The models are used to simulate the current pulse characteristics sufficient to control the input magnet. Oersted switching experiments verify the proposed modeling, visualize controlled nanomagnets in close vicinity of the wire and thereby demonstrate the functionality of the input device.

Nanomagnetic logic: compact modeling of field-coupled computing devices for system investigations

In this paper we investigate error rates of nanomagnetic logic devices with perpendicular magnetization by compact modeling. Two different types of nanomagnets for information propagation and logic computing are introduced. The switching behavior of field-coupled nanomagnets is measured and analyzed. A compact model is derived from physics and experimental results are applied to the magnetic compact model. General requirements for fabrication parameters and clocking fields for reliable operation are extracted. We perform simulations and measurements on single devices to demonstrate the accuracy of the macromodel. Simulations on complex systems show that the error rate of a field-coupled magnetic system strongly depends on the variation of the switching field and the strength of the coupling field between the nanomagnets. The error rate of a 1-bit full adder is investigated for varying dot parameters. The results demonstrate the importance of fast simulation tools for investigations on the design of nanomagnetic computing devices and systems.

Novel rose-type magnetic (Fe 3O 4, γ-Fe 2O 3 and α-Fe 2O 3) nanoplates synthesized by simple hydrothermal decomposition

Rose-type magnetic nanoplates (RTMNPs) were synthesized by a simple hydrothermal decomposition method where FeCl 2·4H 2O was solely used as a precursor. The synthesized nanoplates were characterized using XRD, FE-SEM, UV–vis absorption (reflectance) spectra and magnetic hysteresis loops. The resulting nanoplates were in the ranges of size 350–500 nm and width 60–70 nm with high crystallinity, purity (shown by XRD) and reproducibility. These iron oxide nanoplates have a great potential in magnetic nanodevices and biomagnetic applications.

Synthesis and properties of Fe3O4 nanoparticles by sol-vothermal method using iron(III) acetylacetonate

Well dispersed Fe3O4 nanoparticles were synthesized at 180°C by sol-vothermal method, using iron (III) acetylacetonate as iron source and poly-vinilpyrrolidone (PVP) as special surfactant. The factors affecting reaction system, such as reaction temperature and time, the amount of iron source and surfactant are discussed. The synthesized Fe3O4 particles show excellent saturation magnetization and super-paramagnetic properties, demonstrating their potential applicability in magnetic nanodevices and bio-medicine.

First principle study of the electronic and magnetic properties of a single iron atomic chain encapsulated in boron nitrite nanotubes

The electronic and magnetic properties for a single Fe atom chain wrapped in armchair ( n , n ) boron nitride nanotubes (BNNTs) ( 4 ≤ n ≤ 6 ) are investigated through the density functional theory. By increasing the nanotube diameter, the magnetic moments, total magnetic moments and spin polarization of Fe @ ( n , n ) systems are increased. We have calculated the majority and minority density of states (DOS) of armchair Fe @ ( 6 , 6 ) BNNT. Our results show that the magnetic moment of the system come mostly from the Fe atom chain. The magnetic moment on an Fe atom, the total magnetic moment and spin polarization decrease by increasing the axial separation of the Fe atom chain for the Fe @ ( 6 , 6 ) system. The Fe @ ( 6 , 6 ) BNNT can be used in the magnetic nanodevices because of higher magnetic moment and spin polarization.

Structural, electronic, and magnetic properties of C-doped GaN nanoribbon

First-principles calculations are performed to study the structural, electronic, and magnetic properties of pure and C-doped GaN nanoribbon with both zigzag edge (ZGaNNR) and armchair edge (AGaNNR). With increasing ribbon width, both the indirect bandgap of ZGaNNR and the direct bandgap of AGaNNR decrease monotonically and become closer to each other approaching their asymptotic limit of a single layer of GaN sheet. One C atom substituting for one N atom causes a slight local expansion, while one C atom substituting for one Ga atom results in a large local contraction. Furthermore, the C atom is preferred to substitute for an edge N or Ga atom in either 6-ZGaNNR or 6-AGaNNR, especially edge Ga atoms in 6-AGaNNR. There exists about 0.65 µB magnetic moment, which arises mainly from doped C atom while a single N atom is substituted by one C atom in either 6-ZGaNNR or 6-AGaNNR, which may open a way to design magnetic nanodevices based on GaNNR.