Object Oriented Micromagnetic Framework Research Articles

Magnetic domain structure of La0.7Sr0.3MnO3 nanoislands: Experiment and simulation

We have studied the magnetic domain structure of La0.7Sr0.3MnO3 (LSMO) nanoislands with dimensions on the order of 160 ~ 720 nm. Domain structures of (001)-oriented LSMO rectangular nanoislands exhibit single and multiple flux-closed domain states depending on aspect ratio. (110)-oriented rectangular nanoislands, with a strain-induced magnetically easy axis parallel to the long axis of the rectangle, exhibit single and two domain states depending on aspect ratio. (110) elongated hexagonal islands, with similar aspect ratios, uniformly exhibit single domain states due to the apparent suppression of domain nucleation at tapered edges. These results are consistent with object-oriented micromagnetic framework simulations of LSMO islands.

Tailoring the magnetic properties of Fe asymmetric nanodots

Asymmetric dots as a function of their geometry have been investigated using three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code. The effect of shape asymmetry of the disk on coercivity and remanence is studied. Angular dependence of the remanence and coercivity is also addressed. Asymmetric dots are found to reverse their magnetization by nucleation and propagation of a vortex, when the field is applied parallel to the direction of asymmetry. However, complex reversal modes appear when the angle at which the external field is applied is varied, leading to a non-monotonic behavior of the coercivity and remanence.

Characteristics of signal propagation in magnetic quantum cellular automata circuits

Magnetic quantum cellular automata (MQCA) is a nano-scale computational paradigm that utilises magnetic dipolar interaction and coupling to propagate and process binary information. Characteristics of signal propagation in MQCA circuits with pipelined clocking signals are evaluated for the purposes of fast and reliable logic operation. These circuits are simulated via the Landau–Lifshitz–Gilbert equations and object-oriented micro-magnetic framework tool. It is found that signal propagations in the thin nanomagnet circuits are blocked significantly as the damping parameter is increased; however, their switching times decrease as the damping parameter increases. Switching speeds of MQCA structures are improved as the thickness and damping parameter are increased. The results also show that the majority logic gate performs a complete reliable operation for nanomagnet thicker than about 20 nm, whereas for interconnect wire, thickness ranging from only about 20 to 25 nm introduces a successful operation.

Rapid prototyping of magnetic tunnel junctions with focused ion beam processes

Submicron-sized magnetic tunnel junctions (MTJs) are most often fabricated by time-consuming and expensive e-beam lithography. From a research and development perspective, a short lead time is one of the major concerns. Here, a rapid process scheme for fabrication of micrometre size MTJs with focused ion beam processes is presented. The magnetic properties of the fabricated junctions are investigated in terms of magnetic domain structure, tunnelling magnetoresistance (TMR) and coercivity, with extra attention given to the effect of Ga implantation from the ion beam. In particular, the effect of the implantation on the minimum junction size and the magnetization of the sensing layer are studied. In the latter case, magnetic force microscopy and micromagnetic simulations, with the object-oriented micromagnetic framework (OOMMF), are used to study the magnetization reversal. The fabricated junctions show considerable coercivity both along their hard and easy axes. Interestingly, the sensing layer exhibits two remanent states: one with a single and one with a double domain. The hard axis TMR loop has kinks at about ±20 mT which is attributed to a non-uniform lateral coercivity, where the rim of the junctions, which is subjected to Ga implantation from the flank of the ion beam, is more coercive than the unirradiated centre. The width of the coercive rim is estimated to be 160 nm from the hard axis TMR loop. The easy axis TMR loop shows more coercivity than an unirradiated junction and, this too, is found to stem from the coercive rim, as seen from the simulations. It is concluded that the process scheme has three major advantages. Firstly, it has a high lateral and depth resolution—the depth resolution is enhanced by end point detection—and is capable of making junctions of sizes down towards the limit set by the width of the irradiated rim. Secondly, the most delicate process steps are performed in the unbroken vacuum enabling the use of materials prone to forming oxides in the MTJ film stack. Thirdly, the scheme is both uncomplicated and quick and makes it possible to go from design to characterization in a few hours.

Magnetization reversal induced by irregular shape nanodots in square arrays

Ni 80Fe 20 nanodots in square arrays of irregular shape (C 1h(m) and x-, y-translations symmetry) and circular shape (D 4h (4/mmm)) nanodots of the same area were fabricated under controlled exposure conditions by e-beam lithography, ion beam sputtering coating and further lift-off. The center-to-center nearest dot distances was 700 nm in all the measured arrays. An unpatterned film was fabricated in the same IBS batch for comparison purposes. Structures and magnetic properties were characterized using AFM, SEM and high-sensitivity focused magneto-optical Kerr effect (MOKE). The mechanism of the magnetization reversal of arrays is discussed in two different scenarios: vortex and single-domain. It has been shown that circular dots reverse only through vortex configuration whereas the irregular does either via single-domain and vortex configuration, depending of the dot size. Variable domain phases are confirmed by OOMMF (Object Oriented Micromagnetic Framework) micromagnetic simulations.

Dynamic micromagnetic simulation of permalloy antidot array film

Dynamic magnetic susceptibilities of permalloy antidot array film were studied using three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code with two dimensional periodic boundary condition (2DPBC) extension. Two major resonance peaks associated with different regions were found in the investigated systems. Both resonance frequencies decrease with increasing inter-hole distance. The frequency corresponding to lower resonance peak increases with increasing film thickness for t<20 nm, and then the resonance frequency varies weakly with the thickness. High frequency resonance peak increases with decreasing inner radius, and disappears when the inner radius is below 10 nm. Low resonance frequency varies from 1.72 to 6.4 GHz when inner radius changes from 5 to 40 nm.

Micromagnetic simulation of the magnetic spectrum of two magnetostatic coupled ferromagnetic stripes

The magnetic spectra of two magnetostatic coupled ferromagnetic stripes with different distances were investigated by the three-dimensional object oriented micromagnetic framework (OOMMF) code. The studied systems exhibit two resonance peaks. The higher peak can be identified with the bulk mode predicted by the macroscopic LL model. The lower peak originates from edge magnetic moments. Influence of magnetostatic interaction on the two resonance models was studied in detail. As the distance between two stripes increases, the magnetic spectrum of two stripes is close to the case of a single stripe.

Implementation of Two-Dimensional Polycrystalline Grains in Object Oriented Micromagnetic Framework.

In response to the growing need for a more accurate micromagnetic model to understand switching phenomenon in nanoscale magnets, we developed the capability to simulate two-dimensional polycrystalline grains using the Object Oriented Micromagnetic Framework (OOMMF). This addition allows users full flexibility in determining the magnetocrystalline anisotropy and axe in each grain as well as the inter- and intragranular exchange coupling strength.

Tuning magnetic properties of magnetoelectric BiFeO 3–NiFe 2O 4 nanostructures

Multifunctional thin film nanostructures containing soft magnetic materials such as nickel ferrite are interesting for potential applications in microwave signal processing because of the possibility to shrink the size of device architecture and limit device power consumption. An essential prerequisite to future applications of such a system is a firm understanding of its magnetic properties. We show that nanostructures composed of ferrimagnetic NiFe 2O 4 pillars in a multiferroic BiFeO 3 matrix can be tuned magnetically by altering the aspect ratio of the pillars by depositing films of varying thickness. Magnetic anisotropy is studied using ferromagnetic resonance, which shows that the uniaxial magnetic anisotropy in the growth direction changes sign upon increasing the film thickness. The magnitude of this anisotropy contribution can be explained via a combination of shape and magnetostatic effects, using the object-oriented micromagnetic framework (OOMMF). The key factors determining the magnetic properties of the films are shown to be the aspect ratio of individual pillars and magnetostatic interactions between neighboring pillars.

Preparation and Characterization of Ring-Shaped Co Nanomaterials

Donut- or ring-shaped Co nanomaterials were successfully synthesized using the polymer-modified method, by which PVP is used as a surface modifier reagent. The synthesized products were structurally and morphologically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), whereas the magnetic characterization was performed by temperature- and field-dependent magnetization measurements. The results show that the rings have a h-Co nanostructure. The outer diameter is about 400−500 nm, inner diameter 150−250 nm, and the ring plane thickness ∼120 nm. The magnetization measurements show a typical coercivity of about 112 Oe at 300 K and an almost vanishing coercivity, HC ≈ 0 at T = 5 K. Numerical calculations using the packeage of object oriented micromagnetic framework (OOMMF) reveal a magnetization ground-state of flux closure structure, as expected for the geometric structure of the submicrometer-sized Co rings. This explains the observed hy...

Magnetization ground states and phase diagrams for a nanosized Co hollow sphere: An onion-type magnetization state

The magnetization ground states (MGSs) for a nanosized Co hollow sphere, with the outer radius, R&amp;lt;50 nm, have been studied numerically by micromagnetic simulation using object oriented micromagnetic framework (OOMMF). In addition to the originally known single domain and vortex-curling states, a three dimensional “onion” state with a corresponding analytical expression is proposed and confirmed as one of the ground states. Two phase diagrams, one for a single crystalline and the other for a polycrystalline nanosphere, are obtained for the three MGSs. The result reveals that the magnetic anisotropy has a significant effect on the phase line in the diagrams. The finite temperature effect and the blocking properties of the nanosphere for the magnetization reversal are discussed.

Magnetization reversal in epitaxial Fe nanowires on GaAs(110)

The anisotropy constants of epitaxial Fe films prepared on GaAs(110) are determined using ferromagnetic resonance. The films are structured into wires by employing electron-beam lithography with subsequent Argon-ion etching. Magnetoresistance measurements for wires with widths ranging from 250--2000 nm show the increasing influence of the shape anisotropy with decreasing wire widths. The magnetocrystalline anisotropy constants determined from these measurements agree well with the values obtained for the continuous films. These results are additionally confirmed by magnetic force microscopy measurements and micromagnetic simulations using the object oriented micromagnetic framework (OOMMF). We show that we are able to prepare wires with a remanent magnetization oriented perpendicular to their long axis due to the interplay of cubic and uniaxial in-plane magnetic anisotropies. Moreover, we can tune the effective easy axis by adjusting the width of the wires.

Micromagnetic simulation of the magnetic spectrum of ferromagnetic nanowire

The dynamic susceptibilities of permalloylike nanowires (20×500×20nm3) have been investigated using the three-dimensional object oriented micromagnetic framework (OOMMF) code. The investigated systems show two resonant peaks associated with a “bulk” and an “edge” mode, which are attributed to the local effective field relevant to the local magnetic moments. A local analysis for ferromagnetic resonance calculation has been developed. It handles the local magnetic moments at the edge and the middle of the nanowire, respectively. The resonant frequencies of the edge mode are directly proportional to Ms or A under the same fundamental length scale, i.e., the exchange length.

Nanosecond magnetization reversal in nanocrystalline magnetic films

This paper reports on the investigation of dynamic magnetization reversal process in electrodeposited nanocrystalline Ni and Ni80Fe20 films by employing nanosecond magnetic pulse technique. The surface morphology has been investigated using SEM, EDAX, XRD and AFM analyses and static magnetic properties of the films are characterized by vibrating sample magnetometer (VSM). Two different techniques are designed and employed to study the nanosecond magnetization reversal process in nanocrystalline thin films: Magneto-Optical Kerr Effect (MOKE) and nanosecond pulsed field magnetometer. Results of dynamical behavior as a function of several variables such as magnitude of applied bias magnetic field, amplitude and width of the pulsed magnetic field are analyzed in detail using both techniques. A computer simulation package called Object Oriented Micro-Magnetic Framework (OOMMF) has been used to simulate the magnetic domain patterns of the samples.

Domain Structures and Current-Controlled Switching Characteristics of Micron Sized Permalloy Structures with Varying Aspect Ratio

ABSTRACTPermalloy magnetic structures were fabricated using electron beam lithography and then deposited by evaporation followed by lift-off. The structures were studied for their magnetic domain properties with respect to their aspect ratio. Instead of conventional switching using a magnetic field, a current carrying conductor was used to switch the structures. The current value was gradually increased and magnetic force microscopy was used to study the domain changes of the remanent structures after every increment. At the same time, simulation is done using object-oriented micro-magnetic framework to compare with the experimental results.