Magnetic Nanodisks Research Articles

Superparamagnetic iron oxide nanodiscs for hyperthermia therapy: Does size matter?

For the final realization of magnetic nanoparticles (MNPs) mediated hyperthermia as a viable clinical therapy for cancer treatment, it is necessary to devise novel approaches in order to improve the heating efficiency or Specific Loss Parameter (SLP) of these MNPs. Recently, it has been shown that magnetic nanodiscs with enhanced shape anisotropy, diameters around 200 nm (25 nm thick), and vortex magnetic domain structure exhibit very high SLP values. Despite their high heating efficiency, biomedical applications of these nanodiscs could not be hassle-free due to their relatively big size. Therefore, in this work, we have studied how the heating efficiency of the nanodiscs changes upon size reduction (∼12 nm diameter and ∼3 nm thickness). In addition, we have compared these results with those obtained for more typically studied spherical nanoparticles of similar volume. Transmission Electron microscopy, Atomic Force Microscopy and X-ray Diffraction confirm the disc shape of our MNPs and that they are mostly composed of iron oxide (Fe3O4 or γ-Fe2O3) phase. Magnetometry indicates that the nanodiscs do not exhibit a vortex magnetic domain structure, but still present a superparamagnetic-like behavior, with zero magnetization in the absence of field at room temperature (ideal for biomedical applications) and enhanced effective anisotropy as compared to the spherical nanoparticles. Finally, calorimetric methods based magnetic hyperthermia experiments indicate that the SLP values for these small nanodiscs are much lower than those reported for the bigger disc-shaped nanoparticles, but these superparamagnetic nanodiscs act as better heating mediators than the spherical nanoparticles of similar volume.

Magnetic-vortex Dynamic Quasi-crystal Formation in Soft Magnetic Nano-disks

We report a micromagnetic numerical study on different quasi-crystal formations of magnetic vortices in a rich variety of dynamic transient states in soft magnetic nano-disks. Only the application of spin-polarized dc currents to a single magnetic vortex leads to the formation of topological-soliton quasi-crystals composed of different configurations of skyrmions with positive and negative half-integer numbers (magnetic vortices and antivortices). Such topological object formations in soft magnets, not only in the absence of Dzyaloshinskii- Moriya interaction but also without magnetocrystalline anisotropy, are discussed in terms of two different topological charges, the winding number and the skyrmion number. This work offers an insight into the dynamic topological-spin-texture quasi-crystal formations in soft magnets.

Resonance beyond frequency-matching: multidimensional resonance

Resonance, conventionally defined as the oscillation of a system when the temporal frequency of an external stimulus matches a natural frequency of the system, is important in both fundamental physics and applied disciplines. However, the spatial character of oscillation is not considered in this definition. We reveal the creation of spatial resonance when the stimulus matches the space pattern of a normal mode in an oscillating system. The complete resonance, which we call multidimensional resonance, should be a combination of both the temporal and the spatial resonance. We further elucidate that the spin wave produced by multidimensional resonance drives considerably faster reversal of the vortex core in a magnetic nanodisc. Multidimensional resonance provides insight into the nature of wave dynamics and opens the door to novel applications.

Spontaneous nucleation and topological stabilization of skyrmions in magnetic nanodisks with the interfacial Dzyaloshinskii–Moriya interaction

One of the major societal challenges is reducing the power consumption of information technology (IT) devices and numerous data centers. Distinct from the current approaches based on switching of magnetic single-domain nanostructures or on movement of domain walls under high currents, an original magnetic skyrmion technology offers ultra-low power, fast, high-density, and scalable spintronic devices, including non-volatile random access memory. Using data-driven micromagnetic simulations, we demonstrate the possibility of spontaneous nucleation and stabilization of different skyrmionic states, such as skyrmions, merons, and meron-like configurations, in heavy metal/ferromagnetic nanodisks with the interfacial Dzyaloshinskii–Moriya interaction (iDMI) as a result of quasi-static magnetization reversal only. Since iDMI is not easily modulated in real systems, we show that skyrmion stabilization is easily achievable by manipulating magnetic anisotropy, saturation magnetization, and the diameters of nanodisks. The state diagrams, presented in terms of the topological charge, allow to explicitly distinguish the intermediate states between skyrmions and merons and can be used for developing a skyrmionic medium, which has been recently proposed to be a building block for future spin-orbitronic devices.

Vortex-dynamics-mediated low-field magnetization switching in an exchange-coupled system

A magnetic vortex has attracted significant attention since it is a topologically stable magnetic structure in a soft magnetic nanodisk. Many studies have been devoted to understanding the nature of magnetic vortex in isolated systems. Here we show a new aspect of a magnetic vortex the dynamics of which strongly affects the magnetic structures of environment. We exploit a nanodot of an exchange-coupled bilayer with a soft magnetic Ni81Fe19 (permalloy; Py) having a magnetic vortex and a perpendicularly magnetized L10-FePt exhibiting a large switching field (Hsw). The vortex dynamics with azimuthal spin waves makes the excess energy accumulate in the Py, which triggers the reversed-domain nucleation in the L10-FePt at a low magnetic field. Our results shed light on the non-local mechanism of a reversed-domain nucleation, and provide with a route for efficient Hsw reduction that is needed for ultralow-power spintronic devices.

Surface modification and cellular uptake evaluation of Au-coated Ni80Fe20 nanodiscs for biomedical applications.

A nanofabrication technique based on self-assembling of polystyrene nanospheres is used to obtain magnetic Ni80Fe20 nanoparticles with a disc shape. The free-standing nanodiscs (NDs) have diameter and thickness of about 630 nm and 30 nm, respectively. The versatility of fabrication technique allows one to cover the ND surface with a protective gold layer with a thickness of about 5 nm. Magnetization reversal has been studied by room-temperature hysteresis loop measurements in water-dispersed free-standing NDs. The reversal shows zero remanence, high susceptibility and nucleation/annihilation fields due to spin vortex formation. In order to investigate their potential use in biomedical applications, the effect of NDs coated with or without the protective gold layer on cell growth has been evaluated. A successful attempt to bind cysteine-fluorescein isothiocyanate (FITC) derivative to the gold surface of magnetic NDs has been exploited to verify the intracellular uptake of the NDs by cytofluorimetric analysis using the FITC conjugate.

Ultrafast annular-magnetic-field-driven vortex-core reversals

By micromagnetic numerical simulations, we investigate the dynamics of vortex-core reversal in a soft magnetic nanodisk under the excitation of annular, perpendicular, resonant magnetic fields. The non-fundamental radial modes of the nanodisk are characterized into alternating radial-phase-regions for which two adjacent regions across a node point are in antiphase. We show that radial spin-waves excited by fields applied in the in-phase regions are in phase, and therefore generate strong magnetization oscillations resulting from constructive spin-wave interference. Such annular magnetic fields can substantially speed up the vortex-core reversal and lower the threshold field amplitude in comparison with the global field. Our work provides an efficient mechanism for spin-wave excitation and ultrafast vortex-core switching.

Frequency-based nanoparticle sensing over large field ranges using the ferromagnetic resonances of a magnetic nanodisc

Using finite element micromagnetic simulations, we study how resonant magnetisation dynamics in thin magnetic discs with perpendicular anisotropy are influenced by magnetostatic coupling to a magnetic nanoparticle. We identify resonant modes within the disc using direct magnetic eigenmode calculations and study how their frequencies and spatial profiles are changed by the nanoparticle’s stray magnetic field. We demonstrate that particles can generate shifts in the resonant frequency of the disc’s fundamental mode which exceed resonance linewidths in recently studied spin torque oscillator devices. Importantly, it is shown that the simulated shifts can be maintained over large field ranges (here up to 1 T). This is because the resonant dynamics (the basis of nanoparticle detection here) respond directly to the nanoparticle stray field, i.e. detection does not rely on nanoparticle-induced changes to the magnetic ground state of the disc. A consequence of this is that in the case of small disc-particle separations, sensitivities to the particle are highly mode- and particle-position-dependent, with frequency shifts being maximised when the intense stray field localised directly beneath the particle can act on a large proportion of the disc’s spins that are undergoing high amplitude precession.

Indirect switching of vortex polarity through magnetic dynamic coupling

Magnetic vortex cores exhibit a gyrotropic motion and may reach a critical velocity, at which point they invert their z-component of the magnetization. We performed micromagnetic simulations to describe this vortex core polarity reversal in magnetic nanodisks with a perpendicular anisotropy. We found that the critical velocity decreases with the increase in perpendicular anisotropy, therefore departing from a universal criterion that relates this velocity only to the exchange stiffness of the material. This leads to a critical velocity inversely proportional to the vortex core radius. We have also shown that in a pair of interacting disks, it is possible to switch the core vortex polarity through a non-local excitation; exciting one disk by applying a rotating magnetic field, one is able to switch the polarity of a neighbor disk, with a larger perpendicular anisotropy.

The antitumor effect of magnetic nanodisks and DNA aptamer conjugates.

Here we describe a method of forming large arrays (up to 10(9) pieces) of free magnetic Ni-nanodisks 50 nm thick coated on both sides with layers of 5 nm thick Au. The antitumor effect of the magnetic nickel gold-coated nanodisks and DNA aptamer conjugates was evaluated in vivo and in vitro. Under the influence of rotating magnetic field, the studied nanodisks can cause the death of Ehrlich ascites carcinoma cells.

Vortex Dynamics in Magnetic Nanodisks With a Ring of Magnetic Defects

Recently, nanoscale ferromagnetic disk-shaped elements have aroused intensive investigation for their potential applications in numerous magnetoelectronic devices [1-2]. In a magnetic nanodisk, the equilibrium between the exchange interaction and the magnetostatic energy leads to one special magnetization configuration, called magnetic vortex. Many theoretical and experimental researches have verified that the gyrotropic mode of the vortex core motion can be induced by the external field or spin polarized current [2], and then some works focused on investigating the precession frequency. It is well known that the oscillation frequency is one most important parameter for spin-transfer nano-oscillators (STNOs) based on vortex precession [3]. Nowadays, great efforts have been devoted to study the effect of pointlike magnetic defects in the eigenfrequency of vortex core [2], however, the effect of ring-shaped magnetic defects in the precession frequency has not yet been detailed studied. In this paper, we inserted a ring of magnetic defects into magnetic nano-disk, and studied the dynamics of magnetic vortex by micromagnetic simulation (Fig. 1).

Non-Boolean computing with nanomagnets for computer vision applications.

The field of nanomagnetism has recently attracted tremendous attention as it can potentially deliver low-power, high-speed and dense non-volatile memories. It is now possible to engineer the size, shape, spacing, orientation and composition of sub-100 nm magnetic structures. This has spurred the exploration of nanomagnets for unconventional computing paradigms. Here, we harness the energy-minimization nature of nanomagnetic systems to solve the quadratic optimization problems that arise in computer vision applications, which are computationally expensive. By exploiting the magnetization states of nanomagnetic disks as state representations of a vortex and single domain, we develop a magnetic Hamiltonian and implement it in a magnetic system that can identify the salient features of a given image with more than 85% true positive rate. These results show the potential of this alternative computing method to develop a magnetic coprocessor that might solve complex problems in fewer clock cycles than traditional processors.

Spin motive force driven by skyrmion dynamics in magnetic nanodisks

The spin motive force driven by the dynamics of the skyrmion structure formed in a nanomagnetic disk is numerically investigated. Due to the existence of the magnetic structure along the disk edge, the collective mode of the magnetization is modified from that of the bulk skyrmion lattice obtained by using the periodic boundary condition. For a single-skyrmion disk, the dynamics of the skyrmion core and the edge magnetization induce the spin motive force, and a measurable AC voltage is obtained by two probes on the disk. For a multi-skyrmions disk, the phase-locked collective mode of skyrmions is found in the lowest resonant frequency where the amplitude of the AC voltage is enhanced by the cascade effect of the spin motive force. We also investigate the effect of the Rashba spin-orbit coupling on the spin motive force.

Dynamics and efficiency of magnetic vortex circulation reversal

Author(s): Urbanek, M; Uhliř, V; Lambert, CH; Kan, JJ; Eibagi, N; Vaňatka, M; Flajsman, L; Kalousek, R; Im, MY; Fischer, P; Sikola, T; Fullerton, EE | Abstract: Dynamic switching of the vortex circulation in magnetic nanodisks by fast-rising magnetic field pulse requires annihilation of the vortex core at the disk and reforming a new vortex with the opposite sense of circulation. Here we study the influence of pulse parameters on the dynamics and efficiency of the vortex core annihilation in permalloy (Ni80Fe20) nanodisks. We use magnetic transmission soft x-ray microscopy to experimentally determine a pulse rise time-pulse amplitude phase diagram for vortex circulation switching and investigate the time-resolved evolution of magnetization in different regions of the phase diagram. The experimental phase diagram is compared with an analytical model based on Thiele's equation describing high-amplitude vortex core motion in a parabolic potential. We find that the analytical model is in good agreement with experimental data for a wide range of disk geometries. From the analytical model and in accordance with our experimental finding we determine the geometrical condition for dynamic vortex core annihilation and pulse parameters needed for the most efficient and fastest circulation switching. The comparison of our experimental results with micromagnetic simulations shows that the micromagnetic simulations of ideal disks with diameters larger than ∼250 nm overestimate nonlinearities in susceptibility and eigenfrequency. This overestimation leads to the core polarity switching near the disk boundary, which then in disagreement with experimental findings prevents the core annihilation and circulation switching. We modify the micromagnetic simulations by introducing the boundary region of reduced magnetization to simulate the experimentally determined susceptibility and in these modified micromagnetic simulations we are able to reproduce the experimentally observed dynamic vortex core annihilation and circulation switching.

Ni80Fe20 nanodisks by nanosphere lithography for biomedical applications

A novel nanofabrication technique based on self-assembling of polystyrene nanospheres and aimed to obtain magnetic nanodisks suspended in ethanol is here presented. Free-standing Ni80Fe20 disks having lateral dimension around 650 nm and thickness 30 nm were obtained by using nanosphere lithography on a sputtered continuous thin film. The multi-step nanofabrication process will be explained, in detail. The process end-product can be used as suitable magnetic carriers having nearly monodispersed size and simultaneously displaying high saturation magnetization and low-coercivity. Magnetisation reversal has been studied by room-temperature hysteresis loop measurements in either dot arrays attached on a substrate or in liquid-dispersed free-standing nanodisks. In both samples, the reversal is marked by magnetic vortex nucleation/annihilation. Such a behavior is confirmed for Ni80Fe20 dot arrays by measuring magnetic domain configuration, while numerical simulation is used for confirming magnetization reversal process in nanodisks.

Metrology to support therapeutic and diagnostic techniques based on electromagnetics and nanomagnetics

This paper presents an overview of the recent research activities carried on at INRIM in the field of metrology for healthcare, aiming at supporting therapeutic and diagnostic techniques based on electromagnetics and nanomagnetics. Attention is here specifically focused on three research topics, respectively related to electromagnetic dosimetry for MR-safety, production and characterization of magnetic Ni80Fe20 nanodisks for biomedical applications and development of modeling tools to support the design of novel biosensors.

Non-linear radial spinwave modes in thin magnetic disks

We present an experimental investigation of radial spin-wave modes in magnetic nano-disks with a vortex ground state. The spin-wave amplitude was measured using a frequency-resolved magneto-optical spectrum analyzer, allowing for high-resolution resonance curves to be recorded. It was found that with increasing excitation amplitude up to about 10 mT, the lowest-order mode behaves strongly non-linearly as the mode frequency redshifts and the resonance peak strongly deforms. This behavior was quantitatively reproduced by micromagnetic simulations. Micromagnetic simulations showed that at higher excitation amplitudes, the spinwaves are transformed into a soliton by self-focusing, and collapse onto the vortex core, dispersing the energy in short-wavelength spinwaves. Additionally, this process can lead to switching of the vortex polarization through the injection of a Bloch point.

Azimuthal-spin-wave-mode-driven vortex-core reversals

We studied, by micromagnetic numerical calculations, asymmetric vortex-core reversals driven by the m = −1 and m = +1 azimuthal spin-wave modes' excitations in soft magnetic circular nano-disks. We addressed the similarities and differences between the asymmetric core reversals in terms of the temporal evolutions of the correlated core-motion speed, locally concentrated perpendicular gyrofield, and magnetization dip near the original vortex core. The criterion for the core reversals was found to be the magnetization dip that must reach the out-of-plane magnetization component, mz = −p, with the initial polarization p, where p = +1 (−1) for the upward (downward) core magnetization. The core-motion speed and the associated perpendicular gyrofield, variable and controllable with static perpendicular field, Hz, applied perpendicularly to the disk plane, must reach their threshold values to meet the ultimate core-reversal criterion. Also, we determined the Hz strength and direction dependence of the core-switching time and threshold exciting field strength required for the core reversals, whose parameters are essential in the application aspect. This work offers deeper insights into the azimuthal spin-wave-driven core-reversal dynamics as well as an efficient means of controlling the azimuthal-modes-driven core reversals.

Micromotors with step-motor characteristics by controlled magnetic interactions among assembled components.

In this study, we investigated the control of the rotation dynamics of an innovative type of rotary micromotors with desired performances by tuning the magnetic interactions among the assembled micro/nanoscale components. The micromotors are made of metallic nanowires as rotors, patterned magnetic nanodisks as bearings and actuated by external electric fields. The magnetic forces for anchoring the rotors on the bearings play an essential role in the rotation dynamics of the micromotors. By varying the moment, orientation, and dimension of the magnetic components, distinct rotation behaviors can be observed, including repeatable wobbling and rolling in addition to rotation. We understood the rotation behaviors by analytical modeling, designed and realized micromotors with step-motor characteristics. The outcome of this research could inspire the development of high-performance nanomachines assembled from synthetic nanoentities, relevant to nanorobotics, microfluidics, and biomedical research.

Orientation Mediated Enhancement on Magnetic Hyperthermia of Fe3O4 Nanodisc

A two‐step chemical approach to synthesize high quality Fe3O4 nanodisc is reported. The magnetic hyperthermia properties of the nanodisc and isotropic nanoparticles are investigated systematically. The results suggest that the nanodisc shows much higher specific absorption rate (SAR) than isotropic nanoparticles. This is attributed to the parallel alignment of nanodisc with respect to the alternating current magnetic field, which is confirmed by good agreement between experimental results and micromagnetic simulation. It is found that such parallel alignment could enhance the SAR value by a factor of ≈2 with respect to the randomly oriented case. The above results indicate that the nanodisc provides an excellent thermal seed for magnetic hyperthermia. This study sheds the light on the magnetic hyperthermia mechanism of magnetic nanodisc and it also opens the window to explore high efficiency thermal seeds by controlling the orientation of magnetic nanostructures.