Magnetostatic Stray Field Research Articles

On the Control of Magnetostatic Stray Fields Using Electric Current

On the Control of Magnetostatic Stray Fields Using Electric Current

Control of the Magnetostatic Stray Fields Using Electric Current

Control of the Magnetostatic Stray Fields Using Electric Current

Simultaneous magnetic field and field gradient mapping of hexagonal MnNiGa by quantitative magnetic force microscopy

Magnetic force microscopy (MFM) is a scanning microscopy technique that is commonly employed to probe the sample’s magnetostatic stray fields via their interaction with a magnetic probe tip. In this work, a quantitative, single-pass MFM technique is presented that maps one magnetic stray-field component and its spatial derivative at the same time. This technique uses a special cantilever design and a special high-aspect-ratio magnetic interaction tip that approximates a monopole-like moment. Experimental details, such as the control scheme, the sensor design, which enables simultaneous force and force gradient measurements, as well as the potential and limits of the monopole description of the tip moment are thoroughly discussed. To demonstrate the merit of this technique for studying complex magnetic samples it is applied to the examination of polycrystalline MnNiGa bulk samples. In these experiments, the focus lies on mapping and analyzing the stray-field distribution of individual bubble-like magnetization patterns in a centrosymmetric [001] MnNiGa phase. The experimental data is compared to calculated and simulated stray-field distributions of 3D magnetization textures, and, furthermore, bubble dimensions including diameters are evaluated. The results indicate that the magnetic bubbles have a significant spatial extent in depth and a buried bubble top base.

Stray field and combined effects on device miniaturization of the magnetic tunnel junctions

Magneto-static stray field (H stray) interactions become an important issue when perpendicular CoFeB/MgO magnetic tunnel junctions (MTJs) are miniaturized. This raises the issue of which of the two mainstream etching processes, the pillar structure and the step structure, is better able to retain MTJ performance at extremely small scales. In the current study, we first simulated H stray effects as a function of Ruderman–Kittel–Kasuya–Yosida strength within a synthetic antiferromagnetic structure for the two structures. Our results revealed that H stray interactions were less influential (in terms of offset field) in step MTJs than in pillar MTJs during MTJ miniaturization. This is in good agreement with experimental results. This finding is further supported by adding Dzyaloshinskii–Moriya interactions into the free-layer of the two structures. We further simulated thermal stability with the inclusion of H stray for 30 nm MTJs. We found that adding etching damage effects (i.e. assuming both anisotropy constant and saturation magnetization of the free layer had some degree of loss) into the model of the pillar MTJ was necessary to obtain a trend that is close to the experimental results of thermal stability. This information can provide some guidance on the technical choices for the MTJ miniaturization.

Giant spin-valve effect in yttrium--ion garnet--aluminum structures

The nature of resistive switching in structures of yttrium iron garnet-aluminum (YIG/Al) observed when the current direction is changed relative to the direction of YIG magnetization has been studied. It has been established that the effect observed when the magnetic field is rotated increases with an increase in the current passed through the structure and achieves 100 percent switching from the superconducting to the normal resistive state. The magnitude of the effect depends on geometric factors, namely, on the ratio of the lateral dimensions of the YIG plates. Modeling the configuration of the magnetostatic stray fields showed that the effect is partially determined by these fields. It has been established that the inversion of the current flowing through the structures also leads to a variation in their resistance. The effect reaches 20% near the temperature of superconducting transition. Keywords: magnetic dielectrics, superconductor, proximity effects, spin-orbit coupling, Zeeman field, spin-valve.

Observation of spectral narrowing and mode conversion in two-dimensional binary magnonic crystal

We study binary magnonic crystal, where Ni80Fe20 nanodots of two different sizes are diagonally connected forming a unit and those units are arranged in a square lattice. The magnetization dynamics of the sample is measured by using time-resolved magneto-optical Kerr effect microscope with varying magnitude and orientation (ϕ) of the bias magnetic field, which is applied at about 10° angle from the sample plane. Interestingly, at ϕ = 0°, the spin-wave mode profiles show frequency selective spatial localization of spin-wave power within the array. With the variation of ϕ in the range 0° < ϕ ≤ 45°, we observe spectral narrowing due to localized to extended spin-wave mode conversion. Upon further increase of ϕ, the spin-wave modes slowly lose the extended nature and become fully localized again at 90°. We have extensively demonstrated the role of magnetostatic stray field distribution on the rotational symmetries obtained for the spin-wave modes. From micromagnetic simulations, we find that the dipole-exchange coupling between the nanodots leads to remarkable modifications of the spin-wave mode profiles when compared with arrays of individual small and large dots. Numerically, we also show that the physical connection between the nanodots provides more control points over the spin-wave propagation in the lattice at different orientations of bias magnetic field. In particular, the directional propagation of spin-wave is crucial for the application in coupler. Overall, we envision this binary magnonic crystal to have potential applications in magnonic devices such as spin-wave waveguide, filter, coupler, and other on-chip microwave communication devices.

Influence of asymmetrical location of magnetic impurities on domain walls local deformation

Magnetic ordering in a mixed domain structure composed of a stripe domain and asymmetrically localized magnetic impurity inside the stripe domain was investigated. The analytical expression for domain walls deformation mediated by the influence of the magnetostatic stray field of the asymmetrically localized magnetic impurity was obtained. The calculations were done for the magnetic film with parameters which are typical for films used in magnetic memory. Experimental implementation of the mixed domain structure composed of the stripe domain and asymmetrically localized magnetic impurity inside the stripe domain was done in (BiLu)3(FeGa)5O12 film. Obtained analytical solution for the shape of stripe domain walls distortion is in excellent agreement with the experimental results.

Ferromagnet/Superconductor Hybridization for Magnonic Applications

AbstractIn this work, a new hybridization of superconducting and ferromagnetic orders is demonstrated, promising for magnonics. By measuring the ferromagnetic and spin wave resonance absorption spectra of a magnetostatically coupled permalloy/niobium bilayer at different temperatures, magnetostatic spin wave resonances with unconventional dispersion are observed. The mechanism behind the modified dispersion, confirmed with micromagnetic simulations, implies screening of the alternating magnetostatic stray fields of precessing magnetic moments in the ferromagnetic layer by the superconducting surface in the Meissner state.

A mixed domain structure in magnetic films with large anisotropy

Influence of anisotropy on a bending of a magnetic stripe domain wall due to a magnetostatic stray field of a cylindrical magnetic domain (CMD) that is located within the stripe one is under investigation. It is revealed that for a specific set of physical parameters of the domain structure, energy of domain wall bending anisotropy can suppress the bending. An analytical expression for the bending shape is obtained on account of a change of both magnetostatic energy of the configuration and energy of anisotropy of the domain wall.

Unravelling the tunable exchange bias-like effect in magnetostatically-coupled two dimensional hybrid (hard/soft) composites

Hybrid 2D hard-soft composites have been fabricated by combining soft (Co73Si27) and hard (NdCo5) magnetic materials with in-plane and out-of-plane magnetic anisotropies, respectively. They have been microstructured in a square lattice of CoSi anti-dots with NdCo dots within the holes. The magnetic properties of the dots allow us to introduce a magnetostatic stray field that can be controlled in direction and sense by their last saturating magnetic field. The magnetostatic interactions between dot and anti-dot layers induce a completely tunable exchange bias-like shift in the system’s hysteresis loops. Two different regimes for this shift are present depending on the lattice parameter of the microstructures. For large parameters, dipolar magnetostatic decay is observed, while for the smaller one, the interaction between the adjacent anti-dot’s characteristic closure domain structures enhances the exchange bias-like effect as clarified by micromagnetic simulations.

Magnetic field dependent small-angle neutron scattering on a Co nanorod array: evidence for intraparticle spin misalignment.

The structural and magnetic properties of a cobalt nanorod array have been studied by means of magnetic field dependent small-angle neutron scattering (SANS). Measurement of the unpolarized SANS cross section dΣ/dΩ of the saturated sample in the two scattering geometries where the applied magnetic field H is either perpendicular or parallel to the wavevector ki of the incoming neutron beam allows one to separate nuclear from magnetic SANS, without employing the usual sector-averaging procedure. The analysis of the SANS data in the saturated state provides structural parameters (rod radius and centre-to-centre distance) that are in good agreement with results from electron microscopy. Between saturation and the coercive field, a strong field dependence of dΣ/dΩ is observed (in both geometries), which cannot be explained using the conventional expression of the magnetic SANS cross section of magnetic nanoparticles in a homogeneous nonmagnetic matrix. The origin of the strong field dependence of dΣ/dΩ is believed to be related to intradomain spin misalignment, due to magnetocrystalline and magnetoelastic anisotropies and magnetostatic stray fields.

Condition of the ratchet effect of a magnetic domain wall motion under an asymmetric potential energy

We have investigated the ratchet effect of magnetic domain wall (DW) motion in a straight ferromagnetic nanowire under ac magnetic field by means of micromagnetic simulation. A structure-stable DW ratchet effect along the ferromagnetic nanowire is observed utilizing an asymmetric potential produced by a nonuniform magnetostatic stray field from an array of a periodic non-contact trapezoidal stubs. A diode-like consecutive operation process for a transverse DW motion is examined with variation of the ac field frequency and amplitude, where the necessary conditions for the DW ratchet effect are systematically examined. We have also obtained the empirical relation between a DW velocity of the ratchet effect and the ac field frequency and amplitude.

Temperature-dependent interlayer coupling in Ni/Co perpendicular pseudo-spin-valve structures

The temperature-dependent coupling mechanisms in perpendicular pseudo-spin valves based on the following structure, [Ni/Co]${}_{5}$/Cu(t${}_{\mathrm{Cu}}$)/[Ni/Co]${}_{2}$, are investigated. Despite a thick (t${}_{\mathrm{Cu}}\ensuremath{\ge}3$ nm) Cu spacer, room-temperature measurements reveal complete coupling of the [Ni/Co]${}_{5}$ and [Ni/Co]${}_{2}$ multilayers. This coupling can be attributed to strong long range magnetostatic stray fields that penetrate the spacer layer. This results in magnetic domain imprinting and vertically correlated domains throughout the reversal process. Surprisingly, when the temperature is reduced, a complete decoupling is observed. This somewhat counterintuitive result can be explained by a large difference in the [Ni/Co]${}_{5}$ and [Ni/Co]${}_{2}$ multilayer coercivities at reduced temperatures, which then impedes domain imprinting and promotes decoupling. Finally, the decoupling temperature is found to increase with spacer thickness.

Influence of geometry on current-driven vortex oscillations in nanocontact devices

We present a computational study of current-driven vortex dynamics in a particular geometry, a hybrid Co/Au/Py nanocontact, in which the Co layer is not flat. The experimental measurements validate the numerical results. We identify the Py layer as dynamically active. The nonuniform magnetization configuration in the Co layer, which acts as spin polarizer, and the interlayer magnetostatic stray field, both of which are mostly determined by geometry, are shown to have crucial influence on the dynamic properties of the system. The frequency as a function of current at zero field and also as a function of an out-of-plane field for a fixed current are computed. An excellent quantitative agreement with experimental data is obtained, demonstrating a novel approach for tailoring vortex nano-oscillators.

Magnetization configurations and reversal of thin magnetic nanotubes with uniaxial anisotropy

We present calculations of the magnetization configuration and reversal behavior of magnetic nanotubes with uniaxial anisotropy by means of two-dimensional micromagnetic simulations and analytical methods. The tube radii R from 50 to 150 nm and the tube length /radius aspect ratio L/R≤20 were explored. For a finite length of magnetic nanotubes the magnetization configuration is characterized by a uniformly magnetized along the tube axis middle part and two nonuniform curling states of a length Lc in two ends of the tube with the same or opposite magnetization rotating senses, referring as C-state or B-state, respectively. We found that the magnetization configuration of the C-state exists for thin nanotubes with the tube thickness, ΔR, in the range of ΔR/R≤0.2. For thicker nanotubes the strong magnetostatic stray field forces the change of rotating senses of the end domains in opposite directions (the B-state). The transition from the C-state to a vortex state with in-plane magnetization is described as function of the tube geometrical parameters. The nanotube hysteresis loops and switching fields were calculated. The simple analytical model was developed to describe the nanotube magnetization reversal reducing its description to the Stoner–Wohlfarth model with effective parameters. The equilibrium state of nanotube is described in terms of θ, the angle of the magnetization deviation from the intrinsic tube easy axis. The L/R dependence of the C-state magnetization, the shape of hysteresis loops and the switching field values are described by a dependence of θ on L/R.

Domain overlap in antiferromagnetically coupled [Co∕Pt]∕NiO∕[Co∕Pt] multilayers

Antiferromagnetically coupled magnetic thin films with perpendicular anisotropy exhibit domain overlap regions originating from magnetostatic stray fields localized in the vicinity of the domain walls. Using high resolution magnetic force microscopy, the authors investigate these overlap regions in [Co∕Pt]∕NiO∕[Co∕Pt] multilayers with various strengths of the interlayer exchange coupling. They develop a simple model that provides a quantitative explanation of the formation of these regions and the relationship between the domain overlap width and the coupling strength. Their results are important for application of magnetic layered structures with perpendicular anisotropy in advanced magnetoresistive devices.

Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy

The time and field dependence of the magnetic domain structure at magnetization reversal were investigated by Kerr microscopy in a structure consisting of a hard and a soft ferromagnetic Co∕Pt multilayer stack with perpendicular anisotropy, separated by a thicker nonmagnetic Pt spacer layer. Large local inhomogeneous magnetostatic stray fields appear as soon as a nonuniform magnetic area exists within one of the stacks and induce a correlated domain structure within the other. The long range nature of this magnetostatic interaction gives rise to ultraslow dynamics even in zero applied field, i.e., it affects the long time domain stability. Due to this additional interaction field, the magnetization reversal under short magnetic field pulses differs markedly from the well-known slow dynamic behavior. Namely, in high field, the magnetization of the coupled harder layer has been observed to reverse more rapidly by domain wall motion than the softer layer alone.

Layer-resolved imaging of magnetic interlayer coupling by domain-wall stray fields

Layer-resolved magnetic domain images of epitaxially grown Co/Cu/Ni trilayers on Cu(001) have been studied, taken by photoelectron emission microscopy using x-ray magnetic circular dichroism as a magnetic contrast mechanism. In these trilayers the Ni layers are magnetized perpendicularly to the film plane, whereas the Co magnetization is in the film plane. Comparison of the as-grown magnetic domain images of the Co and Ni layers reveals the influence of the magnetostatic stray fields from Ni domain walls on the Co domain pattern as a lateral displacement of the Co domain wall position compared to the Ni domain walls. The effect is quantified by comparing to the effect of external magnetic fields, and is found to be equivalent to about 250 Oe. Micromagnetic simulations using the Landau-Lifshitz-Gilbert equation confirm that size of the Ni domain wall stray field interaction.

A lower bound for the volume-averaged mean-square magnetostatic stray field

Based on a micromagnetics model, we develop a method through which quantitative information on the volume-averaged mean-square magnetostatic stray field �| H b| 2 � v due to non-zero divergences of the magnetization M within the bulk of a ferromagnetic body can be obtained by analysis of magnetic-field- dependent small-angle neutron scattering data. In the limit of high applied magnetic field Ha, when the direction of M deviates only sligthly from Ha, we have estimated a lower bound for �| H b| 2 � v as a function of the external field, and we have applied the method to bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The root-mean-square magnetostatic stray field, which is inherent to a particular magnetic microstructure, shows a pronounced field dependence, with values ranging from about 5 to 50 mT. Even at applied fields as large as 1. 7T , the quantityµ0�| H b| 2 � 1/2 v of nanocrystalline Co is still 24 mT, which suggests that contributions to the total magnetostatic field originating from the bulk are significant in nanocrystalline ferromagnets; therefore, �| H b| 2 � v cannot be ignored in the interpretation of e.g. measurements of magnetization or spin-wave resonance. A comparison of �| H b| 2 � v with the volume-averaged mean-square anisotropy field reveals that both quantities are of comparable magnitude.

A comparison of torque-strengths in micromagnetics

In the approach-to-magnetic-saturation range, we present experimental data for the magnitude and applied magnetic-field dependence of the volume-averaged mean-square magnetostatic stray field 〈| H d b | 2 〉 v that results from non-zero divergences of the magnetization from within the volume of bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The magnitude of 〈| H d b | 2 〉 v is compared to the volume-averaged mean-square anisotropy field 〈| H p | 2 〉 v , which arises from the combined effect of magnetocrystalline and magnetoelastic anisotropy.