Search

Different models have been used to evaluate the interfacial Dzyaloshinskii–Moriya interaction (DMI) from the asymmetric bubble expansion method using magneto-optics. Here, we investigate the most promising candidates over a range of different magnetic multilayers with perpendicular anisotropy. Models based on the standard creep hypothesis are not able to reproduce the domain wall (DW) velocity profile when the DW roughness is high. Our results demonstrate that the DW roughness and the interface roughness of the sample layers are correlated. Furthermore, we give guidance on how to obtain reliable results for the DMI value with this popular method. A comparison of the results with Brillouin light scattering (BLS) measurements on the same samples shows that the BLS approach often results in higher measured values of DMI. [ABSTRACT FROM AUTHOR]

We have explored dynamic magnetic properties of micron-sized Ni-coated carbon fibers embedded in a polymer matrix for electromagnetic interference shielding applications. These hollow magnetic cylinders exhibit unusual dynamic magnetic properties, which were measured with a broad-band ferromagnetic resonance system (FMR). We observe three families of FMR modes, which are connected to different physical locations within the cylinder. We develop a simple analytic model to explain these results and corroborate resonant mode profiles with micromagnetic simulations. We find excellent agreement between experimental results and theoretical models. Our work indicates that global demagnetizing factors are not appropriate for understanding the spin motions in these hollow cylinders. The FMR absorption observed in these hallow cylinders is very different from those observed in nanowires or solid cylinders. The field-swept envelope of all the observed FMR resonances is very broad, approximately μ 0 H = 1 T, with a linewidth of individual modes around μ 0 Δ H = 250 mT. This can be important for electromagnetic shielding applications. [ABSTRACT FROM AUTHOR]

We determined the spin-transport properties of Pd and Pt thin films by measuring the increase in ferromagnetic resonance damping due to spin-pumping in ferromagnetic (FM)-nonferromagnetic metal (NM) multilayers with varying NM thicknesses. The increase in damping with NM thickness depends strongly on both the spin- and charge-transport properties of the NM, as modeled by diffusion equations that include both momentum- and spin-scattering parameters. We use the analytical solution to the spin-diffusion equations to obtain spin-diffusion lengths for Pt and Pd. By measuring the dependence of conductivity on NM thickness, we correlate the charge- and spin-transport parameters, and validate the applicability of various models for momentum-scattering and spin-scattering rates in these systems: constant, inverse-proportional (Dyakanov-Perel), and linear-proportional (Elliot-Yafet). We confirm previous reports that the spin-scattering time appears to be shorter than the momentum scattering time in Pt, and the Dyakanov-Perel-like model is the best fit to the data. [ABSTRACT FROM AUTHOR]

Motivated by the importance of magnetization dynamics in nanomagnets for the development and optimization of magnetic devices and sensors, we measured and modeled spin wave spectra in patterned elliptical nanomagnets. Ferromagnetic resonance spectra for multiple nanomagnets of Ni80Fe20, fabricated by electron-beam lithography to have nominal short-axes of 200 nm or 100 nm, were measured by use of heterodyne magneto-optical microwave microscopy. Scanning electron microscope images taken of the same nanomagnets were used to define element shapes for micromagnetic simulations. The measured spectra show significant differences between nominally identical nanomagnets, which could be only partially attributed to uncontrolled shape variations in the patterning process, as evidenced by the limited agreement between the measured and simulated spectra. Agreement between measurements and simulations was improved by including a zone of reduced magnetization and exchange at the edges of the nanomagnets in the simulations. Our results show that the reduction of shape variations between individual magnetic random-access memory elements can potentially improve their performance. However, unambiguous determination of materials parameters in nanomagnets based on analysis and modeling of spin wave spectra remains problematic. [ABSTRACT FROM AUTHOR]

Large-angle magnetization precession induced by a short pulsed magnetic field in a 15-nm thick Ni81Fe19 film is observed by use of a time-resolved magneto-optical Kerr effect technique with sensitivity to all three components of the magnetization vector. A reduction of the magnitude of the magnetization vector |M| during large-angle precession is inferred and indicates incoherent dynamics due to excitation of traveling spin waves with wavelengths smaller than the diameter of the sampling area, that is, the laser spot size for the magneto-optical measurements. The reduction in |M| depends on the magnetic bias field Hbias, which can be qualitatively understood by comparison to the theoretical threshold for observing four-magnon scattering in a small time interval t. We estimate that two-magnon scattering cannot explain the experimental results. [ABSTRACT FROM AUTHOR]

X-ray diffraction (XRD), magnetometry, and ferromagnetic resonance (FMR) measurements were performed on sputtered thin films of the nominal Heusler alloy (Co2Mn)1-xGex with varying Ge content and annealing temperatures. XRD indicates some degree of B2 alloy formation, with strong (110) texturing. FMR measurements were performed in a perpendicular geometry that minimized the contribution of two-magnon scattering to the linewidth. The FMR data indicate a significant increase in linewidth for samples that lack a well-defined (220) peak, presumably as a result of inhomogeneous line broadening. Samples annealed at 200?°C exhibit decreasing Landau-Lifshitz damping with increasing Ge content, while samples annealed at 245 and 300?°C have a nonlinear dependence of linewidth on frequency. The nonlinear component of the linewidth data was successfully fit with a generalized theory of slowly relaxing impurities, originally proposed by Van Vleck and Orbach. The modified theory includes the possibility of transverse coherence during the relaxation process. Magnetometry and FMR spectroscopy results were analyzed in the context of Malozemoff's generalized Slater-Pauling (GSP) theory, with the conclusion that the Ge sites support a significant negative-polarized spin density of several tens of percent. The GSP analysis results were consistent with a more conventional analysis of the spectroscopic g-factor that is appropriate for alloys. The proportionality of the strength of the slow-relaxer contribution to the damping suggests that the negatively polarized Ge atoms are acting as the slowly relaxing impurities in question. [ABSTRACT FROM AUTHOR]

Brillouin light scattering shows that shape distortions in Ni80Fe20 nanomagnets can have a dramatic effect on the measured collective linewidth of certain spin-wave modes. The intentional introduction of quantifiable asymmetric egglike shape distortion to an ideal elliptical structure lifts the degeneracy of end modes with concentrated amplitude at the nanomagnet edges. In contrast, modes with concentrated amplitude at the interior are significantly less affected by the distortion. The splitting of end modes by asymmetric distortions explains the large inhomogeneous linewidth broadening in end modes found in large ensembles of nanomagnets that contain a relatively small statistical variation in the degree of distortion. [ABSTRACT FROM AUTHOR]

We studied the microwave assisted magnetic reversal of 65×71 nm2 elliptical Ni80Fe20 nanomagnets. Hysteresis curves were measured by magneto-optical Kerr effect for a range of microwave frequencies and amplitudes. The coercive field Hc was reduced by 26% for an rf field of 0.08Hc when the microwave frequency coincided with the minimum of the experimentally determined ferromagnetic resonance frequency with varying dc field. The experimental results for the fractional reduction in Hc with rf field amplitude are in good agreement with numerical simulations for an array of interacting macrospins with a physically realistic shape anisotropy distribution. [ABSTRACT FROM AUTHOR]

The authors demonstrate the stimulation of the magnetization switching process of a Ni81Fe19 ellipsoid, which is dominated by domain nucleation and propagation, by applying a transverse microwave field. The study of the quasistatic switching behavior under the influence of a microwave field was performed using longitudinal magneto-optic Kerr effect magnetometry. A strong reduction of the coercive field for microwave frequencies between 500 and 900 MHz has been observed, which can be attributed to two different mechanisms: microwave stimulated enhancement of domain nucleation and microwave stimulated growth of the reversed domain. The authors prove that heating is not the origin of the reduction of the coercive field. [ABSTRACT FROM AUTHOR]

We report on the realization of microstructures exhibiting the coexistence of two exchange bias fields with orthogonal directions. The microstructures are made of Fe50Mn50/Ni81Fe19 bilayers and have L-like and crosslike shapes. Above the Néel temperature the magnetic elements are initially saturated along the diagonal of the structures and then cooled down in zero external field. This causes the exchange bias field to be initialized along two different axes. Magneto-optical measurements confirm the simultaneous presence of orthogonal biasing directions. [ABSTRACT FROM AUTHOR]

A crossed coplanar waveguide design using polymer insulation layers is developed as a tool to experimentally investigate fast, precessional switching phenomena in view of applications in magnetic random access memory. The photosensitive polymer Cyclotene 4024™ (benzocyclobutene) is used as an electric insulation layer to realize a multilayer sample consisting of several different conducting and insulating layers. We report on the processing of Cyclotene 4024™ multilayers with optimized surface and electric insulating properties after hard cure. The micropatterned impedance matched coplanar waveguides have a bandwidth of 8.5 GHz according to -2 dB criterion, and for the -3 dB criterion the bandwidth exceeds the measurement range of up to 15 GHz. These values allow for short rise and fall times of the pulse for ultrafast magnetization dynamics measurements. [ABSTRACT FROM AUTHOR]

A writing scheme is presented for Savtchenko-type magnetic random access memory (MRAM) cells, which allows for ultrafast direct writing with high stability against half select switching, using two orthogonally oriented unipolar magnetic field pulses with time delay, which allows for backswitching by reversing the temporal sequence of the two pulses. The numerical simulations are based on the Stoner–Wohlfarth model and a Runge Kutta integration of the Landau–Lifshitz and Gilbert equation. [ABSTRACT FROM AUTHOR]

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

Multilayers of [Co/Ni( t )/Co/Pt] ×8 with varying Ni thickness were investigated for possible use as a free layer in magnetic tunnel junctions and spintronics devices. The thickness t of the Ni sub-layer was varied from 0.3 nm to 0.9 nm and the resulting magnetic properties were compared with (Co/Ni) and (Co/Pt) multilayers. As determined from magnetic force microscopy, magnetometry and ferromagnetic resonance measurements, all multilayers exhibited perpendicular magnetic anisotropy. Compared with (Co/Pt) multilayers, the sample with t of 0.9 nm showed almost the same anisotropy field of μ = 1.15 T but the damping constant was 40% lower. These characteristics make these multilayers attractive for spin torque based magnetoresistive devices with perpendicular anisotropy.0 H k = 1.15 T but the damping constant was 40% lower. These characteristics make these multilayers attractive for spin torque based magnetoresistive devices with perpendicular anisotropy.