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J. R. Childress

Publications and source records attributed to J. R. Childress.

11 recordsLinked to original sources
arXiv2020

Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core

The curling spin wave modes of a ferromagnetic vortex confined to a microscale disc have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest-frequency, azimuthal modes has previously been reported to lead to their splitting and counter propagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disc thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is only permitted with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disc, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disc. The measured spatio-temporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disc. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetised regions.

cond-mat.mes-hall↗
arXiv2020

Ion beam modification of magnetic tunnel junctions

The impact of 400 keV $Ar^+$ ion irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The irradiation-induced changes of the magnetic anisotropy, coupling energies and tunnel magnetoresistance (TMR) exhibited a correlated dependence on the ion fluence, which allowed us to distinguish between two irradiation regimes. In the low-fluence regime, $Φ < 10^{14} cm^{-2}$, the parameters required for having a functioning MTJ were preserved: the anisotropy of the FeCoB free layer (FL) was weakly modulated following a small decrease in the saturation magnetization $M_S$; the TMR decreased continuously; the interlayer exchange coupling (IEC) and the exchange bias (EB) decreased slightly. In the high-fluence regime, $Φ > 10^{14} cm^{-2}$, the MTJ was rendered inoperative: the modulation of the FL anisotropy was strong, caused by a strong decrease in $M_S$, ascribed to a high degree of interface intermixing between the FL and the Ta capping; the EB and IEC were also lost, likely due to intermixing of the layers composing the synthetic antiferromagnet; and the TMR vanished due to the irradiation-induced deterioration of the MgO barrier and MgO/FeCoB interfaces. We demonstrate that the layers surrounding the FL play a decisive role in determining the trend of the magnetic anisotropy evolution resulting from the irradiation, and that an ion-fluence window exists where such a modulation of magnetic anisotropy can occur, while not losing the TMR or the magnetic configuration of the MTJ.

cond-mat.mtrl-sci↗
arXiv2008

Experimental Test of Analytic Theory of Spin Torque Oscillator Dynamics

We make measurements of power spectral density of the microwave voltage emitted by a spin torque nano-oscillator (STNO) and compare our experimental results to predictions of an analytic theory of a single-mode STNO dynamics by V. S. Tiberkevich, A. N. Slavin, J. V. Kim,[Phys. Rev. B 78, 092401 (2008)]. We find that a complete set of the oscillator spectral properties: power, frequency, spectral line width and line shape as functions of current are self-consistently described by the analytic theory for moderate amplitudes of oscillations (< 70 degrees).

cond-mat.other↗
arXiv2008

High-output CPP-GMR sensor with synthetic-ferrimagnet free layer and enhanced spin-torque critical currents

It is shown that the maximum stable output of a CPP-GMR sensor is increased significantly by using a synthetic ferrimagnet free layer, provided the electron current flows from free layer to reference layer. This free layer allows a larger magnetoresistance ratio for a given free layer magnetic moment, and in addition results in a greater than three-fold increase in the critical current above which spin-torque instability of the free layer occurs. In read heads with net free layer moments equivalent to only 4.5nm of Ni80Fe20, this effect is shown to result in sustainable sense current densities above 2e8 A/cm2.

cond-mat.mtrl-sci↗
arXiv2008

Suppression of spin-torque in current perpendicular to the plane spin-valves by addition of Dy cap layers

We demonstrate that the addition of Dy capping layers in current perpendicular to the plane giant magneto-resistive spin-valves can increase the critical current density beyond which spin-torque induced instabilities are observed by about a factor of three. Current densities as high as 5e7 A/cm2 are measured provided that the electron current flows from the free to the reference layer. While Dy capped samples exhibit nonmagnetic 1/f noise, it is sufficiently small to be unimportant for read head operation at practical data rates.

cond-mat.other↗
arXiv2008

Magnetic domain wall propagation in a submicron spin-valve stripe: influence of the pinned layer

The propagation of a domain wall in a submicron ferromagnetic spin-valve stripe is investigated using giant magnetoresistance. A notch in the stripe efficiently traps an injected wall stopping the domain propagation. The authors show that the magnetic field at which the wall is depinned displays a stochastic nature. Moreover, the depinning statistics are significantly different for head to head and tail-to-tail domain walls. This is attributed to the dipolar field generated in the vicinity of the notch by the pinned layer of the spin-valve.

cond-mat.mtrl-sci↗
arXiv2007

Time-resolved investigation of magnetization dynamics of arrays of non-ellipsoidal nanomagnets with a non-uniform ground state

We have performed time-resolved scanning Kerr microscopy (TRSKM) measurements upon arrays of square ferromagnetic nano-elements of different size and for a range of bias fields. The experimental results were compared to micromagnetic simulations of model arrays in order to understand the non-uniform precessional dynamics within the elements. In the experimental spectra two branches of excited modes were observed to co-exist above a particular bias field. Below the so-called crossover field, the higher frequency branch was observed to vanish. Micromagnetic simulations and Fourier imaging revealed that modes from the higher frequency branch had large amplitude at the center of the element where the effective field was parallel to the bias field and the static magnetization. Modes from the lower frequency branch had large amplitude near the edges of the element perpendicular to the bias field. The simulations revealed significant canting of the static magnetization and the effective field away from the direction of the bias field in the edge regions. For the smallest element sizes and/or at low bias field values the effective field was found to become anti-parallel to the static magnetization. The simulations revealed that the majority of the modes were de-localized with finite amplitude throughout the element, while the spatial character of a mode was found to be correlated with the spatial variation of the total effective field and the static magnetization state. The simulations also revealed that the frequencies of the edge modes are strongly affected by the spatial distribution of the static magnetization state both within an element and within its nearest neighbors.

cond-mat.mtrl-sci↗
arXiv2004

Thermal Effects on the Magnetic Field Dependence of Spin Transfer Induced Magnetization Reversal

We have developed a self-aligned, high-yield process to fabricate CPP (current perpendicular to the plane) magnetic sensors of sub 100 nm dimensions. A pinned synthetic antiferromagnet (SAF) is used as the reference layer which minimizes dipole coupling to the free layer and field induced rotation of the reference layer. We find that the critical currents for spin transfer induced magnetization reversal of the free layer vary dramatically with relatively small changes the in-plane magnetic field, in contrast to theoretical predictions based on stability analysis of the Gilbert equations of magnetization dynamics including Slonczewski-type spin-torque terms. The discrepancy is believed due to thermal fluctuations over the time scale of the measurements. Once thermal fluctuations are taken into account, we find good quantitative agreement between our experimental results and numerical simulations.

cond-mat.mtrl-sci↗
arXiv1998

Ferromagnetic resonance force microscopy on microscopic cobalt single layer films

We report mechanical detection of ferromagnetic resonance signals from microscopic Co single layer thin films using a magnetic resonance force microscope (MRFM). Variations in the magnetic anisotropy field and the inhomogeneity of were clearly observed in the FMR spectra of microscopic Co thin films 500 and 1000 angstrom thick and 40 X 200 micron^2 in lateral extent. This demonstrates the important potential that MRFM detection of FMR holds for microscopic characterization of spatial distribution of magnetic properties in magnetic layered materials and devices.

cond-mat.mtrl-sci↗