Searcharxiv⌕ Search

arXiv subjects

J. B. Ketterson

Publications and source records attributed to J. B. Ketterson.

18 recordsLinked to original sources
arXiv2021

Ferromagnetic resonance modes in the exchange dominated limit in cylinders of finite length

We analyze the magnetic mode structure of axially-magnetized, finite-length, nanoscopic cylinders in a regime where the exchange interaction dominates, along with simulations of the mode frequencies of the ferrimagnet yttrium iron garnet. For the bulk modes we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized "cap modes" that are "trapped" at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semi-quantitative explanations are given for some of the modes in terms of a one-dimensional Schrodinger equation which is valid in the exchange dominant case. The assignment of the azimuthal mode number is carefully discussed and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

cond-mat.mes-hall↗
arXiv2016

Spin Transport through the metallic antiferromagnet FeMn

We investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in Ni80Fe20/FeMn/W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. Using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents respectively. The latter can extend to relatively large distances (up to 9 nm) and is enhanced when the antiferromagnetic ordering temperature is close to the measurement temperature.

cond-mat.mtrl-sci↗
arXiv2016

Novel, Retroreflective, Magneto-Optical Trap Near a Surface

We report on a novel Magneto-Optical Trap (MOT) geometry involving the retroreflection of one of the six MOT beams in order to create an atom cloud close to the surface of a prism which does not have optical access along one axis. A MOT of Rb$^{85}$ with $\sim 4 \times 10^7$ atoms can be created 700 um from the surface. The MOT lies close to the minimum of an evanescent Gravito-Optical Surface Trap (GOST) allowing for transfer into the GOST with potentially minimal losses.

physics.atom-ph↗
arXiv2016

Dynamic response of an artificial square spin ice

Magnetization dynamics in an artificial square spin-ice lattice made of Ni80Fe20 with magnetic field applied in the lattice plane is investigated by broadband ferromagnetic resonance spectroscopy. The experimentally observed dispersion shows a rich spectrum of modes corresponding to different magnetization states. These magnetization states are determined by exchange and dipolar interaction between individual islands, as is confirmed by a semianalytical model. In the low field regime below 400 Oe a hysteretic behavior in the mode spectrum is found. Micromagnetic simulations reveal that the origin of the observed spectra is due to the initialization of different magnetization states of individual nanomagnets. Our results indicate that it might be possible to determine the spin-ice state by resonance experiments and are a first step towards the understanding of artificial geometrically frustrated magnetic systems in the high-frequency regime.

cond-mat.mes-hall↗
arXiv2015

Interface-driven spin-torque ferromagnetic resonance by Rashba coupling at the interface between non-magnetic materials

The Rashba-Edelstein effect stems from the interaction between the electron's spin and its momentum induced by spin-orbit interaction at an interface or a surface. It was shown that the inverse Rashba-Edelstein effect can be used to convert a spin- into a charge current. Here, we demonstrate that a Bi/Ag Rashba interface can even drive an adjacent ferromagnet to resonance. We employ a spin-torque ferromagnetic resonance excitation/detection scheme which was developed originally for a bulk spin-orbital effect, the spin Hall effect. In our experiment, the direct Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, Ni80Fe20) layer. Electrical detection of the magnetization dynamics is achieved by a rectification mechanism of the time dependent multilayer resistance arising from the anisotropic magnetoresistance.

cond-mat.mes-hall↗
arXiv2015

Large spin-wave bullet in a ferrimagnetic insulator driven by spin Hall effect

Due to its transverse nature, spin Hall effects (SHE) provide the possibility to excite and detect spin currents and magnetization dynamics even in magnetic insulators. Magnetic insulators are outstanding materials for the investigation of nonlinear phenomena and for novel low power spintronics applications because of their extremely low Gilbert damping. Here, we report on the direct imaging of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ based on the excitation and detection by SHEs. The driven spin dynamics in Y$_3$Fe$_5$O$_{12}$ is directly imaged by spatially-resolved microfocused Brillouin light scattering (BLS) spectroscopy. Previously, ST-FMR experiments assumed a uniform precession across the sample, which is not valid in our measurements. A strong spin-wave localization in the center of the sample is observed indicating the formation of a nonlinear, self-localized spin-wave `bullet'.

cond-mat.mes-hall↗
arXiv2013

Unexpectedly Slow Two Particle Decay of Ultra-Dense Excitons in Cuprous Oxide

For an ultra-dense exciton gas in cuprous oxide (Cu$_2$O), exciton-exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

cond-mat.quant-gas↗
arXiv2009

Perfect coupling of light to surface plasmons with ultra-narrow linewidths

We examine the coupling of electromagnetic waves incident normal to a thin silver film that forms an oscillatory grating embedded between two otherwise uniform, semi-infinite half spaces. Two grating structures are considered, in one of which the mid point of the Ag film remains fixed whereas the thickness varies sinusoidally, while in the other the mid point oscillates sinusoidally whereas the film thicknesses remains fixed. On reducing the light wavelength from the long wavelength limit, we encounter signatures in the transmission, T, and reflection, R, coefficients associated with: i) the short-range surface plasmon mode, ii) the long-range surface plasmon mode, and iii) electromagnetic diffraction tangent to the grating. The first two features can be regarded as generalized (plasmon) Wood's anomalies whereas the third is the first-order conventional (electromagnetic) Wood's anomaly. The energy density at the film surface is enhanced for wavelengths corresponding to these three anomalies, particularly for the long range plasmon mode in thin films. When exciting the silver film with a pair of waves incident from opposite directions, we find that by adjusting the grating oscillation amplitude and fixing the relative phase of the incoming waves to be even or odd, T+R can be made to vanish for one or the other of the plasmon modes; this corresponds to perfect coupling (impedance matching in the language of electrical engineering) between the incoming light and these modes.

physics.optics↗
arXiv2007

Dynamic magnetic response of infinite arrays of ferromagnetic particles

Recently developed techniques to find the eigenmodes of a ferromagnetic particle of arbitrary shape, as well as the absorption in the presence of an inhomogeneous radio-frequency field, are extended to treat infinite lattices of such particles. The method is applied to analyze the results of recent FMR experiments, and yields substantially good agreement between theory and experiment.

cond-mat.mtrl-sci↗
arXiv2005

Excitation of spin waves by spin polarized current

Numerical and analytical analysis is used to explain recently observed experimental phenomenon - excitation of spin waves in spin valves due to the applied spin polarized current. Excited spin waves are being identified and Lyapunov stability analysis is being used to identify different regions of stability depending on the value of the applied current.

cond-mat.mtrl-sci↗
arXiv2005

Switching spin valves using r.f. currents

We show that magnetization reversal in spin-injection devices can be significantly faster when using a chirped r.f. rather than d.c current pulse. Alternatively one can use a simple sinusoidal r.f. pulse or an optimized series of alternating, equal-amplitude, square pulses of varying width (a digitized approximation to a chirped r.f. pulse) to produce switching using much smaller currents than with a d.c. pulse.

cond-mat.mtrl-sci↗
arXiv2005

Controlling the Suhl instability: a numerical study

Magnetization reversal (switching) using either r.f. fields or brute-force precessional switching is currently thought to ultimately be limited by the non-linear excitation of non-uniform spin waves, the so-called Suhl instability. Here we show (numerically, for the case of a sphere) that this instability can be suppressed by choosing the applied field and/or sphere diameter in such a way that the frequencies of the modes that can be excited through non-linear processes are off-resonance. While the results cannot be explained by a traditional model based on plane waves, they can be understood by projecting the actual state onto the small amplitude spin resonant eigenfunctions.

cond-mat.mtrl-sci↗
arXiv2005

Micromagnetic simulations of absoption spectra

Further development of a previously introduced method for numerically simulating magnetic spin waves is presented. Together with significant improvements in speed, the method now allows one to calculate the energy absorbed by the various modes excited by a position- and time-dependent H1 field in a ferromagnetic body of arbitrary shape in the presence of a (uniform or non uniform) static H0 field as well as the internal exchange and anisotropy fields. The method is applied to the case of the single vortex state in a thin disc, a ring, and various square slabs, for which the absorption spectra are calculated and the most strongly excited resonance modes are identified.

cond-mat.mtrl-sci↗
arXiv2005

Magnetization reversal in the anisotropy-dominated regime using time-dependent magnetic fields

We study magnetization reversal using various r.f. magnetic pulses. We show numerically that switching is possible with simple sinusoidal pulses; however the optimum approach is to use a frequency-swept (chirped) r.f. magnetic pulse, the shape of which can be derived analytically. Switching times of the order of nanoseconds can be achieved with relatively small r.f. fields, independent of the anisotropy's strength.

cond-mat.mtrl-sci↗
arXiv2002

H-T phase diagram of the metamagnetic transition in $URu_2Si_2$, measured in high d. c. magnetic fields

We have studied the ultrasonic velocity and ac susceptibility of $URu_2Si_2$ in d. c. magnetic fields up to 45T. A significant difference between the constructed H-T phase diagram and that extracted from earlier pulsed fields measurements can be explained in terms of a large magnetocaloric effect. A hysteresis of metamagnetic transition at low temperatures and a new phase separation line have been observed for the first time.

cond-mat.str-el↗
arXiv2001

Ferromagnetic resonance in periodic particle arrays

We report measurements of the ferromagnetic resonance (FMR) spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography. In contrast to plane ferromagnetic films, the spectra of the arrays show a number of additional resonance peaks, whose position depends strongly on the orientation of the external magnetic field and the interparticle interaction. Time-dependent micromagnetic simulation of the ac response show that these peaks are associated with coupled exchange and dipolar spin wave modes

cond-mat.mes-hall↗
arXiv2001

Distribution of parallel vortices studied by spin-polarized neutron reflectivity and magnetization

We present the studies of non-uniformly distributed vortices in Nb/Al multilayers at applied field near parallel to film surface by using spin-polarized neutron reflectivity (SPNR) and DC magnetization measurements. We have observed peaks above the lower critical field, Hc1, in the M-H curves from the multilayers. Previous works with a model calculation of minimizing Gibbs free energy have suggested that the peaks could be ascribed to vortex line transitions for spatial commensuration in a thin film superconductor. In order to directly determine the distribution of vortices, we performed SPNR measurements on the multilayer and found that the distribution and density of vortices are different at ascending and descending fields. At ascending 2000 Oe which is just below the first peak in the M-H curve, SPNR shows that vortices are mostly localized near a middle line of the film meanwhile the vortices are distributed in broader region at the descending 2000 Oe. That is related to the observation of more vortices trapped at the descending field. As the applied field is sightly tilted (< 3.5degree), we observe another peak at a smaller field. The peak position is consistent with the parallel lower critical field (Hc1||). We discuss that the vortices run along the applied field below Hc1|| and rotate parallel to the surface at Hc1||.

cond-mat.supr-con↗