John Ketterson

Using the time dependent Ginzburg-Landau (TDGL) equations, we simulate the inductive responses of a variety of thin film systems containing patterned anti-dots with differing sizes and shapes. The results for all shapes show that the kinetic inductance diverges as the applied current approaches a critical current, J c, which is below the BCS depairing current, J dp. Exploiting the similarity of the observed EJ behavior to that modeled by a critical exponent near a second phase transition, we obtain an empirical equation that well fits the effective inductivity curve as a function of the applied current.

Systems and methods for reversing a magnetization in a ferromagnet include a nanometer-scale cylindrical ferromagnetic sample having a height to diameter aspect ratio on the order of 2 or greater. A temporally-varying external field comprising an rf Pi pulse is applied to the ferromagnetic sample to cause a precession magnetization vector inclined at an angle with respect to the longest axis of the ferromagnetic sample to continuously rotate around the longest axis. One or more parameters of the temporally-varying external field is continuously adjusted based on at least magnetization dynamics of the ferromagnetic sample and/or an angular dependence of a precession frequency of the ferromagnetic sample.

Thin films of amorphous Mo x Ge 1- x grown with magnetron sputtering are known to exhibit excellent homogeneity and fairly high superconducting transition temperatures (up to 7.5 K). These properties make MoGe thin films suitable to form high-quality tunnel junctions, as demonstrated recently by our group [see Supercond. Sci. Technol. 35, 035008 (2022)]. Here, we report the experimental demonstration of an MoGe/Al/AlO x /(Al)MoGe Josephson oscillator based on amorphous superconductor MoGe, which allows for continuous tuning of the oscillation frequency from about 1 GHz up to tens of GHz. We believe such an oscillator may be useful as a compact on-chip microwave source suitable to control some types of qubits based on Josephson junctions.

We report on flux-flow properties of 50 nm thick thin-film amorphous MoGe bridges of different sizes with and without patterned sub-micron holes with different diameters and spacings. Characterization of the devices was carried out in liquid He at 4.2 K in a magnetic field, H , applied perpendicular to the device plane. Two critical currents, I c1 and I c2 , were studied. The current I c1 is identified as the onset of a low-resistance state, whereas I c2 is the current at which the device switches to a high-resistance state, and the corresponding dependences I c1 ( H ) and I c2 ( H ) were determined. In the absence of the holes, I c1 decreases monotonically with H , whereas I c2 ( H ) manifests lobes resembling those in the Fraunhofer-like pattern characteristic of Josephson junctions. This behavior may be due to formation of an ordered vortex lattice in some current and field ranges. Introducing the hole-line arrays …

We simulate the magnetization dynamics of a permalloy spheroid of nanoscopic size in zero external field, such that both dipolar and exchange interactions are important. Low excitation power is used to obtain the frequencies and mode patterns of many normal modes. At higher power, non-linear three and four mode couplings between magnons carrying orbital angular momentum are observed to give rise to Suhl instabilities. Suhl’s analysis of the selection rules governing the allowed processes is extended to initial states other than uniform precession. These rules are studied and confirmed by the simulations. Both down- and up-conversion are seen as well as three and four-mode processes. General trends are inferred for preferred instabilities among those that are allowed, although the thresholds for some instabilities appear to be very high.

Materials with low damping, such as Yttrium Iron Garnet (YIG), are of interest in connection with spintronic devices. A promising structure for information storage is the Skyrmion, a domain wall quasi-particle. It has been shown that the stabilization of a Skyrmion can be energetically favorable with the addition of spin–orbit coupling (SOC) through the Dzyaloshinskii–Moriya Interaction (DMI). This interaction should be largest in metals, but still present in insulators. To produce spintronic devices using YIG, we must evaluate the DMI interaction inherent in the substrate used to grow the YIG, which is generally Gadolinium Gallium Garnet (GGG). In this paper, we measure nonreciprocal spin-wave propagation in a thick YIG film to place a limit on the DMI parameter in a YIG/GGG bilayer.

An unconventional “heteromorphic” superlattice (HSL) is realized, comprised of repeated layers of different materials with differing morphologies: semiconducting pc‐In2O3 layers interleaved with insulating a‐MoO3 layers. Originally proposed by Tsu in 1989, yet never fully realized, the high quality of the HSL heterostructure demonstrated here validates the intuition of Tsu, whereby the flexibility of the bond angle in the amorphous phase and the passivation effect of the oxide at interfacial bonds serve to create smooth, high‐mobility interfaces. The alternating amorphous layers prevent strain accumulation in the polycrystalline layers while suppressing defect propagation across the HSL. For the HSL with 7:7 nm layer thickness, the observed electron mobility of 71 cm2 Vs‐1, matches that of the highest quality In2O3 thin films. The atomic structure and electronic properties of crystalline In2O3/amorphous MoO3 …

We report experimental and theoretical studies of spin dynamics in lattice structures of permalloy (Ni 80 Fe 20) nano-ellipses, with four different types of networks including honeycomb and square lattices. The lattices are patterned at the center line of the co-planar wave guide and consist of non-contacting or contacting ellipses. Micromagnetic simulations show excellent agreement with the broadband ferromagnetic resonance (FMR) experimental results. We find the existence of a spin-wave mode localized in the vertex region of the contacting nano-ellipse network. Our finding has important implications when designing an artificial spin ice (ASI) network for functional magnonics.

We have fabricated and characterized all-MoGe Josephson junctions with a very thin Al/AlOx/(Al) barrier, where the amorphous MoGe films exhibit superconducting transition temperatures up to 7 K. Due to the uniformity of the surface morphology of the MoGe films, the junctions demonstrate high uniformity of their tunneling properties. The experimental data on the temperature dependence of the subgap current agree well with theoretical calculations. The results obtained imply that Josephson tunnel junctions based on amorphous superconductors are promising candidates for use in superconducting electronics, especially in applications requiring multiple stacked junctions or the creation of a nonequilibrium quasiparticle distribution.

We have fabricated and characterized MoGe/Al/AlO x/(Al) MoGe Josephson junctions (JJs) using amorphous MoGe thin films with superconducting transition temperatures up to 7 K. Amorphous MoGe films are known to have very smooth and uniform surfaces, which allows us to achieve high quality tunnel barriers and excellent uniformity of the junction properties when using a very thin Al overlayer to form the barrier. Our experimental results and comparisons with the theoretical calculations confirm these behaviors. High uniformity of the characteristics of the MoGe junctions, which exceeds that of Nb/Al/AlO x/(Al)/Nb junctions fabricated using the same equipment, is important for practical applications involving superconducting electronics. In particular, we suggest that MoGe junctions are especially suitable for applications where vertical stacking of multiple JJs is desirable, eg, in voltage standards and Josephson …

We simulate the motion of a commensurate vortex lattice in a periodic lattice of artificial circular pinning sites having different diameters, pinning strengths, and spacings using the time-dependent Ginzburg-Landau formalism. Above some critical DC current density J c, the vortices depin, and the resulting steady-state motion then induces an oscillatory electric field E (t) with a defect “hopping” frequency f 0, which depends on the applied current density and the pinning landscape characteristics. The frequency generated can be locked to an applied AC current density over some range of frequencies, which depends on the amplitude of the DC as well as the AC current densities. Both synchronous and asynchronous collective hopping behaviors are studied as a function of the supercell size of the simulated system and the (asymptotic) synchronization threshold current densities determined.

We report simulations of large-amplitude responses of a permalloy ellipsoid with z, x, and y diameters of 100× 50× 50nm 3 driven by an algorithm that constrains the H 1 field to lie the xy plane and perpendicular to the sample-averaged magnetization. For smaller H 1 uniform precession is observed, but as H 1 increases instabilities are encountered. The first, signaled by an abrupt increase in the exchange energy, involves a crossover of the uniform mode and a standing spin wave with a 2Pi phase variation along z, characteristic of the second Suhl 1 processes. The degeneracy arises from a strong dependence of the 2Pi mode frequency on the uniform precession amplitude. Above the crossover these two modes appear to merge into a bound state, in which the spins precess nonuniformly but in phase and the two modes periodically exchange energy back and forth. At the second instability two low frequency …

Large amplitude magnetization dynamics in elliptical permalloy nano-disks are studied via simulation of the Landau-Lifshitz equation. When the external magnetic field is applied in the plane of the disk two low-lying resonant modes exist. The higher frequency mode corresponds to the Kittel-like uniform ferromagnetic resonance mode and is consistent with previous experimental observations. The second mode with the lower frequency, termed the “cap mode”, is localized near the two ends of the ellipse. At certain fields the ratio of the frequencies of these modes is equal to two or three and here one can efficiently generate the Kittel mode as the 2nd or 3rd harmonic of the “cap mode” when the magnitude of the input microwave field is of order 0.1 Oe, which can easily be achieved in practice. The nonlinear responses depend strongly on the equilibrium, field-dep-endent, behavior of the magnetization which is non …

In the presence of a periodic lattice of patterned pinning sites in a superconducting film, together with a proper choice of an external magnetic field, a commensurate vortex array can be prepared. Above some critical applied dc current the lattice breaks loose from these pins and drifts between them, momentarily repinning at each site. This generates an ac current in the film, the fundamental frequency of which increases with the applied dc current. We have simulated this behavior using the time Dependent Ginzburg-Landau formalism (TDGL)[1] for arrays of circular pinning sites having differing diameters, pinning strengths and spacings. Depending on conditions, the harmonic content of the resulting signal can be high. If, in addition, an external ac current is superimposed on the dc current, the vortex site-hopping frequency can be pulled and locked to the applied frequency over some range of applied frequencies …

The method of images is applied to find the field and charge distribution for a conducting prolate spheroid in a uniform electric field via simple one-dimensional integrals. This solution is then repurposed to yield the demagnetization factors for a prolate spheroid.

Using a many spin micromagnetic simulation tool that directly integrates the Landau-Lifshitz equation, we demonstrate that by applying an r.f. pulse, generally referred to as a Pi pulse, it is possible to near-perfectly reverse the direction of the magnetization in a ferromagnet, provided that the sample is sufficiently small and the angular dependence of the precession frequency is continuously matched using an appropriately “chirped” r.f. pulse of the proper length. Simulations are carried out for “prolate” uniaxially symmetric cylindrical samples in the presence of dipole and exchange interactions. Such reversals can be performed in the presence of a static external magnetic field or, importantly, at zero field under the sample’s own internal demagnetization field. However, the ability to perform near-perfect Pi or two-Pi rotations is lost for samples above certain dimensions for which additional internal degrees of freedom …

Chalcogenide-based phase change memory (PCM) is a key enabling technology for optical data storage and electrical nonvolatile memory. Here, we report a new phase change chalcogenide consisting of a 3D network of ionic (K···Se) and covalent bonds (Bi–Se), K2Bi8Se13 (KBS). Thin films of amorphous KBS deposited by DC sputtering are structurally and chemically homogeneous and exhibit a surface roughness of 5 nm. The KBS film crystallizes upon heating at ∼483 K. The optical bandgap of the amorphous film is about 1.25 eV, while its crystalline phase has a bandgap of ∼0.65 eV shows 2-fold difference between the two states. The bulk electrical conductivity of the amorphous and crystalline film is ∼7.5 × 10–4 and ∼2.7 × 10–2 S/cm, respectively. We have demonstrated a phase change memory effect in KBS by Joule heating in a technologically relevant vertical memory cell architecture. Upon Joule …

We report experimental studies of quantum-matter heterostructures based on a ferromagnet/normal metal multilayer proximitized by Nb superconducting electrodes to form a novel Josephson weak-link device that is highly sensitive to magnetic fields. The device is a single Josephson junction containing Al/Ni or Al/Py (Py: Ni 80 Fe 20 ) multilayer structures, which manifests quasi-sinusoidal critical current oscillations resembling the response of a dc Superconducting Quantum Interference Device (SQUID). Our analysis shows that the field sensitivity of this novel device, as measured by the magnetic field needed to form one period of the oscillations, is about twice that reported for recent micro- or nano-SQUIDs. We present an analysis of the temperature dependence of the period of the oscillations and the Josephson critical current, as well as the background current. We believe that our devices are promising …

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. Semiquantitative 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 …

In this work, we realize a physical system—nanoengineered singly connected Josephson junction with a periodic superparamagnetic Ni/Al multilayer—that manifests supercurrent-versus-magnetic field response typical of a dc superconducting quantum interference device (SQUID); however, unlike a SQUID, which involves a superconducting loop occupying significant space, our lumped device is more suitable for miniaturization. In addition, we show that it exhibits enhanced magnetic field sensitivity as compared with conventional superconductor-insulator-superconductor Josephson junctions. SQUID-like oscillatory response to external magnetic fields, analogous to the two-slit optical interference, is explained in terms of the dominance of Andreev bound states localized at the barrier edges in the comparatively thick and strongly anisotropic weak links. Our results may lead to significant advancement in …

Artificial spin ice (ASI) consists of periodic arrays of nanomagnets in the shape of elongated elements where competing interactions between the elements lead to geometric frustration. Recently, non-Ising-like ASI have attracted great attention due to their exotic phase diagrams. One example is an array of ferromagnetic nanodiscs.Here, we report the experimental and theoretical characterization of angular-dependent spin dynamics in arrays of ferromagnetic nanodiscs arranged on a Kagome lattice. The arrangement consists of coupled discs that are 500 nm in diameter. The distance between the Kagome vertices is 76 nm. The magnetic field and microwave frequency dependence obtained by broadband ferromagnetic resonance reveals a rich spectrum of modes that is strongly affected by the microstate of the network. In the high-field range the magnetization is parallel to the applied field, while at low-fields …

We present experimental and theoretical studies of spin-wave mode dynamics in artificial kagome spin ice vertices made of three identical 15-nm thick elongated Ni 80 Fe 20 nanoislands (macrospins). We consider several possible configurations, from completely disjointed macrospins (full dipolar interelement interactions) to fully jointed macrospins (full dipolar-exchange interactions). Using angular-resolved magnetic field dependent broadband ferromagnetic resonance (FMR), we demonstrate the occurrence of a mode localized in the vertex region as indicated by the distinct behavior of the FMR spectra at different angles and configurations. Theoretical calculations using micromagnetic simulations support the existence, origin, and behavior of this mode by interpreting it as a localized, quasi-uniform Kittel mode. Our findings pave the way for designing the most appropriate network consisting of ferromagnetic …

We report the experimental and theoretical characterization of the angular-dependent spin dynamics in arrays of ferromagnetic nanodisks arranged on a honeycomb lattice. The magnetic field and microwave frequency dependence, measured by broadband ferromagnetic resonance, reveal a rich spectrum of modes that is strongly affected by the microstate of the network. Based on symmetry arguments with respect to the external field, we show that certain parts of the ferromagnetic network contribute to the detected signal. A comparison of the experimental data with micromagnetic simulations reveals that different subsections of the lattice predominantly contribute to the high-frequency response of the array. This is confirmed by optical characterizations using microfocused Brillouin light scattering. Furthermore, we find indications that nucleation and annihilation of vortexlike magnetization configurations in the low …

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the Ag vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without …

An efficient way to perform microwave-assisted switching of small ferromagnetic samples is studied. Magnetization reversals in cylindrical Yttrium Iron Garnet nanomagnets are simulated by applying a fixed-frequency transverse microwave field and a time-varying longitudinal applied field along the direction of the static field so as to continually match the precession frequency with the microwave frequency. The ideal form of microwave field is circularly polarized, and we also studied linearly polarized microwave fields since they are simpler to implement. Inhomogeneous modes nucleate in larger samples with dimensions several times larger than an exchange length which leads to incomplete switching.

Using parametric excitation, we have studied the minimum frequency associated with the backward volume (BV) spin wave branch in a thin film of the ferrimagnetic material yttrium iron garnet (YIG) for the magnetic field lying in plane and parallel to the wave vector as well as for out-of-plane field angles. We find that there is a drastic change in the efficiency of parametric excitation between two different pumping frequency regimes.

We report broadband ferromagnetic resonance measurements of the in-plane magnetic field response of three-and four-fold symmetric vertices formed by non-contacting permalloy nano-ellipses together with extended lattices constructed from them. Complementing the experimental data with simulations, we are able to show that, as far as the most intense FMR responses are concerned, the spectra of vertices and lattices can largely be interpreted in terms of a superposition of the underlying hysteretic responses of the individual ellipses, as elemental building blocks of the system. This property suggest that it is possible to understand the orientation of the individual magnetic dipole moments in a dipole network in terms of dynamic measurements alone, thereby offering a powerful tool to analyze the alignment statistics in frustrated systems that are exposed to various magnetic histories.