A. O. Adeyeye

Deterministic Control of Magnetization Dynamics in Reconfigurable Nanomagnetic Networks for Logic Applications

Information processing based on nanomagnetic networks is an emerging area of spintronics, as the energy consumption and integration density of the current semiconductor technology are reaching their fundamental limits. Nanomagnet-based devices rely on manipulating the magnetic ground states for device operations. While the static behavior of nanomagnets has been explored, little information is available on their dynamic behavior. Here, we demonstrate an additional functionality based on their collective dynamic response and explore the concept utilizing networks of bistable rhomboid nanomagnets. The control of the magnetic ground states of the networks was achieved by the geometrical design of the nanomagnets instead of the conventional interelement dipolar coupling. Dynamic responses of both the ferromagnetic and antiferromagnetic ground states were monitored using broadband ferromagnetic resonance spectroscopy, the Brillouin light scattering technique, and direct magnetic force microscopy. Micromagnetic simulations and numerical calculations validate our experimental observations. This method would have potential implications for low-power magnonic devices based on reconfigurable microwave properties.

Robust electric-field tunable opto-electrical behavior in Pt-NiO-Pt planar structures

Capacitor-like metal-NiO-metal structures have attracted large interest in non-volatile memory applications based on electric field control of resistance, known as resistive switching (RS). Formation of conducting nanofilaments by the application of an electric field (electroformation) is considered an important pre-requisite for RS. Besides RS, due to the wide band gap and p-type semiconducting nature, NiO has been used to fabricate heterojunctions for photodetector applications. However, very little is known about the electrical and opto-electrical properties of NiO films in planar structure. Here, we demonstrate intriguing photoresponse and electrical behavior in electroformed Pt-NiO-Pt planar structures. While the pristine devices show ohmic electrical behavior and negligible photoresponse, the electroformed devices exhibit a nonlinear rectification behavior and a remarkable photoresponse at low voltage biases. More interestingly, the devices show a dramatic change of sign of rectification under light illumination at higher voltage biases. A polarity dependent and robust gain phenomenon is demonstrated in these devices. The large sensitivity, fast response, simple design and ease of preparation of these planar structures make them attractive for integration with current circuit technologies and various novel opto-electrical applications.

Microwave magnetic dynamics in ferromagnetic metallic nanostructures lacking inversion symmetry

In this work, we carried out systematic experimental and theoretical investigations of ferromagnetic resonance (FMR) responses of quasi-two-dimensional magnetic objects—macroscopically long stripes with nanoscale cross-section made of ferromagnetic metals. We were interested in the impact of the symmetries of this geometry on the FMR response. Three possible scenarios from which the inversion symmetry break originated were investigated, namely: (1) from the shape of the stripe cross-section, (2) from the double-layer structure of the stripes with exchange coupling between the layers, and (3) from the single-side incidence of the microwave magnetic field on the plane of the stripe array. The latter scenario is a characteristic of the stripline FMR configuration. It was found that the combined effect of the three symmetry breaks is much stronger than the impacts of each of these symmetry breaks separately.

Dynamic behavior of Ni80Fe20 nanowires with controlled periodic width modulation

The magnetization reversal and dynamic behaviors of Ni80Fe20 nanowires (NWs) with controlled periodic width modulation on single and double sides of the wires have been systematically investigated using magneto-optical Kerr effect and broadband ferromagnetic resonance spectroscopy. In contrast with the single resonance mode observed in the homogeneous NWs, the NWs with periodic width modulation display two distinct resonance modes (the fundamental mode at lower frequency and the high frequency mode which is localized in the modulated regions) due to the non-uniform demagnetizing field. An enhancement of the coercive field is observed for the width modulated NWs when compared with homogeneous NWs. We also observed that the high frequency mode and the frequency difference between the two distinct modes are very sensitive to the modulation profile and film thickness. The results obtained from our experimental results agree well with the micromagnetic simulations. The results have potential implications in th...

Large and robust resistive switching in co-sputtered Pt-(NiO-Al2O3)-Pt devices

We have systematically investigated the resistive switching and electroresistance behavior in Pt-[NiO-Al2O3]-Pt (PNAP) capacitor-like structures. The PNAP devices show a large ON-OFF ratio (∼107), which is strongly dependent on the rate of the voltage sweep. Interestingly, the devices exhibit a robust electroresistance behavior in the high resistance OFF state and show an intriguing change of sign of rectification with increasing end voltage. Our direct measurement of the surface temperature of the sample during resistive switching indicates that RESET process is assisted by Joule heating effects. The results are explained on the basis of plausible interplay between Schottky barrier modification due to the trapped charge carriers at the metal–oxide interface and percolation effects of conducting nanofilaments.

Unconventional spin distributions in thick Ni80Fe20 nanodisks

We study the spin distributions in permalloy (Py: Ni80Fe20) nanodisks as a function of diameter D (300 nm ≤ D ≤ 1 μm) and thickness L (30 nm ≤ L ≤ 100 nm). We observed that beyond a certain thickness, for a fixed disk diameter, an unconventional spin topology precipitates which is marked by the presence of a divergence field within the magnetic vortex curl. The strength of this divergence changes anti-symmetrically from negative to positive—depending on the core polarity—along the axis of the cylindrical nanodisk. This is also accompanied by a skyrmion-like out-of-plane bending of the spin vectors farther away from the disk center. Additionally, the vortex core dilates significantly when compared to its typical size. This has been directly observed using magnetic force microscopy. We determined from the ferromagnetic resonance spectroscopy measurements that the unconventional topology in the thicker nanodisks gyrated at a frequency, which is significantly lower than what is predicted by a magnetic vortex ...

Bias field free tunability of microwave properties based on geometrically controlled isolated permalloy nanomagnets

We have investigated the static and dynamic properties of two lithographically patterned bi-stable nanomagnets. Different ground magnetic states were realized using a simple in-plane field initialization technique. These states were directly imaged with magnetic force microscopy. Using the broadband ferromagnetic spectroscopy, we show that different magnetic ground states are associated with distinct microwave absorption spectra due to the variation of the internal magnetic field leading to large shift between the absorption spectra. Our experimental observations are in good agreement with micromagnetic simulations which also indicate the possibility of sub-ns switching between magnetic states using a rectangular pulse field.