Bethanie J. H. Stadler

Magnetic nanowires for acoustic sensors (invited)

Tiny hairlike sensors or cilia play a very important role in detection for many biological species, including humans. This research took inspiration from the packaging and transduction processes of the inner ear’s cochlea and cilia to design acoustic sensors. Specifically, this work uses nanowires of magnetostrictive materials as artificial cilia to sense acoustic signals. Anodic aluminum oxide (AAO) templates with hexagonal spacings were fabricated using a two-step anodization process as well as nanoimprint assisted self-assembly and were characterized using atomic force microscopy. Patterned microelectrodes were also fabricated at the backside of several templates using photolithography. Ni, Co, and Galfenol (Fe1−xGax0.1⩽x⩽0.25at.%) nanowires were fabricated using electrochemical deposition into nanoporous AAO templates where the pores had various geometries and some had large-area ordering as dictated by nanoimprinting. High aspect ratio nanowires with diameters varying from 10 to 200 nm and lengths up...

Integrated Magneto-Optical Materials and Isolators: A Review

Many novel materials and device designs have been proposed as photonic analogs to electrical diodes over the last four decades. This paper seeks to revisit these materials and designs as advanced technologies may enable experimental realization that was not possible upon conception of several of these designs. The background behind integration challenges, including waveguide birefringence, fabrication tolerances, garnet/semiconductor mismatch, and optimized interfaces will hopefully spark new ideas that will finally enable the realization of integrated optical isolators and circulators.

Integrating yttrium iron garnet onto nongarnet substrates with faster deposition rates and high reliability

Magneto-optical garnets (Y3Fe5O12 or YIG) were grown monolithically by a novel reactive radio-frequency sputtering method that used a partial pressure differential to increase sputtering rates. MgO and quartz substrates were used as they are good buffer layers and optical claddings for integration. A wide single-phase field for annealed YIG was found (26.9–43.2at% Y), and the magnetic properties were measured. The films had refractive indices of 2.1 and out-of-plane Faraday rotations up to 0.2°∕μm at 633nm. The dielectric matrix was used to calculate the difference in the propagation constants of forward and backward traveling light (Δβ=1.999×10−5).

Magneto-optical properties of iron oxide films

Iron oxide films have been grown onto MgO and oxidized silicon substrates using pulsed laser deposition with a 248 nm excimer laser. The films were deposited in a vacuum from an α-Fe2O3 target. Films grown epitaxially on MgO consisted of the ferrimagnetic γ-Fe2O3 phase, with an average saturation magnetization of 353±26 kA/m. The Faraday rotation of the γ-Fe2O3 films was measured at 645 nm and 1550 nm wavelengths, and was found to be 4 and 2.5 degrees/μm, respectively. It is assumed that the high Faraday rotation, accompanied by a relatively high absorption, is associated with a transition of Fe2+ ions in octahedral sites.

Integration of magneto-optical garnet films by metal-organic chemical vapor deposition

This paper presents a novel technique for integrating yttrium iron garnets, namely Ce:YIG, onto semiconductor platforms using metal-organic chemical vapor deposition (MOCVD). Large amounts of cerium (Ce) could be incorporated into the garnet structure because of the nonequilibrium nature of the technique. The method can alloy up to 54% Ce, thereby increasing the refractive index and enhancing the Faraday rotation of the YIG films. Faraday rotations as high as 0.4/spl deg///spl mu/m at 1.3 /spl mu/m were achieved in MOCVD-grown garnets, exceeding the rotations of bismuth-doped YIG films (0.15/spl deg///spl mu/m at 1.3 /spl mu/m) grown by liquid-phase epitaxy. The easy axis of magnetization is within the plane of the films. When the garnet films were sputtered onto [100] magnesia (MgO) buffer layers, their hysteresis loops indicated that they were isotropic.

Growth Parameters of Fully Crystallized YIG, Bi:YIG, and Ce:YIG Films With High Faraday Rotations

We report on the growth of thin films of yttrium iron garnet (YIG) on dielectric substrates. Such films have historically been challenging to grow due to either cracking or incomplete crystallization of the films. We have established the proper growth parameters by tuning seed layer thickness to an optimum of 45 nm. These films were then used as seed layers for growth of films of Bi:YIG and Ce:YIG. Bi:YIG films show a Faraday rotation of 1700 °/cm, and Ce:YIG films show a Faraday rotation of 3700 °/cm.

Magneto-optical garnet films made by reactive sputtering

Reactive radio frequency (RF) sputtering was used to grow cerium-doped yttrium iron garnet (YIG) films on magnesium oxide substrates. Magnesium oxide has been proven to be a good buffer material for semiconducting substrates. Reactive sputtering was not effective for cerium doping because the cerium target reacted with the oxygen in the sputtering gas. The films were amorphous as deposited. Stoichiometric compositions yielded smooth, polycrystalline garnet films on annealing. The effects of fluctuations in the yttrium-to-iron ratio were studied. Separate iron and yttrium targets were cosputtered in order to tailor the composition systematically along the iron-yttrium binary phase diagram. Oxygen content was found to be important in the formation of garnet and in the prevention of secondary phases. The garnet films had strong in-plane magnetizations and small coercive fields, which have promise for waveguide and switch devices, respectively.

Low-Resistivity 10 nm Diameter Magnetic Sensors

Resistivities of 5.4 μΩ·cm were measured in 10-nm-diameter metallic wires. Low resistance is important for interconnections of the future to prevent heating, electromigration, high power consumption, and long RC time constants. To demonstrate application of these wires, Co/Cu/Co magnetic sensors were synthesized with 20-30 Ω and 19% magnetoresistance. Compared to conventional lithographically produced magnetic tunnel junction sensors, these structures offer facile fabrication and over 2 orders of magnitude lower resistances due to smooth sidewalls from in situ templated chemical growth.

Electrochemical deposition of Fe1−xGax nanowire arrays

Nanowires of the Fe–Ga alloy system, Fe1−xGax (0.1⩽x⩽0.25at.%), were fabricated using electrochemical deposition into nanoporous anodic alumina. Fe–Ga alloys are magnetically soft, mechanically robust, ductile, and have large magnetostriction. High aspect ratio nanowires with varying diameters and lengths were fabricated in arrays and were collectively and individually characterized using scanning electron microscopy and energy dispersive spectroscopy. The magnetic response of the array was also measured using vibrating-sample magnetometry. The Ga composition was controlled by optimizing deposition parameters such as ion concentration, buffer acids, agitation, and applied voltage. Through the analysis of pure Fe and pure Ga depositions, it was predicted that the Ga3+ ions would be depleted more quickly from the electrolyte than the Fe2+ ions. This hypothesis was supported by compositional analysis along alloy wires showing that the Ga composition decreased along the length of the wires.

Fabrication of BioInspired Inorganic Nanocilia Sensors

In nature, microscale hair-like projections called cilia are used ubiquitously for both sensing and motility. In this paper, biomimetic nanoscale cilia arrays have been fabricated through templated growth of Co in anodized aluminum oxide. The motion of arrays of Co cilia was then detected using magnetic sensors. These signals were used to prove the feasibility of two types of sensors: flow sensors and vibration sensors. The flow sensors were tested in a microfluidic channel. They showed the ability to detect flows from 0.5 ml/min to 6 ml/min with a signal to noise (SNR) of 44 using only 140 μW of power and no amplification. The vibration sensors were tested using a shake table in the low earthquake-like frequency range of 1-5 Hz. The vibration response was a mW signal at twice the frequency of the shake table.