Kevin P. O'brien

Investigation of extrinsic damping caused by magnetic dead layer in Ta-CoFeB-MgO multilayers with perpendicular anisotropy

We systematically investigated the influence of the dead layer, the oxidation degree of naturally oxidized MgO, the structure of adjacent nonmagnetic metal layers on the damping parameter, and the perpendicular anisotropy of Ta(Ru)/Co20Fe60B20/MgO and MgO/Co20Fe60B20/Ta films using the vector network analyzer ferromagnetic resonance measurement technique. MgO/Co20Fe60B20/Ta film shows almost twice larger extrinsic damping than that for Ta/Co20Fe60B20/MgO film, whereas the perpendicular anisotropy is much smaller. This two-fold enhancement of damping parameter is successfully explained by the extrinsic damping arises from the dead layer at the CoFeB-Ta interface through the Elliott-Yafet mechanism in addition to the conventional spin-pumping model. Furthermore, we found that the oxidation degree of naturally oxidized MgO has no significant impact on the damping parameter, while the perpendicular anisotropy for MgO/Co20Fe60B20/Ta film is enhanced by longer oxidation time.

Comparative Investigation of the Effect of Oxygen Adsorbate and Electrode Work Function on Carbon-Nanotube Field-Effect Transistors

The effect of electrode work function (WF) on the device resistance of carbon-nanotube field-effect transistors (CNT-FETs) is less significant than that of device surface chemistry, specifically interactions with molecular oxygen adsorbate. Experimental results are based on over 130 individual devices with electrode WFs varying from 4.17 to 5.21 eV that were tested under standard-atmosphere, ultrahigh-vacuum, and pure-oxygen environments. Oxygen decreased device resistance by an average of 56% and altered majority charge carriers regardless of electrode metal. Variations in CNT-FET performance based on electrode material appeared more closely associated with crystallization and oxidation states than WF.

High frequency microwave on-chip inductors using increased ferromagnetic resonance frequency of magnetic films

The fabrication and characterization of high frequency on-chip inductors using sputtered magnetic films with an improved frequency range is presented. Reducing the sputtering power in the deposition process was found to result in smoother film surfaces and stronger uniaxial magnetic anisotropy and increased the FMR of CoZrTaB from 1.48 GHz to 2.13 GHz. A magnetic-core, on-chip inductor was fabricated using the CoZrTaB films. Results have shown 150% higher inductance and a larger Q-factor up to 1.2 GHz as compared to an air-core inductor.

Circuit modeling of nonlinear lossy frequency dependent thin-film magnetic inductors

Thin-film magnetic inductors are useful for integrated power delivery solutions for microprocessors and mobile devices. In many of these applications, the inductor dimensions have to be optimized simultaneously with the other circuit components, and thus accurate circuit models are required. Hence, we present an accurate physics-based circuit model that can capture the frequency dependency, losses, and the nonlinear behavior of thin-film magnetic inductors both for the uncoupled and coupled configurations. The model is based on the magnetic circuit description of the inductors with additional elements to model the frequency dependency within the magnetic material. The model is verified using vector network analyzer measurements of direct current biased inductors.

Magnetically aligned 2D rectangular micro magnets with uniaxial in-plane anisotropy for high frequency magnetic devices

The design and demonstration of magnetically aligned 2D rectangular micro magnets have been developed for high frequency magnetic devices. It is known that 2D magnetic structures and the induced anisotropy can provide unique properties to achieve high magnetic resonance frequency. A methodology to fabricate the 2D micro magnets using the microfabrication processes with magnetic field alignment has been proposed and successfully demonstrated. In-plane anisotropy from the drop casted magnet arrays has been confirmed. Compared to a blanket NiFe thin film, the 2D magnet arrays is found to increase the anisotropy field 30 times from 10 to 300 Oe, or 5.5 times higher magnetic resonance frequency from 0.45 to 2.5 GHz.

Spatial Identification of Defect Sites in Individual Carbon Nanotubes Using Micro-Raman Spectroscopy

The role of defects in the structural and electrical properties of carbon nanotubes (CNTs) is becoming increasingly important for a number of applications. These defects have been traced to changes in transport behavior (e.g. from metallic to semiconducting) in transistors [1], compressive buckling in thin films [2], and electrochemical sensitivity in sensors [3]. Raman spectroscopy has been successfully used for determining CNT defect density and chirality of bulk samples but it has only recently been applied to individual tubes [4]. The advantages of this technique include its speed, non-destructive nature and ability to characterize disorder in sp 2 carbon materials like CNTs. The disorder-induced D-bands are the principle Raman signal used to determine the presence of defects in nanotubes [5]. However, use of these peaks to map the locations of defects in CNTs is complicated by very weak D-band intensities in single, isolated tubes.