Amal El-Ghazaly

Integrated Transformers With Sputtered Laminated Magnetic Core

Integrated interleaved solenoid transformers with a single layer of laminated magnetic core have been designed and fabricated. A coupling coefficient of > 0.97 was achieved for a device with a primary and secondary inductance of 565 nH, which represents an enhancement of more than 60 times that of air core inductors of identical geometry. By using the laminated magnetic core structure, the quality factor of the primary or secondary inductance was increased to a peak value of 6.3 with 3 μm thick electroplated copper coils. Measured characteristics of the integrated transformers agree very well with simulation results for values of self-resonant frequency, inductance, quality factor, and coupling coefficient.

Kerr-Imaged Edge-Curling Wall Effects of Narrow Magnetic Cores

Magnetic cores produced by laminating alternating magnetic/nonmagnetic layers have been shown to eliminate closure domains by linking flux vertically between magnetic layers at the edge of the film. Closure domains, which do not contribute significantly to the permeability, are undesired in patterned magnetic cores for transformers since they reduce the inductance. The inductance of narrow magnetic core transformers was found to drop with increasing frequency. Kerr images of these cores reveal partial closure domains at the edges, which explain the inductance behavior. Edge domain effects are explained based upon energy models of laminated Permalloy strips.

Influence of Nonuniform Micron-Scale Strain Distributions on the Electrical Reorientation of Magnetic Microstructures in a Composite Multiferroic Heterostructure

Composite multiferroic systems, consisting of a piezoelectric substrate coupled with a ferromagnetic thin film, are of great interest from a technological point of view because they offer a path toward the development of ultralow power magnetoelectric devices. The key aspect of those systems is the possibility to control magnetization via an electric field, relying on the magneto-elastic coupling at the interface between the piezoelectric and the ferromagnetic components. Accordingly, a direct measurement of both the electrically induced magnetic behavior and of the piezo-strain driving such behavior is crucial for better understanding and further developing these materials systems. In this work, we measure and characterize the micron-scale strain and magnetic response, as a function of an applied electric field, in a composite multiferroic system composed of 1 and 2 μm squares of Ni fabricated on a prepoled [Pb(Mg1/3Nb2/3)O3]0.69-[PbTiO3]0.31 (PMN-PT) single crystal substrate by X-ray microdiffraction and X-ray photoemission electron microscopy, respectively. These two complementary measurements of the same area on the sample indicate the presence of a nonuniform strain which strongly influences the reorientation of the magnetic state within identical Ni microstructures along the surface of the sample. Micromagnetic simulations confirm these experimental observations. This study emphasizes the critical importance of surface and interface engineering on the micron-scale in composite multiferroic structures and introduces a robust method to characterize future devices on these length scales.

Domain structure of micro-patterned CoZrTa with 45 degree induced anisotropy for isotropic high permeability

Magnetic materials have been utilized to increase the inductance of integrated inductors by taking advantage of their high-permeability [1]-[3]. It has been shown by simulation of the magnetic flux in a closed-core inductor that magnetic films with isotropic permeability are promising. Utilizing a core with uniaxial magnetic hard axis inevitably produces an effective air-gap, which inefficiently closes the flux loop [4]. In order to obtain isotropic permeability in both longitudinal and transverse directions of a magnetic film, a CoZrTa film was deposited with induced easy axis at a 45° angle from the conventional pattern axis. This paper studies the magnetic properties of this CoZrTa film and the domain structures of both patterned and unpatterned samples to obtain a physical understanding of the magnetization process which determines the permeability frequency spectrum.

Increasing ferromagnetic resonance frequency using lamination and shape

The magnetic permeability frequency spectrum is one of the most critical properties for the operation of high frequency magnetic devices in the gigahertz regime. Permeability is fairly constant up to the ferromagnetic resonance (FMR) frequency, at which point the relative permeability drops to unity. Extending FMR to higher frequencies is thus imperative for developing GHz-range magnetic devices. The simulation and experimental investigations presented in this paper demonstrate how stacking layers to form a laminated film increases the FMR frequency by allowing flux closure between layers along the induced easy-axis direction. This flux closure reduces the demagnetization factor along the easy-axis direction by two orders of magnitude. This effect, however, is only observable in patterned films where the shape anisotropy is enough to result in variation of the FMR frequency. Experiments using patterned magnetic cores were performed to illustrate this effect. Through detailed investigation of the permeability spectra of both single layer and laminated CoTaZr magnetic films patterned into 500 μm × L films (where L ranged from 200 μm to 1000 μm), the FMR frequency was extracted and proven to increase as a result of lamination. The degree to which the frequency is boosted by lamination increases exponentially as the length of the film is decreased. Through a combination of lamination and shape demagnetization, the effective anisotropy, which directly relates to FMR frequency, was shown to increase by about 100%.

Achieving Isotropic Permeability for Integrated Inductors

Partially perpendicular anisotropy in Permalloy thin films was found to produce reliably isotropic permeability in the plane of the film. Isotropic permeability is especially desirable for integrated closed-loop magnetic solenoid inductors in order to create an efficient return path for the flux. Material properties for the Permalloy were verified through hysteresis loop, permeability, and magneto-optic Kerr effect microscopy and utilized to simulate the behavior of the inductor. Laminated Permalloy films with partially perpendicular anisotropy yielded a relatively high permeability value of 860 isotropically in-plane. Simulation results indicate that, depending on the permeability, in-plane closure can yield 2× the inductance or more compared with a rectangular magnetic core with the same permeability. The enhancement due to closure was seen to depend heavily on permeability, increasing at a rate of 6.8 per unit permeability.

Effect of Mg Oxidation Degree on Rashba-Effect-Induced Torques in Ta/CoFeB/Mg(MgO) Multilayer

Control of spin-orbit torques (SOTs) in heavy metal/ferromagnetic metal/oxide structures is critical to the realization of promising SOT magnetic random access memory devices. In this paper, the SOT-induced effective fields in Ta/CoFeB/Mg(MgO) structures with perpendicular magnetic anisotropy (PMA) were investigated through the low-current-induced lock-in technique. This paper demonstrates that the industrially preferred MgO preparation method allows us to readily manipulate the oxidation degree of the Mg(MgO) and produce a significant effect on the SOT as well as PMA. Importantly, the ratio of the field-like torque to the damping-like torque efficiency can be controlled without thinning or oxidizing the bottom Ta layer, which is the source of spinHall-effect-induced damping-like torque. The achieved ratio of ~2.5 satisfies the condition that is necessary for the SOT-induced switching without an external magnetic field based on the recently proposed half-precessional switching mechanism.

Material optimization with perpendicular anisotropy for closed-loop magnetic inductors

Isotropic permeability was produced using partially perpendicular anisotropy in Permalloy. Via simulation, this material demonstrated significant enhancement over its uniaxial anisotropic inductor counterpart. Simulations suggest that even higher enhancements due to magnetic closure can be achieved with increased permeability.

45° Induced Magnetic Anisotropy for Isotropic High-Frequency Permeability

High-frequency permeability spectra and magnetic domain structures of CoZrTa films with 45° induced magnetic anisotropy were investigated in order to realize integrated flux-closed magnetic inductors. Isotropic permeability was experimentally achieved for both in-plane parallel and transversal directions. The obtained relative permeability of ~380 for both the directions of the unpatterned sample is approximately half of the conventional hard-axis permeability as expected from a magnetization rotation model. Once the films are micropatterned, permeability is typically degraded by the formation of spike domains in addition to the demagnetization effect. Through the observations of the magnetic domain structures using magneto-optical Kerr effect microscopy, we found that the formation of the spike domains is suppressed for the smaller patterned structures due to the domain walls along the 45° axis. The relative permeability of ~360 is achieved in the low-frequency range for the micropatterned structure, promising the possibility of flux-closed magnetic inductors with large inductance enhancement.