Margaret Costa

Origin of fourfold anisotropy in square lattices of circular ferromagnetic dots

We discuss the fourfold anisotropy of the in-plane ferromagnetic resonance field Hr, found in a square lattice of circular Permalloy dots when the interdot distance a becomes comparable to the dot diameter d. The minimum Hr along the lattice 11 axes and the maximum along the 10 axes differ by 50 Oe at a/d=1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a mechanism of nonuniform magnetization mr in a dot in response to dipolar forces in the patterned magnetic structure under strong enough applied field. It is well described by an iterative solution of a continuous variational procedure.

Interlayer dipolar interactions in multilayered granular films

Using the ferromagnetic resonance (FMR) technique, the interlayer coupling is studied in granular multilayered system (CoFe∕Al2O3)n with varying number of layers n=1,…,8. The main difference between the cases of n=1 and n⩾3 is a notable splitting of FMR line under external field normal to layers. This is explained by an interlayer dipolar coupling, only possible for discontinuous layers. The relative magnitudes of two absorption peaks and distance between them are described by a simple model of planar square lattices of magnetic dipoles.

Criteria for Long-Range Magnetic Order in Planar Lattices of Dipolar Coupled Magnetic Nanoparticles

The conditions are sought for the dipolar interactions in a planar lattice of separate magnetic nanogranules to bring this system from the initially frustrated ground state to a long-range ferromagnetic (FM) order. This is promoted by lowering the symmetry: either of the lattice (e.g., from square to rectangular (a > b)) for point-like or spherical granules, or of a finite-size granule (e.g., from spherical to rod-like or to disk-like) on square lattice. The FM order turns possible after some critical values of asymmetry parameters, though the system energy may behave non-analytically in these parameters. The comparison is done with the available experimental data.

Collective dynamics and ferromagnetic order in random planar arrays of magnetic granules

A dynamical study is done on existence and stability of ferromagnetically ordered ground state in a positionally disordered planar array of magnetic moments coupled only by dipolar forces. Starting from almost aligned ground state under a strong enough applied field, the excitation energy spectrum and related eigenmodes are found, permitting to develop a mean-field analysis of the static magnetization in function of magnetic field and temperature. In the limit of zero applied field, the stability conditions are obtained for the onset of in-plane spontaneous magnetization, manifesting a specific “order from disorder” mechanism.

Bi-directional extended range parallel plate electrostatic actuator based on feedback linearization

In this paper, we present a bi-directional extended range parallel-plate electrostatic actuator using feedback linearization control. The actuator can have stable displacements up to 90% of the full-gap (limited by mechanical stoppers) on both directions, i.e., the device can move ±2μm within a ±2.25μm gap. The system has successfully tracked references until 1kHz (limited by the dynamics of the device) and it presents a capacitor tuning range of 17, using an actuation voltage from 0 to 10V. The results presented here are a clear advance in respect to the current state-of-the-art in terms of tracking capabilities, total stable displacement and tuning range.

Real-Time Operation and Characterization of a High-Performance Time-Based Accelerometer

An accelerometer based on the electrostatic pull-in time of a microstructure is presented in this paper. The device uses a parallel-plate overdamped microstructure, and real-time control operation is performed using a field programmable gate array and high precision digital-to-analog converters. Both open-loop and closed-loop measurements are presented. The low noise is a key feature of this approach, which is limited only by the mechanical-thermal noise of the microstructure used, 2 μg/√Hz as shown in the open-loop results (3 μg/√Hz in closed-loop operation). The time readout method has extremely high-resolution capabilities. The pull-in time sensitivity can be adjusted up to 1.6 μs/μg, and the electrostatic feedback voltage sensitivity is 61.3 V2/g. The closed-loop control allows operation of the accelerometer in a much larger range than in open-loop (±500 mg have been achieved) and the linearity is greatly improved (<;1%FS). The current closed-loop control algorithm allows operation up to 2 Hz. A bias stability of 50 μg has been measured over 45 h in open loop and 250 μg in closed loop.

3D Magnetic Field Reconstruction Methodology Based on a Scanning Magnetoresistive Probe

The present work provides a detailed description on quantitative 3D magnetic field reconstruction using a scanning magnetoresistance microscopy setup incorporating a 19.5 μm × 2.5 μm magnetoresistive sensor. Therefore, making use of a rotation stage, 11 nm thick ferromagnetic CoFe elements with 20 μm × 5 μm planar size were measured along different sensor axes and converted into cartesian coordinate magnetic field components by use of the analytical coordinate transform equations. The reconstruction steps were followed and validated by numerical simulations based on a field averaging model caused by a non-negligible sensor volume. Detailed in-plane magnetic component reconstruction with ability to reconstruct sub-micrometer features is achieved. A discussion on the limiting factors for optimal resolution is presented.

Towards high-resolution scanning magnetoresistance microscopy

In this report, we present an atomic force microscopy cantilever with µm-sized magnetoresistive sensors integrated at its tip for scanning magnetoresistance applications. Simultaneous topographic and magnetic maps are generated with a spatial resolution better than 1 µm by means of a two-dimensional scan over an area of 100×100 µm2. Experimental results validate that our scanning technique is capable of quantifying accurately the absolute magnitude of magnetic stray fields with a field sensitivity of 85.60 µV/Oe. An extension to this work is reported in which sub-μm magnetic sensors are integrated at the tip of the cantilever.

Novel magnetic readout for hybrid spintronic MEMS devices

This digest reports for the first time on the concept, fabrication and characterization of a spintronics magnetic readout for hybrid MEMS. The method uses magnetic sensors (spin valves) and permanent magnets (on both the movable and the fixed parts of the MEMS structure) to detect displacement. The use of such transduction method has several potential advantages: it allows a small form factor, straightforward readout electronics, xyz integrability, and does not require high aspect ratio silicon gaps, allowing the replacement of conventional capacitive transductors by a mechanism that occupies much less chip area. The proof-of-concept is presented for an in-plane movable structure. Both the full-gap linear displacement and the electrostatic pull-in behavior are monitored and displayed.

Application of SU-8 photoresist as a multi-functional structural dielectric layer in FOWLP

Microelectromechanical Systems (MEMS) are a fast growing technology for sensor and actuator miniaturization finding more and more commercial opportunities by having an important role in the field of Internet of Things (IoT). On the same note, Fan-out Wafer Level Packaging (FOWLP), namely WLFO technology of NANIUM, which is based on Infineon/ Intel eWLB technology, is also finding further applications, not only due to its high performance, low cost, high flexibility, but also due to its versatility to allow the integration of different types of components in the same small form-factor package. Despite its great potential it is still off limits to the more sensitive components as micro-mechanical devices and some type of sensors, which are vulnerable to temperature and pressure. In the interest of increasing FOWLP versatility and enabling the integration of MEMS, new methods of assembling and processing are continuously searched for. Dielectrics currently used for redistribution layer construction need to b...