Fernando M.F. Rhen

Microfabricated inductors for 20 MHz Dc-Dc converters

This paper presents the design and measured results for micro-fabricated inductors suitable for use in high frequency (> 10 MHz), low power (1 -2 W) dc-dc converters. The design has focused on maximizing inductor efficiency for a given converter specification. Inductors in the range of 100 nH to 300 nH have been fabricated and tested. The small signal measurements show a relatively flat inductance profile, with a 10% drop in inductance at 30 MHz. Inductance vs. dc bias current measurements show less than 15% decrease in inductance at 500 mA current. The performance of the micro-inductors have also been compared to a conventional wire-wound inductor in a 20 MHz dc-dc converter. The converter efficiency is shown to be approximately 4% lower when the micro-inductor is used compared to the when the wire- wound inductor is used. The peak efficiency of the micro-inductor in the converter is estimated to be approximately 93%.

Thick-film permanent magnets by membrane electrodeposition

Thick films of nearly equiatomic Co–Pt produced by electrodeposition into nanoporous membranes exhibit an isotropic magnetic phase with square hysteresis loops (Mr∕Ms=0.95) and coercivity of up to 1.3 T. The initial magnetization curve is characteristic of pinning-type magnets. The coercivity can be controlled by the choice of annealing temperature or alloy composition, which depends on the chemical composition of the plating bath, properties which can be exploited to produce fine-pitch pole patterns.

Electrodeposited FePt films

In this paper we reported the preparation of continuous FePt films by electrodeposition from a different bath and present their structural and magnetic characterization. SEM was studied for as deposited and annealed films.

Thin-Film-Integrated Power Inductor on Si and Its Performance in an 8-MHz Buck Converter

This paper presents a microinductor fabricated on silicon using electrochemical techniques that has high efficiency in a low power dc-dc converter. Small signal measurements show a flat frequency response up to 20 MHz with a self resonant frequency of 130 MHz. The inductance at low frequency is approximately 440 nH with a dc resistance of 0.5 Omega , and a high quality factor of 11.7 at 5.5 MHz. The current handling capability test shows less than 10% decrease in inductance at 500-mA current. The performance of the microinductor has been compared to a conventional chip inductor in a commercially available 8-MHz buck converter. The converter maximum efficiency when using the microinductor is shown to be approximately 3% lower than the one using the conventional discrete chip inductor. However, the profile of the microinductor is much lower than that of the discrete chip inductor. The maximum efficiency of the microinductor in the converter is estimated to be approximately 92%.

Dependence of magnetic properties on micro- to nanostructure of CoNiFe films

The magnetic properties of electrodeposited CoNiFe films with thicknesses varying from 0.20 to 10 μm were studied. The films show a single face-centered-cubic CoNiFe phase with grain sizes ranging from 23 to 29 nm. The coercivity is controlled by a combination of the morphology and nanocrystalline structure of the deposits. The nanocrystalline grain size determines the intrinsic coercivity associated with crystalline anisotropy as in the random anisotropy model, whereas an additional morphology term of coercivity is controlled by the thickness inhomogeneity on a submicron scale. The thin films show considerable roughness and a higher coercivity, up to a level of 560 A m−1 (7.0 Oe) in 250 nm films. The thick films show coercivity values of as low as 16 A m−1 (0.2 Oe). The coercivity dependence on thickness was fitted using a simple model combining a morphology dependent additional contribution term to the random anisotropy model as Hc=Hcmorph+Hcanis. Good agreement between the model and the experimental re...

Magnetic properties of nickel nanowires: Effect of deposition temperature

We have investigated the magnetic properties of electroplated nickel nanowires with very distinct nanostructures, which are obtained by simply changing the plating temperature of the electrolyte. Low temperature (40 °C) resulted in larger average grain size comparable to the diameter of the wires, whereas higher temperature (60 °C) revealed self-similar morphology composed of nanogranules. For low temperature samples, a two stage magnetization process is evident in the easy axis direction where grain size is comparable to wire diameter. In contrast, for high temperature samples, nanowires are formed by an agglomeration of particles with average diameter of about 22 nm. In this case each individual particle behaves as a single domain and thereby magnetization reversal occurs by the switching of an ensemble of randomly oriented particles and magnetization saturates quickly with applied field. In the present case, with the high density of disorder caused by the self-similar morphology of the nanogranules, we...

Influence of a Magnetic Field on the Electrochemical Rest Potential

The influence of an external magnetic field on the rest potential Eg of ferromagnetic and nonferromagnetic electrodes is studied. The shift ΔE 0 is measured as a function of magnetic field magnitude and direction, pH, electrolyte composition, solution agitation, and electrode roughness. Anodic shifts can be observed not only for ferromagnetic electrodes (iron, cobalt, nickel), but also for nonmagnetic electrodes (zinc, manganese). The essential condition to observe the shift is that the electrode should be actively corroding. An anodic shift is observed when the cathodic corrosion current is mass-transport limited. The primary mechanism for the effect is agitation of the electrolyte near the electrode surface due to Lorentz force acting on the corrosion currents directly, or via the electrokinetic effect. A smaller influence of magnetic field gradient produced by ferromagnetic electrodes is identified.

Magnetic properties and underlayer thickness in SmCo/Cr films

SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for high density recording media. Magnetron sputtering was used here to produce Cr/SmCox/Cr films on Si (100) substrates. The magnetic films were deposited at a substrate temperature of 350 °C and an Ar pressure of 5 mTorr. Cr underlayers were deposited both at 25 °C and at 350 °C and exhibited different textures. Layer thicknesses were evaluated using Rutherford backscattering spectroscopy, while SmCo grain size and underlayer texture were determined from a Rietveld analysis of x-ray diffraction data. The magnetic properties were measured with a vibrating sample magnetometer. The resulting films had in-plane coercivities in the range 4–8 kOe. For a SmCo layer with a thickness of 200 nm, the Cr underlayer thickness was varied in the range 75–300 nm to study its effect on intergranular coupling in the films. For thermally demagnetized samples, both magnetizing and demagnetizing remanence curves were measured and use...

Core Materials for High Frequency VRM Inductors

The application of PCB integrated electroplated cores for VRM inductors is proposed, where the motivation is to increase frequencies beyond 1 MHz so that VRM transient response can be improved. It is shown that electroplated alloys have loss properties that are at least competitive with those of the highest frequency ferrite material available. Various PCB integrated inductor designs are presented, with toroidal cores providing the smallest solution. Measured losses under non-sinusoidal operating conditions are provided and work is ongoing to characterise the materials further.

Effect of ZrB2 addition on SmCo-1:7 high temperature magnets

This work focuses on a search for magnets that can be used at elevated temperature. We have studied the effect of ZrB2 addition on the magnetic properties and stability of nanocrystalline 1:7 type Sm(Co,Ti,Zr) magnets up to 500 °C. Amorphous ball milled powder was annealed to develop the TbCu7 type structure. The room temperature coercivity of SmCo6.5(ZrB2)0.033Ti0.4 is 2.1 T. The microstructure is stable at 400 °C, where the coercivity is 0.5 T.