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=Hc[morph]+Hc[anis]. Good agreement between the model and the experimental results was obtained

An ac susceptibility study in capped Ni/Ni(OH)(2) core-shell nanoassemblies: dual peak observations

In this study, the ac susceptibility (χ' and χ'') variation with temperature (10–100 K) for oleic acid (OA) capped Ni/Ni(OH)2 core–shell nanoparticle assemblies are reported at frequencies varying from 0.1 to 1000 Hz. Nanoparticle assemblies, with two average particle diameters of ~34 nm and ~14 nm, were synthesized using a wet chemical synthesis approach. Two peaks in the ac susceptibility versus temperature curves are clearly discernable for each of the samples. The first, occurring at ~22 K was attributed to the paramagnetic/antiferromagnetic transition of the Ni(OH)2 present in the shell. The second higher temperature peak was attributed to the superparamagnetic blocking of the pure Ni situated at the core of the nanoparticles. The higher temperature peaks in both the χ' and χ'' curves were observed to increase with increasing frequency. Thus the Néel and the blocking temperatures for such core–shell nanoassemblies were clearly identified from the ac analysis, whereas they were not discernible (superimposed) even from very low dc (FC/ZFC) field measurements. Interparticle interactions within the assemblies were studied through the fitting of phenomenological laws to the experimental datasets. It is observed that even with an OA capping layer, larger Ni/Ni(OH)2 nanoparticles experience a greater degree of sub-capping layer oxidation thus producing lower magnetic interaction strengths

High current inductor design for MHz switching

High current inductor applications operating in the MHz range are generally limited to Voltage Regulator Modules (VRM’s) and Point of Load (POL) power supplies, where the issue of fast transient response demands reduced inductance values. State-of-the-art controller IC’s enable switching frequencies up to 8 MHz for low current POL applications. However, for current levels higher than 1 A, commercial inductors are not generally quoted as suitable for operation beyond 5 MHz. The application of electroplated metal alloy foils to produce competitive discrete cores suitable for operation in the range of 1 – 5 MHz is investigated in this paper

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 nonsinusoidal operating conditions are provided and work is ongoing to characterise the materials further

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 wirewound inductor is used. The peak efficiency of the micro-inductor in the converter is estimated to be approximately 93%