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Ultra-thin soft magnetic amorphous ribbons of Co-Fe-B-Si-Nb alloy were synthesised by a single step rapid-quenching approach to acquire advantage of improved material performance and lower costs over commercial amorphous alloys. The amorphous ribbons of approximately 5.5 µm thicknesses were quenched by a single roller melt spinner in a single-step production process and characterised for their structural and magnetic properties. The disordered atomic structure of amorphous ribbons was confirmed by the X-ray diffraction. A surface morphology study revealed the continuity of ultra-thin ribbons without pores over a large scale. The amorphous alloy showed the ultra-soft magnetic properties in the as-quenched state. The observed thickness dependency of the magnetic properties was attributed to the increased surface roughness and possibly due to a lack of densely packed atomic structure resulting from the extremely high cooling rates experienced by ultra-thin ribbons. We propose that in-situ thinning process of amorphous ribbons significantly reduces the basic material cost and eliminates the need for post-processing steps; hence it provides the opportunity for mass production of high-performance soft magnetic amorphous ribbons at relatively lower costs.

The present work is focused on developing amorphisation capability criteria to predict regions with high amorphous forming ability (AFA) in the Fe-Si-B phase diagram. First, the AFA of Fe-Si-B alloy powders was evaluated by conventional empirical glass forming parameters, which eventually did not guide to the best AFA alloy. Then, AFA analysis was extended to the ternary phase diagram, calculated using CALPHAD, along with superimposed mathematical model based on topological instability factor (λ), estimated critical cooling rate (RC) and critical particle size (dC), to confine the phase diagram regions with larger AFA. The alloy with the highest AFA shows optimum atomic size mismatch when λ = 0.204. Furthermore, the optimal region in the phase diagram to design alloys with high AFA is where Fe2B is the first solid phase under equilibrium solidification. Within these two limits, the alloys with lower liquidus temperatures show the highest AFA for the gas-atomised powders.

This paper demonstrates an object orientated GPU accelerated micromagnetic simulator designed to model the material characteristics of complex magnetic alloy systems. The GPU simulator is written in C++ and uses object orientated programming to create a class that stores the magnetic information required to solve the Landau-Lifshitz-Gilbert (LLG) equation for a distribution of magnetic objects. Each magnetic object can be assigned individual properties based on the alloy distribution of interest. The simulator is verified against standard problem 4 of the “Micromagnetic Modelling Activity Group” (μmag). Standard problem 4 is chosen as it provides a benchmark for the modelling tools ability to accurately predict the magnetisation dynamics of a 500 nm × 125 nm × 3 nm Permalloy thin film.

A thermodynamics-based approach, Calculation of Phase Diagram (CALPHAD), combined with topological instability parameters are proposed and experimentally evaluated, in order to optimise in-situ nanocrystallisation of rapidly quenched CoFeB alloys and exploit their remarkable Bs = 1.57 T. The high Ms of the alloy is attributed to the precipitation of the metastable Co7Fe3 nanocrystalline phase dispersed heterogeneously in the amorphous matrix. High Ms of Co7Fe3 phase can also be inferred from the high hyperfine magnetic field of the Fe nuclei deduced from Mossbauer spectra. It is worth noting that the in-situ nanocrystallisation is a volume phenomenon, instead of surface crystallisation at the air-side of ribbons owning to lower cooling rates. We judge, based on nucleation theory, that the formation of the metastable phase is kinetically favoured, when compared to the equilibrium phases, hence promoting the high Ms, when compared with conventional Co-rich amorphous alloys. The local atomic order of nanocrystallised phase was confirmed by X-ray and electron diffraction techniques. Using Mossbauer spectroscopy and the extracted distribution of the hyperfine magnetic field, it is asserted that cobalt atoms form clusters, as they attract each other to form ordered structures, and boron atoms undergo only short-range ordering, likely due to covalent bond formation, governed by the size and electronegativity differences with the atoms in the amorphous matrix. We suggest the proposed CALPHAD-assisted design of nanostructured alloys, along with an in-situ nanocrystallisation, provides a practical scheme to develop novel functional alloys with the best possible balance of coercivity and saturation, exclusively aimed for a high-tech application.

Soft magnetic amorphous materials with ultra-low power loss are highly desirable for high-frequency drive applications. The present work demonstrates the high-frequency power loss performance and underlying loss mechanisms in ultra-thin amorphous alloys. This is achieved by rapid-quenching amorphous alloys of Co-, CoFe- and Fe-rich systems, investigating their amorphous atomic structure, quantifying the saturation magnetostriction constants (λs), imaging magnetic domains at remanent magnetization, analyzing magnetization reversal from various magnetization levels, and finally, investigating the material loss performance over a broad frequency range (f = 50 kHz–2 MHz) at various excitation levels (Bm = 25–100 mT). The ultra-high performance of ultra-thin Co-rich amorphous ribbons, as compared to CoFe- and Fe-rich alloys, was attributed to the significantly low eddy current loss, due to the reduced thickness, and a minimal amount of excess loss, owning to minimal magnetoelastic contributions and magnetization reversal by rotation. The underlying loss mechanisms were analyzed by decomposing material loss into primary components and identifying the magnetization reversal mechanisms using minor hysteresis loops. In the Co-rich amorphous alloys, we suggest that magnetization reversal by rotation dominates, at least at low excitations, while in CoFe- and Fe-rich alloys domain wall displacement prevails and contributes significantly to the excess loss up to the MHz frequency range. Magnetization reversal by rotation in Co-rich alloys could be attributed to the zero/near-zero λs, and eventually low residual stress, leading to a homogeneous magnetic domain structure, as compared to the inhomogeneous “fingerprint-like” complex domains in highly magnetostrictive CoFe-rich alloys.

The effect of surface roughness in thin film magnetics on magnetic susceptibility and Dynamic Magnetic Hysteresis (DMH) loops are investigated using Brown’s continuous diffusion of magnetic spins model. The magnetic relaxation time of the magnetic thin films is determined by regressing the two-mode approximation for the magnetic susceptibility onto experimental data obtained for Ni45Fe55. There is a moderate linear correlation between the magnetic relaxation time and the surface roughness to the thickness (SRT) ratio. The area of the DMH loops and the non-linear dynamic magnetic susceptibility strongly depend on the magnetic relaxation time and the SRT of the films. The thin films exhibit a strong linear relationship between the coercivity and the SRT ratio, along with a strong logarithmic dependence of the dynamic magnetic susceptibility on the SRT ratio.

The present work demonstrates the high-frequency core loss performance of Fe-based amorphous and nanocrystalline powder cores, initially produced by gas atomised powder, consolidated using sieved particles ≤20 µm, and isolated by a precise insulating layer of polymer to limit the inter- and intra-particle eddy currents to attain enhanced performance. The large glass forming ability (GFA) of the gas atomised powder, reflected by different glass forming instruments, such as the supercooled region (ΔTX = 54 °C) and the reduced glass transition temperature (Trg = 0.56), is consistent with the substantial amorphisation capability of the alloy. To the best of our knowledge, this is the first-ever report to reveal a large ΔTX in the Finemet-type alloy powders, an essential parameter to gas-atomise the amorphous powders with significantly lower cooling rates compared to the melt-spun ribbons. Further, subsequent annealing of the amorphous powders, between the exothermic events guided by differential scanning calorimetry (DSC), lead to the growth of a fine nanocrystalline structure of grains ≤15 nm, thanks to the positive enthalpy of mixing of Cu with the constituents to act as a nucleation agent, to retain the excellent soft magnetic properties. The DC soft magnetic properties of the powders were significantly improved on thermal annealing, confirmed by hysteretic loops, quantified by reduced coercivity HC

The mechanisms that limit the power loss performance in melt-spun amorphous ribbons have been investigated through DC and AC magnetic characterization methods. The measured total power loss is resolved into hysteresis, eddy current, and anomalous losses. The anomalous loss is found to account for more than 90% of the total loss, which significantly reduced by annealing in a transverse magnetic field. This is attributed to the reorientation of preferred magnetisation axis perpendicular to the length of ribbons. Transverse magnetic annealing promotes the relative contribution of domain rotation over domain wall motion during magnetisation reversal process. Magnetic annealing also causes a measurable decrease in the domain width, which promotes pinning and inhibits domain wall motion, thus further favoring coherent domain rotation as the primary mechanism of magnetization. This combination accounts for a 75% decrease in the total power loss in the so-processed ribbons and renders them attractive for applications in mid-and high-frequency power supplies and inverters.

An analytic model for stripline micro-inductors is presented which accurately approximates finite-element solutions while also allowing for variation of a wide range of parameters. Optimized models for magnetic enhancement layers are validated with measured results of different materials. The effects of these device and material models are applied to analyze stripline devices operating above 100 MHz. The effects of geometry on inductance, Q-factor, and current-carrying capability are presented for the materials in question.

As result of its high resistivity and good magnetic properties, electrodeposited amorphous Co–P (a-Co–P) is an attractive material for use in the integration of on-chip inductors into silicon process technology. However as an amorphous material, the properties of a-Co–P are dramatically changed upon crystallization. This paper reports a novel electrodeposited magnetic alloy, a-Co–P–Re that shows a significantly improved thermal performance in comparison to a-Co–P, retaining coercivity, Hc

A novel fluxgate sensor with a single ferromagnetic core capable of measurement in three principal axes is presented. The sensor manufacturing process and internal coil structure is explained and its excitation is modelled using FEM. The sensor was implemented utilizing multilayer printed circuit board copper routes and electroplated through holes to create appropriate coils. The sensor magnetic core was electroplated on the printed circuit board surface and through holes using Permalloy (Ni80Fe20). The core material properties were identified using a PCB based transformer device showing relative permeability of 2300 at 50 kHz. Sensor excitation effects are investigated with special emphasis on sensor axis orthogonality revealing a significant sensitivity axis tilt due to excitation means. The sensor is characterised in terms of sensitivity, linearity and noise. Measurements show a significant influence of core material anisotropy on sensitivity (90 V/T in x axis vs. 112 V/T in y axis). The z-axis sensitivity (198 V/T) is significantly higher due to the different core geometry in corresponding direction and different number of pick-up coil turns.

We have investigated CoNiFe and CoNiFe–C electrodeposited by pulse reverse plating (PRP) and direct current (DC) techniques. CoNiFe(PRP) films with composition Co 59.4 Fe 27.7 Ni 12.8 show coercivity of 95 A m −1 (1.2 Oe) and magnetization saturation flux ( μ 0 M s ) of 1.8 T. Resistivity of CoNiFe (PRP) is about 24 μΩ cm and permeability remains almost constant μ r ′ ∼475 up to 30 MHz with a quality factor ( Q ) larger than 10. Additionally, the permeability spectra analysis shows that CoNiFe exhibits a classical eddy current loss at zero bias field and ferromagnetic resonance (FMR) when biased with 0.05 T. Furthermore, a crossover between eddy current and FMR loss is observed for CoNiFe-PRP when baised with 0.05 T. DC and PRP plated CoNiFe–C, which have resistivity and permeability of 85, 38 μΩ cm, μ r ′=165 and 35 with Q >10 up to 320 MHz, respectively, showed only ferromagnetic resonance losses. The ferromagnetic resonance peaks in CoNiFe and CoNiFe–C are broad and resembles a Gaussian distribution of FMR frequencies. The incorporation of C to CoNiFe reduces eddy current loss, but also reduces the FMR frequency.

Electroplating is compatible with the deposition of relatively thick layers i.e. several μm to 10s- of μm. However eddy current losses mean that thick layers will have an inferior frequency response. We have used a low content phosphorous bath together with pulse reverse plating to generate Co-rich and Co-deficient multi-nanolayers of CoP having improved saturation magnetisation and a better frequency response. The plating parameters have been optimised in order to produce a material with low loss, and a high permeability of around 700 retained up to 103 MHz for a sample with a thickness of 1.7 μm and a resistivity of 136 μΩ cm.

This paper presents an electromagnetic based generator which is suitable for supplying generating power from human body motion and has application in providing energy for body worn sensors or electronics devices. A prototype generator has been built and tested both by a shaker at resonance condition and also by human body motion during walking and slow running. The experimental results will show that the prototype could generate 300 μW to 2.5 mW power from human body motion. The measured results are analyzed and compared with the theoretical model.

This paper discusses the technologies required to produce magnetics on silicon for power supply on chip. Prototypes of micro-inductors have been fabricated using techniques such as thick copper coil deposition and improved magnetic materials. The measured maximum Q factor and inductance value are 5 at 2.5 MHz and 38 nH/mm2, respectively. The impact of seed layers and core overlap has been identified to be significant at high frequencies. Using the validated analytical model, the electrical performance of micro-inductors in a buck converter can be predicted and efficiency of 85.9 with a power density of 15.8 W/cm2 can be achieved.

This paper describes the design and testing of a planar fluxgate current sensor, which is totally embedded in printed circuit board (PCB) and which is suitable for sensing currents in the printed circuit board traces. Fluxgate excitation and pickup coils are implemented as racetrack shaped copper winding in the printed circuit board internal layers. These are surrounded by an electroplated magnetic core consisting of a thin film of nickel iron alloy. The sensor has been tested over a 2 A current range. Sinusoidal excitation at 10 kHz, 30 kHz and 50 kHz has been used. The sensitivity has been measured at 200 mV/A for 50 kHz excitation. The power dissipation has been measured at approximately 2.6 W for a sinusoidal excitation.

The reliability of the eutectic Sn37Pb (63%Sn37%Pb) and Sn3.5Ag (96.5%Sn3.5%Ag) solder bumps with an under bump metallization (UBM) consisting of an electroless Ni(P) plus a thin layer of Au was evaluated following isothermal aging at 150 °C. All the solder bumps remained intact after 1500 h aging at 150 °C. Solder bump microstructure evolution and interface structure change during isothermal aging were observed and correlated with the solder bump shear strength and failure modes. Cohesive solder failure was the only failure mode for the eutectic Sn37Pb solder bump, while partial cohesive solder failure and partial Ni(P) UBM/Al metallization interfacial delamination was the main failure mode for eutectic Sn3.5Ag solder bump.

Power inductors have been fabricated on silicon substrates using low-temperature IC compatible processes. The electrical properties of these micro-inductors have been measured and discussed. A maximum quality factor of 6 at 4 MHz has been achieved with an inductance value of about 160 nH. The DC saturation currents of the non-gapped and gapped inductors are ∼500 and 700 mA, respectively. The relatively high Q factor and the load current characteristics allow these micro-machined inductors to be used in integrated power converters.

Thin film microtransformers have been fabricated on silicon with Ni 45 Fe 55 as a core material. Fractal/dendritic growths are observed in the patterned cores in DC electroplating due to the enhancement of localized current density at defect/nucleation sites. A ‘pulser’ device was made in house to produce forward and reverse current of the required amplitude for a particular duration. The combination of a low amplitude long (millisecond) forward pulse and a short (microsecond) high-amplitude reverse pulse gave dendrite-free plated cores with a uniform thickness and alloy composition over a 3D topology of a microrough substrate surface. Finally, we characterized the material in situ by small signal electrical measurements, and with MOKE hysteresis loops measured on a complete device.

This paper describes the design, fabrication and test of a range of third order LC low pass Butterworth filters in MCM-L technology. There are seven filters in the range, each consisting of two inductors and one capacitor with cut-off frequencies ranging from 10 to 540 MHz. Inductance values range from 9.9 to 530 nH, and capacitance values vary from 14 to 723 pF. Air-core (by which we mean no magnetic material) spiral winding structures are designed to provide the inductive functions. A new FR4 type laminate material, which was developed by ISOLA, is used to form integrated parallel plate capacitor structures. The range of filters was successfully manufactured and electrically characterised using a HP4195A impedance analyser. Some environmental testing was also carried out on the embedded filters. These included thermal shock and humidity testing. There were no obvious changes in device performance after the environmental tests.

The application of a hysteresis model to the prediction of the efficiency of a high-frequency micro-transformer, operating at 5 MHz, is described. The core of the micro-transformer consists of electroplated thin films of nickel iron alloy. Measurements were carried out on both toroidal magnetic core and micro-transformers in order to validate the model.