The ongoing demands for miniaturization in mobile electronic devices as well as steadily increasing frequencies of electronic circuits into the GHz range have stimulated the research on high frequency magnetic components. Our approach is very attractive for the development of high-frequency (up to > 1 GHz) soft-magnetic materials which would be readily integrable in monolithic microwave integrated circuit (MMIC) devices. Eddy currents, one of the main loss mechanisms determining the cut-off frequency of inductor materials, can be effectively suppressed by embedding magnetic nanoparticles in a dielectric matrix. The ferromagnetic resonance frequency, which is the other main parameter determining the high frequency behavior of an inductor, can be maximized by the possibility to tune the magnetic properties of the material by using alloy particles with well controlled composition, size and shape. The figure below shows very promising permeability data of a composite consisting of ternary magnetic nanoparticles in a hydrophobic protecting fluoropolymer matrix. The resonance frequency is around 5 GHz and the quality factor is > 50 at 1 GHz.

Fig. 1  Real (µ’) and imaginary (µ”) part of the permeability as well as the quality factor Q of a sputtered Fe-Ni-Co/Teflon nanocomposite.

One field of application for this type of magnetic materials is as core material of integrated microinductor devices. Current devices for commercial applications are realized as planar spiral inductors without a magnetic core. Their drawbacks are their large lateral size and thus higher costs as well as the spatial propagation of stray fields leading to eddy currents in the substrate and possibly to coupling effects between neighboring devices. A toroidal inductor (figure below) is advantageous because of the minimization of stray fields due to a closed magnetic core ring. In a first series of experiments the aforementioned magnetic core material was successfully integrated into such toroidal microinductors in cooperation with the group of Prof. E. Quandt from material science in Kiel.

Fig. 2  Schematic of a toroidal microinductor (left) and cross-section through a real device (right).

Nanorod based magnetic nanocomposites

The nanocomposites with nanorods build on our finding that by co-evaporation of Fe-Ni-Co alloys together with a fluoropolymer like Teflon AF at elevated substrate temperatures, extremely thin rods with a very large aspect ratio can be obtained above a critical ratio of the metal/organic deposition rates. The growth is attributed to a self-organization process which involves the very low interaction energy between metal and polymer as well as a critical threshold in the flux ratio of the two constituents. The figure blow shows a cross-sectional transmission electron microscopy image of such a nanorod film. The formation mechanism of the nanorods was also investigated by kinetic Monte Carlo simulations in cooperation with Prof. Michael Bonitz from theoretical physics in Kiel. It turned out that the solidification of the initially very small metallic nanoparticles upon growth also plays a key role. For very small liquid particles, the surface tension always enforces a spherical shape.

Fig. 3  TEM image of ternary magnetic Fe-Ni-Co nano-rods in a Teflon AF matrix deposited by co-evaporation (left). Kinetic Monte Carlo showing the metallic filling factor vs the deposition rate ration of the components (right).

Selected publications

J.W. Abraham, N. Kongsuwan, T. Strunskus, F. Faupel, and M. Bonitz, J. Appl. Phys. 117, 014305 (2015).

L. Rosenthal, H. Greve, V. Zaporojtchenko, T. Strunskus, F. Faupel, and M. Bonitz, J. Appl. Phys. 114, 044305 (2013).

H. Greve, C. Pochstein, H. Takele, V. Zaporojtchenko A. Gerber, M. Frommberger, E. Quandt, and F. Faupel, Appl. Phys. Lett. 89, 242501 (2006).

Henry Greve, Abhijit Biswas, Ulrich Schürmann, Vladimir Zaporojtchenko, and Franz Faupel, Appl. Phys. Lett., 88, 123103 (2006).

A. Biswas, Z. Marton, J. Kanzow, J. Kruse, V. Zaporojtchenko, F. Faupel, and T. Strunskus, Nano Letters, 3, 1 (2003).