In recent years, metallic soft magnetic materials, which can be used at high frequency, have been strongly required due to the development of power electronics techniques. When driving the metallic materials at a high frequency, we need to reduce the thickness or control the magnetization process (the anisotropy) to reduce eddy current loss. In the present study, we focused on an electroplating method to prepare thin ribbons and a bilayer structure to control the direction of anisotropy using the magnet elastic effect.
To construct a bilayer structure, we firstly electroplated 8 μm-thick Fe6Ni94 films on a stainless substrate and then electroplated 8 μm-thick Fe56Ni44 films. The plating bath and conditions were almost the same as ref 1. Figure 1 shows a schematic representation of the electroplating, and we prepared 140 mm-long bilayer films. After electroplating, the bilayer film was peeled off from the substrate. The ribbon (peeled film) formed into a toroidal core (D = 10 mm).
Figure 2 shows the hysteresis loop of the core, and the difference between the loops is the inside layer. The blue and red loops are for the inside layer of Fe6Ni94 and Fe56Ni44, respectively. An obvious difference was observed between the two loops. It is well-known that the magnetostriction constant of Fe-Ni alloys depends on the composition. The sing of magnetostriction constant for Ni-rich phase such as Fe6Ni94 is negative, whereas Fe-rich one such as Fe56Ni44 is positive. When the ribbon formed a toroidal shape, bending stress is induced in the ribbon. The stress of the outer side and the inner one is tension and compression, respectively. In the case of the inside layer of Fe56Ni44, the directions of the induced anisotropy are perpendicular to the ribbon axis for both phases, indicating that the easy axis is perpendicular to the ribbon axis. Consequently, the permeability and the hysteresis loss decrease. As the perpendicular anisotropy is effective in reducing the eddy current loss, we found that thin bilayer ribbons, which consist of two phases with positive and negative magnetostriction, are an attractive structure for high-frequency applications.
1T. Yanai, J. Kaji, K. Koda, K. Takashima, M. Nakano, H. Fukunaga, “Magnetic Properties of Exchange-Coupled Fe-Ni/Fe22Ni78 Double-Layered Thick Films”, IEEE Transactions on Magnetics, 54 (2018) #2002503.