Thin Soft Magnetic Tapes in Small High-Frequency Fields

Abstract

The complex permeability of iron-nickel and Permalloy alloys in tape thicknesses down to 2 μ has been measured at frequencies up to 1000 Mc. Toroidal cores with different heat treatments and in the cold-rolled state were used. Curves of complex permeability (Cole-Cole diagrams) differ more and more from the classical eddy-current curve with decreasing thickness. It is concluded that spin relaxations and resonances are effective as known from ferrites. The effects are clearer in alloys with low rather than high initial permeability; and are strongly marked in thin cold-rolled tapes and magnetic films. Curves of complex permeability are analyzed by means of the equations for domain-wall motions and domain rotations, and the significant parameters are discussed. These measurements brought new knowledge about the limiting frequency and the loss factor of thin tapes. The limiting frequency tends clearly towards an upper limiting value with decreasing thickness. This is in contrast to the classical eddy-current theory. The upper limiting frequency is identical with the ``gyromagnetic'' limiting frequency as calculated by Snoek, which is accordingly not valid for ferrites only, but for metals as well. The frequency-dependent part of the loss factor tgδ has a lower limiting value. This loss minimum and the upper limiting frequency are really reached at annealed tapes of some microns, and with dust cores of fine powders. Further improvements are possible by using special anisotropies. An example is the magnetic films which have a direction of hard magnetization and a plane of easy magnetization. Cold-rolled and half-hard thin tapes with Isoperm-like hysteresis loops show effects in similar direction. The results show that a tape thickness of 3–6 μ is normally sufficient to applicate metals in the Mc range. Further decreasing of the tape thickness seems successful only if reduced to the order of 1000 Å and also using the strong shape anisotropy used with the magnetic films.