The invention relates to a magnetic erasing head for use in a radio-receiver/tape recorder combination, comprising a housing having an aperture in a surface thereof, a core unit formed by at least two core parts of a sintered oxidic ferromagnetic material and having ends which enclose an erasing gap between them, and an electric winding provided on at least one of the core parts. The core unit is accommodated in the housing such that the aperture leaves the ends of the core parts free for cooperation with a magnetizable record carrier to be erased.
Magnetic erasing heads serve to magnetize the magnetic material of a record carrier at least into a state of magnetic saturation so as to cause magnetic modulations previously provided on the record carrier to disappear and then subsequently to demagnetize the material of the record carrier by an alternating magnetic field decreasing to zero. The core unit of magnetic erasing heads may be composed in various manners. It is not unusual to construct a head core unit as a magnetic circuit built up from two C-shaped core parts of sintered oxidic ferromagnetic material (so-called ferrite), in which the blunt ends of the C's face each other (see, for example, Winckel "Technik der Magnetspeicher" (1960), pp. 51-52). In the plate where a record carrier is intended to be conveyed past the unit, the magnetic circuit is interrupted by a so-called erasing gap. Usually a foil of non-magnetizable material is provided between the ends of the core parts in the plate. It is also known to press the opposite ends of the core parts, in the place which is not contacted with the record carrier, directly against each other without the interposition of a foil.
When an erasing head of the above-described type is used in a combined radio receiver-tape recorder, undesired interference effects may occur in the radio reception (in particular in the medium wave band range) of the receiver. Similar interference effects can occur when a separate tape recorder is combined with a separate radio receiver, for example, when a number of audio components are accommodated in a rack installation.
Tests have shown that these interference effects are due to the presence of microscopic roughness of the confronting ferrite surfaces in the region where the ferrite core parts are directly pressed against each other. Due to this roughness, the surfaces are not in full contact with each other throughout the whole surface area but only in a multiplicity of point contact areas. When the magnetic circuit is flux-loaded, the point contact areas will rapidly become magnetically saturated. As a result of occurring non-linear phenomena, higher harmonics of the erasing frequency are generated. The resulting radiation may interfere with the radio reception of the receiver.
It was generally assumed in the art to be possible to grind and polish surfaces of ceramic ferrite material to such a high degree of smoothness, that the magnetic resistance in a place where two polished ferrite surfaces contacted each other would be much smaller than in a place where two surfaces of a metallic magnetic material contacted each other (see, for example, Snoek: "New Developments in Ferromagnetic Materials" (1947), p. 69). Therefore, the cause of interference effects detected in radio reception of radio receiver-tape recorder combinations was not sought in radiation of higher harmonics of the erasing frequency caused by local asperities on the contact surfaces of the core parts.