MAGNETIC CORE

Abstract: Suitable for use in a magnetic head or in a transformer, the magnetic core is composed of lamellar core parts (4,4',4") of amorphous, ferromagnetic material coated with a thin film of a material chosen from the group comprising SiO₂, Al₂O₃, TiO₂ and Si₃N₄. The core parts are bonded together by means of an epoxy adhesive.

Description: The invention relates to a magnetic core, comprising a first lamellar core part of amorphous, ferromagnetic material which is bonded by means of an epoxy adhesive to a second lamellar core part or to a substrate. Magnetic cores having core parts of amorphous, ferromagnetic material are used, inter alia , in magnetic heads (paricularly for video applications) and in transformers. When assembling the core parts into one core, one of the difficulties is the realization of a satisfactory bond. Glass is a frequently used bonding material in conventional magnetic heads. However, glass is not suitable in this case, because its high processing temperature (higher than 300°C) degrades the magnetic properties of the amorphous, ferromagnetic metal of the core parts. To prevent this degradation, the bond must be created in a temperature range below 200°C. Epoxy compounds are frequently used as bonding agents when bonding metals. Those epoxy compounds which provide a satisfactory bond have, however, the drawback for this application that they have a high viscosity. This high viscosity prevents the realization of very thin bonding layers. When lamellar cores of amorphous, ferromagnetic material are used in magnetic heads, it is important that the bonding layers between the lamellas are as thin as possible (particularly thinner than 0.1 µm) in order to obtain the best possible writing efficiency. (The thicker the seams, the less magnetic material remains in the case of a given track width). However, in connection with eddy current losses, they must provide satisfactory insulation between the core parts. In addition it is important for the bonding layer between head core and substrate to be as thin as possible. If the bonding layer is thick, there are likely to be problems when grinding a tape contact face profile on the head. Chamfers may be produced both on the substrate and on the head core, resulting in spacing losses and/or efficiency losses. There are epoxy compounds having a low viscosity and which thus provide the possibility of a thin bonding layer. These epoxy compounds have, however, the drawback that they do not bond satisfactorily to an amorphous, ferromagnetic material surface. The invention provides a solution to the above-mentioned problems. This solution is characterized in that the surfaces of the lamellar core parts of amorphous, ferromagnetic material are coated with a film of a material chosen from the group comprising TiO₂, SiO₂, Al₂O₃ and Si₃N₄. An advantage of Si₃N₄ is that it can be sputtered very thinly and yet constitutes a closed film. Coating the lamellas with a thin film of one of these materials enables the use of epoxy adhesives having a low viscosity, particularly a viscosity at room temperature of less than 1 Pa.s and even a viscosity at room temperature of less than 500 mPa.s, while still providing great bonding strength. An embodiment of the invention will be described with reference to the drawing, in which: Figure 1 is a perspective view of a magnetic core for a magnetic head, and Figure 2 is a perspective view of a magnetic head in which the magnetic core of Figure 1 is used. Figure 1 shows an embodiment of a magnetic core 1 for a magnetic head. Magnetic core 1 is formed by two half cores 2, 3 which are bonded together. Each half core consists of three lamellar core parts 4, 4', 4'' of amorphous, ferromagnetic metal having a thickness of approximately 7 µm, placed between two protective blocks 5, 6. A material having the composition Fe₅Co₇₀B₁₀Si₁₅ is suitable, for example, as the amorphous, ferromagnetic material for the lamellar magnetic cores, or more generally a material based on Fe Co Si B. Other amorphous magnetic materials applicable in magnetic head cores are, for example, materials based on Co Zr Mo Cr, or based on Co Mn Si B, or based on Co Nb Zr (for example, Co₁₈Nb₁₃Zr₆). The surfaces of the core parts 4, 4', 4'' are coated with a film of SiO₂ applied by means of plasma deposition or sputtering and having a thickness of between 50 and 500 Å, and they are bonded by means of an epoxy adhesive having a low viscosity, such as Epotek 330 to which 1 vol.% of a primer, such as α-glycidoxy-propyl-trimethoxy-silane has been added (by heating at l30° C for 1 hour). Epotek 330 is the tradename for a 2-component thermohardener from the firm Epoxy Technology Inc. The viscosity at 24° C is 375 mPa.s. An additional advantage of the use of an epoxy adhesive having a low viscosity as a bonding agent is that only little pressure needs to be exerted on the core parts during the bonding process (for example, several hundred grammes per cm²). A drawback of having to exert a high pressure (for example, several tens of kg/cm² as is required in glass cementing techniques) is that the core parts may be locally pressed together because they are often not exactly flat, causing short circuits and an increase of the eddy current losses. The total thickness of the lamellar core part is approximately 22 µm. The space between the two core halves 2, 3 (the gap length) is 0.3 µm. The magnetic core 1 of Figure 1 is shown as a complete magnetic head 9 (Figure 2) by providing it with a winding 8 wound through a winding aperture 7. This head had a writing efficiency of 50%, a quality factor 4 at 5.4 MHz and a signal-to-noise ratio of l05 dB. In addition to other ceramic materials, a suitable material for the protective blocks 5, 6 is Macor. The core parts 4,4'' may be bonded thereto with the aid of the same bonding agent as is used for their mutual bonding. MnZn-ferrite is a suitable protective block material for certain uses. In that case the bonding strength is improved if, in addition to the core parts 4, 4'', the surface of the MnZn-ferrite protective block to be bonded is also coated with an oxide film of SiO₂, TiO₂, Al₂O₃ or Si₃N₄. When an SiO₂ film is used, the bonding strength of the epoxy bond is found to be a factor of 10 higher than the bonding strength without the use of the SiO₂ film. In the case of Al₂O₃ the bonding strength is found to a factor of 7 higher and in the case of TiO₂ it is found to be a factor of 5 higher. In all cases the bonding strength may be further increased by adding a silane to the epoxy adhesive. The conditions under which the aforementioned films were sputtered on the surfaces of the core parts were as follows :

Claims: 1. A magnetic core comprising a first lamellar core part of amorphous, ferromagnetic material which is bonded by means of an epoxy adhesive to a second lamellar core part or to a substrate, characterized in that the surfaces of the lamellar core parts are coated with a thin film of a material chosen from the group comprising TiO₂, SiO₂, Al₂O₃ and Si₃N₄. 2. A magnetic core as claimed in Claim 1, characterized in that the epoxy adhesive used has a viscosity of less than 1 Pa.s at room temperature. 3. A magnetic core as claimed in Claim 1, characterized in that the epoxy adhesive used has a viscosity of less than 500mPa.s at room temperature.

Inventors: VERBUNT, JOHANNES PETRUS MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H01F1
IPC codes: G11B5, H01F1