Magnetic structure

Abstract: A magnetic structure is provided with an alumina layer (26) formed between a ceramic substrate (22) and a soft magnetic layer (24). The alumina layer (26) has a formative energy of oxide lower than that of the soft magnetic layer (24) to protect diffusion of oxygen to the soft magnetic layer, when the soft magnetic layer is heated to be formed. Further, a bonding glass layer (36) is positioned between the ceramic substrate (30b) and the alumina layer (34), and another ceramic substrate (30a) is formed on the soft magnetic layer (32).

Description: This invention relates to a magnetic structure, and more particularly to, a magnetic structure used in a magnetic head. A first conventional magnetic head includes a ceramic substrate and a soft magnetic layer formed on the ceramic substrate. In such a magnetic head, the soft magnetic layer is made of FeSiAl or FeMN (M=Ti, Zr, Nb, Mo, Hf or Ta) having a large saturation magnetization. The ceramic substrate includes a great amount of oxide such as NiO, MnO, CoO, etc. each having a large thermal expansion factor close to that of the soft magnetic layer. In fabrication, FeSiAl or FeMN is grown on a surface of the ceramic substrate by gas phase crystal growth such as sputtering and is heated to more than 500 °C, so that the soft magnetic layer is formed. According to the first conventional magnetic head, however, there are disadvantages in that oxygen in the ceramic substrate is diffused to the soft magnetic layer when the thermal treatment is carried out, because a formative energy of oxide in the soft magnetic layer is lower than that of the ceramic substrate. As the result, magnetic property of the soft magnetic layer is reduced. This disadvantage is noteworthy, when a ceramic substrate includes NiO and CoO. In addition, it is difficult to achieve a flat surface of the ceramic substrate, because a ceramic is difficult to be ground. Therefore, the ceramic substrate and the soft magnetic layer are not fastened closely. A second conventional magnetic head includes a ceramic substrate, a soft magnetic layer, and a bonding glass layer formed between the soft magnetic layer and the ceramic substrate. According to the second conventional magnetic head, however, there is a disadvantage in that a yield of the magnetic head is not high, because it is difficult to provide the bonding glass layer with a sufficient bonding strength on the soft magnetic layer. JP-A-2 257404 describes a magnetic structure according to the preamble of claim 1, wherein the protection layer is an iron silicon alloy barrier thin film formed by a sputtering method. It is an object of the invention to provide a magnetic structure keeping a high magnetic property and which can be fabricated with high yield. This object is achieved by a magnetic structure as defined in claim 1; the dependent claims are related to further developments of the invention. The invention will be explained in more detail in conjunction with appended drawings; wherein: Fig. 1 is a perspective view showing a conventional magnetic head; Figs. 2A and 2B are cross sectional views showing a part of a first conventional magnetic head, respectively; Fig. 3 is a cross sectional view showing a second conventional magnetic head; Figs. 4A and 4B are cross sectional views showing a part of a magnetic head of a first preferred embodiment according to the invention, respectively; Figs. 5 and 6 are graphs showing magnetization curves for explaining operation of the first preferred embodiment, respectively; Fig. 7 is a graph showing formative energy of oxide for explaining operation of the first preferred embodiment; Fig. 8 is a graph showing output levels of a magnetic head relative to frequency for explaining operation of the first preferred embodiment; Fig. 9 is a cross sectional view showing a magnetic head of a second preferred embodiment according to the invention; and Figs. 10A and 10B are cross sectional views showing samples for explaining operation of the second preferred embodiment, respectively. Before describing a magnetic head according to the invention, the briefly aforementioned conventional magnetic heads will be explained in conjunction with Figs. 1, 2A, 2B and 3. Fig. 1 shows an approximate structure of a conventional magnetic head which includes two ceramic substrates 10a and 10b and a magnetic core 12 formed between the two ceramic substrates 10a and 10b. Fig. 2A shows a part of a first conventional magnetic head which includes a ceramic substrate 10 made of MnO and NiO shown in Fig. 2B and a soft magnetic layer 14 of FeTaN formed on the ceramic substrate 10. The soft magnetic layer 14 becomes the magnetic core of the magnetic head. According to the first conventional magnetic head, however, there is a disadvantage in that oxygen in the ceramic substrate 10 is diffused to the soft magnetic layer 14 when the thermal treatment is carried out to form the soft magnetic layer 14, because formative energy of oxide in the soft magnetic layer 14 is lower than that of the ceramic substrate 10. As a result, magnetic property of the soft magnetic layer 14 is deteriorated. Fig. 3 shows a second conventional magnetic head which is composed of two ceramic substrates 16a and 16b, a soft magnetic layer 18 formed on a surface of the ceramic substrate 16a, and a bonding glass layer 18 positioned between the soft magnetic layer 18 and the ceramic substrate 16b. According to the second conventional magnetic head, however, there is a disadvantage in that 20 it is difficult to provide a sufficient bonding strength to the bonding glass layer on the soft magnetic layer 18, so that a yield of the magnetic head is low. This is because the wet property of the bonding glass layer 20 on a metal is deteriorated as compared to that on a ceramic substrate or a glass substrate, so that a predetermined bonding strength is not obtained between the soft magnetic layer 18 and the bonding glass layer 20. For this reason, a separation between the soft magnetic layer 18 and the bonding glass layer 20 tends to occur during manufacture, so that a fabrication yield is lowered. Fig. 4A shows a part of a magnetic head according to the first preferred embodiment of the invention which includes a ceramic substrate 22 made of MnO and NiO, a soft magnetic layer 24 made of FeTaN and an alumina layer 26 of Al₂O₃ formed between the ceramic substrate 22 and the soft magnetic layer 24. In fabrication, the alumina layer 26 having a thickness of 0.1 µm is formed on a surface of the ceramic substrate 22 as shown in Fig. 4B, and the soft magnetic layer 24 is formed on the surface of the alumina layer 26 by thermal treatment at 550 °C. The alumina layer 26 may have a thickness of 0.05 µm. Figs. 5 and 6 show magnetization curves of the first conventional magnetic structure shown in Fig. 2A and the first preferred embodiment shown in Fig. 4A, respectively. As shown the magnetization curves, according (the value of "M" at zero of "H") to the first preferred embodiment, the magnetism is reduced to substantially suppress the hysteresis property as compared to that of the first conventional magnetic structure. Thus, the magnetic property of the first preferred embodiment is improved to be higher than the first conventional magnetic structure. Fig. 7 shows formative energy of oxide in relation to temperature. As shown in the figure, the formative energy of oxide of Ni included in the ceramic substrate 22 is higher than that of Fe included in the soft magnetic layer 24, so that, according to the first conventional magnetic structure, oxygen of the ceramic substrate 22 is diffused to the soft magnetic layer 24 by the thermal treatment. When the phenomenon is shown by AES (Auger electron spectroscopy), it can be found that oxygen is diffused in the thickness direction of the magnetic head. According to the first preferred embodiment, oxygen does not diffuse to the soft magnetic layer 24, because the formative energy of oxide of Al included in the alumina layer 26 is lower than that of the soft magnetic layer 24. In the first preferred embodiment, even if the ceramic substrate 22 is made of CaO, TiO₂ and NiO, the same effect can be obtained. Fig. 8 shows output level of a magnetic head. In this graph, the output of a magnetic head of the first preferred embodiment is shown by a solid line, and the output of the first conventional magnetic head is shown by a dashed line. Each of the magnetic heads has a soft magnetic layer of FeTaN having a thickness of 5 µm, and is provided with a gap having a width of 0.3 µm. As shown by the graph, the first preferred embodiment provides an output higher than that of the conventional one. The effect is caused by the structure in that the soft magnetic layer 24 of the first preferred embodiment has a high magnetic property. In the first preferred embodiment, the ceramic substrate may be made of composite material including CoO. A layer made of CaO, MgO, TiO₂, V₂O₃, SiO₂, MnO, Cr₂O₃, ZnO, etc. can be used instead of Al₂O₃ of the alumina layer 26, and nitride, carbide and composite material thereof can be used instead of oxide. The soft magnetic layer 24 may be made of FeMN (M=Ti, Zr, Nb, Mo, Hf or Ta) system layer or FeSiAl layer. As described above, according to the first preferred embodiment, the magnetic structure has a high magnetic property, because the magnetic structure is provided with the alumina layer 26 having a formative energy of oxide lower than that of the soft magnetic layer 24. In addition, a surface faced to the soft magnetic layer 24 can be flattened with high precision by abrasion of the alumina layer 26. Therefore, the ceramic substrate 22 and the soft magnetic layer 24 are fastened closely. Fig. 9 shows a structure of a magnetic head of the second preferred embodiment according to the invention. The magnetic head includes two ceramic layers 30a and 30b made of MnO and NiO, a soft magnetic layer 32 of FeTaN formed on a surface of the ceramic substrate 30a, an alumina layer 34 of Al₂O₃ having a thickness of 0.1 µm formed on the other surface of the soft magnetic layer 32, and a bonding glass layer 36 formed between the alumina layer 34 and the ceramic substrate 30b. Next, experiment for the second preferred embodiment will be explained in conjunction with Figs. 10A and 10B. In a first experiment, first and second samples are used. The first sample is composed of a ceramic substrate 30 of MnO and NiO and a soft magnetic layer 32 of FeTaN formed on the ceramic substrate 30 shown in Fig. 10A, and the second sample is composed of a ceramic substrate 30, a soft magnetic layer 32 formed on the ceramic substrate 30 and an alumina layer 34 of Al₂O₃ having a thickness of 0.1 µm formed on the soft magnetic layer 32 shown in Fig. 10B. Bonding glass layers are formed on the surface of the soft magnetic layer 32 of the first sample and the surface of the alumina layer 34 of the second sample, and each of the samples is heated to 550°C under the pressure of 1kg to the bonding glass layer. After that, shear strength of each ten of the samples are determined, and average of that is obtained as shown in the following table. As the result, it can be found that shear strength of a magnetic structure is increased by the provision of the alumina layer 34. Next, yields of magnetic heads using the first and second samples are determined. As the result, a magnetic head using the first sample has a yield of 30%, while a magnetic head using the second sample has a yield of 70%. In this experiment, the gap of the magnetic head is determined to be 0.3 µm. In a second experiment, third and fourth samples are used. The third sample is composed of a ceramic substrate 30 made of CaO, TiO₂ and NiO and a soft magnetic layer 32 of FeTaN formed on the ceramic substrate 30 shown in Fig. 10A, and the fourth sample is composed of a ceramic substrate 30, a soft magnetic layer 32 formed on the ceramic substrate 30 made of CaO, TiO₂ and NiO and an alumina layer 34 of Al₂O₃ having a thickness of 0.1 µm formed on the soft magnetic layer 32 shown in Fig. 10B. Bonding glass layers are formed on the surface of the soft magnetic layer 32 of the third sample and a surface of the alumina layer 34 of the fourth sample, and each of the samples is heated to 680°C under the pressure of 1kg to the bonding layer. After that, shear strength of each ten of the samples are determined, and average of that is obtained as shown in the following table. As the result, it can be found that shear strength of a magnetic structure is increased by the provision of the alumina layer 34. Next, yields of magnetic heads using the third and fourth samples are determined. As the result, a magnetic head using the third sample has a yield of 70%, while a magnetic head using the fourth sample has a yield of 95%. In this case, a gap of the magnetic head is determined to be 0.3 µm. In the second preferred embodiment, a layer made of CaO, MgO, TiO₂, V₂O₃, SiO₂, MnO, Cr₂O₃, ZnO, etc. can be used instead of Al₂O₃ of the alumina layer 32, and nitride, carbide and composite material thereof can be used instead of oxide material. The soft magnetic layer 32 may be made of FeMN (M=Ti, Zr, Nb, Mo, Hf or Ta) system layer or FeSiAl layer including Ti, Cr and Ru. As described above, according to the second preferred embodiment, the magnetic head has a high yield, because the bonding glass layer 36 can be bonded to the soft magnetic layer 32 tightly by the alumina layer 34 formed between them.

Claims: A magnetic structure, comprising: a first substrate (22, 30b); a magnetic layer (24, 32); and a protection layer (26, 34) formed between said first substrate and said magnetic layer, and having a formative energy of oxide lower than that of said magnetic layer, characterized in that said first substrate (22, 30b) includes NiO and CoO; said protection layer (26, 34) is a thin film made of oxide, nitride or carbide. A magnetic structure according to claim 1, wherein: said protection layer (26, 34) is made of Al₂O₃, CaO, MgO, TiO₂, V₂O₃, SiO₂, MnO, Cr₂O₃ or ZnO. A magnetic structure according to claim 1, wherein: said magnetic layer (24, 32) is a soft magnetic layer. A magnetic structure according to claim 3, wherein: said magnetic layer includes Fe, Si and Al. A magnetic structure according to claim 3, wherein: said magnetic layer (24, 32) is made of FeMN, wherein "M" is an element including at least one of Ti, Zr, Nb, Mo, Hf and Ta, and N is nitrogen. A magnetic structure according to claim 1, further comprising: a bonding glass layer (36) formed between said first substrate (30b) and said protection layer (34); and a second substrate (30a) formed on another surface of said magnetic layer (32), so that said magnetic structure is shaped to be covered with said first and second substrates (30b, 30a).

Inventors: ISHIWATA NOBUYUKI C O NEC CORP, ISHIWATA, NOBUYUKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01F10
IPC codes: H01F10, G11B5