Process for producing Mn-Zn ferrite

Abstract: A process for producing Mn-Zn ferrite having large electrical resistance and being durable to the use in high frequency region exceeding 1 MHz easily and at low cost is disclosed. The process comprises pressing a mixed powder comprising a composition of 44.0 to 50.0 mol% of Fe 2 O 3 , 4.0 to 26.5 mol% of ZnO, 0.1 to 8.0 mol% of at least one member selected from the group consisting of TiO 2 and SnO 2 , and the remainder being MnO, and if desired 0.1 to 16.0 mol% of CuO, sintering the resulting green compact in the air or an atmosphere containing an appropriate amount of oxygen, and then cooling the green compact, thereby securing the estimated high initial permeability even in a high frequency region of 1 MHz or more.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a process for producing an oxide magnetic material having soft magnetism, particularly Mn-Zn ferrite. More particularly, the invention relates to a process for producing Mn-Zn ferrite which is suitable for use as low loss materials used in switching power supply transformer, flyback transformer or deflection yoke, various inductance elements, impedance elements for EMI countermeasure, electromagnetic wave absorbers, and the like. 2. Background of Related Art There is Mn-Zn ferrite as the representative oxide magnetic material having soft magnetism. This Mn-Zn ferrite generally has a composition comprising basic components of more than 50 mol% of Fe 2 O 3 , 52 to 55 mol% of Fe 2 O 3 on the average, 10 to 24 mol% of ZnO and the remainder being MnO. EP-A-0 98056 and EP-A-0 980 857 relate to Mn-Zn ferrites comprising 44 to 50 mol% of Fe 2 O 3 and DE-A-1 177 538 discloses a process for producing a dielectric material by calcining a mixture comprising 44 to 47 mol% of Fe 2 O 3 . The Mn-Zn ferrite has conventionally been produced by mixing each raw material powder of Fe 2 O 3 , ZnO and MnO in predetermined proportion, forming the resulting mixture into a predetermined shape through each step of calcination, milling, component adjustment, granulation, pressing and the like, and then subjecting the green compact to sintering treatment such that the green compact is maintained at 1,200 to 1,400°C for 3 to 4 hours in a reduced atmosphere having greatly decreased oxygen concentration by flowing of nitrogen gas. The reason for sintering in a reduced atmosphere is that since the green compact contains Fe 2 O 3 in a large amount of 50 mol% or more, if it is sintered in the air, densification does not proceed sufficiently, and as a result, good soft magnetism is not obtained. Further, Fe 2+ to be formed by reduction of Fe 3+ has a positive crystal magnetic anisotropy, and therefore has the effect that it offsets a negative crystal magnetic anisotropy of Fe 3+ , thereby increasing soft magnetism. However, if sintered in the air, formation of Fe 2+ by such a reduction reaction cannot be expected. However, it is known that the densification depends on oxygen concentration at the time of temperature rising in sintering, and formation of Fe 2+ depends on oxygen concentration at the time of temperature dropping after sintering. Accordingly, if setting of oxygen concentration in sintering is mistaken, it is difficult to secure good soft magnetism. For this reason, conventionally the following equation (1) has been established experimentally, and oxygen concentration in sintering has been administered according to this equation (1). log Po 2 = -14,540/(T + 273) + b wherein T is temperature (°C), Po 2 is oxygen concentration (-), and b is a constant. The numerical value of about 7-8 has conventionally been employed as the constant b. The constant b being 7-8 means that oxygen concentration during sintering must be controlled to a narrow range, and due to this fact, there have conventionally been the problems that sintering treatment is very complicated and production cost is very high. On the other hand, where Mn-Zn ferrite is used as a magnetic core material, eddy current flows with becoming a frequency used high, and loss by such an eddy current increases. Therefore, in order to raise the upper limit of the frequency that can be used as a magnetic core material, it is necessary to make its electrical resistivity large as much as possible. However, there have been the problems that the electrical resistivity in the above-described conventional Mn-Zn ferrite is a value smaller than 1 Ωm due to transfer of electron between Fe 3+ and Fe 2+ (interionic) as mentioned above, frequency that can be used is up to about several hundred kHz, and initial permeability remarkably decreases in a high frequency region exceeding 1 MHz, resulting in losing properties as soft magnetic material. SUMMARY OF THE INVENTION The present invention has been made in view of the above-described conventional problems. An object of the present invention is to provide a production process which can easily and inexpensively obtain Mn-Zn ferrite which has large electrical resistivity and is sufficiently durable to the use in a high frequency region exceeding 1 MHz. The above object can be achieved by the following aspects. According to the present invention, there is provided a process for producing Mn-Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol% of Fe 2 O 3 , 4.0 to 26.5 mol% of ZnO, 0.1 to 8.0 mol% of at least one member selected from the group consisting of TiO 2 and SnO 2 , and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300°C: log Po 2 = -14,540/(T + 273) + b wherein T is temperature (°C), Po 2 is oxygen concentration (-), and b is a constant selected from the range of 6 to 21. In this case, if the temperature is lower than 300°C, since the reaction of oxidation and reduction can be disregarded without depending on the oxygen concentration, the adjustment of the atmosphere is sufficient such that the cooling after sintering advances to the point of 300°C. According to another aspect of the present invention, there is provided a process for producing Mn-Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol% of Fe 2 O 3 , 4.0 to 26.5 mol% of ZnO, 0.1 to 8.0 mol% of at least one member selected from the group consisting of TiO 2 and SnO 2 , 0.1 to 16.0 mol% of CuO and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300°C: log Po 2 = -14,540/(T + 273) + b wherein T is temperature (°C), Po 2 is oxygen concentration (-), and b is a constant selected from the range of 6 to 21. In this case, if the temperature is lower than 300°C, since the reaction of oxidation and reduction can be disregarded without depending on the oxygen concentration, the adjustment of the atmosphere is sufficient such that the cooling after sintering advances to the point of 300°C. It is known that iron component in Mn-Zn ferrite is present in the form of Fe 3+ and Fe 2+ , but Ti and Sn form Fe 2+ by receiving an electron from Fe 3+ . Therefore, Fe 2+ can be formed even by sintering in the air or an atmosphere containing an appropriate amount of oxygen by containing Ti and Sn. The aspects of the present invention make it possible to obtain good soft magnetism by that the content of TiO 2 and/or SnO 2 in the basic components is adjusted to 0.1 to 8.0 mol% to control the amount of Fe 2+ to be formed and optimize a co-existence ratio of Fe 3+ and Fe 2+ , thereby canceling out positive and negative crystal magnetic anisotropy. Further, according to the first to fourth aspects of the present invention, since Ti 4+ and Sn 4+ having the stable number of valency are present in large amounts exchange of electrons between Fe 3+ and Fe 2+ is substantially inhibited, and as a result, an electrical resistance that is considerably larger than the conventional electrical resistance can be obtained (about 10 3 times). However, if the content of TiO 2 and/or SnO 2 is less than 0.1 mol%, such an effect is small. On the other hand, if the content is larger than 8.0 mol%, the initial permeability decreases. For this reason, the content of TiO 2 and/or SnO 2 is adjusted to the range of 0.1 to 8.0 mol%. In the present invention, since the content of Fe 2 O 3 is suppressed to 50 mol% or less, even if sintered in the air or an atmosphere containing an appropriate amount of oxygen, densification proceeds sufficiently, so that the desired soft magnetism is obtained. However, if the content of Fe 2 O 3 is too small, it results in a decrease in initial permeability. Therefore, Fe 2 O 3 should contain in an amount of at least 44.0 mol%. ZnO affects Curie temperature or saturation magnetization. If the content of ZnO is too large, Curie temperature lowers, resulting in practical problem. On the other hand, if the content of ZnO is too small, saturation magnetization decreases. For this reason, the content of ZnO is adjusted to the range of 4.0 to 26.5 mol%. In an aspect of the present invention, CuO is contained as described above. This CuO has the effect which enables low temperature sintering. However, if the content of CuO is too small, the above-described effect is small. On the other hand, if the content is too large, the initial permeability decreases. Therefore, the content of CuO is adjusted to the range of 0.1 to 16.0 mol%. The aspects of the present invention can contain a slight amount of at least one member selected from the group consisting of CaO, SiO 2 , ZrO 2 , Ta 2 O 5 , HfO 2 , Nb 2 O 5 and Y 2 O 3 as additives. Those additives have the function to make grain boundary highly resistant. Further, at least one member selected from the group consisting of V 2 O 5 , Bi 2 O 3 , In 2 O 3 , PbO, MoO 3 and WO 3 can be contained as the additives. Those additives each are oxides having low melting point and have the function to accelerate sintering. Additionally, at least one of Cr 2 O 3 and Al 2 O 3 may further be contained as the additive. Those additives have the function to improve temperature characteristics of initial permeability. In the present invention, cooling after sintering is conducted in an atmosphere having an oxygen concentration determined using an optional value within the range of 6 to 21 as the constant b in the above-described equation (1). If a value larger than 21 is selected as the constant b in the equation (1), the atmosphere is substantially the same atmosphere as the air, and thus it is meaningless to define the oxygen concentration. Further, in order to increase the initial permeability at low frequency of Mn-Zn ferrite to be obtained, it is desirable to select a value as small as possible for the constant b. However, if the value is smaller than 6, electrical resistance becomes too small, and as a result, the initial permeability in high frequency region deteriorates. Therefore, the constant b is set to 6 or more. BRIEF DESCRIPTION OF THE DRAWING Fig 1 is a graph showing frequency characteristics to initial permeability in Samples of the present invention and Comparison Samples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The process for producing Mn-Zn ferrite comprises weighing each raw material of Fe 2 O 3 , ZnO, TiO 2 and/or SnO 2 , CuO, MnO and the like as main components in advance so as to become the predetermined respective proportion, mixing those to obtain a mixed powder, and if necessary, calcining and finely milling the mixed powder. The calcination temperature can be selected at an appropriate temperature within the temperature range of 850 to 950°C, although slightly varying depending on the objective composition. Further, the general-purpose ball mill can be used for fine milling of the mixed powder. If desired, powder of various additives as described above is added to and mixed with the fine mixed powder in the predetermined proportion to obtain the mixed powder having the objective components. After that, granulation and pressing are conducted according to the conventional process for producing ferrite, and sintering is further conducted at 900 to 1,300°C. Incidentally, for the granulation, a method of adding a binder such as polyvinyl alcohol, polyacrylamide, methyl cellulose, polyethylene oxide, glycerin or the like can be used, and for the pressing, for example, a method in which pressing is conducted by applying a pressure of 80 MPa or more can be used. The above-described sintering and cooling after sintering is conducted in an atmosphere having an oxygen concentration defined based on the above-described equation (1) wherein the constant b is within the range of 6 to 21. However, in a case where those are conducted in an atmosphere containing oxygen, it is desirably to control oxygen concentration by flowing of an inert gas such as nitrogen gas in a sintering furnace. In this case, the constant b in the equation (1) can select an optional value within a wide range of 6 to 21, and this makes it possible to easily control the oxygen concentration. The Mn-Zn ferrite thus obtained contains TiO 2 and/or SnO 2 as the main components. Therefore, electrical resistance of the Mn-Zn ferrite greatly increases as compared with that of the conventional Mn-Zn ferrite (about 10 3 times). Further, the limit of initial permeability µ in soft magnetic ferrite is generally in inversely proportional to a frequency f (MHz) in which the ferrite is used, and is estimated by the value given by the following equation (2): µ = K/f (K = 1,500-2,000) According to the Mn-Zn ferrite of the present invention, the initial permeability p of 1,500 to 2,000 at 1 MHz and 300 to 400 at 5 MHz can be obtained as estimated. Thus, the Mn-Zn ferrite according to the present invention is suitable for use as magnetic core materials and electromagnetic wave absorbers for high frequency exceeding 1 MHz. The present invention is described in more detail by referring to the following Examples, but it should be understood that the invention is not limited thereto. EXAMPLE 1 Each raw material powder was mixed with a ball mill so as to obtain a composition such that Fe 2 O 3 was 48.0 mol%, TiO 2 or SnO 2 was 2.0 mol%, and the remainder was MnO and ZnO in a molar ratio of 26:25. The resulting mixture was calcined in the air at 900°C for 2 hours, and then milled with a ball mill for 20 hours to obtain a mixed powder. Next, this mixed powder was subjected to component adjustment so as to have the above-described composition, and then mixed with a ball mill for 1 hour. Polyvinyl alcohol was added to the mixed powder, and the resulting mixture was granulated and then pressed into a toroidal core (green compact) having an outer diameter of 18 mm, an inner diameter of 10 mm and a height of 4 mm under a pressure of 80 MPa. The green compact was introduced in a sintering furnace. The atmosphere in the furnace was adjusted so as to have an oxygen concentration to be obtained when the constant b in the equation (1) was 8 by flowing nitrogen gas, and sintering at 1,300°C for 3 hours and cooling after sintering were conducted to obtain Samples of the present invention 1-2 and 1-3 as shown in Table 1 below. For the sake of comparison, each raw material powder was mixed with a ball mill so as to obtain a composition such that Fe 2 O 3 was 52.5 mol%, MnO was 24.2 mol% and ZnO was 23.3 mol%. The resulting mixture was calcined in the air at 900°C for 2 hours, and then milled with a ball mill for 20 hours to obtain a mixed powder. Next, this mixed powder was subjected to component adjustment so as to have the above-described composition, 0.05 mass% of CaO was added as an additive, and the resulting mixture was then mixed with a ball mill for 1 hour. Next, polyvinyl alcohol was added to the mixed powder, and the resulting mixture was granulated and then pressed into a toroidal core (green compact) having an outer diameter of 18 mm, an inner diameter of 10 mm and a height of 4 mm under a pressure of 80 MPa. The green compact was introduced in a sintering furnace. The atmosphere in the furnace was adjusted so as to have an oxygen concentration such that the constant b in the equation (1) was 7.7, and sintering at 1,300°C for 3 hours and cooling after sintering were conducted to obtain Comparison Sample 1-1 containing Fe 2 O 3 in an amount larger than 50 mol% as the same as conventional. On each of Samples 1-1 to 1-3 thus obtained, final components were confirmed by fluorescent X ray analysis, and initial permeability and electrical resistivity at 100 kHz, 1 MHz and 5 MHz were measured. The results obtained are shown in Table 1 below. Sample No. Classification Basic component composition (mol%) Initial permeability Electrical resistivity (Ωm) Fe 2 O 3 MnO ZnO TiO 2 SnO 2 100 kHz 1MHz 5MHz 1-1 Comparison 52.5 24.2 23.3 - - 7010 1380 1 1.5 x 10 -1 1-2 Invention 48.0 25.5 24.5 2.0 - 7030 1930 310 6.5 x 10 3 1-3 Invention 48.0 25.5 24.5 - 2.0 7010 1940 300 6.2 x 10 3 From the results shown in Table 1 above, Samples of the present invention 1-2 and 1-3 in which Fe 2 O 3 is 50.0 mol% or less have a large electrical resistivity of the order of 10 4 as compared with Comparison Sample 1-1 in which Fe 2 O 3 is more than 50.0 mol%, and corresponding to the large electrical resistivity, the initial permeability at a high frequency region of 1 MHz and 5 MHz is also remarkably high. On the other hand, the initial permeability of Comparison Sample 1-1 is 1 at a frequency of 5 MHz. Consequently, Comparison Sample 1 entirely loses properties as soft magnetic material. EXAMPLE 2 Each raw material powder was mixed with a ball mill so as to obtain a composition such that Fe 2 O 3 was 48.0 mol%, TiO 2 was 2.0 mol%, CuO was 0-20.0 mol%, and the remainder was MnO and ZnO in a molar ratio of 26:25. After that, the resulting mixture was calcined in the air at 900°C for 2 hours, and then milled with a ball mill for 20 hours to obtain a mixed powder. Next, this mixed powder was subjected to component adjustment so as to have the above-described composition, and then mixed with a ball mill for 1 hour. Next, polyvinyl alcohol was added to the mixed powder, and the resulting mixture was granulated and then pressed into a toroidal core having an outer diameter of 18 mm, an inner diameter of 10 mm and a height of 4 mm under a pressure of 80 MPa. Each of the green compacts was introduced in a sintering furnace. The atmosphere in the furnace was adjusted so as to have an oxygen concentration such that the constant b in the equation (1) was 8 by flowing nitrogen gas, and sintering at 900 to 1,300°C for 3 hours and cooling after sintering were conducted to obtain Samples 2-1 to 2-4 as shown in Table 2 below. On each of Samples 2-1 to 2-4 thus obtained, final components were confirmed by fluorescent X-ray analysis, and initial permeability at 1 MHz was measured. The results obtained are shown in Table 2 below. From the results shown in Table 2, in Samples 2-1 (Samples of the present invention) which does not contain CuO at all, the sintering temperature must be set to high temperature of 1,200°C or more in order to obtain high initial permeability of 1,000 or more, but in Samples 2-2 and 2-3 (Samples of the present invention) which contain an appropriate amount of CuO, high initial permeability of 1,000 or more is obtained even though the sintering temperature is set to low temperature of, for example, about 1,000°C. However, in Samples 2-4 (Comparison Samples) containing relatively large amount (20.0 mol%) of CuO, where high temperature sintering of 1,200°C or more is conducted, the initial permeability greatly decreases, and also high permeability exceeding 1,500 is not obtained even though sintered at relatively low temperature (1,100°C). Thus, it was apparent that an appropriate amount of CuO contained is effective to decrease the optimum sintering temperature and improve initial permeability at high frequency region. EXAMPLE 3 Each raw material powder was mixed with a ball mill so as to obtain a composition such that Fe 2 O 3 was 48.0 mol%, TiO 2 was 2.0 mol%, CuO was 0 or 8.0 mol% and the remainder was MnO and ZnO in a molar ratio of 26:25. After that, the resulting mixture was calcined in the air at 900°C for 2 hours, and then milled with a ball mill for 20 hours to obtain a mixed powder. Next, this mixed powder was subjected to component adjustment so as to have the above-described composition, and then mixed with a ball mill for 1 hour. Next, polyvinyl alcohol was added to the mixed powder, and the resulting mixture was granulated and then pressed into a toroidal core having an outer diameter of 18 mm, an inner diameter of 10 mm and a height of 4 mm under a pressure of 80 MPa. After that, each of the green compacts was introduced in a sintering furnace. Sintering at 1,300°C or 1,100°C (1,100°C was for only a green compact containing CuO) for 3 hours and cooling after sintering were conducted in the air or in an atmosphere which was adjusted so as to be oxygen concentration to be obtained when the constant b in the equation (1) was changed within the range of 5.5 to 21 by flowing nitrogen gas to obtain Samples 3-1 to 3-7 as shown in Table 3 below. For each of Samples 3-1 to 3-7 thus obtained, final components were confirmed by fluorescent X ray analysis, and initial permeability and electrical resistivity at 100 kHz, 1 MHz and 5 MHz were measured. The results obtained are shown in Table 3 below. Incidentally, it should be noted the results of Samples of the present invention 1-2 in Example 1 are also shown in Table 3. Further, on Samples of the present invention 3-3 and 3-5 obtained in Example 3, and Samples of the present invention 1-2 and Comparison Sample 1-1 obtained in Example 1, change of the initial permeability was examined by changing the frequency over a wide range. The results obtained are shown in Fig. 1. From the results shown in Table 3 above, Samples of the present invention 3-2 to 3-4, 3-6 and 1-2 in which sintering was conducted in an atmosphere having oxygen concentration such that the constant b in the equation was 6 or more, and Samples of the present invention 3-5 and 3-7 in which sintering was conducted in the air, each shows large electrical resistivity, and according to this, the initial permeability in the high frequency region of 1 MHz and 5 MHz is also high. In particular, Samples 3-5 and 3-7 in which sintering was conducted in the air show that both electrical resistivity and initial permeability at high frequencies are high, as compared with the samples in which sintering was conducted in another atmosphere. On the other hand, Comparison Sample 3-1 in which sintering was conducted in an atmosphere having an oxygen concentration when the constant b was 5.5 shows that the initial permeability in the high frequency region of 1 MHz and 5 MHz is the lowest. Further, from the results shown in Fig. 1, Samples of the present invention 3-3, and 1-2, sample 3-5 and Comparison Sample 1-1 show that the initial permeability does not substantially change up to the frequency of about 500 kHz even if the frequency changes, whereas Comparison Sample 1-1 shows that the initial permeability rapidly drops if the frequency exceeds about 500 kHz, and the initial permeability decreases to 1 at the frequency of 5,000 kHz (5 MHz). EXAMPLE 4 Each raw material powder was mixed with a ball mill so as to obtain a composition such that Fe 2 O 3 was 48.0 mol%, TiO 2 was 2.0 mol%, CuO was 0 or 8.0 mol% and the remainder was MnO and ZnO in a molar ratio of 26:25. After that, the resulting mixture was calcined in the air at 900°C for 2 hours, and then milled with a ball mill for 20 hours to obtain a mixed powder. Next, this mixed powder was subjected to component adjustment so as to have the above-described composition, and 0.05 mass% of MoO 3 , 0.05 mass% of CaO and 0.10 mass% of ZrO 2 or 0.15 mass% of Al 2 O 3 were added thereto as additives. The resulting mixture was then mixed with a ball mill for 1 hour. Next, polyvinyl alcohol was added to the mixed powder, and the resulting mixture was granulated and then pressed into a toroidal core having an outer diameter of 18 mm, an inner diameter of 10 mm and a height of 4 mm under a pressure of 80 MPa. Each of the green compacts was introduced in a sintering furnace. Sintering at 1,300°C or 1,100°C (1,100°C was for only a green compact containing CuO) for 3 hours and cooling after sintering were conducted in an atmosphere which was adjusted so as to be oxygen concentration to be obtained when the constant b in the equation (1) was 8 by flowing nitrogen gas to obtain Samples 4-1 to 4-4 as shown in Table 4 below. On each of Samples 4-1 to 4-4 thus obtained, final components were confirmed by fluorescent X-ray analysis, and initial permeability and electrical resistivity at 100 kHz, 1 MHz and 5 MHz were measured. The results obtained are shown in Table 4 below. Sample No. Classification Basic component composition (mol%) Additive (mass%) Initial permeability Fe 2 O 3 MnO ZnO TiO 2 CuO 100kHz 1MHz 5MHz 4-1 Invention 48.0 25.5 24.5 2.0 - MoO 3 0.05 7050 1920 300 4-2 Invention 48.0 25.5 24.5 2.0 - CaO 0.05 7020 1930 330 4-3 Invention 48.0 21.4 20.6 2.0 8.0 ZrO 2 0.10 7000 1950 360 4-4 Invention 48.0 21.4 20.6 2.0 8.0 Al 2 O 3 0.15 7020 1900 310 From the results shown in Table 4 above, it is found that high initial permeability is obtained at each frequency level by adding slight amounts of MoO 3 , CaO, ZrO 2 , Al 2 O 3 and the like as additives, and thus those additives contribute to improvement of the initial permeability. As described above, according to the process for producing Mn-Zn ferrite of the present invention, Mn-Zn ferrite which are durable to the use in wide frequency region of from a low frequency region of about 100 kHz to a high frequency region exceeding 1 MHz can be obtained in a stable manner even if sintered in the air or an atmosphere containing an appropriate amount of oxygen, by using a mixed powder containing specific components such that TiO 2 or SnO 2 is contained and the amount of Fe 2 O 3 is suppressed to 50 mol% or less. Further, since sintering does not require complicated administration of atmosphere, cost required for the sintering is decreased, making it possible to provide inexpensive Mn-Zn ferrite useful for use in magnetic core materials, electromagnetic wave absorbers, and the like. In particular, when CuO is contained in a mixed powder, low temperature sintering is possible. Consequently, the cost required for the sintering is further decreased, and this greatly contributes to inexpensive production of Mn-Zn ferrite.

Claims: A process for producing Mn-Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol% of Fe 2 O 3 , 4.0 to 26.5 mol% of ZnO, 0.1 to 8.0 mol% of at least one member selected from the group consisting of TiO 2 and SnO 2 , and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300°C by adjusting the atmosphere as defined by the following equation until the cooling has advanced to 300°C to obtain Mn-Zn ferrite having an initial permeability of 1,500 to 2,000 at 1 MHz: log Po 2 = -14,540/(T + 273) + b wherein T is temperature (°C), Po 2 is oxygen concentration (-), and b is a constant selected from the range of 6 to 21. A process for producing Mn-Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol% of Fe 2 O 3 , 4.0 to 26.5 mol% of ZnO, 0.1 to 8.0 mol% of at least one member selected from the group consisting of TiO 2 and SnO 2 , 0.1 to 16.0 mol% of CuO and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300°C by adjusting the atmosphere as defined by the following equation until the cooling has advanced to 300°C to obtain Mn-Zn ferrite having an initial permeability of 1,500 to 2,000 at 1MHz: log Po 2 = -14,540/(T + 273) + b wherein T is temperature (°C), Po 2 is oxygen concentration (-), and b is a constant selected from the range of 6 to 21.

Inventors: HONDA KOJI, KAWASAKI SHUNJI, KOBAYASHI OSAMU, YAMADA OSAMU, HONDA, KOJI, KAWASAKI, SHUNJI, KOBAYASHI, OSAMU, YAMADA, OSAMU, Honda, Koji, c/o Minebea Co., Ltd., Kawasaki, Shunji, c/o Minebea Co., Ltd., Kobayashi, Osamu, c/o Minebea Co., Ltd., Yamada, Osamu, c/o Minebea Co., Ltd.
Applicants: MINEBEA KK, MINEBEA CO., LTD.
Application year: 2000
Main classification: C04B35
IPC codes: H01F1, C04B35, C01G49, C22C29