Abstract:
Disclosed is a MnZnCo-based ferrite consisting of base constituents, accessory constituents, and inevitable impurities, which MnZnCo-based ferrite is characterized by adding silicon oxide (SiO2 conversion): 50-400 mass ppm and calcium oxide (CaO conversion): 1000-4000 mass ppm as secondary constituents to base constituents consisting of iron oxide (Fe2O3 conversion): 51.0-53.0 mol %, zinc oxide (ZnO conversion): greater than 12.0 mol % and less than 18.0 mol %, cobalt oxide (CoO conversion): 0.04-0.60 mol %, and manganese oxide (MnO conversion): remainder, and keeping phosphorus, boron, sulfur, and chlorine of the inevitable impurities to phosphorous: less than 3 mass ppm, boron: less than 3 mass ppm, sulfur: less than 5 mass ppm, and chlorine: less than 10 mass ppm. This MnZnCo-based ferrite has the superior characteristics of always having incremental permeability [mu]? of 2000 or greater across a wide temperature range of −40 DEG C. to 85 DEG C. when a 33 A/m direct current magnetic field is applied, even in cases when formed into a small core with outside diameter of approximately 2-6 mm.

Description:
TECHNICAL FIELD 
     The present invention relates to a Mn—Zn—Co ferrite that is suitable as a material for, for example, a magnetic core for a pulse transformer in an Ethernet (registered trademark) device. 
     BACKGROUND ART 
     Ethernet devices include pulse transformers for the purpose of achieving impedance match and electrical insulation in input/output terminals. Such transformers include magnetic cores generally composed of soft magnetic materials. Such pulse transformers are required to have a high incremental permeability μΔ under the application of a direct-current magnetic field in a temperature range of −40 to 85° C., for example, as defined in American standards ANSI X3.263-1995[R2000]. The incremental permeability μΔ is a value that indicates the degree of magnetization of a magnetic core under the application of a magnetic field. 
     A soft magnetic material used in the application is generally Mn—Zn ferrite. Mn—Zn ferrite is advantageous, for example, in that a high permeability and a high inductance can be easily achieved for a soft magnetic material and Mn—Zn ferrite is less expensive than amorphous metals and the like. Developments of Mn—Zn ferrites suitable for the application are performed and described in, for example, Patent Literatures 1 and 2. 
     However, Mn—Zn ferrite is an oxide magnetic material and hence has drawbacks in that, compared with metal magnetic materials, magnetic characteristics considerably vary with change in temperature and a material having a high permeability has a low saturation flux density. 
     Accordingly, stable magnetic characteristics are less likely to be achieved in a wide temperature range, in particular, in a high temperature range, which is problematic. 
     To overcome the temperature dependency of characteristics of Mn—Zn ferrite, it is known that addition of CoO having positive magnetic anisotropy is effective. For example, Patent Literature 1 states that a Mn—Zn—Co ferrite for the application allows a high permeability under the application of a direct-current magnetic field of about 33 A/m. 
     The inventors previously developed 
     “A Mn—Co—Zn ferrite comprising a basic component, additional components, and impurities, wherein the basic component consists of Fe 2 O 3 : 51.0 to 53.0 mol %, ZnO: 13.0 to 18.0 mol %, CoO: 0.04 to 0.60 mol %, and the balance being MnO; the ferrite contains, as the additional components, SiO 2 : 0.005 to 0.040 mass % and CaO: 0.020 to 0.400 mass % relative to an entirety of the ferrite; the ferrite contains, as the impurities, P: less than 3 mass ppm and B: less than 3 mass ppm relative to the entirety of the ferrite; and an average pulverized particle size is 1.00 to 1.30 μm”, which is suitably used for a magnetic core for a pulse transformer in an Ethernet device, and disclosed the ferrite in Patent Literature 3. 
     The development of such a Mn—Zn—Co ferrite provides a ferrite core having a high incremental permeability of 2300 or more under the application of a direct-current magnetic field of 33 A/m in a wide temperature range of −40° C. to 85° C. 
     However, a Mn—Zn—Co ferrite core in the application mainly has the form of a closed magnetic circuit with a small size represented by a toroidal core having an outer diameter of about 2 to 6 mm. In the case of such a small size, since there is a high probability of mold breakage in compaction, it is impossible to apply a high compaction pressure. Accordingly, when a surface of a core that is a fired compact as illustrated in  FIG. 1  is observed with a scanning electron microscope (SEM), there are cases where cavities due to insufficient disintegration of granulated powder remain as illustrated in  FIG. 2(   b ). 
     When a core includes such cavities, the volume occupied by the magnetic material is small and hence magnetic fluxes concentrate in the magnetic material region and a magnetic flux density locally increases. Accordingly, the same phenomenon as an increase in a superposed magnetic field seemingly occurs in the magnetic material region and, as a result, the incremental permeability decreases. Thus, it is difficult to continuously maintain an incremental permeability of 2000 or more in a wide temperature range of −40° C. to 85° C. 
     CITATION LIST 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2004-196632 
     Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2007-197246 
     Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2008-143744 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention advantageously overcomes the above-described problem. An object of the present invention is to propose a Mn—Zn—Co ferrite that has an excellent characteristic: even when the ferrite is formed into a small core having an outer diameter of about 2 to 6 mm, the core has an incremental permeability μΔ of 2000 or more continuously in a wide temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field of 33 A/m. 
     Solution to Problem 
     Features of the present invention are as follows.
     1. A Mn—Zn—Co ferrite comprising a basic component, sub-components, and unavoidable impurities,
 
wherein, as the sub-components,
   

     silicon oxide (in terms of SiO 2 ): 50 to 400 mass ppm and 
     calcium oxide (in terms of CaO): 1000 to 4000 mass ppm are added to the basic component consisting of 
     iron oxide (in terms of Fe 2 O 3 ): 51.0 to 53.0 mol %, 
     zinc oxide (in terms of ZnO): more than 12.0 mol % and 18.0 mol % or less, 
     cobalt oxide (in terms of CoO): 0.04 to 0.60 mol %, and 
     manganese oxide (in terms of MnO): balance; and amounts of phosphorus, boron, sulfur, and chlorine in the unavoidable impurities are reduced as follows 
     phosphorus: less than 3 mass ppm, 
     boron: less than 3 mass ppm, 
     sulfur: less than 5 mass ppm, and 
     chlorine: less than 10 mass ppm.
     2. The Mn—Zn—Co ferrite according to 1 above, wherein the sub-components further include one or more selected from   

     zirconium oxide (in terms of ZrO 2 ): 0.005 to 0.075 mass %; 
     tantalum oxide (in terms of Ta 2 O 5 ): 0.005 to 0.075 mass %; 
     hafnium oxide (in terms of HfO 2 ): 0.005 to 0.075 mass %; and 
     niobium oxide (in terms of Nb 2 O 5 ): 0.005 to 0.075 mass %. 
     Advantageous Effects of Invention 
     According to the present invention, a Mn—Zn—Co ferrite can be provided that has an excellent characteristic: even when the ferrite is formed into a small core having an outer diameter of about 2 to 6 mm, the core has an incremental permeability μΔ of 2000 or more continuously in a wide temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field of 33 A/m. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a section of a Mn—Zn—Co ferrite core, the section being observed for cavities. 
         FIG. 2(   a ) illustrates a state of a core according to the present invention in which no cavities remain. 
         FIG. 2(   b ) illustrates a state of an existing core in which cavities remain due to insufficient disintegration of a granulated powder. 
         FIG. 3  illustrates the way of calculating an ideal specific surface. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To overcome the above-described problem, the inventors have performed thorough studies. As a result, the inventors have achieved findings described below. 
     When a magnetic field is superposed, domain walls having been in a state prior to the superposition of the magnetic field move within the core being magnetized. When the core has a content of impurities more than a specific value, exaggerated grain growth is caused in the core, which significantly hampers the motion of domain walls. An incremental permeability is a value that indicates the degree of magnetization of the core under the superposition of a magnetic field. Under the superposition of a magnetic field of 33 A/m, magnetization mainly proceeds through domain wall motion. Accordingly, in a state where, for example, segregation of components in exaggerated grains hampers domain wall motion, the incremental permeability significantly decreases. Thus, to achieve a higher incremental permeability under the superposition of a magnetic field of 33 A/m, exaggerated grain growth needs to be strictly suppressed. 
     The inventors have studied how to suppress exaggerated grain growth and have found for the first time that, by more strictly limiting the content of impurities than before, a Mn—Zn—Co ferrite core having a high incremental permeability even under the superposition of a high magnetic field can be achieved. 
     The present invention is based on the findings. 
     Hereinafter, the present invention will be specifically described. 
     The reasons why the composition of the basic component of a Mn—Zn—Co ferrite according to the present invention is limited to the above-described ranges will be described. 
     Iron Oxide (in Terms of Fe 2 O 3 ): 51.0 to 53.0 Mol % 
     When iron oxide in the basic component is less than 51.0 mol % or is more than 53.0 mol %, the incremental permeability μΔ under the application of a direct-current magnetic field decreases in a low-temperature range and a high-temperature range. Accordingly, the content of iron oxide in terms of Fe 2 O 3  is made in the range of 51.0 to 53.0 mol %, preferably in the range of 52.0 to 53.0 mol %. 
     Zinc Oxide (in Terms of ZnO): More than 12.0 Mol % and 18.0 Mol % or Less 
     As the content of zinc oxide increases, the incremental permeability μΔ under the application of a direct-current magnetic field increases. Accordingly, the content of zinc oxide is made more than 12.0 mol %. However, when the content of zinc oxide is more than 18.0 mol %, in a low-temperature range, the incremental permeability μΔ under the application of a direct-current magnetic field decreases; and, in a high-temperature range, the Curie temperature at which a ferromagnetic material loses its magnetism decreases and the incremental permeability μΔ under the application of a direct-current magnetic field also decreases. Accordingly, the content of zinc oxide in terms of ZnO is made in the range of more than 12.0 mol % and 18.0 mol % or less, preferably in the range of 14.0 to 17.0 mol %. 
     Cobalt Oxide (in Terms of CoO): 0.04 to 0.60 Mol % 
     Only when an appropriate amount of cobalt oxide having positive magnetic anisotropy is contained, a high incremental permeability μΔ of 2000 or more can be maintained in a wide temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field. However, when the content of cobalt oxide is less than 0.04 mol %, the effect of adding cobalt oxide is not sufficiently provided. When the content of cobalt oxide is more than 0.60 mol %, the incremental permeability μΔ decreases in the entire temperature range. Accordingly, the content of cobalt oxide in terms of CoO is made in the range of 0.04 to 0.60 mol %, preferably in the range of 0.08 to 0.50 mol %. 
     Manganese Oxide (in Terms of MnO): Balance 
     The present invention relates to a Mn—Zn—Co ferrite and the balance in the composition of the basic component needs to be manganese oxide. This is because, by making the ferrite contain manganese oxide, a high incremental permeability μΔ of 2000 or more cannot be achieved under the application of a direct-current magnetic field of 33 A/m. The preferred range of manganese oxide in terms of MnO is 28.0 to 33.0 mol %. 
     The contents of iron oxide, zinc oxide, cobalt oxide, and manganese oxide that constitute the basic component are adjusted such that the total amount of iron oxide in terms of Fe 2 O 3 , zinc oxide in terms of ZnO, cobalt oxide in terms of CoO, and manganese oxide in terms of MnO is 100 mol %. 
     The reasons why the composition of the sub-components of a Mn—Zn—Co ferrite according to the present invention is limited to the above-described ranges will be described. 
     Silicon Oxide (in Terms of SiO 2 ): 50 to 400 Mass Ppm 
     Silicon oxide has an effect of reducing the number of vacancies remaining in crystal grains to thereby increase the incremental permeability μΔ under the application of a direct-current magnetic field. However, when the content of silicon oxide is less than 50 mass ppm, the effect of adding silicon oxide is not sufficiently exhibited; and when the content of silicon oxide is more than 400 mass ppm, exaggerated grains appear and the incremental permeability considerably decreases under the application of a direct-current magnetic field. Accordingly, the content of silicon oxide in terms of SiO 2  is made in the range of 50 to 400 mass ppm, preferably in the range of 100 to 250 mass ppm. 
     Calcium Oxide (in Terms of CaO): 1000 to 4000 Mass Ppm 
     Calcium oxide segregates in crystal grain boundaries of a Mn—Zn—Co ferrite to provide an effect of suppressing growth of crystal grains. As a result, the initial permeability μ i  is appropriately decreased and the incremental permeability μΔ under the application of a direct-current magnetic field is effectively increased. However, when the content of calcium oxide is less than 1000 mass ppm, the effect of suppressing growth of grains is not sufficiently provided; and when the content of calcium oxide is more than 4000 mass ppm, exaggerated grains appear and the incremental permeability μΔ considerably decreases under the application of a direct-current magnetic field. Accordingly, the content of calcium oxide in terms of CaO is made in the range of 1000 to 4000 mass ppm, preferably in the range of 1000 to 2000 mass ppm. 
     Note that the initial permeability μ i  at 23° C. is preferably made about 3900 to 5000. 
     In the present invention, to increase the incremental permeability μΔ under the application of a direct-current magnetic field, it is important to limit the contents of impurities in a ferrite, in particular, phosphorus, boron, sulfur, and chlorine simultaneously to ranges below. 
     Phosphorus: Less than 3 Mass Ppm, Boron: Less than 3 Mass Ppm 
     Phosphorus and boron are unavoidable impurities derived from raw material iron oxide. When the content of phosphorus or boron is 3 mass ppm or more, exaggerated grain growth is induced and the incremental permeability μΔ considerably decreases under the superposition of a magnetic field of 33 A/m. Accordingly, the contents of phosphorus and boron are limited to less than 3 mass ppm. 
     The contents of phosphorus and boron can be limited to less than 3 mass ppm by, for example, a method of using raw material powders of iron oxide, zinc oxide, and manganese oxide that have a high purity and contain phosphorus and boron as less as possible. In addition, the contents of phosphorus and boron in a medium used in mixing and pulverization such as a ball mill or an attritor are preferably low to avoid entry of phosphorus and boron due to abrasion of the medium. 
     Note that all the values defined herein are quantified by an analytical procedure defined in “Molybdophosphate extraction-separation/molybdophosphoric blue spectrophotometric method” in JIS G 1214 (1998) in terms of the P component and by a procedure defined in “Curcumin spectrophotometric method” in JIS G 1227 (1999) in terms of the B component. 
     Sulfur: Less than 5 Mass Ppm. 
     Sulfur is an unavoidable impurity derived from raw material iron oxide obtained from ferrous sulfide. When the content of sulfur is 5 mass ppm or more, exaggerated grain growth is induced and the incremental permeability considerably decreases under the superposition of a high magnetic field of 33 A/m. Accordingly, the content of sulfur is limited to less than 5 mass ppm. The content of sulfur is preferably limited to less than 4 mass ppm. 
     The content of sulfur can be limited to less than 5 mass ppm by, for example, a method in which time for calcination performed at 800° C. or more in the air atmosphere in the production of a Mn—Zn—Co ferrite is increased to allow reaction between sulfur and oxygen to sufficiently proceed to thereby reduce the content of sulfur. 
     Note that the S values defined herein are quantified by an analytical procedure for the S component defined in “Methylene blue spectrophotometric method after separation of hydrosulfide” in JIS G 1215 (1994). 
     Chlorine: Less than 10 Mass Ppm 
     Chlorine is an unavoidable impurity derived from raw material iron oxide obtained from iron chloride. When the content of chlorine is 10 mass ppm or more, exaggerated grain growth is induced and the incremental permeability μΔ considerably decreases under the application of a direct-current magnetic field of 33 A/m. Accordingly, the content of chlorine is limited to less than 10 mass ppm. The content of chlorine is preferably limited to less than 8 mass ppm. 
     The content of chlorine can be limited to less than 10 mass ppm by, for example, a method in which raw material iron oxide is sufficiently washed with pure water in the production of a Mn—Zn—Co ferrite to dissolve chlorine highly ionizable in pure water to thereby reduce the content of chlorine. 
     Note that the Cl values defined herein are quantified by an analytical procedure for the Cl component: “nitric acid decomposition-iron chloride nephelometry”. 
     The content of each unavoidable impurity other than the above-described phosphorus, boron, sulfur, and chlorine is not particularly limited, but is preferably reduced to 50 mass ppm or less. 
     A Mn—Zn—Co ferrite according to the present invention may contain, in addition to the above-described components, other components described below as additional sub-components appropriately. 
     One or more selected from zirconium oxide (in terms of ZrO 2 ): 0.005 to 0.075 mass %; tantalum oxide (in terms of Ta 2 O 5 ): 0.005 to 0.075 mass %; hafnium oxide (in terms of HfO 2 ): 0.005 to 0.075 mass %; and niobium oxide (in terms of Nb 2 O 5 ): 0.005 to 0.075 mass % 
     All these components are compounds having a high melting point and, in a Mn—Zn—Co ferrite, decrease the size of crystal grains. Thus, the components suppress generation of coarse crystal grains and increase the incremental permeability μΔ under the application of a direct-current magnetic field. Such an effect is not sufficiently provided when the content of such a component is too low. When the content of such a component is too high, exaggerated grain growth is caused and the incremental permeability μΔ decreases under the application of a direct-current magnetic field. Accordingly, the components are made to be contained in the above-described ranges. 
     By using such a Mn—Zn—Co ferrite as a raw material, even when the Mn—Zn—Co ferrite is formed into a small core having an outer diameter of about 2 to 6 mm, the core can have an excellent characteristic: the incremental permeability μΔ is 2000 or more continuously in a wide temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field of 33 A/m. 
     The inventors have also found that, in the practical production of a ferrite core, when the core product includes a large number of cavities, there are cases where an intended good incremental permeability is not necessarily achieved. 
     The inventors have also thoroughly performed studies on this respect. As a result, the inventors have found that cavities in a core are represented by numerical variation of an increase in the specific surface of the core; and, when an ideal specific surface of the core is calculated from the size and form of the core on the assumption that the surface of the core is in an ideal state of complete flatness, and the relationship represented by the following formula (I) in terms of a ratio of the measured specific surface to the ideal specific surface is satisfied,
 
Measured specific surface/ideal specific surface&lt;1500  (1)
 
     the core has a small number of cavities and a desirable incremental permeability. 
     The measured specific surface is a value determined by a BET method (multipoint method) in JIS Z 8830 (2001) and the unit thereof is m 2 /g. The ideal specific surface is a value obtained by, on the basis of the size and mass of the ferrite core, dividing the surface area of the core calculated on the assumption that the core is in an ideal state of having no cavities, by the mass of the core; and the unit thereof is also m 2 /g. 
     For reference, the way of calculating the ideal specific surface is illustrated in  FIG. 3 . 
     When the assumption is made that the core surface is in an ideal state of complete flatness, the specific surface can be calculated with the following formula.
 
Ideal specific surface=[2×{(outer diameter) 2 −(inner diameter) 2 }/4×π+{(outer diameter)+(inner diameter)}×π×height]/core mass
 
     When a ferrite core includes a large number of cavities, which results in a problem of a decrease in the incremental permeability, a large number of cavities also remain in the surface of the core and hence the measured specific surface is large. Thus, the ratio of measured specific surface/ideal specific surface is high. Thorough studies have been performed on the ratio of measured specific surface/ideal specific surface. As a result, it has been revealed that, when the ratio can be reduced to less than 1500, it can be regarded that a core is obtained in which the incremental permeability does not decrease, that is, a dense core is obtained in which the number of cavities remaining in the surface of the core is small. The ratio of (measured specific surface/ideal specific surface) is preferably 1150 or less. 
     When an appropriate amount of the sub-component such as zirconium oxide, tantalum oxide, hafnium oxide, or niobium oxide is made to be contained, the above-described requirement of the ratio of (measured specific surface/ideal specific surface) is not necessarily strictly applied. Specifically, when such a sub-component is not contained, the ratio of (measured specific surface/ideal specific surface) needs to be reduced to less than 1500; however, when such a sub-component is contained, the allowable range of the ratio of (measured specific surface/ideal specific surface) is widened to 1850 or less as represented by the following formula (1)′
 
Measured specific surface/ideal specific surface≦1850  (1)′
 
     In addition, the allowable range of the compressive rupture strength of a granulated powder is also widened to 1.30 MPa or less. 
     Thus, the production requirements are relaxed and a Mn—Zn—Co ferrite core having desired characteristics are readily obtained. 
     To reduce the ratio of measured specific surface/ideal specific surface to less than 1500, it is important to optimize granulation conditions in the production process of a Mn—Zn—Co ferrite to provide a soft granulated powder. The production process of a Mn—Zn—Co ferrite is a known technique in which, as the granulation method, a spray drying method is mainly employed, which is described in detail in documents, for example, p. 52 in “Ferrite” (Hiraga, Okutani, and Oshima; Maruzen: 1986). 
     The hardness of a granulated powder can be represented in a numerical value by the measurement of the compressive rupture strength of a granulated powder defined in JIS Z 8841 (1993). It has been revealed that, when the compressive rupture strength measured by the procedure defined in JIS Z 8841 is 1.10 MPa or less, the ratio of measured specific surface/ideal specific surface can be reduced to less than 1500. The compressive rupture strength is preferably 1.00 MPa or less. 
     Since Mn—Co—Zn ferrite has a low resistivity of less than 10 2  Ωm, it is often used after a surface thereof is subjected to an insulation coating treatment. However, the measured specific surface defined in the present invention is a measured value of a core that is not subjected to the coating treatment. This is because the coating treatment makes the surface be smooth and hence it becomes impossible to accurately measure the specific surface of a Mn—Zn—Co ferrite. 
     A preferred crystal grain size of a Mn—Zn—Co ferrite according to the present invention will be described. As described above, generation of exaggerated grains decreases the incremental permeability μΔ under the application of a direct-current magnetic field. Accordingly, an average crystal grain size is preferably 5 μm to less than 15 μm. 
     Next, a preferred method for producing a Mn—Zn—Co ferrite according to the present invention will be described. 
     The powders of iron oxide, zinc oxide, cobalt oxide, and manganese oxide that constitute the basic component are weighed so as to achieve predetermined percentages. The powders are sufficiently mixed and then calcined. The calcined powder is then pulverized. The sub-components are added to the calcined powder so as to achieve predetermined proportions and are pulverized together with the calcined powder. In this process, the powder needs to be made sufficiently uniform such that concentrations of the added components are not localized and the particle size of the calcined powder needs to be decreased to a target average size. 
     Thus, a Mn—Zn—Co ferrite having a desired composition can be obtained. 
     Such a Mn—Zn—Co ferrite is used to produce a ferrite core as follows. The resultant powder is mixed with an organic binder such as polyvinyl alcohol and granulated by a spray drying method or the like into a soft granulated powder having a compressive rupture strength of less than 1.10 MPa. The granulated powder is then compacted into a desired form and subsequently fired under appropriate firing conditions. The pressure applied in the compaction is preferably about 115 to 120 MPa. The firing conditions are preferably a temperature of 1200 to 1400° C. and a time for about 18 to 30 hours. 
     The compressive rupture strength of the granulated powder can be effectively reduced to less than 1.10 MPa by decreasing the temperature in granulation: specifically, to about 150 to 200° C., which is about 50 to 100° C. lower than the conventional temperature of 250 to 300° C. 
     When granulation is performed at the conventional temperature of 250 to 300° C., the granulated powder has a compressive rupture strength of about 1.2 to 1.4 MPa. As described above, a target condition of the present invention
 
Measured specific surface/ideal specific surface&lt;1500  (1)
 
cannot be satisfied with such a compressive rupture strength.
 
     The thus-obtained Mn—Zn—Co ferrite core can have, under the application of a direct-current magnetic field of 33 A/m, a high incremental permeability μΔ of 2000 or more in a temperature range of −40° C. to 85° C., which cannot be achieved in existing Mn—Zn—Co ferrite cores. 
     Note that existing Mn—Zn—Co ferrite cores have a low incremental permeability μΔ of about 1700 under the application of a direct-current magnetic field of 33 A/m in a wide temperature range of −40° C. to 85° C. 
     EXAMPLE 1 
     Raw material powders were weighed such that the compositions of iron oxide, zinc oxide, cobalt oxide, and manganese oxide that constituted the basic component satisfied percentages in Table 1 in terms of Fe 2 O 3 , ZnO, CoO, and MnO. The raw material powders were mixed with a ball mill for 16 hours and then calcined in the air at 925° C. for 3 hours. The calcined powders were then mixed with, as sub-components, silicon oxide and calcium oxide having been respectively weighed so as to achieve percentages in Table 1 in terms of SiO 2  and CaO, and pulverized with the ball mill for 12 hours. Thus, Mn—Zn—Co ferrite powders were produced. 
     The resultant Mn—Zn—Co ferrite powders were mixed with water to be turned into slurries, then mixed with polyvinyl alcohol, granulated at 180° C. by a spray drying method, and compacted into toroidal cores under the application of a pressure of 118 MPa. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm were obtained. 
     A wire was wound ten turns around each specimen obtained in this way. While the core was under the application of a direct-current magnetic field of 33 A/m with a direct-current application apparatus (42841A, manufactured by Agilent Technologies, Inc.), the specimen was measured in terms of incremental permeability μΔ at −40° C., 0° C., 23° C., 70° C., and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz with a LCR meter (4284A, manufactured by Agilent Technologies, Inc.). 
     Note that the initial permeability μ i  was measured at 23° C. with the LCR meter (4284A). 
     In the preparation of the specimens, all the raw materials including iron oxide were highly pure; the ball mill serving as the mixing and pulverization medium had low contents of phosphorus and boron; the calcination was performed under sufficient air flow; and pure water used scarcely contained Cl. As a result, in all the specimens, the final contents of P, B, S, and Cl were respectively 2 mass ppm, 2 mass ppm, 3 mass ppm, and 6 mass ppm. 
     The compressive rupture strength of the granulated powders measured in accordance with JIS Z 8841 was 0.90±0.05 MPa and hence the number of cavities remaining in the surfaces of the ferrite cores was small. Accordingly, the measured specific surface was 0.453 to 0.493 m 2 /g and the ideal specific surface was 4.44×10 −4  m 2 /g. All the ratios of (measured specific surface/ideal specific surface) were 1020 to 1110, which were less than 1500. 
     As for the crystal grain size of each specimen, the core was cut; the fracture section was polished; micrographs of three different fields of view of the polished surface at 500× magnification were taken with an optical microscope; size of grains in the micrographs was measured; and the average crystal grain size was calculated from the measured grain size. 
     The results are also described in Table 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                   
                   
                 Sub- 
                 Initial 
                 Incremental permeability under 
                   
               
               
                   
                 Basic component 
                 components 
                 permeability 
                 application of magnetic field of 33 A/m 
               
               
                 Specimen 
                 (mol %) 
                 (mass %) 
                 μ i   
                 (μΔ) 
               
             
          
           
               
                 No. 
                 Fe 2 O 3   
                 ZnO 
                 CoO 
                 MnO 
                 SiO 2   
                 CaO 
                 23° C. 
                 −40° C. 
                 0° C. 
                 23° C. 
                 70° C. 
                 85° C. 
               
               
                   
               
               
                 1-1 
                 
                   54.0 
                 
                 16.0 
                 0.40 
                 Balance 
                 0.010 
                 0.150 
                 4700 
                 2700 
                 4000 
                 3900 
                 2600 
                 
                   1900 
                 
               
               
                 1-2 
                 
                   50.5 
                 
                 16.0 
                 0.40 
                 Balance 
                 0.010 
                 0.150 
                 4200 
                 
                   1900 
                 
                 3200 
                 3500 
                 3000 
                 2500 
               
               
                 1-3 
                 52.9 
                 16.0 
                 0.04 
                 Balance 
                 0.010 
                 0.150 
                 4600 
                 2700 
                 3900 
                 3900 
                 2900 
                 2500 
               
               
                 1-4 
                 52.0 
                 16.0 
                 
                   0.00 
                 
                 Balance 
                 0.010 
                 0.150 
                 5400 
                 
                   1900 
                 
                 3800 
                 4300 
                 2700 
                 
                   1900 
                 
               
               
                 1-5 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 0.010 
                 0.150 
                 4500 
                 2600 
                 3800 
                 4000 
                 3300 
                 2700 
               
               
                 1-6 
                 52.0 
                 16.0 
                 
                   0.80 
                 
                 Balance 
                 0.010 
                 0.150 
                 3400 
                 
                   1900 
                 
                 2600 
                 2600 
                 2100 
                 2000 
               
               
                 1-7 
                 52.0 
                 
                   18.5 
                 
                 0.40 
                 Balance 
                 0.010 
                 0.150 
                 6000 
                 
                   1900 
                 
                 2500 
                 3800 
                 2100 
                 
                   1700 
                 
               
               
                 1-8 
                 52.0 
                 
                   11.5 
                 
                 0.40 
                 Balance 
                 0.010 
                 0.150 
                 3500 
                 
                   1800 
                 
                 3000 
                 3300 
                 2000 
                 
                   1900 
                 
               
               
                 1-9 
                 51.8 
                 16.0 
                 0.55 
                 Balance 
                 0.010 
                 0.150 
                 4400 
                 2600 
                 3700 
                 3900 
                 3300 
                 2800 
               
               
                 1-10 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 
                   0.002 
                 
                 0.150 
                 6400 
                 
                   1900 
                 
                 3100 
                 3000 
                 2600 
                 
                   1800 
                 
               
               
                 1-11 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 0.010 
                 
                   0.080 
                 
                 6600 
                 
                   1900 
                 
                 3000 
                 3000 
                 2600 
                 
                   1800 
                 
               
               
                 1-12 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 0.025 
                 0.250 
                 4300 
                 2500 
                 3600 
                 3800 
                 3200 
                 2600 
               
               
                 1-13 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 
                   0.045 
                 
                 0.150 
                 3400 
                 
                   1900 
                 
                 2700 
                 2700 
                 2000 
                 
                   1900 
                 
               
               
                 1-14 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 0.010 
                 
                   0.450 
                 
                 3300 
                 
                   1800 
                 
                 2600 
                 2600 
                 2000 
                 
                   1800 
                 
               
               
                 1-15 
                 52.0 
                 16.0 
                 0.40 
                 Balance 
                 
                   0.045 
                 
                 
                   0.450 
                 
                 2600 
                 
                   1300 
                 
                 
                   1800 
                 
                 
                   1800 
                 
                 
                   1600 
                 
                 
                   1300 
                 
               
               
                   
               
             
          
           
               
                   
                   
                   
                 Compressive 
                 Measured 
                   
               
               
                   
                   
                   
                 rupture 
                 specific 
               
               
                   
                   
                 Average 
                 strength of 
                 surface/ 
               
               
                   
                   
                 crystal 
                 granulated 
                 ideal 
               
               
                   
                 Specimen 
                 grain size 
                 powder 
                 specific 
               
               
                   
                 No. 
                 (μm) 
                 (MPa) 
                 surface 
                 Remarks 
               
               
                   
                   
               
               
                   
                 1-1 
                 8 
                 0.90 ± 0.05 
                 1020-1110 
                 Comparative example 
               
               
                   
                 1-2 
                 8 
                   
                   
                 Comparative example 
               
               
                   
                 1-3 
                 8 
                   
                   
                 Example of invention 
               
               
                   
                 1-4 
                 8 
                   
                   
                 Comparative example 
               
               
                   
                 1-5 
                 8 
                   
                   
                 Example of invention 
               
               
                   
                 1-6 
                 8 
                   
                   
                 Comparative example 
               
               
                   
                 1-7 
                 8 
                   
                   
                 Comparative example 
               
               
                   
                 1-8 
                 8 
                   
                   
                 Comparative example 
               
               
                   
                 1-9 
                 8 
                   
                   
                 Example of invention 
               
               
                   
                 1-10 
                 17 
                   
                   
                 Comparative example 
               
               
                   
                 1-11 
                 23 
                   
                   
                 Comparative example 
               
               
                   
                 1-12 
                 6 
                   
                   
                 Example of invention 
               
               
                   
                 1-13 
                 143 
                   
                   
                 Comparative example 
               
               
                   
                 1-14 
                 137 
                   
                   
                 Comparative example 
               
               
                   
                 1-15 
                 209 
                   
                   
                 Comparative example 
               
               
                   
                   
               
             
          
         
       
     
     As described in Table 1, Specimen Nos. 1-3, 1-5, 1-9, and 1-12 serving as examples of the invention all had an excellent characteristic in which the incremental permeability μΔ was continuously 2000 or more in a wide temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field of 33 A/m. 
     In contrast, in a comparative example (Specimen No. 1-1) in which the content of Fe 2 O 3  was more than 53.0 mol % and a comparative example (Specimen No. 1-2) in which the content of Fe 2 O 3  was less than 51.0 mol %, the incremental permeability μΔ at 85° C. and −40° C. was less than 2000. 
     In a comparative example (Specimen No. 1-4) without containing CoO, the incremental permeability μΔ at −40° C. and 85° C. was less than 2000. In contrast, in a comparative example (Specimen No. 1-6) in which the content of CoO was high, the incremental permeability μΔ decreased in the entire temperature range and, in particular, the incremental permeability μΔ at −40° C. and 85° C. was less than 2000. 
     In a comparative example (Specimen No. 1-7), in which the content of ZnO was high and more than the upper limit, under the application of a direct-current magnetic field of 33 A/m, the incremental permeability μΔ at −40° C. and 85° C. was less than 2000. In contrast, in a comparative example (Specimen No. 1-8) in which the content of ZnO was less than the lower limit, the incremental permeability μ0 decreased in the entire temperature range and, in particular, the incremental permeability μΔ at −40° C. and 85° C. was less than 2000. 
     As for SiO 2  and CaO, in comparative examples (Specimen Nos. 1-10 and 1-11) in which the content of SiO 2  or CaO was less than the appropriate ranges, the initial permeability μ i  excessively increased and, as a result, the incremental permeability μΔ decreased in the entire temperature range, compared with the examples of the invention: the incremental permeability μΔ at −40° C. and 85° C. was less than 2000. In contrast, in comparative examples (Specimen Nos. 1-13, 1-14, and 1-15) in which the content of SiO 2  and/or CaO was more than the appropriate ranges, exaggerated grains appeared and, as a result, the incremental permeability μΔ considerably decreased in the entire temperature range. 
     EXAMPLE 2 
     Several iron oxide raw materials having different contents in terms of P, B, S, and Cl were used. The raw materials were weighed on the basis of calculation for achieving the contents of P, B, S, and Cl in Table 2 such that the composition of iron oxide (in terms of Fe 2 O 3 ), zinc oxide (in terms of ZnO), cobalt oxide (in terms of CoO), and manganese oxide (in terms of MnO) that constituted the basic component respectively satisfied Fe 2 O 3 : 52.0 mol %, ZnO: 16.0 mol %, CoO: 0.40 mol %, and MnO: the balance. The raw materials were mixed with a ball mill for 16 hours and then calcined in the air at 925° C. for 3 hours. Some of the calcined powders were then mixed with, as sub-components, silicon oxide (100 mass ppm in terms of SiO 2 ) and calcium oxide (500 mass ppm in terms of CaO). All the raw material powders were then pulverized with the ball mill for 12 hours to produce Mn—Zn—Co ferrite powders. 
     The resultant Mn—Zn—Co ferrite powders were subsequently mixed with water to be turned into slurries, then mixed with polyvinyl alcohol, granulated at 180° C. by a spray drying method, and compacted into toroidal cores under the application of a pressure of 118 MPa. At this time, the granulation temperature was varied such that the resultant granulated powders had various compressive rupture strengths. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm were obtained. 
     A wire was wound ten turns around each specimen obtained in this way. The specimen was measured in terms of incremental permeability μΔ at −40° C., 0° C., 23° C., 70° C., and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz under the application of a direct-current magnetic field of 33 A/m with the same direct-current application apparatus and LCR meter as in EXAMPLE. The compressive rupture strength of the granulated powders was measured in accordance with requirements of JIS Z 8841. The measured specific surface was also measured by a BET method (multipoint method) in JIS Z 8830 (2001). The ideal specific surface was calculated from size and weight that were measured in accordance with JIS C 2560. The ratio of (measured specific surface/ideal specific surface) was calculated from the ideal specific surface of 4.44×10 −4  m 2 /g). The initial permeability μ i  and the average crystal grain size were measured as in EXAMPLE 1. 
     The results are also described in Table 2. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                   
                 Sub- 
                 Contents of 
                 Initial 
                 Incremental permeability under 
               
               
                   
                 components 
                 impurities 
                 permeability 
                 application of magnetic field of 33 A/m 
               
               
                 Specimen 
                 (mass %) 
                 (mass ppm) 
                 μ i   
                 (μΔ) 
               
             
          
           
               
                 No. 
                 ZrO 2   
                 Nb 2 O 5   
                 P 
                 B 
                 S 
                 Cl 
                 23° C. 
                 −40° C. 
                 0° C. 
                 23° C. 
                 70° C. 
                 85° C. 
               
               
                   
               
               
                 1-5 
                 0 
                 0 
                 2 
                 2 
                 3 
                 6 
                 4500 
                 2600 
                 3800 
                 4000 
                 3300 
                 2700 
               
               
                 2-1 
                   
                   
                 2 
                 2 
                 4 
                 8 
                 4400 
                 2500 
                 3700 
                 3900 
                 3200 
                 2600 
               
               
                 2-2 
                   
                   
                 
                   5 
                 
                 2 
                 3 
                 6 
                 3600 
                 
                   1900 
                 
                 2900 
                 3000 
                 2500 
                 
                   1800 
                 
               
               
                 2-3 
                   
                   
                 2 
                 
                   5 
                 
                 3 
                 6 
                 3500 
                 
                   1800 
                 
                 2800 
                 3000 
                 2400 
                 
                   1700 
                 
               
               
                 2-4 
                   
                   
                 2 
                 2 
                 
                   8 
                 
                 6 
                 3700 
                 
                   1900 
                 
                 2900 
                 3100 
                 2500 
                 
                   1900 
                 
               
               
                 2-5 
                   
                   
                 2 
                 2 
                 3 
                   15   
                 3800 
                 
                   1900 
                 
                 2900 
                 3200 
                 2500 
                 
                   1900 
                 
               
               
                 2-6 
                   
                   
                 
                   5 
                 
                 
                   5 
                 
                 3 
                 6 
                 3400 
                 
                   1700 
                 
                 2600 
                 2800 
                 2100 
                 
                   1500 
                 
               
               
                 2-7 
                   
                   
                 2 
                 2 
                 
                   8 
                 
                   15   
                 3400 
                 
                   1600 
                 
                 2600 
                 2800 
                 2100 
                 
                   1500 
                 
               
               
                 2-8 
                   
                   
                 
                   5 
                 
                 2 
                 
                   8 
                 
                   15   
                 3000 
                 
                   1500 
                 
                 
                   2200 
                 
                 2400 
                 
                   1900 
                 
                 
                   1400 
                 
               
               
                 2-9 
                   
                   
                 
                   5 
                 
                 
                   5 
                 
                 3 
                   15   
                 2700 
                 
                   1300 
                 
                 
                   1900 
                 
                 2100 
                 
                   1800 
                 
                 
                   1300 
                 
               
               
                 2-10 
                   
                   
                 
                   5 
                 
                 
                   5 
                 
                 
                   8 
                 
                 6 
                 2700 
                 
                   1300 
                 
                 
                   1900 
                 
                 2100 
                 
                   1700 
                 
                 
                   1200 
                 
               
               
                 2-11 
                   
                   
                 
                   5 
                 
                 
                   5 
                 
                 
                   8 
                 
                   15   
                 2300 
                 
                   1000 
                 
                 
                   1500 
                 
                 
                   1700 
                 
                 
                   1500 
                 
                 
                   1000 
                 
               
               
                 2-12 
                   
                   
                   10   
                   10   
                   15   
                   30   
                 2000 
                   800   
                 
                   1300 
                 
                 
                   1500 
                 
                 
                   1200 
                 
                   500   
               
               
                 2-13 
                 0 
                 0 
                 2 
                 2 
                 
                   8 
                 
                 6 
                 3500 
                 
                   1700 
                 
                 2700 
                 2900 
                 2300 
                 
                   1700 
                 
               
               
                 2-14 
                 0 
                 0 
                 2 
                 2 
                 3 
                   15   
                 3600 
                 
                   1700 
                 
                 2700 
                 3000 
                 2300 
                 
                   1700 
                 
               
               
                 2-15 
                 0 
                 0 
                 2 
                 2 
                 
                   8 
                 
                   15   
                 3200 
                 
                   1400 
                 
                 2400 
                 2600 
                 
                   1900 
                 
                 
                   1400 
                 
               
               
                 2-16 
                 0.050 
                 0 
                 2 
                 2 
                 
                   8 
                 
                 6 
                 3400 
                 
                   1700 
                 
                 2700 
                 2900 
                 2300 
                 
                   1700 
                 
               
               
                 2-17 
                 0 
                 0.050 
                 2 
                 2 
                 3 
                   15   
                 3500 
                 
                   1700 
                 
                 2700 
                 3000 
                 2300 
                 
                   1700 
                 
               
               
                 2-18 
                 0.030 
                 0.030 
                 2 
                 2 
                 
                   8 
                 
                   15   
                 3100 
                 
                   1400 
                 
                 2400 
                 2600 
                 2000 
                 
                   1400 
                 
               
               
                 2-19 
                 0.050 
                 0 
                 2 
                 2 
                 
                   8 
                 
                 6 
                 3100 
                 
                   1900 
                 
                 2900 
                 3100 
                 2500 
                 
                   1900 
                 
               
               
                 2-20 
                 0 
                 0.050 
                 2 
                 2 
                 3 
                   15   
                 3700 
                 
                   1900 
                 
                 2900 
                 3200 
                 2500 
                 
                   1900 
                 
               
               
                 2-21 
                 0.030 
                 0.030 
                 2 
                 2 
                 
                   8 
                 
                   15   
                 3200 
                 
                   1600 
                 
                 2600 
                 2800 
                 2100 
                 
                   1500 
                 
               
               
                   
               
             
          
           
               
                   
                   
                   
                 Compressive 
                   
                 Measured 
                   
               
               
                   
                   
                 Average 
                 rupture 
                   
                 specific 
               
               
                   
                   
                 crystal 
                 strength of 
                 Measured 
                 surface/ 
               
               
                   
                   
                 grain 
                 granulated 
                 specific 
                 ideal 
               
               
                   
                 Specimen 
                 size 
                 powder 
                 surface 
                 specific 
               
               
                   
                 No. 
                 (μm) 
                 (MPa) 
                 (m 2 /g) 
                 surface 
                 Remarks 
               
               
                   
                   
               
               
                   
                 1-5 
                 8 
                 0.90 ± 0.05 
                 0.448-0.493 
                 1020-1110 
                 Example of invention 
               
               
                   
                 2-1 
                 10 
                   
                   
                   
                 Example of invention 
               
               
                   
                 2-2 
                 21 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-3 
                 32 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-4 
                 28 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-5 
                 23 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-6 
                 39 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-7 
                 45 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-8 
                 53 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-9 
                 59 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-10 
                 73 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-11 
                 205 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-12 
                 232 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-13 
                 28 
                 1.25 ± 0.05 
                 0.730-0.808 
                 
                   1640-1800 
                 
                 Comparative example 
               
               
                   
                 2-14 
                 23 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-15 
                 45 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-16 
                 27 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-17 
                 22 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-18 
                 42 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-19 
                 27 
                 0.90 ± 0.05 
                 0.448-0.493 
                 1020-1110 
                 Comparative example 
               
               
                   
                 2-20 
                 22 
                   
                   
                   
                 Comparative example 
               
               
                   
                 2-21 
                 42 
                   
                   
                   
                 Comparative example 
               
               
                   
                   
               
             
          
         
       
     
     As described in Table 2, examples of the invention (Specimen Nos. 1-5 and 2-1) in which the content of S was less than 5 mass ppm, the content of Cl was less than 10 mass ppm, and the contents of P and B were less than 3 mass ppm, had an excellent characteristic in which, under the application of a direct-current magnetic field of 33 A/m, the incremental permeability μΔ was continuously 2000 or more in a temperature range of −40° C. to 85° C. 
     In contrast, in all the comparative examples (Specimen Nos. 2-2 to 2-21) in which the content of at least one of P, B, S, and Cl was more than the appropriate ranges, the incremental permeability μΔ at a temperature of at least one of −40° C., 0° C., 23° C., 70° C., and 85° C. was less than 2000. 
     EXAMPLE 3 
     A pulverized powder having the same composition as in Specimen No. 1-5 was used as a raw material. By changing the temperature condition in granulation by a spray drying method, granulated powders having different compressive rupture strengths in the range of 0.70 to 1.40 MPa were obtained. 
     The granulated powders were then compacted and fired under the same conditions as in EXAMPLE 1 to obtain sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm (ideal specific surface: 4.44×10 −4  m 2 /g). 
     A wire was wound ten turns around each specimen obtained in this way. While the core was under the application of a direct-current magnetic field of 33 A/m, the specimen was measured in terms of incremental permeability μΔ at −40° C., 0° C., 23° C., 70° C. and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz with the same direct-current application apparatus and LCR meter as in EXAMPLE. 
     The compressive rupture strength of the granulated powders was measured in accordance with requirements of JIS Z 8841. The measured specific surface was also measured by a BET method (multipoint method) in JIS Z 8830 (2001) and the ratio of (measured specific surface/ideal specific surface) was determined. The initial permeability μ i  and the average crystal grain size were measured as in EXAMPLE 1. 
     The results are described in Table 3. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                   
                   
                 Compressive 
                   
                   
                   
               
               
                   
                   
                 rupture strength 
                   
                 Measured 
                 Initial 
               
               
                   
                 Granulation 
                 of granulated 
                 Measured 
                 specific surface/ 
                 permeability 
               
               
                 Specimen 
                 temperature 
                 powder 
                 specific surface 
                 ideal specific 
                 μi 
               
               
                 No. 
                 (° C.) 
                 (MPa) 
                 (m 2 /g) 
                 surface 
                 23° C. 
               
               
                   
               
               
                 1-5 
                 180 
                 0.90 
                 0.475 
                 1070 
                 4500 
               
               
                 3-1 
                 150 
                 0.70 
                 0.426 
                  960 
                 4700 
               
               
                 3-2 
                 170 
                 0.80 
                 0.444 
                 1000 
                 4600 
               
               
                 3-3 
                 190 
                 1.03 
                 0.559 
                 1260 
                 4400 
               
               
                 3-4 
                 200 
                 1.08 
                 0.635 
                 1430 
                 4400 
               
               
                 3-5 
                 220 
                 1.14 
                 0.684 
                 
                   1540 
                 
                 4100 
               
               
                 3-6 
                 250 
                 1.25 
                 0.768 
                 
                   1730 
                 
                 4000 
               
               
                 3-7 
                 290 
                 1.40 
                 0.875 
                 
                   1970 
                 
                 3800 
               
               
                   
               
             
          
           
               
                   
                 Incremental permeability under 
                 Average 
                   
               
               
                   
                 application of magnetic field of 33 A/m 
                 crystal 
               
               
                 Specimen 
                 (μΔ) 
                 grain size 
               
             
          
           
               
                 No. 
                 −40° C. 
                 0° C. 
                 23° C. 
                 70° C. 
                 85° C. 
                 (μm) 
                 Remarks 
               
               
                   
               
               
                 1-5 
                 2600 
                 3800 
                 4000 
                 3300 
                 2700 
                 8 
                 Example of invention 
               
               
                 3-1 
                 2800 
                 4000 
                 4200 
                 3500 
                 2900 
                 9 
                 Example of invention 
               
               
                 3-2 
                 2800 
                 3900 
                 4100 
                 3400 
                 2800 
                 9 
                 Example of invention 
               
               
                 3-3 
                 2600 
                 3700 
                 3900 
                 3100 
                 2600 
                 8 
                 Example of invention 
               
               
                 3-4 
                 2500 
                 3700 
                 3900 
                 3000 
                 2500 
                 8 
                 Example of invention 
               
               
                 3-5 
                 
                   2400 
                 
                 3300 
                 3600 
                 2800 
                 
                   2300 
                 
                 8 
                 Comparative example 
               
               
                 3-6 
                 
                   2300 
                 
                 3000 
                 3400 
                 2500 
                 
                   2200 
                 
                 8 
                 Comparative example 
               
               
                 3-7 
                 
                   2100 
                 
                 2800 
                 3100 
                 
                   2200 
                 
                 
                   2000 
                 
                 8 
                 Comparative example 
               
               
                   
               
             
          
         
       
     
     Examples of the invention (Specimen Nos. 1-5 and 3-1 to 3-4) in which the granulated powders had a compressive rupture strength of less than 1.10 MPa and, as a result, the specific surfaces of the cores decreased and the ratios of measured specific surface/ideal specific surface were less than 1500, had an excellent characteristic in which, under the application of a direct-current magnetic field of 33 A/m, the incremental permeability μΔ was 2500 or more in a temperature range of −40° C. to 85° C. 
     However, comparative examples (Specimen Nos. 3-5 to 3-7) in which the granulated powders had a compressive rupture strength of 1.10 MPa or more and the ratios of measured specific surface/ideal specific surface were 1500 or more, that is, a large number of cavities were contained due to insufficient disintegration of the granulated powders, did not achieve an incremental permeability μΔ of 2500 or more in the entire temperature range of −40° C. to 85° C. 
     EXAMPLE 4 
     A calcined powder having the same composition as in Specimen No. 1-5 (note that adjustment was performed so as to achieve P: 2 mass ppm, B: 2 mass ppm, S: 3 mass ppm and Cl: 6 mass ppm) was mixed with zirconium oxide (in terms of ZrO 2 ), tantalum oxide (in terms of Ta 2 O 5 ), hafnium oxide (in terms of HfO 2 ), and niobium oxide (in terms of Nb 2 O 5 ) as sub-components such that the final compositions satisfied percentages in Table 4, and pulverized with a ball mill for 12 hours to produce Mn—Zn—Co ferrite powders. 
     The resultant Mn—Zn—Co ferrite powders were then mixed with water to be turned into slurries, then mixed with polyvinyl alcohol, granulated at 180° C. by a spray drying method, and compacted into toroidal cores under the application of a pressure of 118 MPa. The compacts were then put into a kiln and fired at a maximum temperature of 1350° C. Thus, sintered cores having an outer diameter of 6.0 mm, an inner diameter of 3.0 mm, and a height of 4.0 mm (ideal specific surface: 4.44×10 −4  m 2 /g) were obtained. 
     A wire was wound ten turns around each specimen obtained in this way. The specimen was measured in terms of incremental permeability μΔ at −40° C., 0° C., 23° C., 70° C., and 85° C. at a measurement voltage of 100 mV and at a measurement frequency of 100 kHz under the application of a direct-current magnetic field of 33 A/m with the same direct-current application apparatus and LCR meter as in EXAMPLE. The compressive rupture strength of the granulated powders was measured in accordance with requirements of JIS Z 8841. The measured specific surface was also measured by a BET method (multipoint method) in JIS Z 8830 (2001). The ideal specific surface was calculated from size and weight that were measured in accordance with JIS C 2560. The ratio of (measured specific surface/ideal specific surface) was calculated from the ideal specific surface of 4.44×10 −4  m 2 /g). The initial permeability μ i  and the average crystal grain size were measured as in EXAMPLE 1. 
     The results are described in Table 4. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                   
                   
                 Initial 
                 Incremental permeability under application 
               
               
                   
                 Sub-components 
                 permeability 
                 of magnetic field of 33 A/m 
               
               
                 Specimen 
                 (mass %) 
                 μ i   
                 (μΔ) 
               
             
          
           
               
                 No. 
                 ZrO 2   
                 Ta 2 O 5   
                 HfO 2   
                 Nb 2 O 5   
                 23° C. 
                 −40° C. 
                 0° C. 
                 23° C. 
                 70° C. 
                 85° C. 
               
               
                   
               
               
                 1-5 
                 0    
                 0    
                 0    
                 0    
                 4800 
                 2600 
                 3800 
                 4000 
                 3300 
                 2700 
               
               
                 4-1 
                 0.050 
                 0    
                 0    
                 0    
                 4700 
                 2800 
                 4000 
                 4200 
                 3500 
                 2900 
               
               
                 4-2 
                 0    
                 0.050 
                 0    
                 0    
                 4700 
                 2800 
                 4000 
                 4200 
                 3500 
                 2900 
               
               
                 4-3 
                 0    
                 0    
                 0.050 
                 0    
                 4600 
                 2700 
                 3900 
                 4100 
                 3400 
                 2800 
               
               
                 4-4 
                 0    
                 0    
                 0    
                 0.050 
                 4700 
                 2800 
                 4000 
                 4200 
                 3600 
                 2900 
               
               
                 4-5 
                 0.030 
                 0.030 
                 0    
                 0    
                 4700 
                 2800 
                 4000 
                 4200 
                 3500 
                 3000 
               
               
                 4-6 
                 0.030 
                 0    
                 0.030 
                 0    
                 4600 
                 2800 
                 3900 
                 4100 
                 3400 
                 2800 
               
               
                 4-7 
                 0.030 
                 0    
                 0    
                 0.030 
                 4700 
                 2900 
                 4100 
                 4300 
                 3500 
                 3000 
               
               
                 4-8 
                 0    
                 0.030 
                 0.030 
                 0    
                 4600 
                 2800 
                 3900 
                 4100 
                 3400 
                 2800 
               
               
                 4-9 
                 0    
                 0.030 
                 0    
                 0.030 
                 4700 
                 2900 
                 4000 
                 4200 
                 3600 
                 2900 
               
               
                 4-10 
                 0    
                 0    
                 0.030 
                 0.030 
                 4600 
                 2800 
                 3900 
                 4100 
                 3400 
                 2800 
               
               
                 4-11 
                 0.020 
                 0.020 
                 0.020 
                 0    
                 4600 
                 2800 
                 3900 
                 4100 
                 3400 
                 2800 
               
               
                 4-12 
                 0.020 
                 0.020 
                 0    
                 0.020 
                 4700 
                 3000 
                 4100 
                 4300 
                 3600 
                 3000 
               
               
                 4-13 
                 0.020 
                 0    
                 0.020 
                 0.020 
                 4600 
                 2800 
                 3900 
                 4100 
                 3400 
                 2800 
               
               
                 4-14 
                 0    
                 0.020 
                 0.020 
                 0.020 
                 4600 
                 2800 
                 3800 
                 4000 
                 3400 
                 2800 
               
               
                 4-15 
                 0.020 
                 0.020 
                 0.020 
                 0.020 
                 4600 
                 2900 
                 3900 
                 4200 
                 3600 
                 3000 
               
               
                 4-16 
                 
                   0.080 
                 
                 0    
                 0    
                 0    
                 3700 
                 
                   1900 
                 
                 2800 
                 3100 
                 2300 
                 
                   1900 
                 
               
               
                 4-17 
                 
                   0.080 
                 
                 0    
                 0    
                 
                   0.080 
                 
                 3400 
                 
                   1700 
                 
                 2700 
                 2900 
                 
                   2100 
                 
                 
                   1800 
                 
               
               
                 4-18 
                 
                   0.080 
                 
                 
                   0.080 
                 
                 
                   0.080 
                 
                 
                   0.080 
                 
                 2400 
                 
                   1100 
                 
                 
                   1500 
                 
                 
                   1600 
                 
                 
                   1400 
                 
                 
                   1100 
                 
               
               
                 4-19 
                 0.05  
                 0    
                 0    
                 0    
                 3900 
                 2400 
                 3200 
                 3400 
                 2600 
                 2200 
               
               
                 4-20 
                 0    
                 0    
                 0    
                 0.050 
                 3900 
                 2400 
                 3200 
                 3500 
                 2600 
                 2200 
               
               
                 4-21 
                 0.030 
                 0    
                 0.030 
                 0    
                 3900 
                 2400 
                 3100 
                 3400 
                 2600 
                 2200 
               
               
                 4-22 
                 0    
                 0.030 
                 0    
                 0.030 
                 4000 
                 2400 
                 3100 
                 3400 
                 2600 
                 2200 
               
               
                 4-23 
                 0.020 
                 0.020 
                 0    
                 0.020 
                 4000 
                 2400 
                 3100 
                 3400 
                 2600 
                 2200 
               
               
                 4-24 
                 
                   0.080 
                 
                 0    
                 0    
                 
                   0.080 
                 
                 3200 
                 
                   1500 
                 
                 2400 
                 2600 
                 
                   1800 
                 
                 
                   1500 
                 
               
               
                   
               
             
          
           
               
                   
                   
                   
                 Compressive 
                   
                 Measured 
                   
               
               
                   
                   
                 Average 
                 rupture 
                   
                 specific 
               
               
                   
                   
                 crystal 
                 strength of 
                 Measured 
                 surface/ 
               
               
                   
                   
                 grain 
                 granulated 
                 specific 
                 ideal 
               
               
                   
                 Specimen 
                 size 
                 powder 
                 surface 
                 specific 
               
               
                   
                 No. 
                 (μm) 
                 (MPa) 
                 (m 2 /g) 
                 surface 
                 Remarks 
               
               
                   
                   
               
               
                   
                 1-5 
                 7 
                 0.90 ± 0.05 
                 0.448-0.493 
                 1020-1110 
                 Example of invention 
               
               
                   
                 4-1 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-2 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-3 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-4 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-5 
                 6 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-6 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-7 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-8 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-9 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-10 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-11 
                 6 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-12 
                 6 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-13 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-14 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-15 
                 6 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-16 
                 121 
                   
                   
                   
                 Comparative example 
               
               
                   
                 4-17 
                 140 
                   
                   
                   
                 Comparative example 
               
               
                   
                 4-18 
                 193 
                   
                   
                   
                 Comparative example 
               
               
                   
                 4-19 
                 7 
                 1.25 ± 0.05 
                 0.730-0.808 
                 1640-1800 
                 Example of invention 
               
               
                   
                 4-20 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-21 
                 7 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-22 
                 6 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-23 
                 6 
                   
                   
                   
                 Example of invention 
               
               
                   
                 4-24 
                 190 
                   
                   
                   
                 Comparative example 
               
               
                   
                   
               
             
          
         
       
     
     As is clear from Table 4, all the examples of the invention (Specimen Nos. 4-1 to 4-15) in which appropriate amounts of one or more selected from ZrO 2 , Ta 2 O 5 , HfO 2 , and Nb 2 O 5  were added had excellent values: under the application of a direct-current magnetic field of 33 A/m, the incremental permeability μΔ was continuously 2700 or more in a temperature range of −40° C. to 85° C. Thus, these examples had characteristic values equivalent to or better than those of the example of the invention (Specimen No. 1-5) without such components. 
     However, in comparative examples (Specimen Nos. 4-16 to 4-18) in which at least one of the four components was added in large amounts more than the upper limit, exaggerated grain growth was caused and the incremental permeability μΔ under the application of a direct-current magnetic field of 33 A/m considerably decreased in the entire temperature range. 
     When appropriate amounts of one or more selected from ZrO 2 , Ta 2 O 5 , HfO 2 , and Nb 2 O 5  were added, as demonstrated in, the examples of the invention (Specimen Nos. 4-19 to 4-23), although the ratios of (measured specific surface/ideal specific surface) were 1500 or more, as long as the ratios were 1850 or less, an excellent characteristic was achieved: the incremental permeability μΔ was continuously 2200 or more in a temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field of 33 A/m. In contrast, in Specimen No. 4-24, since the amounts of ZrO 2  and Nb 2 O 5  added were more than the appropriate amounts, the incremental permeability decreased. 
     Industrial Applicability 
     By using a Mn—Zn—Co ferrite according to the present invention, a Mn—Zn—Co ferrite core having an excellent characteristic can be obtained in which the incremental permeability μΔ is continuously 2000 or more in a wide temperature range of −40° C. to 85° C. under the application of a direct-current magnetic field of 33 A/m. Such a Mn—Zn—Co ferrite core is advantageously applicable to, for example, a magnetic core of a pulse transformer in an Ethernet device.