Method for producing sintered ferrite magnet, and sintered ferrite magnet

Abstract

A sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca.sub.1-x-yR.sub.xA.sub.yFe.sub.2n-zCo.sub.z, wherein the atomic ratios (1-x-y), x, y and z of these elements and the molar ratio n meet the relations of 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7, and (b) a grain boundary phase indispensably containing Si, the amount of Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO.sub.2) based on the entire sintered ferrite magnet, and its production method.

Claims

1. A method for producing a sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure, and (b) a grain boundary phase indispensably containing Si, said method comprising a step of preparing calcined ferrite containing a ferrite phase having a hexagonal M-type magnetoplumbite structure, which comprises Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca.sub.1-x-yR.sub.xA.sub.yFe.sub.2n-zCo.sub.z, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:
0.3≦(1-x-y)≦0.65,
0.2≦x≦0.65,
0≦y≦0.2,
0.25≦z≦0.65, and
4≦n≦7; a step of pulverizing said calcined body to powder; a step of molding said powder to a green body; and a step of sintering said green body to obtain a sintered body; more than 1% by mass and 1.6% or less by mass of SiO.sub.2 and 1.2-2% by mass (calculated as CaO) of CaCO.sub.3 being added to 100% by mass of said calcined body, before the pulverization step, a ratio of the amount (calculated as CaO) of CaCO.sub.3 to the amount of SiO.sub.2 being 0.9-1.1 when 0.25≦z≦0.3 and 1.1-1.4 when 0.3≦z≦0.65.

2. The method for producing a sintered ferrite magnet according to claim 1, wherein the amount of SiO.sub.2 added is 1.1-1.6% by mass.

3. The method for producing a sintered ferrite magnet according to claim 1, wherein (1-x-y), x, y and z and n meet the relations of
0.35≦(1-x-y)≦0.55,
0.4≦x≦0.6,
0≦y≦0.15,
0.25≦z≦0.4, and
4.5≦n≦6.

4. The method for producing a sintered ferrite magnet according to claim 1, wherein said pulverization step comprises a first fine pulverization step, a step of heat-treating powder obtained by said first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again.

5. A sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca.sub.1-x-yR.sub.xA.sub.yFe.sub.2n-zCo.sub.z, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:
0.3≦(1-x-y)≦0.65,
0.2≦x≦0.65,
0≦y≦0.2,
0.25≦z≦0.65, and
4≦n≦7, and (b) a grain boundary phase indispensably containing Si, and Ca in amounts determined by more than 1% by mass and 1.6% or less by mass of SiO.sub.2 powder and 1.2-2% by mass (calculated as CaO) of CaCO.sub.3 powder added to 100% by mass of said ferrite phase, a ratio of the amount (calculated as CaO) of CaCO.sub.3 to the amount of SiO.sub.2 being 0.9-1.1 when 0.25≦z≦0.3, and 1.1-1.4 when 0.3≦z≦0.65.

6. The sintered ferrite magnet according to claim 5, wherein the amount of SiO.sub.2 powder added is 1.1-1.6% by mass based on 100% by mass of said ferrite phase.

7. The sintered ferrite magnet according to claim 5, wherein (1-x-y), x, y and z and n meet the relations of
0.35≦(1-x-y)≦0.55,
0.4≦x≦0.6,
0≦y≦0.15,
0.25≦z≦0.4, and
4.5≦n≦6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the relation between the amount of SiO.sub.2 added and a residual magnetic flux density B.sub.r in the sintered ferrite magnet of Example 1.

(2) FIG. 2 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 1.

(3) FIG. 3 is a graph showing the relation between the amount of SiO.sub.2 added and a squareness ratio H.sub.k/H.sub.cJ in the sintered ferrite magnet of Example 1.

(4) FIG. 4 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 1.

(5) FIG. 5 is a graph showing the relation between the amount of CaO added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 1.

(6) FIG. 6 is a graph showing the relation between the amount of SiO.sub.2 added and a residual magnetic flux density B.sub.r in the sintered ferrite magnet of Example 2.

(7) FIG. 7 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 2.

(8) FIG. 8 is a graph showing the relation between the amount of SiO.sub.2 added and a squareness ratio H.sub.k/H.sub.cJ in the sintered ferrite magnet of Example 2.

(9) FIG. 9 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 2.

(10) FIG. 10 is a graph showing the relation between the amount of SiO.sub.2 added and a residual magnetic flux density B.sub.r in the sintered ferrite magnets of Examples 1-3.

(11) FIG. 11 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnets of Examples 1-3.

(12) FIG. 12 is a graph showing the relation between the amount of SiO.sub.2 and a squareness ratio H.sub.k/H.sub.cJ in the sintered ferrite magnets of Examples 1-3.

(13) FIG. 13 is a graph showing the relation between CaO/SiO.sub.2 and coercivity H.sub.cJ in the sintered ferrite magnets of Examples 1-3.

(14) FIG. 14 is a graph showing the relation between the amount of SiO.sub.2 added and a residual magnetic flux density B.sub.r in the sintered ferrite magnet of Example 4.

(15) FIG. 15 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 4.

(16) FIG. 16 is a graph showing the relation between the amount of SiO.sub.2 added and a squareness ratio H.sub.k/H.sub.cJ in the sintered ferrite magnet of Example 4.

(17) FIG. 17 is a graph showing the relation between CaO/SiO.sub.2 and coercivity H.sub.cJ in the sintered ferrite magnet of Example 4.

(18) FIG. 18 is a graph showing the relation between the amount of SiO.sub.2 added and a residual magnetic flux density B.sub.r in the sintered ferrite magnet of Example 5.

(19) FIG. 19 is a graph showing the relation between the amount of SiO.sub.2 added and coercivity H.sub.cJ in the sintered ferrite magnet of Example 5.

(20) FIG. 20 is a graph showing the relation between the amount of SiO.sub.2 added and a squareness ratio H.sub.k/H.sub.cJ in the sintered ferrite magnet of Example 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(21) [1] Sintered Ferrite Magnet

(22) The sintered ferrite magnet of the present invention comprises

(23) (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca.sub.1-x-yR.sub.xA.sub.yFe.sub.2n-zCo.sub.z, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:
0.3≦(1-x-y)≦0.65,
0.2≦x≦0.65,
0≦y≦0.2,
0.03≦z≦0.65, and
4≦n≦7; and

(24) (b) a grain boundary phase indispensably containing Si, the amount of the above Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO.sub.2), based on the entire sintered ferrite magnet.

(25) The Ca content (1-x-y) is 0.3≦(1-x-y)≦0.65. When Ca is less than 0.3, the amounts of the elements R and A are relatively large, undesirably resulting in low B.sub.r and H.sub.k/H.sub.cJ. When Ca is more than 0.65, the amounts of the elements R and A are relatively small, undesirably resulting in low B.sub.r and H.sub.k/H.sub.cJ. The range of (1-x-y) is preferably 0.35≦(1-x-y)≦0.55, more preferably 0.42≦(1-x-y)≦0.5.

(26) The element R is at least one of rare earth elements, indispensably including La. To obtain high magnetic properties, the percentage of La in the element R is preferably 50 atomic % or more, more preferably 70 atomic % or more, most preferably La alone except for inevitable impurities. because La is most dissolvable in the M phase among the elements R, the larger percentage of La provides larger effects of improving magnetic properties. The amount (x) of the element R is 0.2≦x≦0.65. When x is less than 0.2 or more than 0.65, low B.sub.r and H.sub.k/H.sub.cJ are provided. The range of x is preferably 0.4≦x≦0.6, more preferably 0.45≦x≦0.55.

(27) The element A is Ba and/or Sr. The amount (y) of the element A is 0≦y≦0.2. Although the effects of the present invention would not be deteriorated even if the element A were not added, the addition of the element A makes crystals finer in the calcined body with smaller aspect ratios, resulting in improved H.sub.cJ. The range of y is 0≦y≦0.15, more preferably 0≦y≦0.08.

(28) The amount (z) of Co is 0.03≦z≦0.65. When z is less than 0.03, the addition of Co does not provide effects of improving magnetic properties. Also, because unreacted α-Fe.sub.2O.sub.3 remains in the calcined body, a slurry tends to leak from a mold cavity during wet molding. When z is more than 0.65, undesired phases containing a large amount of Co are formed, resulting in largely decreased magnetic properties. The range of z is preferably 0.1≦z≦0.4, more preferably 0.2≦z≦0.3.

(29) Part of Co may be substituted by at least one of Zn, Ni and Mn. Particularly, the partial substitution of Co with Ni and Mn reduces production cost without lowering the magnetic properties. Also, the partial substitution of Co with Zn improves B.sub.r, despite slightly reduced H.sub.cJ. The total amount of substituting Zn, Ni and Mn is preferably 50 mol % or less of Co.

(30) In the Ca—La—Co ferrite, increase in the amounts of Co and La essentially results in improvement in H.sub.cJ. However, because Co and La are rare metals, which are rare and expensive, their contents are desirably as small as possible for the purposes of reducing resource consumption and lowering the prices of sintered ferrite magnets. Because H.sub.cJ can be extremely improved while maintaining high B.sub.r and squareness ratio H.sub.k/H.sub.cJ by the present invention, the amounts of Co and La can be reduced to provide magnets having the same H.sub.cJ as that of the conventional sintered Ca—La—Co ferrite magnets. It makes it possible to obtain H.sub.cJ on a practical level even in compositions containing Co at an atomic ratio of 0.2, whose practical use has been difficult because high H.sub.cJ cannot be expected, thereby providing inexpensive, high-performance sintered ferrite magnets containing reduced amounts of Co and La.

(31) The value of n represents a molar ratio of (Fe+Co) to (Ca+R+A), 2n=(Fe+Co)/(Ca+R+A). The molar ratio n is preferably 4≦n≦7. When n is less than 4, the magnet contains a large percentage of non-magnetic portions with excessively flat calcined particles, resulting in drastically reduced H.sub.cJ. When n is more than 7, the unreacted α-Fe.sub.2O.sub.3 remains in the calcined body, resulting in the leakage of a slurry from a mold cavity during wet molding. The range of n is preferably 4.5≦n≦6, more preferably 4.8≦z≦5.2.

(32) Si is an indispensable element, and contained in an amount of more than 1% by mass and 1.8% or less by mass (calculated as SiO.sub.2) based on the entire magnet. Si is preferably added in the form of SiO.sub.2 to the calcined ferrite, and the amount of SiO.sub.2 added is preferably more than 1% by mass and 1.8% or less by mass based on 100% by mass of the calcined body. SiO.sub.2 added to the calcined body essentially remains in the sintered magnet, too, without change. It should be noted, however, that some of SiO.sub.2 may flow out in the pulverization step and the molding step, resulting in a smaller amount in the sintered magnet than in the calcined body. Si essentially forms the grain boundary phase, without being contained in the ferrite phase having a hexagonal M-type magnetoplumbite structure. The preferred range of Si is 1.1-1.6% by mass (calculated as SiO.sub.2).

(33) A molar ratio range x/z of the element R to Co is preferably 0.73≦x/z≦15.62, more preferably 1≦x/z≦3, most preferably 1.2≦x/z≦2. By selecting the composition meeting these values, the magnetic properties can be improved.

(34) In the present invention, large effects of improving magnetic properties are obtained when the amount of the element R>the amount of Co>the amount of the element A, namely, x>z>y. Also, when the amount of Ca>the amount of the element A, namely, (1-x-y)>y, large effects of improving magnetic properties are obtained.

(35) The sintered ferrite magnet of the present invention, and the calcined body used for producing it have a ferrite phase having a hexagonal M-type magnetoplumbite structure. The term “having a hexagonal M-type magnetoplumbite structure” used herein means that the X-ray diffraction pattern of the hexagonal M-type magnetoplumbite structure is mainly observed when the X-ray diffraction of the sintered ferrite magnet or the calcined body is measured under general conditions. There may be undesired phases (orthoferrite phase, spinel phase, etc.) and impurity phases in extremely small amounts (about 5% or less by mass) observed by X-ray diffraction, etc. The quantitative measurement of undesired phases by X-ray diffraction can be conducted by a Rietveld analysis method.

(36) The sintered ferrite magnet of the present invention indispensably comprises the ferrite phase having a hexagonal M-type magnetoplumbite structure, and the grain boundary phase indispensably containing Si. Because the grain boundary phase cannot easily be observed by an X-ray diffraction pattern, it is preferably confirmed by a transmission electron microscope, etc.

(37) In the present invention, both the calcined ferrite and the sintered ferrite magnet have the same ferrite phase, which has a hexagonal M-type magnetoplumbite structure. The calcined ferrite preferably contains the ferrite phase as a main component for the orientation of crystals in the molding and structure control in the sintering, the existence of the grain boundary phase being not critical. On the other hand, the sintered ferrite magnet contains the ferrite phase as a main component, and needs the grain boundary phase indispensably containing Si for structure control and densification in the sintering process.

(38) With the above preferred ranges, the sintered ferrite magnet of the present invention has magnetic properties such as coercivity H.sub.cJ of 450 kA/m or more, a residual magnetic flux density B.sub.r of 0.4 T or more, and a squareness ratio H.sub.k/H.sub.cJ of 80% or more. With more preferred ranges, it has magnetic properties such as coercivity H.sub.cJ of 460 kA/m or more, a residual magnetic flux density B.sub.r of 0.44 T or more, and a squareness ratio H.sub.k/H.sub.cJ of 80% or more.

(39) [2] Production Method of Sintered Ferrite Magnet

(40) The sintered ferrite magnet is produced by a step of preparing calcined ferrite, a step of pulverizing the calcined ferrite to powder, a step of molding the powder to green body, and a step of sintering the green body to a sintered body. Before the pulverization step, more than 1% by mass and 1.8% or less by mass of SiO.sub.2 can be added to 100% by mass of the calcined body to provide the sintered magnet with remarkably improved coercivity H.sub.cJ.

(41) The calcined ferrite comprises a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, the element R which is at least one of rare earth elements and indispensably includes La, the element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca.sub.1-x-yR.sub.xA.sub.yFe.sub.2n-zCo.sub.z, wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations: 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7.

(42) The composition of the calcined ferrite including 0 (oxygen) is represented by the general formula of Ca.sub.1-x-yR.sub.xA.sub.yFe.sub.2n-zCo.sub.zO.sub.α, wherein (1-x-y), x, y, z, α represents the atomic ratios of Ca, the element R, the element A, Co and O, and n represents a molar ratio, meeting 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7, and α=19 at a stoichiometric composition ratio in which the element R and Fe are trivalent, Co is bivalent, x=z, and n=6.

(43) In the composition of the above calcined ferrite including O (oxygen), the mole ratio of oxygen may differ depending on the valences of Fe and Co, the value of n, and the kind of the element R. Also, in the sintered ferrite magnet, a ratio of oxygen to the metal element may change depending on the vacancy of oxygen when sintered in a reducing atmosphere, the change of the valences of Fe and Co in the ferrite phase, etc. Accordingly, the actual molar ratio α of oxygen may be deviated from 19. Therefore, the composition of metal elements, which are most easily identified, are used in the present invention.

(44) (1) Step of Preparing Calcined Ferrite

(45) The calcined ferrite is produced by mixing oxide powders and powders of compounds such as Ca compounds, compounds of the elements R, Ba and/or Sr compounds, if necessary, iron compounds, and Co compounds, which are converted to oxides by calcining, to the above composition; and calcining (ferritizing) the resultant mixture. The composition ranges of Ca, the element R, the element A, Fe and Co are restricted for the same reasons as in the case of the sintered ferrite magnet.

(46) Usable as the starting material powders are powders of metals in the form of oxides, carbonates, hydroxides, nitrates, chlorides, etc., regardless of their valences. Solutions of starting material powders may also be used. Used as the Ca compounds are carbonates, oxides, chlorides, etc. of Ca. Used as the compounds of the elements R are oxides such as La.sub.2O.sub.3, hydroxides such as La(OH).sub.3, carbonates such as La.sub.2(CO.sub.3).sub.3.8H.sub.2O, etc. Particularly, the oxides, hydroxides, carbonates, etc. of mixed rare earth elements (La, Nd, Pr, Ce, etc.) are preferable because they are inexpensive, enabling cost reduction. Used as the compounds of the elements A are carbonates, oxides, chlorides, etc. of Ba and/or Sr. Used as the iron compounds are iron oxides, iron hydroxides, iron chlorides, mill scales, etc. Used as the Co compounds are oxides such as CoO, Co.sub.3O.sub.4, etc., hydroxides such as CoOOH, Co(OH).sub.2, Co.sub.3O.sub.4.m.sub.1H.sub.2O (m.sub.1 is a positive number), etc., carbonates such as CoCO.sub.3, and basic carbonates such as m.sub.2CoCO.sub.3.m.sub.3Co(OH).sub.2.m.sub.4H.sub.2O, etc. (m.sub.2, m.sub.3 and m.sub.4 are positive numbers).

(47) Other starting material powders than CaCO.sub.3, Fe.sub.2O.sub.3 and La.sub.2O.sub.3 may be added at the time of mixing starting materials or after the calcination. For example, the sintered ferrite magnet can be produced, (1) by mixing CaCO.sub.3, Fe.sub.2O.sub.3, La.sub.2O.sub.3 and Co.sub.3O.sub.4, calcining the resultant mixture, and pulverizing, molding and sintering the calcined body, or (2) by mixing CaCO.sub.3, Fe.sub.2O.sub.3 and La.sub.2O.sub.3, calcining the resultant mixture, adding Co.sub.3O.sub.4 to the calcined body, and pulverizing, molding and sintering the calcined body.

(48) To accelerate a reaction during calcining, about 1% or less by mass of a B-containing compound such as B.sub.2O.sub.3, H.sub.3BO.sub.3, etc. may be added, if necessary. Particularly, the addition of H.sub.3BO.sub.3 is effective for improving H.sub.cJ and B.sub.r. The amount of H.sub.3BO.sub.3 added is more preferably 0.3% or less by mass, most preferably about 0.2% by mass. Less than 0.1% by mass of H.sub.3BO.sub.3 provides only a small effect of improving B.sub.r, and more than 0.3% by mass of H.sub.3BO.sub.3 reduces B.sub.r. Because H.sub.3BO.sub.3 has effects of controlling the shapes and sizes of crystal grains during sintering, it may be added after calcining (before fine pulverization or before sintering), or both before and after calcining.

(49) The mixing of the starting material powders may be conducted in a wet or dry state. When stirred with media such as steel balls, etc., the starting material powders can be mixed more uniformly. In the case of wet mixing, the solvent is preferably water. To increase the dispersibility of the starting material powders, known dispersants such as ammonium polycarboxylate, calcium gluconate, etc. may be used. A slurry of the starting materials is dewatered to obtain mixed starting material powders.

(50) The mixed starting material powders are heated in an electric furnace, a gas furnace, etc. to cause a solid-phase reaction, thereby forming a ferrite compound having a hexagonal M-type magnetoplumbite structure. This process is called “calcining,” and the resultant compound is called “calcined body.”

(51) The calcining step is conducted preferably in an atmosphere having an oxygen concentration of 5% or more. The oxygen concentration of less than 5% results in abnormal grain growth, the formation of undesired phases, etc. More preferably, the oxygen concentration is 20% or more.

(52) In the calcining step, the solid-phase reaction forming the ferrite phase proceeds as the temperature is elevated, and is completed at about 1100° C. When the calcining temperature is lower than 1100° C., the unreacted hematite (iron oxide) remains, resulting in low magnetic properties. On the other hand, when the calcining temperature is higher than 1450° C., crystal grains grow excessively, likely needing a long period of time for the pulverization step. Accordingly, the calcining temperature is preferably 1100-1450° C., more preferably 1200-1350° C. The calcining time is preferably 0.5-5 hours.

(53) When H.sub.3BO.sub.3 is added before calcining, the ferritization reaction is accelerated, enabling calcining at 1100-1300° C.

(54) (2) Addition of SiO.sub.2

(55) The production method of the present invention is characterized by adding more than 1% by mass and 1.8% or less by mass of SiO.sub.2 to 100% by mass of the calcined body before the pulverization step. This specifically improves H.sub.cJ. The addition of 1% or less by mass of SiO.sub.2 fails to obtain an effect of improving H.sub.cJ, and the addition of more than 1.8% by mass of SiO.sub.2 undesirably reduces H.sub.cJ, B.sub.r and H.sub.k/H.sub.cJ. The more preferred amount of SiO.sub.2 added is 1.1-1.6% by mass.

(56) Though SiO.sub.2 is most preferably added to the calcined body, part of SiO.sub.2 may be added before calcining (when mixing the starting material powders). With SiO.sub.2 added before calcining, the size control of crystal grains can be conducted in calcining.

(57) (3) Addition of CaCO.sub.3

(58) Depending on the amount of SiO.sub.2 added, 1-2% by mass (calculated as CaO) of CaCO.sub.3 is preferably added to 100% by mass of the calcined body before the pulverization step. The addition of CaCO.sub.3 prevents the decrease of B.sub.r and H.sub.k/H.sub.cJ as much as possible, resulting in sintered ferrite magnets having high H.sub.cJ with high B.sub.r and H.sub.k/H.sub.cJ maintained, which have not been achieved conventionally. When the amount of CaCO.sub.3 (calculated as CaO) is less than 1% by mass or more than 2% by mass, B.sub.r and H.sub.k/H.sub.cJ are undesirably low. The amount of CaCO.sub.3 added is more preferably 1.2-2% by mass.

(59) The preferred amount of SiO.sub.2 added may slightly change depending on the amount of CaCO.sub.3 (calculated as CaO). As shown in Examples below, it slightly changes also depending on the amount (z) of Co. For example, from the aspect of improving H.sub.cJ, increase in the amount of CaCO.sub.3 added tends to shift the preferred amount of SiO.sub.2 to a higher side, regardless of the amount (z) of Co. Also, decrease in the amount of Co tends to shift the preferred amount of SiO.sub.2 to a higher side. However, too much SiO.sub.2 reduces B.sub.r and H.sub.k/H.sub.cJ. To achieve high H.sub.cJ while maintaining high B.sub.r and H.sub.k/H.sub.cJ, it is preferable to add SiO.sub.2 in an amount of 1.1-1.5% by mass and CaCO.sub.3 in an amount of 1.2-2% by mass (calculated as CaO) when the amount (z) of Co is z≧0.3, and to add SiO.sub.2 in an amount of 1.4-1.6% by mass and CaCO.sub.3 in an amount of 1.5-2% by mass (calculated as CaO) when the amount (z) of Co is z<0.3. Considering both cases, as described above, the amount of SiO.sub.2 added is preferably 1.1-1.6% by mass. In this case, the amount (calculated as CaO) of CaCO.sub.3 added is preferably 1.2-2% by mass.

(60) In the present invention, when both SiO.sub.2 and CaCO.sub.3 are added, the amounts of SiO.sub.2 and CaCO.sub.3 may be properly determined within the above ranges. As described above, it is preferable to add SiO.sub.2 in a range of 1.1-1.6% by mass and CaCO.sub.3 in a range of 1.2-2% by mass (calculated as CaO), providing sintered ferrite magnets with high H.sub.cJ, with high B.sub.r and H.sub.k/H.sub.cJ maintained.

(61) When both SiO.sub.2 and CaCO.sub.3 are added, the magnetic properties can be improved by adjusting a ratio of the amount (calculated as CaO) of CaCO.sub.3/the amount of SiO.sub.2 to 0.8-2. In this case, the preferred range of [CaCO.sub.3 (as CaO)/SiO.sub.2] slightly changes depending on the amount (z) of Co as shown in Examples below. When Z≧0.3, the ratio is preferably 1-1.7, more preferably 1.1-1.4. When z<0.3, the ratio is preferably 0.8-1.4, more preferably 0.9-1.1. Considering both cases of z≧0.3 and z<0.3, the ratio is preferably 0.8-1.7, more preferably 0.9-1.4. With [CaCO.sub.3 (as CaO)/SiO.sub.2] set in the above range, the sintered ferrite magnets has high H.sub.cJ, with high B.sub.r and H.sub.k/H.sub.cJ maintained.

(62) (4) Pulverization Step

(63) The calcined body is pulverized by a vibration mill, a ball mill, an attritor, etc. to powder. The pulverized powder preferably has an average particle size of about 0.4-0.8 μm (measured by an air permeation method). The pulverization step may be either dry pulverization or wet pulverization, though their combination is preferable.

(64) The wet pulverization uses water and/or non-aqueous solvents (organic solvents such as acetone, ethanol, xylene, etc.). The wet pulverization produces a slurry containing the calcined body in water (solvent). The slurry preferably contains known dispersant and/or surfactant in an amount of 0.2-2% by mass on a solid basis. After the wet pulverization, the slurry is preferably concentrated and kneaded.

(65) In the pulverization step, Cr.sub.2O.sub.3, Al.sub.2O.sub.3, etc. may be added together with SiO.sub.2 and CaCO.sub.3 described above to improve the magnetic properties. Each of them is preferably 5% or less by mass.

(66) Because the pulverized powder contains ultra-fine powder of less than 0.1 μm, which cause poor dewatering and molding defects, the pulverized powder is preferably heat-treated to remove the ultra-fine powder. The heat-treated powder is preferably pulverized again. Thus, using a pulverization step comprising a first fine pulverization step, a step of heat-treating powder obtained by the first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again, which is called “heat-treating, repulverizing step,” H.sub.cJ can be further improved in addition to the effect of improving H.sub.cJ by the addition of SiO.sub.2 and CaCO.sub.3, thereby providing sintered ferrite magnets with extremely high H.sub.cJ, with high B.sub.r and H.sub.k/H.sub.cJ maintained, which have not been obtained so far.

(67) Ultra-fine powder of less than 0.1 μm is likely to be inevitably formed in a usual pulverization step, and the existence of ultra-fine powder lowers H.sub.cJ, results in poor dewatering in the molding step, provides green bodies with defects, and deteriorates pressing cycles because too much time is needed for dewatering. When the powder containing ultra-fine powder, which is obtained by the first fine pulverization step, is heat-treated, a reaction occurs between powder having relatively large particle sizes and the ultra-fine powder, resulting in decrease in the amount of the ultra-fine powder. In the second fine pulverization step, the particle sizes are controlled with necking removed, to produce powder with predetermined particle sizes. Thus, powder having an excellent particle size distribution with a small percentage of ultra-fine powder can be obtained, thereby improving H.sub.cJ, and thus solving the above problems in the molding step.

(68) Utilizing the effect of improving H.sub.cJ by the heat-treating, repulverizing step, the same H.sub.cJ as when using powder having an average particle size of about 0.4-0.8 μm, which is produced by a usual pulverization step, is obtained, even if powder obtained by the second fine pulverization step has relatively large particle sizes (for example, an average particle size of about 0.8-1.0 μm). Accordingly, time saving can be achieved by the second fine pulverization step, with improved dewatering and pressing cycle.

(69) As described above, though the heat-treating, repulverizing step provides various advantages, cost increase due to increased production steps cannot be avoided. However, magnetic properties are much more improved by the heat-treating, repulverizing step in the production of the sintered ferrite magnet of the present invention than in the production of conventional sintered ferrite magnets, canceling the above cost increase. Accordingly, the heat-treating, repulverizing step is practically significant in the present invention.

(70) The first fine pulverization may be the same as the above-described usual pulverization, using a vibration mill, a jet mill, a ball mill, an attritor, etc. The pulverized powder preferably has an average particle size of about 0.4-0.8 μm (measured by an air permeation method). The pulverization step may be either dry pulverization or wet pulverization, though their combination is preferable.

(71) After the first fine pulverization step, a heat treatment is conducted preferably at 600-1200° C., more preferably 800-1100° C. Though not particularly restricted, the heat treatment time is preferably 1 second to 100 hours, more preferably about 1-10 hours.

(72) The second fine pulverization after the heat treatment step uses a vibration mill, a jet mill, a ball mill, an attritor, etc., like the first fine pulverization. Because the desired particle sizes are already substantially obtained by the first fine pulverization step, the control of particle sizes and the removal of necking are mainly conducted in the second fine pulverization step. Accordingly, milder pulverization conditions due to the shortened pulverization time, etc. are preferably used in the second fine pulverization step than in the first fine pulverization step. Pulverization under the same conditions as in the first fine pulverization step undesirably forms ultra-fine powder again.

(73) The average particle size of the powder after the second fine pulverization is preferably about 0.4-0.8 μm (measured by an air permeation method) as in a usual pulverization step when seeking higher H.sub.cJ than that of sintered ferrite magnets obtained by the usual pulverization step, and preferably 0.8-1.2 μm, more preferably about 0.8-1.0 μm (measured by an air permeation method) when seeking the saving of time in the pulverization step, improving dewatering and a pressing cycle, etc.

(74) (5) Molding Step

(75) A slurry after the pulverization is press-molded with or without a magnetic field while removing water (solvent). Press-molding in a magnetic field aligns the crystal orientation of powder particles, thereby drastically improving the magnetic properties. To improve the orientation further, a dispersant and a lubricant may be added in an amount of 0.01-1% by mass. Also, the slurry may be concentrated before molding, if necessary. The concentration is preferably conducted by centrifugal separation, filter pressing, etc.

(76) (6) Sintering Step

(77) The press-molded green body is degreased if necessary, and then sintered. The sintering is conducted in an electric furnace, a gas furnace, etc.

(78) The sintering is conducted preferably in an atmosphere having an oxygen concentration of 10% or more. When the oxygen concentration is less than 10%, abnormal grain growth, the formation of undesired phases, etc. occur, resulting in deteriorated magnetic properties. The oxygen concentration is more preferably 20% or more, most preferably 100%.

(79) The sintering temperature is preferably 1150-1250° C. The sintering time is preferably 0.5-2 hours. The sintered magnet has an average crystal particle size of about 0.5-2 μm.

(80) After the sintering step, the sintered ferrite magnet is obtained through known production steps such as a machining step, a washing step, an inspection step, etc.

(81) The present invention will be explained in more detail referring to Examples without intention of restricting it thereto.

EXAMPLE 1

(82) CaCO.sub.3 powder, La(OH).sub.3 powder, Fe.sub.2O.sub.3 powder and Co.sub.3O.sub.4 powder were mixed to have the formula of Ca.sub.1-x-yLa.sub.xA.sub.yFe.sub.2n-zCO.sub.3O.sub.19-δ, wherein x=0.5, y=0, z=0.3, n=5.2, and δ≧0, and 0.1% by mass of H.sub.3BO.sub.3 powder was added to the total amount (100% by mass) of the mixed powders to produce a starting material powder. This starting material powder was mixed by a wet ball mill for 4 hours, and granulated by drying. It was calcined at 1300° C. for 3 hours in the air, and the calcined body was coarsely pulverized by a hammer mill to obtain coarse powder.

(83) SiO.sub.2 powder and CaCO.sub.3 powder (calculated as CaO) were added in the amounts shown in Table 1 to the above coarse powder, and fine pulverization was conducted by a wet ball mill using water as a solvent until its average particle size (measured by an air permeation method) became 0.55 μm. Each of the resultant fine powder slurries was molded in a magnetic field of about 1.3 T under pressure of about 50 MPa with a compression direction parallel to the direction of the magnetic field, while removing the solvent. Each of the resultant green bodies was sintered at 1200° C. for 1 hour in the air to obtain a sintered magnet.

(84) Each sintered magnet was measured with respect to a residual magnetic flux density B.sub.r, coercivity H.sub.cJ, and a squareness ratio H.sub.k/H.sub.cJ, wherein H.sub.k was the value of H at J of 0.95B, on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in FIGS. 1-3. In FIGS. 1-3, the axes of abscissas represent the amount (% by mass) of SiO.sub.2 added, and the axes of ordinates represent a residual magnetic flux density B.sub.r (T) (FIG. 1), coercivity H.sub.cJ (FIG. 2) and a squareness ratio H.sub.k/H.sub.cJ (FIG. 3). In the figures, data obtained with the same amount of CaO were connected by straight lines. A dotted line at H.sub.k/H.sub.cJ=80% in FIG. 3 is a line as a measure for practical use. Even if a sintered magnet had high B.sub.r and H.sub.cJ, it would not be able to be made thin when its H.sub.k/H.sub.cJ is less than 80%, substantially failing to be useful for various applications such as electronic parts for automobiles, parts for electric appliances, etc. The same is true of Examples below.

(85) TABLE-US-00001 TABLE 1 Sample CaO SiO.sub.2 No. (% by mass) (% by mass) CaO/SiO.sub.2 101 0.7 0.0 — 102 0.7 0.3 2.33 103 0.7 0.6 1.17 104 1.0 0.0 — 105 1.0 0.3 3.33 106 1.0 0.6 1.67 107 1.0 0.9 1.11 108* 1.0 1.2 0.83 109* 1.3 1.1 1.18 110 1.5 0.3 5.00 111 1.5 0.6 2.50 112 1.5 0.9 1.67 113* 1.5 1.2 1.25 114* 1.5 1.5 1.00 115 1.5 1.9 0.80 116 2.0 0.6 3.33 117 2.0 0.9 2.22 118* 2.0 1.2 1.67 119* 2.0 1.5 1.33 120* 2.0 1.8 1.11 Note: *Within the present invention.

(86) As is clear from FIGS. 1-3, high magnetic properties are obtained when more than 1% by mass of SiO.sub.2 and 1% or more by mass of CaO are added, and higher magnetic properties are obtained when 1.1-1.5% by mass of SiO.sub.2 and when 1.3-2% by mass of CaO are added. Particularly, Sample 113 to which 1.2% by mass of SiO.sub.2 and 1.5% by mass of CaO were added, and Sample 119 to which 1.5% by mass of SiO.sub.2 and 2.0% by mass of CaO were added had specifically improved H.sub.cJ with maximally suppressed decrease in B.sub.r and KAU.

(87) FIG. 4 compares Samples 113 and 119, and samples containing 1% or less by mass of SiO.sub.2, which are shown in FIG. 2. It is clear that as compared with samples to which 1% or less by mass of SiO.sub.2 was added, which have conventionally been considered optimum, Sample 113 to which 1.2% by mass of SiO.sub.2 and 1.5% by mass of CaO were added, and Sample 119 to which 1.5% by mass of SiO.sub.2 and 2.0% by mass of CaO were added had H.sub.cJ improved by about 20% or more.

(88) As described above, because Ca—La—Co ferrite in which the atomic ratio of Co was 0.3 had an anisotropic magnetic field H.sub.A of 2.1 MA/m, the H.sub.cJ (400 kA/m or less) of the samples containing 1% or less by mass of SiO.sub.2 was about 19% or less of the above anisotropic magnetic field H.sub.A, but the H.sub.cJ of Samples 113 and 119 was about 23% of the above anisotropic magnetic field H.sub.A, closer to the inherent potential of the material. Further, taking into consideration Samples 113 and 119 having CaO/SiO.sub.2 of 1.25 and 1.33, and Sample 109 having better magnetic properties, it has been found that excellent magnetic properties can be obtained at a ratio of CaO/SiO.sub.2 in a range of about 1.1-1.4.

(89) FIG. 5 compares Sample 103 (CaO=0.7% by mass, and SiO.sub.2=0.6% by mass), Sample 107 (CaO=1.0% by mass, and SiO.sub.2=0.9% by mass), Sample 109 (CaO=1.3% by mass, and SiO.sub.2=1.1% by mass), Sample 113 (CaO=1.5% by mass, and SiO.sub.2=1.2% by mass), and Sample 119 (CaO=2.0% by mass, and SiO.sub.2=1.5% by mass), which exhibit the highest H.sub.cJ at the CaO content of 0.7% by mass, 1.0% by mass, 1.3% by mass, 1.5% by mass, and 2.0% by mass, respectively, in FIG. 2. In FIG. 5, the axis of abscissas represents the amount (% by mass) of CaO added, and the axis of ordinates represents coercivity H.sub.cJ. As is clear from FIG. 5, as the amount of CaO added increases, the coercivity H.sub.cJ is improved. Particularly when the amount of CaO added was 1.5% by mass and 2% by mass, the highest coercivity H.sub.cJ was obtained. Also, when the amount of CaO added was in a range of 1.2-2% by mass, the coercivity H.sub.cJ was about 20% or more of the anisotropic magnetic field H.sub.A of Ca—La—Co ferrite in which the atomic ratio of Co was 0.3, and when the amount of CaO added was in a range of 1.5-2% by mass, the coercivity H.sub.cJ was about 23% of the above anisotropic magnetic field H.sub.A.

(90) With respect to the sintered magnets of Samples 113 and 114, the amounts of SiO.sub.2 and CaO added and those measured by inductively coupled plasma (ICP) atomic emission spectroscopy are shown in Table 2. The amounts of SiO.sub.2 and CaO added are expressed by “% by mass” determined based on the entire composition.

(91) TABLE-US-00002 TABLE 2 Amount of SiO.sub.2 Amount of CaO Sample (% by mass) (% by mass) No. Measured Added Measured Added 113 1.19 1.17 3.98 4.37 114 1.66 1.46 3.96 4.36

(92) As shown in Table 2, SiO.sub.2 added to the calcined body remained in the sintered magnet. The amount of SiO.sub.2 measured was larger than that added, presumably because the amounts of other elements than SiO.sub.2 became smaller than when added.

EXAMPLE 2

(93) Sintered magnets were produced in the same manner as in Example 1, except that SiO.sub.2 powder and CaCO.sub.3 powder (calculated as CaO) were added in the amounts shown in Table 3 to a composition having the formula of Ca.sub.1-x-yLa.sub.xA.sub.yFe.sub.2n-zCo.sub.zO.sub.19-δ, wherein x=0.5, y=0, z=0.2, n=4.8, and δ≧0.

(94) The sintered magnets were measured with respect to a residual magnetic flux density B.sub.r, coercivity H.sub.cJ, and a squareness ratio H.sub.k/H.sub.cJ. The measurement results are shown in FIGS. 6-8. As in Example 1, in FIGS. 6-8, the axes of abscissas represent the amount (% by mass) of SiO.sub.2 added, and the axes of ordinates represent a residual magnetic flux density B.sub.r (T) (FIG. 6), coercivity H.sub.cJ (FIG. 7), and a squareness ratio H.sub.k/H.sub.cJ (FIG. 8), data at the same CaO content being connected by a straight line.

(95) TABLE-US-00003 TABLE 3 Sample CaO SiO.sub.2 No. (% by mass) (% by mass) CaO/SiO.sub.2 201 0.6 0.2 3.00 202 0.6 0.4 1.50 203 0.6 0.6 1.00 204 0.6 0.8 0.75 205 0.6 1.0 0.60 206 1.0 0.2 5.00 207 1.0 0.4 2.50 208 1.0 0.6 1.67 209 1.0 0.8 1.25 210 1.0 1.0 1.00 211 1.5 1.0 1.50 212* 1.5 1.2 1.25 213* 1.5 1.4 1.07 214* 1.5 1.5 1.00 215* 1.5 1.6 0.94 216* 1.5 1.8 0.83 217 1.5 2.0 0.75 218 2.0 1.0 2.00 219* 2.0 1.5 1.33 220 2.0 2.0 1.00 Note: *Within the present invention.

(96) As is clear from FIGS. 6-8, even in the compositions in which the amount of Co was changed, as in Example 1, high magnetic properties were obtained when more than 1% by mass of SiO.sub.2 and 1% or more by mass of CaO were added, and better magnetic properties were obtained when 1.4-1.6% by mass of SiO.sub.2 and 1.5-2% by mass of CaO were added. Particularly, Sample 215 to which 1.6% by mass of SiO.sub.2 and 1.5% by mass of CaO were added had specifically improved H.sub.cJ, while suppressing decrease in B.sub.r and H.sub.k/H.sub.cJ as much as possible.

(97) FIG. 9 compares Sample 215 to which 1.6% by mass of SiO.sub.2 and 1.5% by mass of CaO were added, and samples to which 1% or less by mass of SiO.sub.2 was added, which are shown in FIG. 7. As is clear from FIG. 9, Sample 215 to which 1.6% by mass of SiO.sub.2 and 1.5% by mass of CaO were added had H.sub.cJ improved by about 27%, as compared with samples containing 1% or less by mass of SiO.sub.2, which have conventionally been considered optimum.

(98) SPD measurement revealed that Ca—La—Co ferrite in which the atomic ratio of Co was 0.2 had an anisotropic magnetic field H.sub.A of 1.9 MA/m (about 23.9 kOe). Accordingly, samples to which 1% or less by mass of SiO.sub.2 was added had H.sub.cJ (300 kA/m or less), which was about 16% or less of the above anisotropic magnetic field H.sub.A, Sample 215 to which 1.6% by mass of SiO.sub.2 and 1.5% by mass of CaO were added had H.sub.cJ, which was about 20% of the above anisotropic magnetic field H.sub.A, close to the inherent potential of the material. Further, CaO/SiO.sub.2 was 0.94 in Sample 215. It was found that excellent magnetic properties were obtained at a CaO/SiO.sub.2 ratio in a range of about 0.9-1.1.

EXAMPLE 3

(99) Sintered magnets were produced in the same manner as in Example 1, except that SiO.sub.2 powder and CaCO.sub.3 powder (calculated as CaO) were added in the amounts shown in Table 4 to a composition having the formula of Ca.sub.1-x-yLa.sub.xA.sub.yFe.sub.2n-zCO.sub.zO.sub.19-δ, wherein x=0.5, y=0, z=0.25, n=5.0, and δ≧0.

(100) The sintered magnets were measured with respect to a residual magnetic flux density B.sub.r, coercivity H.sub.cJ, and a squareness ratio H.sub.k/H.sub.cJ. The measurement results are shown in FIGS. 10-12. As in Example 1, in FIGS. 10-12, the axes of abscissas represent the amount (% by mass) of SiO.sub.2 added, and the axes of ordinates represent a residual magnetic flux density B.sub.r (T) (FIG. 10), coercivity H.sub.cJ (FIG. 11), and a squareness ratio H.sub.k/H.sub.cJ (FIG. 12). To investigate influence by the Co content (z) in FIGS. 10-12, Samples 111-115 in Example 1 and Samples 211-217 in Example 2 were plotted, and data at the same Co content (z) were connected by a straight line.

(101) TABLE-US-00004 TABLE 4 Sample CaO SiO.sub.2 No. (% by mass) (% by mass) CaO/SiO.sub.2 301 1.5 0.9 2.00 302* 1.5 1.2 1.33 303* 1.5 1.5 1.00 304 1.5 1.9 0.78 Note: *Within the present invention.

(102) As is clear from FIGS. 10-12, even at the Co content (z) of 0.25, as in Examples 1 and 2, high magnetic properties were obtained when more than 1% by mass of SiO.sub.2 and 1% or more by mass of CaO were added. Particularly when SiO.sub.2 was 1.5% by mass, H.sub.cJ was specifically improved while suppressing decrease in B.sub.r and H.sub.k/H.sub.cJ as much as possible.

(103) It is also clear from FIGS. 10-12 that the preferred amount of SiO.sub.2 differs at z of 0.3, 0.25 and 0.2, and that as the Co content (z) decreases, the preferred amount of SiO.sub.2 tends to shift toward a higher side. Namely, the amount of SiO.sub.2 providing high H.sub.cJ while maintaining high B.sub.r and H.sub.k/H.sub.cJ was in a range of 1.1-1.2% by mass when the Co content (z) was 0.3, and in a range of 1.4-1.6% by mass when the Co content (z) was 0.25 and 0.2, respectively.

(104) FIG. 13 shows these data, the axis of abscissas representing CaO/SiO.sub.2, and the axis of ordinates representing coercivity H.sub.cJ, and data at the same Co content (z) being connected by a straight line. As is clear from FIG. 13, when both SiO.sub.2 and CaO are added, magnetic properties can be improved by setting CaO/SiO.sub.2 in a range of 0.8-2.0. In this case, it is clear that the preferred range of CaO/SiO.sub.2 slightly differs between when z=0.3 and when z<0.3 (z=0.25, z=0.2). Namely, the range of CaO/SiO.sub.2 for achieving high H.sub.cJ while maintaining high B.sub.r and H.sub.k/H.sub.cJ is preferably 1-1.7, more preferably 1.2-1.4, when z=0.3, and preferably 0.8-1.4, more preferably 0.9-1.1, when z<0.3 (z=0.25, z=0.2). Considering both cases of z≧0.3 and z<0.3, it is preferably 0.8-1.7, more preferably 0.9-1.4.

(105) It is clear from FIG. 11 that a lower Co content (z) tends to provide a lower maximum value of H.sub.cJ. However, as considered in Examples 1 and 2, H.sub.cJ was improved by 20% or more in the same Co content (z), as compared with samples to which 1% or less by mass of SiO.sub.2 (conventionally considered optimum) was added, the improvement of H.sub.cJ being 4% or more of the anisotropic magnetic field H.sub.A. It was thus confirmed that the sintered magnets had H.sub.cJ close to potential inherent in the materials. Accordingly, when magnets having the H.sub.cJ same as that of conventional sintered Ca—La—Co ferrite magnets are provided, the amounts of rare and expensive Co and La can be reduced. As in Example 2, even for compositions having a Co content z of 0.2, which are not expected to have high H.sub.cJ so that it has been difficult to put them into practical use, H.sub.cJ on a practical level can be obtained, thereby providing high-performance sintered ferrite magnets with reduced amounts of Co and La at a low cost.

EXAMPLE 4

(106) Sintered magnets were produced in the same manner as in Example 1, except that CaCO.sub.3 powder, La(OH).sub.3 powder, SrCO.sub.3 powder, BaCO.sub.3 powder, Fe.sub.2O.sub.3 powder and Co.sub.3O.sub.4 powder were mixed to provide the formula of Ca.sub.1-x-y1-y2La.sub.xSr.sub.y1Ba.sub.y2Fe.sub.2n-zCo.sub.zO.sub.19-δ wherein x=0.5, y1+y2=0.05, z=0.3, n=5.3, and δ≧0, and that the amounts of SiO.sub.2 powder and CaCO.sub.3 powder (calculated as CaO) relative to the total amount of these mixed powders were changed as shown in Table 5. The atomic ratios of Sr (y1) and Ba (y2) are also shown in Table 5. Samples 111-115 are sintered magnets evaluated in Example 1.

(107) The sintered magnets were measured with respect to a residual magnetic flux density B.sub.r, coercivity H.sub.cJ and a squareness ratio H.sub.k/H.sub.cJ, wherein H.sub.k is the value of H, at which J is 0.95B.sub.r, on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in FIGS. 14-16. In FIGS. 14-16, the axes of abscissas represent the amount (% by mass) of SiO.sub.2 added, and the axes of ordinates represent a residual magnetic flux density B.sub.r (T) (FIG. 14), coercivity H.sub.cJ (FIG. 15) and a squareness ratio H.sub.k/H.sub.cJ (FIG. 16), respectively. In the figures, data connected by a straight line are denoted by “Unsubstituted” representing Samples 111-115, “Ba” representing Samples 401-404 to which only Ba was added, “Sr” representing Samples 405-408 to which only Sr was added, and “Sr+Ba” representing Samples 409-412 to which both Sr and Ba were added.

(108) TABLE-US-00005 TABLE 5 Atomic Atomic Sample Ratio Ratio CaO SiO.sub.2 No. of Sr (y1) of Ba (y2) (% by mass) (% by mass) CaO/SiO.sub.2 401 0 0.05 1.5 0.6 2.50 402 0 0.05 1.5 0.9 1.67 403* 0 0.05 1.5 1.2 1.25 404* 0 0.05 1.5 1.5 1.00 405 0.05 0 1.5 0.9 1.67 406* 0.05 0 1.5 1.2 1.25 407* 0.05 0 1.5 1.5 1.00 408 0.05 0 1.5 1.9 0.80 409 0.025 0.025 1.5 0.9 1.67 410* 0.025 0.025 1.5 1.2 1.25 411* 0.025 0.025 1.5 1.5 1.00 412 0.025 0.025 1.5 1.9 0.80 111 0 0 1.5 0.6 2.50 112 0 0 1.5 0.9 1.67 113* 0 0 1.5 1.2 1.25 114* 0 0 1.5 1.5 1.00 115 0 0 1.5 1.9 0.80 Note: *Within the present invention.

(109) As is clear from FIGS. 14-16, the compositions to which Sr and/or Ba were added also had high magnetic properties when more than 1% by mass of SiO.sub.2 and 1% or more by mass of CaO were added, as in Examples 1-3 not containing Sr and Ba.

(110) FIG. 17 is a graph showing these data plotted with the axis of abscissas being CaO/SiO.sub.2, and the axis of ordinates being coercivity H.sub.cJ. As is clear from FIG. 17, the compositions to which Sr and/or Ba were added also had high magnetic properties at a CaO/SiO.sub.2 ratio of 0.8-2.0 when both SiO.sub.2 and CaO were added, as in Examples 1-3 not containing Sr and Ba.

EXAMPLE 5

(111) Sintered magnets were produced in the same manner as in Example 1, except that SrCO.sub.3 powder, La(OH).sub.3 powder, Fe.sub.2O.sub.3 powder and Co.sub.3O.sub.4 powder were mixed to have the formula of Sr.sub.1-xLa.sub.xFe.sub.2n-zCO.sub.zO.sub.19-δ, wherein x=0.2, z=0.2, n=5.8, and δ≧0, and that the amounts of SiO.sub.2 powder and CaCO.sub.3 powder (calculated as CaO) were changed as shown in Table 6. These sintered magnets are so-called Sr—La—Co ferrite magnets having basic compositions outside the present invention.

(112) The sintered magnets were measured with respect to a residual magnetic flux density B.sub.r, coercivity H.sub.cJ, and a squareness ratio H.sub.k/H.sub.cJ. The measurement results are shown in FIGS. 18-20.

(113) TABLE-US-00006 TABLE 6 Sample CaO SiO.sub.2 No. (% by mass) (% by mass) CaO/SiO.sub.2 501 0.7 0.0 — 502 0.7 0.3 2.33 503 0.7 0.6 1.17 504 1.0 0.0 — 505 1.0 0.3 3.33 506 1.0 0.6 1.67 507 1.0 0.9 1.11 508 1.0 1.2 0.83 509 1.5 0.3 5.00 510 1.5 0.6 2.50 511 1.5 0.9 1.67 512 1.5 1.2 1.25 513 1.5 1.5 1.00

(114) It is clear from FIGS. 18-20 that when more than 1% by mass of SiO.sub.2 was added, Sr—La—Co ferrite magnets do not have good magnetic properties, with H.sub.cJ not extremely improved, and the squareness ratio lowered to less than 80%.

EXAMPLE 6

(115) CaCO.sub.3 powder, La(OH).sub.3 powder, Fe.sub.2O.sub.3 powder and Co.sub.3O.sub.4 powder were mixed to have the formula of Ca.sub.1-x-yLa.sub.xA.sub.yFe.sub.2n-zCO.sub.zO.sub.19-δ, wherein x=0.5, y=0, z=0.3, n=5.2, and δ≧0, and 0.1% by mass of H.sub.3BO.sub.3 powder was added to 100% by mass of the mixed powders to obtain a starting material powder. This starting material powder was mixed for 4 hours by a wet ball mill, and granulated by drying. It was calcined at 1300° C. for 3 hours in the air, and the calcined body was pulverized to coarse powder by a hammer mill.

(116) 1.2% by mass of SiO.sub.2 powder and 1.5% by mass (calculated as CaO) of CaCO.sub.3 powder were added to the coarse powder, and the first fine pulverization was conducted to an average particle size of 0.5 μm (measured by an air permeation method) by a wet ball mill with water as a solvent. The resultant powder was dried, and heat-treated at 1000° C. for 5 hours in the air. The heat-treated powder had an average particle size of 1.4 μm (measured by an air permeation method). The heat-treated powder was subject to the second fine pulverization to an average particle size of 0.8 μm by a wet ball mill. While removing the solvent, a fine powder slurry obtained by the above heat-treating, repulverizing step was molded in a magnetic field of about 1.3 T under pressure of about 50 MPa, with a compression direction parallel to the direction of a magnetic field. The resultant green body was sintered at 1200° C. for 1 hour in the air to obtain a sintered magnet.

(117) In addition, a sintered magnet was produced in the same manner as above, except for conducting only the first fine pulverization to an average particle size of 0.8 μm (measured by an air permeation method) without the heat treatment and the second fine pulverization.

(118) The sintered magnets were measured with respect to a residual magnetic flux density B.sub.r, coercivity H.sub.cJ, and a squareness ratio H.sub.k/H.sub.cJ, wherein H.sub.k was the value of H at J of 0.95B.sub.r on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in Table 7. Table 7 shows Sample 601 pulverized in the heat-treating, repulverizing step, and Sample 602 pulverized only in the first fine pulverization.

(119) TABLE-US-00007 TABLE 7 Sample Br H.sub.cJ Hk/H.sub.cJ No. (T) (kA/m) (%) 601 0.438 494.0 80.1 602 0.436 387.9 80.4

(120) As is clear from Table 7, the use of the heat-treating, repulverizing step drastically improved H.sub.cJ with B.sub.r and H.sub.k/H.sub.cJ unchanged, than the use of only one usual pulverization.

(121) Sample 113 in Example 1 shown in Table 1 was obtained in the same manner as in this Example except for using only one usual pulverization to an average particle size of 0.55 μm, having magnetic properties of H.sub.cJ=476 kA/m, Br=0.443 T, and H.sub.k/H.sub.cJ=81.6% as shown in FIGS. 1-3. Comparison of this sample with Sample 601 in this Example revealed that Sample in this Example had slightly improved H.sub.cJ with B.sub.r and H.sub.k/H.sub.cJ substantially unchanged than the sample (average particle size 0.55 μm) in Example 1, despite the fact that the pulverized powder had a large average particle size (0.8 μm). Namely, even though powder after the second fine pulverization step has relatively large particle sizes, the use of the heat-treating, repulverizing step provides as small H.sub.cJ as when using powder having relatively small particle sizes, which is obtained by a usual pulverization step. Accordingly, the second fine pulverization step can be conducted in a shortened period of time, thereby improving dewatering and pressing cycles.

EFFECT OF THE INVENTION

(122) Because the production method of the present invention can extremely improve H.sub.cJ while maintaining high B.sub.r and squareness ratio H.sub.k/H.sub.cJ, even if the amounts of rare and expensive Co and La indispensable for Ca—La—Co ferrite are reduced than before, economically advantageous Ca—La—Co ferrite magnets can be obtained. Ca—La—Co ferrite magnets obtained by the production method of the present invention do not suffer demagnetization by a demagnetizing field generated by thinning the magnet. Accordingly, the sintered ferrite magnet of the present invention can be used for electronic parts for automobiles and parts for electric appliances, such as small, light-weight, high-performance motors, power generators, speakers, etc.