Patent Description:
A method for producing a ferrite powder for bonded magnet according to the preamble of claim <NUM> and a ferrite powder according to the preamble of claim <NUM> are known from Patent Document <NUM>.

As magnets having strong magnetic force, such as magnets used for small motors for use in audio visual equipment, office automation equipment, automotive electric components and so forth, and magnets used for magnet rolls of copying machines and so forth, ferritic sintered magnets are conventionally used. However, in ferritic sintered magnets, there is a problem in that it is difficult to process them in complicated shapes, in addition to the problems in that the productivity thereof is bad since they may be broken and since it is required to polish them.

For that reason, in recent years, as magnets having strong magnetic force, such as magnets used for small motors for use in office automation equipment, audio visual equipment, automotive electric components and so forth, bonded magnets of rare earth magnets are used. However, there are problems in that the costs of rare earth magnets are higher than about twenty times as high as the costs of ferrite sintered magnets and that rare earth magnets are easy to rust off, so that it is desired to use ferrite bonded magnets in place of ferrite sintered magnets.

As such a ferrite powder for bonded magnet, there is proposed a strontium ferrite particle powder for bonded magnet, which is a magnetoplumbite-type strontium ferrite particle powder which has a composition of (Sr<NUM>-xAx)O·n[(Fe<NUM>-y-zCoyZnz)<NUM>O<NUM>] (wherein A is La, La-Nd, La-Pr or La-Nd-Pr, n = <NUM> to <NUM>, x = <NUM> to <NUM>, y = <NUM> to <NUM>, <NUM> ≦ z < <NUM>), a saturation magnetization value σ s of not less than <NUM> Am<NUM>/kg (<NUM> emu/g) and an average particle diameter of <NUM> to <NUM> µm and which contains <NUM> % or more of plate-shaped particles in a number-based percentage in the strontium ferrite particle powder for bonded magnet (see, Patent Document <NUM>).

Further ferrite powders for a bonded magnet are disclosed in Patent Documents <NUM> and <NUM>.

However, the strontium ferrite particle powder for bonded magnet disclosed in Patent Document <NUM> contains many plate-shaped particles. For that reason, if the particle powder is attempted to be arranged in the magnetic field direction thereof by the magnetic field orientation thereof, each of the plated-particles interferes with the orientation thereof to each other, so that it is difficult to produce a bonded magnet having a high orientation.

It is therefore an object of the present invention to eliminate the aforementioned conventional problems and to provide a ferrite powder for bonded magnet, which can obtain a bonded magnet having a high residual magnetization Br by the magnetic field orientation thereof, and a method for producing the same.

In order to accomplish the aforementioned object, the inventors have diligently studied and found that it is possible to produce a ferrite powder for bonded magnet, which can obtain a bonded magnet having a high residual magnetization Br by the magnetic field orientation thereof, if a complex oxide powder of iron, strontium, lanthanum and cobalt is mixed with iron oxide to be granulated to be fired. Thus, the inventors have made the present invention.

According to the present invention, there is provided a method for producing a ferrite powder for bonded magnet, the method comprising the steps of: mixing a complex oxide powder of iron, strontium, lanthanum and cobalt, with iron oxide to obtain a mixture; granulating the mixture to obtain a granulated mixture; and firing the granulated mixture.

This method for producing a ferrite powder for bonded magnet, preferably further comprises the steps of: coarsely pulverizing a fired substance, which is obtained by firing the granulated mixture, to obtain a coarsely pulverized powder; pulverizing the coarsely pulverized powder to obtain a pulverized powder; and annealing the pulverized powder. According to the invention, the complex oxide powder is obtained by a method comprising the steps of: mixing strontium carbonate, lanthanum oxide, iron oxide and cobalt oxide to obtain a mixture; granulating the mixture to obtain a granulated mixture; firing the granulated mixture at a temperature of <NUM> to <NUM> to obtain a fired substance; and pulverizing the fired substance. The firing after granulating the mixture of the complex oxide powder with the iron oxide is preferably carried out at a temperature of <NUM> to <NUM>. The complex oxide powder is preferably mixed with the iron oxide so that a molar ratio (Fe / (Sr + La)) of Fe in the iron oxide to the total of Sr and La is in the range of from <NUM> to <NUM>, when the complex oxide powder is mixed with the iron oxide.

According to the present invention, there is provided a ferrite powder for bonded magnet, which has a composition of (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z (wherein <NUM> < x ≦ <NUM>, <NUM> < y ≦ <NUM>, <NUM> ≦ n ≦ <NUM>, -<NUM> ≦ z ≦ <NUM>) and which has an average particle diameter of <NUM> to <NUM> µm.

This ferrite powder for bonded magnet preferably has a specific surface area of <NUM> to <NUM><NUM>/g. The average value of ratios is not greater than <NUM>, each of the ratios being a ratio (length of major axis / length of minor axis) of a length of a major axis of a corresponding one of particles of the ferrite powder for bonded magnet to a length of a minor axis of the corresponding one of the particles, the length of the major axis of the corresponding one of the particles being <NUM>µm or more. If a bonded magnet, which is a cylindrical bonded magnet having a diameter of <NUM> x a height of <NUM> (the direction of magnetic field orientation of the bonded magnet being a direction along the central axis thereof), is produced by injection-molding a kneaded pellet having an average diameter of <NUM> at a temperature of <NUM> at a molding pressure of <NUM> N/mm<NUM> in a magnetic field of <NUM> kOe (<NUM> kOe = <NUM> x <NUM><NUM> A/m), the kneaded pellet being produced by kneading a mixture at <NUM>, the mixture being obtained by filling and mixing <NUM> parts by weight of the ferrite powder for bonded magnet, <NUM> parts by weight of a silane coupling agent, <NUM> parts by weight of a lubricant and <NUM> parts by weight of a polyamide resin powder in a mixer, the bonded magnet has a residual magnetization Br of not less than <NUM> (<NUM> = <NUM>-<NUM> T) when the residual magnetization Br is measured in a measuring magnetic field of <NUM> kOe. The bonded magnet has a maximum energy product BHmax of not less than <NUM> MGOe (<NUM> MGOe = <NUM> x <NUM><NUM> J/m<NUM> = <NUM> kJ/m<NUM>) when the maximum energy product BHmax is measured in a measuring magnetic field of <NUM> kOe.

According to the present invention, there is provided a bonded magnet comprising: the above-described ferrite powder for bonded magnet; and a binder.

According to the present invention, it is possible to produce a ferrite powder for bonded magnet, which can obtain a bonded magnet having a high residual magnetization Br by the magnetic field orientation thereof.

In the preferred embodiment of a method for producing a ferrite powder for bonded magnet according to the present invention, a complex oxide powder of iron, strontium, lanthanum and cobalt is mixed with iron oxide (preferably hematite (α - Fe<NUM>O<NUM>)) (so that the molar ratio (Fe / (Sr + La)) of Fe in the iron oxide to the total of Sr and La is preferably in the range of from <NUM> to <NUM> and more preferably in the range of from <NUM> to <NUM>), to be granulated to be fired (preferably at a temperature of <NUM> to <NUM>, more preferably at a temperature of <NUM> to <NUM> and most preferably at a temperature of <NUM> to <NUM>) to obtain a fired substance which is coarsely pulverized to obtain a coarsely pulverized powder which is pulverized to be annealed (preferably at a temperature of <NUM> to <NUM>).

The above-described complex oxide powder of iron, strontium, lanthanum and cobalt is obtained by a method comprising the steps of: mixing strontium carbonate, lanthanum oxide, iron oxide and cobalt oxide to obtain a mixture; granulating the mixture to obtain a granulated mixture; firing the granulated mixture preferably at a temperature of <NUM> to <NUM>, more preferably at a temperature of <NUM> to <NUM> and most preferably at a temperature of <NUM> to <NUM> to obtain a fired substance; and pulverizing the fired substance.

The above-described coarsely pulverized powder is preferably pulverized (wet-pulverized) (preferably for <NUM> to <NUM> minutes) by means of a wet attritor or the like to obtain a slurry. The slurry thus obtained is preferably filtered to obtain a solid body. The solid body thus obtained is preferably dried to obtain a dry cake. The dry cake thus obtained is preferably crushed by means of a mixer to obtain a crushed substance. The crushed substance thus obtained is preferably pulverized by means of a vibratory ball mill or the like to obtain a pulverized substance. The pulverized substance thus obtained is preferably annealed.

Thus, it is possible to produce a ferrite powder for bonded magnet, which has a composition expressed by (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z (wherein <NUM> < x ≦ <NUM>, <NUM> < y ≦ <NUM>, <NUM> ≦ n ≦ <NUM>, -<NUM> ≦ z ≦ <NUM>).

The ferrite powder for bonded magnet according to the present invention has a composition of (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z wherein <NUM> < x ≦ <NUM> (preferably <NUM> ≦ x ≦ <NUM>, more preferably <NUM> ≦ x ≦ <NUM>), <NUM> < y ≦ <NUM> (preferably <NUM> ≦ y ≦ <NUM>), <NUM> ≦ n ≦ <NUM> (preferably <NUM> ≦ n ≦ <NUM>), -<NUM> ≦ z ≦ <NUM> (preferably -<NUM> ≦ z ≦ <NUM>), the ferrite powder having an average particle diameter of <NUM> to <NUM> µm (preferably <NUM> to <NUM> µm).

The specific surface area of this ferrite powder for bonded magnet is preferably in the range of from <NUM><NUM>/g to <NUM><NUM>/g and more preferably in the range of from <NUM><NUM>/g to <NUM><NUM>/g.

After <NUM> of the ferrite powder for bonded magnet is filled in a cylindrical die having an inside diameter of <NUM>ϕ, when a pressure of <NUM> ton/cm<NUM> (<NUM> ton/cm<NUM> = <NUM> MPa) is applied to the ferrite powder for bonded magnet, the density of the ferrite powder for bonded magnet is measured as the compression density (CD) of the ferrite powder for bonded magnet. The compression density (CD) thus measured is preferably in the range of from <NUM>/cm<NUM> to <NUM>/cm<NUM> and more preferably in the range of from <NUM>/cm<NUM> to <NUM>/cm<NUM>.

After <NUM> of the ferrite powder for bonded magnet and <NUM> cc of a polyester resin are kneaded in a mortar, <NUM> of the kneaded substance thus obtained is filled in a die having an inside diameter of <NUM>ϕ to obtain a filled substance, and a pressure of <NUM> ton/cm<NUM> (<NUM> MPa) is applied to the filled substance for <NUM> seconds. The molded article thus obtained is taken out from the die to be dried at <NUM> for <NUM> minutes to produce a pressurized powder body. As the magnetic characteristics of this pressurized powder body, the coercivity iHc and residual magnetization Br of the pressurized powder body are measured in a measuring magnetic field of <NUM> kOe by means of a BH tracer. The coercivity iHc thus measured is preferably in the range of from <NUM> Oe to <NUM> Oe and more preferably in the range of from <NUM> Oe to <NUM> Oe (<NUM> Oe = <NUM>,<NUM> A/m). The residual magnetization Br thus measured is preferably in the range of from <NUM> to <NUM> and more preferably <NUM> to <NUM>.

It is known that the residual magnetization Br of a ferrite magnetic material having a magnetoplumbite-type crystal structure generally has a negative temperature coefficient, and the coercivity Hc thereof generally has a positive temperature coefficient, and that the temperature coefficient of the coercivity Hc thereof is in the range of from about <NUM> %/°C to about <NUM> %/°C. That is, the coercivity Hc of the ferrite magnetic material having the magnetoplumbite-type crystal structure is lower as the temperature is lower. For that reason, when the ferrite magnetic material having the magnetoplumbite-type crystal structure is used for a bonded magnet, unless the ferrite magnetic material has a higher coercivity Hc than necessary, there is a problem in that irreversible demagnetization (low-temperature demagnetization) is caused by a temperature cycle from a low temperature to a high temperature. There is particularly a problem in the low-temperature demagnetization of such a ferrite magnetic material, when the ferrite magnetic material is used as the material of a bonded magnet, which is used for an outdoor unit for an air conditioner, a motor for an automotive vehicle or the like, which are greatly influenced by the variation in outdoor air temperature. For that reason, the temperature coefficient of the coercivity Hc of the ferrite powder for bonded powder is preferably not higher than <NUM> %/°C, the temperature coefficient of the coercivity Hc thereof being calculated from the rate of variability in the coercivity Hc which is measured at three points of -<NUM>, <NUM> and <NUM> by a full loop (at a sweep applying rate of <NUM> Oe/second) in which a magnetic field is applied to <NUM> T (<NUM>,<NUM> Oe), the ferrite powder for bonded powder being fixed in a measuring cell by cooling <NUM> of the ferrite powder for bonded powder and <NUM> of paraffin, which are filled in the measuring cell to be held at <NUM> for <NUM> minutes.

After <NUM> parts by weight of the ferrite powder for bonded magnet, <NUM> parts by weight of a silane coupling agent, <NUM> parts by weight of a lubricant and <NUM> parts by weight of a polyamide resin powder are filled and mixed in a mixer to obtain a mixture which is kneaded at <NUM> to produce a kneaded pellet having an average diameter of <NUM>, the kneaded pellet is injection-molded at a temperature of <NUM> at a molding pressure of <NUM> N/mm<NUM> in a magnetic field of <NUM> kOe to produce a cylindrical bonded magnet having a diameter of <NUM> x a height of <NUM> (the direction of magnetic field orientation of the bonded magnet being a direction along the central axis thereof). When the coercivity iHc, residual magnetization Br and maximum energy product BHmax of the bonded magnet thus produced are measured in a measuring magnetic field of <NUM> kOe, the bonded magnet has a coercivity iHc of preferably <NUM> to <NUM> Oe (more preferably <NUM> to <NUM> Oe), a residual magnetization Br of preferably not less than <NUM> (more preferably of not less than <NUM>) and a maximum energy product BHmax of preferably not less than <NUM> MGOe (more preferably of <NUM> to <NUM> MGOe).

After the above-described bonded magnet is cut in parallel to the direction of the applied magnetic field, the shape of particles on the cut surface of the bonded magnet is observed at a magnification of <NUM>,<NUM> by means of an electron microscope to obtain an electron micrograph. The electron micrograph thus obtained is binarized to obtain a ratio (length of major axis / length of minor axis) (aspect ratio) of a length of a major axis (the maximum value of distances between two parallel straight lines (each of the distances being the length of a line segment drawn in a direction perpendicular to the two parallel straight lines) when one particle is put between the straight lines) of a corresponding one of the particles to a length of a minor axis (the minimum value of distances between two parallel straight lines when one particle is put between the straight lines) of the corresponding one of the particles, as a shape index of the particles, the length of the major axis of the corresponding one of the particles being <NUM> µm or more. That is, a volume average aspect ratio weighted by volume is calculated as volume = length of major axis x length of major axis x length of minor axis. The aspect ratio thus obtained is preferably not greater than <NUM>. If the aspect ratio is not greater than <NUM>, the particles of the ferrite powder for bonded magnet are easily arranged in the direction of the magnetic field by the magnetic field orientation, so that it is possible to easily produce a bonded magnet having a high orientation of particles, a high residual magnetization Br and a high maximum energy product BHmax.

Examples of a ferrite powder for bonded magnet and a method for producing the same according to the present invention will be described below in detail.

First, strontium carbonate (SrCO<NUM>, specific surface area = <NUM><NUM>/g), lanthanum oxide (La<NUM>O<NUM>, specific surface area = <NUM><NUM>/g), hematite (α - Fe<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) (serving as iron oxide) and cobalt oxide (Co<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) were weighed to be mixed in a molar ratio of Sr : La : Fe : Co = <NUM> : <NUM> : <NUM> : <NUM>. The mixture thus obtained was granulated while water was added thereto in a pan pelletizer. The granulated spherical particles having diameters of <NUM> to <NUM> thus obtained were put into an internal-combustion rotary kiln to be fired (primary-fired) at <NUM> for <NUM> minutes in the atmosphere to obtain a fired substance. This fired substance was pulverized by means of a roller mill to obtain a complex oxide powder of iron, strontium, lanthanum and cobalt. The specific surface area (SSA) of the complex oxide powder was measured by means of a specific surface area measuring apparatus (Monosorb produced by Quantachrome Corporation) using the single point BET method. As a result, the specific surface area of the complex oxide powder was <NUM><NUM>/g.

This complex oxide powder and hematite (α - Fe<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) (serving as iron oxide) were weighed to be mixed so that the molar ratio (Fe / (Sr + La)) of Fe in the oxide to the total of Sr and La was <NUM>. This mixture thus obtained was mixed with <NUM> % by weight of boric acid and <NUM> % by weight of potassium chloride. Then, water was added thereto to granulate the mixture. The granulated spherical particles having diameters of <NUM> to <NUM> thus obtained were put into an internal-combustion rotary kiln to be fired (secondary-fired) at <NUM> (firing temperature) for <NUM> minutes in the atmosphere. The fired substance thus obtained was pulverized by means of a roller mill to obtain a coarsely pulverized powder.

First, <NUM> parts by weight of the coarsely pulverized powder thus obtained and <NUM> parts by weight of water were put into a wet attritor to be pulverized for <NUM> minutes to obtain a slurry. The slurry thus obtained was filtered to obtain a solid body. The solid body thus obtained was dried at <NUM> for <NUM> hours in the atmosphere to obtain a dry cake. The dry cake thus obtained was crushed by means of a mixer to obtain a crushed substance. The crushed substance thus obtained was pulverized at a rotation number of <NUM> rpm in an amplitude of <NUM> for <NUM> minutes by means of a vibratory ball mill (Uras Vibrator KEC-<NUM>-YH produced by MURAKAMI SEIKI MFG CO. ) using steel balls having a diameter of <NUM> as media. The pulverized substance thus obtained was annealed at <NUM> for <NUM> minutes in the atmosphere by means of an electric furnace to obtain a ferrite powder for bonded magnet.

The composition analysis of the ferrite powder for bonded magnet was carried out by calculating the ingredient amount of each element by the fundamental parameter method (FP method) by means of a fluorescent X-ray analyzer (ZSX 100e produced by Rigaku Corporation). In this composition analysis, the ferrite powder for bonded magnet was filled in a measuring cell to be molded by applying a pressure of <NUM> tons/cm<NUM> thereto for <NUM> seconds, to carry out the qualitative analysis thereof at a measuring diameter of <NUM> in a measuring mode of EZ scan mode in the sample form of an oxide for a measuring time of standard time in a vacuum, and then, to carry out the quantitative analysis of the detected constituent elements. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr, Mn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>. Furthermore, z was calculated so that the total valence of the chemical formula was <NUM> (zero) assuming that the valence of Sr was +<NUM> and the valence of La was +<NUM>, the valence of Fe being +<NUM>, the valence of Co being +<NUM> and the valence of O being -<NUM>.

With respect to this ferrite powder for bonded magnet, a powder X-ray diffractometer (Miniflex <NUM> produced by Rigaku Corporation) was used for carrying out measurement based on the powder X-ray diffraction (XRD) analysis on conditions containing a tube voltage of <NUM> kV, a tube current of <NUM> mA, a measuring range of <NUM>° to <NUM>° , a scan speed of <NUM>° /minute and a scan width of <NUM>°. The results of the measurement are shown in <FIG>. Furthermore, the peak positions of SrFe<NUM>O<NUM> having a usual M-type ferrite structure are shown on the lower side of <FIG>. It can be seen from <FIG> that all of the peaks are measured at the same positions as those of SrFe<NUM>O<NUM>, so that it is confirmed that the ferrite powder for bonded magnet in this example has the M-type ferrite structure. These results are the same as those in Examples <NUM> through <NUM> and Comparative Examples <NUM> through <NUM> which will be described later.

The average particle diameter (APD) of the ferrite powder for bonded magnet was measured by an air transmission method by means of a specific surface area measuring apparatus (SS-<NUM> produced by Shimadzu Corporation). As a result, the average particle diameter was <NUM> µm. The specific surface area (SSA) of the ferrite powder for bonded magnet was measured by the same method as the above-described method. As a result, the specific surface area was <NUM><NUM>/g.

After <NUM> of the ferrite powder for bonded magnet was filled in a cylindrical die having an inside diameter of <NUM>ϕ, when a pressure of <NUM> ton/cm<NUM> was applied to the ferrite powder for bonded magnet, the density of the ferrite powder for bonded magnet was measured as the compression density (CD) of the ferrite powder for bonded magnet. As a result, the compression density (CD) was <NUM>/cm<NUM>.

After <NUM> of the ferrite powder for bonded magnet and <NUM> cc of a polyester resin (P-resin produced by Nichika Inc. ) were kneaded in a mortar, <NUM> of the kneaded substance thus obtained was filled in a die having an inside diameter of <NUM> ϕ, and a pressure of <NUM> ton/cm<NUM> was applied to the filled substance for <NUM> seconds. The molded article thus obtained was taken out from the die to be dried at <NUM> for <NUM> minutes to obtain a pressurized powder body. As the magnetic characteristics of the pressurized powder body thus obtained, the coercivity iHc and residual magnetization Br thereof were measured in a measuring magnetic field of <NUM> kOe by means of a BH tracer (TRF-5BH produced by Toei Industry Co. As a result, the coercivity iHc was <NUM> Oe, and the residual magnetization Br was <NUM>.

After <NUM> of the ferrite powder for bonded powder and <NUM> of paraffin were filled in a measuring cell to be held at <NUM> for <NUM> minutes, they were cooled to be fixed in the measuring cell. Then, a vibrating sample magnetometer (VSM-5HSC produced by Toei Industry Co. ) was used for measuring the coercivity Hc of the ferrite powder for bonded magnet at three points of -<NUM>, <NUM> and <NUM> by a full loop (at a sweep applying rate of <NUM> Oe/second) in which a magnetic field was applied to <NUM> T (<NUM>,<NUM> Oe). From the variability in the coercivity H, the temperature coefficient of the coercivity Hc was calculated. As a result, the temperature coefficient of the coercivity Hc of the ferrite powder for bonded powder was - <NUM> %/°C. Furthermore, this temperature coefficient was calculated as the inclination of a related expression of y and x, the related expression being obtained by the least-square method assuming that the coercivity Hc was y and that the temperature was x.

First, <NUM> parts by weight of the ferrite powder for bonded magnet, <NUM> parts by weight of a silane coupling agent (Z-6094N produced by Toray Dow Corning, Co. ), <NUM> parts by weight of a lubricant (VPN-212P produced by Henkel AG & Co. KGaA) and <NUM> parts by weight of a polyamide resin powder (P-1011F produced by Ube Industries, Inc. ) were weighed to be filled in a mixer to be mixed to obtain a mixture. The mixture thus obtained was kneaded at <NUM> to obtain a kneaded pellet having an average diameter of <NUM>. The kneaded pellet thus obtained was loaded in an injection molding machine (produced by Sumitomo Heavy Industries, Ltd. ) to be injection-molded at a temperature of <NUM> at a molding pressure of <NUM> N/mm<NUM> in a magnetic field of <NUM> kOe to obtain a cylindrical bonded magnet (F. <NUM> % by weight, <NUM> kOe) having a diameter of <NUM> x a height of <NUM> (the direction of magnetic field orientation of the bonded magnet was a direction along the central axis thereof).

As the magnetic characteristics of the bonded magnet thus obtained, a BH tracer (TRF-5BH produced by Toei Industry Co. ) was used for measuring the coercivity iHc, residual magnetization Br and maximum energy product BHmax of the bonded magnet in a measuring magnetic field of <NUM> kOe. As a result, the coercivity iHc was <NUM> Oe, the residual magnetization Br was <NUM>, and the maximum energy product BHmax was <NUM> MGOe.

After the bonded magnet was cut in parallel to the direction of the magnetic field applied thereto, the shape of particles on the cut surface of the bonded magnet was observed at a magnification of <NUM>,<NUM> by means of a scanning electron microscope (SEM) to obtain an SEM image. The SEM image thus obtained was binarized to obtain an average value (aspect ratio) of ratios (length of major axis / length of minor axis) as a shape index of the particles, each of the ratios being a ratio of a length of a major axis (the maximum value of distances between two parallel straight lines when one particle was put between the straight lines) of a corresponding one of <NUM> particles or more (the whole outline of each of the particles being observed in one field of vision or more on the SEM image) to a length of a minor axis (the minimum value of distances between two parallel straight lines when one particle was put between the straight lines) of the corresponding one of the particles, the length of the major axis of the corresponding one of the particles being <NUM>µm or more. As a result, the aspect ratio was <NUM>. Furthermore, as this aspect ratio, a volume average aspect ratio weighted by volume was calculated as volume = length of major axis x length of major axis x length of minor axis assuming that each of the particles was a plate particle.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that the pulverizing time using the wet attritor was <NUM> minutes.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of ZnO, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr, Mn, Zn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

With respect to this ferrite powder for bonded magnet, the average particle diameter, specific surface area and compression density (CD) thereof, and the coercivity iHc and residual magnetization Br of the pressurized powder body thereof were measured by the same methods as those in Example <NUM>. As a result, the average particle diameter was <NUM> µm, the specific surface area was <NUM><NUM>/g, and the compression density (CD) was <NUM>/cm<NUM>. The coercivity iHc of the pressurized powder body was <NUM> Oe, and the residual magnetization Br thereof was <NUM>.

This ferrite powder for bonded magnet was used for obtaining a bonded magnet by the same method as that in Example <NUM>. With respect to this bonded magnet, the coercivity iHc, residual magnetization Br and maximum energy product BHmax thereof were measured by the same methods as those in Example <NUM>, and the aspect ratio thereof was calculated by the same method as that in Example <NUM>. As a result, the coercivity iHc was <NUM> Oe, the residual magnetization Br was <NUM>, and the maximum energy product BHmax was <NUM> MGOe. The aspect ratio was <NUM>.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that the temperature of the secondary firing was <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr, Mn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that the same complex oxide powder as that in Example <NUM> and hematite (α -Fe<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) (serving as iron oxide) were weighed to be mixed so that the molar ratio (Fe / (Sr + La)) of Fe in the iron oxide to the total of Sr and La was <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of ZnO, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Mn, Zn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = -<NUM>.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that strontium carbonate (SrCO<NUM>, specific surface area = <NUM><NUM>/g), lanthanum oxide (La<NUM>O<NUM>, specific surface area = <NUM><NUM>/g), hematite (α - Fe<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) and cobalt oxide (Co<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) were weighed to be mixed in a molar ratio of Sr : La : Fe : Co = <NUM> : <NUM> : <NUM> : <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr, Mn, and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

With respect to this ferrite powder for bonded magnet, the average particle diameter, specific surface area and compression density (CD) thereof, and the coercivity iHc and residual magnetization Br of the pressurized powder body thereof were measured by the same methods as those in Example <NUM>. As a result, the average particle diameter was <NUM> µm, the specific surface area was <NUM><NUM>/g, and the compression density (CD) was <NUM>/cm<NUM>. The coercivity iHc of the pressurized powder body was <NUM> Oe, and the residual magnetization Br thereof was <NUM>. By the same methods as those in Example <NUM>, the coercivity Hc of the ferrite powder for bonded magnet was measured, and the temperature coefficient of the coercivity Hc was calculated. As a result, the temperature coefficient was -<NUM> %/°C.

With respect to this ferrite powder for bonded magnet, the average particle diameter, specific surface area and compression density (CD) thereof, and the coercivity iHc and residual magnetization Br of the pressurized powder body thereof were measured by the same methods as those in Example <NUM>. As a result, the average particle diameter was <NUM>, the specific surface area was <NUM><NUM>/g, and the compression density (CD) was <NUM>/cm<NUM>. The coercivity iHc of the pressurized powder body was <NUM> Oe, and the residual magnetization Br thereof was <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of ZnO, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Mn, Zn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that the complex oxide powder and hematite (serving as iron oxide) were weighed to be mixed so that the molar ratio (Fe / (Sr + La)) of Fe in the oxide to the total of Sr and La was <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Mn, Zn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of ZnO, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Mn, Zn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr, Mn, and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

With respect to this ferrite powder for bonded magnet, the average particle diameter, specific surface area and compression density (CD) thereof, and the coercivity iHc and residual magnetization Br of the pressurized powder body thereof were measured by the same methods as those in Example <NUM>. As a result, the average particle diameter was <NUM> µm, the specific surface area was <NUM><NUM>/g, and the compression density (CD) was <NUM>/cm<NUM>. The coercivity iHc of the pressurized powder body was <NUM> Oe, and the residual magnetization Br thereof was <NUM>. By the same methods as those in Example <NUM>, the coercivity Hc of the ferrite powder for bonded magnet was measured, and the temperature coefficient of the coercivity Hc was calculated. As a result, the temperature coefficient was <NUM> %/°C.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO, <NUM> % by weight of BaO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr, Mn and Ba, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

With respect to this ferrite powder for bonded magnet, the average particle diameter, specific surface area and compression density (CD) thereof, and the coercivity iHc and residual magnetization Br of the pressurized powder body thereof were measured by the same methods as those in Example <NUM>. As a result, the average particle diameter was <NUM>, the specific surface area was <NUM><NUM>/g, and the compression density (CD) was <NUM>/cm<NUM>. The coercivity iHc of the pressurized powder body was <NUM> Oe, and the residual magnetization Br thereof was <NUM>. By the same methods as those in Example <NUM>, the coercivity Hc of the ferrite powder for bonded magnet was measured, and the temperature coefficient of the coercivity Hc was calculated. As a result, the temperature coefficient was <NUM> %/°C.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that the temperature of the primary firing was <NUM>.

A ferrite powder for bonded magnet was obtained by the same method as that in Example <NUM>, except that strontium carbonate (SrCO<NUM>, specific surface area = <NUM><NUM>/g), lanthanum oxide (La<NUM>O<NUM>, specific surface area = <NUM><NUM>/g), hematite (α - Fe<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) and cobalt oxide (Co<NUM>O<NUM>, specific surface area = <NUM><NUM>/g) were weighed to be mixed in a molar ratio of Sr : La : Fe : Co = <NUM> : <NUM> : <NUM> : <NUM>, that the temperature of the primary firing was changed to <NUM> from <NUM> and that the secondary firing was not carried out.

The composition analysis of the ferrite powder for bonded magnet was carried out by the same method as that in Example <NUM>. As a result, the ferrite powder for bonded magnet was detected to contain <NUM> % by weight of Cr<NUM>O<NUM>, <NUM> % by weight of MnO, <NUM> % by weight of Fe<NUM>O<NUM>, <NUM> % by weight of Co<NUM>O<NUM>, <NUM> % by weight of SrO and <NUM> % by weight of La<NUM>O<NUM>, so that Sr, La, Fe and Co being the main components of the ferrite powder for bonded magnet were detected. Furthermore, although elements, such as Cr and Mn, considered to be derived from impurities in the raw materials, were detected, the amount of each of the elements was a very small amount of <NUM> % when the elements were converted into oxides. Assuming that the very small amounts (<NUM> % by weight or less when the elements were converted into oxides) of the elements were impurities, if the chemical formula of the ferrite powder for bonded magnet was expressed as (Sr<NUM>-xLax) · (Fe<NUM>-yCoy)nO<NUM>-z from the analyzed values of Sr, La, Fe and Co being the main components, x, y, n and z were calculated to be x = <NUM>, y = <NUM>, n = <NUM> and z = <NUM>.

A ferrite powder for bonded magnet was obtained by the same method as that in Comparative Example <NUM>, except that the temperature of the primary firing was <NUM>.

The results of these examples and comparative examples are shown in Tables <NUM>-<NUM>. The scanning electron microscope (SEM) images of the cross-sections of the bonded magnets obtained in Example <NUM> and Comparative Example <NUM> are shown in <FIG> and <FIG>, respectively.

It can be seen from the results of Examples <NUM>-<NUM> and Comparative Examples <NUM>-<NUM> that it is possible to produce a ferrite powder for bonded magnet, which can obtain a bonded magnet having a high residual magnetization Br by the magnetic field orientation thereof.

Claim 1:
A method for producing a ferrite powder for bonded magnet, the method comprising the steps of:
mixing a complex oxide powder of iron, strontium, lanthanum and cobalt, with iron oxide to obtain a mixture;
granulating the mixture to obtain a granulated mixture; and
firing the granulated mixture, characterized in that
the complex oxide powder is obtained by a method comprising the steps of:
mixing strontium carbonate, lanthanum oxide, iron oxide and cobalt oxide to obtain a mixture;
granulating the mixture to obtain a granulated mixture;
firing the granulated mixture at a temperature of <NUM> to <NUM> to obtain a fired substance; and
pulverizing the fired substance.