Patent Publication Number: US-2002007874-A1

Title: Magnetic powder and isotropic bonded magnet

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to magnetic powder and an isotropic bonded magnet. More particularly, the present invention relates to magnetic powder and an isotropic bonded magnet which is produced, for example, using the magnetic powder.  
       [0003] 2. Description of the Prior Art  
       [0004] For reduction in size of motors, it is desirable that a magnet has a high magnetic flux density (with the actual permeance) when it is used in the motor. Factors for determining the magnetic flux density of a bonded magnet include magnetization of the magnetic powder and the content of the magnetic powder contained in the bonded magnet. Accordingly, when the magnetization of the magnetic powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is raised to an extremely high level.  
       [0005] At present, most of practically used high performance rare-earth bonded magnets use the isotropic bondedmagnets which are made using MQP-B powder manufactured by MQI Inc. as the rare-earth magnetic powder thereof. The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. On the other hand, however, the conventional isotropic bonded magnets represented by those manufactured using the MQP-B powder involve the following problems.  
       [0006] (1) The conventional isotropic bonded magnets do not have a sufficiently high magnetic flux density. Namely, because the magnetic powder that has been being used has poor magnetization, the content of the magnetic powder to be contained in the bonded magnet has to be increased. However, the increase in the content of the magnetic powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnetic powder is somehow managed to be increased by changing the molding conditions or the like, there still exists a limit to the obtainable magnetic flux density. For these reasons, it is not possible to reduce the size of the motor by using the conventional isotropic bonded magnets.  
       [0007] (2) Although there are reports concerning nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux densities (for the permeance in the actual use) obtainable when they are practically used in motors are very low. Further, these magnets have poor heat stability due to their small coercive forces.  
       [0008] (3) The conventional bonded magnets have low corrosion resistance and heat resistance. Namely, in these magnets, it is necessary to increase the content of the magnetic powder to be contained in the bonded magnet in order to compensate the low magnetic properties (magnetic performance) of the magnetic powder. This means that the density of the bonded magnet becomes extremely high. As a result, the corrosion resistance and heat resistance of the bonded magnet are deteriorated, thus resulting in low reliability.  
       SUMMARY OF THE INVENTION  
       [0009] It is therefore an object of the present invention to provide magnetic powder that can produce a bonded magnet having excellent magnetization and having excellent reliability especially excellent temperature characteristics (that is, heat resistance and heat stability), and provide an isotropic bonded magnet formed from the magnetic powder.  
       [0010] In order to achieve the above object, the present invention is directed to magnetic powder composed of an alloy composition represented by R x (Fe 1−y Co y ) 100−x−z−w B z Nb w  (where R is at least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9 at %, and w is 0.2-3.5 at %), the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein the magnetic powder has magnetic properties in which, when the magnetic powder is formed into an isotropic bonded magnet by mixing with a binding resin and then molding it, the irreversible susceptibility (χ irr ) which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing the magnetic properties at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10 −6  H/m as a starting point is equal to or less than 5.0×10 −7  H/m, and the intrinsic coercive force (H CJ ) of the bonded magnet at the room temperature is in the range of 320-720 kA/m.  
       [0011] According to the magnetic powder as described above, it is possible to provide bonded magnets having excellent magnetic properties as well as excellent heat resistance (heat stability) and corrosion resistance.  
       [0012] In the present invention, it is preferred that when the magnetic powder is formed into an isotropic bonded magnet having a density ρ [Mg/m 3 ] by mixing with a binding resin and then molding it, the remanent magnetic flux density Br[T] at the room temperature satisfies the relationship represented by the formula of Br/ρ [×10 −6  T·m 3 /g]≦0.125.  
       [0013] This makes it possible to further improve magnetic properties as well as heat resistance (heat stability) and corrosion resistance.  
       [0014] Further, in the present invention, it is preferred that when the magnetic powder is formed into an isotropic bonded magnet by mixing with a binding resin and then molding it, the absolute value of the irreversible flux loss (initial flux loss) is equal to or less than 6.2%.  
       [0015] This makes it possible to provide bonded magnets having especially excellent heat resistance (heat stability).  
       [0016] In these cases, it is preferred that said R comprises rare-earth elements mainly containing Nd and/or Pr. This makes it possible to improve saturation magnetization of the hard phase of the composite structure (in particular, nanocomposite structure), and thereby coercive force is further enhanced.  
       [0017] Further, it is also preferred that said R includes Pr and its ratio with respect to the total mass of said R is 5-75%. This makes it possible to improve the coercive force and rectangularity with less drop of the remanent magnetic flux density.  
       [0018] Further, it is also preferred that said R includes Dy and its ratio with respect to the total mass of said R is equal to or less than 14%. This makes it possible to improve the coercive force and heat resistance (heat stability) without markedly lowering the remanent magnetic flux density.  
       [0019] In the present invention, it is also preferred that the magnetic powder is obtained by quenching the alloy of a molten state. According to this, it is possible to refine the microstructure (crystalline grains) relatively easily, thereby enabling to further improve the magnetic properties of the bonded magnet.  
       [0020] Further, it is also preferred that the magnetic powder is obtained by milling a melt spun ribbon of the alloy which is manufactured by using a cooling roll. According to this, it is possible to refine the microstructure (crystalline grains) relatively easily, thereby enabling to further improve the magnetic properties of the bonded magnet.  
       [0021] Furthermore, it is also preferred that the magnetic powder is subjected to a heat treatment for at least once during the manufacturing process or after its manufacture. According to this, homogeneity (uniformity) of the structure can be obtained and influence of stress introduced by the milling process can be removed, thereby enabling to further improve the magnetic properties of the bonded magnet.  
       [0022] In the magnetic powders described above, it is preferred that the average particle size lies in the range of 0.5-150 μm. This makes it possible to further improve the magnetic properties. Further, when the magnetic powder is used in manufacturing bonded magnets, it is possible to obtain bonded magnets having a high content of the magnetic powder and having excellent magnetic properties.  
       [0023] Another aspect of the present invention is directed to an isotropic bonded magnet formed by binding a magnetic powder containing Nb with a binding resin, wherein the isotropic bonded magnet is characterized in that the irreversible susceptibility (χ irr ) which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing the magnetic properties at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10 6  H/m as a starting point is less than 5.0×10 −7  H/m, and the intrinsic coercive force (H CJ ) of the bonded magnet at the room temperature is in the range of 320-720 kA/m.  
       [0024] According to the structure as described above, it is possible to provide an isotropic bonded magnet having excellent magnetic properties as well as excellent heat resistance (heat stability) and corrosion resistance.  
       [0025] In this isotropic bonded magnet, it is preferred that the density of the isotropic bonded magnet is ρ [Mg/m 3 ], the remanent magnetic flux density Br[T] at the room temperature satisfies the relationship represented by the formula Br/σ&gt;0.125 [×10 −6  T·m 3 /g].  
       [0026] This makes it possible to provide an isotropic bonded magnet having particularly excellent magnetic properties as well as particularly excellent heat resistance (heat stability) and corrosion resistance.  
       [0027] Further, in this isotropic bonded magnet, it is preferred that said magnetic powder is formed of R—TM—B—Nb based alloy (where R is at least one rare-earth element and TM is a transition metal containing Iron as a major component thereof). This also makes it possible to provide an isotropic bonded magnet having particularly excellent magnetic properties as well as particularly excellent heat resistance (heat stability) and corrosion resistance.  
       [0028] Furthermore, in this isotropic bonded magnet, it is also preferred that the magnetic powder is composed of an alloy composition represented by R x (Fe 1−y Co y ) 100−x−z−w B z Nb w B z Nb w  (where R is at least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9 at %, and w is 0.2-3.5 at %). This also makes it possible to provide an isotropic bonded magnet having particularly excellent magnetic properties as well as particularly excellent heat resistance (heat stability) and corrosion resistance.  
       [0029] Moreover, in this isotropic bonded magnet, it is also preferred that said R comprises rare-earth elements mainly containing Nd and/or Pr. This makes it possible to improve the coercive force of the bonded magnet.  
       [0030] In this case, it is preferred that said R includes Pr and its ratio with respect to the total mass of said R is 5-75%. This makes it possible to improve the coercive force and rectangularity with less drop of the remanent magnetic flux density.  
       [0031] Further, it is also preferred that said R includes Dy and its ratio with respect to the total mass of said R is equal to or less than 14%. This makes it possible to improve the coercive force and heat resistance (heat stability) without markedly lowering the remanent magnetic flux density.  
       [0032] In the isotropic bonded magnets as described above, it is preferred that the average particle size of the magnetic powder lies in the range of 0.5-150 μm. This makes it possible to obtain an isotropic bonded magnet having a high content of the magnetic powder and having excellent magnetic properties.  
       [0033] Further, in the isotropic bonded magnets as described above, it is also preferred that the absolute value of the irreversible flux loss (initial flux loss) is equal to or less than 6.2%. This makes it possible to provide particularly excellent heat resistance (heat stability).  
       [0034] Furthermore, in the isotropic bonded magnets as described above, it is also preferred that the magnetic powder is constituted from a composite structure having a soft magnetic phase and a hard magnetic phase. This improves magnetizability and heat resistance (heat stability), thus leading to less deterioration in the magnetic properties with elapse of time.  
       [0035] Preferably, the isotropic bonded magnets as described above are to be subjected to multipolar magnetization or have already been subjected to multipolar magnetization. According to this, satisfactory magnetization can be made even in the case where sufficient magnetizing magnetic field is not obtained, thereby enabling to obtain sufficient magnetic flux density.  
       [0036] Further, preferably, the isotropic bonded magnets as described above are used for a motor. By using the bonded magnet to motors, it becomes possible to provide small and high performance motors.  
       [0037] These and other objects, structures and advantages of the present invention will be apparent from the following detailed description of the invention and the examples taken in conjunction with the appended drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0038]FIG. 1 is an illustration which schematically shows one example of a composite structure (nanocomposite structure) of magnetic powder according to the present invention.  
     [0039]FIG. 2 is an illustration which schematically shows one example of a composite structure (nanocomposite structure) of magnetic powder according to the present invention.  
     [0040]FIG. 3 is an illustration which schematically shows one example of a composite structure (nanocomposite structure) of magnetic powder according to the present invention.  
     [0041]FIG. 4 is a perspective view showing an example of the configuration of an apparatus (melt spinning apparatus) for manufacturing a magnet material.  
     [0042]FIG. 5 is a sectional side view showing the situation in the vicinity of colliding section of the molten metal with a cooling roll in the apparatus shown in FIG. 4.  
     [0043]FIG. 6 is a J-H diagram (coordinate) for explaining the irreversible susceptibility.  
     [0044]FIG. 7 is a J-H diagram (coordinate) that represents demagnetization curves and recoil curves. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0045] In the following, embodiments of the magnetic powder and the isotropic bonded magnet according to this invention will be described in detail.  
     [0046] General Description of the Invention  
     [0047] At present, a magnet having high magnetic flux density is practically required in order to reduce the size of motors or other electrical devices. In a bonded magnet, factors that determine the magnetic flux density are the magnetization of magnetic powder and the content (compositional ratio) of the magnetic powder contained in the bonded magnet. When the magnetization of the magnetic powder itself is not so high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is increased to an extremely high level.  
     [0048] As described in the above, the MQP-B powder made by MQI Inc. which is now being widely used can not provide sufficient magnetic flux density depending on its use. As a result, in manufacturing the bonded magnets, it is required to increase the content of the magnetic powder in the bonded magnet, that is, it is required to increase the magnetic flux density. However, this in turn leads to the lack of reliability in the corrosion resistance, heat resistance and mechanical strength thereof and the like. Further, there is also a problem in that the obtained magnet has a poor magnetizability due to its high coercivity.  
     [0049] In contrast, the magnetic powder and the isotropic bonded magnet according to this invention can obtain a sufficient magnetic flux density and an adequate coercive force. As a result, without extremely increasing the content of the magnetic powder in the bonded magnet, it is possible to provide a bonded magnet having high strength and having excellent moldability, corrosion resistance and magnetizability. This makes it possible to reduce the size of the bondedmagnet and increase its performance, thereby contributing to reduction in size of motors and other devices employing magnets.  
     [0050] Further, the magnetic powder of the present invention may be formed so as to constitute a composite structure having a hard magnetic phase and a soft magnetic phase.  
     [0051] While the MQP-B powder made by MQI Inc. is a single phase structure of a hard magnetic phase, the magnetic powder of the present invention has the composite structure which has a soft magnetic phase with high magnetization. Accordingly, it has an advantage that the total magnetization of the system as a whole is high. Further, since the recoil permeability of the bonded magnet becomes high, there is an advantage that, even after a reverse magnetic field has been applied, the demagnetizing factor remains small.  
     [0052] Alloy Composition of Nagnetic Powder  
     [0053] Preferably, the magnetic powder according to this invention is formed of R—TM—B—Nb based alloys (where R is at least one rare-earth element and TM is a transition metal containing Iron as a major component thereof). Among these alloys, an alloy having alloy compositions represented by R x (Fe 1−y Co y ) 100−x−z−w B z Nb w  (where R is at least one kind of rare-earth element, x is 7.1-9.9 at %, y is 0-0.30, z is 4.6-6.9 at %, and w is 0.2-3.5 at %) is particularly preferred.  
     [0054] Examples of the rare-earth elements R include Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and a misch metal. In this connection, R may include one kind or two or more kinds of these elements.  
     [0055] The content of R is set at 7.1-9.9 at %. When the content of R is less than 7.1 at %, sufficient coercive force cannot be obtained, and addition of Nb enhances the coercive force only to a small extent. On the other hand, when the content of R exceeds 9.9 at %, sufficient magnetic flux density fails to be obtained because of the drop in the magnetization potential.  
     [0056] Here, it is preferable that R includes the rare-earth elements Nd and/or Pr as its principal ingredient. The reason for this is that these rare-earth elements enhance the saturation magnetization of the hard magnetic phase which constitutes the composite structure (especially, nanocomposite structure), and are effective in realizing satisfactory coercive force as a magnet.  
     [0057] Moreover, it is preferable that R includes Pr and its ratio to the total mass of R is 5-75%, and more preferably 20-60%. This is because when the ratio lies within this range, it is possible to improve the coercive force (coercivity) and the rectangularity by hardly causing a drop in the remanent magnetic flux density.  
     [0058] Furthermore, it is also preferable that R includes Dy and its ratio to the total mass of R is equal to or less than 14%. When the ratio lies within this range, the coercive force can be improved without causing marked drop in the remanent magnetic flux density, and the temperature characteristic (such as heat stability) can be also improved.  
     [0059] Cobalt (Co) is a transition metal element having properties similar to Fe. By adding Co, that is by substituting a part of Fe by Co, the Curie temperature is elevated and the temperature characteristic of the magnetic powder is improved. However, if the substitution ratio of Fe by Co exceeds 0.30, both of the coercive force and the magnetic flux density tend to fall off. The range of 0.05-0.20 of the substitution ratio of Fe by Co is more preferable since in this range not only the temperature characteristic of the magnetic powder but also the magnetic flux density thereof are improved.  
     [0060] Boron (B) is an element which is important for obtaining high magnetic properties, and its content is set at 4.6-6.9 at %. When the content of B is less than 4.6 at %, the rectangularity of the B-H (J-H) loop is deteriorated. On the other hand, when the content of B exceeds 6.9 at %, the nonmagnetic phase increases and the magnetic flux density drops sharply.  
     [0061] Niobium (Nb) is an element which is advantageous for improving the coercive force, and the effect of improvement of the coercive force is conspicuous when its content lies in the range of 0.2-3. 5 at %. In addition, the rectangularity and the maximum magnetic energy product are also improved in this range in accompanying with the improvement in the coercive force, and the heat resistance and corrosion resistance also become satisfactory. In this connection, however, when the content of R is less than 7.1 at %, these effects due to addition of Nb are very small as described above. Further, when the content of Nb exceeds 3.5 at %, the drop in the magnetization occurs.  
     [0062] Further, another important effect obtained by containing 0.2-3. 5 at % of Nb is that the irreversible susceptibility (ω irr ) described hereinafter can be made small and the irreversible flux loss can be improved so that the heat resistance (heat stability) of the magnet is improved. In this connection, it is to be noted that if the amount of Nb is less than 0.2 at %, such effect is hardly realized and the effect for improving coercive force described above is small.  
     [0063] Of course, Nb itself is a known substance. However, in the present invention, it has found through repeatedly conducted experiments and researches that by containing 0.2-3.5 at % of Nb to the magnetic powder constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, the following four effects are realized, in particular these four effects are realized at the same time, and this is the significance of the present invention.  
     [0064] (1) The coercive force of the magnetic powder can be improved while maintaining the excellent rectangularity and the maximum magnetic energy product.  
     [0065] (2) The irreversible susceptibility (χ irr ) described below can be made small.  
     [0066] (3) The irreversible flux loss can be improved, that is the absolute value thereof can be lowered.  
     [0067] (4) Better corrosion resistance can be maintained.  
     [0068] In this connection, the preferred range of the content of Nb is 0.2-3.5 at % as described above. In this case, it is more preferable that the upper limit of the range is 3.0 at %, and it is the most preferable that the upper limit is 2.5 at %.  
     [0069] In addition, for the purpose of further improving the magnetic properties, at least one other element selected from the group comprising Al, Cu, Si, Ga, Ti, V, Ta, Zr, Nb, Mo, Hf, Ag, Zn, P, Ge, Cr and W (hereinafter, referred to as “Q”) may be contained as needed. When containing the element belonging to Q, it is preferable that the content thereof should be equal to or less than 2 at %, and it is more preferable that the content thereof lies within the range of 0.1-1.5 at %, and it is the most preferable that the content thereof lies within the range of 0.2 -1.0 at %.  
     [0070] The addition of the element belonging to Q makes it possible to exhibit an inherent effect of the kind of the element. For example, the addition of Al, Cu, Si, Ga, V, Ta, Zr or Cr exhibits an effect of improving corrosion resistance.  
     [0071] Composite Structure  
     [0072] As described above, the magnetic material of the present invention has a composite structure having a soft magnetic phase and a hard magnetic phase.  
     [0073] In this composite structure (nanocomposite structure), a soft magnetic phase  10  and a hard magnetic phase  11  exist in a pattern (model) as shown in, for example, FIG. 1, FIG. 2 or FIG. 3, where the thickness of the respective phases and the particle diameter are on the order of nanometers (for example, 1-100 nm). Further, the soft magnetic phase  10  and the hard magnetic phase  11  are arranged adjacent to each other (this also includes the case where these phases are adjacent through intergranular boundary phase), which makes it possible to perform magnetic exchange interaction therebetween. In this regard, it is to be noted that the patterns illustrated in FIG. 1 to FIG. 3 are only specific examples, and are not limited thereto. For example, the soft magnetic phase  10  and the hard magnetic phase  11  in FIG. 2 are interchanged to each other.  
     [0074] The magnetization of the soft magnetic phase readily changes its orientation by the action of an external magnetic field. Therefore, when the soft magnetic phase coexists with the hard magnetic phase, the magnetization curve for the entire system shows a stepped “serpentine curve” in the second quadrant of the B-H diagram. However, when the soft magnetic phase has a sufficiently small size of less than several tens of nm, magnetization of the soft magnetic body is sufficiently and strongly constrained through the coupling with the magnetization of the surrounding hard magnetic body, so that the entire system exhibits functions like a hard magnetic body.  
     [0075] A magnet having such a composite structure (nanocomposite structure) has mainly the following five features.  
     [0076] (1) In the second quadrant of the B-H diagram (that is, J-H diagram), the magnetization springs back reversively (in this sense, such a magnet is also referred to as a “spring magnet”).  
     [0077] (2) It has a satisfactory magnetizability, and it can be magnetized with a relatively low magnetic field.  
     [0078] (3) The temperature dependence of the magnetic properties is small as compared with the case where the system is constituted from a hard magnetic phase alone.  
     [0079] (4) The changes in the magnetic properties with the elapse of time are small.  
     [0080] (5) No deterioration in the magnetic properties is observable even if it is finely milled.  
     [0081] In the alloy composition described in the above, the hard magnetic phase and the soft magnetic phase are respectively composed of the following, for instance.  
     [0082] The hard magnetic phase: R 2 TM 14 B system (where, TM is Fe or Fe and Co), or R 2 (TM, Nb) 14 B system (or R 2 (TM, Q) 14 B system, or R 2 (TM, Nb, Q) 14 B system).  
     [0083] The soft magnetic phase: TM (α-Fe or α-(Fe, Co) in particular), or an alloy phase of TM and Nb, a composite phase of TM and B, or a composite phase of TM, B and Nb (or these phases containing Q).  
     [0084] Manufacture of Magnetic Powders  
     [0085] As for the magnetic powders according to this invention, it is preferable that they are manufactured by melt-spinning (quenching) a molten alloy, and more preferable that they are manufactured by milling a melt spun (quenched) ribbon obtained by quenching and solidifying the molten metal of the alloy. An example of such a method will be described in the following.  
     [0086]FIG. 4 is a perspective view showing an example of the configuration of an apparatus (melt spinning apparatus) for manufacturing a magnet material by the melt spinning (quenching) method using a single roll, and FIG. 5 is a sectional side view showing the situation in the vicinity of colliding section of the molten metal with the cooling roll in the apparatus shown in FIG. 4.  
     [0087] As shown in FIG. 4, the melt spinning apparatus  1  is provided with a cylindrical body  2  capable of storing the magnet material, and a cooling roll  5  which rotates in the direction of an arrow  9 A in the figure relative to the cylindrical body  2 . A nozzle (orifice)  3  which injects the molten metal of the magnet material alloy is formed at the lower end of the cylindrical body  2 .  
     [0088] In addition, a heating coil  4  is arranged on the outer periphery of the cylindrical body  2  in the vicinity of the nozzle  3 , and the magnet material in the cylindrical body  2  is melted by inductively heating the interior of the cylindrical body  2  through application of, for example, a high frequency wave to the coil  4 .  
     [0089] The cooling roll  5  is constructed from a base part  51  and a surface layer  52  which forms a circumferential surface  53  of the cooling roll  5 .  
     [0090] The base part  51  may be formed either integrally with the surface layer  52  using the same material, or formed using a material different from that of the surface layer  52 .  
     [0091] Although there is no particular limitation on the material of the base part  51 , it is preferable that the base part  51  is formed of a metallic material with high heat conductivity such as copper or a copper alloy in order to make it possible to dissipate heat of the surface layer  52  as quickly as possible.  
     [0092] Further, it is preferable that the surface layer  52  is formed of a material with heat conductivity equal to or lower than that of the base part  51 . Examples of the surface layer  52  include a metallic thin layer of Cr or the like, a layer of metallic oxide and a ceramic layer.  
     [0093] Examples of the ceramics for use in the ceramic layer include oxide ceramics such as Al 2 O 3 , SiO 2 , TiO 2 , Ti 2 O 3 , ZrO 2  Y 2 O 3 , barium titanate, and strontium titanate and the like; nitride ceramics such as AlN, Si 3 N 4 , TiN, and BN and the like; carbide ceramics such as graphite, SiC, ZrC, Al 4 C 3 , CaC 2 , and WC and the like; and mixture of two or more of these ceramics.  
     [0094] The melt spinning apparatus  1  is installed in a chamber (not shown), and it is operated preferably under the condition where the interior of the chamber is filled with an inert gas or other kind of gas. In particular, in order to prevent oxidation of a melt spun ribbon  8 , it is preferable that the gas is an inert gas such as argon gas, helium gas, nitrogen gas or the like.  
     [0095] In the melt spinning apparatus  1 , the magnet material (alloy) is placed in the cylindrical body  2  and melted by heating with the coil  4 , and the molten metal  6  is discharged from the nozzle  3 . Then, as shown in FIG. 5, the molten metal  6  collides with the circumferential surface  53  of the cooling roll  5 , and after the formation of a puddle  7 , the molten metal  6  is cooled down rapidly to be solidified while dragged along the circumferential surface  53  of the rotating cooling roll  5 , thereby forming the melt spun ribbon  8  continuously or intermittently. A roll surface  81  of the melt spun ribbon  8  thus formed is soon released from the circumferential surface  53 , and the melt spun ribbon  8  proceeds in the direction of an arrow  9 B in FIG. 4. The solidification interface  71  of the molten metal is indicated by a broken line in FIG. 5.  
     [0096] The optimum range of the circumferential velocity of the cooling roll  5  depends upon the composition of the molten alloy, the wettability of the circumferential surface  53  with respect to the molten metal  6 , and the like. However,for the enhancement of the magnetic properties, a velocity in the range of 1 to 60 m/s is normally preferable, and 5 to 40 m/s is more preferable. If the circumferential velocity of the cooling roll  5  is too small, the thickness t of the melt spun ribbon  8  is too large depending upon the volume flow rate (volume of the molten metal discharged per unit time), and the diameter of the crystalline grains tends to increase. On the contrary, if the circumferential velocity is too large, amorphous structure becomes dominant. Further, in these cases, enhancement of the magnetic properties can not be expected even if a heat treatment is given in the later stage.  
     [0097] Thus obtained melt spun ribbon  8  may be subjected to at least one heat treatment for the purpose of, for example, acceleration of recrystallization of the amorphous structure and homogenization of the structure. The conditions of this heat treatment may be, for example, a heating in the range of 400 to 900° C. for 0.5 to 300 min.  
     [0098] Moreover, in order to prevent oxidation, it is preferred that this heat treatment is preferable to be performed in a vacuum or under a reduced pressure (for example, in the range of 1×10 −1  to 1×10 −6  Torr), or in a nonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas, helium gas or the like.  
     [0099] The melt spun ribbon (thin ribbon-like magnet material)  8  obtained as in the above has a microcrystalline structure or a structure in which microcrystals are included in an amorphous structure, and exhibits excellent magnetic properties. The magnetic powder of this invention is obtained by milling thus obtained melt spun ribbon  8 .  
     [0100] The milling method of the melt spun ribbon is not particularly limited, and various kinds of milling or crushing apparatus such as ball mill, vibration mill, jet mill, and pin mill maybe employed. In this case,in order to prevent oxidation, the milling process may be carried out under vacuum or reduced pressure (for example, under a reduced pressure of 1×10 −1  to 1××10 −6  Torr), or in a nonoxidizing atmosphere of an inert gas such as nitrogen, argon, helium, or the like.  
     [0101] The average particle size of the magnetic powder is not particularly limited. However, in the case where the magnetic powder is used for manufacturing isotropic bonded magnets described later, in order to prevent oxidation of the magnetic powder and deterioration of the magnetic properties during the milling process, it is preferred that the average particle size lies within the range of 0.5 to 150 μm, more preferably the range of 0.5 to 80 μm, and still more preferably the range of 1 to 50 μm.  
     [0102] In order to obtain a better mold ability of the bonded magnet, it is preferable to give a certain degree of dispersion to the particle size distribution of the magnetic powder. By so doing, it is possible to reduce the void ratio (porosity) of the bonded magnet obtained. As a result, it is possible to raise the density and the mechanical strength of the bonded magnet as compared with a bonded magnent having the same content of the magnetic powder, thereby enabling to further improve the magnetic properties.  
     [0103] Thus obtained magnetic powder may be subjected to a heat treatment for the purpose of, for example, removing the influence of stress introduced by the milling process and controlling the crystalline grain size. The conditions of the heat treatment are, for example, heating at a temperature in the range of 350 to 850° C. for 0.5 to 300 min.  
     [0104] In order to prevent oxidation of the magnetic powder, it is preferable to perform the heat treatment in a vacuum or under a reduced pressure (for example, in the range of 1×10 −1  to 1××10 −6  Torr), or in a nonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas, and helium gas.  
     [0105] Thus obtained magnetic powder has a satisfactory bindability with the binding resin (wettability of the binding resin). Therefore, when a bonded magnet is manufactured using the magnetic powder described above, the bonded magnet has a high mechanical strength and excellent thermal stability (heat resistance) and corrosion resistance. Consequently, it can be concluded that the magnetic powder is suitable for the manufacture of the bonded magnet.  
     [0106] In the above, the melt spinning (quenching) method is described in terms of the single roll method, but the twin roll method may also be employed. Besides, other methods such as the atomizing method which uses gas atomization, the rotating disk method, the melt extraction method, and the mechanical alloying method (MA) may also be employed. Since such a melt spinning method can refine the microstructure (crystalline grains), it is effective for enhancing the magnetic properties, especially the coercive force or the like, of the bonded magnet.  
     [0107] Bonded Magnets and Manufacture thereof  
     [0108] Next, the isotropic bonded magnets (hereinafter, referred to simply also as “bonded magnets”) according to this invention will be described.  
     [0109] Preferably, the bonded magnets of this invention is formed by binding the above described magnetic powder using a binding resin (binder).  
     [0110] As for the binder, either of a thermoplastic resin or a thermosetting resin may be employed.  
     [0111] Examples of the thermoplastic resin include polyamid (example: nylon 6, nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12, nylon 6-12, nylon 6-66, nylon 6T and nylon 9T); thermoplastic polyimide; liquid crystal polymer such as aromatic polyester; poly phenylene oxide; poly phenylene sulfide; polyolefin such as polyethylene, polypropylene and ethylene-vinyl acetate copolymer; modified polyolefin; polycarbonate; poly methyl methacrylate; polyester such as poly ethylen terephthalate and poly butylene terephthalate; polyether; polyether ether ketone; polyetherimide; polyacetal; and copolymer, blended body, and polymer alloy having at least one of these materials as a main ingredient. In this case, a mixture of two or more kinds of these materials may be employed.  
     [0112] Among these resins, a resin containing polyamide as its main ingredient is particularly preferred from the viewpoint of especially excellent moldability and high mechanical strength. Further, a resin containing liquid crystal polymer and/or poly phenylene sulfide as its main ingredient is also preferred from the viewpoint of enhancing the heat resistance. Furthermore, these thermoplastic resins also have an excellent kneadability with the magnetic powder.  
     [0113] These thermoplastic resins provide an advantage in that wide range of selection can be made. For example, it is possible to provide a thermoplastic resin having a good moldability or to provide a thermoplastic resin having good heat resistance and mechanical strength by appropriately selecting their kinds, copolymerization or the like.  
     [0114] On the other hand, examples of the thermosetting resin include various kinds of epoxy resins of bisphenol type, novolak type, and naphthalene-based, phenolic resin, urea resin, melamine resin, polyester (or unsaturated polyester) resin, polyimide resin, silicone resin, polyurethane resin, and the like. In this case, a mixture of two or more kinds of these materials may be employed.  
     [0115] Among these resins, the epoxy resin, phenolic resin, polyimide resin and silicone resin are preferable from the viewpoint of their special excellence in the moldability, high mechanical strength, and high heat resistance. In this case, the epoxy resin is especially preferable. These thermosetting resins also have an excellent kneadability with the magnetic powder and homogeneity (uniformity) in kneading.  
     [0116] The unhardened thermosetting resin to be used may be either in liquid state or in solid (powdery) state at the room temperature.  
     [0117] The bonded magnet according to this invention described in the above may be manufactured, for example, as in the following. First, the magnetic powder, a binding resin and an additive (antioxidant, lubricant, or the like) as needed are mixed and kneaded (warm kneading) to form a bonded magnet composite (compound). Then, thus obtained bonded magnet composite is formed into a desired magnet form in a space free from magnetic field by a molding method such as compaction molding (press molding), extrusion molding, or injection molding. When the binding resin used is a thermosetting type, the obtained green compact is hardened by heating or the like after molding.  
     [0118] In these three types of molding method, the extrusion molding and the injection molding (in particular, the injection molding) have advantages in that the latitude of shape selection is broad, the productivity is high, and the like. However, these molding methods require to ensure a sufficiently high fluidity of the compound in the molding machine in order to obtain satisfactory moldability. For this reason, in these methods it is not possible to increase the content of the magnetic powder, namely, to make the bonded magnet having high density, as compared with the case of the compaction molding method. In this invention, however, it is possible to obtain a high magnetic flux density as will be described later, so that excellent magnetic properties can be obtained even without making the bonded magnet high density. This advantage of the present invention can also be extended even in the case where bonded magnets are manufactured by the extrusion molding method or the injection molding method.  
     [0119] The content of the magnetic powder in the bonded magnet is not particularly limited, and it is normally determined by considering the kind of the molding method and the compatibility of moldability and high magnetic properties. More specifically, it is preferable to be in the range of 75-99.5 wt %, and more preferably in the range of 85-97.5 wt %.  
     [0120] In particular, in the case of a bonded magnet to be manufactured by the compaction molding method, the content of the magnetic powder should preferably lie in the range of 90-99.5 wt %, and more preferably in the range of 93-98.5 wt %.  
     [0121] Further, in the case of a bonded magnet to be manufactured by the extrusion molding or the injection molding, the content of the magnetic powder should preferably lie in the range of 75-98 wt %, and more preferably in the range of 85-97 wt %.  
     [0122] The density ρ of the bonded magnet is determined by factors such as the specific gravity of the magnetic powder contained in the magnet and the content of the magnetic powder, and void ratio(porosity) of the bonded magnet and the like. In the bonded magnets according to this invention, the density ρ is not particularly limited to a specific value, but it is preferable to be in the range of 5.3-6.6 Mg/m 3 , and more preferably in the range of 5.5-6.4 Mg/m 3 .  
     [0123] In this invention, since the magnetic flux density and the coercive force of the magnetic powder are high, the bonded magnet formed from the magnetic powder provides excellent magnetic properties (especially, high maximum magnetic energy product (BH) max ) even when the content of the magnetic powder is relatively low. In this regard, it goes without saying that it is possible to obtain the excellent magnetic properties in the case where the content of the magnetic powder is high.  
     [0124] The shape, dimensions, and the like of the bonded magnet manufactured according to this invention are not particularly limited. For example, as to the shape, all shapes such as columnar, prism-like, cylindrical (ring-shaped), circular, plate-like, curved plate-like, and the like are acceptable. As to the dimensions, all sizes starting from large-sized one to ultraminuaturized one are acceptable. However, as repeatedly described in this specification, the present invention is particularly advantageous in miniaturization and ultraminiaturization of the bonded magnet.  
     [0125] Further, in view of the advantages described above, it is preferred that the bonded magnet of the present invention is subject to multipolar magnetization has been magnetized so as to have multipoles.  
     [0126] The bonded magnet of this invention as described in the above has magnetic properties in which the irreversible susceptibility (χ irr ) which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram (that is, coordinate where the longitudinal axis represents magnetization (J) and the horizontal axis represents magnetic field (H)) representing the magnetic characteristics at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10 −6  H/m as a starting point is equal to or less than 5.0×10 −7  H/m, and the intrinsic coercive force (H CJ ) of the magnet at the room temperature is in the range of 320-720 kA/m. Hereinafter, explanation will be made with regard to the relationship among the irreversible susceptibility (χ irr ), the intrinsic coercive force (H CJ ), the remanent magnetic flux density (Br) and the density (ρ).  
     [0127] Irreversible Susceptibility (χ irr )  
     [0128] As shown in FIG. 6, the irreversible susceptibility (χ irr ) is the parameter which is represented by the following formula (unit is Henry/m, which is represented by H/m in this specification), wherein a gradient of a tangential line of the demagnetization curve at a certain point P on the demagnetization curve in the J-H diagram is defined by differential susceptibility (χ dif ) and a gradient of a recoil curve when the recoil curve from the point P is drawn with the demagnetization field being once reduced (that is, a gradient connecting the both ends of the recoil curve) is defined by reversible susceptibility (χ irr ).  
     [0129] Irreversible Susceptibility (χ irr )=differential susceptibility (χ dif )−reversible susceptibility (χ rev )  
     [0130] In this connection, it is to be noted that in the present invention, the point P is defined as an intersectioning point of the demagnetization curve and the straight line y which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10 −6  H/m.  
     [0131] In the present invention, the reason why the upper limit value of the irreversible susceptibility (χ irr ) at the room temperature is defined as 5.0×10 −7  H/m is as follows.  
     [0132] As stated in the above, the irreversible susceptibility (χ irr ) represents the changing ratio of the magnetization, which is not returned even if the absolute value thereof is reduced once after demagnetization field is applied, with respect to the magnetic field. Accordingly, by restraining the irreversible susceptibility (χ irr ) to a relatively small value, it is possible to improve heat stability of the bonded magnet and especially to reduce the absolute value of the irreversible flux loss. Actually, within the range of the irreversible susceptibility (χ irr ) of the present invention, the irreversible flux loss obtained when the bonded magnet is being left in the atmosphere of 100° C. for one hour and then the temperature is lowered into room temperature is equal to or less than 5% in its absolute value, which means that practically satisfactory heat resistance (in particular when used in motors or the like), that is satisfactory heat stability can be obtained.  
     [0133] In contrast, when the irreversible susceptibility (χ irr ) exceeds 5.0×10 −7  H/m, the absolute value of the irreversible flux loss is increased, so that it is not possible to obtain satisfactory heat stability. Further, since the intrinsic coercive force (H CJ ) is lowered and the rectangularity thereof becomes poor, use of the obtained bonded magnet is limited to the case where the permeance coefficient (Pc) becomes large (e. g. Pc≧5). Furthermore, the lowered coercive force reduces the heat stability.  
     [0134] The reason why the irreversible susceptibility (χ irr ) at the room temperature is defined as 5.0×10 −7  H/m is described above. However, it is preferable that the value of the irreversible susceptibility (χ irr ) is as smaller as possible. Therefore, in the present invention, it is preferable that the irreversible susceptibility (χ irr ) is equal to or less than 4.5×10 −7  H/m, and it is more preferable that the irreversible susceptibility (χ irr ) is equal to or less than 4.0×10 −7  H/m.  
     [0135] Intrinsic Coercive Force (H CJ )  
     [0136] It is preferred that the intrinsic coercive force (H CJ ) of the bonded magnet at room temperature is 320 to 720 kA/m, and 400 to 640 kA/m is more preferable.  
     [0137] If the intrinsic coercive force (H CJ ) exceeds the above upper limit value, magnetizability is deteriorated and therefore satisfactory magnetic flux density can not be obtained.  
     [0138] On the other hand, if the intrinsic coercive force (H CJ ) is lower than the lower limit value, demagnetization occurs conspicuously when a reverse magnetic field is applied depending upon the usage of the motor and the heat resistance at a high temperature is deteriorated. Therefore, by setting the intrinsic coercive force (H CJ ) to the above range, in the case where the bonded magnet (cylindrical magnet in particular) is subjected to multipolar magnetization, a satisfactory magnetization can be accomplished even when a sufficiently high magnetizing field cannot be secured, which makes it possible to obtain a sufficient magnetic flux density, and to provide a high performance bonded magnet, especially a bonded magnet for motor.  
     [0139] Relationship between Remanent Magnetic Flux Density (Br) and Density (ρ)  
     [0140] In the present invention, it is preferred that the following formula (I) is satisfied between the remanent magnetic flux density Br(T) and the density ρ(Mg/m 3 ). 
     0.125≦ Br/ρ[× 10 −6   T·m   3   /g]   (I) 
     [0141] In this connection, it is more preferable that the following formula (II) is satisfied between the remanent magnetic flux density Br(T) and the density ρ (Mg/m 3 ), and it is most preferable that the following formula (III) is satisfied therebetween. 
     0.128≦ Br/ρ[× 10 −6   T·m   3   /g]≦ 0.16  (II) 
     0.13≦ Br/ρ[× 10 −6   T·m   3   /g]≦ 0.155  (III) 
     [0142] When the value of Br/ρ[×10 −6   T·m   3   g] is less than the lower limit value of the formula (I), it is not possible to obtain a sufficient magnetic flux density unless otherwise the density of the magnet is increased, that is the content of the magnetic powder in the magnet is increased. However, this in turn leads to problems in that available molding methods are limited, manufacturing cost is increased, and moldability is lowered due to a reduced amount of the binding resin. Further, when a magnetic flux density of a certain level is to be obtained, a volume of the magnet is necessarily increased, which results in difficulty in miniaturizing devices such as motors.    
     [0143] Maximum Magnetic Energy Product (BH) max    
     [0144] In the present invention, it is preferable that the maximum magnetic energy product (BH) max  of the bonded magnet is equal to or greater than 60 kJ/m 3 , more preferably equal to or greater than 65 kJ/m 3 , and most preferably in the range of 70 to 130 kJ/m 3 . When the maximum magnetic energy product (BH) max  is less than 60 kJ/m 3 , it is not possible to obtain a sufficient torque when used for motors depending on the types and structures thereof.  
     [0145] Irreversible Flux Loss  
     [0146] In the present invention, it is preferable that the absolute value of the irreversible flux loss (that is, initial flux loss) is equal to or less than 6.2%, it is more preferable that it is equal to or less than 5.0%, and it is most preferable that it is equal to or less than 4%. This makes it possible to obtain a bonded magnet having excellent heat stability (heat resistance).  
     EXAMPLES  
     [0147] Hereinbelow, the actual examples of the present invention will be described.  
     Example 1  
     [0148] Magnetic powders with alloy compositions (Nd 0.7 Pr 0.25 Dy 0.05 ) 8.7 Fe ba1 Co 7.0 B 5.6 Nb w  (that is, various types of magnetic powders in which the content w of Nb is changed variously) were obtained by the method described below.  
     [0149] First, each of the materials Nd, Pr, Dy, Fe, Co, B and Nb was weighed, and then they were cast to produce a mother alloy ingot, and a sample of about 15 g was cut out from the ingot.  
     [0150] A melt spinning apparatus  1  as shown in FIG. 4 and FIG. 5 was prepared, and the sample was placed in a quartz tube  2  having a nozzle  3  (circular orifice of which diameter is 0. 55 mm) at the bottom. After evacuating the interior of a chamber in which the melt spinning apparatus  1  is housed, an inert gas (Ar gas) was introduced to obtain an atmosphere with desired temperature and pressure.  
     [0151] The cooling roll  5  of the melt spinning apparatus  1  is provided with a surface layer  52  on the outer periphery of the base part  51  made of Cu. The surface layer  52  is formed of WC and has a thickness of about 5 μm.  
     [0152] Then, the ingot sample in the quartz tube  2  was melted by high frequency induction heating. Further, the jetting pressure (difference between the inner pressure of the quartz tube  2  and the pressure of the atmosphere) and the circumferential velocity were adjusted to obtain a melt spun ribbon.  
     [0153] Thus obtained melt spun ribbon was then coarsely crushed, and the powder was subjected to a heat treatment in an argon gas atmosphere at 710° C. for 300 sec. In this way, the various types of magnetic powders each having different contents w of Nb were obtained.  
     [0154] Next, for the purpose of adjustment of the particle size, each magnetic powder is milled by a milling machine in an argon gas atmosphere to obtain a magnetic powder having an average particle size of 60 μm.  
     [0155] To analyze the phase structure of the obtained magnetic powders, the respective magnetic powder was subjected to X-ray diffraction using Cu-Kα line at the diffraction angle of 20°-60°. From the thus obtained diffraction pattern, the presence of diffracted peaks of a hard magnetic phase, R 2 (Fe.Co) 14 B phase, and a soft magnetic phase, α-(Fe,Co) phase, were confirmed. Further, from the observation result using a transmission electron microscope (TEM), the formation of a composite structure (nanocomposite structure) was confirmed in each magnetic powder.  
     [0156] A composite (compound) for bonded magnet was prepared by mixing the respective magnetic powder with a polyamide resin (Nylon  12 ) and a small amount of hydrazine antioxidant and lubricant, and then kneading them under the temperature of 250° C. for 15 min. In this case, the compounding ratio (mixing ratio by weight) of the magnetic powder with respect to the polyamide resin was common to the respective bonded magnets. Specifically, in each of the bonded magnets, the content of the magnetic powder was about 97 wt %.  
     [0157] Then, each of the thus obtained compounds was crushed to be granular. Then, the granular substance (particle) was weighed and filled into a die of a press machine, and then it was subjected to a compaction molding (in the absence of a magnetic field) under the temperature of 210° C. and the pressure of 800 MPa, to obtain an isotropic bonded magnet of a columnar shape having a diameter of 10 mm and a height of 7 mm. Next, the density ρ of each bonded magnet was measured by means of the Archimedes&#39; method. The results thereof were shown in the Table 1.  
     [0158] Evaluation for Magnetic Properties and Irreversible Susceptibility (χ irr )  
     [0159] After pulse magnetization is performed for the respective bonded magnets under the magnetic field strength of 3.2 MA/m, magnetic properties (remanent magnetic flux density Br, intrinsic coercive force (H CJ ), and maximum magnetic energy product (BH) max ) were measured using a DC recording flux meter (manufactured and sold by Toei Industry Co. Ltd with the product code of TRF-5BH) under the maximum applied magnetic field of 2.0 MA/m. The temperature at the measurement was 23° C. (that is, room temperature).  
     [0160] As shown in FIG. 7, in the measured demagnetization curve of J-H diagram, a recoil curve having a starting point at an intersectioning point P between the demagnetization curve and a straight line which passes an origin and has a gradient of −3.8×10 −6  H/m was produced with the magnetic field being once changed to zero and then being returned the original state, and then the gradient of the recoil curve (that is, the gradient of the straight line connecting the both ends of the recoil curve) was obtained and then it was defined as the reversible susceptibility(χ rev ). Further, the gradient of a tangential line of the demagnetization curve at the intersectioning point P was obtained and then it was defined as the differential susceptibility (χ dif ). The irreversible susceptibility (χ irr ) was obtained by the formula of χ irr =χ dif −χ rev . The results of them are shown in the attached Table 1.  
     [0161] Evaluation for Heat Resistance  
     [0162] Next, the heat resistance (heat stability) of each of the bonded magnets (each having the column shape having the diameter of 10 mm and the height of 7mm) was examined. The heat resistance was obtained by measuring the irreversible flux loss (initial flux loss) obtained when the bonded magnet was being left in the atmosphere of 100° C. for one hour and then the temperature was lowered to the room temperature, and then it was evaluated. The results thereof are shown in the attached Table 1. In this connection, it is to be noted that smaller absolute value of the irreversible flux loss (initial flux loss) means more excellent heat resistance (heat stability).  
     [0163] Total Evaluation  
     [0164] As seen from the attached Table 1, the isotropic bonded magnets (No. 2 to No. 6) formed of the magnetic powders in which the content w of Nb is 0.2 to 3.5 at % and the irreversible susceptibility (χ irr ) is equal to or less than 5.0×10 −7  H/m exhibit excellent magnetic properties (remanent magnetic flux density, intrinsic coercive force and maximum magnetic energy product) and have small absolute value of the irreversible flux loss, so that the heat resistance of these magnets is high and the magnetizability thereof is excellent.  
     [0165] As described above, according to the present invention, it is possible to obtain bonded magnets having high performance and high reliability (especially, heat resistance). In particular, these high performances are exhibited when the bonded magnets are used in motors.  
     Example 2  
     [0166] In the same manner as Example 1, magnetic powders with alloy compositions (Nd 0.75 Pr 0.20 Dy 0.05 ) 8.9 Fe bal Co 6.9 B 5.5 Nb 1.5  were obtained by the method described below.  
     [0167] A composite (compound) for bonded magnet was prepared by mixing the respective magnetic powder with a polyamide resin (Nylon  12 ) and a small amount of hydrazine antioxidant and lubricant, and then kneading them under the temperature of 200-230° C. for 15 min. In this case, the content of the magnetic powder to be contained in each of the bonded magnets was variously changed to obtain seven types of compounds.  
     [0168] Among thus obtained compounds, the compounds having a relatively high content of the magnetic powder were crushed to be granular, and then they were subjected to a compaction molding (in the absence of a magnetic field), while the compounds having a relatively low content of the magnetic powder were crushed to be granular, and then they were subjected to an injection molding (in the absence of a magnetic field), thereby forming bonded magnets.  
     [0169] In this connection, it is to be noted that each bonded magnet was formed into a columnar shape having a diameter of 10 mm and a height of 7 mm.  
     [0170] Further, it is also to be noted that the compaction molding was carried out by filing each granular substance (particle) into a die of a press machine and then it was subjected to a compaction molding under the temperature of 210-220° C. and the pressure of 800 MPa. Further, the injection molding was carried out under the conditions that the die temperature at molding was 90° C. and the temperature inside the injection cylinder was 230-280° C.  
     [0171] For each of thus obtained bonded magnets, magnetic properties thereof were measured and heat resistance thereof was also tested in the same manner as the Example 1. The results thereof are shown in the attached Table 2.  
     [0172] Total Evaluation  
     [0173] As seen from the attached Table 2, the bonded magnets according to the present invention exhibit, over the wide range of the density ρ, excellent magnetic properties (remanent magnetic flux density Br,maximum magnetic energy product(BH) max , and coercive force (H CJ )) and have a small absolute value of the irreversible flux loss, so that the heat stability (heat resistance) of these magnets is also excellent.  
     [0174] In particular, the bonded magnets according to the present invention exhibit excellent magnetic properties even in the case where the bonded magnets are low density bonded magnets (that is, bonded magnets having a small content of the magnetic powder) which can be obtained by means of an injection molding. The reason of this is supposed as follows.  
     [0175] When bonded magnets are low density, that is bonded magnets have a relatively large content of the binding resin, fluidity of the compound during the kneading process or molding process is high. This makes it possible to knead the magnetic powder and the binding resin at a relatively low temperature within a short time period, so that it is possible to easily accomplish that the magnetic powder and the binding resin are uniformly mixed during the kneading process. Further, such a high fluidity of the compound makes it possible to easily carry out the molding at a relatively low temperature within a short time period. In other words, molding conditions can be moderated. As a result, it becomes possible to hold the deterioration (e.g. oxidization) of the magnetic powder during the kneading process and molding process at the minimum level, which results in production of bonded magnets having high magnetic properties as well as improvement of the moldability.  
     [0176] Further, the high fluidity of the compound makes it possible to lower a void ratio of the obtained bonded magnets, so that mechanical strength and magnetic properties thereof are also improved.  
     Example 3  
     [0177] Using the magnetic powders obtained by Example 1, cylindrical (ring-shaped) isotropic bonded magnets having outer diameter of 22 mm, inner diameter of 20 mm and height of 4 mm were manufactured in the same manner as Example 1. Then, thus obtained bonded magnets were subjected to a multi-pole magnetization so as to have eight poles. At the magnetization process, an electric current of 16 kA was flowing through a magnetizing coil.  
     [0178] In this case, a magnitude of the magnetizing magnetic field for achieving 90% magnetizing ratio was relatively small, and this means that the magnetizability was excellent.  
     [0179] Further, using each bonded magnet as a magnet for a rotor, a spindle motor for CD-ROM drive was assembled. Then, each of the DC motors was rotated at 1000 rpm to measure a back electromotive force generated in the coil winding thereof. As a result, it has been confirmed that a voltage equal to or less than 0. 80V can be obtained in the motors using the bonded magnets of the samples No. 1 and No. 7 (Comparative Examples), while a voltage equal to or greater than 0. 96V which is more than 20% higher value can be obtained in the motors using the bonded magnets of the samples No. 2 to No. 6 (Example of present invention).  
     [0180] With this result, it has found that it is possible to manufacture high performance motors by using the bonded magnets of the present invention.  
     [0181] In addition to the above, bonded magnets same as those of Examples 1 to 3 were manufactured excepting that they are formed by means of an extrusion molding (the content of the magnetic powder in each bonded magnet was 92 to 95 wt %). Then, the performance of these bonded magnets were examined. As a result, it has found that the same results can be obtained by the motors using the bonded magnets  
     [0182] Further, bonded magnets same as those of Examples 1 to 3 were manufactured excepting that they are formed by means of an injection molding (the content of the magnetic powder in each bonded magnet was 90 to 93 wt %). Then, the performance of these bonded magnets were examined. As a result, it has found that the same results can be obtained by the motors using the bonded magnets  
     [0183] As described above, according to the present invention, the following effects can be obtained.  
     [0184] Since each of the magnetic powders contains a predetermined amount of Nb and has a composite structure having a soft magnetic phase and a hard magnetic phase, they have high magnetization and exhibit excellent magnetic properties. In particular, intrinsic coercive force and rectangularity thereof are improved.  
     [0185] The absolute value of the irreversible flux loss is small and excellent heat resistance (heat stability) can be obtained.  
     [0186] Because of the high magnetic flux density that can be secured by this invention, it is possible to obtain a bonded magnet with high magnetic performance even if it is isotropic. In particular, since magnetic properties equivalent to or better than the conventional isotropic bonded magnet can be obtained with a magnet of smaller volume as compared with the conventional isotropic bonded magnet, it is possible to provide a high performance motor of a smaller size.  
     [0187] Moreover, since a higher magnetic flux density can be secured, in manufacturing a bonded magnet sufficiently high magnetic performance is obtainable without pursuing any means for elevating the density of the bonded magnet. As a result, the dimensional accuracy, mechanical strength, corrosion resistance, heat resistance (heat stability) and the like can be improved in addition to the improvement in the moldability, so that it is possible to readily manufacture a bonded magnet with high reliability.  
     [0188] Since the magnetizability of the bonded magnet according to this invention is excellent, it is possible to magnetize a magnet with a lower magnetizing field. In particular, multipolar magnetization or the like can be accomplished easily and surely, and further a high magnetic flux density can be obtained.  
     [0189] Since a high density is not required to the bonded magnet, the present invention is adapted to the manufacturing method such as the extrusion molding method or the injection molding method by which molding at high density is difficult as compared with the compaction molding method, and the effects described in the above canal also be realized in the bonded magnet manufactured by these molding methods. Accordingly, various molding method can be selectively used and thereby the degree of selection of shape for the bonded magnet can be expanded.  
     [0190] Finally, it is to be understood that the present invention is not limited to Examples described above, and many changes or additions may be made without departing from the scope of the invention which is determined by the following claims.  
                                               TABLE 1                                                       Irreversible               p   Br   H cJ     (BH) max     Br/p   X irr     Flux Loss       Sample No.   W   (Mg/m 3 )   (T)   (kA/m)   (kJ/m 3 )   (×10 −6 T · m 3 /g)   (×10 −7 H/m)   (%)                                                                    1 (Comp. Ex.)   0.1   6.27   0.83   345   75.6   0.132   7.5   −6.5       2 (This Invention)   0.2   6.26   0.87   415   104.8   0.139   4.8   −4.7       3 (This Invention)   0.5   6.32   0.90   478   113.2   0.142   3.7   −4.0       4 (This Invention)   1.2   6.29   0.92   496   115.9   0.146   3.2   −3.6       5 (This Invention)   2.5   6.30   0.90   530   112.0   0.143   3.0   −3.2       6 (This Invention)   3.3   6.33   0.81   561   102.7   0.128   2.7   −2.7       7 (Comp. Ex.)   3.6   6.31   0.76   553   79.1   0.120   3.3   −3.5                  
 
     [0191]                                                       TABLE 2                           Kneading       Molding                           Irreversible           Temp.   Molding   Temp.   p   Br   H cJ     (BH) max     Br/p   X irr     Flux Loss       Sample No.   (° C.)   Method   (° C.)   (Mg/m 3 )   (T)   (kA/m)   (kJ/m 3 )   (×10 −6 T · m 3 /g)   (×10 −7 H/m)   (%)                                                                             8 (This Invention)   200   Injection   230   5.30   0.78   563   83.4   0.147   2.1   −2.2               Molding        9 (This Invention)   203   Injection   245   5.50   0.80   551   88.3   0.146   2.3   −2.5               Molding       10 (This Invention)   211   Injection   260   5.67   0.82   542   92.6   0.145   2.5   −2.9               Molding       11 (This Invention)   216   Injection   275   5.80   0.84   535   96.2   0.144   2.7   −3.1               Molding       12 (This Invention)   220   Compaction   210   5.95   0.85   531   100.5   0.143   2.9   −3.4               Molding       13 (This Invention)   224   Compaction   215   6.21   0.88   517   108.8   0.142   3.2   −3.7               Molding       14 (This Invention)   230   Compaction   220   6.48   0.92   510   118.4   0.142   3.8   −4.2               Molding