New types of particle feedstocks and heterogeneous grain structures are provided for rare earth permanent magnets (REPMs) and their production in a manner to significantly enhance toughness of the magnet with little or no sacrifice in the hard magnetic properties. The novel tough REPMs made from the feedstock have heterogeneous grain structures, such as bi-modal, tri-modal, multi-modal, laminated, gridded, gradient fine/coarse grain structures, or other microstructural heterogeneity and configurations, without changing the chemical compositions of magnets.

A ferrite sintered magnet includes a composition expressed by a formula (1) of Ca.sub.1-w-xLa.sub.wA.sub.xFe.sub.zCo.sub.mMn.sub.aO.sub.19. In the formula (1), w, x, z, m, and a satisfy a formula (2) of 0.21w0.62, a formula (3) of 0.02x0.46, a formula (4) of 7.43z11.03, a formula (5) of 0.18m0.41, and a formula (6) of 0.046a0.188. In the formula (1), A is at least one kind of element selected from a group consisting of Sr and Ba.

A ferrite sintered body contains from 48.2% by mole to 49.7% by mole Fe in terms of Fe.sub.2O.sub.3, from 2.0% by mole to 8.0% by mole Cu in terms of CuO, from 12.0% by mole to 19.0% by mole Ni in terms of NiO, and from 28.5% by mole to 33.0% by mole Zn in terms of ZnO, in which when Fe, Cu, Ni, and Zn are converted to Fe.sub.2O.sub.3, CuO, NiO, and ZnO, respectively, and when the total amount of the Fe.sub.2O.sub.3, the CuO, the NiO, and the ZnO is 100 parts by weight, the ferrite sintered body contains from 5 ppm to 25 ppm B in terms of elemental B and 6 ppm to 25 ppm Nb in terms of elemental Nb.

A method comprising sintering a Mn and Bi powder compact at a first temperature for a first predetermined duration, based on the first temperature, and sintering the compact at a second temperature, less than the first temperature, for a second predetermined duration, greater than the first duration, is disclosed. The sintering at a first temperature for a first predetermined duration generates a predetermined MnBi LTP transition driving force to decrease a formation energy barrier for transition to MnBi LTP. Sintering the compact at the second temperature for the second predetermined duration forms a magnet containing the MnBi LTP.

This invention provides for a rare-earth permanent magnet-forming sintered body having an integral sintered structure of magnet material particles containing a rare-earth substance. The integral sintered structure is formed in a three-dimensional shape having: a cross-section with a shape defined by a radially outer-side arc-shaped surface having a first curvature radius, a radially inner-side arc-shaped surface having a second curvature radius less than the first curvature radius and having an arc shape concentric with the outer-side arc-shaped surface; and a first end face and a second end face each of which is a radially-extending face along a virtual radial line extending from a curvature center of the arc shapes; and an axial length extending in a direction perpendicular to the cross-section.

A method for producing rare-earth magnets is provided in which, when a slurry 2 having a rare-earth-compound powder dispersed therein is applied to sintered magnet bodies 1 and dried to apply the powder thereto, the magnet bodies 1 are accommodated and conveyed in holding pockets 42 of a conveyance drum 4 which rotates in a state of being partially immersed in the slurry 2, and, as a result, the magnet bodies 1 are immersed in the slurry 2, withdrawn from the slurry 2, and dried to apply the powder to the sintered magnet bodies 1. According to this production method, the powder can be uniformly and efficiently applied, wastage of the rare-earth compound can be effectively suppressed, and a reduction in the surface area of equipment for performing an application step can also be achieved.

The present invention provides a method for producing a rare earth sintered magnet and a molding device therefor that can stably mold molded bodies with less variation in unit weight. The method includes: 1) preparing a slurry that includes an alloy powder containing a rare earth element, and a dispersion medium; 2) disposing an upper punch and a lower punch in respective through holes provided in a die, thereby preparing a plurality of cavities; 3) applying a magnetic field in each of the cavities by an electromagnet in a direction substantially parallel to a direction in which at least one of the upper punch and the lower punch is movable, and then supplying the slurry into the plurality of cavities; 4) producing a molded body of the alloy powder in each of the cavities by press molding in the magnetic field; and 5) sintering the molded body.

A manufacturing method of a rare-earth magnet includes: manufacturing a first sealing body by filling a graphite container with a magnetic powder to be a rare-earth magnet material and by sealing the graphite container; manufacturing a sintered body by sintering the first sealing body to manufacture a second sealing body in which the sintered body is accommodated; and manufacturing a rare-earth magnet by performing hot plastic working on the second sealing body to give magnetic anisotropy to the sintered body.

Provided is a heretofore non-existing, novel rare-earth sintered magnet having both of an extremely low carbon content and an extremely small average particle size of magnet material particles. The sintered body for forming a rare-earth magnet comprises a large number of magnet material particles sintered together, wherein each of the magnet material particles contains a rare-earth substance and has an easy magnetization axis. This sintered body for forming a rare-earth magnet has a carbon content of 500 ppm or less, and the magnet material particles have an average particle size of 2 m or less.

The present invention provides a rare earth based magnet that inhibits the high temperature demagnetization rate even when less or no heavy rare earth elements such as Dy, Tb and the like than before are used. The rare earth based magnet according to the present invention is a sintered magnet which includes R.sub.2T.sub.14B crystal grains as main phase and grain boundary phases between the R.sub.2T.sub.14B crystal grains. when evaluating the cross-sectional area distribution of the main phase crystal grains by histogram in any cross-section of the rare earth based magnet, the crystal grains with large particle size and the crystal grains with small particle size are controlled so that the cross-sectional area distribution becomes the one which respectively has at least one peak at two sides of the average value of the cross-sectional area.