The present application relates to a technical field of determining an irreversible demagnetization of a grain boundary diffusion NdFeB magnet, and more particularly, to a method for identifying an irreversible demagnetization of a grain boundary diffusion NdFeB magnet by magnetic field distribution. After applying a reverse magnetic field to a saturatedly magnetized grain boundary diffusion NdFeB magnet, if a number of magnetic poles on a non-diffusion face of the grain boundary diffusion NdFeB magnet is increased, it is determined that there is an irreversible demagnetization in the grain boundary diffusion NdFeB magnet.

An R-T-B based permanent containing main phase grains with a composition of (R.sub.1-xY.sub.x).sub.2T.sub.14B (R is rare earth element(s) composed of one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, T is one or more transition metal elements including Fe or a combination of Fe and Co as essential elements, and 0.2x0.7), wherein the residual magnetic flux density Br is 1.1 T or more, the coercivity HcJ is 400 kA/m or less, and the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ is 1.10 or less.

A manufacturing method for a sintered compact includes a first step in which magnetic powder is fabricated by rapid solidification, a second step in which a mass of the magnetic powder is housed in a forming mold, and preliminary heating is performed by placing the mass of the magnetic powder in a preliminary heating part of the forming mold at first temperature that is lower than coarse crystal particle generation temperature, and a third step in which main heating is performed by placing the preliminarily heated mass of the magnetic powder at second temperature that is lower than the coarse crystal particle generation temperature and higher than the first temperature, and press forming is performed while keeping temperature of the magnetic powder at densification temperature or higher.

The present invention provides a producing method of R-T-B-based sintered magnets in which, the recovery chamber 40 includes inert gas introducing means 42, evacuating means 43, a carry-in port, a discharge port 40a, and a recovery container 60. The recovery step includes a carrying-in step of conveying a processing container 50 into the recovery chamber 40, a discharging step of discharging coarsely pulverized powder in the processing container 50 into the recovery chamber 40, a gas introducing step of introducing inert gas into the recovery chamber 40, and an alloy accommodating step of recovering the coarsely pulverized powder into the recovery container 60. Addition of pulverization aid is carried out in the alloy accommodating step. A remaining amount of coarsely pulverized powder in the recovery chamber 40, an oxygen-containing amount of the R-T-B-based sintered magnet is reduced, and magnetic properties are enhanced.

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 R-T-B based alloy powders, wherein R represents at least one rare earth element, and T represents at least one element selected from the group consisting of ferrum and cobalt. The R-T-B based alloy powders have main phase grains, grain boundary phases and additive phases. The main phase grains are composed of R.sub.2T.sub.14B and have an average grain size of 200 nm or more and 500 nm or less. The grain boundary phases are richer in R than the main phase grains. With respect to any cross section of the R-T-B based alloy powders, the coverage of the main phase grains defined by equation 1 with the grain boundary phases with a roundness defined by equation 2 being 0.1 or more and 0.6 or less, is 10% or more and 40% or less. $\begin{array}{c}\mathrm{coverage}\frac{\underset{i}{\overset{}{.Math.}}{l}_i}{\underset{j}{\overset{}{.Math.}}{L}_j}{l}_i\text{:}\begin{array}{c}\mathrm{Circumference}\mathrm{of}\end{array}\end{array}$

A method of preparing a magnet powder, and a magnet powder, are disclosed. The method includes: preparing a neodymium praseodymium (Nd, Pr) mixed oxide containing Nd and Pr; preparing iron (Fe) oxide; preparing boron (B) oxide; mixing the prepared (Nd, Pr) mixed oxide, iron oxide, and boron oxide to prepare a first mixture; mixing the first mixture with calcium (Ca) to prepare a second mixture; inducing diffusion while shaping and pressing the second mixture; reducing the shaped and pressed second mixture to prepare a magnetic substance containing Nd, Fe, and B; powdering the reduced magnetic substance; and removing reduction by-products from the powdered magnetic substance.

A nano magneto-rheological fluid, comprising nano-scale magnetizable magnetic particles, wherein an average particle size or a minimum size in one dimension is less than 100 nanometers; and fluids used as carrier liquids, wherein the magnetic particles are dispersively distributed in the fluids. An apparatus for making the nanometric magnetorheological fluid including a ball mill, a settling separator located downstream of the ball mill for receiving the primary magnetic particles, a magnetic separator located downstream of and connected to the settling separator for receiving the upper layer of fluid containing fine magnetic particles, and an agitator for mixing the desired secondary magnetic particles with a carrier liquid and an additive. A method for making the nano magneto-rheological fluid wherein the nano magneto-rheological fluid has performance advantages such as no remanent magnetization, non-settlement, low viscosity, low abrasive rate for components, long service life, high reliability and fast and clear response.

In a method of manufacturing a permanent magnet having a curved surface, a permeating material including metal particles and a flux is applied to the curved surface of a magnet. The magnet to which the permeating material is applied is then positioned within a furnace and the furnace is placed in a vacuum or filled with inert gas to volatilize a solvent and the like of the flux contained in the permeating material. The furnace is set to be a temperature within a range of 300 through 500 degrees C. to heat the permeating material. This enables the flux to be carbonized to form reticulated carbon. The furnace is then set to be a temperature within a range of 500 through 800 degrees C. to melt the metal particles in the permeating material, thereby permeating the melted metal particles into the magnet through the reticulated carbon uniformly.

The present invention is provided with a quenched alloy for rare earth magnet and a manufacturing method of rare earth magnet. It comprises an R.sub.2T.sub.14B main phase, wherein R is selected from at least one rare earth element including Nd. The average grain diameter of the main phase in the brachyaxis direction is in a range of 1015 m and the average interval of the Nd rich phase is in a range of 1.03.5 m. In the fine powder of the above-mentioned quenched alloy, the number of magnet domains of a single grain decreases. Thus, it is easier for external magnetic field orientation to obtain high performance magnet, and the squareness, coercivity and the thermal resistance of the magnet are sufficiently improved.