Setsuo Fujimura

New Material for Permanent Magnets on a Base of Nd and Fe

A new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm. This phase, which has the approximate composition, 12 at. % Nd, 6 at. % B and balance Fe, possesses remarkable magnetic properties. From the approach to saturation an anisotroy constant of about 3.5 MJ/m3 can be calculated, while saturation magnetization amounts to 1.35 T. The magnetization versus temperature curve shows a Curie temperature of 585 K, which is much higher than those of the Fe and light rare earth binary compounds. Based on the new compound, sintered permanent magnets have been developed which have a record high energy product. Permanent magnet properties and physical properties of a typical specimen which has the composition Nd15B8Fe77 are as follows: Br =1.23 T, HcB =880 kA/m, HcI =960 kA/m, (BH)max =290 kJ/m3, temperature coefficient of Br =−1260 ppm/K, density=7.4 Mg/m3, specific resistivity=1.4 μΩm, Vickers hardn...

Permanent magnet materials based on the rare earth-iron-boron tetragonal compounds

Structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the Nd-Fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique. The study on the new magnet has also been extended to other R-Fe-B componds containing various rare earths (R) and to R-Fe-Co-B alloys. The results are as follows; (1) The sintered Nd-Fe-B magnet is composed of mainly three phases, the Nd 2 Fe 14 B matrix phase plus Nd-rich phase and B-rich phase ∼ Nd 2 Fe 7 B 6 ) as minor phases. (2) Nd 2 Fe 14 B has the space group of P4 2 /mnm. The crystal structure of this phase can be described as a layer structure with alternate stacking sequence of a Nd-rich layer and a sheet formed only by Fe atoms. The sheet of Fe atoms has a structure similar to the σ-phase found in Fe-Cr and Fe-Mo systems. (3) The Nd-rich phase containing more than 95 at.% Nd, 3∼5 at.% Fe and a trace of B has fcc structure with a=0.52 nm. This phase is formed around grain boundaries of the matrix phase. Nd 2 Fe 7 B 6 phase has an one-dimentional incommensurate structure with a=a o and c\simeq8 C o , based on a tetragonal structure with a o =0.716 nm and c o =0.391 nm. (4) In the as sintered Nd 15 Fe 77 B 8 alloy periodic strain contrasts are observed along grain boundaries, which disappear after annealing at 870K. This may be related to the enhancement of the intrinsic coercivity of the sintered magnet by post sintering heat treatment. (5) Stable R 2 Fe 14 B phases are formed by various rare earths except La. Of all the R 2 Fe 14 B compounds, Nd 2 Fe 14 B has the maximum saturation magnetization as high as 1.57 T. Dy and Tb form R 2 Fe 14 B phases with the highest anisotropies. Small additions of these elements greatly enhance the coercive force of the Nd 2 Fe 14 B base magnet. (6) Partial replacement of Fe by Co raises the Curie temperature of the Nd 2 Fe 14 B compound, which improves the temperature coefficient of the remanence of the magnet. But the intrinsic coercive force is decreased by the Co addition.

Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals

The temperature dependence of the saturation magnetization and the magnetocrystalline anisotropy field have been measured on single‐crystal samples of the R2Fe14B compounds for R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm from 4.2 K to the magnetic ordering temperatures. A spin reorientation transition of the Nd2Fe14B type has been found in Ho2Fe14B at 57.6 K in zero field. Another type of spin reorientation caused by anisotropy compensation between the Fe and the R sublattices exists in Er2Fe14B and Tm2Fe14B. The temperature dependence of the angle of the easy direction of magnetization from the c axis has been measured for R=Nd, Ho, Er, and Tm. The relation between the magnetocrystalline anisotropy and the sublattice magnetization is investigated by employing a simplified two‐sublattice molecular field model.

Nd-Fe-B Permanent Magnet Materials

A comprehensive review of the Nd–Fe–B permanent magnet material is given. A historical survey, the basic magnetic properties of Nd2Fe14B and isomorphous compounds, a phase diagram for the ternary Nd–Fe–B system, processing techniques and magnetic properties of sintered Nd–Fe–B magnets, and the future directions for improvements of this type of permanent magnet are discussed.

Magnetic properties of rare‐earth‐iron‐boron permanent magnet materials

Static magnetic measurements have been carried out on single crystals of Nd2Fe14B, Sm2Fe14B, and Y2Fe14B from 4.2 to 590 K. Values of K1 estimated from high field measurements at room temperature are 4.5, −12, and 1.1 MJ/m3 for Nd2Fe14B, Sm2Fe14B, and Y2Fe14B, respectively. Anisotropic behavior of the magnetization versus magnetic field curves in the basal plane has been observed for Sm2Fe14B, indicating large amplitude of the high order coefficients, K2 and K3. In Nd2Fe14B, the magnetization has been found to tilt from the c axis and simultaneously increase in magnitude. Average Fe moment is estimated to be 2.23 μB/Fe at 4.2 K from the saturation magnetization of Y2Fe14B.

Phase Diagram of the Nd-Fe-B Ternary System

An experimental study of the liquidus projection in a phase diagram of the Nd-Fe-B ternary system has been carried out. In the Nd-poor region, it is found that two phases, Fe2B and Nd2Fe7B6(T2), coexist with the liquid. Two monovariant curves, L\rightleftharpoonsFe+Fe2B and L\rightleftharpoonsFe2B+T2, join and form the transition reaction, L+Fe2B\rightleftharpoonsT2+Fe. The end point of the liquid in the Nd-poor regions is the ternary eutectic point, L\rightleftharpoonsFe+Nd2Fe14B(T1)+T2. The T2 and Nd2FeB3(T3) phases simultaneously solidify from the liquid. In the Nd rich region. T1 phase, T2 phase and (Nd) solid solution solidify from the liquid at the end point of the liquid.

Dependence of coercivity on the anisotropy field in the Nd2Fe14B-type sintered magnets

The temperature dependence of the coercivity (HcI)of Nd15(Fe1−xCox)77B8 and (Nd1−xDyx)15Fe77B8 sintered magnets and that of the saturation magnetization (Ms) and the anisotropy field (HA) of Nd2(Fe1−xCox)14B and (Nd1−xDyx)2Fe14B single crystals have been observed in the temperature range between 295 and 800 K. The dependence of the coercivity on the major magnetic properties of the matrix phase in the Nd‐Fe‐B based magnets are investigated using the μ0HA vs μ0HcI+Ms plot. It is demonstrated that this method of analysis is useful in studying the coercivity mechanism of the Nd‐Fe‐B based sintered magnets.

Magnetization and magnetic anisotropy of R2Co14B and Nd2(Fe1−x Cox)14B measured on single crystals

Magnetization and anisotropy field of R2Co14B with R being La, Nd, and Gd and of Nd2(Fe1−xCox)14B have been measured on single‐crystal specimens. The cobalt sublattice has a planar anisotropy: K1=−1.30 MJ/m3 at 4.2 K for La2Co14B with the [110] direction being the room‐temperature easy direction of magnetization. The rare‐earth ions in R2Co14B experience a smaller exchange field than in R2Fe14B, which results in a rapid reduction of the rare‐earth sublattice magnetization and anisotropy with increasing temperature. Because of the planar anisotropy of cobalt and the weaker R‐Co interactions than R‐Fe, the anisotropy field of Nd2(Fe1−xCox)14B turns out to be almost independent of the cobalt concentration in a temperature range between 300 and 500 K.

Metallographic study on Nd-Fe-Co-B sintered magnets

Metallographic study has been carried out on the Nd-Fe-Co-B alloys to clarify the change in coercivity with the Co composition substituted for Fe in the Nd 2 Fe 14 B type sintered magnets. The decrease in coercivity caused by the Co substitution is attributed to the formation of soft-magnetic phases such as Nd(Fe, Co) 2 and Nd(Fe, Co) 3 . The mole ratio of Nd to (Fe+Co) of these soft-magnetic phases decreases with increasing the Co substitution. The Co concentration is much higher in these soft-magnetic phases than in Nd 2 (Fe, Co) 14 B matrix. The A1 addition in Nd-Fe-Co-B magnet makes the Nd(Fe, Co) 2 compound unstable to form the non-magnetic Nd 3 (Fe, Co, Al) compound, resulting in the increase in the coercivity of Nd-Fe-Co-B magnet.

Magnetic properties of the BCC phase at grain boundaries in the Nd-Fe-B permanent magnet

Structural and magnetic studies have been carried out on a bcc phase that appears in melt-spun rare earth-iron based alloys during a heat treatment at temperatures ranging from 700K to 770K. It has been found that the bcc phase has a lattice constant very close to that for α-Fe and that it is a soft magnet with Curie temperatures in a 400-450K range. Relation of this phase to a bcc boundary phase in sintered Nd-Fe-B magnet and its role in the magnetic hardening mechanism in the Nd-Fe-B magnets is discussed.