K. J. Strnat

A Family of New Cobalt‐Base Permanent Magnet Materials

The magnetocrystalline anisotropy of several intermetallic phases of the type RCo5 (R = Y, Ce, Pr, Sm, Y‐rich and Ce‐rich mischmetals) has been investigated, and it is concluded that these alloys are promising candidates for fine‐particle permanent magnets. They have extremely high uniaxial anisotropy (K = 5.4 to 7.7 × 107 erg/cm3), single easy axis, high saturation (Bs = 8500 to 11 200 G) and Curie point (tc = 464° to 747°C). Approximate upper limits for the possible energy product lie between 18 and 31.3 MGOe. Experimentally, coercive forces of over 8000 Oe and (BH)max = 5.1 MGOe have been observed in SmCo5 merely ground at room temperature. Grinding of YCo5 and (Ce‐MM) Co5 produces an increase of MHc to 2200 and 2700 Oe, respectively, followed by a decrease as particle size continues to decrease.

Rare earth-cobalt permanent magnets

Abstract This paper reviews the historical background and the development of rare earth-cobalt-based permanent magnets from basic science studies on rare earth-transition metal alloys in the 1960s to todays broad spectrum of commercial magnet types and their applications. It puts the RE-Co magnets in perspective relative to older magnet types and also traces the path to the subsequent development of the related Nd-Fe-B magnets. The treatment is qualitative, with emphasis on the relationship between fundamental properties of the compounds and the interaction between microstructure and magnetic domain walls that makes high coercivity and the exceptional hard magnetic properties of the rare-earth magnets possible. The various kinds of RE-Co magnets in production and use today, some of their engineering properties, and economic aspects governing their applicability, cost and availability are also discussed. Many references provide a guide to the special literature regarding the physics, metallurgy, manufacture, product selection and properties of rare earth-cobalt magnets.

Magnetic properties of rare--Earth--Iron intermetallic compounds

Curie points, the temperature dependence of the high-field magnetization below room temperature, saturation moments, and densities are reported for the most iron-rich compounds in the rare earth-iron systems. The heavy rare earth compounds have the composition A 2 Fe 14 and the hexagonal Th 2 Ni 17 -type structure, while the light rare earths have the stoichiometry AFe 7 and a modified rhombohedral Th 2 Zn 17 structure. From the saturation of Y 2 Fe 17 and Lu 2 Fe 17 , an iron moment of 2.0\mu_{B} is calculated. In these two compounds and in CeFe 7 , PrFe 7 , and, perhaps, NdFe 7 , all atomic moments couple ferromagnetically, while for Gd 2 Fe 17 through Tm 2 -Fe 17 the magnetizations of the rare earth and iron sublattices subtract. Curie temperatures range from -180°C (CeFe 7 ) through a peak of +186°C (Gd 2 Fe 17 ) to -41°C (Tm 2 Fe 17 ). This suggests that two exchange mechanisms are active which make comparable contributions to the interaction energy: direct exchange between Fe atoms and indirect coupling via conduction electrons between iron and the rare earth atoms.

The hard-magnetic properties of rare earth-transition metal alloys

The intermetallic phases formed between the rare earth metals (R) and the elements of the 3-d transition series are a large group of new substances with interesting magnetic properties. They have been studied intensely in the last decade. Their properties are reviewed with a view toward their usefulness for permanent magnets. Some aspects of the recently developed magnets based on certain RCo 5 compounds are discussed. The prediction is made that second-generation rare earth magnets can be developed using R 2 (Co,Fe) 17 alloys. These should have energy products and thermal stability superior even to sintered SmCo 5 , while being less expensive. Experimental evidence supporting this claim is discussed.

Ferrimagnetism of the Rare‐Earth‐Cobalt Intermetallic Compounds R2Co17

Saturation moments, Curie temperatures, and supporting crystallographic data are reported for a series of ferrimagnetic intermetallic compounds of the stoichiometry R2Co17, where R is any of the rare‐earth elements Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The saturation moment of Y2Co17 is 27.2, that of Lu2Co17 is 27.6 Bohr magnetons per formula unit, indicating a cobalt moment of 1.61 μB in this series. Substitution of Pr and Nd for R raises the saturation to 32.8 μB/F.U. for Pr2Co17; all other lanthanides lower it (minimum: 5.7 μB/F.U. for Ho2Co17). Except for Sm2Co17, the values agree well with the concept that the Co moments couple antiparallel with the rare‐earth spin sublattice. For Ce and Pr, a valence>3 and electron transfer from R to Co must be assumed. The Curie temperatures lie between 795° and 940°C. The variation of Tc with the atomic number suggests that two coupling mechanisms are active: direct Co‐Co exchange interaction and indirect exchange between R and Co via polarized conduc...

The recent development of permanent magnet materials containing rare earth metals

The prospect that permanent magnets with previously unattainable coercivities and energy products might be made from cobalt-rare earth alloys has caused intense research efforts in the last three years. Alternative ways of preparing magnets from powders and by casting were demonstrated in several laboratories. (BH)_{\max} \approx 20 MG.Oe and M H_{c} > 25 000 Oe have been achieved with SmCo 5 , and the development of manufacturing processes for magnets made from this alloy has begun. This paper reviews the basic concepts, properties of the alloys of interest, and the physical factors influencing the coercive force. Approaches to alloy, powder, and magnet fabrication are discussed, with their merits and drawbacks; also problems incurred in the materials development and their possible solutions. Application areas are reviewed and some economic factors considered. It is concluded that the R Co 5 magnets are indeed beginning to live up to their promise, but that more materials research, process development, and circuit redesign are needed if their potential is to be fully utilized.

Permanent magnet properties of sintered Nd‐Fe‐B between −40 and +200 °C

Easy and hard‐axis magnetization curves, and second quadrant demagnetization lines were measured on two early commercial Nd‐Fe‐B based pilot products at temperatures between −40 and +200 °C. Open‐circuit flux losses during short‐term heating were determined. The high‐Hc grade 30‐H shows generally better temperature stability of magnetic properties than the lower‐coercivity grade 35. The (negative) temperature coefficients of Br, BHc, and (BH)max at 100 °C of Nd‐Fe‐B are approximately twice those of SmCo5, that of MHc is three times larger. After thermal demagnetization by cycling to the Curie temperature all losses are fully recoverable by remagnetizing. The required recharging field strength of previously field‐demagnetized magnets can be reduced by the simultaneous application of field and heat.

Domain behavior in sintered Nd-Fe-B magnets during field-induced and thermal magnetization change

On sintered Nd‐Fe‐B permanent magnets, magnetic domain pattern changes in applied fields and at elevated temperatures were observed by the Kerr effect. Patterns on pole and side faces were recorded for different magnetization states in fields up to 17 kOe at 20 °C, and remanent patterns at temperatures up to TC. They are qualitatively interpreted. On pole faces, most grains are multidomain even at remanence. But the surface domains seen are not characteristic of the magnet interior. Side faces show mostly single‐domain grains at remanence. These images appear to reflect bulk behavior. Walls are strongly pinned at grain boundaries but move easily in the main magnetic phase, a behavior analogous to that of sintered SmCo5.

Demagnetization curves of four rare‐earth‐cobalt magnet types at temperatures 300–1000 K

Intrinsic demagnetization curves and the temperature dependence of specific magnetization states were measured with a dc magnetometer on four types of sintered rare‐earth‐cobalt permanent magnets at temperatures from 300 to 1000 K. All magnetic properties vary reversibly with temperatures up to limits of 650–850 K, depending on the alloy. Precipitation‐hardened magnets of the 1–7 or the 1–5 types and the common sintered SmCo5 all exhibit quite different temperature characteristics. The MHc of 1–7 type magnets decreases gradually with increasing temperature. Its temperature dependence can be well described by a simple exponential function. The MHc of SmCo5 drops sharply with increasing temperature and becomes very small at about 750 K. The cerium magnet shows a distinct kink in MHc versus T at 550 K, separating two temperature regions in which clearly different mechanisms are responsible for the coercivity.

The effects of various alloying elements on modifying the elevated temperature magnetic properties of sintered Nd‐Fe‐B magnets

The effects of the alloying elements Co, Dy, Er, Al, and Nb on modifying the elevated temperature properties of sintered Nd‐Fe‐B–based permanent magnets have been studied in our laboratory. The results are summarized and analyzed. Small Nb additions increase coercivity and improve elevated temperature properties. Higher Nb additions lead to decomposition of the 2:14:1 phase into a 2:17 phase and a new magnetic phase with a Curie point around 350 °C. The effectiveness of Nb additions appears to be associated with the rare‐earth content of the magnets.