The phase transformation and the exchange coupling in (Ndo095Lao005)9.5FebaICOsNb 2BI05 have been investigated. Nanocomposites were obtained by treating amorphous precursors at temperatures ranging from 650TC to 9500C for 10 minutes. The magnetic properties were characterized via the vibrating sample magnetometer (VSM). X-ray diffraction (XRD), thermomagnetic analysis (TMA), and transmission electron microscopy (TEM) were used to perform phase identification, measure grain size, and analyze phase distribution. The strength of the exchange coupling between the magnetically hard and soft phases in the corresponding nanocomposite was analyzed via the AM-versus-H plot. It was found that the remanence (Br), coercivity (Hci), and maximum energy product (BHmax) obtained were affected by the magnetic phases present as well as the grain size of constituent phases and their distribution. The optimal magnetic performance, BHm, occurred between 700°C to 750°C, where the crystallization has completed without excessive grain growth. TMA and TEM indicated that the system was composed of three phases at this point, Nd2(Fe Co) 14B, ca-Fe, and Fe3B. The exchange coupling interaction among these phases was consistently described via the AM-versus-H plot up to 750°C. The Br, Hci, and BHmax degraded severely when the thermal treatment temperature increased from 750°C. This degradation may be attributed to the grain growth of the main phases, from 45 to 68nm, and the development of precipitates, which grew from 5nm at 750°C to 12nm at 850°C. Moreover, the amount of the precipitates was found to increase with the thermal treatment temperatures. The precipitates, presumably borides, may cause a decrease in the amount of the a-Fe and Fe 3B and result in a redistribution of the Co in the nanocomposites. The increase of the Co content in the Nd 2(Fe Co) 14B may explain the increase of its Curie temperature with the thermal treatment temperatures. In this paper, we examine the impacts of these factors on the magnetic properties of (Ndo 95Lao 05)9 5FebaICosNb2B10.5 nanocomposite.

Polycrystalline refractory metal-substituted Pr3(Fe0.6M0.1Co0.3)27.5Ti1.5 (M=V, Ti, Zr, Mo, Nb,Cr) and Pr3(Fe0.5Co0.5)27.5Ti1.5 have been studied for high-temperature permanent magnetic materials. X-ray diffraction showed the main phase to be the 3:29phase. We observed the highest reported TC (Curie temperature) of 640°C for the 3:29 system in the Pr3(Fe0.5Co0.5)27.5Ti1.5. In the refractory metal-substituted systems, the highest TC of 480°C was observed for the Nb-substituted alloy. SEM measurements showed that Ti in Pr3(Fe0.6Ti0.1Co0.3)27.5Ti1.5 is deposited near the grain boundary. HA (anisotropy energy) of V-substituted alloy is as high as 72kOe, the highest reported in the 3:29 system and is ∼200% higher than 24kOe observed in Pr3(Fe0.7Co0.3)27.5Ti1.5. Cr and Ti substitutions show an increase of 65% (40kOe) and 45% (35kOe) in HA respectively. MS (saturation magnetization) values were ∼100emu/g and are lower than that observed in Pr3(Fe0.7Co0.3)27.5Ti1.5.

Structure and magnetic properties of both melt-spun and hot deformed bulk Sm–Co type nanocomposite magnets have been investigated with various metal additions, including Zr, Cu, and Nb. The Zr and Nb additions play important roles in constraining grain growth, resulting in an increase of coercivity Hc. The Cu addition significantly improves the squareness of BH loops as well as the energy product (BH)max. A typical hot deformed bulk anisotropic nanocomposite SmCo type magnet with Mr(hard)∕Mr(easy)∼0.4, Hc∼9kOe and (BH)max of 13.2MGOe was obtained.

Melt-spun alloys of Pr3(Fe0.7Co0.3)27.5Ti1.5, Pr3(Fe0.6Co0.4)27.5Ti1.5, and Pr3(Fe0.5Ga0.1Co0.4)27.5Ti1.5 prepared at linear wheel speeds of 20, 30, and 40m∕s were found to contain nanocrystalline, a mixture of nanocrystalline and amorphous, and mostly amorphous phases, respectively. Grain sizes of <10nm were observed for the crystalline phases. Annealed alloys showed an increase in grain size to 15–20nm. The Curie temperature (TC) values observed in melt-spun alloys were lower than those observed in fully crystalline alloys. A reduction of ∼55°C in TC was observed for the Pr3(Fe0.7Co0.3)27.5Ti1.5 composition compared to ∼15°C for Pr3(Fe0.6Co0.4)27.5Ti1.5. For the Ga substituted alloy with Co=0.4, a large difference of ∼50°C in TC was noted. MS values of 41 and 35emu∕g were observed in alloys with Co=0.4 and Ga=0.1, and Co=0.4, respectively, at room temperature. Ga substitution increased the anisotropy field value, HA, but lowered the intrinsic coercivity value, Hci. In the alloys with Co=0.4 and Ga=0.1, and Co=0.4, spin reorientation transitions (TSR) were observed at 220 and 140K respectively. The low values of Hci observed in melt-spun ribbons are attributed to large amounts of a soft α-Fe phase.

The effect of microstructural variation on magnetic properties of Sm(Co0.56Fe0.31Cu0.04Zr0.05B0.04)z (z=8, 10 and 12) melt-spun ribbons was investigated. A high coercivity (iHc) of 11kOe was obtained in as-spun ribbons of z=8. The ribbon had nanosized grains of ∼10nm with 1:7H (TbCu7 type) as a major phase. The ribbon with a high z (z=12) displayed large grain size (∼50nm) having 2:17H (Th2Ni17 type) as a major phase. Boron enriched precipitates were observed in both z=8 and 12 ribbons. The boron solubility in the 1:7H phase was more than that in 2:17H phase. The variation of coercivity with z was related to the grain size.

The microstructure and magnetic properties of melt-spun RE/sub 2/Fe/sub 14/B//spl alpha/-Fe and RE/sub 2/Fe/sub 14/B/Fe/sub 3/B nanocomposite powders were studied using Pr and Nd versions of RE/sub 9/Fe/sub 86/B/sub 5/(RE/sub 2/Fe/sub 14/B//spl alpha/-Fe) and RE/sub 9/Fe/sub 79/B/sub 12/(RE/sub 2/Fe/sub 14/B/Fe/sub 3/B) alloys. It was found that the RE/sub 9/Fe/sub 86/B/sub 5/ exhibited a finer and more uniform microstructure with an average grain size of /spl sim/10 nm, whereas a relatively coarse microstructure (grain size up to 100 nm) was observed in the RE/sub 9/Fe/sub 79/B/sub 12/. It was also found that the RE/sub 9/Fe/sub 86/B/sub 5/ yielded higher B/sub r/ and (BH)/sub max/, and a more square demagnetization loop, although the H/sub ci/ was slightly lower. The superior magnetic properties observed in the RE/sub 9/Fe/sub 86/B/sub 5/ can be attributed to the more favorable intrinsic magnetic properties of /spl alpha/-Fe (higher M/sub s/ and lower K) and the much finer microstructure, which produces a stronger intergrain exchange coupling between the RE/sub 2/Fe/sub 14/B and /spl alpha/-Fe phases. The results indicate that RE/sub 2/Fe/sub 14/B//spl alpha/-Fe nanocomposites are more desirable than RE/sub 2/Fe/sub 14/B/Fe/sub 3/B for future development of RE-Fe-B nanocomposite magnets.

Fully dense anisotropic nanocomposite Sm(Co0.58Fe0.31Zr0.05Cu0.04B0.02)z (z=7.5–12) magnets have been synthesized via rapid hot pressing and hot deformation processes. The highest (BH)max∼10.6MGOe was observed for a magnet with z=10. X-ray diffraction and M-H measurements indicated that the easy magnetization direction of magnets prefers to be in the hot pressing direction. Transmission electron microscopy investigation confirmed that plastic deformation is an important route for forming magnetic anisotropy in the Sm–Co-type nanocomposite magnets. Some stripe and/or platelike patterns have been observed inside the nanograins (50–200nm), which may present as twins, and stacking faults. The (0001) twins have been observed in the 2:17R phase.

Exchange spring magnet particles of Nd2(FeCo)14B∕α-FeCo were prepared by spark erosion. X-ray diffraction and Mössbauer studies showed that the particles are composed of about ∼85vol% of Nd2(FeCo)14B and ∼13vol% of α-FeCo with negligible other phases. No oxide was found in these particles. Transmission electron micrographs indicated that the grain sizes of the Nd2(FeCo)14B and α-FeCo phases are ∼10–50nm, and are compatible with effective exchange coupling between the hard and soft phases. The intergrain exchange coupling was also observed in ΔM measurements.

The microstructure and magnetic properties of melt-spun RE2Fe14B/alpha-Fe and RE2Fe14B/Fe3B nanocomposite powders; were studied using Pr and Nd versions of RE9Fe86B5(RE2Fe14B/alpha-Fe) and RE9Fe79B12(RE2Fe14B/Fe3B) alloys. It was found that the RE9Fe86B5 exhibited a finer and more uniform microstructure with an average grain size of similar to 10 nm, whereas a relatively coarse microstructure (grain size up to 100 nm) was observed in the RE9Fe79B12. It was also found that the RE9Fe86B5 yielded higher B-r and (BH)(max), and a more square demagnetization loop, although the H-ci was slightly lower. The superior magnetic properties observed in the RE9Fe86B5 can be attributed to the more favorable intrinsic magnetic properties of alpha-Fe (higher M-s and lower K) and the much finer microstructure, which produces a stronger intergrain exchange coupling between the RE2Fe14B and alpha-Fe phases. The results indicate that RE2Fe14B/alpha-Fe nanocomposites are more desirable than RE2Fe14B/Fe3B for future development of RE-Fe-B nanocomposite magnets.

The behavior of Nb atoms in Nd12Fe82-xB6Nbx (x = 0 to 3) nanocrystalline alloys has been investigated by using atom probe tomography (APT) technique on near-atomic scale in the present paper. Three-dimensional atom probe (3DAP) analyses on the Nb substituted alloys clearly reveal that Nb atoms are enriched at grain boundaries with a peak concentration about 4 times higher than the average for Nd2Fe14B grains. A grain boundary region with a chemical composition near the Nb: Fe: B stoichiometry of 3: 3: 5 is also measured for the x = 3 alloy. The results provide direct evidence of microstructural refinement due to solute drag of Nb atoms during solidification, resulting in Nb enrichment at grain boundaries and possibly the formation of a Nb-rich interfacial phase which give rise to enhanced magnetic properties.

Hot deformed magnets can be commercially produced from hyper-stoichiometric nanocrystalline Nd-Fe-B compositions. This paper summarizes the influence of the rare-earth component (Nd, Pr, Dy) and gallium to magnetic properties and hot workability of die-upset magnets based upon hyper-stoichiometric compositions. Increasing the Nd from 13.3 to 14.4 at.% raises the H/sub ci/ from 770 to 930 kAm/sup -1/. B/sub r/ values are highest (1.31 T) at lower levels of RE (/spl sim/13 at.%), although such compositions exhibit a decrease in hot workability. The substitution of Dy and Pr for Nd promotes higher H/sub ci/ values at some expense in B/sub r/. Gallium additions up to 0.5 at.% significantly increase both B/sub r/ and H/sub ci/.

We have investigated the microstructures of melt-spun Sm(Co0.58Fe0.31Cu0.04Zr0.05B0.02)z (z=7.5 and 12) ribbons by transmission electron microscopy (TEM) and three dimensional atom probe (3DAP) to correlate them with the hard magnetic properties. The ribbon with z=7.5 has a high coercivity (iHc∼12 kOe) while the ribbon with z=12 possesses a low coercivity (iHc∼4.9 kOe). The high Hci ribbon comprises of nanocrystals (∼10 nm) having 1:7H (TbCu7) type structure as main phase. However, in the low Hci ribbon, the microstructure consists of both 1:7H and 2:17H (Th2Ni17) phases with crystal size ranging from 50 to 80 nm. The low coercivity ribbon also contains (CoFeZr)23B6 (M23B6 type) soft magnetic phase. The low coercivity obtained in the melt-spun ribbon with z=12 is due to: (i) the formation of Th2Ni17 type low anisotropy phase, (ii) the presence of boron-rich soft magnetic phase, and (iii) larger crystal size. Realization of high coercivity in the ribbon with z=7.5 is due to the formation of 1:7H, a high anisotropy phase, as well as reduced grain size.

Cast alloys and melt-spun ribbons with nominal compositions of Sm(Co/sub bal/Fe/sub 0.31/Zr/sub 0.05/Cu/sub 0.04/B/sub x/)/sub z/ (x=0.02-0.04,z=7.5-12) have been synthesized and characterized in a temperature range of 10-1273 K and at fields up to 5T. The main phase in the as-cast alloys exhibited a Th/sub 2/Ni/sub 17/ type structure, with a strong uniaxial anisotropy. Minor phases with a TbCu/sub 7/ and/or CaCu/sub 5/ structure emerged as z (3d/R) decreased. As a result, the anisotropy field (H/sub A/) increased from 67 to 120 kOe, while 4/spl pi/M/sub s/ fell from 12.8 to 10.5 kG at 300 K when z was decreased from 12 to 7.5. For melt-spun ribbons, they are nano-structured in nature and magnetically hard, even in the as-spun state. By lowering the value of z(3d/R) and raising the B content, a finer microstructure and a higher H/sub ci/ were obtained. Hard magnetic properties of H/sub ci/=4.9-12 kOe, 4/spl pi/M/sub s/=9.0-12.0 kG at 300 K have been obtained from ribbon samples. Among them, Sm(Co/sub bal/Fe/sub 0.31/Zr/sub 0.05/Cu/sub 0.04/B/sub 0.02/)/sub 10/ ribbon showed the highest (BH)/sub max/ of 10.8 MGOe at 300 K. A Henkel plot analysis suggested the existence of exchange-coupling interaction between the magnetically hard and soft phases in the ribbon materials.

Cast alloys and melt-spun ribbons with nominal compositions of Sm(CobalFe0.31Zr0.05Cu0.04Bx)(z) (x = 0.02-0.04, z = 7.5-12) have been synthesized and characterized in a temperature range of 10-1273 K and at fields up to 5T. The main phase in the as-cast alloys exhibited a Th2Ni17 type structure, with a strong uniaxial anisotropy. Minor phases with a TbCu7 and/or CaCu5 structure emerged as z (3d/R) decreased. As a result, the anisotropy field (HA) increased from 67 to 120 kOe, while 4piM(s) fell from 12.8 to 10.5 kG at 300 K when z was decreased from 12 to 7.5. For melt-spun ribbons, they are nano-structured in nature and magnetically hard, even in the as-spun state. By lowering the value of z(3d/R) and raising the B content, a finer microstructure and a higher H-ci were obtained. Hard magnetic properties of H-ci = 4.9-12 kOe, 4piM(s) = 9.0-12.0 kG at 300 K have been obtained from ribbon samples. Among them, Sm(CobalFe0.31Zr0.05Cu0.04B0.02)(10) ribbon showed the highest (BH)(max) of 10.8 MGOe at 300 K. A Henkel plot analysis suggested the existence of exchange-coupling interaction between the magnetically hard and soft phases in the ribbon materials.

The effect of Nb substitution on the thermal stability of melt-spun powders with near stoichiometric 2:14:1 composition of Nd12Fe82−xNbxB6 (x=0, 1.5 and 3) has been investigated. It has been found that the thermal stability is significantly improved with 1.5at% Nb substitution. As compared with the ternary Nd12Fe82B6, the Nb-substituted Nd12Fe80.5Nb1.5B6 powder exhibits remarkably increased coercivity (Hci) over a wide temperature range of 22–180°C. The temperature coefficient of coercivity (β) is reduced from −0.36%/°C at x=0 to −0.32%/°C at x=1.5. As a result of improved coercivity and its temperature dependence, the irreversible flux aging loss (δirr), measured on the epoxy bonded magnets after being exposed at 180°C for 100h, is also significantly decreased from −13.7% at x=0 to −5.0% with x=1.5. Microstructure studies using X-ray diffraction and transmission electron microscopy have shown a significant microstructure refinement with Nb substitution. Nb substitution also increases the amount of α-Fe phase in the alloys. For x=0, the average grain size of the magnetically soft phase (α-Fe) is 15nm, while the hard phase (Nd2Fe14B) has an average grain size of 30nm. The soft and hard magnetically phases are reduced to <10 and 10–20nm, respectively for x=1.5 and 3. Enriched Nb along the grain boundaries is believed to be the main reason for the observed improved thermal stability in Nb-substituted powders.

The recoil behavior of nanocrystalline materials based on Nd2Fe14B has been investigated in melt-spun alloys with the stoichiometric composition Nd2Fe14B and in the nanocomposite Nd9Fe86B5. The onset of recoil hysteresis, or attainment of appreciable area of the recoil loop, was determined and followed as a function of the reverse applied magnetic field. Its behavior was correlated with that of two other independent parameters: the reversible portion of the recoil susceptibility and the recoil remanence, which both provide information on the ease of remagnetization of the materials. The simultaneous appearance of extrema in a restricted internal field range in all three parameters strongly suggests that the area of the recoil loops in these materials may be taken as a measure of the extent of the intergranular exchange. The temperature dependence of the results provides information about the thermal evolution of the interphase exchange coupling; this information is not evident in major loop measurements. Preliminary results indicate that the off-stoichiometric Nd9Fe86B5 maintains a higher degree of interphase exchange coupling but only at low reverse fields; the stoichiometric Nd2Fe14B maintains a lower degree of magnetic reversibility but sustains it to higher reverse fields. Comparison of the results obtained from different forms of the Nd2Fe14B-based nanocrystalline alloys highlights the origins of the differences in their technical magnetic behavior and suggests structural modification pathways to improve their properties.