The “anomalous” nonmonotonic temperature dependence of coercivity, reported in Sm–Zr–Co–Cu magnets, has also been observed in bulk-hardened Y–Zr–Co–Fe–Cu alloys with a similar microstructure. The phenomenon appears to be universal for all R–Co magnets (R=rare earth) having a microstructure consisting of R2Co17 cells surrounded by the RCo5 phase. The effect of R and Cu on the temperature dependence of coercivity cannot be simply explained by traditional domain-wall pinning model based on the difference in a domain wall energy. Possibility that the coercivity is controlled by nucleation of reversed domains in magnetically isolated R2Co17 cells is discussed.

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Anisotropic nanocomposite R–Fe–B/Fe magnets (R=Pr, Tb) were synthesized by hot pressing and subsequent die upsetting blends of R-rich and R-lean melt-spun ribbons. The magnets have a layered structure, in which alternating layers of the two starting alloys lay perpendicularly to the pressing direction. A crystallographic alignment of the R2Fe14B grains is observed in the R-rich layers, whose microstructure is identical to that of the conventional die-upset magnets. The R-lean layers consisting of exchange-coupled R2Fe14B and α-Fe grains retain the random crystallographic orientation. The obtained bulk R-lean magnets show better properties than magnets of the same overall composition prepared from a single alloy.

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Composite magnets were prepared by hot pressing followed by hot deformation of blends composed of Nd14Fe79.5Ga0.5B6 or (Nd0.75Dy0.25)14Fe79.5Ga0.5B6 ribbon powders as a high-coercivity component and Fe powder as a high-magnetization component. The addition of 15 wt % α-Fe to (Nd0.75Dy0.25)14Fe79.5Ga0.5B6 increases the remanent magnetization of the hot-deformed magnets from 10.6 to 12.04 kG, while the maximum energy product is also increased from 27.3 to 29.5 MG Oe for hot-deformed magnets with 10 wt % α-Fe addition. Microstructure investigations of the composite magnets revealed the size of the Fe particles in the micrometer range, exceeding by far the size for effective exchange interactions. Despite a less refined microstructure, the particular layered configuration of the composite magnets gives rise to a positive magnetostatic coupling of the grains and therefore a unitary magnetic behavior with enhanced magnetic properties. The cooperative demagnetization process, together with the magnetic coupling of the grains, was pointed out through a smooth demagnetization curve and a sharp single peak of the irreversible susceptibility.

The demagnetization behavior of the hard-soft composite magnet has been simulated with a simple model in order to understand better the magnetization reversal of die-upset composite magnets fabricated from blends of Nd–Fe–B ribbons and coarse Fe powders. The calculations show that soft magnetic inclusions of any size can be fully magnetically coupled with the hard matrix by long-range magnetostatic interactions provided that the inclusions form layers perpendicular to the magnetization direction. Though the magnetostatic coupling along does not lead to enhanced hard magnetic properties of the composite magnets, it makes the full exchange coupling between the hard and soft phases unnecessary and, therefore, relaxes the strict requirements for the size of the soft inclusions. The combination of magnetostatic coupling and partial exchange coupling in a die-upset magnet with layered morphology may facilitate the development of anisotropic hard-soft composite magnets with properties superior to the single-phase permanent magnets.

Dysprosium-added sintered magnets were prepared from blends of Nd15.5(Fe,Co,Ga)78.2B6.3 and Dy2S3 powders; their microstructure and magnetic properties were compared to those of the magnets made with Dy2O3 additions or from single (Nd,Dy)-(Fe,Co,Ga)-B alloys. The addition of Dy2S3 leads to replacement of the neodymium oxide phases in the sintered magnets by the Nd2O2S and NdS phases. The magnets prepared with both the Dy2S3 and Dy2O3 powders exhibited inhomogeneous distribution of Dy within the (Nd,Dy)2Fe14B grains with Dy-rich outer grain regions. However, in a marked difference from the Dy2O3-added and single-alloy magnets, where the grain-boundary oxide phases were Dy-rich, the magnets prepared with Dy2S3 had their sulfur-containing grain-boundary phases depleted of Dy. With the larger fraction of Dy atoms available for alloying the main (Nd,Dy)2Fe14B phase, the magnets prepared with Dy2S3 showed the largest coercivity gain per 1 at.% of the added Dy.