Enhancement of Curie temperature of the 2:17 phase in nanocomposite Sm2(Fe,Co)15Cr2C2/(Fe,Co) magnets

The proximity effect on the Curie temperature of exchange-coupled melt-spun nanocomposite Sm-2(Fe,Co)(15)Cr2C2/(Fe,Co) magnets consisting of a fine mixture of hard and soft phases has been examined using thermomagnetic measurements, x-ray diffraction (XRD), Mossbauer and transmission electron microscopic studies. A significant enhancement in the Curie temperature of the 2:17 phase is observed in these magnets (27 degrees C in Sm2Fe15Cr2C2 and 102 degrees C in Sm2Fe11Co4Cr2C2) as compared to their patent cast alloys. XRD and Mossbauer analysis confirm that both the cast alloys and the nanocomposite magnets are comprised of the same phases [rhombohedral 2:17 and bcc Fe(Co)]. The unit cell volume of the 2:17 phase is slightly larger in ribbons and it is found to contribute to about 15% of the total Curie temperature enhancement. Microstructural studies revealed that the grain size is reduced from 10 to 20 mu m in the cast alloy to 20-60 nm in the melt-spun magnets. The enhancement of the Curie temperature is believed to arise mainly from the exchange coupling effect between the 2:17 phase and the Fe(Co) phase which results in an extra exchange field at the 2:17 grains. Furthermore, the enhancement of the Curie temperature is found to increase with the volume fraction of the Fe(Co) phase in the magnets. It is also interesting to note that the Curie temperature of the Fe(Co) phase in the magnets remains unchanged as compared with their parent alloys, because the exchange coupling effect disappears completely at temperature above the Curie temperature of the 2:17 phase. (C) 1999 American Institute of Physics. [S0021- 8979(99)06719-5].