Anomalous high-temperature coercivities in hard nanocomposite alloys

To elucidate the interphase interactions inherent to nanocomposite magnetic alloys, measurements of remanence B-r, and coercivity H-ci were made on a series of four meltspun, remanence-enhanced nanocomposite alloys consisting solely of various amounts of Nd2Fe14B and alpha-Fe. The phase constitution and microstructural scale of the alloys were characterized with synchrotro n x-ray diffraction, Magnetic measurements were made using superconducting quantum interference device (SQUID) magnetometry on evacuated and encapsula ted samples in the temperature range of 300 K less than or equal to T less than or equal to 750 K, in order to characterize the alpha-Fe component ind ependently of the Nd2Fe14B component. The high-temperature coercivities of the samples increase with the amount of alpha-Fe present in the samples, ra nging from an average value of approximately 75 Oe for the sample with 14 w t % excess Fe to over 400 Oe at 700 K for the sample with 27 wt % excess Fe , The relatively high coercivities of the samples found at elevated tempera tures imply that a tabular morphology of the alpha-Fe grains is conferring anisotropy to the phase; this conclusion is supported by transmission elect ron microscopy. It is concluded that while the significant coercivity of th e alpha-Fe phase likely reduces the room-temperature remanence enhancement of the alloy below its theoretical ideal, the favorable interphase interfac e orientation promotes exchange coupling. (C) 1998 American Institute of Ph ysics. [S0021-8979(98)20811-5].