LANTHANIDE AND BORON OXIDE-COATED ALPHA-FE PARTICLES

To increase the coercivity of alpha-Fe particles, needlelike goethite crystals were first coated with different Ln(3+) ions (Ln=La,Nd,Sm,Gd, Dy,Eu) plus B3+, then reduced to alpha-Fe under H-2. The magnetic mome nt of the particles depends upon the Ln and B contents and temperature . Both powder x-ray diffraction and Mossbauer spectra indicate the exi stence of a crystalline FeO phase; the FeO content decreased as the re duction temperature increased. A phase with no hyperfine splitting was also detected in the Mossbauer spectra, which may be superparamagneti c alpha-Fe or amorphous iron oxide; the amount of this phase decreased as the reduction temperature increased. The temperature dependence of the coercivity of these B- and Ln-coated particles was steeper than f or particles coated with silica: this suggests a coercivity mechanism other than shape anisotropy. (C) 1996 American Institute of Physics.