Lanthanide and boron oxide-coated α-Fe particles

Abstract

To increase the coercivity of α‐Fe particles, needlelike goethite crystals were first coated with different Ln³⁺ ions (Ln=La,Nd,Sm,Gd,Dy,Eu) plus B³⁺, then reduced to α‐Fe under H₂. The magnetic moment of the particles depends upon the Ln and B contents and temperature. Both powder x‐ray diffraction and Mössbauer spectra indicate the existence of a crystalline FeO phase; the FeO content decreased as the reduction temperature increased. A phase with no hyperfine splitting was also detected in the Mössbauer spectra, which may be superparamagnetic α‐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 for particles coated with silica: this suggests a coercivity mechanism other than shape anisotropy.