We synthesized ultrafine particles in R (rare earth element)-Fe-O systems by a thermal plasma evaporation method. Amorphous citrate gel powders containing R (R=Y, Yb, Er, Ho, Dy, Gd, Eu, Sm and Nd) and Fe were supplied to rf Ar-O
2 thermal plasmas. In the cases of R=Y, Yb, Er, Ho, Dy, Gd and Eu, the synthesized ultrafine particles contained new compounds of RFe
(3+x)O
1.5(4+x). RFe
(3+x) O
1.5(4+x) were synthesized when R ionic radius was smaller than 0.0947nm (that of Eu). The accurate cubic lattice constants of RFe
(3+x)O
1.5(4+x) were calculated from 2θ of the maximum XRD peak (111). In the case of Eu, the lattice constant was at maximum (=0.4866nm). Synthesis of other products such as h-RFeO
3, ε-Fe
2O
3 or γ-Fe
2O
3 were also dependent on R ionic radius. γ-Fe
2O
3 enhanced the saturated magnetization values of ultrafine particles at room temperature. However, γ-Fe
2O
3 did not have such an effect.
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