Phase stability, EXAFS investigation and correlation between nanostructure and extrinsic magnetic properties of nanocrystalline Pr2(Co,Fe)7

Abstract Nanocrystalline Pr 2 Co 7− x Fe x ( x ⩽ 3.5 ) powder was synthesized by high energy milling and was subsequently annealed at 973 K for 30 min. Their crystalline structures were investigated by means of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). Magnetic properties were also studied at room temperature and 10 K as a function of the iron composition. This compounds structure depends on the iron content; for x ⩽ 1 they crystallize in the Ce 2 Ni 7 -type hexagonal structure (space group: P 6 3 / m m c ). For 1 x ⩽ 3, we observe a mixture of three phases, Pr 2 (Co,Fe) 7 , Pr(Co,Fe) 3 and Pr 2 (Co,Fe) 17 while for larger values of x the Pr 2 (Co,Fe) 7 phase disappears completely. Since the substitution of cobalt by iron in a pure Pr 2 Co 7− x Fe x phase is limited to x ⩽ 1, a site preference can be suggested. Unfortunately standard XRD cannot be used unambiguously because Co and Fe have too close normal X-ray atomic scattering factors. Among the local and Fe selective available experimental methods, we have chosen to explore the P 6 3 / m m c space group Wyckoff positions for iron by EXAFS at the Fe edge. This study shows that the local iron radial distribution function is much larger than expected for all the specific sites, at least two Fe–Pr distances, except for 12 k which gives the best fit. A preferential substitution of cobalt by iron in Ref. 12 k site is coherent with (i) the EXAFS result (ii) the substitution rate x k , 6 h and 2 a cannot be excluded. However, if we assume a unique preferential site, 12 k is the only solution. For x ⩽ 1, the magnetocryslalline anisotropy is observed in Pr 2 Co 7− x Fe x alloys with fairly strong anisotropy fields at room temperature in range from 123 to 136 kOe for x  = 0 and x  = 1, respectively. For x  > 1, the magnetocrystalline anisotropy field of the 2:17 phase is lower than that of the 2:7 phase, which causes a further reduction in anisotropy and coercivity.