Structure and Magnetic Property Correlation in Nanocrystalline SmFe 9
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95 Mat. Res. Soc. Syrup. Proc. Vol. 581 ©2000 Materials Research Society
where
Ik (O)is
the observed Bragg intensity and Ik(C) is the calculated one. The Curie temperature Tc were measured on a differential sample magnetometer Manics, in a field of 1000 Oe. M6ssbauer spectra at room temperature were collected with a conventional constant acceleration 512 channel spectrometer with a Fe line-width of 25mm/s and Fe sample concentration of 12mg/cm 2 .The spectra were least-square-fitted in the assumption of Lorentzian lines. RESULTS Structure studies The x-ray diagrams of the as-milled samples show a diffuse maximum at low angle assigned to an amorphous Sm based phase and nanocrystalline Fe with diffraction domain size around 7 nm. The diagrams of the three samples A, B, C annealed at temperature between 600
and 1200 TC, can be classified into three types (Fig. 1). In the range 600 °C < Ta < 820 °C , the 76-
547
32 0 6- B 5-'4, •,,_3. 750"rC 600,C
5432-
C
0-
30
35
40
45
50
55
60
Fig. 1.X-ray diagrams for samples A,B, Cannealed at various temperatures. (hid) indices are related to the ordered Sm2Fe 17 phase
96
hexagonal P6/mmm derived from TbCu7 is observed the main phase.toAbove 820°C up to 1000 °C,phase the emergence of extra-lines (104), (211), (122),as(024) attributed the superlattice R-3m can be observed. Their intensity is governed by the Sm content. From 1000°C up to 1200 °C, the R 3 m phase is unambiguously obtained. It must be pointed out that the broadening of the diffraction lines attests for the nanocrystalline state preserved up to 1200°C (100nm).The amount of Fe decreases fi'om A to C as the nominal Sm content increases but, it always coexists with small quantities of Sm203 or SmO-N and the hexagonal phase composition cannot be deduced from the nature of the coexisting phases, like in binary system equilibrium diagrams. The problem concerning the TbCu7 hexagonal phase is effectively its stoichiometry domain RxT0.x). The structure is derived from RTs, it might lay theoretically from x=0.16 (RTs) down to x=0.07 prelude to the 1:12 structure. Due to the nanoscale of the hexagonal phases preventing from a pertinent x-ray dispersive analysis, the phase composition had to be deduced from a Rietveld refinement. We have explained the disordered TbCu7 type structure according to the general model developed previously by Givord [9]. Let be (l-s) the occupation rate of Sm (site la in 0,0,0 position), with decreasing Sm content, 2s Fe in 2e site (site of Fe dumbbells in 0,0,Z position) will be distributed on both sides of the empty Sm position. It results that only 2(1-3s) atoms remain located in the 2c special position 1/3,2/3,0, and due to the presence of the 2e Fe dumbbells, 6s Fe are shifted towards the c axis in the more general 61 position X,2X,0, (X
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