Influence of Ion Irradiation on the Microstructure of Fe/Ti Multilayers
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In order to induce ion-beam mixing the multilayers were irradiated with 200 keV Ar++ or 350
2 keV Kr"+ ions with doses (0) ranging from 5×10 " to 7×1016 Ar/cm and a) = 5x×103
+
1.6.1016
2
Kr/cm . The ranges of Ar and Kr ions, calculated by TRIM [7], were about 105 and 95 nm, respectively. The beam current density was less than 1 jaA/cm 2 to avoid extensive heating and keep the target temperature to less than about 50'C. After each irradiation step the samples were analyzed by conversion electron Mossbauer spectroscopy (CEMS). The CEMS measurements were performed at room temperature by using a He-6%CH4 gas-flow electron counter. A conventional M6ssbauer spectrometer with a "5'Co-in-Rh source was used. The isomer shift data are given with respect to the aX-Fe standard. RESULTS As-deposited Fe/Ti multilayers It was shown recently [3], that the as-deposited Fe/Ti ML with A=20 and 60 nm prepared by rf sputtering consist of predominantly crystalline close packed bcc (110) planes of Fe and (100) hcp Ti. The plane spacings were about 0.2028 nm and 0.2558 nm for Fe and Ti, respectively. The CEMS spectrum of the as-deposited film with A=20 nm consists of 3 spectral components (Fig. Ia): (1) a sextet with Hhf=32.9T and IS=0.00 mm/s corresponding to the aC-Fe layers, (2) a broad sextet with Hhf=29T and IS=0.00 mm/s corresponding to the interfacial crystalline region, most probably bcc-FeTi, and (3) a quadrupole doublet with QS=0.40 mm/s and IS= -0.09 mm/s corresponding to the amorphous iron-poor FeTi phase [3]. The A=60 nm sample is almost completely crystalline; the CEMS spectrum consists of the dominating oa-Fe sextet and the QS doublet does not exceed 2% of the total spectral area (Fig. Ig). Irradiations with Ar and Kr ions The dose dependence of the ion-beam mixing was studied for MLs with A=20 and 60 nm irradiated with Ar" ions with doses up to 7x 1016 Ar/cm 2 (Fig. 1). The relative fractions of various phases detected in CEMS spectra vs. (D"zare shown in Fig. 2. The CEMS results for A=20 nm (Figs. lb-1f) reveal three distinct ranges of ion dose in which different phases dominate: (i) at low ion doses (up to about 1.3 x 015 Ar/cm 2 ) the sextet corresponding to a-Fe, which dominated in the as-deposited ML (Fig. I a), decreases markedly with increasing ion dose and the phase formed due to ion-beam mixing is the bcc-FeTi solid solution identified by its characteristic broad sextet (Hhf-29T, IS=0.00 mm/s, Fig. lb). (ii) At intermediate ion doses (1.5±8x 1015 Ar/cm 2) the bcc-FeTi component dominates strongly in the spectra (Figs. Ic, Id) which were fitted with the combination of the distribution of the hyperfine fields P(H) and the ax-Fe sextet. The P(H) distributions were calculated by using a constrained Hesse-Ruibartsch method [8,9] with the NORMOS program [10]. The contribution of the a-Fe sextet decreases further with increasing ion dose and for ML irradiated with 8x 1015 Ar/cm 2 (Fig. Id) it contributes to only 3% of the total spectral area (Fig. 2a). In this dose range no other phases were detected. (iii) At high
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