Point Defects And The B2 To Fcc Transformation In Milled FeRh
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and the transition to a nonequilibrium phase. Detection of point defects was not possible in the earlier M6ssbauer measurements on FeRh [4] because linewidths were very broad. Resolution is much better than in our earlier work on FeAl [2] because the defects can be resolved here by magnetic hyperfine field shifts that they induce. EXPERIMENT Samples of FeRh having 52,54,56 and 58 at.% Fe were made by melting high-purity foils together (>3N5) under argon in a small arc-furnace and splat-quenching in order to make foil samples appropriate for M6ssbauer absorbers. Samples were given a crystallizing anneal at 1000 C for 1 hour and cooled in furnace prior to measurement. Measurements reported here were made in triangular, constant-acceleration mode at 293(2) K using a Ranger MS-1200 spectrometer, with spectra measured at positive and negative accelerations folded together. A gamma-ray source of 57Co in Rh purchased from Amersham International was used that yielded a very narrow linewidth when extrapolated to zero absorber-thickness, only 0.21(1) mm/sec. Milling was carried out under argon in a tungsten carbide vial using a SPEX 8000 vibrator mill. Two WC ball bearings were used and the ball to sample mass ratio was always greater than 20, making for very high-energy impacts. FeRh is exceedingly hard and, unlike for FeAl or PdIn, the milled material never fragmented into a powder even after milling for 15 minutes. Milled foil fragments were withdrawn from the milling vial after milling for time intervals up to 15 minutes and assembled into a mosaic absorber for M6ssbauer and x-ray measurements. X-ray measurements made using a Siemens 5000 diffractometer showed that annealed samples were entirely B2 phase and milled samples contained B2 and fcc phases, with the fcc phase increasing with milling time. The mean B2 crystallite size determined by x-rays was 8 nm after 15 minutes of milling. A search for L10 distortions of the fcc phase showed broadening of the reflection consistent with a range of c/a distortions of the fcc structure between 1.00 and 1.01. ANALYSIS AND RESULTS Annealed samples. Fig. 1 shows M6ssbauer spectra for annealed samples having 52-58 at.% Fe. All spectra are fitted well assuming only the presence of ferromagnetic B2 FeRh. Spectra consist of a superposition of magnetic sextets, each attributed to a discrete configuration of probe atoms and defects in the first two atomic shells. In the perfect B2 structure, Fe atoms are surrounded by 8 Rh atoms in the first near-neighbor (nn) shell and 6 Fe atoms in the second shell. For Fe-rich samples, excess Fe atoms go onto the Rh sublattice where they are surrounded by 8 Fe atoms in the first shell and 6 Rh atoms in the second shell. The large-area sextet with outer-line splitting of 8.8 mnm/sec and the small-area sextet with splitting of 12.5 mm/sec correspond to majority FeFR and minority Fein probes without defects in their first two shells. The Fein probe has a larger splitting (and hyperfine field) because it is surrounded by 8 Fe atoms, as in pure iron metal
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