Field-induced local magnetic moments in γ (FCC) Fe–Ni anti-Invar alloys

Abstract Mossbauer spectroscopy in longitudinal external fields (up to 7 T) and SQUID magnetometry (up to 5 T) measurements have been carried out on mechanically alloyed (MA) γ (FCC) Fe 100− x Ni x ( x =21, 24, and 27 at%) alloys at room temperature. The zero-field Mossbauer spectra of these alloys show only singlets. The high field Mossbauer results indicate that large amounts of the material is in the paramagnetic state, giving rise to two spectral components with their effective fields almost linearly depend on the external field, but with slopes that are smaller than unity. The in-field Mossbauer spectra of the x =27 at% alloy show an additional component with a hyperfine field of ≈21 T, which is attributed to Ni-rich (>30 at% Ni) clusters (domains) of ferromagnetically ordered HM phase that behaves superparamagnetically at room temperature and shows a non-linear character in the magnetization (M–H) curves at low fields. This HM phase is also present in the x =21 and 24 at% samples but with smaller amounts. The results suggest induced hyperfine fields and hence induced moments in the paramagnetic components, which increases with increasing Ni contents. Taenite-enriched samples from the metal particles of two stony meteorites, Al Kidirate (H6) and New Halfa (L4), are also studied by high field Mossbauer spectroscopy and the results are compared to that of MA samples.