Reentrant magnetism, antiferromagnetism, and domain wall pinning in nominally ferromagnetic Fe-Ni invar

Abstract

We report measurements of the low field ( ≈ 10 Oe) near zero field quenched dc magnetization, M ZFQ, measured on warming in the interval 4.2–250 K for three Fe-Ni Invar alloys of nominal compositions, Fe 66 Ni 34, Fe 65 Ni 35, and Fe 64 Ni 36. The samples were characterized by chemical analysis, X-ray diffraction, Mössbauer spectroscopy, and high field magnetization, prior to and subsequent to various annealing and aging treatments which affected the measured M ZFQ curves. These curves showed two distinct behaviours: typical reentrant spin glass (RSG) behaviour was seen in virgin-state samples whereas a new behaviour involving linear in temperature dependencies was seen in all samples which contained crystallographic precipitates. Both ferromagnetic α-phase (bcc, T C≈1000 K) and antiferromagnetic low-spin γ'-phase (fcc, T N≈20 K) precipitates were identified. Their occurrence and production by heat treatment and/or aging were studied in detail and their amounts, sizes, and magnetic properties were inferred from their influence on M ZFQ. Both the RSG behaviour and the new behaviour are understood in terms of thermally activated depinning of domain walls, as is shown more fully in an accompanying paper. There, it is also shown that the new behaviour is predicted to occur under precisely the same conditions which lead to a logarithmic time dependence in the magnetic viscosity. It should therefore be termed normal viscous (NV) behaviour. A complete review of the evidence for low temperature antiferromagnetism in Fe-Ni Invar is given.