Phase Transitions, 2002, Vol. 75, No. 1–2, pp. 211–219 FIRST OBSERVATION OF A COMPOSITION- CONTROLLED LOW-MOMENT/HIGH-MOMENT TRANSITION IN THE FCC Fe–Ni SYSTEM: IMPLICATIONS REGARDING INVAR AND ANTI-INVAR BEHAVIORS K. LAGAREC a , D.G. RANCOURT a, *, S.K. BOSE b and R.A. DUNLAP c a Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5; b Department of Physics, Brock University, St. Catharines, Ontario, Canada L2S; c Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 (Received in final form 29 March 2001) We report the first direct observation of a high-moment (HM)/low-moment (LM) transition occurring in face centered cubic (FCC) Fe–Ni alloys. 57 FeM€ ossbauer isomer shifts (ISs) give local electronic densities that exhi- bit a large discontinuity of  0:4 el./a 3 0 at the transition that spans the concentration range 65–75 apc (atomic percent) Fe, in agreement with ab initio predictions. In the most Fe-rich alloys that have LM ground states (including -Fe), we show that thermal stabilization of the HM state occurs at high tempera- tures, thereby providing an experimental proof that anti-Invar behavior is due to such HM stabilization. In Invar (Fe 65 Ni 35 ) and at near-Invar compositions, we observe temperature-induced changes in electronic density that follow the spontaneous magnetization curves and find that Invar is predominantly a HM phase at all temperatures where an Invar effect occurs. We show that LM phase thermal excitation cannot cause the Invar effect and that such excitation would cause a contraction instead of the required expansion, relative to normal behavior. Keywords: Invar; Anti-Invar; Electronic structure; Low-moment/High-moment transition; Mo¨ssbauer; Isomer shift 1. INTRODUCTION A predominant view that has emerged in recent years (Wassermann, 1987, 1990, 1991; Moruzzi, 1989, 1990, 1992; Moroni and Jarlborg, 1989, 1990; Buchholz et al., 1994) is that Invar is at a composition that places it near a high-moment (HM)/low-moment (LM) transition, analogous to the one that occurs as a function of lattice parameter in pure FCC iron (Rancourt and Scorzelli, 1995), and that the LM state is thermally populated to produce the Invar effect. Other work involving approximate high- temperature extensions of ab initio calculations is more refined but is also based on a similar moment instability to explain the Invar effect (Mohn et al., 1989, 1991; *Corresponding author. Tel.: 613-562-5800 x6774; Fax: 613-562-5190; E-mail: dgr@physics.uottawa.ca ISSN 0141-1594 print: ISSN 1029-0338 online/01/010000-00 ß 2002 Taylor & Francis Ltd DOI: 10.1080/01411590290023102