First Observation of a Composition-Controlled Low-Moment/High-Moment Transition in the Fcc Fe-Ni System: Implications Regarding Invar and Anti-Invar Behaviors |
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Authors: | K Lagarec DG Rancourt SK Bose RA Dunlap |
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Institution: | 1. Department of Physics , University of Ottawa , Ottawa , Ontario , Canada , K1N 6N5;2. Department of Physics , Brock University , St. Catharines , Ontario , Canada , L2S;3. Department of Physics , Dalhousie University , Halifax , Nova Scotia , Canada , B3H 3J5 |
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Abstract: | 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 Fe Mössbauer isomer shifts (ISs) give local electronic densities that exhibit a large discontinuity of , 0.4 el./ $ a_{0}^3 $ 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 n -Fe), we show that thermal stabilization of the HM state occurs at high temperatures, 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. |
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Keywords: | Invar Anti-Invar Electronic Structure Low-moment/High-moment Transition Mo¨ssbauer Isomer Shift |
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