Interplay of spin structures, hyperfine magnetic field distributions and chemical order-disorder phenomena in face centered cubic Fe-Ni alloys studied by Mossbauer spectroscopy measurements and Monte Carlo simulations.
Creators: name: Dang, Mei-Zhen.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1996
Subjects: subject: Physics, Condensed Matter.

description: The magnetic properties of fcc Fe-Ni alloys are studied by Mossbauer spectroscopy and Monte Carlo (MC) simulations. Both macroscopic (magnetization, paraprocess susceptibility, Curie points, etc.) and microscopic properties (hyperfine fields) are used to test simple local moment models under various assumptions. A non-linear composition dependence of the average hyperfine field is observed by Fe-57 Mossbauer spectroscopy. A microscopic vector hyperfine field model is proposed and used to model the measured average hyperfine fields and hyperfine field distributions (HFDs) in the collinear ferromagnetic Fe-Ni alloys (y $\le$ 0.45 in Fe$\sb{y}$Ni$\sb{1-y}).$ Modeling the liquid helium temperature average hyperfine fields and HFDs resolves the coupling parameters in the proposed hyperfine field model:$$\langle\vec H\sb{k}\rangle\sb{T}=A\langle\vec\mu\sb{k}\rangle\sb{T}+ B\sum\sb{j}\langle\vec\mu \sb{j}\rangle\sb{T}.$$To the extent that chemical short range order can be neglected in our rapidly quenched samples, the coupling parameters are $\rm A=A\sb0+A\sb1y\ (A\sb0=89$ kOe/$\mu\sb{B},$ A$\sb1={-}20$ kOe/$\mu\sb{B})$ and B = B$\rm\sb0=B\sb1y\ (B\sb0=4.4$ kOe/$\mu\sb{B},$ B$\sb1=3.2$ kOe/$\mu\sb{B}).$ MC simulations show the success and the limits of a simple local moment model, in characterizing the bulk magnetic properties of Fe-Ni. A new approach for simulating HFDs is developed. It combines MC simulation for the spin structure and the above phenomenological hyperfine field model for the site-specific hyperfine field values. Using this method, we calculated spin structures and HFDs in Fe-Ni alloys at different compositions and temperatures. Finally, interplay between the magnetic and the atomic ordering phenomena is studied in FeNi$\sb3,$ FeNi and Fe$\sb3$Ni, by considering the magnetic and chemical interactions simultaneously using MC simulations. Serveral new features that are not predicted by mean-field theory or MC simulations with chemical interactions only arise: (1) chemical order can be induced where using chemical interactions only leads to the prediction of no chemical order (2) chemical segregation can be induced where using chemical interactions only leads to the prediction of no chemical segregation, (3) FeNi$\sb3$ and Fe$\sb3$Ni are found to have significantly different chemical ordering temperatures where chemical interactions only lead to equal ordering temperatures, (4) chemical ordering temperatures are significantly shifted from their chemical interactions only values, even when the chemical ordering temperature is larger than the magnetic ordering temperature, (5) abrupt steps can occur in the spontaneous magnetization at the chemical ordering temperature, when the latter is smaller than the magnetic ordering temperature, and (6) nonlinear relations arise between the chemical ordering temperature and the chemical exchange parameter U $\equiv$ 2U$\sb{FeNi}-{\rm U}\sb{FeFe}-{\rm U}\sb{NiNi},$ where the U$\sb{ij}$ are the near-neighbour pair-wise chemical bonds.