[Paper Review] Predicting magnetization of ferromagnetic binary Fe alloys from chemical short range order

Van-Truong Tran, Chu‐Chun Fu+1 more|arXiv (Cornell University)|Mar 27, 2019

Magnetic Properties and Applications1 references4 citations

TL;DR

This study develops a DFT-based analytical model that predicts the magnetization of ferromagnetic Fe-Co and Fe-Ni alloys by correlating local magnetic moments with chemical short-range order (SRO) and Co concentration in the first and second coordination shells. The model accurately reproduces the higher magnetization in ordered Fe-Co alloys compared to random solid solutions, attributing the effect primarily to local Co content rather than detailed atomic arrangements, with strong agreement to experimental data for 30–70 at.% Co.

ABSTRACT

Among the ferromagnetic binary alloys, body centered cubic (bcc) Fe-Co is the one showing the highest magnetization. It is known experimentally that ordered Fe-Co structures show a larger magnetization than the random solid solutions with the same Co content. In this work, based on density functional theory (DFT) studies, we aim at a quantitative prediction of this feature, and point out the role of the orbital magnetic moments. Then, we introduce a DFT-based analytical model correlating local magnetic moments and chemical compositions for Co concentrations ranging from 0 to 70 at.%. It is also extended to predict the global magnetization of both ordered and disordered structures at given concentration and chemical short range orders. The latter model is particularly useful for interpreting experimental data. Based on these models, we note that the local magnetic moment of a Fe atom is mainly dictated by the Co concentration in its first two neighbor shells. The detailed local arrangement of the Co atoms has a minor effect. These simple models can fully reproduce the difference in magnetization between the ordered and disordered Fe-Co alloys between 30% and 70% Co, in good agreement with experimental data. Finally, we show that a similar model can be established for another bcc binary Fe alloy, the Fe-Ni, also presenting ferromagnetic interactions between atoms.

Motivation & Objective

• To quantitatively predict the magnetization difference between ordered and disordered Fe-Co alloys with varying Co concentrations.
• To identify the dominant structural factors—specifically local chemical environment—governing local magnetic moments in ferromagnetic Fe-based binary alloys.
• To develop a transferable analytical model based on density functional theory (DFT) that links local composition and short-range order to global magnetization.
• To extend the model to Fe-Ni alloys to test its applicability across different bcc ferromagnetic systems.

Proposed method

• Density functional theory (DFT) calculations are used to compute local magnetic moments and orbital contributions in Fe-Co and Fe-Ni alloys across Co concentrations from 0 to 70 at.%.
• A local magnetic moment model is derived based on the number of Co atoms within the first and second coordination shells of each Fe atom.
• The model correlates the average local magnetic moment with the total Co concentration in the first two shells, treating atomic arrangements as secondary effects.
• The model is validated against DFT data and extended to predict global magnetization in both disordered solid solutions and ordered superstructures.
• The approach is generalized to Fe-Ni alloys, demonstrating transferability across bcc Fe-based ferromagnetic systems.
• Orbital magnetic moments are explicitly considered to explain deviations in magnetization trends.

Experimental results

Research questions

• RQ1 What is the role of chemical short-range order in determining the magnetization of Fe-Co alloys?
• RQ2 How do local Co concentrations in the first and second coordination shells affect the local magnetic moment of Fe atoms?
• RQ3 Why do ordered Fe-Co structures exhibit higher magnetization than random solid solutions with the same Co content?
• RQ4 Can a simple analytical model based on local composition accurately predict global magnetization in both ordered and disordered Fe-based alloys?
• RQ5 To what extent can this model be generalized to other bcc ferromagnetic binary alloys like Fe-Ni?

Key findings

• The local magnetic moment of a Fe atom is primarily determined by the total number of Co atoms in its first and second coordination shells, with detailed atomic arrangements having only minor influence.
• The model successfully reproduces the experimentally observed higher magnetization in ordered Fe-Co alloys (30–70 at.% Co) compared to random solid solutions, with quantitative agreement.
• Orbital magnetic moments contribute significantly to the overall magnetization and are captured in the model’s predictive framework.
• The model’s predictive accuracy for magnetization is maintained across both disordered and ordered structures, validating its use for interpreting experimental data.
• The same modeling approach is successfully extended to Fe-Ni alloys, confirming its transferability to other bcc ferromagnetic binary systems.
• The study demonstrates that chemical short-range order effects can be captured through a simple composition-based local model, bypassing the need for complex structural enumeration.