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We theoretically study the magnetic properties of iron germanium, known as one of canonical helimagnets. For this purpose we use the real-space spin Hamiltonian and micromagnetic model, derived in terms of Andersen’s “local force theorem”, to describe the low-lying magnetic excitations via spin-polarized Green’s function, obtained from the first-principles calculations. The model was designed to numerically evaluate the spin stiffness constant in reciprocal space, in order for assessment of the contributing itinerant mechanisms. The calculated pairwise exchange interactions reveal the essentiality of Ruderman-Kittel-Kasuya-Yosida coupling in FeGe. Thus provided mean-field estimation of magnetic transition temperature agrees good with the experimental measurement, underlining the necessity of the comprehensive real/reciprocal space-based approach for a proper description of magnetic excitations in FeGe. [ABSTRACT FROM AUTHOR]

Fe 3 GeTe 2 attracts significant attention due to technological perspectives of realizing room temperature ferromagnetism in two-dimensional materials. Here we show that due to structural peculiarities of the Fe 3 GeTe 2 monolayer, short distance between the neighboring iron atoms induces a strong exchange coupling. This strong coupling allows us to consider them as an effective cluster with a magnetic moment ∼ 5 μ B , giving rise to a simplified spin model on a bipartite honeycomb lattice with the reduced number of long-range interactions. The simplified model perfectly reproduces the results of the conventional spin model, but allows for a more tractable description of the magnetic properties of Fe 3 GeTe 2 , which is important, e.g., for large-scale simulations. Also, we discuss the role of biaxial strain in the stabilization of ferromagnetic ordering in Fe 3 GeTe 2. [Display omitted] • Alternative magnetic model on a bipartite honeycomb lattice is proposed for monolayer Fe 3 GeTe 2. • The simplified model perfectly reproduces the results of the conventional three-site model. • At the same time, it allows for a more tractable description of the magnetic properties of Fe 3 GeTe 2. • The role of biaxial strain in the stabilization of ferromagnetic ordering is discussed. [ABSTRACT FROM AUTHOR]