Electronic topological transition and noncollinear magnetism in compressed hcp Co

Abstract

Recent experiments showed that Co undergoes a phase transition from the ferromagnetic hcp phase to the nonmagnetic fcc one around 100 GPa. Since the transition is of first order, a certain region of coexistence of the two phases is present. By means of ab initio calculations, we found that the hcp phase itself undergoes a series of electronic topological transitions (ETTs), which affects both elastic and magnetic properties of the material. Most importantly, we propose that the sequence of ETTs lead to the stabilization of a noncollinear spin arrangement in highly compressed hcp Co. Details of this noncollinear magnetic state and the interatomic exchange parameters that are connected to it are presented here.

Temperature-dependent exchange stiffness and domain wall width in Co

“…The Curie temperatures for both HCP (T c = 1250 K) and FCC (T c = 1300 K) phases calculated with our parameters are also close to the experimental one (see Table I) although they are below it. At the same time the calculations by Kvashnin et al [26]) give a smaller Curie temperature T C = 1100 K but in agreement with the value of 1131 K cited in the book by Cullity [43]. We note that our estimations of T c do not include the effects of longitudinal spin fluctuations at high temperatures and renormalization of the exchange constants due to the spin disorder in a paramagnetic state [35].…”

“…The exchange constants were estimated in ferromagnetic ground state at T = 0 K, which give a reasonable estimation also at non-zero temperatures for both fcc and hcp Co. Our method of the exchange constants calculation is essentially the same as that of Pajda et al [25], however, we used a more extended basis for partial wave expansion (l max = 3) than Pajda et al who used l max = 2. Because of that our exchange constants are closer to Kvashnin et al [26], who used a full potential methodology. Previously, the application of our approach has allowed a successful description of magnetism in various transition metal systems [30,31].…”

“…The first set of these parameters for HCP Co at T = 0 K was evaluated by Turek et al [24] and for FCC Co by Pajda et al [25] with experimental lattice parameters. Very recently a new set data has been published by Kvashnin et al [26]. Since the calculated exchange parameters in these two works are different, we have performed our own calculations of the Heisenberg exchange parameters (see Table I).…”

Anisotropic distortion and Lifshitz transition inα-Hf under pressure

“…As a matter of fact, the anomalous phenomena with compression that occurs for c/a is also present in Fe [40], Co [41], Zn [42], and Os [43], which as reported results from ETT. According to the above discussions, a perturbation on d ln(c/a)/dP appears at 10 GPa, and a drastic change takes place at 17 GPa in α Hf.…”

Evidence of ferromagnetic ground state and strong spin phonon coupling in Zr2TiAl with bi-axial strain: first principles study

“…In the case of CsCl-type FeSe a transition from AFM phase to NM phase is observed with intermediate FM phase [32]. HCP Co is observed to become non-magnetic at a pressure of 180 Gpa with a series of electronic topological transitions (ETT) at different pressures [33].…”