From the density functional theory to the model Hamiltonian studies on the magnetic skyrmion in MnSi and MnGe

Speaker: Hongchul Choi
Date & Time: Friday April 7th, 2017 - 4:00pm - 4:30pm
Location: AMPEL 311
Local Contact: George Sawatzky
Intended Audience:

Since I came to Los Alamos, I have studied the magnetic skyrmion, performing the density functional theory (DFT)calculation and model Hamiltonian studies from the DFT inputs. This time I will summarize my work on the skyrmion.  As a topologically stabilized spin object, the skyrmion shows more than several tens nanometers of a collective spin texture in magnetic materials. This meta-stable state has been only observed near the phase boundary between helical and ferromagnetic states at finite temperatures and external magnetic fields. For the device applications, it is important to understand the interaction between the skyrmion and impurity, which plays a significant role in controlling the skyrmion dynamics. To explore this problem, we have performed first-principles simulations of magnetic states with non-collinear spin configurations in MnSi. We first demonstrated that the skyrmion spin texture could indeed be stabilized with an optimized skyrmion size in a supercell of MnSi. The skyrmion-pinning effect was then studied by calculating the total energies of the system for varying location of impurities in MnSi. Although the magnetic skyrmion in B20 structures such as MnSi is mostly observed in two-dimensional (2D) space, MnGe recently showed the variation of the skyrmion topology along c axis as a 3D skyrmion. We explored the nature of the 3D skyrmion in MnGe, performing the DFT calculation and the tight-binding model simulation constructed from the DFT result. Presumably the dual nature of d electrons, a strong coupling model was proposed with the antiferromagnetic coupling between the localized and itinerant moments. Our model could capture the large (MnGe) and reduced (MnSi) moment states together by varying the strength of the coupling. We found the competition between the localized and itinerant moments is associated with whether a 3D or 2D skyrmion occurs. Furthermore, I will summarize what I have studied on in the condensed matter with DFT and DFT+DMFT calculations. I will  address what I am interested in doing in future.