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MA: Fachverband Magnetismus
MA 33: Non-Skyrmonic Magnetic Textures I
MA 33.4: Vortrag
Donnerstag, 20. März 2025, 10:15–10:30, H16
Quantum Bloch points in magnetic systems — •Vladyslav Kuchkin, Štefan Liščák, Andreas Haller, Andreas Michels, and Thomas Schmidt — University of Luxembourg
A Bloch point represents a three-dimensional hedgehog singularity of a magnetic vector field in which the magnetization vanishes at the center. Experimentally, the appearance of such points is well-established; at the same time, the standard micromagnetic theory is only suitable for fixed-length continuous magnetization vector fields and is thus not applicable to such singularities. To approach this problem, we study a Bloch point in a quantum Heisenberg model for the case of spin-1/2 particles. Such a state can be stabilized by adding a Zeeman term that imposes a boundary condition. We obtain the ground state and the corresponding magnetization profile by performing an exact diagonalization and using density matrix renormalization group techniques. Our findings show a smooth change of the spin length in the quantum model, leading to zero magnetization at the Bloch point. This behavior is generic for different system sizes. Our results indicate the necessity of generalizing the classical micromagnetic model, relying on a magnetization vector field of constant length, by adding a third degree of freedom of the spin: the ability to change its length. We achieve this by introducing a regularized S3 model that describes a four-dimensional order parameter of unit length. In contrast to earlier attempts to describe magnetization profiles of varying lengths, our approach satisfies the quantum mechanical constraints on spin operators.
Keywords: Bloch point; Heisenberg model; Regularized micromagnetic model; Density matrix renormalization group method; Quantum fluctuations