PhD students talks
SKYRMION PHASE IN A FRUSTRATED HEISENBERG ANTIFERROMAGNET WITH DZYALOSHINSKII-MORIYA INTERACTION
Vector field textures of chiral and topological nature wrapping a unit sphere once called magnetic skyrmions are known for their exotic behavior and seem promising in electronic applications due to their small size, stability and sensitivity in terms of current-driven motion with surprisingly low current density [1, 2]. They were first experimentally identified in 2009 by means of neutron scattering and spin-polarized scanning tunneling microscopy and since then have been found in many magnetic, multiferroic and ferroelectric materials and superconductors [3, 4, 5]. Skyrmion’s formation can be driven by different mechanisms either in the systems with broken inversion symmetry or in the systems that are inversion symmetric  (so-called bubble skyrmions). It has been proven that such spin textures can be stabilized not only in the ferromagnetic states, but also in antiferromagnetic ones due to the weak next-nearest neighbor interaction .
We focus on the antiferromagnetic Heisenberg model with the broken symmetry due to the presence of the Dzyaloshinskii-Moriya interaction (DMI skyrmions) in the varying external magnetic field. By considering topological order parameters it is possible to identify the formation and stability of skyrmion phase (SkX) on three different interpenetrating sublattices at low temperatures. We also can observe the transition to SkX directly from the snapshots. We aim to identify the lowest boundary for the strength of DM interaction starting from which such transition becomes possible. We try to build a phase diagram to show the transition from a spiral phase to SkX at certain field, temperature and DM interaction ranges by means of hybrid Monte-Carlo simulations. Finally, the stability of SkX is studied in terms of the time skyrmions remain present in the system after the external field is switched off.
 X.C. Zhang, Y. Zhou, M. Ezawa et al., Scientific Reports, 5 (2015).
 S. Mulbauer, B. Binz, F. Jonietz et al., Science, 323 (2009).
 S. Heinze, K. von Bergmann, M. Menzel et al., Nature Physics, 7 (2011).
 I. Kezsmarki et al., Nat. Mater. 14 (2015).
 U. K. Rossler, A. N. Bogdanov, C. Pfleiderer, Nature, 442 (2006).
 T. Okubo, S. Chung, and H. Kawamura, Phys. Rev. Lett., 108 (2012).
RIGOROUS INVESTIGATION OF MAGNETIZATION PROCESSES IN CLASSICAL AND QUANTUM SPIN MODEL
The magnetization process and adiabatic demagnetization of antiferromagnetic spin-1/2 XXZ Heisenberg clusters in the shape of regular polyhedra (octahedron and cuboctahedron) are examined using the exact diagonalization method. It is demonstrated that a quantum (xy) part of the XXZ exchange interaction is a primary cause for the presence of additional intermediate magnetization plateaux and steps, which are totally absent in the limiting Ising case. It is shown that spin-1/2 XXZ Heisenberg regular polyhedra exhibit an enhanced magnetocaloric effect in the proximity of magnetization steps and jumps, which are accompanied with a rapid drop (rise) of temperature just above (below) the level-crossing field when the magnetic field is removed adiabatically.
In addition, the mixed spin-1 and spin-1/2 Heisenberg octahedral chain with regularly alternating monomeric spin-1 sites and square-plaquette spin-1/2 sites is investigated using variational technique, localized-magnon approach, exact diagonalization and density-matrix renormalization group method. The investigated model has in a magnetic field an extraordinarily rich ground-state phase diagram, which includes the uniform and cluster-based Haldane phases, two ferrimagnetic phases of Lieb-Mattis type, two quantum spin liquids and two bound magnon crystals in addition to the fully polarized ferromagnetic phase. The lowest-energy eigenstates in a highly-frustrated parameter region belong to flat bands and hence, low-temperature thermodynamics close to discontinuous field-driven quantum phase transitions related to the bound-magnon crystals and ferromagnetic ground states can be satisfactorily described within the localized-magnon approach. The variational method provides an exact evidence for the magnon-crystal phase with a character of the monomer-tetramer ground state at zero magnetic field, while another magnon-crystal phase involving a single magnon bound state at each square plaquette is found in a high-field region.