Frustration-driven magnetic fluctuations as the origin of the low-temperature skyrmion phase in Co7Zn7Mn6

In chiral cubic helimagnets, phases of magnetic skyrmions—topologically protected spin whirls—are stabilized by thermal fluctuations over a narrow region directly below the magnetic ordering temperature Tc. Due to often being touted for use in applications, there is a high demand to identify new ways to stabilize equilibrium skyrmion phases far below Tc where they may display an enhanced robustness against external perturbation due to a larger magnetic order parameter. Here, from quantum beam experiments on the chiral magnet Co7Zn7Mn6, we unveil a direct correlation between the stability of its second skyrmion phase-stable far from Tc, and a concomitant enhancement of an underlying magnetic fluctuation rate that is driven by geometric magnetic frustration. The influences of other leading skyrmion stability mechanisms, such as those derived from thermal fluctuations and low T cubic anisotropies, are shown to be weak in this system. We therefore advance the existence of a fundamental mechanism for stabilizing topological skyrmions in Co7Zn7Mn6 chiral magnet that draws upon magnetic frustration as the key ingredient.

Influence of Magnetic Order Parameter Symmetry and Spin Fluctuations on Forming of Ferroelectric Order in Mn2O5 Oxides

The magnetic phase transitions from paramagnetic to magnetic phases have studied in RMn2O5 oxides. Using a symmetry analysis and renormalization group method it was shown that an appearance of the electric polarization or lack whereof in the magnetic phases are determined the symmetry of these phases and spin fluctuations in the vicinity of the phase transitions.

Macroscopic degeneracy and order in the 3D plaquette Ising model

The purely plaquette 3D Ising Hamiltonian with the spins living at the vertices of a cubic lattice displays several interesting features. The symmetries of the model lead to a macroscopic degeneracy of the low-temperature phase and prevent the definition of a standard magnetic order parameter. Consideration of the strongly anisotropic limit of the model suggests that a layered, “fuki-nuke” order still exists and we confirm this with multi-canonical simulations. The macroscopic degeneracy of the low-temperature phase also changes the finite-size scaling corrections at the first-order transition in the model and we see this must be taken into account when analyzing our measurements.

Antiferromagnetism in two-dimensional quasicrystals

Quasiperiodic structures possess long range positional order, but are freed of constraints imposed by translational invariance. For spins interacting via Heisenberg couplings, one may expect therefore to find novel magnetic configurations in such structures. We have studied magnetic properties for simple two dimensional models, as a first step towards understanding experimentally studied magnetic quasicrystals such as the Zn–Mg–R(rare earth) compounds. We analyse properties of the antiferromagnetic groundstate and magnon excitation modes for bipartite tilings such as the octagonal and Penrose tilings. In the absence of frustration, one has an inhomogeneous Neel-ordered ground state, with local quantum fluctuations of the local staggered magnetic order parameter. We study the spin wave spectrum and wavefunctions of such antiferromagnets within the linear spin wave approximation. Some results for spin-spin correlations in these systems are discussed.

Thermodynamic Properties of One-Dimensional Hubbard Model at Finite Temperatures

The phase diagram and thermodynamic properties of the Hubbard model in one dimension are calculated numerically within the generalized self-consistent approach over a wide range of temperature T, magnetic field h, interaction strength U/t and electron concentration n. The temperature variation of the transverse magnetization, double occupied sites and chemical potential at various n, U/t and h is also analyzed. The wave vector q for the magnetic order parameter in the vicinity of the half-filling in the metallic phase shows a crossover from incommensurate magnetic phase (0<q<π) into the state with strong antiferromagnetic correlations (q=π) as temperature is increased. Overall our numerical results in the approximate theory at finite temperatures are in good quantitative and qualitative agreement with the Bethe-ansatz results.