Oxide electronics

Abstract:

Magnetic skyrmions are nanosized magnetization whirls that exhibit topological robustness and nontrivial magnetoelectrical properties, such as emergent electromagnetism and intriguing spin dynamics in the microwave-frequency region. In chiral magnets, skyrmions are usually found at a pocket in the phase diagram in the vicinity of the ordering temperature, wherein they order in the form of a hexagonal skyrmion lattice (SkL). It is generally believed that this equilibrium SkL phase is a uniform, long-range-ordered magnetic structure with a well-defined lattice constant. Here, using high-resolution small angle resonant elastic x-ray scattering, we study the field- and temperature-dependence of the skyrmion lattice in FeGe and membranes. Indeed, shows the expected rigid skyrmion lattice, known from bulk samples, that is unaffected by tuning field and temperature within the phase pocket. In stark contrast, the lattice constant and skyrmion size in FeGe membranes undergo a continuous evolution within the skyrmion phase pocket, whereby the lattice constant changes by up to 15% and the magnetic scattering intensity varies significantly. Using micromagnetic modeling, it is found that for FeGe the competing energy terms contributing to the formation of the skyrmion lattice fully explain this breathing behavior. In contrast, for this stabilizing energy balance is less affected by the smaller field variation across the skyrmion pocket, leading to the observed rigid lattice structure.

Abstract:

Magnetic skyrmions are localized swirls of magnetization with a non-trivial topological winding number. This winding increases their robustness to superparamagnetism and gives rise to a myriad of novel dynamical properties, making them attractive as next-generation information carriers. Recently the equation of motion for a skyrmion was derived using the approach pioneered by Thiele, allowing for macroscopic skyrmion systems to be modeled efficiently. This powerful technique suffers from the prerequisite that one must have a priori knowledge of the functional form of the interaction between a skyrmion and all other magnetic structures in its environment. Here we attempt to alleviate this problem by providing a simple analytic expression which can generate arbitrary repulsive interaction potentials from the micromagnetic Hamiltonian. We also discuss a toy model of the radial profile of a skyrmion which is accurate for a wide range of material parameters.

Abstract:

Two members of the isostructural R2CoGa8 intermetallic series, Er2CoGa8 and Tm2CoGa8, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed Er2CoGa8 to adopt a three-dimensional universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium- and thulium-based compounds. The corresponding magnetic space groups were determined to be P(2a)mmm’ and P(C)mmm. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

Abstract:

In multiferroic TbMn2O5, the behavior of the terbium ions forms a crucial part of the magnetoelectric coupling. The result is a magnetically induced reversal of the electric polarization at 2 T. In this article we present the first direct measurement of the terbium magnetic structure under applied magnetic fields. Contrary to the current interpretation of the magnetic properties of TbMn2O5, we show that upon the electric polarization reversal the terbium sublattice adopts a canted antiferromagnetic structure with a large component of magnetic moment parallel to the a axis. Furthermore, we provide evidence for a coupling between the manganese 3d magnetic structure and the terbium 4f magnetism, which is of great significance in the elusive magnetoelectric mechanisms at play.