Jessica E. Bickel

Writing and Deleting Single Magnetic Skyrmions

Controlling Skyrmions Magnetic skyrmions—tiny vortex patterns of spins—hold promise for information storage because of their robustness to perturbations. Skyrmions have been observed experimentally, but manipulating them individually remains a challenge. Romming et al. (p. 636; see the cover) used spin-polarized electrons generated by a scanning tunneling microscope to reversibly create and destroy skyrmions in a thin iron film covered by a layer of palladium. The energy of the tunneling electrons was the decisive factor determining the probability of the process; atomic defects in the film acted as pinning sites for the skyrmions. The work demonstrates the feasibility of using spin-polarized tunnel currents for the controlled manipulation of individual skyrmions. Spin-polarized currents delivered by a scanning tunneling microscope can be used to create and destroy spin whirlpools. Topologically nontrivial spin textures have recently been investigated for spintronic applications. Here, we report on an ultrathin magnetic film in which individual skyrmions can be written and deleted in a controlled fashion with local spin-polarized currents from a scanning tunneling microscope. An external magnetic field is used to tune the energy landscape, and the temperature is adjusted to prevent thermally activated switching between topologically distinct states. Switching rate and direction can then be controlled by the parameters used for current injection. The creation and annihilation of individual magnetic skyrmions demonstrates the potential for topological charge in future information-storage concepts.

Information Transfer by Vector Spin Chirality in Finite Magnetic Chains

Vector spin chirality is one of the fundamental characteristics of complex magnets. For a one-dimensional spin-spiral state it can be interpreted as the handedness, or rotational sense of the spiral. Here, using spin-polarized scanning tunneling microscopy, we demonstrate the occurrence of an atomic-scale spin spiral in finite individual bi-atomic Fe chains on the (5×1)-Ir(001) surface. We show that the broken inversion symmetry at the surface promotes one direction of the vector spin chirality, leading to a unique rotational sense of the spiral in all chains. Correspondingly, changes in the spin direction of one chain end can be probed tens of nanometers away, suggesting a new way of transmitting information about the state of magnetic objects on the nanoscale.

Atomic Size Mismatch Strain Induced Surface Reconstructions

The effects of lattice mismatch strain and atomic size mismatch strain on surface reconstructions are analyzed using density functional theory. These calculations demonstrate the importance of an explicit treatment of alloying when calculating the energies of alloyed surface reconstructions. Lattice mismatch strain has little impact on surface dimer ordering for the α2(2×4) reconstruction of GaAs alloyed with In. However, atomic size mismatch strain induces the surface In atoms to preferentially alternate position, which, in turn, induces an alternating configuration of the surface anion dimers. These results agree well with experimental data for α2(2×4) domains in InGaAs∕GaAs surfaces.

The creation of 360° domain walls in ferromagnetic nanorings by circular applied magnetic fields

Switching behavior in ferromagnetic nanostructures is often determined by the formation and annihilation of domain walls (DWs). In contrast to the more familiar 180° DWs found in most nanostructures, 360° DWs are the proposed transition state of nanorings. This paper examines the formation of 360° DWs created by the application of a circular magnetic field using micromagnetic simulations. 360° DWs form from pairs of canting moments that are oppositely aligned, which each grow to form rotated domains bounded by two 180° DWs and the 180° DWs combine to form 360° DWs. The resulting 360° DWs occur in pairs of opposite topological winding number due to these domains of opposite canting direction. The final number of DWs formed is greatly impacted by symmetry, both of the ring and of the placement of the circular magnetic field.

A multi-level single-bit data storage device

One method to increase bit density in magnetic memory devices is to use larger structures that have multiple states in which to encode information rather than the typical two state system. A ferromagnetic nanoring with multiple domain walls that annihilate at different applied magnetic fields could serve as such a bit. This paper examines the formation and annihilation of four 360° domain walls (DWs) using micromagnetic simulations. To create the walls, one can apply circular magnetic fields to asymmetric nanoring structures. Nanorings with circular notches on a centered elliptical hole enable the formation of stable DWs in specific locations with known characteristics. By considering the impacts of both domain wall length and topological winding number on domain wall energy, one can create a nanostructure with four stable domain walls that annihilate at different applied magnetic fields. With two stable vortex configurations, such nanorings could theoretically encode up to ten different states.