E. Y. Vedmedenko

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.

Stability of single skyrmionic bits.

The switching between topologically distinct skyrmionic and ferromagnetic states has been proposed as a bit operation for information storage. While long lifetimes of the bits are required for data storage devices, the lifetimes of skyrmions have not been addressed so far. Here we show by means of atomistic Monte Carlo simulations that the field-dependent mean lifetimes of the skyrmionic and ferromagnetic states have a high asymmetry with respect to the critical magnetic field, at which these lifetimes are identical. According to our calculations, the main reason for the enhanced stability of skyrmions is a different field dependence of skyrmionic and ferromagnetic activation energies and a lower attempt frequency of skyrmions rather than the height of energy barriers. We use this knowledge to propose a procedure for the determination of effective material parameters and the quantification of the Monte Carlo timescale from the comparison of theoretical and experimental data.

Noncollinear magnetic order in quasicrystals.

Based on Monte Carlo simulations, the stable magnetization configurations of an antiferromagnet on a quasiperiodic tiling are derived theoretically. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that the superposition of geometric frustration with the quasiperiodic ordering leads to a three-dimensional noncollinear antiferromagnetic spin structure. The structure can be divided into several ordered interpenetrating magnetic supertilings of different energy and characteristic wave vector. The number and the symmetry of subtilings depend on the quasiperiodic ordering of atoms.

Magnetic structures of Ising and vector spins monolayers by Monte-Carlo simulations

Abstract We derive the optimal magnetic structures for monolayers of either square or triangular lattice symmetry with evidence for morphological differences. The interplay between short-range exchange and long-range dipolar forces leads to quite different results for Ising spins and vector spins. For the Ising model, spin domains with parallel stripes, chevron patterns and labyrinths at different scales and with thermal disorder are deduced. For the vector model with a weak perpendicular anisotropy, the spins are planar and form a lattice of vortices of both signs. Such a structure remains stable even under a large perpendicular magnetic field, whereas a weak in-plane magnetic field is sufficient to obtain a uniform magnetic domain. For a sufficiently large perpendicular anisotropy, a mixed structure appears that includes spin vortex areas surrounding spin-up and spin-down areas.

Size-dependent magnetic properties in nanoplatelets

We demonstrate that the discrete dipolar sums can be separated into two contributions: thickness- and geometry-dependent parts. The geometry-dependent part is analogous to the shape dependence of the continuum approach. The correct normalization of the dipolar energy eliminates the apparent discrepancies of the discrete summation with the experimental results and continuum Maxwell theory. The superposition of the two contributions explains a new phenomenon, i.e. the size-dependent spin reorientation transition and/or enhancement of the effective perpendicular anisotropy.

Topologically Protected Magnetic Helix for All-Spin-Based Applications

The recent years have witnessed an emergence of the field of all-spin-based devices without any flow of charge. An ultimate goal of this scientific direction is the realization of the full spectrum of spin-based networks as in modern electronics. The concept of energy-storing elements, indispensable for those networks, are so far lacking. Analyzing analytically the size dependent properties of magnetic chains that are coupled via either exchange or long-range dipolar or Ruderman-Kittel-Kasuya-Yosida interactions, we discover a particularly simple law: magnetic configurations corresponding to helices with integer number of twists, which are commensurate with the chains length, are energetically stable. This finding, supported by simulations and an experimentally benchmarked model, agrees with the study [R. Skomski et al., J. Appl. Phys. 111, 07E116 (2012)] showing that boundaries can topologically stabilize structures that are not stable otherwise. On that basis, an energy-storing element that uses spin at every stage of its operation is proposed.

Dipolar magnetic anisotropy energy of laterally confined ultrathin ferromagnets: multiplicative separation of discrete and continuum contributions

Very recent exact summation has indicated that the lateral confinement of ultrathin ferromagnetic islands brings about significant deviations from the usually assumed laterally infinite sample so far as the dipolar magnetic anisotropy is concerned. Here, it is demonstrated that the phenomenological rescaling of the structural detail leads to a fundamental micromagnetic (continuum theory) quantity, namely, the demagnetizing energy for the assumed shape of the mesoscopic island. The derivation of a compact analytical formula for the demagnetization factor of any right circular cylinder has been instrumental for this insight. The effects of discrete geometry (lattice and substrate orientation), thickness, and overall shape of the ultrathin structure are thus distilled into a form which exhibits a great deal of universality.

Interplay between magnetic and spatial order in quasicrystals

The stable magnetization configurations of antiferromagnets on quasiperiodic tilings are investigated theoretically. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that the combination of geometric frustration and the quasiperiodic order of atoms leads to complicated non-collinear ground states. The structure can be divided into subtilings of different energies. The symmetry of the subtilings depends on the quasiperiodic order of magnetic moments. The subtilings are spatially ordered. However, the magnetic ordering of the subtilings in general does not correspond to their spatial arrangements. While subtilings of low energy are magnetically ordered, those of high energy can be completely disordered due to local magnetic frustration.

Magnetic properties of single atoms of Fe and Co on Ir(111) and Pt(111)

In using the fully relativistic versions of the embedded cluster and screened Korringa-Kohn-Rostoker methods for semi-infinite systems the magnetic properties of single adatoms of Fe and Co on Ir(111) and Pt(111) are studied. It is found that for Pt(111) Fe and Co adatoms are strongly perpendicularly oriented, while on Ir(111) the orientation of the magnetization is only out of plane for a Co adatom; for an Fe adatom it is in plane. For comparison, the so-called band energy parts of the anisotropy energy of a single layer of Fe and Co on these two substrates are also shown. The obtained results are compared to recent experimental studies using, e.g., the spin-polarized STM technique.

Multipole interaction of polarized single-domain particles

The multipole moments and multipole–multipole interactions of uniformly polarized particles have been calculated based on the fundamental theory of electrostatics. As the polarization of the particles is uniform, only surface charges are considered. The polarization may have its origin in magnetization or ferroelectricity or be an intrinsic property of molecules. It is demonstrated that, depending on the geometry of the particles, the higher order interactions can be comparable to or even stronger than the dipole–dipole interaction. The higher order moments give rise to an additional energy contribution in arrays of close packed polarized nanoparticles. The influence of particle aspect ratios as well as array periodicity is discussed.