Thin film nanoscale elements with a curling magnetic structure (vortex) are a promising candidate for future nonvolatile data storage devices. Their properties are strongly influenced by the spin structure in the vortex core. We have used spin-polarized scanning tunneling microscopy on nanoscale iron islands to probe for the first time the internal spin structure of magnetic vortex cores. Using tips coated with a layer of antiferromagnetic chromium, we obtained images of the curling in-plane magnetization around and of the out-of-plane magnetization inside the core region. The experimental data are compared with micromagnetic simulations. The results confirm theoretical predictions that the size and the shape of the vortex core as well as its magnetic field dependence are governed by only two material parameters, the exchange stiffness and the saturation magnetization that determines the stray field energy.

https://science.sciencemag.org/content/sci/298/5593/577.full.pdf

Direct Observation of Internal Spin Structure of Magnetic Vortex Cores

Two-dimensional systems, such as ultrathin epitaxial films and superlattices, display magnetic properties distinct from bulk materials. A challenging aim of current research in magnetism is to explore structures of still lower dimensionality. As the dimensionality of a physical system is reduced, magnetic ordering tends to decrease as fluctuations become relatively more important. Spin lattice models predict that an infinite one-dimensional linear chain with short-range magnetic interactions spontaneously breaks up into segments with different orientation of the magnetization, thereby prohibiting long-range ferromagnetic order at a finite temperature. These models, however, do not take into account kinetic barriers to reaching equilibrium or interactions with the substrates that support the one-dimensional nanostructures. Here we demonstrate the existence of both short- and long-range ferromagnetic order for one-dimensional monatomic chains of Co constructed on a Pt substrate. We find evidence that the monatomic chains consist of thermally fluctuating segments of ferromagnetically coupled atoms which, below a threshold temperature, evolve into a ferromagnetic long-range-ordered state owing to the presence of anisotropy barriers. The Co chains are characterized by large localized orbital moments and correspondingly large magnetic anisotropy energies compared to two-dimensional films and bulk Co.

Ferromagnetism in one-dimensional monatomic metal chains.

Spin mapping at the nanoscale and atomic scale

In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

https://iopscience.iop.org/article/10.1088/0034-4885/71/5/056501/pdf

Magnetism in ultrathin film structures

Combining relativistic first-principles calculations with a micromagnetic model, we establish the Dzyaloshinskii-Moriya interaction as an important mechanism in thin-film magnetism, determining the orientation of magnetic domains relative to the lattice, the type of domain wall, and the rotational direction of the magnetization in the wall. Applying the analysis to two monolayers Fe on W(110), we provide an explanation for puzzling experimental data obtained by spin-polarized scanning tunneling microscopy.

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Dzyaloshinskii-Moriya interaction accounting for the orientation of magnetic domains in ultrathin films: Fe/W(110)

Fe nanostripes on W(110) are investigated by Kerr magnetometry and spin-polarized scanning tunneling microscopy (SP-STM). An Arrhenius law is observed for the temperature dependent magnetic susceptibility indicating a one-dimensional magnetic behavior. The activation energy for creating antiparallel spin blocks indicates extremely narrow domain walls with a width on a length scale of the lattice constant. This is confirmed by imaging the domain wall by SP-STM. This information allows the quantification of the exchange stiffness and the anisotropy constant.

Atomic-scale magnetic domain walls in quasi-one-dimensional Fe nanostripes.

Strain-induced magnetic anisotropies in Co films on Mo(110)

The magnetic susceptibility in Fe/W(110) nanostripes decreases exponentially with increasing temperature according to an Arrhenius law which indicates a quasi-one-dimensional behavior. The interface energy of the Arrhenius law corresponds to the domain wall energy of a domain wall across a single stripe, separating fluctuating regions of homogeneous magnetization. The domain wall energy increases linearly with the width of the stripes, revealing a negative offset which we attribute to boundary effects. Domain wall energies have been determined for Fe/W(110) nanostripes coated with Au and Pd and are compared to values for uncoated Fe/W(110) nanostripes in ultrahigh vacuum.

Domain wall energy in quasi-one-dimensional Fe/W(110) nanostripes

We investigated the growth of Co submonolayers on bcc W(110) by scanning tunneling microscopy. Due to the strong Co-W bonding, monolayers of Co grow in equilibrium pseudomorphically on W(110) until the monolayer is almost completed. When excess atoms are deposited atop the pseudomorphic monolayer, it transforms to a close-packed (cp) monolayer with misfit dislocation lines parallel to $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ (Nishiyama-Wassermann orientation). The structure of the cp monolayer, as concluded from atomically resolved STM images, deviates from the rigid hard-sphere model of a cp (111) layer. Details of the structure are compared to a structure model previously proposed by van der Merwe et al. based on the embedded atom model (EAM).

Heteroepitaxial growth of Co on W(110) investigated by scanning tunneling microscopy

We present an experimental investigation of ${\mathrm{C}\mathrm{o}}_{x}{\mathrm{F}\mathrm{e}}_{1\ensuremath{-}x}$ monolayers grown on flat and stepped W(110) surfaces. Atomically resolved scanning tunneling microscopy and low energy electron diffraction reveal continuous miscibility and pseudomorphic growth of Co and Fe for $0\ensuremath{\le}xl0.6$. We observe short range ordered ${\mathrm{C}\mathrm{o}\mathrm{F}\mathrm{e}}_{3}$-$p(2\ifmmode\times\else\texttimes\fi{}2)$ and CoFe-$c(2\ifmmode\times\else\texttimes\fi{}2)$ structures in the pseudomorphic monolayer. High Co concentrations lead to a phase separation. The Curie temperature and the ferromagnetic Kerr signal at low temperatures decrease monotonically with increasing Co concentration, finally vanishing at $x=0.5$.

M. Pratzer

Papers

Atomic-scale magnetic domain walls in quasi-one-dimensional Fe nanostripes.

Strain-induced magnetic anisotropies in Co films on Mo(110)

Domain wall energy in quasi-one-dimensional Fe/W(110) nanostripes

Heteroepitaxial growth of Co on W(110) investigated by scanning tunneling microscopy

Structural and magnetic properties of Co-Fe binary alloy monolayers on W(110).