Direct Observation of the Single-Domain Limit of Fe Nanomagnets by Spin-Polarized Scanning Tunneling Spectroscopy

We have investigated the magnetic structure of self-organized Fe islands on W(001) by means of spin-polarized scanning tunneling spectroscopy (Sp-STS). Single-domain, simple vortex, and distorted vortex states have been observed. The high resolution magnetic images were used to experimentally determine the single-domain limit. The experimental structures were compared with results of micromagnetic calculations confirming the ground state nature of the experimental configurations. The single-domain limit directly observed with Sp-STS is consistent with theoretical predictions.

Figure 1

(a) STS spectra for an Fe whisker. Solid and dotted lines show spectra with parallel and antiparallel orientation between the in-plane component of the magnetization of the tip and the magnetization of the whisker, respectively. (b) Topographic image of low coverage Fe islands on W(001). The image is differentiated to make the edge of the islands clearer. The differentiation leads to an illumination of the islands from the bottom right. (c) Topographic image of an Fe island on W(001) at $U=0.20\mathrm{V}$. (d) Magnetic $dI/dU$ image of the same Fe island with a magnetic vortex state on W(001) at $U=0.20\mathrm{V}$. The arrows illustrate the orientation of the magnetization.

Figure 2

Magnetic $dI/dU$ images of Fe islands with magnetic single-domain (onion) states on W(001) at $U=0.20\mathrm{V}$. (a) and (b) show the Fe islands (marked by ellipses) which have the different orientations of the magnetization.

Figure 3

(a) Magnetic $dI/dU$ image of an Fe island with modified vortex state on W(001) at $U=0.15\mathrm{V}$. (b) Calculated vortex state for irregular Fe island. About 35 500 elements were used in the micromagnetic simulation.

Figure 4

A phase diagram of the magnetic states for different diameters and thicknesses of the Fe islands on W(001). Solid and open circles show the single-domain and vortex state, respectively. The solid line is the dividing line following calculations by Cowburn et al. [13].