Stress measurements at monolayers and surfaces

Schematic of the stress measurement set-up. We monitor the crystal curvature by reflecting a laser beam from the surface onto a position sensitive detector. The stress, which induces the curvature, is derived from the position signal.

We study the correlation between mechanical stress, magnetic anisotropy and structural transitions in ferromagnetic monolayers. To this end we developed a stress measurement technique, which monitors the stress-induced curvature of 0.1 mm thin single crystal metal substrates with an optical deflection technique. The high sensitivity of the setup allows to measure film stress during growth with sub-monolayer sensitivity and to study adsorbate-induced surface stress changes. Our results indicate the significant role of stress as the driving force of structural transition in monolayers and we shed fresh light on the role of surface stress for surface reconstruction (Fig. 1 and 2).

Stress Measurements - Movie 1
Optical two-beam laser stress measurement. Note, that a tiny stress-induced crystal deflection in the sub-nm range is detectable. This corresponds to a radius of curvature as large as 1000 km. This is more than sufficient to detect stress in the sub-monolayer coverage regime. The light of a laser diode (1) is split in two beams, which are reflected from the sample surface (2) onto position sensitive detectors (3), Ref. [5]

The interplay between stress and magnetic anisotropy is given by magneto-elasticity. We measure the stress change in ferromagnetic films upon a reorientation of the film magnetization, as sketched in Fig. 3, and from this we extract the corresponding magneto-elastic coupling-coefficients. The resulting magneto-elastic stress changes are typically three orders of magnitude smaller than epitaxial misfit stress, and we determine the corresponding coupling coefficients in films as thin as 5 atomic layers. Here, magneto-elastic coupling coefficients deviate in sign and magnitude from the respective bulk values. These measurements reveal the decisive role of film strain for the peculiar magneto-elastic properties of epitaxial monolayers. For detailed description of the method and for a discussion of selected experimental results please consult [1-6].

Sress Measurements - Movie 2
Movie 2: Schematic of magneto-elastic stress measurements. The movie illustrates a film with a negative magneto-elastic coupling. The film has a tendency to expand into the direction of magnetization, and thus it exhibits a compressive magneto-elastic stress along the magnetization direction.