R. Schmidt

We demonstrate that magnetic exchange force spectroscopy allows for a quantitative determination of the distance-dependent magnetic exchange interaction across a vacuum gap. Experiments were performed on the antiferromagnetic Fe monolayer on W(001) with magnetically sensitive tips and compared to first-principles calculations performed for different cluster tip models. For stable tips, which can be distinguished from unstable tips by analyzing the dissipation signal, very good agreement with theory is observed.

Applying magnetic exchange force microscopy with an Fe-coated tip, we experimentally resolve the atomic-scale antiferromagnetic structure of the Fe monolayer on W(001). On the basis of first-principles calculations, using an Fe nanocluster as a tip, we determine the distance dependence of the magnetic exchange forces. Significant relaxation of tip and sample atoms occurs, which depend sensitively on the local magnetic configuration. This shifts the onset of magnetic interactions toward larger separations and facilitates their observation. Implementing a multiatom tip in the calculations and accounting for relaxation effects are crucial to obtain the correct sign and distance dependence of the magnetic exchange interaction. By comparison with our calculations, we show that the experimentally observed contrast is due to a competition between chemical and magnetic forces.

We demonstrate the feasibility of observing and inducing magnetization switching using the distance dependence of the magnetic exchange interaction. The experiments were performed employing an atomic force microscopy setup on the antiferromagnetic iron monolayer on the (001) surface of tungsten with magnetic tips that behaved like independent superparamagnetic clusters with uniaxial anisotropy. Applying the N\'eel-Brown law, we were able to determine energy barriers from lifetimes measured at different distances with and without external magnetic field. Our findings suggest that the distance dependence of the magnetic exchange interaction can be utilized to monitor and control magnetization dynamics on the atomic level.

A NdCo5 (37 nm)/Fe (22 nm) hard/soft bilayer was grown by pulsed laser deposition on a MgO (1 1 0) substrate, and investigated by vibrating sample magnetometry and ferromagnetic resonance (FMR). Due to the direct exchange coupling at the NdCo5/Fe interface, the spin reorientation transition (SRT) typical of NdCo5 is observed also in the bilayer by means of global magnetometry. Concerning the magnetization dynamics, the interlayer magnetic coupling is weak, thus allowing the identification of the FMR signals in the sample as those originating mainly from the individual responses of the Fe and NdCo5 layers. In the NdCo5 layer, the effective coupling field is negligible compared to the internal anisotropy, and the magnetization precession is similar to the one found in a NdCo5 single layer. In the magnetically soft Fe layer, however, the precession of the moments occurs in the exchange field stemming from the dynamically fixed NdCo5 layer, giving rise to a partial transfer of magnetic anisotropy from the latter and enabling us to follow the SRT of NdCo5 by measuring the Fe FMR peak field position. Controlling the anisotropy direction in the soft layer by making use of the SRT in the hard layer can find applications in future spintronic devices.

We study the effect of hydrogen adsorption on gadolinium islands epitaxially grown on W(110) utilizing atomic force microscopy operated in the non-contact regime. In constant force images, gadolinium islands exhibit two height levels, corresponding to hydrogen covered and clean gadolinium areas, respectively. The experimentally measured height differences are strongly bias dependent, showing that the contrast pattern is dominated by electrostatic tip-sample forces. We interpret our experimental findings in terms of a local reduction of the work function and the presence of localized charges on hydrogen covered areas. Both effects lead to a variation of the contact potential difference between tip and surface areas, which are clean or hydrogen covered gadolinium. After clarifying the electrostatic contrast formation, we can unambiguously identify regions of clean gadolinium on the islands. These results are important for further magnetic exchange force microscopy based studies, because hydrogen also alters the magnetic properties locally.