The thermal switching behavior of individual in-plane magnetized Fe/W(110) nanoislands is investigated by a combined study of variable-temperature spin-polarized scanning tunneling microscopy and Monte Carlo simulations. Even for islands consisting of less than 100 atoms the magnetization reversal takes place via nucleation and propagation. The Arrhenius prefactor is found to strongly depend on the individual island size and shape, and based on the experimental results a simple model is developed to describe the magnetization reversal in terms of metastable states. Complementary Monte Carlo simulations confirm the model and provide new insight into the microscopic processes involved in magnetization reversal of smallest nanomagnets.

A theoretical concept of local manipulation of magnetic domain walls is introduced. In the proposed procedure, a domain wall is driven by a spin-polarized current induced by a magnetic tip, as used in a scanning tunneling microscope, placed above a magnetic nanostripe and then moved along its long axis with a current flowing through the vacuum barrier. The angular momentum from the spin-polarized current exerts a torque on the magnetic moments underneath the tip and leads to a displacement of the domain wall. Particularly, the manipulation of a ferromagnetic 180° transverse domain wall has been studied by means of Landau-Lifshitz-Gilbert dynamics and Monte Carlo simulations. Different relative orientations of the tip and the sample magnetization have been considered.

We calculated the real-time non-equilibrium dynamics of quantum spin systems at finite temperatures. The mathematical framework originates from the C * -approach to quantum statistical mechanics and was applied to the samples investigated by means of spin-polarized scanning tunneling microscopy. Quantum fluctuations around thermal equilibrium were analyzed and calculated. The time-averaged expectation values agree with the time-averaged experimental data for magnetization curves. The method was used to investigate the dynamics of a sample for shorter times than the resolution time of the experimental setup. Furthermore, predictions of the relaxation times of single spins on metallic and semiconductor surfaces are made. To check the validity of our model, we compared our results with experimental data obtained from Fe adatoms on InSb and from Co adatoms on Pt(111) and found good agreement. Approximated thermalization was found numerically for the expectation values of the spin operators.

Using exact diagonalization, Monte-Carlo, and mean-field techniques, characteristic temperature scales for ferromagnetic order are discussed for the Ising and the classical anisotropic Heisenberg model on finite lattices in one and two dimensions. The interplay between nearestneighbor exchange, anisotropy and the presence of surfaces leads, as a function of temperature, to a complex behavior of the distance-dependent spin-spin correlation function, which is very different from what is commonly expected. A finite experimental observation time is considered in addition, which is simulated within the Monte-Carlo approach by an incomplete statistical average. We find strong surface effects for small nanoparticles, which cannot be explained within a simple Landau or mean-field concept and which give rise to characteristic trends of the spin-correlation function in different temperature regimes. Unambiguous definitions of crossover temperatures for finite systems and an effective method to estimate the critical temperature of corresponding infinite systems are given.

The manipulation of a ferromagnetic domain wall using a Spin-Polarized Scanning Tunneling Microscope (SP-STM) has been studied by means of a Landau-Lifshitz-Gilbert spin dynamics. We demonstrate that a ferromagnetic transverse domain wall can be shifted by the injection of a spin-polarized tunnel current. Depending on the tip polarization different scenarios occur where the domain wall will be pushed or pulled. Further, we show the possibility to reverse the magnetization inside the domain wall by using the current of the SP-STM tip and describe the predictions for the model system Fe/W(110).