Magnetic and interatomic forces measured by low temperature scanning force microscopy

Abstract

Magnetic and interatomic forces have been investigated using low temperature scanning force microscopy (LTSFM) in a dynamic mode where the probing tip at the end of a microlever is oscillated with a constant amplitude above the sample and the resonance frequency of the lever is tracked. First, magnetic domains in ultrathin Fe �lms sandwiched between Ag layers have been investigated at room temperature. These �lms only 3.5 atomic layers thick show a thickness dependent reorientation transition whereby the orientation of the magnetization changes from perpendicular at small thicknesses to parallel to the �lm at large �lm thicknesses. In the perpendicular regime, magnetic domains could be imaged although the coercive �eld of the sample was less than 1 mT, i.e. of the same order of magnitude as the �eld generated by the magnetic tip. As expected, the domain size varies strongly with �lm thickness, namely between 580 nm and 190 nm. The value of the magnetic surface anisotropy parameter KS obtained from the domain size variation agrees well with literature values. For this study, it was essential to prepare a magnetic tip that generates a weak magnetic �eld: Ideally the magnetic �eld generated by the tip should be smaller than the coercive �eld of the sample. In order to investigate the magnetic �elds generated by magnetic tips and in order to analyse their resolution, a quantitative analysis has been carried out. This analysis relies on the calibration of the magnetic tip using a known test sample with perpendicular anisotropy. Magnetic tips coated with di�erent �lm thicknesses of Fe were studied, showing that above a nominal Fe thickness of 1:3 nm the tip has good imaging properties and that the �eld generated by the tip increases with growing Fe thickness. Ni coated tips do not have good imaging properties. The resolution of magnetic tips can be improved using ultrasharp tips instead of standard ones. Ultrasharp tips are well described using a monopole model, in contrast to standard tips, where an extended charge model is necessary. Interatomic forces have been studied on single crystalline KBr and NiO (001) surfaces. The ionic crystalline KBr (001) surface is a model system for the study of interaction forces, as atomic resolution images are easily obtained and calculations can be performed using a shell-model and a dedicated program developed by L. Kantorovich, University College, London. After obtaining atomic resolution, the frequency shift was measured as a function of tip-sample distance above characteristic surface sites. The obtained data was converted to force versus distance data. The total interaction force is expressed as a sum of long-range and short-range force components. The long-range force is well described by a van-der-Waals force between a conical tip with spherical cap and a at surface. The short-range interaction was computed using atomistic simulations. The maximum value of the calculated attractive force and its decay length are in good agreement with our experiments. The corrugation is overestimated in the calculations: a corrugation of only 0:025 nm was measured, while the calculations predict a corrugation of 0:2 nm. One possible reason is that the atomistic tip apex model assumed for the calculations may not represent the experimental tip well, for example due to multiatom interactions. The experimental observation of an atomic scale defect shows that multiatom interactions are negligible at the mean imaging distance, but at closer tip to sample distances, these interactions can become important. Finally the water contaminated KBr (001) surface has been studied. Atomic resolution images of an ordered phase have been obtained. NiO is an antiferromagnetic insulator studied by several SFM groups in order to measure shortrange magnetic exchange interactions. Atomic resolution was obtained on the NiO (001) surface using a non-magnetic tip. The surface forces have been studied with site-speci�c frequency versus distance measurements. The long-range force-distance data are well modelled with a capacitive force in the distance range of 0:5 to 20 nm. These capacitive forces arise from electric charges localized on the insulating surface. Multiatom interactions become important at close tip-sample distances in agreement with calculations and lead to an oscillation in the frequency versus distance. As shown by calculations, such multiatom interactions are possible because of large relaxation e�ects. These relaxation e�ects prevent unfortunately imaging at close tipsample distances, where short-range magnetic interactions are expected to be of a measurable magnitude. Furthermore, the dissipated energy per oscillation cycle increases sharply at the tip-sample distance where the interaction between several atoms on the tip and on the surface becomes important. This is an indication that the induced displacements of tip and surface atoms becomes hysteretic at this tip-sample distance. The observation of a point defect on the NiO surface shows that the tip is atomically sharp at the tip-sample distances where stable images could be obtained.