H. J. Hug

QUANTITATIVE MAGNETIC FORCE MICROSCOPY ON PERPENDICULARLY MAGNETIZED SAMPLES

We present a transfer-function approach to calculate the force on a magnetic force microscope tip and the stray field due to a perpendicularly magnetized medium having an arbitrary magnetization pattern. Under certain conditions, it is possible to calculate the magnetization pattern from the measured force data. We apply this transfer function theory to quantitatively simulate magnetic force microscopy data acquired on a CoNi/Pt multilayer and on an epitaxially grown Cu/Ni/Cu/Si(001) magnetic thin film. The method described here serves as an excellent basis for (i) the definition of the condition for achieving maximum resolution in a specific experiment, (ii) the differences of force and force z-derivative imaging, (iii) the artificial distinction between domain and domain wall contrast, and finally (iv) the influence of various tip shapes on image content.

A method for the calibration of magnetic force microscopy tips

A method is presented for the analysis of magnetic force microscopy(MFM) measurements that allows the quantitative determination of the sample stray field. It is shown how measurements on an easily obtainable calibration sample can be used to calibrate the MFM measurement as a function of the wavelength of the sample stray field at the tip apex. It is demonstrated that the thus obtained MFMcalibration can be used to determine the stray field distribution of the tip. Furthermore, the effect of the tip on the MFM imaging mechanism is analyzed by comparing it to some simple tip models. From the analysis, it is shown that the point–monopole and dipole models do not accurately describe the MFM imaging mechanism.

Operation characteristics of piezoelectric quartz tuning forks in high magnetic fields at liquid helium temperatures

Piezoelectric quartz tuning forks are investigated for use as force sensors in dynamic mode scanning probe microscopy at temperatures down to 1.5 K and in magnetic fields up to 8 T. The mechanical properties of the forks are extracted from the frequency dependent admittance and simultaneous interferometric measurements. The performance of the forks in a cryogenic environment is investigated. Force-distance studies performed with these sensors at low temperatures are presented.

Comparing the resolution of magnetic force microscopes using the CAMST reference samples

A set of reference samples for comparing the results obtained with different magnetic force microscopes (MFM) has been prepared. These samples consist of CoNi/Pt magneto-optic multilayers with di¤erent thicknesses. The magnetic properties of the multilayer are tailored in such a way that a very Þne stripe domain structure occurs in remanence. On top of this intrinsic domain structure, bits were written thermomagnetically using di¤erent laser powers. These samples have been imaged in six di¤erent laboratories employing both home-built and commercial magnetic force microscopes. The resolution obtained with these different microscopes, tips and measurement methods varies between 30 and 100 nm.

MAGNETIC DOMAIN STRUCTURE IN ULTRATHIN CU/NI/CU/SI(001) FILMS (INVITED)

We present a series of magnetic force microscope (MFM) images of epitaxial magnetic thin films. The films studied, Ni/Cu/Si(001) capped by 2 nm of Cu, exhibit perpendicular anisotropy over an exceptionally broad thickness range, 2 nm<h<14 nm. The magnetic domain structure of the as‐grown films shows a sharp transition to a finer length scale above a finite critical thickness of 12 nm. Micromagnetic theory provides the first quantitative description for these general but previously unexplained phenomena. Further we discuss MFM data obtained on films with a thickness larger than 14 nm. These films show a pronounced in‐plane anisotropy.

A miniature fibre optic force microscope scan head

The authors have built a fibre optic force microscope scan head which scans the cantilever relative to a fixed sample. The design enables the fibre-to-cantilever distance to be altered by moving the fibre relative to the fixed cantilever. This design feature is particularly important when using the variable deflection mode and for acquiring force versus distance curves. The interferometer system reaches a resolution of 0.01 nm (peak-to-peak).

Force–distance studies with piezoelectric tuning forks below 4.2 K

Abstract Piezoelectric quartz tuning forks have been employed as the force sensor in a dynamic mode scanning force microscope operating at temperatures down to 1.7 K at He-gas pressures of typically 5 mbar. An electrochemically etched tungsten tip glued to one of the tuning fork prongs acts as the local force sensor. Its oscillation amplitude can be tuned between a few angstroms and tens of nanometers. Quality factors of up to 120xa0000 allow a very accurate measurement of small frequency shifts. Three calibration procedures are compared which allow the determination of the proportionality constant between frequency shift and local force gradient based on the harmonic oscillator model and on electrostatic forces. The calibrated sensor is then used for a study of the interaction between the tip and a highly oriented pyrolytic graphite (HOPG) substrate. Force gradient and dissipated power can be recorded simultaneously. It is found that during approaching the tip to the sample considerable power starts to be dissipated although the force gradient is still negative, i.e., the tip is still in the attractive regime. This observation concurs with experiments with true atomic resolution, which seem to require the same tip–sample separation.

Low temperature magnetic force microscopy on YBa2Cu3O7-δ thin films

Abstract Recent low temperature magnetic force microscope measurements on YBa2Cu3O7−δ thin films are summarized. The high spatial resolution and the high sensitivity of the low temperature magnetic force microscope make it well suited for probing the local state of a magnetic sample. Single vortices have been imaged and their pinning locations determined. In addition, the tip can be used to locally perturb the magnetic state of the sample to move vortices out of their pinning centers or create vortex bundles.

Mechanical manifestations of rare atomic jumps in dynamic force microscopy.

The resonance frequency and the excitation amplitude of a silicon cantilever have been measured as a function of distance to a cleaved KBr(001) surface with a low-temperature scanning force microscope (SFM) in ultrahigh vacuum. We identify two regimes of tip-sample distances. Above a site-dependent critical tip-sample distance reproducible data with low noise and no interaction-induced energy dissipation are measured. In this regime reproducible SFM images can be recorded. At closer tip-sample distances, above two distinct atomic sites, the frequency values jump between two limiting curves on a timescale of tens of milliseconds. Furthermore, additional energy dissipation occurs wherever jumps are observed. We attribute both phenomena to rarely occurring changes in the tip apex configuration which are affected by short-range interactions with the sample. Their respective magnitudes are related to each other. A specific candidate two-level system is also proposed.

Temperature dependence of large exchange-bias in TbFe-Co/Pt

The exchange-bias effect provides unidirectional anisotropy to ferromagnetic thin-films in GMR and TMR sensors. It is weak because it relies on an inherently compensated coupling across a ferromagnet/antiferromagnet interface. We replace the antiferromagnet with a TbFe-based perpendicular-anisotropy amorphous ferrimagnet, coupling to an adjacent ferromagnet with 13-5u2009mJ/m2 for temperatures within 7.9–280u2009K. A large exchange-bias between 1.1 and 0.7u2009T in that temperature range ensues. The temperature dependence of the bias is step-like, and thus different from that of antiferromagnet-based exchange-bias systems. It suggests using this material in exchange-biased temperature assisted magnetic writing.