Ferromagnetic Tip Research Articles

Magnetic force microscopy investigation of superconductors: first results

We have performed our first magnetic force microscope (MFM) measurements on the high- T c superconductor Bi 2Sr 2CaCu 2O x and conventional type-1 superconductor Sn, with a specially designed low-temperature MFM. The magnetic interaction between ferromagnetic tip and Sn sample was detected and recorded as a function of the tip-sample distance. It is shown that the low-temperature MFM may be used to investigate the distribution of the superconductive phase on the surface of the superconductors. All MFM images of Bi 2Sr 2CaCu 2O x at T< T c show some perturbations and smooth regions, which may be treated as the regions with the disturbance of the superconductivity of the sample. The possible physical reason for the existence of such regions could be imperfections in the sample structure or more complex phenomena of the magnetic field penetration into the high- T c superconductor, which is caused by the tip.

Scanning tunneling microscopy study of the degree of dimer asymmetry on the Si(001)-(2 × 1) surface

The degree of dimer asymmetry on the Si(001)-(2 × 1) surface as seen in high resolution scanning tunneling microscopy (STM) images has been studied in detail. It is found that the degree of dimer asymmetry can change drastically from one terrace to another. The degree of dimer asymmetry has also been studied as a function of tip material using non-magnetic W and ferromagnetic Fe tips.

Progress towards spin-polarized scanning tunneling microscopy

Solutions to the main problems in operating a spin-polarized scanning tunneling microscope are discussed. Preliminary experimental results obtained in the course of implementing these solutions are reported. Atomic resolution on Si(111) and Si(100) is achieved with a scanning tunneling microscope (STM) using chromium and iron tips. Fabrication of antiferromagnetic tips of Cr, MnNi, and MnPt is described. A technique of preparation of clean (100) surfaces of Fe3O4 (magnetite) is given. Low-energy electron diffraction patterns were obtained on Fe3O4 for the first time. The first STM experimental results obtained on magnetite in air and in ultrahigh vacuum are reported. Atomic resolution is obtained on Fe3O4 (100) with an ultrahigh vacuum scanning tunneling microscope using iron and tungsten tips. This is the first successful observation of atomic resolution on a ferromagnetic sample using a ferromagnetic tip.

Observation of spin-polarized-electron tunneling from a ferromagnet into GaAs

Experimental evidence is presented for the tunneling of polarized electrons from the apex of a ferromagnetic Ni tip into GaAs(110). The polarization is found to be negative and of highest magnitude at very low injection energies, which shows that highly polarized minority 3d electrons are preferentially extracted from the Fermi level of the tip.

Magnetic imaging at the atomic level

Magnetic contrast at the atomic level has been observed for the first time in scanning tunneling microscopy experiments on a magnetite (Fe3O4(001)) surface using in-situ prepared ferromagnetic Fe tips. A periodic corrugation with a 12 A periodicity is clearly observed along the rows of FeB-sites which corresponds to the repeat period of Fe2+ and Fe3+ along these rows. This periodicity is not observed by using non-magneticW tips although the rows of FeB-sites can be resolved as well. The magnetic contrast observed with Fe tips is attributed to the different spin configurations of the magnetic ions Fe2+ and Fe3+ in Fe3O4.

Topographic and Magnetic-Sensitive Scanning Tunneling Microscope Study of Magnetite

The topographic and magnetic surface structure of a natural single crystal of magnetite (Fe(3)0(4)), a common mineral, has been studied from the submicrometer scale down to the atomic scale with a scanning tunneling microscope having nonmagnetic tungsten as well as ferromagnetic iron probe tips. Several different (001) crystal planes were imaged to atomic resolution with both kinds of tips. A selective imaging of the octahedrally coordinated Fe B-sites in the Fe-O planes, and even a selective imaging of the different magnetic ions Fe(2+) and Fe(3+), has been achieved, demonstrating for the first time that magnetic imaging can be realized at the atomic level.

Theoretical estimates of forces acting on a magnetic force microscope tip over a high temperature superconductor

We have calculated the force between the tip of a magnetic force microscope and a high temperature superconductor due to the diamagnetic response of the superconductor to the stray field of the tip. First we compare macroscopic and microscopic levitation experiments. Then a model is presented, simulating the screening effect of the superconductor by completely screening the flux. These results are compared to a new model based on London theory. Finally we have calculated the force acting on an idealized ferromagnetic tip in the presence of a c-axis oriented high temperature superconductor.

Vacuum tunneling of spin-polarized electrons detected by scanning tunneling microscopy

We show that the scanning tunneling microscope (STM), operated in ultrahigh vacuum, can be made sensitive to the electron spin and therefore to magnetic structures down to the atomic scale. The experiments were performed with a ferromagnetic CrO2 tip providing electrons of high spin polarization. As sample, we used a Cr(001) single crystal which offers an ideal magnetic test structure. The experimental results obtained with the ferromagnetic CrO2 tip characteristically differ from those obtained with a nonmagnetic tungsten tip. We give an expression, relating the local electron spin polarization of the tunneling junction with quantities directly measurable with the STM. We also discuss possible contributions from magnetic dipole and exchange forces as well as the feasibility of magnetic exchange force microscopy.

Scanning tunneling microscopy of the Abrikosov flux lattice with ferromagnetic probes

Using a low-temperature scanning tunneling microscope spatial variations of the current-voltage characteristics have been investigated on NbSe2 single crystals employing PtIr and ferromagnetic Ni tips. At 4.2 K a clear superconducting energy gap is visible even when tunneling through the Ni tip. After field-induced transfer of the sample into the superconducting mixed state a complete Abrikosov flux lattice is imaged for both types of probes by recording the tunneling current at a fixed voltage within the superconducting gap. Comparison of the images obtained by the two probe materials clearly shows that no distortion of the flux lattice is produced by magnetostatic interactions between the ferromagnetic tip and the individual vortices. This provides the basis for future investigations of flux distributions in superconductors by magnetic force microscopy.

On the possibility of resolving quantization axes of surface spins by means of a scanning tunneling microscope with a magnetic tip

It is shown that a scanning tunneling microscope with a magnetic tip gives us, in principle, the possibility to determine the spin directions of surface magnetic ions with atomic resolution. The tip should not necessarily be a ferromagnet. For instance, the tip can just as well be an antiferromagnet with a magnetic structure of the spin density wave type such as observed in chromium. The advantage of an anti ferromagnetic tip is that it does not influence the studied surface through the magnetostatic field. An analogous consideration based on exchange interaction is valid for an atomic force microscope with a magnetic tip.

Spin-polarized scanning tunneling microscope: Concept, design, and preliminary results from a prototype operated in air

We describe the concept of the spin-polarized scanning tunneling microscope. It consists of a ferromagnetic tip with saturated magnetization from which spin-polarized electrons tunnel into a ferromagnetic sample which has its axis of magnetization aligned with that of the tip. When the magnetization of the sample is alternated periodically from parallel to antiparallel, a portion of the tunnel current is predicted to oscillate at the same frequency, with an amplitude linearly proportional to the average tunnel current. The construction of a prototype microscope, operated in air, is described. When the sample magnetization is alternated, a modulation of the tunnel current is observed at the same frequency. This signal satisfies criteria developed to characterize spin-polarized tunneling. Spurious signals are also observed and their probable origins identified. A number of improvements are suggested that should eliminate the spurious effects.

Theory of magnetic force microscope images

A model is presented for magnetic force microscope (MFM) images obtained with a ferromagnetic tip and a ferromagnetic sample. Results of calculations are compared to recent MFM experiments and show good agreement using known values for magnetic parameters characterizing tip and sample. The model describes a limit in resolution determined by the demagnetizing effect of tip and sample, predicts the appearance of a novel image artifact which is particularly observable in images of narrow domains of reversed magnetization, and suggests an explanation for the observed contrast between regions of opposite magnetization.

Scanning tunneling microscopy with spin-polarized electrons

We present a short outline of the first STM experiments with spin-polarized electrons performed in ultrahigh vacuum by using ferromagnetic CrO2 tips and a Cr(001) single crystal surface. A clear distinction can be made between topographic STM line scans obtained with a non-magnetic tungsten tip and those obtained with a ferromagnetic CrO2 tip, which are modified due to an additional contribution from spin-dependent vacuum tunneling. STM therefore has the potential to measure the local electron spin polarization of the free surface as well as the spatial distribution of spins on the atomic scale.

The point dipole approximation in magnetic force microscopy

Image interpretation in magnetic force microscopy (MFM) requires detailed information about the internal microstructure of the ferromagnetic tip used for probing the surface microfield of a sample. Since these informations are generally not experimentally available, image interpretation is more speculative than rigorously quantitative at the present time. This theoretical analysis confirms by a simple criterion that MFM image interpretation can be performed in terms of point dipole probing provided that some experimental constraints are satisfied. The validity of the criterion is demonstrated for various experimentally relevant examples.

Investigation of Bloch wall fine structures by magnetic force microscopy

SUMMARYUsing a magnetic force microscope (MFM), measurements have been performed on single crystal iron whiskers. These samples exhibit a comparatively simple magnetic domain structure and a high degree of crystallographic perfection. STM measurements yield an average surface corrugation of about 3 nm. These nearly ideal boundary conditions permit an investigation of the undistorted surface configuration of Bloch walls in a cubic bulk crystal. Interactions between the stray field configuration of the sample and a ferromagnetic microscope tip have been measured with a new MFM device which is based on direct compliance detection. Measurements have been performed in the constant‐current mode, as well as in the constant‐compliance, mode. Control observation by means of the Kerr magneto‐optic effect permit an interpretation of the obtained MFM data in terms of isolated 180° Bloch walls. The fine structure of these walls has been analysed and compared with results obtained from model calculations.

Calculation of the Bloch wall contrast in magnetic force microscopy

SUMMARYThe magnetic force microscope (MFM) is a promising analytical tool for the mapping of magnetic microfield distributions on a nanometre scale. The detailed interpretation of experimentally obtained data requires a rigorous micromagnetic analysis of the underlying magnetostatic tip‐sample interactions. We have performed model calculations yielding the MFM image contrast obtained for 180° Bloch walls using ferromagnetic microscope tips. The actually observed wall image is shown to depend critically upon the mesoscopic tip design. The spatial resolution and also values for the resulting interaction forces and tip‐sample compliances are derived.

Spin-polarized electron microscopy

A current scientific challenge with many ramifications for magnetic technology is to image magnetic microstructure with the highest possible spatial resolution in order to observe magnetic domains or even spin configurations within a domain wall. Ultimately one can envision imaging the magnetic moment of individual atoms which would also make possible the observation of antiferromagnetic structures. The measurement of the spin polarization of secondary electrons generated by a finely focussed (unpolarized) scanning electron microscope (SEM) beam to obtain high-resolution magnetization images is presented. An alternative measurement, using a spin-polarized incident beam in an SEM, has many difficulties which are discussed. To measure spin configurations with higher spatial resolution, the possibility of introducing electron spin polarization in scanning field-emission and tunneling microscopy is considered. The measurement of the spin polarization of secondary electrons generated by a specially prepared single-atom scanning field-emission tip looks promising. High-resolution imaging of spin configurations in scanning tunneling microscopy appears possible if the tip itself is a source of spin-polarized electrons. The potential advantages and unsolved problems involved in using a ferromagnetic tip or an optically pumped semiconductor tip are described.

Magnetic imaging by atomic force microscope (invited) (abstract)

A new method of imaging the surface of magnetic samples on a submicron scale is described and the observation of magnetic domain walls is presented. Measurements in air on a Co-Ni recording media, polycrystalline Ni foil and rapidly quenched alloys are shown. This magnetic microscope is based on the idea of measuring magnetic forces with the recently developed atomic force microscope (AFM). Forces acting on a tip are recorded by the measurement of the deflection of a lever to which this tip is attached. In order to measure ultra small forces (less than nN), the spring constant of the lever has to be small and the deflection of the lever has to be measured with great sensitivity, i.e., by using interferometry or scanning tunneling microscopy (STM). The lever is made from ferromagnetic Ni foil with an integrated, electrochemically etched tip. Different measuring modes of the AFM are described. The images obtained by the AFM using a para- or ferromagnetic tip and by the STM are compared. Furthermore, the results obtained by the magnetic AFM on Co-Ni recording media are compared with previous studies by scanning electron microscopy with polarization analysis (SEMPA).

Investigation of Bloch wall fine structures by magnetic force microscopy

Using a magnetic force microscope (MFM), measurements have been performed on single crystal iron whiskers. These samples exhibit a comparatively simple magnetic domain structure and a high degree of crystallographic perfection. STM measurements yield an average surface corrugation of about 3 nm. These nearly ideal boundary conditions permit an investigation of the undistorted surface configuration of Bloch walls in a cubic bulk crystal. Interactions between the stray field configuration of the sample and a ferromagnetic microscope tip have been measured with a new MFM device which is based on direct compliance detection. Measurements have been performed in the constant-current mode, as well as in the constant-compliance, mode. Control observation by means of the Kerr magneto-optic effect permit an interpretation of the obtained MFM data in terms of isolated 180° Bloch walls. The fine structure of these walls has been analysed and compared with results obtained from model calculations.

Calculation of the Bloch wall contrast in magnetic force microscopy

The magnetic force microscope (MFM) is a promising analytical tool for the mapping of magnetic microfield distributions on a nanometre scale. The detailed interpretation of experimentally obtained data requires a rigorous micromagnetic analysis of the underlying magnetostatic tip-sample interactions. We have performed model calculations yielding the MFM image contrast obtained for 180° Bloch walls using ferromagnetic microscope tips. The actually observed wall image is shown to depend critically upon the mesoscopic tip design. The spatial resolution and also values for the resulting interaction forces and tip-sample compliances are derived.