Radiant energy – Radiation tracer methods
Reexamination Certificate
2000-03-22
2002-11-05
Berman, Jack (Department: 2881)
Radiant energy
Radiation tracer methods
Reexamination Certificate
active
06476386
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention concerns a method for tunnel microscopy, especially a tunnel microscopy method for recording the magnetic, spin or susceptibility structure of a sample, and a tunnel micro scope (a so-called “spin-polarized tunnel microscope”) for implementation of the method as well as methods for use of such a tunnel microscope.
PRIOR ART
A traditional tunnel microscope, for instance as described by G. Binnig et al. in “Phys. Rev. Lett.”, Vol. 49, 1992, p. 57 ff., is shown in the diagram in FIG.
6
. The tunnel microscope
10
′ is designed for non-contact scanning of a sample
50
′ on the basis of the tunnel effect. The tunnel microscope
10
′ specifically includes a tunnel tip
20
′ with a piezo-electrical drive
21
′, a control circuit
30
′ and a display and evaluation system
40
′. The tunnel tip
20
′ typically consisting of tungsten is moved across the surface of the sample
50
′ using the piezo-electrical drive
21
′ for scanning in the x and y directions. During scanning, the tunnel current between the tunnel tip
20
′ and the surface is continuously measured and using the control loop
31
′ of the control circuit
30
′, the z coordinate of the tunnel tip is set so that the tunnel current remains constant during scanning. The two-dimensional dependency of the z coordinate from the x and y coordinates represents the topography of the sample surface, which may be displayed using the display and evaluation system
40
′ and be subject to further image processing. Using a tunnel tip
20
′ sharpened in atomic dimensions, for this mapping of the topography, local resolutions below the nm range may be achieved.
Further investigation methods are known which use scanning microscopes, for which for local resolution not the tunnel current, but for instance optical characteristics or electrical field effects are measured at the sample surface. There is specifically also interest in recording the spin structure of a sample, i.e. the magnetic sample characteristics using a local resolution typical to tunnel microscope examination methods. In this respect, in particular, the following three magnetic scanning microscopy techniques are known.
The tunnel microscope known from U.S. Pat. No. 5,266,897 is based on magnetic force microscopy (MFM). The tunnel microscope is operated using a vibrating cantilever tip, which is subject to effects of magnetic forces with reference to the sample surface. The magnetic force between the sample and the cantilever tip is effected depending on the magnetic sample characteristics, so that the distance of the tip to the sample and, therefore, the tunnel current through the cantilever tip changes. This technology has the following disadvantages.
For magnetic force microscopy, the magnetic recording of the structure is based on the local resolution of measurement of force exerted on a magnetic tip by the magnetic stray field of a sample. But the stray field of the sample is not a local surface characteristic. To the contrary, it is created in the sample volume. The local resolution is limited by the so-called “magnetic volume” of the tip. The best local resolution for the MFM method is in the range of 20 to 40 nm. The MFM method is furthermore disadvantageous because contrast formation is effected by magnetization of a layer reaching into the sample volume, and therefore possibly the image of the sample surface is adulterated. Also, MFM is usually operated as non-contacting. Due to the long-range magnetic interaction, the local resolution even for a working distance between tip and sample of some nanometers is limited. Only in case the of provision of specific precautions, for instance as described by P. Grutter et al. in “J. Appl. Phys.”, Vol. 67, 1990, p. 1437 ff., the may local resolution be improved to up to 10 nm.
In the case of magnetooptic near field microscopy (SNOM), the sample surface is also scanned using a detector tip. The detector tip essentially includes a sharp glass fiber which locally resolved measures the magnetooptic Kerr effect at a resolution of below the wavelength of the light used. The local resolution of the magnetooptic SNOM method is defined laterally by focussing of the light field and vertically by the penetration depth into the sample. Up to now, only a local resolution of up to 60 nm has been achieved (refer to C. Durkan et al. in “Appl. Phys. Lett.”, Vol. 70, 1997, p. 1323 ff.).
Scanning electronic microscopy with polarization analysis (SEM-PA) is based on recording of the spin polarization of secondary electrons shot out of the top atomic layers of a sample using a scanning electronic microscope. The disadvantage of this method is again the limited local resolution, which is laterally limited by the focus of the primary electron beam and achieves about up to 20 nm (refer to H. Matsuyama et al. in “J. of Electron Microscopy”, Vol. 43, 1994, p. 157 ff.).
From U.S. Pat. No. 4,939,363 (corresponding to EP 0 348 239 A1), a tunnel microscope for investigation of the magnetic characteristics of a sample surface is known, which is partially shown in schematic FIG.
7
. The tunnel microscope
10
″ specifically comprises the tunnel tip
20
″ with piezo-electrical drive
21
″ as well as (not shown) the control circuit and the display and evaluation system. Below the sample
50
″, a permanent magnet
60
″ is arranged, which cooperates with magnetic coils
61
″ with reversable polarity as follows. Using the permanent magnet
60
″ and the magnetic coils
61
″, the tunnel tip
20
″ is magnetized in such a manner that a magnetic field forms in axial the direction from the tunnel tip to the sample. On the basis of the so-called “magnetic tunnel effect” (refer to M. Julliere in “Phys. Lett. A”, Vol. 54, 1975, p. 225 ff.), the tunnel current depends on the orientation of the spins of the electrons in the sample relative to the spins of the tip. The magneto tunnel effect is based on the dependency of tunnel probability from the energetically split state densities of the electrons in the sample. Locally resolved spin measurement is achieved by performing two tunnel current measurements. Within each case different tip magnetization is performed at each measuring point of the tunnel tip
20
″. The spin state at the sample location may be derived from the tunnel current difference for the two orientations of the magnetic field. Even though, using this technology, in comparison with the above stated technologies, theoretically a substantially better local resolution of up to a few Ångström may be achieved, the spin-polarized tunnel microscope according to U.S. 4,939,363 is disadvantageous for the following reasons.
For a conventional tunnel microscope, in contrary to the spin-independent microscopy (according to
FIG. 6
, see above), the z position of the tunnel tip is not controlled so that the tunnel current is constant (“constant current mode”). Instead, the tunnel current is in each case measured for the different magnetizations for constant z coordinate. Measurement with variable tunnel current is disadvantageous due to the deviation of the measuring principle from the traditional spin-independent microscopes. Also, signal evaluation with great effort must be performed to separately derive the topographical properties and the magnetic properties from the measured current values. This evaluation is performed subsequently using numerical means so that additional time is required for recording the surface image. An important disadvantage furthermore consists in the fact that for the magnetic tunnel effect the dependency of the tunnel current on the magnetization direction is substantially lower than the dependency of the tunnel current from the distance tip-sample. Because the dependency of magnetization only amounts to about 20% of the dependency on topography, when recording a tunnel image of the surface of a ferromagnet using a magnetic tip, the magnetic dom
Kirschner Jürgen
Wulfhekel Wulf
Berman Jack
Max-Planck- Gesselschaft zur Forderung der Wissenschaften e.V.
Schnader Harrison Segal & Lewis LLP
Smith Johnnie L
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