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Imaging Magnetic Domains On Ferromagnetic Thin Films On III-V Compounds by Tunneling Luminescence Microscopy

IP.com Disclosure Number: IPCOM000120424D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 189K

Publishing Venue

IBM

Related People

Alvarado, SF: AUTHOR

Abstract

High levels of luminescence radiation can occur in highly doped GaAs (p = 1018 to 1019 cm-3) with up to 104 c.p.s. per nA of incident tunneling current at tunneling voltages of VT ~ 2 V. Based on this effect, a technique is proposed for studying magnetic properties of thin films 1 deposited on semiconductors 2 or other suitable materials. One interesting possibility is the study of epitaxial ferromagnetic materials, e.g., of epitaxially grown Fe on GaAs (001) surfaces. To analyze the magnetic structure of films 1 with nanometer resolution, the tip 3 of a Scanning Tunneling Microscope (STM) is used as a source of low-energy electrons (Fig. 1). These electrons create a cascade of spin-polarized secondary electrons, with kinetic energies below ca. 5 eV, within the material 1.

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Imaging Magnetic Domains On Ferromagnetic Thin Films On III-V Compounds
by Tunneling Luminescence Microscopy

      High levels of luminescence radiation can occur in highly
doped GaAs (p = 1018 to 1019 cm-3) with up to 104 c.p.s. per nA
of incident tunneling current at tunneling voltages of VT ~ 2 V.
Based on this effect, a technique is proposed for studying magnetic
properties of thin films 1 deposited on semiconductors 2 or other
suitable materials.  One interesting possibility is the study of
epitaxial ferromagnetic materials, e.g., of epitaxially grown Fe on
GaAs (001) surfaces. To analyze the magnetic structure of films 1
with nanometer resolution, the tip 3 of a Scanning Tunneling
Microscope (STM) is used as a source of low-energy electrons (Fig.
1).  These electrons create a cascade of spin-polarized secondary
electrons, with kinetic energies below ca. 5 eV, within the material
1.  For example, for the alloy Fe82 Bi12 Si6 the polarization of the
secondary electrons is P ~ 0.40.  Secondary electrons moving in the
forward direction can penetrate into the III-V compound, thermalize
into the conduction band and recombine directly into the valence band
(Fig. 2).  The luminescence arising from the re-combination of
spin-polarized electrons will then exhibit a circular polarization
which depends on the magnetization of that region of the ferromagnet
where the electrons penetrate the surface.  A necessary condition is
that a non- zero projection of the magnetization along the optical
axis is given.  Other physical effects, such as the rotation of the
electron spin in the band-bending region at the interface, could be
used to manipulate the electron spin orientation.  The circularly
polarized photons arising from the recombining electrons are detected
by a photomultiplier 4 equipped with a collector lens 5 and a filter
and polarizer 6.

      Obvious advantages of this approach for imaging magnetic
structures are: (1) ruggedness, because the ferromagnet, e.g. a
metal, is much less sensitive to damage associated with the tunneling
process, e.g., heating. Therefore, by at least two orders of
magnitude higher current densities and, thus, higher luminescence
intensities can be realized.  This allows faster scans to image
magnetic domains in a reasonably short time.  (2) Note that the
incident electrons do not have to be polarized. The source of
polarized electrons is the thin magnetic film being analyzed.

      Consider a beam of electrons incident perpendicularly upon a
thin ferromagnetic film with a primary energy Ep . The attenuation of
the incident beam at a point z below the surface is:

                            (Image Omitted)

 (1)
where ip0 is the electron current incident from the tip of an STM in
field emission mode, and gp is the mean free path. For Ep ~ 102 eV
in a metal one has typically gp ~ 0.5 nm. Inelastic scattering
produces a cascade of secondary electrons of low energy Es, thu...