IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

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Characterization of Fe/W Spin-Polarized Tips
by Means of Holographic TEM andSpin-Polarized STS of OpticallyPumped p-GaAs
T. Matsuda1 , A. Tonomura2 , T. K. Yamada3 , D. Okuyama3 , N. Mizuno3 , A. L. Vazquez V. de Parga4 ,H. van Kempen5 , and T. Mizoguchi3Frontier Research System, The Institute of Physical and Chemical Research, Hatoyama, Saitama 350-0395,
Japan, and Naka Customer Center, Hitachi Science Systems, Ltd., Ishikawa, Hitachinaka, Ibaraki 312-0057, Japan
Advanced Research Laboratory, Hitachi Ltd., Hatoyama, Saitama 350-0395, Japan
Faculty of Science, Gakushuin University, 1-5-1 Mejiro, 171-8588 Tokyo, Japan
Departimento Fisica de la Materia Condensada, Universidad Autonoma de Madrid, Madrid E-28049, Spain
Institute for Molecules and Materials, Radboud University, 6525 ED Nijmegen, The Netherlands
Holographic transmission electron microscopy showed that magnetic flux does not diverge from the top of an Fe-coated W tip indicating the in-plane magnetization at the apex of successful spin-polarized tips. A dependence of differential conductivities of GaAs withFe-coated W tips on the helicity of illuminating laser light was confirmed at negative sample bias where partially spin-polarized electrons
in the conduction band of p-GaAs tunnel into spin-dependent Fe empty states while no dependence of those with nonmagnetic W tips.
Index Terms—Magnetic field measurement, optical pumping, spectroscopy, tunneling.

I. INTRODUCTIONAMONG remarkable progress in wide application of scanning probe microscopy, spin polarized scanning tunnelingmicroscopy and spectroscopy (SP-STM/STS) is now of greatinterest as it gives us the local spin density of states with atomic
scale resolution [1]–[3].
There is a limited number of groups that have established theSP-STM/STS technique with which they succeed to get magnetic images of samples, but the importance of the characterization of spin-polarized tips is not yet recognized satisfactorily.
The asymmetry of magnetic contrast in SP-STM/STS is considered to be a product of the polarization of the sample surface, where is the angle
and that of the apex of the tip andbetween the quantization axes of the polarization of a sample
and the apex of a tip. We have succeeded in obtaining images ofmagnetic contrast on antiferromagnetic Mn layer stackings onFe(001) [2] with Fe-coated W tips which are widely used to getmagnetic contrast on samples with in-plane magnetizations.
In order to investigate nanoscale local magnetization at theapex of the Fe/W tip, we observed directly the magnetic flux
from the tip by means of electron-holographic interference microscopy.
On the other hand, after reports of spin-dependent tunnelingof optically excited electrons for junctions [4], [5], extensive efforts [6]–[9] have been dedicated to develop a spin-polarized
STM with a GaAs microprobe tip. The GaAs tips made by

Digital Object Identifier 10.1109/TMAG.2005.854672

cleavage in air seem to have a serious problem since even nonmagnetic Au test samples showed helicity-dependent opticalscattering in the STM-junction [9]. However, use of optically
pumped spin-polarized electrons from GaAs is still attractive
since its spin polarization is theoretically known. Thus, the spinpolarization of Fe-coated W tips is investigated in this study
using polarized electrons from optically pumped p-GaAs(110)as the standard sample.
II. EXPERIMENTAL DETAILSElectron holographic interference microscopy measurements
were performed by transmission electron microscopy (TEM)with a 300-keV field emission electron source and a TEM witha 1-MeV cold emission electron source at a pressure of about
Pa. For holographic TEM measurements blunt W tips
10coated with 10-nm Fe film were prepared in ultrahigh vacuum(UHV) in the same way as we prepared our SP-STS experiments [1], [2]. They were covered by 2-nm Au film to preventoxidization. Phase amplified ( 16) interferogram images show
magnetic force lines as black–white contrasts.
A clean p-GaAs(110) surface was obtained by cleavage of ap-GaAs(001)-wafer in UHV (2 10 Pa) [10]. SP-STS experiments on this p-GaAs(110), illuminated by circularly polarizednm) incident with an angle
laser light ( 0.2 mW/mmof 75 relative to the surface normal with Fe-coated (4–10 nm)
W tips and nonmagnetic W tip were performed at room temperature in UHV in a current imaging tunneling spectroscopy
curve obtained by averaging over about 1000
mode. Anpixels in a clean terrace of p-GaAs(110) was numerically difcurve.
ferentiated to get a differential conductivity

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

2Fig. 2. (a) shows an STM image (54 24 nm ; V = +2:0 V, I = 0:06
nA) obtained on p-GaAs(110). (b) shows an I (V ) map at 1 V obtained on thesame area as (b).

20Fig. 1. Electron-holographic phase-amplified ( 16) interference microscopy
results of an Fe-coated W tip. Black–white contrast corresponds to magneticforce lines. (a) was obtained by a TEM with a 300-keV field emission electronsource. (b) was obtained by a TEM with a 1-MeV cold emission electron source.
Single fringe corresponds to a magnetic flux of h=(16e), where h denotes thePlank’s constant and e the electron charge.

III. HOLOGRAM TEM RESULTS
We might imagine that flux lines diverge from an end of asharp ferromagnetic needle, but we should keep in mind that theSP-STM tip is not a bulk ferromagnet, but a very thin layer offerromagnetic film, i.e., 10-nm Fe film on a nonmagnetic blunt
tip of tungsten. This peculiar shape of ferromagnetic thin film,i.e., a thin pipe of radius about 130 nm terminated by a hemispherical cap, makes it possible to realize in-plane magnetization at the top of the apex. Holographic images of an Fe-coated
W tip are shown in Fig. 1. Magnetic flux lines diverge from one
side of the tip and go into the other side as seen in the overall
view [Fig. 1(a)]. It is clear that the flux does not diverge at thetop of apex of the tip as seen in the enlarged view [Fig. 1(b)],where the magnetization at the top of the tip is in-plane, i.e.,
perpendicular to the axis ( direction) of the tip. We measured
four different Fe-coated W tips and obtained similar results.
If we take the direction of local magnetization at the top of
apex of the tip as the axis, the magnetization of one sideof thin Fe film on tungsten tip goes upward to and goesto thedirection. Thus,
downward on the other side
there must be magnetic domain walls along direction between. Since Fe film on the tungsten tip is thin enough,them atthe domain walls must be Néel type with cross tie structure, fromwhich the magnetic flux lines diverge or converge. The width ofa Néel wall is expressed in a limiting case of thin films as [11]for film thickness(1)

Inserting rough values close to those for -Fe,J/m and
J/m , we get
nm,which is about a one-tenth part of a circle of the Fe thin filmon the top part of the tungsten tip. It would be interesting toinvestigate the micromagnetic structure of this peculiar shapeof ferromagnetic thin sheath as a function of the curvature ofthe apex and thickness of the film.
Our approach is rather practical. We made successful Fe/Wtips in SP-STM and investigate the micromagnetic structure bymeans of holographic TEM. This result explains why we cannot
get good SP-STM results with sharp tips as used in conventional
STM because the magnetization is undefined at an apex of a tip.

Fig. 3. (a) dI =dV curves obtained on GaAs(110) with a W tip (gray) and anFe-coated W tip (black) without illumination of light. (b) Scheme of tunnelingwith illumination of circular polarized light at a negative sample bias voltage.
Two channels of tunneling electrons are considered: electrons from the valenceband (gray arrow) in addition to electrons from the conduction band (blackarrow).

IV. SPIN-POLARIZED STM/STS TIPS STUDIED BY
SPIN-POLARIZED ELECTRONS FROM OPTICALLY PUMPEDp-GaAs(110)
A topographic image of p-GaAs(110) cleaved in UHV[Fig. 2(a)] shows flat terraces of atomic planes. A spectroscopic
image obtained at the same area [Fig. 2(b)] shows, however,
several sites, not only steps, which look different from a pureatomic array of GaAs(110). We carefully select a pure clean
area of an atomic plane for further spectroscopic study.
We are interested in electron tunneling from a p-GaAs sample
to the W or Fe/W tips at negative sample bias voltage for characterizations of the tips. Without light illumination, electrons
tunnel from valence states of p-GaAs to empty states of a tip atcurves obtained with W
negative sample bias. Averagedor Fe/W tips from the clean GaAs surface without illumination
belowof light are shown in Fig. 3(a). A rapid increase ofthe Fermi level and a bandgap of about 1.5 V above the Fermi
energy are observed. The difference between W and Fe/W tips
is likely caused by different work functions of W and Fe. Simcurves were observed on Mn(001) withilar difference inW and Fe/W tips [1]. Fig. 3(b) illustrates a scheme of tunnelingfrom p-GaAs to Fe/W tip.
curves over the clean part of GaAs(110)
The averaged
with illumination of right and left circular-polarized light obtained with a nonmagnetic W tip and a magnetic Fe-coated W
tip are shown in Fig. 4(a) and (b), respectively. It is clear that
there exists a dependence of the differential conductivities ofGaAs with an Fe-coated W tip on the helicity of illuminating
laser light at negative sample bias where partially spin-polarized
electrons in the conduction band of p-GaAs tunnel into spin-dependent Fe empty states, while no difference exists in those witha nonmagnetic W tip. This shows that a new channel of tunneling of optically pumped electrons in the conduction band, inMATSUDA et al.: CHARACTERIZATION OF FE/W SPIN-POLARIZED TIPS BY MEANS OF HOLOGRAPHIC TEM

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a ferromagnetic tip at the Fermi level is necessary. It is desirable
that the polarization of a spin polarized tip be calibrated in each
SP-STS experiment with this method.
V. CONCLUSION

Fig. 4. (a) and (b) show dI =dV curves obtained with a nonmagnetic W tip
and an Fe-coated W tip, respectively, on GaAs surface with illumination of right(black) and left (gray) circular-polarized light at the same set point of VV, InA.
:= 0 15= +2addition to the tunneling from valence band, is opened in thisSTS. Since the conduction band is about 1.5 V higher than thevalence band, a transition probability from the former is about
one order of magnitude larger than that from the latter.
The asymmetry is defined as, wheredenotes
the normalized differential conductivity by the transition probability, , with illumination of right (left) circular polarized light[2]. This is considered to be equal to, whereandare the spin polarizations of tunneling electrons of sample
and tip, respectively, and is the angle between the polarization
axes of the apex of the tip and the sample [2], [3]. Circular-polarized laser light incident with an angle of 75 to the surface
normal is refracted to 15 to the surface normal inside the GaAs
[6]. Thus, the unit vector in the direction
crystalof polarization axes of conduction electrons in p-GaAs(110), where denotes
can be written asa unit vector of the surface normal, and and denote unit
vectors perpendicular to the surface normal. The unit vector
in the direction of spin at the apex of the tip can be written as. Thus, we get, andfrom the ellipticity of pumping light in GaAs [6].
Due to the uncertainty of angle of spin direction of a tip
obtained with different Fe-coated Wapex, asymmetries
tips differ each other. If we assume a maximum observed value
corresponds to a case for, we know . The tunof
neling transition probability must be independent of the helicityof pumping laser light, but, so far, we have experimental difficulty adjusting the distance between a tip and a sample, which
depends on light intensity to get equal transition probability forboth helicities.
It should be noted also that in usual SP-STS studies for magnetic samples with Fe-coated W tips, electrons tunnel from theFermi level of the tip to empty states of a sample in a positive
sample bias. In this case, information about spin-polarization of

Holographic TEM indicates the in-plane magnetization at theapex of successful spin-polarized Fe/W tips. The differential
conductivities obtained with Fe-coated W tips on a GaAs surface depend on the helicity of the illuminating laser light at anegative sample bias where partially spin-polarized electrons
tunnel into spin-dependent Fe empty states, while there is nodependence of those with nonmagnetic W tips.
ACKNOWLEDGMENT
This work was supported by the High Technology ResearchCenter, Ministry of Education Science, Sports, and Culture. The
work of T. K. Yamada was supported by a Grant-in-Aid for JSPS
Fellows.The authors would like to thank Dr. K. Harada and Dr.
Y. Togawa of Frontier Research System, the Institute of Physicaland Chemical Research, for making the magnetic force images
from hologram images.
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Manuscript received February 7, 2005.