of spin-polarized STM approach for measuring of spin distribution
on a magnetic surface
(this review reflects the situation in the field of spin polarized STM, which occured in 1998. For the most recent information, please refer to the R. Wiesendanger group web page)
The main aim is to obtain the map of spin distribution on the surface and to characterize the spin properties of selected nanoscale features of surface.
Now it is possible to formulate three ideas how (in principle) to measure the spin orientation of atoms on surface.
|The realizations (papers) and obtained limitations|
Magnetic tip/magnetic surfaces.
In this paper they obtain the difference in atomic steps height on antiferromagnetic ordered planes (100) of Cr(100) measured by ferromagnetic CrO2 -tip and nonmagnetic (W) one: 1.2 and 1.6 A by CrO2- tip, and 1.4 A by W-tip. ( From my conversation with Dr. I. Shvets from Dublin Trinity Colledge in 1996: "Nobody can reproduce this result")
They observed the difference in STM images of Fe3O4 obtained with Fe- and W-tips. For W-tip only atomic rows were in images, no corrugation along the rows; for Fe-tip a strong (comparable with inter-row corrugation) atomic corrugation was obtained along the row. They interpreted this phenomenon as interaction of spin moments of Fe2+ and Fe3+ ions in Fe3O4 and Fe-tip.
My request to Dr. I. Shvets on 19 March 1998 to comment the situation in this field of research was answered that they not published anything in this field since 1995 and he is not aware of any recent breakthroughs by other groups either. Limitation Since 1990 there have been published only two papers in this direction. The main limitation of this approach is the need to compare the STM images recorded both by magnetic and nonmagnetic tips. In this sense the mentioned papers has a demonstrative character only. It seams as not convenient approach for real use in SP-STM measurements. Perhaps it could be applied for surface objects with spatially variable magnetic moments like domain walls or something similar without comparison magnetic -nonmagnetic tip images.
III-V semiconductor tips (GaAs, InAs, etc.) excited by circularly polarized light/magnetic surfaces
Semiconductor tips (GaAs, InAs, etc.) excited by circularly polarized light/magnetic surfaces (Direct scheme A).
Atomic resolution STM images recorded with cleaved GaAs tips were demonstrated for Si(111)-(7x7) surface in ultrahigh vacuum. The tips were cleaved from single-crystal (100) oriented 450 mm wafers along (110) direction in air. The tip size was 2mm. The tip apex radius was smaller than 100 nm (SEM resolution limitation) . In vacuum tips were prepared by heating at 630 oC. The sample was biased at +2 V, tunnel current was 1 nA.
They used GaAs tip and Pt/Co multi-layer film deposited on a glass. The tip was made by cleavage (100) wafers in (110 and 1-10) directions. The light excitation was directed from the tip side with angle of 45a°. 1.96 eV He-Ne laser. The polarization of photocurrent from the tip was estimated as 10%. The amplitude of modulated current was of 4 pA in comparison of total tunnel current 1 nA or 0.4% polarized.
The analysis of possible phenomena in tunnel junction and in semiconductor tip itself under polarized light illumination has been done. The most convenient regime for tunnel current polarization could be obtained at the following conditions:
Part of these conclusions has been already confirmed in experimental works of Prins et al.(1995, 96)
They performed experiments with GaAs tips under illumination by circularly polarized light (5 mW He-Ne laser , 633 nm/1.96 eV, 10% polarized) in UHV at room temperature. Ferromagnetic ultrathin Co layers with perpendicular magnetization were employed as a sample. The light gone from the side of transparency sample. The measured modulated current shown (-2 pA) and +2pA for the locations with different direction of magnetization at tunnel parameters (-1 V, 1 nA). The current polarization was 0.2% for selected spin orientation. The size of magnetization location is 200-500 nm typically. The MFM measurements of the same sample showed the similar result.
III-V semiconductor sample (GaAs, InAs, etc.) excited by circularly polarized light/magnetic tip (Direct scheme B).
GaAs thin film sample pumped by circularly polarized light (from back side of the sample) and a ferromagnetic polycrystalline Ni tip were used. The tunnel current was perturbed by modulating the power and polarization of the pumping light. The perturbation arise due to three dominant effects: the thermal expansion of tip - surface junction, the variation in excited carriers concentration in GaAs and spin-polarized tunneling effect. The spin polarized effect was distinguished by observing the current dependence on the bias. The experiment was performed in air. Wavelength was 830 nm or 1.49 eV(single-mode LD, 30 mW), the tip was magnetized in H=1T ex situ. Tip bias was -1.2 V, It=1 nA. Current polarization was 10% in average. The observed tunneling current modulation depends upon both helicity of circular polarization and tip magnetization. The estimated spin polarization of the tip was negative.(?)
Measure of circular polarization of recombination radiation from the semiconductor tip appeared as a result of tunneling of spin-oriented electrons from magnetic surface. (Reverse scheme A)
Measure of circular polarization of recombination radiation from the semiconductor sample appeared as a result of tunneling of spin-oriented electrons from magnetic tip. (Reverse scheme B)
They used ferromagnetic Ni-tip for GaAs(110) surface tunneling study in UHV. Magnetization of tip was made in situ by means of small electromagnet . As a measuring value they analyzed the circular polarization of the recombination luminescence excited by electrons tunneling from a ferromagnetic Ni tip. The degree of negative spin polarization of electron extracted from tip was large, Pm= -31%. The degree of polarization decrease with increase of kinetic energy of injected electrons.
He studied the dependence of degree of circularly polarized light emitted from Al0.06Ga0.94As sample as a result of tunneling of electron from Ni tip versus the tunnel barrier height. There was a competition in tunneling from 3d (polarized) and 4sp (unpolarized) states. For narrower and lower barrier the Pm increase and reach 50 %. It means that coupling between 3D-like states and the semiconductor increases faster than for the 4sp-like states. Important point: maybe the UHV conditions permit to keep low density of surface states in semiconductor with large value of constant of spin relaxation.
The system Fe tip/GaAS surface is studied. The 43% of circularly polarized recombinative radiation from tip/surface tunnel junction was obtained. The 95% electron magnetization in Fe was proposed. Limitations In principle there is no limitation.
Comparison of direct and reverse schemes
A very small amount of polarized tunnel current is observed in direct scheme A for GaAs tip/magnetic sample combination : 0.2-0.4% (Prins et al., Suzuki et al.). The original degree of spin polarization of tip electrons was estimated as 10% in both cases. For the Co/Pt sample (Prins et al) the magnetization of sample was not equal to 100%.
In direct scheme B a larger degree of tunnel current polarization was observed, 6-10 % (Sueoki et al). Taking into account that original degree of polarization of semiconductor sample carriers is about 50%, we can compare the results of schemes A and B. A : 0.4%x5=2% is less than in B (6%) but of the same order of magnitude.
Opposite in reverse scheme B (Alvarado et al) the degree of circular polarization of recombination light can reach 50%. It means that spin relaxation time in semiconductor surface was rather large so the spin-oriented electrons injected from magnetic tip were able to recombine with the holes before the loss of spin orientation. For 95% magnetization in Fe they obtained 43% in STM measurement with Fe tip.
They obtain the optimal conditions upon STM measurements for very narrow tunnel barrier. For Ut=1.8 V and change of It from 20pA to 500 pA the polarization in Ni tip/AlGaAs system was changed from 25% to 50 %. The change of tunnel barrier thickness was estimated as 1.8 A.
Only papers of Alvarado et al and Suzuki et al were made in UHV conditions. Alvarado et al pay strong attention for study the processes of radiative recombination in GaAs samples (they made special publications in this field) and prepared the MBE grown sample with optimized parameters. Also clean atomically flat sample was produced by cleaving in situ in their works.
In Suzuki paper the authors used the oxide GaAs tips prepared by cleaving in air for UHV STM. No information about semiconductor surface states, spin relaxation time, etc. was presented in the paper. We think that spin relaxation time in oxidized GaAs tip was short enough and degree of spin polarization of semiconductor electrons was very low. The similar problems probably were in the papers of Prins et al and Sueoka et al.
|Conclusion and recommendations|
The main conclusion from data presented is that this field of STM activity is very difficult in experimental realization. The experimental papers presented here are only the first steps. Not all possible combinations of tip/sample/light system are yet realized. The key problem lies in the spin relaxation processes in semiconductor surface.
Special note about the teams involved in this activity in present time.
van Kempen team from Holland is broken because M. Prins is working in another field (Philips Lab) and R. Jansen continues this activity in MTI, USA.
Two teams from Japan have been involved in this field recently. It is Suzuki et al. (APL 1997) and Musaka et al.
So, only three teams (USA and Japan, no from Europe) continue activity in this field.
|STS on a magnetic surface|
The basic idea here is to study STS dependence for selected nanosize objects (up to atoms) in a magnetic surface. With the data from other methods measurements and from theory to try to understand the electronic structure (including its spin orientation) of magnetic surfaces. The first papers in this field have been published recently.
Using the STS information obtained for Fe(001) and Cr(001) samples separately (PRL 1995) the authors could distinguish the Cr and Fe atoms in a alloy. There were no atomic resolution in STM images but bright places in STM images were identified by STS as Cr atoms.
In Fe-Si alloy they could identified the boundaries between Fe-Si domains enriched by Fe atoms by means of analysis of STS data and comparing it with STS on FE(001) previously obtained. Moreover they determined the one-dimensional character of this Fe state.
For Fe film of different thickness (1, 2, or 3 ML) the different STS curves have been observed. They explained the STS results by strain-induced change of electronic structure of heteroepitaxially grown film.
By STM a set of different superstructures was observed in Gd film. The own STS curves were observed for each superstructure. They observed peaks in STS both for empty and occupied states.
This direction seems to be interesting for the materials with magnetism like rear earth elements and transition metals. In combination with XPS, UPS and theoretical predictions it could be very useful. The most interesting application for the systems with low dimensions: rear earth/transition metals quantum dots and wires.
|Modern situation in SP-STM field|
special features of STM setups and research activity.
The possible experiments.
One of our STM (Room temperature STM) has a very open scanner construction which permits to put the light excitation in tunnel junction in parallel with tip axis. It can be used for experiments with polarized light excitation of tunnel junction semiconductor tip/magnetic surface.
Another STM (Variable temperature STM, 120-800 K) is combined with optical spectrometer (400-800 nm). It permits to realize the experiments with measure the polarized recombinative radiation from tunnel junction semiconductor tip/magnetic surface.
Taking into account that the spin relaxation in III-V semiconductor tips seems to be central problem in SP-STM the experience of Moscow team in chemical surface reaction is useful for tip preparation in situ. Optical education of its members (as a rule) and experience in laser, nonlinier optics, light polarization are also important.
|The tentative research program and tasks.|
Semiconductor (GaAs, InAs, etc) tips testing and characterization.
Spin polarized measurements with semiconductor tip
Reverse scheme A.
All measurements are carry out in VT STM/optical spectrometer setup.The experiment conditions:
Direct scheme A.
All measurements are carry out in RT STM setup. The experiment conditions:
Magnetic tips fabrication and testing.
Magnetic tip measurements:
Nonmagnetic tip STS measurements:
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