Using ring-shaped and magnetically coated tungsten wire as the probe of spin-polarized scanning tunneling microscopy
Chii-Bin Wu, Pin-Jui Hsu, and Hong-Yu Yen
Department of Physics, National Taiwan University, Taipei 10617, Taiwan Minn-Tsong Lina兲
Department of Physics, National Taiwan University, Taipei 10617, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
共Received 20 July 2007; accepted 26 October 2006; published online 15 November 2007兲 We report a method of magnetic probe fabrication using ring-shaped and iron-coated tungsten wire for spin-polarized scanning tunneling microscopy. Magneto-optic Kerr effect measurement on the probe front end shows that by controlling the saturating field direction, we can fix the probe magnetization in the specific in-plane direction. The ring is applied to the scanning tunneling microscopy and spectroscopy experiment on 6.8 ML Mn/ Fe共001兲, and spin contrast in the in-plane direction is demonstrated. © 2007 American Institute of Physics. 关DOI:10.1063/1.2813614兴
Spin-polarized scanning tunneling microscopy 共SP- STM兲 has been proved to be a powerful tool for magnetic domain imaging with spatial resolution down to the nanom- eter scale.1–3According to the theory of spin-dependent tun- neling, the image contrast is proportional to the projected probe magnetization along the sample magnetization direction.4Therefore, to control the probe magnetization di- rection is one crucial issue in attaining domain contrast. By coating the probe surface with different magnetic materials, SP-STM with out-of-plane or in-plane sensitivity can be readily prepared.5However, to identify which in-plane direc- tion the probe magnetization points to is still difficult so far.
This is because traditionally a sharp probe is preferred for STM to obtain high spatial resolution so that much effort has been taken to sharpen the probe, either by electrochemical etching, ion milling, or using single-atom tip.6–10 All these methods result in needle-shaped probes with axial symmetry, which makes it difficult to specify the in-plane direction of the probe magnetization. To solve this problem, a method using magnetically soft material as STM probe in hollow disk shape has been proposed to bind the magnetization of the coated iron film in the disk-plane direction.11 Two coils are wound around the probe to periodically switch the mag- netization, and the lock-in amplifier is used to extract the dI / dM signal, as compared to another often used spectros- copy method, which extracts the dI / dV signal.5Although the probe prepared in such way can bind the in-plane tip mag- netization, this advantage was not reportedly applied to spin- polarized scanning tunneling spectroscopy共SP-STS兲 experi- ment. Usually a flashed W tip, coated with Fe, is taken to do SP-STS experiment. Although flashing is not the only way to clean the tip surface, it is reported to be an efficient and crucial step to realize stable SP-STS experiment.5However, the probe of soft magnetic material cannot be flashed be- cause of two concerns. Firstly, the alloy concentration of the soft material might change at high temperature, and sec- ondly, the insulating coating of the coils would be destroyed at high temperature.
In light of this, we propose a method of SP-STM probe preparation by bending fine tungsten wire into a ring shape and coating it with 20 ML iron thin film. For iron film as thick as 20 ML on tungsten surface, its magnetization easy axis lies in the sample plane because of shape anisotropy.12,13 The in-plane magnetization might be further determined by the shape anisotropy because of the asymmetric geometry of the ring probe. This shape anisotropy alone might not be strong enough to automatically align the magnetization of the as-deposited iron film along ring periphery, but a saturat- ing field can help to align and stabilize the tip magnetization along the ring, as shown by our magneto-optic Kerr effect 共MOKE兲 measurement below. A similar field-aligned tech- nique was also reported in Refs. 5 and 14. In this way the orientation of probe magnetization in the sample plane can be predetermined, and the projected sample in-plane domain structure can be imaged by spectroscopy method.
To prepare such a ring probe, a tungsten wire with diam- eter of 125m is bent to a ring shape with radius of curva- ture of about 1 mm and spot welded onto the probe carrier such that the periphery is parallel to the sample, as shown in Fig.1. Loaded into an ultrahigh vacuum chamber, the ring is electron bombarded to get rid of the surface oxide layer and contaminate, an important step to performing SP-STS mea- surement. After that, 20 ML Fe is head-on deposited on it at room temperature using electron bombardment evaporator with calibrated deposition rate.15
a兲Author to whom correspondence should be addressed. Electronic mail:
mtlin@phys.ntu.edu.tw
FIG. 1. 共a兲 Photography of the ring probe made of 125-m-wide tungsten wire,共b兲 schematics of the parallel magnetizations of the sample and iron film coated on the ring tip, and共c兲 Kerr hysteresis loop for the field applied parallel to the ring plane.
APPLIED PHYSICS LETTERS 91, 202507共2007兲
0003-6951/2007/91共20兲/202507/3/$23.00 91, 202507-1 © 2007 American Institute of Physics
To verify the magnetic property of such probe, it is capped with several monolayers of Cu and the MOKE mea- surement at ambient conditions is performed. With the laser beam focused on the most front end of the ring, the hyster- esis loop for field applied parallel to the ring plane is mea- sured and shown in Fig. 1共c兲. One can clearly observe a square hysteresis loop with coercivity around 80 Oe. This indicates that the probe magnetization with the help of shape anisotropy can be easily stabilized along the ring periphery after applying saturating field. As a result, it can serve as a spin-sensitive probe for mapping out the in-plane domain structure of the sample.
We choose Mn/ Fe共001兲 as the test system because Mn exhibits layer-wise antiferromagnetism when the thickness is larger than 3 ML,16–18and its spin direction is determined by the ferromagnetic iron whisker substrate, whose dimension is 2 mm wide and 10 mm long, preferring the magnetization parallel to the long axis. The whole experiment is conducted in an ultrahigh vacuum chamber whose base pressure is 2
⫻10−10mbar and equipped with various thin film deposition and characterization tools.19The Fe共001兲 substrate is cleaned by cycles of sputtering and annealing until a clear low energy electron diffraction pattern could be observed. After that, the routine process is to sputter the substrate for 1 h at room temperature and then at 1000 K for another half hour, fol- lowed by cooling to 400 K and depositing Mn film on it at the same temperature with the deposition rate around 1.5 ML/ min. After the deposition, the sample is transferred to the STM chamber, inside which the STM and STS experi- ments are performed at room temperature with the field- aligned magnetic ring probe prepared by the method men- tioned above. In the spectroscopy mode, 101 data points of tunneling current from a bias voltage of −0.5– + 0.5 V are taken at each pixel with feedback loop open, and numerically differentiated to get the conductance spectra.
The STM constant current image of 6.8 ML Mn/ Fe共001兲 is shown in Fig. 2共a兲, whose scanning parameters are a sample bias voltage of +0.1 V and feedback current of 0.1 nA. Mn grows in a layer-by-layer way so that we can observe the sixth and seventh layers on the surface. The in- terlayer distance is measured to be 0.19± 0.03 nm from the line profile shown in Fig.2共c兲. The current map of the same area at +0.5 V is also shown in Fig.2共b兲. Contrast between the sixth and seventh layers can be observed in the current map, which is further justified by the line profiles of topog- raphy and current map at the same place in Fig. 2共c兲. The seventh layer has current of about 5% higher than the sixth layer. Since there is no chemical difference between these two Mn layers, the contrast can only be attributed to the spin difference. The spin-dependent densities of states result in the different I共V兲 curves of the parallel and antiparallel configurations.
The averaged I-V spectra of the sixth and seventh layers are shown in Fig.3. It is observed that the seventh layer has slightly higher current all the way from 0 to + 0.5 V than the sixth layer. To express the difference clearer, the numerically differentiated spectra are shown in the same figure. We note that the conductance of positive bias voltage is higher than that of negative bias, which reflects the difference between the unoccupied and occupied densities of states of the sample. Furthermore, the conductance increases almost lin- early with bias voltage from 0 to + 0.4 V, and then rises with steep slope. This trend could be decomposed into an expo-
nential background plus a surface state peak, as pointed out by Bischoff et al. in Ref. 20. These spin polarized surface states can enhance the contrast in the current map.
Blunt it may be, the spatial resolution of the ring probe made by this simple method is not a problem in this system.
Actually, since it is the very end of the probe atom that contributes overwhelmingly most of the tunneling current, even a blunt tip can give spatial resolution down to the na- nometer scale if the sample is flat enough,21which is exactly the case for Mn thin films on iron whisker. The atomic step edge is resolved to be within 2 nm in Fig.2共c兲.
According to the theory of spin-dependent tunneling, the differential conductance at r and bias voltage V can be ex- pressed as dI / dV共r,V兲=C共1+ PtPscos兲, where is the angle between Ptand Ps, C is spin averaged differential con- ductance at共r,V兲, and Pt, and Psare the polarizations of the probe and sample, respectively.22By applying the saturating field, we can adjust the direction of probe magnetization par- allel to the sample and maximize the spin contrast. The cur-
FIG. 2. 共Color online兲 共a兲 STM constant current image of 6.8 ML Mn/ Fe共001兲 with feedback parameters of V= +0.1 V, I=0.1 nA, and 共b兲 its current map at +0.5 V. Line profiles along the black dashed lines in共a兲 and 共b兲 are shown in 共c兲.
FIG. 3. I-V spectra, averaged over the sixth and seventh layers, respectively, with their numerically differentiated conductance spectra shown in the lower part. The seventh layer has larger current than the sixth layer over the whole voltage range.
202507-2 Wu et al. Appl. Phys. Lett. 91, 202507共2007兲
rent asymmetry of 5% measured from Fig. 2共c兲 is compa- rable to the previous result in Ref.23.
In conclusion, the success of obtaining spin contrast demonstrates the capability of tungsten ring probe made by bending the fine tungsten wire with a diameter of 125m.
Such probe is easy to prepare. After the magnetization of the probe is saturated by field, it can give a well defined in-plane magnetization direction of the coated ferromagnetic thin film, and thus can be applied to image the magnetic domain structure with spin sensitivity in a specific in-plane direction down to nanometer scale.
This work was partially supported by the National Sci- ence Council of Taiwan and ITRI.
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