In situ magnetization switching of magnetic probes applied to spin-polarized scanning tunneling microscopy
Pin-Jui Hsu,1Chun-I. Lu,1Szu-Wei Chen,1Wang-Jung Hsueh,1Yu-Hsun Chu,1 Chuang-Han Hsu,1Christopher John Butler,1and Minn-Tsong Lin1,2,a兲
1Department of Physics, National Taiwan University, 10617 Taipei, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, 10617 Taipei, Taiwan
共Received 25 December 2009; accepted 11 March 2010; published online 9 April 2010兲
Soft magnetic tip was utilized to be the probe of spin-polarized scanning tunneling microscopy. It was demonstrated that the spin contrast can be reversed by in situ switching tip magnetization through varying tip-substrate distance for resolving perpendicular magnetic domain images. With this in situ magnetization direction switching of the soft magnetic tip, it is conceivable to separate magnetic from chemical and topographic contributions without applying external magnetic field.
This provides an effective tool for the study of complex magnetic spin structures with various nonmagnetic impurities or compositions involved. © 2010 American Institute of Physics.
关doi:10.1063/1.3380711兴
With the development of spin-polarized scanning tunnel- ing microscopy共SP-STM兲 techniques, numerous fascinating magnetic spin structures have been unveiled and lateral res- olution down to atomic scale of magnetic images can be achieved.1–6 In the SP-STM experiments, spin-polarized probes play the crucial role and different probes were devel- oped according to different approaches of operation modes.
For example, there were optically pumped GaAs tip,7 mag- netic materials coated W tip,8,9 coils wound CoFeSiB tip,10 etc., reported in previous studies. Besides, in order to have in-plane or out-of-plane spin direction identification, tips de- signed with different geometric shapes11,12 or coated with varied magnetic materials13,14have been demonstrated.
However, there are still several critical issues on spin- polarized tips designed for SP-STM experiments.13,15One of them is to separate the magnetic from topographic and chemical contributions with the opportunity of magnetization direction switching of either the sample or the tip. As re- ported by the previous SP-STM experiments,6,8,16,17owing to both sample and tip were under strong external magnetic field applied, magnetization direction rotation needed to be carefully controlled to have magnetization switching of sample or tip only. Instead of applying external magnetic field, recent theoretical studies also reported that varying tip- substrate distance was capable to switch a single spin through the competition of direct and indirect exchange coupling.18–20 In light of this, soft magnetic materials with low coercivity field for spin-polarized probe arise to be an appropriate candidate for in situ magnetization switching in the SP-STM technique.
In this letter, soft magnetic tip from FeMnC alloy mate- rial has been applied to be the spin-polarized probe with out-of-plane spin sensitivity for the Co nanoislands grown on Cu共111兲 with out-of-plane magnetization.16,21 Most impor- tantly, in order to have magnetic signals distinguished, an in situ switching of the tip magnetization direction can be achieved through varying tip-substrate distance, revealing
the magnetic domain of Co nanoislands with reversed spin contrast.
The experiment was carried out in a UHV chamber with base pressure of ⬇3⫻10−11 mbar. The Cu共111兲 substrate was cleaned by cycles of 1 keV Ar+sputtering and annealing at 850 K. After that, submonolayer Co was evaporated on the Cu共111兲 at room temperature through molecular beam epitaxy technique with the deposition rate of 0.6 ML min−1 calibrated from STM. After sample preparation, it was subsequently transferred into low temperature STM 共Omicron LT-STM兲 cooled at 4.4 K with pressure of
⬇2⫻10−12 mbar.
The tip probe of FeMnC alloy material was first pre- pared by mechanical milling and the corresponding magnetic properties were checked with the magneto-optic Kerr effect 共MOKE兲 measurements. As shown in the Fig.1共a兲, the small coercivity and switching field共⬍15⫾1 Oe兲 can be charac- terized from the magnetic hysteresis loop. In addition, the tip image taken from scanning electron microscopy 共SEM兲 is shown in the Fig.1共b兲.
After the tip transferred into UHV chamber, it was fur- ther sputtered by 3 KeV Ar+bombardment. Furthermore, in order to improve the spin contrast10,12and keep coercivity to remain small available for magnetization switching, a few monolayers of Co was coated. The magnetic stability was warranted in such Co coated tip at regular tunneling distance, so that they can be applied to spin-resolved tunneling spec- troscopy measurements with reliability. As for the tunneling
a兲Author to whom correspondence should be addressed. Electronic mail:
(a) (b)
FIG. 1.共Color online兲 共a兲 Magnetic hysteresis loop of FeMnC alloy tip from the MOKE measurements and the switching field smaller than 15⫾1 Oe can be characterized.共b兲 Magnified image of soft magnetic tip taken by the SEM.
APPLIED PHYSICS LETTERS 96, 142515共2010兲
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spectroscopy measurements, the feedback loop was open and the sample bias ramped from +1.0 to ⫺1.0 V whereas the distance between the tip and sample was stabilized typically at scanning parameters of +1.0 V and 1.0 nA. The differen- tial conductance spectra were recorded simultaneously to the topographic images by adding a voltage modulation of 20 mVrmsto the sample bias and detecting the signals by the lock-in technique.
A typical STM topography of 0.35 ML Co grown on Cu共111兲 is shown in Fig. 2共a兲. There are two kinds of Co nanoisland arrangements, faulted and unfaulted fcc stack- ings, protruding with bilayer hight from the Cu surface.22,23 By using the magnetic tip, the spin-polarized conductance mapping resolved at bias of ⫺316 mV and ⫺494 mV are presented in Figs. 2共b兲 and 2共c兲, respectively. In order to prevent the influence from crystalline and size dependent peak position of surface state,16,24 we discuss the conduc- tance curves on Co nanoislands with equal stackings and sizes, as the two sets of them marked from A to D in the Figs. 2共b兲 and 2共c兲. And their corresponding conductance curves depicted with different colors are shown in top of Fig.
2共d兲. The prominent surface state peak of conductance curve at around ⫺0.31 V is consistent with previous studies16,23 and contributes to the significant spin-polarization amplitude due to the hybridization of s-p states with the minority d3z2−r2 band of Co nanoislands. The structural and magnetic asym- metry curves defined as13,16
Astructural⬅dI/dVunfalted↑↑共↓兲− dI/dVfaulted↑↑共↓兲
dI/dVunfalted↑↑共↓兲+ dI/dVfaulted↑↑共↓兲,
Amagnetic⬅dI/dV共un兲faulted↑↑− dI/dV共un兲faulted↑↓
dI/dV共un兲faulted↑↑+ dI/dV共un兲faulted↑↓
,
are shown in the bottom of Fig. 2共d兲. The conductance curves from these two different derivations can thus be clearly distinguished. Besides, the characteristic oscillatory behavior of magnetic asymmetry due to the sign reversal of spin polarization is also observed, being consistent with pre- vious studies.16
After the magnetic domain images and spin-polarized conductance spectra have been resolved, the reversal spin contrast recorded at bias of ⫺0.3 V and 1.0 nA are conse- quentially demonstrated in Figs. 3共a兲–3共c兲. At first, the to- pography of 0.66 ML Co nanoislands and the spin-polarized conductance mapping, recorded simultaneously, are shown in the inset of Fig. 3共a兲 and Fig. 3共a兲, respectively. The spin contrast of these nanoislands can be clearly observed accord- ing to the magnetization direction parallel or antiparallel to that of the magnetic tip.25 Besides the spin contrast, some black spots on the surface of Co nanoislands are also ob- served due to the segregation of Cu atoms.26Afterward we move the tip to one of Co nanoislands, as shown by F in the Fig.3共a兲, with the magnetization antiparallel to the tip mag- netization, and decrease the tip-substrate distance by reduc- ing the bias to 3 mV and increasing the current to 30 nA, which is much smaller than the current used in the spin in- jection of previous SP-STM experiments.27The correspond- ing conductance is ⬃0.258G0, where G0⬅e2/h is for two magnetic electrodes without spin degeneracy.28,29After that, we go back to the normal scanning parameters of ⫺0.3 V and 1.0 nA, and the reversal spin contrast of all Co islands can be clearly observed in the Fig. 3共b兲. By repeating this (a)
C D
A B (c)
A B C
D (b)
(d)
FIG. 2. 共Color online兲 共a兲 Topographic image of 0.35 ML bilayer high Co nanoislands grown on Cu共111兲 共image size 100⫻100 nm2, U = +1.0 V, and I = 1.0 nA兲. 共b兲 and 共c兲 are the spin-polarized conductance mapping images recorded at⫺316 mV and ⫺494 mV, respectively. 共d兲 Top: spin-polarized conductance curves recorded from two set of Co nanoislands with same stackings and nearly same sizes as indicated from A to D in the inset of共b兲 and共c兲. Bottom: structural and magnetic asymmetry curves.
E F (a)
E′′
F′′
(c)
E′
F′
(b)
(d)
FIG. 3. 共Color online兲 共a兲 Spin polarized conductance mapping image of 0.66 ML bilayer high Co nanoislands and the black spots on the surface of Co nanoislands are segragated Cu atoms. The corresponding topography is shown in the inset.共b兲 Reversed spin contrast image taken after the magne- tization switching of front tip end.共c兲 Similar spin contrast image to the 共a兲 taken after tip magnetization reverse again.共d兲 Tunneling spectra of a set of two Co nanoislands taken before and after magnetization switchings of tip end. 共image sizes are all 85⫻85 nm2, taken at U = −0.3 V and I = +1.0 nA兲.
142515-2 Hsu et al. Appl. Phys. Lett. 96, 142515共2010兲
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process at another Co nanoisland E⬘ of Fig. 3共b兲, we can reverse again the spin contrast images as shown in the Fig.
3共c兲similar to the Fig.3共a兲. This demonstrates the magneti- zation direction of tip end can conceivably be reversed back by such kind of operation. In addition, for the segregated Cu atoms, i.e., black spots on the surface of Co nanoislands in the spin contrast images, there is no contrast between them during the magnetization switchings of tip end. Separation of magnetic signal from topographic or chemical contributions could be thus achieved in such way.
The tunneling spectroscopy measurements before and af- ter tip magnetization switchings are also presented in the Fig.
3共d兲. From the comparison of spin-polarized conductance curves of E and F taken from Co nanoislands indicated in the Fig. 3共a兲, curve E has stronger amplitude than curve F at around ⫺0.3 V and curve F has stronger amplitude than curve E at around⫺0.5 V on the contrary.16 Such behavior reverses in the curves of E⬘and F⬘due to the magnetization switching of front tip end and reverses back again in the curves of E⬙ and F⬙. Besides, from the asymmetry curves illustrated in the Fig.3共d兲, the major contribution to reversal spin contrast image can be indicated by the arrows in the bias voltage region of spin-polarized surface state. The slight dif- ference in the asymmetry curves, especially at around⫺0.7 V共Ref. 16兲 which is not reversed accordingly, might come from the complex intra-atomic noncollinear magnetism of the magnetic tip end30 and requires further studies in the future. Nevertheless, the tip magnetization switching behav- ior is much more clear and can be identified from the pro- nounced spin-polarized surface state at around ⫺0.3 V.
In summary, FeMnC alloy materials with low coercivity field have been applied to be the spin-polarized probe with the capability of in situ magnetization direction switching.
According to the consequential reversal spin contrast of Co nanoislands, tip magnetization switching can be achieved through reducing tip-substrate distance. This provides an ef- fective method to distinguish magnetic signals from chemi- cal or topographic contributions without applying external magnetic field in the SP-STM.
We acknowledge financial support from the National Science Council of Taiwan under Grant No. NSC 98-2120- M-002-010.
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