The precession of individual spins on partially oxidized SiC111) surfaces has been detected using a scanning tunneling microscope. The spin precession in a constant magnetic field induces a modulation in the tunneling current at the Larmor frequency. This radio-frequency signal is shown to be localized over distances less than 10 A and follows the expected magnetic field dependence.PACS numbers: 07.58.+g, 61.16.Di The characteristic precession frequencies of paramagnetic atoms and defects can provide unique information about their physical and chemical environments. The most commonly used technique for detection of paramagnetic spin centers, electron-spin resonance (ESR), measures the precession of an induced macroscopic magnetization (which is a result of the alignment of many electron spins) around an external magnetic field. 1 In many cases, valuable spectroscopic information about the properties of individual spins is lost as a result of inhomogeneous line broadening. In addition, ESR measurements typically require a minimum of about 10 10 spins in order to be detectable.The presence of small quantities of magnetic impurities or other localized spin centers in bulk tunnel junctions has been shown to significantly affect tunneling characteristics. 2 " 5 Thus, one may expect that it should be possible to observe this and other spin-related tunneling effects with the scanning tunneling microscope (STM). The STM 6 is unique because it can provide truly local information with atomic resolution about the measured sample and should therefore be sensitive to any local perturbation which is close enough to the tunneling region to affect the tunneling probability. Such a perturbation could be caused, for example, by the magnetic moment of an electron-spin center at a surface interacting with the tunneling electrons via dipolar or exchange interactions. In a constant magnetic field, this magnetic moment will precess around the field direction at the classical Larmor frequency. If the magnetic moment affects the tunneling probability in surrounding regions, then we expect that the precession of the spin around the magnetic field will induce a modulation in the tunneling current at the same frequency, providing a way of detecting and studying individual spin centers using the STM.It is shown here that the rf component of the tunneling current indeed contains information about the paramagnetic centers located at the surface. Moreover, with the atomic resolution we have obtained with the STM, we believe that we have detected the precession of individual spins.The experiments were performed using an STM on which two parallel bar magnets were mounted to establish a magnetic field perpendicular to the surface. The strength of the field could be varied by changing the separation between the magnets. The tip was a polycrystalline tungsten wire etched to a sharp point and thermally annealed in vacuum for cleaning. The surfaces were partially oxidized Si(l 11) which were prepared by first outgassing and annealing a commercial ...

Electron-spin-resonance-scanning tunneling microscopy ͑ESR͒-͑STM͒ of iron atoms in silicon was observed: Si͑111͒ surfaces were covered with iron atoms and annealed. This gives surfaces covered with small islands of different silicide phases. We could detect an ESR-STM signal that corresponds to a g value of 2.07. The signal was split by magnetic field modulation, and phase-sensitive detection was applied. This shows that electron-spin-resonance ͑ESR͒-STM signals can be detected on a surface that is imaged with atomic resolution and that it is possible to observe a g 2 spin center. Getting absorption ͑instead of the expected derivative͒ line shapes with phase-sensitive detection is explained by the asymmetry in the line shape, which is a result of a rapid scan of the signal. Then, when the integration time of the phase sensitive detector is close to the time it takes to sweep the linewidth of the signal, absorption line shapes are observed.

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ESR-STM is an emerging technique which is capable of detecting the precession of a single spin. We discuss the mechanism of ESR-STM based on a direct exchange coupling between the tunneling electrons and the local precessing spin S. We claim that since the number of tunneling electrons in a single precessing period is small (∼ 20) one may expect a net temporary polarization within this period that will couple via exchange interaction to the localized spin. This coupling will randomly modulate the tunneling barrier and create a dispersion in the tunneling current which is a product of a Larmor frequency component due to the precession of the single spin and the dispersion of the spin of the tunneling electrons. This noise component is spread over the whole frequency range for random white noise spin polarization of electrons. In opposite case the power spectrum of the spins of the tunneling electrons has a peak at zero frequency an elevated noise in the current at ωL will appear. We discuss the possible source of this spin polarization. We find that for relevant values of parameters signal to noise ratio in the spectral characteristic is 2-4 and is comparable to the reported signal to noise ratio [1,2]. The magnitude of the current fluctuation is a relatively weak increaing function of the DC current and the magnetic field. The linewidth produced by the back action effect of tunneling electrons on the precessing spin is also discussed.There is a growing realization that the technique of ESR-STM is capable of detecting the precession of a single surface spin by modulating the tunneling current at the Larmor frequency. This technique was successful in measuring Larmor frequency modulations in defects in semiconductor surfaces [1] and in paramagnetic molecules [2]. The increasing interest in this technique is due to the possibility to detect and manipulate a single spin [3].The alternative technique that allows one to detect single spin is the optically detected magnetic resonance (ODMR) spectroscopy in a single molecule [5]. In comparison, ESR-STM has the unique ability to correlate the spectroscopic information with the spatial information, detected at the atomic level. It also allows one to manipulate the position of the spin centers at the atomic level [4].There has been several proposals for the mechanism of detection. One is a polarization of the mobile carriers through spin orbit coupling, and modulation of the LDOS as a result of the precession [6]. Another one is the interference between two resonant tunneling components through the magnetic field splitted Zeeman levels [7]. Both of these mechanisms rely on a spin orbit coupling to couple a local spin S to the conduction electrons and have assumed no spin polarization of tunneling electrons. Recently however, Durkan and Welland [2] observed a strong signal in a system with a substantially smaller spin orbit coupling than what was assumed in the calculations [6], [7]. Motivated by these experiments we addressed a question: what is the role of the direct excha...