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Z. Phys. B 104, 183–184 (1997)ZEITSCHRIFT

FUR PHYSIK Bc Springer-Verlag 1997

 Rapid note

Faraday-rotation imaging by near-field optical microscopyThilo Lacoste, Thomas Huser, Harry Heinzelmann

Institut f ur Physik, Universitat Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland (e-mail: heinzelmann@ubaclu.unibas.ch)

Received: 9 July 1997

Abstract. Scanning near-field optical microscopy with po-larization modulation (PM-SNOM) has been applied to im-age the surface of a yttrium-iron-garnet (YIG) film. Lock-indetection of the phase of the transmitted light directly givesthe magnitude of the Faraday rotation angle.

PACS: 07.79.Fc; 61.16.Ch; 75.70.Kw; 78.20.Ls

The increasing performance of magnetic storage makes agood understanding of novel recording media necessary. Ex-perimental methods to observe the domain structure of mag-netic materials include decoration techniques [1], scanningelectron microscopy with polarization analysis (SEMPA) [2],magnetic force microscopy (MFM) [3] and magneto-optics,dealing with the phenomenon that the polarization of lightis changed when it interacts with magnetized matter.

Scanning near-field optical microscopy (SNOM) [4] usesa sub-wavelength light source which is raster-scanned acrossthe sample. The spatial resolution is determined by the sizeof the aperture and its distance to the sample surface, ratherthan the probing wavelength. SNOM imaging of magneticdomains in reflection (Kerr effect) is difficult [5] since mul-tiple reflections between sample surface and SNOM probecorrupt all polarization information. A more complicated ap-proach is based on the detuning of the plasmon resonanceof small silver particles in a magnetic field [6, 7]. How-ever, imaging of domains in Co/Pt multilayer thin films [8]and in YIG films [9, 10] in transmission (Faraday effect) ispossible.

In this contribution, we demonstrate that by modulating

the linear input polarization state over 180 degrees and syn-chronous detection of the transmitted light, it is possible todirectly image the distribution of Faraday rotation angle, andthus the magnetization.

The setup of our polarization-modulation SNOM (Fig. 1),a home-built instrument placed on top of the sample stageof an inverted optical microscope, is described in detail else-where [11, 12]. The SNOM probe consists of a tapered andAl coated singlemode fiber, which is controlled at approx-imately 5 nm above the surface by means of a shear-force

Fig. 1. Setup of the polarization-modulation SNOM

distance feedback-loop. For polarization-modulation exper-iments, care is taken to use only fiber tips with high andhomogeneous extinction ratios for arbitrary linear input po-larization states. For polarization modulation, light from aHe-Ne laser (λ = 633 nm) with a polarization ratio of 500:1is passed through an electro-optical modulator (driven by

a 1 kHz sawtooth signal) and a quarterwave plate. Thesecomponents’ main axes are mounted at 45 degrees and 0degrees respectively to the input polarization. Thus only lin-ear polarization states from 0 to 180 degrees are coupledinto a singlemode fiber. A fiber-loop polarization controllerhas to be adjusted to compensate fiber birefringence. Aftertransmission through the sample the light is passed throughan analyzer at arbitrary orientation onto a photomultiplier.The phase signal of a lock-in amplifier analyzing the lightintensity signal gives directly the Faraday rotation relative

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Fig. 2. 15×15 µm2 image of the natural domain structure of a YIG film: a topography, b magnitude of Faraday rotation measured by phase modulation,

c transmitted light intensity measured with fixed linear polarization and varying analyzer settings

to some arbitrary zero. The phase resolution is about 0.1degrees, limited by fiber birefringence noise.

Modulating the light impinging on the sample rather thanafter transmission through the sample offers more possibil-ities, since here all different polarization directions can in-teract with the sample. First demonstrated to detect samplebirefringence [13], it also allows to measure simultaneouslythe magnitude and the orientation of optical anisotropies of absorbing materials [14, 12]. For applications such as theFaraday effect, which is isotropic in the plane of the samplesurface, both schemes are equivalent.

The sample for these studies is a 3.62 µm thick YIG filmon a Gd3Ga5O12 substrate. The material is soft magnetic witha magnetization perpendicular to the surface. A 15×15 µm2

image of the domain structure is shown in Fig. 2. The shear-force signal (a) shows a flat surface topography with somerandom contamination. The polarization modulation phaseimage (b) shows the domain structure of the YIG film, witha Faraday rotation angle ranging over 2.3 degrees over thewhole image, i.e. including both up and down domains. A

cross-section through the domain structure shows a sine-typevariation of the signal with a domain width at half maximumof 680 nm. This is contrary to our expectation of a signalvarying only over the domain walls. There are two effectsleading to a blurred signal. First, in this modulation setup wehave to optimize the fibers for their extinction ratios, ratherthan for small apertures. Larger apertures reduce resolution.Second, light diverging into the YIG film (3.62 µm thick)beyond the near-field averages over multiple domains, thusleading to a signal background of long spatial wavelength.The measured signal corresponds to the Faraday rotationgenerated in the top several hundred nanometers, allowingto assess sample magnetization or Verdet constant of a thinsurface layer with sub-wavelength resolution.

Images with fixed input polarization were taken for com-parison (Fig. 2c). A change of contrast for different analyzersettings (rotation of +2 degrees in line A and −2 degrees inline B) is demonstrated on the same sample location. Thisbehavior is similar to the contrast reversal observed in con-ventional polarization microscopy.

One advantage of the modulation experiment is that theFaraday rotation angle can be imaged directly, rather thanby a series of experiments at different analyzer orientations.Furthermore, lock-in detection and the use of higher lightintensities on the photodetector reduce noise substantially.Phase detection of the Faraday rotation angle appears to beless sensitive to features in topography. The intensity record-ing at fixed polarization, however, clearly shows a responseto these features (marked by white circles in Figs. 2a and c).

We would like to thank Hubert Bruckl from the Institute of Solid State

and Materials Research in Dresden for providing the sample, and Hans

Hug for helpful discussions. The work was financially supported by the

Swiss Priority Programs OPTIQUE, MINAST and the National Science

Foundation program CHiral2.

References

1. Bitter, F.: Phys. Rev. 38, 1903 (1931)

2. Oepen, H.P., Kirschner, J.: Phys. Rev. Lett. 62, 819 (1989)

3. Hug, H.J., Stiefel, B., Moser, A., Parashikov, I., Klicznik, A., Lipp,D., Guntherodt, H.-J., Bochi, G., Paul, D.I., O’Handley, R.C.: J. Appl.

Phys. 79, 5609 (1996)

4. Paesler, M., van Hulst, N. (eds): Near Field Optics, vol. 61 of Ultra-

microscopy. Elsevier (1995) Proc. NFO-3, Brno

5. Durkan, C., Shvets, I.V., Lodder, J.C.: Appl. Phys. Lett. 70:1323 (1997)

6. Silva, T.J., Schultz, S., Weller, D.: Appl. Phys. Lett. 65:658 (1994)

7. Silva, T.J., Schultz, S.: Rev. Sci. Instrum. 67, 715 (1996)

8. Betzig, E., Trautman, J.K., Wolfe, R., Gyorgy, E.M., Finn, P.L.: Appl.

Phys. Lett. 61, 142 (1992)

9. Betzig, E., Trautman, J.K., Weiner, J.S., Harris, T.D., Wolfe, R.: Appl.

Opt. 31, 4563 (1992)

10. Hartmann, U.: J. Magn. Magn. Mat. 157/158, 545 (1996)

11. Lacoste, Th., Huser, Th., Heinzelmann, H., Guntherodt, H.-J., In: Marti,

O., Moller, R. (eds) Photons and Local Probes. NATO ASI Series E:

Applied Sciences, Vol. 300, p. 123 (1995)

12. Lacoste, Th., Huser, Th., Prioli, R., Heinzelmann, H.: Proc. NFO-4,Jerusalem, to be published in Ultramicroscopy (1997)

13. Vaez-Iravani, M., Toledo-Crow, R.: Appl. Phys. Lett. 63, 138 (1993)

14. Higgins, D.A., Vanden Bout, D.A., Kerimon, J., Barbara, P.F.: J. Phys.

Chem. 100, 13794 (1996)