Eur. Phys. J. B (2014) 87: 219DOI: 10.1140/epjb/e2014-50257-3

Regular Article

THE EUROPEANPHYSICAL JOURNAL B

Kerr and Faraday microscope for space- and time-resolvedstudies

Oliver Schmitt1,a, Daniel Steil1, Sabine Alebrand1, Fabian Ganss2, Michel Hehn3, Stephane Mangin3,Manfred Albrecht2, Stefan Mathias1, Mirko Cinchetti1, and Martin Aeschlimann1

1 Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin-Schroedinger-Strasse 46,67663 Kaiserslautern, Germany

2 Institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany3 Institut Jean Lamour, UMR 7198, Departement Physique de la matiere et des materiaux (P2M) CNRS, Nancy-Universite,

UPV-Metz, France

Received 17 April 2014 / Received in final form 31 July 2014Published online 23 September 2014 – c© EDP Sciences, Societa Italiana di Fisica, Springer-Verlag 2014

Abstract. We present a multi-purpose scanning magneto-optical microscope for the investigation of mag-netic thin films. The setup can be used for both static and time-resolved (pump-probe) measurements. It ismoreover compatible with samples with arbitrary magnetic anisotropy, as it allows Kerr measurements inpolar and longitudinal geometry as well as in transmission (Faraday geometry). We demonstrate that themicroscope can be used in the following modi: (i) static imaging mode (in polar Kerr and Faraday geom-etry) with a spatial resolution of 1.7 μm; (ii) time-resolved mode (polar Kerr geometry) with a temporalresolution of 300 femtoseconds.

1 Introduction

Magnetization dynamics is a topic of constant interestand research both from a fundamental and technologicalpoint of view. The research area covers state-of-the-art ap-plications with its prominent example of giant-magneto-resistance devices in hard disks and magnetic sensors, butalso strongly extends into multidisciplinary scientific re-search fields, for instance by the prospects of using singlemolecular magnets in future advanced devices. To date, itis known that the fundamental processes and interactionsthat determine magnetic dynamics in complex materialsoccur on timescales of attoseconds-to-femtoseconds andon length scales of micrometers-to-nanometers. Therefore,huge efforts were being made over the last decades to ex-tend experimental capabilities of magnetic domain imag-ing into the nanometer spatial and femtosecond tempo-ral regime. The used magnetic imaging methods reachfrom scanning to full field experimental techniques [1–3],while femtosecond time resolution in microscopy requiresa combination of these imaging techniques with ultra shortlight pulses in a pump-probe configuration. Femtosecondtime-resolved scanning Kerr microscopy with ultra shortvisible light pulses has been pioneered by the Freemangroup in 2001 [3,4]. Also, magnetic imaging with vis-ible ultrashort light pulses and photoemission electronmicroscopy (PEEM) has been developed [5,6], however,time-resolved data in the femtosecond regime is not yetavailable. Recent advances in the production of ultrashortextreme ultraviolet and X-ray pulses via high-harmonic

a e-mail: oschmitt@rhrk.uni-kl.de

generation [7–9] and free-electron lasers (FEL) open thedoor to another class of ultrafast magnetic imaging ex-periments. In particular femtosecond spatially resolvedX-ray magnetic circular dichroism (fs-XMCD) [10] andfemtosecond X-ray PEEM (fs-X-PEEM) schemes are fea-sible these days. These techniques allow for coherent mag-netic imaging and holography with nanometer spatial res-olution [11–13] and add the benefit of element-specificmagnetic probing [14,15]. Here, we present our setup of afemtosecond magnetic scanning microscope with variableKerr and Faraday geometry, which enables us to measuremagnetic structures with in- or out-of-plane orientation,on a femtosecond time scale, and with micrometer spatialresolution.

2 Experimental setup

The light source of our two-color pump-probe setup isa Ti:sapphire oscillator (Spectra Physics Tsunami) witha repetition rate of 80 MHz and a pulse duration ofabout 100 fs. The laser beam with a central wavelengthof 840 nm (infrared) is frequency-doubled to 420 nm (blue)in a BBO crystal. The beam with the fundamental wave-length has a power of up to 350 mW at the sample posi-tion and serves as the pump beam to excite the sample.The frequency-doubled beam with a power of about 1 mWserves as the probe beam.

Figure 1 shows the beam paths used in both the po-lar/Faraday (Fig. 1a) and longitudinal (Fig. 1b) geometry.Only the blue (probe) beam is used in the case of staticmeasurements. For time-resolved measurements a delay

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Fig. 1. Beam paths for the setup in polar/Faraday (a) andlongitudinal (b) geometry. For time-resolved measurements thechopper wheel is in position (B), meaning that just the pump-beam is chopped. The frequency of the chopper wheel acts as areference trigger for the lock-in amplifier. For static measure-ments, the chopper wheel is in position (A) and just the bluebeam is used. The 840 nm pump beam is delayed with respectto the 420 nm (blue) probe beam by means of an optical delayline.

stage is utilized to introduce a time delay between thepump and probe pulses. A variation of the optic path of50mm allows a total time delay between pump and probebeam of more than 300 ps. The two beams are parallel po-larized and superimposed collinearly by a dichroic mirrorbefore being focused on the sample through an objective.A combination of half-wave plate and Glan-Taylor polar-izer is used to tune the power of both beams. For a bettersignal-to-noise ratio (SNR) we use signal modulation bymeans of a chopper wheel (see (A) Fig. 1) with a frequencyof 600 to 800 Hz. This frequency is also used as a refer-ence for the lock-in amplifier. For static measurements thechopper wheel is in position (A). For time-resolved mea-surements two possibilities of signal modulation exist: inthe first case, the chopper wheel is positioned at position(A) and both beams are chopped. Second, to further im-prove the signal quality it is also possible to chop the pumpbeam only (see (B) in Fig. 1). In this case the lock-in am-plifier detects only the pump-induced changes of the Kerrsignal, whereby the absolute values are lost.

The sample is mounted on a non-magnetic scanningpiezo table (PXY 200SG from Piezo Systems Jena) with

Fig. 2. Picture of the magnet yoke in Faraday geometry. Theiron yoke is build around the microscope objectives and per-mits a closed magnetic flux.

a scanning range of 160 μm × 160 μm in closed-loopoperation. The light is focused through a polarization-preserving Nikon LU-Plan ELWD objective with a 50×magnification and a numerical aperture (NA) of 0.55. Thisallows a high working distance of ≈10 mm, which is nec-essary in the longitudinal geometry due to spatial restric-tions by a second, equal objective which parallelizes thelight again. In Faraday geometry the second objective isplaced behind the sample.

The polarization change of the reflected light is typ-ically lower than 0.1◦, hence a high sensitive detectionmechanism is required. This is performed by means ofa balanced optical bridge detection which consists of aWollaston prism and two photodiodes (DET36A fromThorlabs). The two signals are further digitized and sub-tracted with a lock-in amplifier (Stanford Research Sys-tems 830). In the polar and Faraday geometry, the mag-netic field is provided by a coil with an iron yoke aroundthe objectives (see Figs. 1a and 2). A small hole in eachpole shoe allows the transmission of the focused light.Their influence on the out-of-plane field strength in a dis-tance of 200 μm around the laser spot position on the sam-ple is about 1% and is therefore negligible for our purpose.As the magnetic field at the objective positions is neglibleit does not lead to a measurable Faraday rotation. If thelongitudinal geometry is used (see Fig. 1b) the completesample holder is rotated by 45◦ and the coil is alignedparallel to the sample surface. The magnetic field at thesurface of the sample reaches up to 300 mT in the polarand up to 100 mT in the longitudinal setup. The measure-ments were performed on transition metal-alloy samplesnamely an Fe52Pt48:Cu sample for polar MOKE and on aTb26Co74 sample for Faraday observations [16]. The ferro-magnetic Fe52Pt48:Cu sample has a thickness of 5 nm andit was deposited on a Si substrate with a 100 nm SiO2

underlayer. It has a 9% copper content and L10 struc-ture with out-of-plane magnetization. The ferrimagneticTb26Co74 sample has a thickness of 20 nm and also out-of-plane magnetization. It was deposited on a transparentglass substrate with a 5 nm Ta buffer layer. To avoid oxi-dation a capping layer consisting of 2 nm Cu and 4 nm Ptwas applied [17]. A 15 nm thick Ni sample on a siliconsubstrate with in-plane magnetization was measured inlongitudinal geometry. A USAF 1951 target from EdmundOptics (see inset of Fig. 3a) was used to determine thenon-magnetic spatial resolution. This target has defined

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Fig. 3. (a) Line scan over an edge of the USAF 1951 tar-get with the Kerr microscope to test the spatial resolution.The colored inset represents a cutout of this structure withthe performed line scan as indicated by the white bar. (b) Thegraph shows the result of an AFM line scan over another edgeof the target (group –2, element 4) to determine the width ofthe edge. Measurements at different structure positions showeda width of about 230 nm. The inset shows the arrangement ofthe structures on the target.

chromium structures of varying dimensionality on a glasssubstrate.

3 Static imaging mode

3.1 Spatial resolution

To determine the non-magnetic spatial resolution of ourmicroscope, we recorded a topography image of the struc-tured USAF target. The image is obtained by moving thesample on the xy positioning table in front of the objec-tive. The scan of one part of such a structure is shownin the inset of Figure 3a. As indicated by the white bar,we performed a line scan across the border of a structure.The distance in which the signal increases from 10 to 90%is about 1.7 μm (see Fig. 3a). An optical line scan sig-nal is always a convolution between the spatial laser spotprofile and the shape of the scanned area [18]. This shapewas determined by AFM to be an edge with the width of

about 230 nm (see Fig. 3b). This signal is again a con-volution between the resolution of the AFM and the edgeprofile and it was measured at different structure positionson the target with comparable results. So we determinethe resolution of our microscope to be lower than 1.7 μm.It should be mentioned that the resolution is not limitedby the minimal step size of our piezo table which is in theorder of some nanometers.

3.2 Magnetic contrast

To test the magnetic lateral resolution, we scanned theborder of two magnetic domains of a Tb26Co74 sample.This was done twice, once in polar MOKE geometry andonce again in Faraday geometry. The domain structurewas created before the measurement by approaching thesample with a small strong permanent magnet. The re-sult can be seen in Figures 4a and 4b. One clearly seesthe two opposite domains colored in red and blue in bothscans. The good agreement of both measurements con-firms the function of the microscope in both geometries.The contrast of the domain wall is similar in both pictures.The small vertical inhomogeneities are measurement ar-tifacts due to long term laser intensity fluctuations. ForFaraday measurements, the scanning laser beam musthave a higher intensity compared to MOKE measurementsbecause most of the light is reflected from the sample sur-face. To ensure that the shape of the domain wall orig-inates purely from different magnetization patterns andnot from optical artifacts, we replaced the balanced pho-todiode arrangement by a single photodiode. With thatconfiguration, no magnetic information can be measured.A scan over the same area as in Figure 4 exhibited afeatureless contrast pattern (not shown here). Figures 4cand 4d show line scans over the domain wall in MOKE andFaraday geometry. Determination of the distance in whichthe signal increases from 10 to 90% as in Figure 3 yieldsa magnetic spatial resolution of (1.6± 0.1) μm for MOKEand (1.8± 0.1) μm for Faraday setup. The measured signalis a convolution of the magnetic response of the domainwall and the shape of the laser spot. The slightly lowerresolution in the transmission mode can be explained bylight scattering in the glass substrate of the sample. Bothvalues for the spatial resolution are consistent with thespot size of the laser within the error bars. So we deter-mine our all-optical resolution to be (1.7 ± 0.1) μm.

4 Time-resolved mode

4.1 Hysteresis

In all geometries, it is possible to measure hysteresis loops.This is done prior to time-resolved measurements to checkfor a sufficient signal quality and a suitable alignment ofthe sample. Figures 5 and 6 show the hysteresis cycle of theNi film in longitudinal geometry and of the Fe52Pt48:Cusample in out-of-plane geometry. The constant saturationlevels as well as the steep flanks of the hysteresis curvesconfirm the correct measurement geometry for each sam-ple magnetization direction.

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Fig. 4. Static scan of the magnetization of an area including adomain structure of a Tb26Co74 sample. (a) Scan in reflectivegeometry with polar MOKE. The domain wall separates theopposite oriented magnetic domains colored in red and blue.The small yellow areas in the red region result from an averag-ing of the laser spot over both magnetization directions. Theextracted line scan is indicated by the white bar. (b) Scan intransmission using Faraday geometry. The two domains col-ored red and blue can clearly be revealed. The scanned sectionwas the same as in (a). (c) Line scan across the domain borderin MOKE and (d) Faraday geometry. Both measurements wereperformed at the same sample positions.

4.2 Time-delay scans

The dynamic mode is used to probe the magnetization be-havior after the excitation with an intense laser pulse [19].To exclude optical artifacts in the demagnetization traceone has to perform two measurements, one for each di-rection of the saturation magnetization. Subsequent sub-traction of these signals eliminates non-magnetic contri-butions which have the same algebraic sign for differentorientations of the magnetization.

To quantify the temporal resolution we demagnetizedthe Fe52Pt48:Cu film as depicted in Figure 7. This sig-

Fig. 5. Longitudinal geometry: hysteresis of a Ni sample within-plane magnetization.

Fig. 6. Hysteresis of the Fe52Pt48:Cu sample with out-of-planemagnetization recorded in polar geometry (see Fig. 1a).

nal is a convolution of the temporal shape of pump andprobe pulse with the magnetic material response signal.We determined the pump-probe cross-correlation width tobe 280±15 fs using the electro-optical Kerr effect [20] (notshown). This can thus be regarded as the upper limit forour time resolution. Including the cross-correlation widthin a fit to the demagnetization curve of Fe52Pt48:Cu deliv-ers a demagnetization time of τM = 199 ± 7 fs (see blackcurve in the inset of Fig. 7). The demagnetization curve forthe first 100 ps after laser excitation is shown in Figure 7.The chopper wheel during both measurements was in po-sition (A) (see Fig. 1a) to measure only the pump-inducedchanges of the Kerr signal. Due to the lack of absolute val-ues for the demagnetization in this mode, the signal is nor-malized between 1 and 0. A measurement using the otherchopping position (with a lower SNR, not shown here)exhibited that the absolute magnetization is quenched upto 45%. In all time-resolved measurements we took onecycle and applied an excitation fluence of 11 mJ cm−2.

5 Limits

The temporal and spatial resolutions which have been pre-sented above are adequate to characterize a wide rangeof magnetic samples. To reach the physical limits someadditional steps have to be taken. For the temporal res-olution, the relatively high pulse length of 100 fs limitsour resolution. To improve our time resolution a laser sys-tem with shorter pulses would be necessary. But for pulses

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Fig. 7. Ultrafast dynamics of the magnetization of theFe52Pt48:Cu sample after an intense laser pulse. Inset: short-time demagnetization trace which was normalized between zeroand one. The black curve shows the fit to the data which deliv-ers a demagnetization time τM = 199±7 s. Long-term dynam-ics: the magnetization is quenched and slowly begins to relaxback after about 20 ps.

shorter than 100 fs the group velocity dispersion (GVD)stretches the pulses substantially in the optical compo-nents like the objectives. Therefore a GVD compensationpath with prisms would be necessary to compensate thedispersion at the sample position as it was done by refer-ence [4]. The GVD values for the objectives are not knownwhich makes it hard to calculate the pulse broadening.

The fundamental limit in case of the spatial resolutionis caused by diffraction. The minimal spot radius dmin

which can be reached depends on the NA of the objective(Rayleigh criterion): dmin = 0.61λ

NA . This would lead in ourcase to a minimal spot radius of about 466 nm. Never-theless Figure 3b presents a spot diameter of more than amicron. The reason for that is on the one hand that thelaser output beam has a finite divergence. On the otherhand we have a spot diameter of about one millimeterin front of the objective which does not fill the completeentrance aperture.

6 Conclusions

We showed proof of principle measurements of our multi-purpose Kerr microscope in the static and the dynamicmode. The actual time resolution is better than 300 fs.This can be improved by using a Ti:sapphire oscillatorwith shorter pulses, accompanied by an external prismcompensation. The spatial resolution of our instrumentis 1.7 μm and can be enhanced by expanding the laserbeam in front of the objective with a telescope. A strongmagnetic field (up to 300 mT) can be applied evenin Faraday geometry by an advanced yoke design. Intotal, our microscope provides a variety of measuringmodes to investigate the static and dynamic magnetiza-tion properties.

We thank T. Lober from the TU Kaiserslautern for AFMmeasurements.

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