REVIEW OF SCIENTIFIC INSTRUMENTS 82, 023709 (2011)
Apparatus for vectorial Kerr confocal microscopy M. Savoini, F. Ciccacci, L. Duò, and M. Finazzia) CNISM – Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
(Received 17 November 2010; accepted 26 January 2011; published online 28 February 2011) We present a confocal microscopy setup that is able to record magneto-optical hysteresis cycles separating the in-plane and out-of-plane magnetization components. This apparatus is based on a modified commercial microscope, where the light beam has been deviated from the cylindrical symmetry axis of the objective lenses by inserting a translating plate in the optical path. The instrument allows for the magneto-optical imaging with a lateral resolution of 600 nm at λ = 635 nm light wavelength. © 2011 American Institute of Physics. [doi:10.1063/1.3555757] The magneto-optic Kerr effect (MOKE) is employed as a widespread analysis tool for studying the magnetic properties in ferromagnetic materials. In particular, it can be combined with an optical microscope in order to obtain spatially resolved magnetic information. Although many other magnetic imaging techniques exist, as for example, magnetic force microscopy,1 scanning and transmission electron microscopy,2, 3 and scanning Hall probe microscopy,4 magneto-optical microscopy still presents advantages in terms of simplicity and rapidity. Moreover, MOKE can be combined with ultrafast laser sources to extract time-resolved information concerning the magnetization dynamics of the sample.5 In this frame, the lateral resolution is determined by the diffraction limit. By using high-numerical aperture objectives, resolutions of the order of 500–700 nm have been demonstrated for magneto-optical imaging with visible light.6, 7 A MOKE experiment can be realized in three different configurations (polar, longitudinal, and transverse), which are sensitive to different combinations of the components of the magnetization vector. Although Vavassori proposed an effective method to recover the complete magnetization vector using a single setup with three combined measurements at different incoming polarization directions,8 it is a common practice to measure such components separately. In this respect, normal-incidence illumination is generally used to detect the polar (off-plane) component, while oblique incidence is mandatory for measuring in-plane magnetization components: in particular, in longitudinal MOKE the incidence plane and the external magnetic field are parallel, while they are perpendicular in the transverse configuration. Today’s state-of-the-art MOKE microscopy uses “pseudonormal” illumination, in which a normal light beam is strongly focused onto the sample surface. Since the geometry of this setup is generally characterized by axial symmetry, only the magnetization component perpendicular to the sample surface can be measured. A possible solution to measure in-plane magnetization components with spatially-resolved MOKE consists in the use of two microscope objectives, one to illuminate the sample and the other to collect the reflected light, as for example, a) Author to whom correspondence should be addressed.
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shown in Refs. 9–11. This configuration has the advantage of providing a better focusing and hence a better spatial resolution than with a collimated beam. However, the simultaneous use of two objectives restricts the available space near the sample, limiting the maximum numerical aperture (NA) of the optics and hence the lateral resolution that can be achieved. It is also possible to detect in-plane components while keeping a pseudonormal configuration. This has been demonstrated some decades ago, breaking the axial symmetry by exploiting half-in half-out asymmetric illumination conditions.12 In this case a single illumination/detection lens has been used together with spatial filters in order to fill only half of the lens for the illumination path and the other half for the collection of the reflected beam. The axial symmetry can be also broken at the detection level, as shown by Keatley et al.13 However, it has been pointed out that asymmetric configurations cannot be employed in conjunction with the photoelastic modulation scheme, because any asymmetry in the illumination will result in a false background signal.14 Wang and Yang have demonstrated how to circumvent this problem by using the symmetric half-in half-out illumination associated with an aperture. Their setup allows one to use standard phase-modulation techniques, clearly enhancing the sensitivity of the setup and measuring in-plane components of the magnetization vector.7 Although this setup configuration is able to detect in-plane information, it lacks the possibility of distinguishing the two components (parallel and perpendicular to the direction of the external magnetic field). In this work we describe how it is possible to combine the phase modulation technique with a commercial scanning microscope to probe each of the two in-plane components of the magnetization vector. In particular, the configuration uses a shared objective. To select the portion of the objective illuminated by the laser light, we employ a thick (8 mm) fusedsilica plate mounted onto a high precision biaxial goniometer. By adjusting the angle of this plate we can enter the optical system with an off-axis beam, breaking the axial symmetry as required by nonpolar MOKE. This allows one to compensate for any possible misalignment in the optical system. In such a way we are able to control and strongly reduce the problematic background signals that usually arise in combining this configuration with phase modulation. With this technique it is also possible to acquire polar MOKE cycles, simply by
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Rev. Sci. Instrum. 82, 023709 (2011) My
(c)
[010]
Mx
[100]
M
H
My /My,max
1.0 (b)
0.0
H
-1.0
Mx /Mx,max
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H
-60 FIG. 1. (Color online) Experimental setup of the single objective MOKE microscope. The translation plate is mounted on a biaxial goniometer in order to displace the laser beam along two perpendicular directions by varying the angles δ and β.
recovering the axial symmetry of the illumination/collection system by properly turning the plate. We employ the commercially available scanning nearfield optical microscope α-SNOM (WITec GmbH, Germany) in its confocal configuration, which has been modified to insert the optical elements needed for off-axis light phase modulation, as shown in Fig. 1. In particular, a continuous wave laser diode (emission wavelength λ = 635 nm) is coupled through a single mode optical fiber into the microscope. The beam dimension is varied with an iris, which is used to reduce the beam diameter to 1 mm. The light beam is then deflected off axis with the fused-silica plate producing a 2 mm-displacement and, finally, linearly polarized with a Glan–Taylor polarizer (extinction ratio 1:106 ). Subsequently, it is focused onto the sample with a Nikon strain-free microscope objective (NA = 0.65). By entering the microscope with an off-axis laser beam, the NA of the objective is reduced as a consequence of the partial filling of the lens. Nevertheless, this allows us to break the axial symmetry of the setup and collect the beam reflected by the sample surface by using the same objective. The collected light is then sent through a photoelastic modulator (modulation frequency 50 kHz) coupled to a Glan–Taylor analyzer and collected with a photomultiplying tube (Hamamtsu H7732– 01), connected to a lock-in amplifier. The amplitude of the signal at the second harmonic of the modulation frequency is recorded as a function of the external magnetic field, in order to measure hysteresis cycles. It is possible to select any plane
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External Magnetic Field [Gauss] FIG. 2. (Color online) Hysteresis cycles representing the externally applied magnetic-field dependence of the magnetization components along (a) the in-plane x-axis (parallel to the magnetic field), and (b) the in-plane y-axis perpendicular to the x-axis. The sample is oriented to have the magnetic field between the hard [110] and the easy [100] axes. The two components have been measured in longitudinal MOKE geometry. The insets in (a) and (b) give a sketch of the measurement configuration. (c) Diagram showing the orientation of the sample magnetic moment M with respect to the coordinated axes and the in-plane crystallographic directions. The panels in (c) refer to the magnetic field at correspondence with the dotted vertical lines.
of incidence by a combination of the azimuthal and horizontal angle of the plate. Following this procedure, we have recorded hysteresis cycles on a 20 nm-thick Fe/MgO(001) single crystal sample, which is characterized by a very strong in-plane magnetic anisotropy with [100] and [010] as easy axes. By using collimated-beam MOKE we have not been able to measure any sign of uniaxial anisotropy, meaning that the two easy (hard) axes are basically equivalent to each other. The external field is applied between the hard [110] and the easy [100] direction. This geometry has been chosen in order to have a nonvanishing in-plane magnetization component perpendicular to the applied field during the magnetization reversal cycle. The integration time for each point is 200 ms, and the resulting hysteresis loops shown in Fig. 2 are averaged over 10 scans. In accordance with Ref. 15, we can write the sample reflectivity matrix r as follows:16 r=
r pp + α M⊥ β M|| + γ Mz
−β M|| + γ Mz rss
,
(1)
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where rpp and rss are the Fresnel reflection coefficients for p- and s-polarized light, while α and β are complex coefficients accounting for the geometry of the setup and the sample magnetic properties. M and M⊥ are the in-plane magnetization components parallel and perpendicular to the scattering plane, respectively, while Mz is the component perpendicular to the surface, which can be neglected since no out-of-plane magnetization is present in our thin-film Fe single-crystal samples. By using p-polarized light and setting the analyzer angle near extinction we maximize the sensitivity of the setup with respect to the off-diagonal terms in r , which are proportional to M , and we minimize at the same time the contribution from M⊥ . Figure 2(a) shows the typical MOKE hysteresis cycle with the scattering plane oriented along the external magnetic field, which defines the x direction (see inset). In this way we measure the Mx component of the sample magnetization. By simply rotating the scattering plane it is possible to measure in the same way the other component, My , of the magnetization, the one perpendicular to the external field [Fig. 2(b)]. The applied magnetic field is measured with an in situ Hall probe. The results show the typical features and shape characteristic of the magnetization reversal of a system displaying biaxial magnetic anisotropy, as also shown on a similar sample by Carpene et al., in a collimated beam MOKE experiment.17 The data reported in Figs. 2(a) and 2(b) allow reconstructing how the sample magnetization vector reverses as a function of the magnetic field, as sketched in Fig. 2(c). Compared with Refs. 8 and 17, our method gives the advantage that no post processing of the acquired cycles is necessary, giving directly the desired component of the magnetization vector. The optical resolution of our setup is evaluated by collecting a magnetic image of a TbFeCo film where out-ofplane magnetic domains had previously been recorded. The image displayed in Fig. 3(a) has been obtained in the same geometry employed to measure the hysteresis cycle reported in Fig. 2(a). From Eq. (1), one can see that this geometry is sensitive both to the longitudinal in-plane component M of the sample magnetization [the one measured during the hysteresis cycle plotted in Fig. 2(a)] and to the component perpendicular to the sample surface Mz [the one responsible for the magnetic contrast shown in Fig. 3(a)].18 The lateral dimension of the light spot on the sample surface has been evaluated by fitting line profiles obtained from the magnetic contrast map with the convolution between a step function (the wall between the two domains, with a thickness d 100 nm, is much thinner than the expected spatial resolution19 ) and a Gaussian-shaped beam. The small value of the domain wall thickness with respect to the spatial resolution also justifies that fact that no features associated to the domain wall have been detected. The fitted function allows extracting the FWHM size of the beam. As it can be expected, the partial filling of the objective lens induces aberrations, making the laser spot elliptical in his focal plane. Indeed, as shown in Figs. 3(b) and 3(c), the beam size measured along the two axes is slightly different, being 600 nm along the direction perpendicular to the scattering plane, and 800 nm along the direction parallel to it.
Rev. Sci. Instrum. 82, 023709 (2011)
FIG. 3. (Color online) (a) Magnetic mapping of an area of the TbFeCo sample where opposite out-of-plane magnetic domains had previously been recorded. The scattering plane is parallel to the x-axis. (b) and (c) Line profiles evaluated along the yellow lines in (a). The resolution has been evaluated by fitting the data with the convolution between a step function and a Gaussian beam. The extrapolated FWHM size of the light spot in the microscope focal plane is indicated in (b) and (c) by the vertical dashed lines. The spots in the brighter domain are surface defects.
In conclusion, we have presented a microscopy setup based on a commercial scanning microscope and employing phase modulation and a shared-objective geometry to perform MOKE measurements able to separate the contribution of the in-plane components of the sample magnetization. We have demonstrated that such an instrument is able to retrieve information about all the components of the magnetization of the sample with a lateral resolution that can be as low as 600 nm. The authors acknowledge A. Kimel for providing the TbFeCo sample, P. Biagioni and E. Carpene for fruitful discussions. Financial support from the European Union (EU) Nano Sci- European Research Associates (ERA) project FENOMENA, EU-grant UltraMagnetron (UltraMagnetron Grant No. NMP3-SL-2008-214469), Fondazione Cariplo IMMAGINA (Grant No. Rif. 2008.2412), and PONDER (Grant No. Rif. 2009.2726), is gratefully acknowledged. 1 G. 2 G.
Binning, C. Quate, and C. Geber, Phys. Rev. Lett. 56, 930 (1986). A. Jones, J. Magn. Magn. Mater. 8, 263 (1978).
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E. Hale, H. W. Fuller, and H. Rubinstein, J. Appl. Phys. 30, 789 (1959). 4 A. Oral, S. J. Bending, and M. Henini, J. Vac. Sci. Technol. B 14, 1202 (1996). 5 E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot, Phys. Rev. Lett. 76, 4250 (1996). 6 M. Cormier, J. Ferré, A. Mougin, J.-P. Cromières, and V. Klein, Rev. Sci. Instrum. 79, 033706 (2008). 7 C. H. Wang and Z. Yang, Rev. Sci. Instrum. 80, 073107 (2009). 8 P. Vavassori, Appl. Phys. Lett. 77, 1605 (2000). 9 R. P. Cowburn, D. K. Koltsov, A. O. Adeyeye, and M. E. Welland, Appl. Phys. Lett. 73, 3947 (1998). 10 J. Unguris, R. J. Celotta, and D. T. Pierce, Phys. Rev. Lett. 79, 2734, (1997). 11 C. Nistor, G. Beach, and J. Erskine, Rev. Sci. Instrum. 77, 103901 (2006). 12 A. Green and M. Prutton, J. Sci. Instrum. 39, 244 (1962).
Rev. Sci. Instrum. 82, 023709 (2011) 13 P. S. Keatley, V. V. Kruglyak, R. J. Hicken, J. R. Childress, and J. A. Katine,
J. Magn. Magn. Mater. 306, 298 (2006). Koopmans, P. V. Santos, and M. Cardona, Phys. Status Solidi A 170, 307 (1998). 15 S. Polisetty, J. Scheffler, S. Sahoo, Y. Wang, T. Mukherjee, X. He, and C. Binek, Rev. Sci. Instrum. 79, 055107 (2008). 16 R. Hunt, J. Appl. Phys. 38, 1652 (1967). 17 E. Carpene, E. Mancini, C. Dallera, E. Puppin, and S. De Silvestri, J. Appl. Phys. 108, 063919 (2010). 18 There are several ways to separate the two components in the case they are both present at the same time. One possibility would be to record an image in polar configuration, which is sensitive just to the out-of-plane component. The magnetic contrast in Fig. 3 could have been obtained in polar geometry. This would give a better resolution than the extrapolated one, but here the aim is to characterize the resolution associated to the measure of the in-plane components. 19 M. Savoini, Ph.D. thesis, Politecnico di Milano, Milano, Italy (2010). 14 B.
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