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Magneto-optical spectroscopy of surface/interfaces in Co/garnet heterostructures M. Pashkevich a,c , A. Stupakiewicz a,∗ , A. Kirilyuk b , A. Stognij c , A. Maziewski a , Th. Rasing b a b c
Laboratory of Magnetism, Faculty of Physics, University of Bialystok, Lipowa 41, Bialystok, Poland Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, Nijmegen, The Netherlands Scientific-Practical Materials Research Centre of the NASB, P. Brovki 19, Minsk, Belarus
a r t i c l e
i n f o
Article history: Received 4 February 2014 Accepted 28 February 2014 Available online xxx Keywords: Second harmonic generation Garnet Magnetic anisotropy Cobalt
a b s t r a c t Here we report on combined linear and nonlinear magneto-optical studies of Co/garnet heterostructures. Both the crystallographic symmetry and the magnetization of buried Co/garnet interfaces were deduced from an analysis of the rotational anisotropy of spectroscopic magnetization-induced second-harmonic generation (MSHG) measured in the transverse (Voigt) geometry. Unexpected non-zero components of the magnetic MSHG tensor were found, that do not correspond to the original 4mm point group symmetry of (0 0 1)-plane oriented garnet layers. They may be tentatively assigned to the anisotropic distribution of the Co impurities. For Co/garnet interfaces, the magnetic MSHG contrast shows a maximum at the fundamental wavelength of about 890 nm which can be correlated with Faraday rotation spectra at the second harmonic frequency. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, the interest to gain a better understanding of magnetism at surfaces and interfaces have triggered the development of new approaches to study local magnetic phenomena with high spatial and temporal resolution. These advances have been partly motivated by the high technological relevance of surface and interface phenomena in areas such as spintronics or magnetic storage. On the other hand, many new interesting phenomena were discovered. For example, the recent reports of spin-wave excitation, spin-current Hall and Seebeck effects in yttrium iron garnet (YIG)based heterostructures [1–3] have attracted a lot of experimental and theoretical interest in such systems. Ferrimagnetic garnets are a well-known group of thin films characterized by an interesting combination of magnetic and magneto-optical properties [4,5]. These properties can be tailored to a large degree by substitutions of either magnetic or nonmagnetic ions in the garnet films. Interesting light-induced behavior of magnetic effects have been observed in Bi-Lu- and Co-substituted YIG films, such as the inverse Faraday effect [6] and photomagnetism at room temperature [6,7]. Recent experiments have furthermore demonstrated that a combination of thin
∗ Corresponding author. Tel.: +48 857457228. E-mail address: [email protected] (A. Stupakiewicz).
metal layers and dielectric garnet films leads to new effects, which can be strongly dependent on the interface/surface properties. For example, localized surface plasmons resulted in an enhanced magneto-optical response in a patterned Au/YIG heterostructure [8]. A modification of the magnetic anisotropy and the domain structure in garnet films was revealed after the deposition of ultrathin ferromagnetic layers (Fe, Co, Py) [9–11] due to magnetic coupling. In all these cases, the interface behavior was found to dominate the properties of the coupling. Therefore, a detailed study of the magnetic behavior of such interfaces is required for the understanding of the magnetic parameters of the heterostructures. In the past decades, the surface and interface sensitive magneto-optical technique of optical second harmonic generation (SHG) has been developed. This technique combines an extreme surface/interface sensitivity with giant magneto-optical effects [12,13], and is therefore particularly suitable for studies of interface-related magnetic properties. The interface sensitivity originates from a disappearance of electric-dipole SHG in centrosymmetric media. In the presence of a magnetization, additional SHG contributions appear, resulting in a magnetizationsensitive SHG (MSHG) response. Note that the interface sensitivity is preserved in magnetic media, because an axial (magnetization) vector does not change sign at the inversion operation. In the dipole approximation the nonlinear optical response yielding the SHG intensity I2ω ∼ |P(2ω)|2 is given by the polarization Pi (2ω) = ijk (2ω)Ej Ek . Since ijk is a polar tensor of rank 3, it
http://dx.doi.org/10.1016/j.apsusc.2014.02.185 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Pashkevich, et al., Magneto-optical spectroscopy of surface/interfaces in Co/garnet heterostructures, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.02.185
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vanishes in centrosymmetric media. Only from surfaces and interfaces, where the inversion symmetry is broken, SHG arises, thus resulting in the well-known interface sensitivity of SHG [14]. The number of non-zero tensor components depends on the crystallographic and magnetic symmetry of the sample [13,15]. In addition we should note that the use of MSHG spectroscopy can give the possibility to separate different contributions from the various interfaces/surfaces in metal/dielectric heterostructures [16]. In addition, the strong absorption of the garnet film at the second harmonic frequency provides a possibility to study the metal/garnet interface and garnet/substrate interface separately, by illuminating it from either the substrate or surface side, respectively [6]. Although the SHG process is very different from linear optical absorption, a certain correspondence between optical absorption and the SHG intensity was found in a spectroscopic study of garnet films [17]. The influence of the symmetry on the MSHG rotational anisotropy in different garnets has been investigated in detail [18]. In particular, several experiments for (0 0 1)-plane oriented garnets with 4mm point group symmetry showed the independence of the SHG intensity on the magnetization direction in non-magnetic substituted garnet films [6,18,19]. Generally, the magnetism in pure or non-magnetic substituted garnet systems is determined by the anisotropic Fe3+ ions [4]. In the case of Co-substituted garnets, new magnetic effects have been found, such as light-induced magnetic anisotropy in quasistatic [7] and ultrafast dynamics [20] studies. Even though qualitatively similar in behavior, the dynamic modification of the anisotropy differed from the static one, raising questions about their origins. The physical nature of these effects is related to light-induced charge transfer from Co impurity centers to the neighboring Fe ions in the garnet lattice, which are responsible for the magnetic anisotropy. The band gap of Co-doped garnets is about 2.9 eV and the energy levels of the Co ions do not coincide with the 3d level of the Fe ions [4,5]. Therefore one can expect that optical excitation leads to effective redistributions of Co and Fe ions between different positions, which can lead to magnetic effects with spectral sensitivity. Additionally, one can expect that the MSHG rotational anisotropy at Co-doped YIG interfaces may be different in comparison with non-magnetic substituted or pure garnets. In this paper we report experimental studies of both linear and nonlinear magneto-optical effects in Co/garnet heterostructures. Significant changes of the MSHG intensity in Co-substituted garnet films after deposition of an ultrathin Co layer are observed in the fundamental wavelength range from 760 to 920 nm. Changes of the sign and value of magnetic contrast on the interface at about 830 nm and 890 nm, respectively were deduced from the analysis of the spectral MSHG azimuthal dependences for garnet-only and Co/garnet films. In the case of (0 0 1)-plane oriented garnet films we found non-zero components of the magnetic MSHG tensor with a maximum magnetic contrast at about 890 nm for the fundamental wavelength. This indicates anisotropic distribution of the Co substitution in the otherwise symmetric crystallographic structure. For 1.8 m garnet films after 2 nm Co deposition we observe a 45◦ shift of the maxima of SHG intensity between [1 0 0] and [1 1 0] orientations, which means a sudden resonance-like change of the non-magnetic SHG components. The paper is organized as follows: sample preparation and sample characterization are given in Section 2. In Section 3 we present the experimental results and analysis obtained from linear Kerr/Faraday and nonlinear MSHG magneto-optical techniques, and finally one can find the conclusions in Section 4. 2. Sample characterization Our experimental studies were performed on Co/garnet heterostructures and reference bare garnet films. The initial 5.8 m
thick garnet film of Y2 CaFe3.9 Co0.1 GeO12 composition was grown by liquid phase epitaxy on a (0 0 1)-plane oriented paramagnetic gadolinium gallium garnet (GGG) substrate. The miscut angle of the GGG substrate was not larger than 0.1◦ [21]. The lattice parameter mismatch in our garnet samples was a = 9.7 × 10−4 nm or about 0.1% [22]. At room temperature the saturation magnetization was 7 G and the Neel temperature TN = 445 K. One initial garnet sample was thinned down to 1.8 m and surface smoothed by oxygen ion beam etching in the low energy regime. In the next step, an ultrathin 2 nm Co layer was deposited on both the 1.8 m and 5.8 m garnet films by ion beam sputtering [10]. And finally, a 4 nm Au film was used to protect the Co layer from oxidation. Magnetic anisotropy constants of Co and the garnet layers were deduced from ferromagnetic resonance and magnetization reversal measurements. The growth-induced anisotropy constant KUYIG of the garnet was different for the 1.8 m thick films as compared to that of the 5.8 m thick ones: −0.1 × 103 erg/cm3 and 1 × 103 erg/cm3 , respectively. The cubic anisotropy constant for both garnet thicknesses was the same and equal to KC = −2 × 103 erg/cm3 . In the case of garnet films, the easy magnetization axes are along 1 1 1-type directions, corresponding to a fourfold symmetry in the plane. An effective anisotropy constant of the polycrystalline Co layer was Co = −9.9 × 106 erg/cm3 , that corresponds to an easy-plane magKeff netic configuration [10]. 3. Results and discussion The process of magnetization reversal in Co/garnet heterostructures has been studied at room temperature in reflection with the linear magneto-optic Kerr effect (MOKE) and in transmission with the Faraday effect. From the data, we separated different magneto-optical contributions from the Co layer and garnet-only films. Changes of the crystallographic symmetry and magnetization of buried surface/interfaces of such heterostructures were deduced from MSHG dependencies in a wide spectral range of fundamental wavelengths. 3.1. Magneto-optical Faraday and Kerr measurements The MOKE and Faraday magnetometry measurements were performed using light from a mode-locked Ti-sapphire laser (MaiTai HP, Spectra-Physics) operating within the 690–1040 nm range and a repetition rate of 80 MHz. For the detection of the signal (Kerr rotation K ), a lock-in amplifier was used in combination with a standard modulation technique with a photoelastic modulator (PEM-100, Hinds Instruments). The perpendicular magnetization component of the ultrathin Co layer was measured using the polar MOKE (P-MOKE) geometry, with the angle of incidence of the laser light close to the sample normal and the external magnetic field Hz perpendicular to the surface of the sample (see Fig. 1(a)). The measurements of the in-plane magnetization components of the Co layer were performed in the longitudinal MOKE (L-MOKE) geometry, with a 49◦ angle of incidence of the light (see Fig. 1(d)). The magnetic field Hx was applied in the sample plane for various orientations with respect to the garnet [1 0 0] direction. The process of magnetization reversal to determination of the Faraday rotation angle F of the garnet-only films was studied in the magneto-optical Faraday geometry, with perpendicular and in-plane magnetic field orientation (Fig. 1(a) and (d)). 3.2. Linear optical and magneto-optical studies Fig. 1 shows the hysteresis loops obtained with MOKE and Faraday effects in different geometries for the Co/garnet heterostructures. The P-MOKE hysteresis loops observed for the 2 nm thick Co film grown on garnet films of 1.8 m thickness indicate an
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Fig. 1. Hysteresis loops for Co/1.8 m garnet film measured as magneto-optical Kerr rotation in polar (b) and longitudinal (e) geometry and Faraday rotation with magnetic field perpendicular Hz (c) and parallel Hx (f) to the sample plane. Top panels show the experimental configuration with Kerr and Faraday effects for perpendicular (a) and in-plane (d) magnetic field orientations.
in-plane magnetization of Co (see Fig. 1(b)) (the same behavior has been observed for 5.8 m garnet films). This result correlates well with in-plane magnetic anisotropy deduced from FMR experiments [10]. The L-MOKE magnetization curve for the Co layer measured with the in-plane external magnetic field Hx are shown in Fig. 1(e). The shape of this loop is practically independent on the azimuthal sample orientation and confirms the easy plane type of the magnetic anisotropy with a saturation in-plane field of about 0.3 kOe. Fig. 1(c) and (f) shows Faraday rotation hysteresis loops measured for 1.8 m (similar in 5.8 m) thick garnet films and a perpendicular applied field Hz and an in-plane field Hx , respectively. From the hysteresis loop shown in Fig. 1(c), one deduces a Faraday rotation from the garnet layer of about F = 0.08◦ and a paramagnetic linear contribution from the GGG substrate. For the in-plane applied magnetic field in the garnet [1 0 0] direction, the saturating field is about 0.6 kOe. In Fig. 2 both the optical transmittance and magnetooptical Faraday rotation spectra are shown for the 1.8 m and 5.8 m thick garnet films. At photon energies below about 1.5 eV (>800 nm) the absorption is small and equal to about 102 cm−1 . However at higher photon energies >2.5 eV (<500 nm), the absorption is caused by charge transfer transitions between the oxygen and the Fe3+ and Co2+ ions. In such case to separate the contributions from Fe and Co is very difficult [23]. In garnets the exchange interaction between Fe or Co in tetrahedral positions is stronger than that in the octahedral ones, because of a different inter-ion distance [24]. In general case, the optical absorption is correlated with the magneto-optical Faraday rotation. In particular, it was determined that the broad line in the Faraday rotation spectrum is due
to the crystal field transition of the low spin octahedral Co ions [25]. Note, that the transitions in octahedral Co ions have an oscillator strength that is weaker than that for the tetrahedral ones, and in the spectral region under study the transitions in the Fe ions also contribute to the spectrum of Co-substituted YIG [26]. In addition, the inhomogeneous distribution of the Co impurities, combined with the lattice mismatch between Co layer and garnet film and magnetostriction, may lead to a spread of the easy axis direction across
Fig. 2. Transmittance (solid lines) and Faraday rotation (dashed lines) spectra of 1.8 m (red) and 5.8 m (black) garnet films. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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optical absorption (105 –106 cm−1 ), the 4 nm thick Au and 2 nm thick Co layers are thin enough to be transparent (transmittance >65%). Both the 1.8 m and 5.8 m thick garnet films are transparent at the fundamental wavelengths within the measured spectral range, but have a strong absorption at the second harmonic frequencies, as shown in Fig. 2. Such strong absorption of the garnet films at the second harmonic frequency provides a possibility to study the Co/garnet interfaces and YIG/GGG interfaces separately by illuminating it from either the substrate or surface side, respectively [6].
3.4. Interface-sensitivity MSHG spectroscopy
Fig. 3. Rotational anisotropy patterns of the MSHG intensity from Co/1.8 m garnet sample obtained by illumination of the Co side, measured at 860 nm fundamental wavelength.
the sample due to the interface roughness which is significantly contributes to the uniaxial in-plane magnetic anisotropy. 3.3. MSHG measurements The MSHG measurements were performed using the same femtosecond laser system as described above. The laser beam was focused onto a 40 m diameter spot with an average power of about 50 and 120 mW for Co/garnet and garnet-only films, respectively. It was verified, that such optical power does not affect the magnetic behavior. The transmitted SHG signal at normal incidence of the laser light was detected by a cooled photomultiplier (Hamamatsu) using a photon counting technique after filtering out the fundamental wavelength. We applied an in-plane magnetic field Hy of about 1.5 kOe in a transverse magneto-optical configuration (see Fig. 3) which saturates the in-plane magnetization in our samples. The MSHG intensity was recorded for various input–output polarization combinations (Xin Xout , Xin Yout , Yin Xout , Yin Yout ), at various azimuthal angles within 0–360◦ range. This angle was measured from the crystallographic [1 0 0] direction. In addition, MSHG hysteresis curves for selected values of azimuthal angles and polarization combinations were recorded. The comparison of the MSHG intensity measured for different magnetic field orientations, light polarization combinations and wavelength, enables to separate the crystallographic and magnetic contributions coming from either the surface or from the interface of the Co/garnet heterostructures. In spite of the strong
The azimuthal dependences of the MSHG intensity measured in the Co/1.8 m garnet sample for different polarization combinations are shown in Fig. 3. The magnetic contribution in MSHG is observed as a difference in the SHG intensity for two opposite directions of the applied magnetic field Hy . In the case of the fourfold symmetry, the magnetic contribution to SHG should actually be zero because of symmetry reasons, as reported in garnet films with non-magnetic substituted components, such as (YbPr)3 (FeGa)5 O12 [19] and Lu2.5 Bi0.5 Fe5 O12 [6]. However, in our samples the magnetic contrast is clearly visible. In principle, the lattice mismatch [27] and miscut of the substrate can lead to a macroscopic symmetry breaking and therefore an appearance of a bulk magnetic contribution to the SHG intensity. However, both these parameters in our samples are very small [22] as compared to the undoped garnets. We thus should assume that it is the Co impurity of the garnet film that is the main factor of the magnetic contribution to the MSHG azimuthal anisotropy. The transmission spectra show strong optical absorption in the = 400–500 nm range (see Fig. 2). In the 5.8 m film, this would lead to a total extinction of the transmitted light at these wavelengths and allows distinguishing the SHG responses from the garnet surface or from the garnet/substrate interface, by choosing the direction of the light incidence. In contrast, for the 1.8 m garnet thickness the transmittance is between 3% and 50% within the measured wavelength range as shown in Fig. 2. In this spectral range for the 1.8 m garnet we can distinguish the contributions from the surface and/or interface by tuning the wavelength. Therefore, in the next part of the paper we will represent mainly the results obtained on the Co/1.8 m garnet heterostructure. For illumination from the substrate side of the Co/garnet sample in the Yin Xout polarization combination, the recorded MSHG azimuthal dependences are shown in Fig. 4. In general, these dependences confirm the (0 0 1) garnet orientation, with somewhat distorted 4mm point group symmetry. Interestingly, the rotational anisotropy pattern rotates 45◦ in the wavelength range between 800 nm and 900 nm, so that the main peaks of the intensity turn from the 1 0 0 to the 1 1 0 direction. Such rotation could be connected with different balance of surface/interface contributions. For the comparison of these contributions, illumination from both the sample surface and substrate side has further been used. The MSHG intensity as a function of the applied magnetic field, for Co/garnet and garnet only films are shown in Fig. 5, measured for the magnetic field applied along the [1 0 0] and [1 1 0] garnet crystallographic axes. A good agreement between these SHG intensities and the magnetization reversal process deduced from both Faraday and Kerr rotation (see Fig. 1) of the Co/garnet heterostructure is observed. For the garnet only film the saturation field of the SHG intensity curve is about 0.6 kOe (Fig. 5(a)), which is exactly the same as obtained by Faraday rotation from Fig. 1(f). The easy magnetization axis in the garnet is along the 1 1 1 direction, therefore for Hy || [1 1 0] the change of the SHG intensity is practically negligible. However for Hy || [1 0 0], along the hard axis, a coherent rotation of
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relative changes in the MSHG intensity occur are determined from the MSHG magnetic contrast: (ϕ) =
I2ω (+M) − I2ω (−M) I2ω (+M) + I2ω (−M)
(1)
where I2ω (±M) is the MSHG intensity for opposite orientations of the magnetization M. For both 1.8 m and 5.8 m thick garnet films the magnetic contrast from the garnet/substrate interface was measured at ϕ = 90◦ . At the fundamental wavelength of about 890 nm the magnetic contrast shows maxima (see Fig. 6(a)) that are roughly in agreement with the maxima of the Faraday spectra close to 445 nm (Fig. 1). At the fundamental wavelength of about 800 nm, a slight increase of was found due to the surface contribution from the 1.8 m garnet films (Fig. 6(a)). For the 1.8 m garnet films an SHG intensity maximum is observed at about 860 nm fundamental wavelength (Fig. 6(b)). However, for 5.8 m garnet films, Fig. 6(b) shows SHG intensity maxima at the fundamental wavelengths of 820 nm and 860 nm. Note that in this case, the SHG response comes purely from the surface of the garnet, which is obtained by illuminating the sample from the substrate side. The observed spectra show a good correlation with the strong optical absorption at the crystal field transitions of Co3+ and Fe3+ ions in octahedral positions at 410 nm, 416 nm, and 432 nm [23]. For a more quantitative discussion, the MSHG azimuthal dependences shown in Fig. 4 were fitted by the following formula:
2
I2ω (ϕ, ±−→ M) = A4 sin(4ϕ + ϕ4 ) + A1 (ϕ + ϕ1 ) + A0
Fig. 4. Rotational anisotropy patterns of the MSHG intensity with (right column) and without (left column) Co layer on 1.8 m garnet films obtained by illumination of the substrate side for Yin Xout polarization combination, measured at different fundamental wavelengths. The solid lines represent the fits to the measured data points. Multiplication factors scaling the MSHG intensity data with respect to garnet surface measured at 795 nm are shown in the plots.
magnetization is observed. In contrast, for the Co/garnet interface (saturation field of SHG is about 0.3 kOe) the independent behavior of the SHG intensity on magnetic field directions is correlated with the in-plane magnetic anisotropy of the Co layer and the magnetization process shown in Fig. 1(e). The values for which the largest
(2)
The A1 and A4 coefficients correspond to the one- and fourfold symmetry components, that arise from the general 4-fold symmetry of the (0 0 1) surface, and the distortion from it, respectively. Only the A1 coefficient is taken as dependent on the magnetization direction. We have verified that possible contributions of two- and threefold symmetry are negligible. The phases ϕi used from the fits were separately fixed within the whole wavelength range. The A0 coefficient corresponds to an isotropic contribution and is not sensitive to the crystallographic symmetry of any interfaces of the measured samples. The fitted coefficients derived for both Co/1.8 m garnet interface and reference garnet surfaces are plotted as a function of the fundamental wavelength in Fig. 6(c) and (d). For the 1.8 m garnet surface in Fig. 6(c), the A1 coefficient is close in value, but opposite in sign for opposite magnetic field directions, and changes sign at about 830 nm. Note that the same phase ϕ1 is obtained at these fittings. These dependences of A1 for M+ and M− are symmetric with respect to the horizontal zero axis (Fig. 6(c)). However, for the Co/1.8 m garnet interface, the asymmetrical dependence of A1 is visible (see Fig. 6(d)). For 5.8 m garnet films with and without Co layer, practically the same behavior of the A1
Fig. 5. SHG intensity curves of 1.8 m garnet (a) and Co/1.8 m garnet (b) samples measured for 840 nm wavelength fundamental light and Xin Xout polarization combination for [1 0 0] (open points) and [1 1 0] (full points) garnet crystallographic orientation. The sample was illuminated from the substrate side.
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Fig. 6. Spectral plots of magnetic contrast (a) and normalized MSHG intensity (b) for garnet/substrate interface for Xin Xout polarization combination as a function of fundamental wavelength. Spectral dependences of coefficients A1 and A4 , obtained from the fits of the rotational anisotropy data for Yin Xout polarization combination for 1.8 m garnet surface (c) and Co/1.8 m garnet interface (d). The sample was illuminated from the substrate side.
coefficients on function of the fundamental wavelength has been observed. For the Co/1.8 m garnet film, the A4 coefficient changes sign at about 900 nm, which for the garnet surface occurs at about 820 nm of fundamental wavelength. Note that for the Co/1.8 m garnet film the A4 parameter above 840 nm is approximately one order of magnitude smaller than for the 1.8 m garnet film (Fig. 6(c) and (d)). This difference could arise from the presence of the interface modification of the garnet by the Co cover layer. In this case the Co layer has induced a 45◦ -shift between the 1 0 0 and 1 1 0 orientations of the SHG rotational anisotropy, due to the optical transitions between Co ions in tetra- and octahedral positions. This behavior can be clearly seen by a comparison of the MSHG azimuthal dependences of Co/1.8 m garnet and Co/5.8 m garnet interfaces,
Fig. 7. SHG rotational anisotropy of Co/5.8 m garnet (a) and Co/1.8 m garnet (b) heterostructures obtained by illumination of the sample from the Co side for Yin Xout polarization combination at 840 nm fundamental wavelength. Multiplication factors scaling the SHG intensity data with respect to the Co/5.8 m sample are shown in the plot.
shown in Fig. 7. In this case, the lattice mismatch between the garnet and the substrate should be relaxed at the surface of the garnet film. Thus, for the Co/garnet interface the probability of Co ions in octahedral positions is much higher than at the garnet/substrate interface. 4. Conclusions We have studied ultrathin Co layer on Co-substituted YIG/GGG(0 01) heterostructures by linear Faraday and Kerr and nonlinear MSHG spectroscopy within a wide spectral range. Different polarization combinations of the MSHG in the Voigt geometry show normally forbidden non-zero magnetic contribution due to the Co impurities in the YIG crystals with 4mm point group symmetry. This contribution reaches a maximum at about 890 nm of fundamental wavelength that is correlated with a Faraday rotation maximum at about 445 nm. Therefore the Faraday spectra at the SHG frequency are shown to play an important role in the formation of the SHG anisotropic azimuthal dependences. The spectral MSHG rotational anisotropy could be interpreted as due to the influence of electronic magnetic impurities, such as Co dopants, on either surface or interface in the Co/garnet heterostructures. We also observe a 45◦ rotation of the MSHG rotational anisotropy pattern from 1 0 0 to 1 1 0 orientation after depositing the Co layer on the 1.8 m garnet films. We assume that this rotation of the anisotropy pattern has been induced by the optical transitions between octahedral and tetrahedral positions in the garnet. Depending on the photon energy on the second-harmonic frequency, these transitions may give rise to optical absorption at magnetic impurities that is very strong at interfaces close to 445 nm of fundamental wavelength. These results will help to understand the mechanism of the ultrafast light-induced anisotropy [20] which is assumed to
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be related to the redistribution of the Co ions between tetrahedral positions (that corresponds to the 1 0 0 directions of the garnet film). Thus, MSHG spectroscopy method could also be a powerful tool to study the ultrafast dynamics of light-induced anisotropy in the Co/YIG heterostructures. Acknowledgment This work was supported by the National Science Centre Poland for OPUS project DEC-2013/09/B/ST3/02669. References [1] H. Kurebayashi, O. Dzyapko, V. Demidov, D. Fang, A.J. Ferguson, S.O. Demokritov, Nat. Mater. 10 (2011) 660. [2] Y. Kajiwara, K. Harii, S. Takahashi, J. Ohe, K. Uchida, M. Mizuguchi, H. Umezawa, H. Kawai, K. Ando, K. Takanashi, S. Maekawa, E. Saitoh, Nature 464 (2010) 262. [3] K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G.E.W. Bauer, S. Maekawa, E. Saitoh, Nat. Mater. 9 (2010) 894. [4] G. Winkler, Magnetic Garnets, Friedr. Vieweg & Sohn, Braunschweig, 1981. [5] Landolt-Börnstein, Numerical Data and Functional Relationships in Science and Technology, NewSeries, Group III , 27/e, Springer-Verlag, Berlin, 1991. [6] F. Hansteen, O. Hunderi, T.H. Johansen, A. Kirilyuk, T. Rasing, Phys. Rev. B 70 (2004) 094408. [7] A.B. Chizhik, I.I. Davidenko, A. Maziewski, A. Stupakiewicz, Phys. Rev. B 57 (1998) 14366.
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[8] V.I. Belotelov, I.A. Akimov, M. Pohl, V.A. Kotov, S. Kasture, A.S. Vengurlekar, A. Venu Gopal, D.R. Yakovlev, A.K. Zvezdin, M. Bayer, Nat. Nanotechnol. (2011) 370. [9] Y.S. Chun, K.M. Krishnan, J. Appl. Phys. 95 (2004) 6858. [10] M. Pashkevich, A. Stupakiewicz, A. Kirilyuk, A. Maziewski, A. Stognij, N. Novitskii, A. Kimel, Th. Rasing, J. Appl. Phys. 111 (2012) 023913. [11] N. Vukadinovic, J. Ben Youssef, V. Castel, M. Labrune, Phys. Rev. B 79 (2009) 184405. [12] A. Kirilyuk, J. Phys. D: Appl. Phys. 35 (2002) R1–R19. [13] A. Kirilyuk, Th. Rasing, J. Opt. Soc. Am. B 22 (2005) 148. [14] Y.R. Shen, The Principles of Nonlinear Optics, Wiley, New York, 1984. [15] W. Cheikh-Rouhou, L.C. Sampaio, B. Bartenlian, P. Beauvillain, A. Brun, J. Ferré, P. Georges, J.P. Jamet, V. Mathet, A. Stupakewicz, Appl. Phys. B 74 (2002) 665. [16] A. Stupakiewicz, A. Kirilyuk, A. Fleurence, R. Gieniusz, T. Maroutian, P. Beauvillain, A. Maziewski, Th. Rasing, Phys. Rev. B 80 (2009) 094423. [17] V.V. Pavlov, R.V. Pisarev, A. Kirilyuk, Th. Rasing, J. Appl. Phys. 81 (1997) 4631. [18] V.N. Gridnev, V.V. Pavlov, R.V. Pisarev, A. Kirilyuk, Th. Rasing, Phys. Rev. B 63 (2001) 184407. [19] V.V. Pavlov, R.V. Pisarev, A. Kirilyuk, Th. Rasing, Phys. Rev. Lett. 78 (1997) 2004. [20] F. Atoneche, A.M. Kalashnikova, A.V. Kimel, A. Stupakiewicz, A. Maziewski, A. Kirilyuk, Th. Rasing, Phys. Rev. B 81 (2010) 214440. [21] A. Maziewski, J. Magn. Magn. Mater. 88 (1990) 325. ˇ sová, W. Andrä, W. Schüppel, P. Sumˇ ˇ [22] P. Görnert, M. Nevˇriva, J. Simˇ sál, R. Bubáková, Phys. Status Solidi A 74 (1982) 107. [23] D.L. Wood, J.P. Remeika, J. Chem. Phys. 46 (1967) 3595; D.L. Wood, J.P. Remeika, J. Appl. Phys. 38 (1967) 1038. [24] S. Visnovsky, R. Krishnan, Appl. Phys. 18 (1979) 243. ˇ sa, J. Simˇ ˇ sová, P. Görnert, A. Maziewski, Acta Phys. Pol. A 76 (1989) 277. [25] Z. Simˇ [26] W. Wettling, B. Andlauer, P. Koidl, J. Schneider, W. Tolksdorf, Phys. Status Solidi B 559 (1973) 63. [27] I.L. Lyubchanskii, J.-W. Jeong, S.-C. Shin, N.N. Dadoenkova, M.I. Lyubchanskii, Th. Rasing, J. Appl. Phys. 87 (2000) 6794.
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Magneto-optical spectroscopy of surface/interfaces in Co/garnet heterostructures M. Pashkevich a,c , A. Stupakiewicz a,∗ , A. Kirilyuk b , A. Stognij c , A. Maziewski a , Th. Rasing b a b c
Laboratory of Magnetism, Faculty of Physics, University of Bialystok, Lipowa 41, Bialystok, Poland Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, Nijmegen, The Netherlands Scientific-Practical Materials Research Centre of the NASB, P. Brovki 19, Minsk, Belarus
a r t i c l e
i n f o
Article history: Received 4 February 2014 Accepted 28 February 2014 Available online xxx Keywords: Second harmonic generation Garnet Magnetic anisotropy Cobalt
a b s t r a c t Here we report on combined linear and nonlinear magneto-optical studies of Co/garnet heterostructures. Both the crystallographic symmetry and the magnetization of buried Co/garnet interfaces were deduced from an analysis of the rotational anisotropy of spectroscopic magnetization-induced second-harmonic generation (MSHG) measured in the transverse (Voigt) geometry. Unexpected non-zero components of the magnetic MSHG tensor were found, that do not correspond to the original 4mm point group symmetry of (0 0 1)-plane oriented garnet layers. They may be tentatively assigned to the anisotropic distribution of the Co impurities. For Co/garnet interfaces, the magnetic MSHG contrast shows a maximum at the fundamental wavelength of about 890 nm which can be correlated with Faraday rotation spectra at the second harmonic frequency. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, the interest to gain a better understanding of magnetism at surfaces and interfaces have triggered the development of new approaches to study local magnetic phenomena with high spatial and temporal resolution. These advances have been partly motivated by the high technological relevance of surface and interface phenomena in areas such as spintronics or magnetic storage. On the other hand, many new interesting phenomena were discovered. For example, the recent reports of spin-wave excitation, spin-current Hall and Seebeck effects in yttrium iron garnet (YIG)based heterostructures [1–3] have attracted a lot of experimental and theoretical interest in such systems. Ferrimagnetic garnets are a well-known group of thin films characterized by an interesting combination of magnetic and magneto-optical properties [4,5]. These properties can be tailored to a large degree by substitutions of either magnetic or nonmagnetic ions in the garnet films. Interesting light-induced behavior of magnetic effects have been observed in Bi-Lu- and Co-substituted YIG films, such as the inverse Faraday effect [6] and photomagnetism at room temperature [6,7]. Recent experiments have furthermore demonstrated that a combination of thin
∗ Corresponding author. Tel.: +48 857457228. E-mail address: [email protected] (A. Stupakiewicz).
metal layers and dielectric garnet films leads to new effects, which can be strongly dependent on the interface/surface properties. For example, localized surface plasmons resulted in an enhanced magneto-optical response in a patterned Au/YIG heterostructure [8]. A modification of the magnetic anisotropy and the domain structure in garnet films was revealed after the deposition of ultrathin ferromagnetic layers (Fe, Co, Py) [9–11] due to magnetic coupling. In all these cases, the interface behavior was found to dominate the properties of the coupling. Therefore, a detailed study of the magnetic behavior of such interfaces is required for the understanding of the magnetic parameters of the heterostructures. In the past decades, the surface and interface sensitive magneto-optical technique of optical second harmonic generation (SHG) has been developed. This technique combines an extreme surface/interface sensitivity with giant magneto-optical effects [12,13], and is therefore particularly suitable for studies of interface-related magnetic properties. The interface sensitivity originates from a disappearance of electric-dipole SHG in centrosymmetric media. In the presence of a magnetization, additional SHG contributions appear, resulting in a magnetizationsensitive SHG (MSHG) response. Note that the interface sensitivity is preserved in magnetic media, because an axial (magnetization) vector does not change sign at the inversion operation. In the dipole approximation the nonlinear optical response yielding the SHG intensity I2ω ∼ |P(2ω)|2 is given by the polarization Pi (2ω) = ijk (2ω)Ej Ek . Since ijk is a polar tensor of rank 3, it
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vanishes in centrosymmetric media. Only from surfaces and interfaces, where the inversion symmetry is broken, SHG arises, thus resulting in the well-known interface sensitivity of SHG [14]. The number of non-zero tensor components depends on the crystallographic and magnetic symmetry of the sample [13,15]. In addition we should note that the use of MSHG spectroscopy can give the possibility to separate different contributions from the various interfaces/surfaces in metal/dielectric heterostructures [16]. In addition, the strong absorption of the garnet film at the second harmonic frequency provides a possibility to study the metal/garnet interface and garnet/substrate interface separately, by illuminating it from either the substrate or surface side, respectively [6]. Although the SHG process is very different from linear optical absorption, a certain correspondence between optical absorption and the SHG intensity was found in a spectroscopic study of garnet films [17]. The influence of the symmetry on the MSHG rotational anisotropy in different garnets has been investigated in detail [18]. In particular, several experiments for (0 0 1)-plane oriented garnets with 4mm point group symmetry showed the independence of the SHG intensity on the magnetization direction in non-magnetic substituted garnet films [6,18,19]. Generally, the magnetism in pure or non-magnetic substituted garnet systems is determined by the anisotropic Fe3+ ions [4]. In the case of Co-substituted garnets, new magnetic effects have been found, such as light-induced magnetic anisotropy in quasistatic [7] and ultrafast dynamics [20] studies. Even though qualitatively similar in behavior, the dynamic modification of the anisotropy differed from the static one, raising questions about their origins. The physical nature of these effects is related to light-induced charge transfer from Co impurity centers to the neighboring Fe ions in the garnet lattice, which are responsible for the magnetic anisotropy. The band gap of Co-doped garnets is about 2.9 eV and the energy levels of the Co ions do not coincide with the 3d level of the Fe ions [4,5]. Therefore one can expect that optical excitation leads to effective redistributions of Co and Fe ions between different positions, which can lead to magnetic effects with spectral sensitivity. Additionally, one can expect that the MSHG rotational anisotropy at Co-doped YIG interfaces may be different in comparison with non-magnetic substituted or pure garnets. In this paper we report experimental studies of both linear and nonlinear magneto-optical effects in Co/garnet heterostructures. Significant changes of the MSHG intensity in Co-substituted garnet films after deposition of an ultrathin Co layer are observed in the fundamental wavelength range from 760 to 920 nm. Changes of the sign and value of magnetic contrast on the interface at about 830 nm and 890 nm, respectively were deduced from the analysis of the spectral MSHG azimuthal dependences for garnet-only and Co/garnet films. In the case of (0 0 1)-plane oriented garnet films we found non-zero components of the magnetic MSHG tensor with a maximum magnetic contrast at about 890 nm for the fundamental wavelength. This indicates anisotropic distribution of the Co substitution in the otherwise symmetric crystallographic structure. For 1.8 m garnet films after 2 nm Co deposition we observe a 45◦ shift of the maxima of SHG intensity between [1 0 0] and [1 1 0] orientations, which means a sudden resonance-like change of the non-magnetic SHG components. The paper is organized as follows: sample preparation and sample characterization are given in Section 2. In Section 3 we present the experimental results and analysis obtained from linear Kerr/Faraday and nonlinear MSHG magneto-optical techniques, and finally one can find the conclusions in Section 4. 2. Sample characterization Our experimental studies were performed on Co/garnet heterostructures and reference bare garnet films. The initial 5.8 m
thick garnet film of Y2 CaFe3.9 Co0.1 GeO12 composition was grown by liquid phase epitaxy on a (0 0 1)-plane oriented paramagnetic gadolinium gallium garnet (GGG) substrate. The miscut angle of the GGG substrate was not larger than 0.1◦ [21]. The lattice parameter mismatch in our garnet samples was a = 9.7 × 10−4 nm or about 0.1% [22]. At room temperature the saturation magnetization was 7 G and the Neel temperature TN = 445 K. One initial garnet sample was thinned down to 1.8 m and surface smoothed by oxygen ion beam etching in the low energy regime. In the next step, an ultrathin 2 nm Co layer was deposited on both the 1.8 m and 5.8 m garnet films by ion beam sputtering [10]. And finally, a 4 nm Au film was used to protect the Co layer from oxidation. Magnetic anisotropy constants of Co and the garnet layers were deduced from ferromagnetic resonance and magnetization reversal measurements. The growth-induced anisotropy constant KUYIG of the garnet was different for the 1.8 m thick films as compared to that of the 5.8 m thick ones: −0.1 × 103 erg/cm3 and 1 × 103 erg/cm3 , respectively. The cubic anisotropy constant for both garnet thicknesses was the same and equal to KC = −2 × 103 erg/cm3 . In the case of garnet films, the easy magnetization axes are along 1 1 1-type directions, corresponding to a fourfold symmetry in the plane. An effective anisotropy constant of the polycrystalline Co layer was Co = −9.9 × 106 erg/cm3 , that corresponds to an easy-plane magKeff netic configuration [10]. 3. Results and discussion The process of magnetization reversal in Co/garnet heterostructures has been studied at room temperature in reflection with the linear magneto-optic Kerr effect (MOKE) and in transmission with the Faraday effect. From the data, we separated different magneto-optical contributions from the Co layer and garnet-only films. Changes of the crystallographic symmetry and magnetization of buried surface/interfaces of such heterostructures were deduced from MSHG dependencies in a wide spectral range of fundamental wavelengths. 3.1. Magneto-optical Faraday and Kerr measurements The MOKE and Faraday magnetometry measurements were performed using light from a mode-locked Ti-sapphire laser (MaiTai HP, Spectra-Physics) operating within the 690–1040 nm range and a repetition rate of 80 MHz. For the detection of the signal (Kerr rotation K ), a lock-in amplifier was used in combination with a standard modulation technique with a photoelastic modulator (PEM-100, Hinds Instruments). The perpendicular magnetization component of the ultrathin Co layer was measured using the polar MOKE (P-MOKE) geometry, with the angle of incidence of the laser light close to the sample normal and the external magnetic field Hz perpendicular to the surface of the sample (see Fig. 1(a)). The measurements of the in-plane magnetization components of the Co layer were performed in the longitudinal MOKE (L-MOKE) geometry, with a 49◦ angle of incidence of the light (see Fig. 1(d)). The magnetic field Hx was applied in the sample plane for various orientations with respect to the garnet [1 0 0] direction. The process of magnetization reversal to determination of the Faraday rotation angle F of the garnet-only films was studied in the magneto-optical Faraday geometry, with perpendicular and in-plane magnetic field orientation (Fig. 1(a) and (d)). 3.2. Linear optical and magneto-optical studies Fig. 1 shows the hysteresis loops obtained with MOKE and Faraday effects in different geometries for the Co/garnet heterostructures. The P-MOKE hysteresis loops observed for the 2 nm thick Co film grown on garnet films of 1.8 m thickness indicate an
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Fig. 1. Hysteresis loops for Co/1.8 m garnet film measured as magneto-optical Kerr rotation in polar (b) and longitudinal (e) geometry and Faraday rotation with magnetic field perpendicular Hz (c) and parallel Hx (f) to the sample plane. Top panels show the experimental configuration with Kerr and Faraday effects for perpendicular (a) and in-plane (d) magnetic field orientations.
in-plane magnetization of Co (see Fig. 1(b)) (the same behavior has been observed for 5.8 m garnet films). This result correlates well with in-plane magnetic anisotropy deduced from FMR experiments [10]. The L-MOKE magnetization curve for the Co layer measured with the in-plane external magnetic field Hx are shown in Fig. 1(e). The shape of this loop is practically independent on the azimuthal sample orientation and confirms the easy plane type of the magnetic anisotropy with a saturation in-plane field of about 0.3 kOe. Fig. 1(c) and (f) shows Faraday rotation hysteresis loops measured for 1.8 m (similar in 5.8 m) thick garnet films and a perpendicular applied field Hz and an in-plane field Hx , respectively. From the hysteresis loop shown in Fig. 1(c), one deduces a Faraday rotation from the garnet layer of about F = 0.08◦ and a paramagnetic linear contribution from the GGG substrate. For the in-plane applied magnetic field in the garnet [1 0 0] direction, the saturating field is about 0.6 kOe. In Fig. 2 both the optical transmittance and magnetooptical Faraday rotation spectra are shown for the 1.8 m and 5.8 m thick garnet films. At photon energies below about 1.5 eV (>800 nm) the absorption is small and equal to about 102 cm−1 . However at higher photon energies >2.5 eV (<500 nm), the absorption is caused by charge transfer transitions between the oxygen and the Fe3+ and Co2+ ions. In such case to separate the contributions from Fe and Co is very difficult [23]. In garnets the exchange interaction between Fe or Co in tetrahedral positions is stronger than that in the octahedral ones, because of a different inter-ion distance [24]. In general case, the optical absorption is correlated with the magneto-optical Faraday rotation. In particular, it was determined that the broad line in the Faraday rotation spectrum is due
to the crystal field transition of the low spin octahedral Co ions [25]. Note, that the transitions in octahedral Co ions have an oscillator strength that is weaker than that for the tetrahedral ones, and in the spectral region under study the transitions in the Fe ions also contribute to the spectrum of Co-substituted YIG [26]. In addition, the inhomogeneous distribution of the Co impurities, combined with the lattice mismatch between Co layer and garnet film and magnetostriction, may lead to a spread of the easy axis direction across
Fig. 2. Transmittance (solid lines) and Faraday rotation (dashed lines) spectra of 1.8 m (red) and 5.8 m (black) garnet films. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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optical absorption (105 –106 cm−1 ), the 4 nm thick Au and 2 nm thick Co layers are thin enough to be transparent (transmittance >65%). Both the 1.8 m and 5.8 m thick garnet films are transparent at the fundamental wavelengths within the measured spectral range, but have a strong absorption at the second harmonic frequencies, as shown in Fig. 2. Such strong absorption of the garnet films at the second harmonic frequency provides a possibility to study the Co/garnet interfaces and YIG/GGG interfaces separately by illuminating it from either the substrate or surface side, respectively [6].
3.4. Interface-sensitivity MSHG spectroscopy
Fig. 3. Rotational anisotropy patterns of the MSHG intensity from Co/1.8 m garnet sample obtained by illumination of the Co side, measured at 860 nm fundamental wavelength.
the sample due to the interface roughness which is significantly contributes to the uniaxial in-plane magnetic anisotropy. 3.3. MSHG measurements The MSHG measurements were performed using the same femtosecond laser system as described above. The laser beam was focused onto a 40 m diameter spot with an average power of about 50 and 120 mW for Co/garnet and garnet-only films, respectively. It was verified, that such optical power does not affect the magnetic behavior. The transmitted SHG signal at normal incidence of the laser light was detected by a cooled photomultiplier (Hamamatsu) using a photon counting technique after filtering out the fundamental wavelength. We applied an in-plane magnetic field Hy of about 1.5 kOe in a transverse magneto-optical configuration (see Fig. 3) which saturates the in-plane magnetization in our samples. The MSHG intensity was recorded for various input–output polarization combinations (Xin Xout , Xin Yout , Yin Xout , Yin Yout ), at various azimuthal angles within 0–360◦ range. This angle was measured from the crystallographic [1 0 0] direction. In addition, MSHG hysteresis curves for selected values of azimuthal angles and polarization combinations were recorded. The comparison of the MSHG intensity measured for different magnetic field orientations, light polarization combinations and wavelength, enables to separate the crystallographic and magnetic contributions coming from either the surface or from the interface of the Co/garnet heterostructures. In spite of the strong
The azimuthal dependences of the MSHG intensity measured in the Co/1.8 m garnet sample for different polarization combinations are shown in Fig. 3. The magnetic contribution in MSHG is observed as a difference in the SHG intensity for two opposite directions of the applied magnetic field Hy . In the case of the fourfold symmetry, the magnetic contribution to SHG should actually be zero because of symmetry reasons, as reported in garnet films with non-magnetic substituted components, such as (YbPr)3 (FeGa)5 O12 [19] and Lu2.5 Bi0.5 Fe5 O12 [6]. However, in our samples the magnetic contrast is clearly visible. In principle, the lattice mismatch [27] and miscut of the substrate can lead to a macroscopic symmetry breaking and therefore an appearance of a bulk magnetic contribution to the SHG intensity. However, both these parameters in our samples are very small [22] as compared to the undoped garnets. We thus should assume that it is the Co impurity of the garnet film that is the main factor of the magnetic contribution to the MSHG azimuthal anisotropy. The transmission spectra show strong optical absorption in the = 400–500 nm range (see Fig. 2). In the 5.8 m film, this would lead to a total extinction of the transmitted light at these wavelengths and allows distinguishing the SHG responses from the garnet surface or from the garnet/substrate interface, by choosing the direction of the light incidence. In contrast, for the 1.8 m garnet thickness the transmittance is between 3% and 50% within the measured wavelength range as shown in Fig. 2. In this spectral range for the 1.8 m garnet we can distinguish the contributions from the surface and/or interface by tuning the wavelength. Therefore, in the next part of the paper we will represent mainly the results obtained on the Co/1.8 m garnet heterostructure. For illumination from the substrate side of the Co/garnet sample in the Yin Xout polarization combination, the recorded MSHG azimuthal dependences are shown in Fig. 4. In general, these dependences confirm the (0 0 1) garnet orientation, with somewhat distorted 4mm point group symmetry. Interestingly, the rotational anisotropy pattern rotates 45◦ in the wavelength range between 800 nm and 900 nm, so that the main peaks of the intensity turn from the 1 0 0 to the 1 1 0 direction. Such rotation could be connected with different balance of surface/interface contributions. For the comparison of these contributions, illumination from both the sample surface and substrate side has further been used. The MSHG intensity as a function of the applied magnetic field, for Co/garnet and garnet only films are shown in Fig. 5, measured for the magnetic field applied along the [1 0 0] and [1 1 0] garnet crystallographic axes. A good agreement between these SHG intensities and the magnetization reversal process deduced from both Faraday and Kerr rotation (see Fig. 1) of the Co/garnet heterostructure is observed. For the garnet only film the saturation field of the SHG intensity curve is about 0.6 kOe (Fig. 5(a)), which is exactly the same as obtained by Faraday rotation from Fig. 1(f). The easy magnetization axis in the garnet is along the 1 1 1 direction, therefore for Hy || [1 1 0] the change of the SHG intensity is practically negligible. However for Hy || [1 0 0], along the hard axis, a coherent rotation of
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relative changes in the MSHG intensity occur are determined from the MSHG magnetic contrast: (ϕ) =
I2ω (+M) − I2ω (−M) I2ω (+M) + I2ω (−M)
(1)
where I2ω (±M) is the MSHG intensity for opposite orientations of the magnetization M. For both 1.8 m and 5.8 m thick garnet films the magnetic contrast from the garnet/substrate interface was measured at ϕ = 90◦ . At the fundamental wavelength of about 890 nm the magnetic contrast shows maxima (see Fig. 6(a)) that are roughly in agreement with the maxima of the Faraday spectra close to 445 nm (Fig. 1). At the fundamental wavelength of about 800 nm, a slight increase of was found due to the surface contribution from the 1.8 m garnet films (Fig. 6(a)). For the 1.8 m garnet films an SHG intensity maximum is observed at about 860 nm fundamental wavelength (Fig. 6(b)). However, for 5.8 m garnet films, Fig. 6(b) shows SHG intensity maxima at the fundamental wavelengths of 820 nm and 860 nm. Note that in this case, the SHG response comes purely from the surface of the garnet, which is obtained by illuminating the sample from the substrate side. The observed spectra show a good correlation with the strong optical absorption at the crystal field transitions of Co3+ and Fe3+ ions in octahedral positions at 410 nm, 416 nm, and 432 nm [23]. For a more quantitative discussion, the MSHG azimuthal dependences shown in Fig. 4 were fitted by the following formula:
2
I2ω (ϕ, ±−→ M) = A4 sin(4ϕ + ϕ4 ) + A1 (ϕ + ϕ1 ) + A0
Fig. 4. Rotational anisotropy patterns of the MSHG intensity with (right column) and without (left column) Co layer on 1.8 m garnet films obtained by illumination of the substrate side for Yin Xout polarization combination, measured at different fundamental wavelengths. The solid lines represent the fits to the measured data points. Multiplication factors scaling the MSHG intensity data with respect to garnet surface measured at 795 nm are shown in the plots.
magnetization is observed. In contrast, for the Co/garnet interface (saturation field of SHG is about 0.3 kOe) the independent behavior of the SHG intensity on magnetic field directions is correlated with the in-plane magnetic anisotropy of the Co layer and the magnetization process shown in Fig. 1(e). The values for which the largest
(2)
The A1 and A4 coefficients correspond to the one- and fourfold symmetry components, that arise from the general 4-fold symmetry of the (0 0 1) surface, and the distortion from it, respectively. Only the A1 coefficient is taken as dependent on the magnetization direction. We have verified that possible contributions of two- and threefold symmetry are negligible. The phases ϕi used from the fits were separately fixed within the whole wavelength range. The A0 coefficient corresponds to an isotropic contribution and is not sensitive to the crystallographic symmetry of any interfaces of the measured samples. The fitted coefficients derived for both Co/1.8 m garnet interface and reference garnet surfaces are plotted as a function of the fundamental wavelength in Fig. 6(c) and (d). For the 1.8 m garnet surface in Fig. 6(c), the A1 coefficient is close in value, but opposite in sign for opposite magnetic field directions, and changes sign at about 830 nm. Note that the same phase ϕ1 is obtained at these fittings. These dependences of A1 for M+ and M− are symmetric with respect to the horizontal zero axis (Fig. 6(c)). However, for the Co/1.8 m garnet interface, the asymmetrical dependence of A1 is visible (see Fig. 6(d)). For 5.8 m garnet films with and without Co layer, practically the same behavior of the A1
Fig. 5. SHG intensity curves of 1.8 m garnet (a) and Co/1.8 m garnet (b) samples measured for 840 nm wavelength fundamental light and Xin Xout polarization combination for [1 0 0] (open points) and [1 1 0] (full points) garnet crystallographic orientation. The sample was illuminated from the substrate side.
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Fig. 6. Spectral plots of magnetic contrast (a) and normalized MSHG intensity (b) for garnet/substrate interface for Xin Xout polarization combination as a function of fundamental wavelength. Spectral dependences of coefficients A1 and A4 , obtained from the fits of the rotational anisotropy data for Yin Xout polarization combination for 1.8 m garnet surface (c) and Co/1.8 m garnet interface (d). The sample was illuminated from the substrate side.
coefficients on function of the fundamental wavelength has been observed. For the Co/1.8 m garnet film, the A4 coefficient changes sign at about 900 nm, which for the garnet surface occurs at about 820 nm of fundamental wavelength. Note that for the Co/1.8 m garnet film the A4 parameter above 840 nm is approximately one order of magnitude smaller than for the 1.8 m garnet film (Fig. 6(c) and (d)). This difference could arise from the presence of the interface modification of the garnet by the Co cover layer. In this case the Co layer has induced a 45◦ -shift between the 1 0 0 and 1 1 0 orientations of the SHG rotational anisotropy, due to the optical transitions between Co ions in tetra- and octahedral positions. This behavior can be clearly seen by a comparison of the MSHG azimuthal dependences of Co/1.8 m garnet and Co/5.8 m garnet interfaces,
Fig. 7. SHG rotational anisotropy of Co/5.8 m garnet (a) and Co/1.8 m garnet (b) heterostructures obtained by illumination of the sample from the Co side for Yin Xout polarization combination at 840 nm fundamental wavelength. Multiplication factors scaling the SHG intensity data with respect to the Co/5.8 m sample are shown in the plot.
shown in Fig. 7. In this case, the lattice mismatch between the garnet and the substrate should be relaxed at the surface of the garnet film. Thus, for the Co/garnet interface the probability of Co ions in octahedral positions is much higher than at the garnet/substrate interface. 4. Conclusions We have studied ultrathin Co layer on Co-substituted YIG/GGG(0 01) heterostructures by linear Faraday and Kerr and nonlinear MSHG spectroscopy within a wide spectral range. Different polarization combinations of the MSHG in the Voigt geometry show normally forbidden non-zero magnetic contribution due to the Co impurities in the YIG crystals with 4mm point group symmetry. This contribution reaches a maximum at about 890 nm of fundamental wavelength that is correlated with a Faraday rotation maximum at about 445 nm. Therefore the Faraday spectra at the SHG frequency are shown to play an important role in the formation of the SHG anisotropic azimuthal dependences. The spectral MSHG rotational anisotropy could be interpreted as due to the influence of electronic magnetic impurities, such as Co dopants, on either surface or interface in the Co/garnet heterostructures. We also observe a 45◦ rotation of the MSHG rotational anisotropy pattern from 1 0 0 to 1 1 0 orientation after depositing the Co layer on the 1.8 m garnet films. We assume that this rotation of the anisotropy pattern has been induced by the optical transitions between octahedral and tetrahedral positions in the garnet. Depending on the photon energy on the second-harmonic frequency, these transitions may give rise to optical absorption at magnetic impurities that is very strong at interfaces close to 445 nm of fundamental wavelength. These results will help to understand the mechanism of the ultrafast light-induced anisotropy [20] which is assumed to
Please cite this article in press as: M. Pashkevich, et al., Magneto-optical spectroscopy of surface/interfaces in Co/garnet heterostructures, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.02.185
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Please cite this article in press as: M. Pashkevich, et al., Magneto-optical spectroscopy of surface/interfaces in Co/garnet heterostructures, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.02.185