FeCo multilayers

Journal of Magnetismand Magnetic Materials 165 (1997) 52-55

Journal of magnetism and magnetic materials

ELSEVIER

Circular polarized soft X-ray resonant magnetic scattering studies of FeCo/Mn/FeCo multilayers V. Chakarian a,*, Y.U. Idzerda

a,

C.-C. Kao

b,

C.T. Chen c

a Naval Research Laboratory, Code 6345, Washington, DC 20375, USA b Brookhaven National Laboratory, NSLS, Upton, N Y 11973, USA c A T a n d TBell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974, USA

Abstract We present some of our recent results of circular polarized soft X-ray resonant magnetic scattering studies, a technique which combines the power of X-ray scattering and magnetic circular dichroism. The energy, angle, and polarization dependence of the reflectivity near the L-edges of the each element provides a new means for determining multilayer magnetic ordering in a heteromagnetic multilayer in an element-specific manner. As an example, the results from a FeEsCO75/Mn/Fe25Co75 trilayer are presented. Keywords: X-ray scattering;Magnetic dichroism;Multilayers;Magnetic ordering

During the last five years, there has been an enormous increase in the use of synchrotron radiation sources in the study of magnetic materials [1]. One of the new techniques that experienced a tremendous growth is magnetic circular dichroism (MCD) [2,3]. In MCD, one determines the difference in the absorption cross sections of left- and right-circular polarized (soft) X-rays at the absorption edges of each element of the system. This difference can then be used to identify magnetic orientations [4,5] and to determine the orbital and spin contributions to the total magnetic moment of each element in the system [6]. Concurrent to the advances in MCD, there has been significant advances in the area of resonant X-ray scattering (RXS) using circular and linear polarized light in the study of magnetic materials. This latter technique provides magnetic and structural information via the angular and energy dependence of the scattering. The absorption and reflection are closely related through the optical theorem, which relates the absorption (extinction) coefficient to the imaginary part of the forward scattering amplitude. One should note, however, that the dichroic effects exhibited in the reflection are not only due to absorption cross section (i.e., the imaginary part of the dielectric tensor), but are also due

* Corresponding author. Naval Research Laboratory, c/o Brookhaven National Laboratory, Bldg. 510E, Upton, NY 11973, USA. Fax: +1-516-344-2823;email: [email protected].

to the real part of the dielectric tensor, making the interpretation of the data less straightforward. The correlation of the structural and magnetic properties is a long-standing challenge in thin film and multilayer technology. It is, therefore, very appealing to combine these two techniques (MCD and RXS) to provide a new means for the study of element-specific, depth-dependent chemical and magnetic structure of (hetero)magnetic thin films and multilayers. This new type of measurement, which can be described as circular polarized (soft) X-ray resonant magnetic scattering (CP-XRMS), combines the structural and elemental sensitivity of RXS and the elemental and magnetic sensitivity of MCD. It offers the possibility of determining multilayer magnetic ordering, film thicknesses, interface quality, and can identify layer switching in a heteromagnetic multilayer in an elementspecific manner. Its feasibility has been recently demonstrated for a Co thin film by Kao et al. [7] and more recently for a Ni(110) single crystal by Sacchi et al. [8]. In this paper, we present our recent results of CP-XRMS studies of a heteromagnetic trilayer: FeEsCOvs/Mn/ Fe25Co75. The experiments were conducted at the NRL/NSLS U4B beamline located at the National Synchrotron Light Source [9,10]. A ( 0 - 2 0 ) spectrometer with 1° resolution and a gas proportional avalanche counter with an overall efficiency of 10% and an energy bandwidth of ~ 4 0 % [11] was attached to the beamline for these measurements. The sample consists of two single crystal Fe25Co75 (106 ,~) alloy films, separated by a thin Mn interlayer (4.5 ML). The trilayer was grown on a thick

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V. Chakarian et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 52-55

~ 660 A ZnSe(001) buffer layer and capped with a 30 AI film to prevent oxidation [12]. This class of trilayers displays a distribution of coupling angles between the two FeCo films as a function of Mn interlayer thickness [13]; the sample used for this study displays a coupling angle of ~ 90 °. In addition, the Mn interlayer has a net magnetic moment and a layered antiferromagnetic helical structure [5]. The experimental arrangement is shown schematically in Fig. 1. The magnetization vector of the sample lies in the sample plane. The magnetic field is applied parallel to the sample surface and in the scattering plane (i.e., plane defined by the incident and the specularly scattered beams). E v and E H are vertical and horizontal components of the incident electric field vector, respectively, and the circular polarization rate of the incoming photon beam was fixed at 75% [14]. A schematic representation of the sample structure is also shown. For the fields utilized in this experiment (up to 400 Oe), because of the strong 90 ° coupling, the magnetization vectors for the two FeCo layers are oriented at _ 45 ° with respect to the applied field direction (and hence the scattering plane). Two types of measurements were made for this experiment: (1) fixed energy ( 0 - 2 0 ) scans in which both the sample and the detector were rotated in the specular geometry with the energy of the incoming photon beam fixed at a particular value (typically at and away from the L 3 a n d / o r L 2 resonances); and (2) fixed 0 energy scans in which the energy of the incident photon beam was swept through the absorption edge of a given element. For both types measurements, the applied magnetic field direction was reversed at each measurement point to minimize the effects of decreasing photon flux and changes in the scatter/background light intensity. Representative energy dependent reflectivity spectra along various incidence angles in the Fe and Co L-edge

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regions are shown in Figs. 2 and 3, respectively. Also shown are the Fe and Co absorption spectra measured with the same degree of circular polarization. As it is clearly apparent, the reflectivity spectra display a much more complex and intriguing interference characteristics than the absorptivity spectra. This complicated behavior is due to the fact that the reflected intensity depends not only the imaginary part of the dielectric tensor (proportional to absorptivity) but also the real part. Comparing the spectra for Fe and Co, one readily observes that, although the Fe and Co atoms are alloyed together and therefore ferromagnetically coupled, the energy and angle dependence of the reflectivity near the Fe and Co L-edges is drastically different, reflecting the differences in the chemical and magnetic environment in the vicinity of each element. More insight can be obtained regarding the chemical and magnetic structure of the sample via the ( 0 - 2 0) scans. Representative ( 0 - 2 0 ) scans for Mn, Fe, and Co are shown in Figs. 4-6, respectively. In each figure, the top panel shows the resonant scattering while the bottom panel shows the off-resonance scattering scans, in which the incoming photon energy was set at the L 3 resonance

V. Chakarian et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 52-55

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maximum and a few eV away from it, respectively (in Figs. 2 and 3, the vertical dot-dashed lines mark these energy positions for Fe and Co). These angle scans, while complementary to the energy scans like those shown in Figs. 2 and 3, offer a way of obtaining depth or layer

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v. Chakarian et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 52-55

sensitivity. A comparison of the resonance and off-resonance angle scans immediately reveals the layer sensitivity of the CP-XRMS technique. Since, in the on-resonance case, the photons are strongly absorbed in the top FeCo layer [15], the measured interference, i.e., the frequency of the intensity oscillations, depends expressly on the physical parameters of the top layer. In the off-resonance case, the incident photon beam penetrates deeper into the sample and hence the reflected intensity has contributions from the bottom film as well. Since, in this case, the thicknesses of the top and bottom layer are nearly the same, one would expect the frequency of the intensity oscillations to double, as is observed. The off-resonance scans for Co, shown in the bottom panel of Fig. 6, display extra oscillations not seen in the on-resonance scans (top panel). The layer sensitivity is further confirmed in the Mn (Fig. 4) and Fe data (Fig. 5) where the resonant and off-resonant reflectivities express the same periodicity. Recall that since Fe is dilute in the FeCo alloy layers, its stopping power is significantly smaller than that of Co. Hence, near Fe L 3 energies, the reflected intensity has contributions from both layers. The results presented demonstrate the richness of the information content of CP-XRMS measurements. Through the use of ( 0 - 2 0 ) and energy scans, collected near the L-edges of Fe, Co, and Mn, we illustrated the chemical and magnetic structure sensitivity of the technique as well as its layer selectivity. What we have not fully discussed here is that the energy and angular dependence of CPXRMS can be compared with model calculations to elucidate the correlation of magnetization and structure. The modeling consists of solving Maxwell's equations for layered media utilizing experimentally determined energy dependent, complex dielectric tensors with no free parameters. This method was proven to work very well in the case of single Co films of reasonable quality [7]. The results of a similar modeling applied to these F e C o / M n / F e C o trilayers reproduce this observed period doubling as well as many other details of these rich reflectivity scans. The comparison between the measured and modeled spectra, including the details of how the complex dielectric tensors are generated, will be described at a later date.

55

Acknowledgements. One of the authors (VC) is supported by the Office of Naval Research. Work done at National Synchrotron Light Source was supported by DOE, under contract No. DE-AC02-76CH00016.

References [1] D.B. McWhan, J. Synch. Radiat. 1 (1994) 83. [2] C.T. Chen, F. Sette, Y. Ma and S. Modesti, Phys. Rev. B 42 (1990) 7262. [3] V. Chakarian, Y.U. Idzerda, C.T. Chen, G. Meigs and C.-C. Kao, in: Applications of Synchrotron Radiation in Industrial, Chemical, and Materials Science, eds. L.J. Terminello, K.L. D'Amico and D.K. Shuh (Plenum, New York, 1996). [4] V. Chakarian, Y.U. Idzerda, G. Meigs, E.E. Chaban and C.T. Chen, Appl. Phys. Lett. 66 (1995) 3368. [5] V. Chakarian, Y.U. Idzerda, H.-J. Lin, C.J. Gutierrez, G.A. Prinz, G. Meigs and C.T. Chen, Phys. Rev. B 53 (1996) 11313. [6] C.T. Chen, Y.U. Idzerda, H.-J. Lin, N.V. Smith, G. Meigs, E. Chaban, G.H. Ho, E. Pellegrin and F. Sette, Phys. Rev. Lett. 75 (1995) 152. [7] C.-C. Kao, C.T. Chen, E.D. Johnson, J.B. Hastings, H.-J. Lin, G.H. Ho, G. Meigs, J.-M. Brot, S.L. Hulbert, Y.U. Idzerda and C. Vettier, Phys. Rev. B 50 (1994) 9599. [8] M. Sacchi, J. Vogel and S. Iacobucci, J. Magn. Magn. Mater. 147 (1995) Lll. [9] C.T. Chen and F. Sette, Rev. Sci. Instr. 60 (1989) 1616. [10] C.T. Chen, Rev. Sci. Instr. 63 (1992) 1229. [11] E.D. Johnson, C.-C. Kao and J.B. Hastings, Rev. Sci. Instr. 63 (1992) 1443. [12] C.J. Gutierrez, J.J. Krebs and G.A. Prinz, J. Appl. Phys. 61 (1992) 2476. [13] M.E. Filipkowski, J.J. Krebs, G.A. Prinz and C.J. Gutierrez, Phys. Rev. Lett. 75 (1995) 1847. [14] The photon beam is elliptical polarized, expressed as a sum of circular and linear polarization. The linear polarization rate for this circular polarization setting is approximately 60%. [15] Unlike the hard X-rays, the mean-free-path of the soft X-ray photons at the L 3 peak, i.e., on-resonance, is relatively short. Our L-edge transmission measurements from both Fe and Co indicate these values to be approximately 230/k and 130 ,~ for the 1+ and I_ absorption, respectively. The corresponding values off-resonance are 1000-5000 A.