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W(110 ) across the spin-reorientation transition-temperature
Nuclear Instruments and Methods in Physics Research B 200 (2003) 210–214 www.elsevier.com/locate/nimb
Spin and orbital moments in Au/Co/Au(1 1 1)/W(1 1 0) across the spin-reorientation transition-temperature K. Removic-Langer a, J. Hunter Dunn b, J. Langer a, D. Arvanitis c, H. Maletta a, E. Holub-Krappe a,* a
Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany b MAX-lab, Lund University, P.O. Box 118, S-221 00 Lund, Sweden c University of Uppsala, Box 530, S-75121 Uppsala, Sweden
Abstract The temperature-driven spin-reorientation transition (SRT) has been investigated in a layer of 2.1 nm Co in Au/Co/ Au. We studied the evolution and stability of the in-plane magnetization components in the temperature range from 300 to 90 K by means of polarized neutron reflectometry (PNR) and Co L-edge X-ray magnetic circular dichroism (XMCD) measurements. The PNR measurement provides evidence for the occurrence of a SRT in this temperature range. XMCD can be used as a vector magnetometer that yields both the in- and out-of-plane magnetization components per Co atom. Combining the results of both techniques (PNR and XMCD) allows separating the orbital and spin magnetic moments using magneto-optical sum rules. We observed that already above the SRT the in-plane orbital moment component changes significantly stronger than the spin component. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.10.Ht; 61.12.Ha; 75.25.+z; 75.30.Gw; 75.70.Ak; 78.70.Dm Keywords: XMCD; PNR; SRT; Co thin film
1. Introduction Transition metal thin films and surfaces may exhibit a variety of interesting magnetic phenomena such as enhanced spin and orbital magnetic moments or perpendicular magnetic anisotropy (PMA) [1]. These effects have their microscopic origin in the reduced symmetry experienced by magnetic atoms near interfaces or surfaces. In any
*
Corresponding author. Tel.: +49-30-8062-2557; fax: +4930-8062-2389. E-mail address: [email protected] (E. Holub-Krappe).
magnetic system the magnetic anisotropy plays an important role, since it describes the relation between the free energy of the magnetic system and the direction of the magnetization. The expression for the magnetic anisotropy consists predominantly of two terms, one of them is the shape anisotropy due to magnetic dipole–dipole interactions within the volume, and the other is the magneto-crystalline anisotropy due to spin–orbit interactions. The first one is strongly dependent on the shape of the sample, and the second one depends on the symmetry of the crystal lattice. The magneto-crystalline anisotropy of surface atoms leads here to a PMA. In thin films with
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 7 2 1 - 4
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
predominating shape anisotropy the magnetization direction is parallel to the film plane. When the anisotropy term due to surface atoms becomes larger than the shape anisotropy term the orientation of the magnetization will be perpendicular to the film plane. The change of the easy axis of magnetization from in-plane to out-of-plane orientation is the manifestation of the spin-reorientation transition (SRT). The magnetic anisotropy can be described with Eq. (1) in polar coordinates [2]: f ðhÞ ¼ f0 þ K2 sin2 h þ K4 sin4 h þ
ð1Þ
with the thickness- and temperature-dependent anisotropy coefficients of second (K2 ) and fourth (K4 ) order. The separation of volume and surface contributions is given with Eq. (2) [3,4]: Ki ðd; T Þ ¼ KiV ðT Þ þ 2KiS ðT Þ=d;
i ¼ 2; 4
ð2Þ
with the film thickness d, the temperature T and the volume and surface anisotropy coefficients of ith order KiV and KiS . It is obvious that the free energy of thin films can be changed either by changing the film thickness (inducing a thicknessdriven SRT) or the temperature (temperaturedriven SRT). A PMA was indeed observed in systems like Co/Ru(0 0 0 1) [5], Co/CeH2 [6], Co/Pd(1 1 1) [7], Co/W(1 1 0) [8], and Co/Au(1 1 1) [9–12] at small values of the film thickness and/or at low temperature. In the Co/Au(1 1 1) system the SRT was investigated versus Co thickness as well as versus temperature [1,10–12]. The change of the orbital and spin magnetic moments for different thicknesses of Co layer was observed in the region of the SRT [1]. In this paper, we present results of our investigation on the temperature-driven SRT in Au/Co/ Au(1 1 1) with emphasis on the changes of the inplane orbital and spin magnetic moments of the Co atoms by lowering the temperature across the SRT. First we describe the preparation of the sample. Then the results from the study of the total in-plane magnetic moment (by polarized neutron reflectometry – PNR), and the in-plane orbital and spin magnetic moments of the Co atom (by X-ray magnetic circular dichroism – XMCD) were presented and discussed. Finally the main results were summarized.
211
2. Sample preparation The sample was prepared by molecular beam epitaxy in an ultrahigh-vacuum chamber (base pressure <3 1010 mbar) [10]. The film thickness and deposition rate were monitored by a quartz microbalance. As substrate a one-side polished sapphire Al2 O3 (1 1 2 0) single crystal with a area of 39 15 mm2 was used. In PNR studies it is advantageous for reasons of intensity to choose substrates with a large area. Prior to film deposition, the sapphire substrate was annealed until it had a contaminant-free surface, as checked by Auger electron spectroscopy (AES). Before the growth of an epitaxial Au(1 1 1) substrate layer, an epitaxial W(1 1 0) buffer layer was deposited keeping the substrate temperature T 1200 K. A distance of 0.60 m between the evaporator and facing substrate ensures a homogeneous film thickness across the substrate area. Thickness of W was dW ¼ 10 nm, of Au dAu ¼ 5:1 nm, of Co dCo ¼ 2:1 nm and Au-cap dAu ¼ 2:8 nm. A detailed study of the deposition and growth mechanism of the layers can be found in [13].
3. Experiments X-ray absorption and XMCD spectroscopy measurements were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) using an elliptical polarized undulator EPU and the beam line 5.2 with an UHV chamber specially designed for spectroscopy experiments [14]. The EPU in combination with beam line 5.2 is capable to produce highly brilliant, linear or circular X-ray light in the soft energy range of 0.2–1.5 keV [15]. The EPU delivers nearly 100% circularly polarized light, and the beam line optics deliver a beam spot of about 1.5 mm2 on the sample. The sample was remanently magnetized by means of magnetic field pulses of 170 Oe, well beyond the in-plane coercive field of the sample (Hc 20 to 30 Oe). All absorption spectra were taken in the remanent state. To analyze the XMCD spectra a well established standard procedure was used [16] giving results on a per atom basis. All spectra were taken at an grazing angle of incidence of 10° and therefore
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
(a)
300 K
500
σ
+
σ
250
0
XMCD [a.u.]
100
(b) 500
275 K
250
σ σ
+
0 100
(c)
300 K
50 0 -50 -100 -150 760
Comparing the ml =meff values in Table 1 at difs ferent temperatures one realizes the biggest change (46%) from 300 to 275 K. Obviously it is due to the observed big change in the orbital moment component Dml of 0.057 lB . The orbital moment ml decreases by 51% in this temperature range, compared to only 8% in the decrease of the spin moment ms . The PNR measurements were carried out on the ) [20] at the BERII V6 reflectometer (k ¼ 4:66 A reactor of the Hahn-Meitner-Institute (HMI) in Berlin in the temperature range 300–90 K. Polarized neutrons with spin parallel (spin-up) and antiparallel (spin-down) with respect to the external magnetic field H were reflected off the surface of the sample at grazing incidence in order to detect the reflected intensities of the spin-up (Rþ ) and spindown (R ) neutrons, respectively. The external field H was applied parallel to the film plane and perpendicular to the incoming beam. The experimental run was started on a fully in-plane saturated sample in 0.4 T at room temperature. Then the field was switched off, and the sample was measured in the remanent state at different temperatures. However, to prevent a depolarization of the neutrons outside the sample, a small guide field of
Norm. XAS [a.u.]
Norm. XAS [a.u.]
corrected for saturation effects which are pronounced at small incidence angles [17]. An effective electron escape depth of 1.7 nm was used for the saturation correction. The values of local magnetic moments for the system Au/Co/Au were determined by XMCD according to the magnetooptical sum rules [17–19]. XMCD spectra were measured by switching the helicity of the circular polarization of the beam while the sample magnetization was fixed. In order to calculate the spin magnetic moment ms from the effective spin magnetic moment meff s , the magnetic dipole term of mT ¼ 0:0486 lB [1] was taken into account. In Fig. 1 are shown XAS and XMCD spectra for the 8.4 ML Co in Au/Co/Au(1 1 1) measured at grazing incidence at 300 and 275 K. The data from Fig. 1 indicate a qualitatively different magnetic behavior of the Co layer depending on the temperature. The magnetic splitting is the biggest at 300 K and it decreases with temperature. Finally no magnetic splitting is observed at 90 K. In Fig. 2 and Table 1 the results from the XMCD measurements are summarized. The ratio of the orbital and spin moments, the separated components of the spin and orbital moments, and the total moment decrease with decreasing temperature.
780
800
Photon Energy [eV]
820
XMCD [a.u.]
212
(d)
275 K
50 0 -50 -100 -150 760
780
800
820
Photon Energy [eV]
Fig. 1. XAS spectra of Au/8.4 ML Co/Au(1 1 1) taken with different beam helicity rþ or r at an angle of 10° with respect to the surface (grazing incidence) are presented for 300 K (a) and 275 K (b). The corresponding XMCD spectra corrected for the saturation effects are displayed in (c) and (d).
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
(a)
0.08
213
(b)
0.10 0.08
0.06
0.06 0.04
0.04
0.02
0.02
0.00
0.00
(c) 1.0
eff
ms
1.5
(d)
ms
XMCD PNR
1.0 0.5
0.5 0.0
0.0 0
100
200
300
0
100
200
300
eff Fig. 2. XMCD sum rule results for (a) ml =meff s , (b) ml and (c) ms , ms of the Co layer plotted versus temperature, for an incidence angle h ¼ 10° from the surface of the circularly polarized X-rays. In (d) ltot determined by XMCD and PNR are compared showing a good agreement.
Table 1 Magnetic parameters determined by XMCD measurements using the sum rules T (K)
ml =meff s
ml ½lB
meff s ½lB
ms ½lB
ltot ½lB
300 (rem) 275 (rem) 250 (rem) 240 (rem) 222 (rem) 90 (rem)
0.091 0.049 0.045 0.044 0.032 0
0.111 0.054 0.029 0.025 0.013 0
1.219 1.109 0.645 0.564 0.401 0
1.299 1.189 0.725 0.644 0.481 0
1.41 1.243 0.754 0.669 0.494 0
ml is the orbital moment; meff s is the effective spin moment; ms is the spin moment and ltot the total magnetic moment of the Co atom; (rem) means remanent state.
20 Oe was always present. Simulations were performed using the HMI Parrat code [21]. First the analysis of the PNR data was done for the fully saturated sample using the tabulated (bulk) densities and coherent scattering lengths of the individual layers under the assumption to stay constant. Only the magnetic moments, layer thickness and roughness parameters were varied. In the model which has been applied, the PNR best-fit thickness parameter is an average value, since it is assumed that there is a region of roughness associated with each interface. The magnetic moment of 1.76 lB was estimated during simultaneous variation of other parameters. In the simulations of the temperature dependent data the values of the layer
thickness and interface roughness were fixed and only the magnetic moment was varied until a best fit was achieved. Table 2 summarizes the ltot values of the total in-plane magnetization derived from the PNR experiment. In the remanent state the film shows a reduced magnetization compared to the saturated film at room temperature. It is a good indication that the sample consists of magnetic domains, which are all aligned in-plane at high enough fields, i.e. in the saturated sample. The key observation from PNR measurements is the decreasing values of the so called spin asymmetry (Rþ R Þ=ðRþ þ R ) with decreasing temperature from 300 to 90 K, which directly indicate the decrease of the total in-plane magnetic moment per
214
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
Table 2 Magnetic parameters determined by PNR T (K)
ltot ½lB
300 (sat) 300 (rem) 275 (rem) 250 (rem) 225 (rem) 200 (rem) 90 (rem)
1.76 1.64 1.18 0.82 0.61 0.49 0.12
ltot is the total in-plane magnetic moment of the Co atom; (rem) and (sat) mean remanent and saturated state, respectively.
atom in the Co layer. The Au/Co/Au(1 1 1) sample shows almost no in-plane component of magnetization at a temperature of 90 K.
4. Conclusions The change of the total in-plane magnetic moment as well as the in-plane orbital and spin magnetic moment components were studied using PNR and XMCD as complementary methods in the region of the temperature-driven SRT for the Au/Co/Au(1 1 1) system from 300 to 90 K. By lowering the temperature one expects the magnetocrystalline anisotropy near the interfaces to become larger than the shape anisotropy, inducing the magnetization (which is in-plane at high temperatures) to turn perpendicular to the film plane at and below the SRT. From PNR measurements a significant change in the total in-plane magnetization component was observed immediately below room temperature, confirmed by XMCD measurements. In addition, XMCD gives further insight into the change of the total in-plane magnetization since one can separate the contributions from the orbital and spin magnetic moments. From synchrotron hysteresis loops measured on the same sample (to be published) and results presented in [10] we estimate the SRT temperature of about 230 K. Our main result, observed for the first time, is that already above the SRT the in-plane orbital moment component changes significantly stronger than the spin com-
ponent. The strong change of the orbital magnetic moment is a precursor of the SRT. It clearly starts already above the transition and triggers the transition to the perpendicular magnetization at the SRT temperature.
References [1] J. St€ ohr, J. Magn. Magn. Mater. 200 (1999) 470. [2] C. Chappert, P. Bruno, J. Appl. Phys. 64 (1988) 5736. [3] H.J.G. Draaisma, W.J.M. de Jonge, J. Appl. Phys. 64 (1988) 3610. [4] H. Fritzsche, J. Kohlhepp, H.J. Elmers, U. Gradmann, Phys. Rev. B 49 (1994) 15665. [5] G. Garreau, E. Beaurepaire, K. Ounadjela, M. Farle, Phys. Rev. B 53 (1996) 1083. [6] T. Nawrath, B. Damaske, O. Schulte, W. Felsch, Phys. Rev. B 55 (1997) 3071. [7] J. Kohlhepp, U. Gradmann, J. Magn. Magn. Mater. 139 (1995) 347. [8] R. Sellmann, H. Fritzsche, H. Maletta, V. Leiner, R. Siebrecht, Phys. Rev. B 63 (2001) 224415. [9] R. Allenspach, M. Stampanoni, A. Bischof, Phys. Rev. Lett. 65 (1990) 3344. [10] R. Sellmann, H. Fritzsche, H. Maletta, V. Leiner, R. Siebrecht, Phys. Rev. B 64 (2001) 054418. [11] R. Sellmann, H. Fritzsche, H. Maletta, V. Leiner, R. Siebrecht, Physica B 276–278 (2000) 578. [12] J. Langer, R. Sellmann, J. Hunter Dunn, A. Hahlin, O. Karis, D. Arvanitis, H. Maletta, J. Magn. Magn. Mater 226–230 (2001) 1675. [13] R. Sellmann, H. Fritzsche, H. Maletta, Surf. Sci. 495 (2001) 185. [14] J. Hunter Dunn, Ph.D. Thesis Uppsala University, 1998, ISBN 91-554-4245-5. [15] D. Arvanitis, J. Hunter Dunn, O. Karis, A. Hahlin, B. Brena, R. Carr, N. M artensson, J. Synchr. Rad. 8 (2001) 120. [16] D. Arvanitis, M. Tischer, J. Hunter Dunn, F. May, N. M artensson, K. Baberschke, in: Springer Lecture Notes in Physics, vol. 466, Springer-Verlag, Berlin, 1996, p. 145. [17] J. Hunter Dunn, D. Arvanitis, N. M artensson, M. Tischer, F. May, M. Russo, K. Baberschke, J. Phys. Condens. Matter 7 (1995) 1111. [18] W.L. OÕ Brian, B.P. Tonner, Phys. Rev. B 30 (1994) 12672. [19] R. Wu, D. Wang, A.J. Freeman, Phys. Rev. Lett 71 (1993) 3581. [20] F. Mezei, R. Golub, F. Klose, H. Toews, Physica B 213– 214 (1995) 898. [21] Model calculations were carried out with the software REFLECTIVITY TOOL PARRAT 32, HMI Berlin, 1999.
Spin and orbital moments in Au/Co/Au(1 1 1)/W(1 1 0) across the spin-reorientation transition-temperature K. Removic-Langer a, J. Hunter Dunn b, J. Langer a, D. Arvanitis c, H. Maletta a, E. Holub-Krappe a,* a
Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany b MAX-lab, Lund University, P.O. Box 118, S-221 00 Lund, Sweden c University of Uppsala, Box 530, S-75121 Uppsala, Sweden
Abstract The temperature-driven spin-reorientation transition (SRT) has been investigated in a layer of 2.1 nm Co in Au/Co/ Au. We studied the evolution and stability of the in-plane magnetization components in the temperature range from 300 to 90 K by means of polarized neutron reflectometry (PNR) and Co L-edge X-ray magnetic circular dichroism (XMCD) measurements. The PNR measurement provides evidence for the occurrence of a SRT in this temperature range. XMCD can be used as a vector magnetometer that yields both the in- and out-of-plane magnetization components per Co atom. Combining the results of both techniques (PNR and XMCD) allows separating the orbital and spin magnetic moments using magneto-optical sum rules. We observed that already above the SRT the in-plane orbital moment component changes significantly stronger than the spin component. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.10.Ht; 61.12.Ha; 75.25.+z; 75.30.Gw; 75.70.Ak; 78.70.Dm Keywords: XMCD; PNR; SRT; Co thin film
1. Introduction Transition metal thin films and surfaces may exhibit a variety of interesting magnetic phenomena such as enhanced spin and orbital magnetic moments or perpendicular magnetic anisotropy (PMA) [1]. These effects have their microscopic origin in the reduced symmetry experienced by magnetic atoms near interfaces or surfaces. In any
*
Corresponding author. Tel.: +49-30-8062-2557; fax: +4930-8062-2389. E-mail address: [email protected] (E. Holub-Krappe).
magnetic system the magnetic anisotropy plays an important role, since it describes the relation between the free energy of the magnetic system and the direction of the magnetization. The expression for the magnetic anisotropy consists predominantly of two terms, one of them is the shape anisotropy due to magnetic dipole–dipole interactions within the volume, and the other is the magneto-crystalline anisotropy due to spin–orbit interactions. The first one is strongly dependent on the shape of the sample, and the second one depends on the symmetry of the crystal lattice. The magneto-crystalline anisotropy of surface atoms leads here to a PMA. In thin films with
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 7 2 1 - 4
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
predominating shape anisotropy the magnetization direction is parallel to the film plane. When the anisotropy term due to surface atoms becomes larger than the shape anisotropy term the orientation of the magnetization will be perpendicular to the film plane. The change of the easy axis of magnetization from in-plane to out-of-plane orientation is the manifestation of the spin-reorientation transition (SRT). The magnetic anisotropy can be described with Eq. (1) in polar coordinates [2]: f ðhÞ ¼ f0 þ K2 sin2 h þ K4 sin4 h þ
ð1Þ
with the thickness- and temperature-dependent anisotropy coefficients of second (K2 ) and fourth (K4 ) order. The separation of volume and surface contributions is given with Eq. (2) [3,4]: Ki ðd; T Þ ¼ KiV ðT Þ þ 2KiS ðT Þ=d;
i ¼ 2; 4
ð2Þ
with the film thickness d, the temperature T and the volume and surface anisotropy coefficients of ith order KiV and KiS . It is obvious that the free energy of thin films can be changed either by changing the film thickness (inducing a thicknessdriven SRT) or the temperature (temperaturedriven SRT). A PMA was indeed observed in systems like Co/Ru(0 0 0 1) [5], Co/CeH2 [6], Co/Pd(1 1 1) [7], Co/W(1 1 0) [8], and Co/Au(1 1 1) [9–12] at small values of the film thickness and/or at low temperature. In the Co/Au(1 1 1) system the SRT was investigated versus Co thickness as well as versus temperature [1,10–12]. The change of the orbital and spin magnetic moments for different thicknesses of Co layer was observed in the region of the SRT [1]. In this paper, we present results of our investigation on the temperature-driven SRT in Au/Co/ Au(1 1 1) with emphasis on the changes of the inplane orbital and spin magnetic moments of the Co atoms by lowering the temperature across the SRT. First we describe the preparation of the sample. Then the results from the study of the total in-plane magnetic moment (by polarized neutron reflectometry – PNR), and the in-plane orbital and spin magnetic moments of the Co atom (by X-ray magnetic circular dichroism – XMCD) were presented and discussed. Finally the main results were summarized.
211
2. Sample preparation The sample was prepared by molecular beam epitaxy in an ultrahigh-vacuum chamber (base pressure <3 1010 mbar) [10]. The film thickness and deposition rate were monitored by a quartz microbalance. As substrate a one-side polished sapphire Al2 O3 (1 1 2 0) single crystal with a area of 39 15 mm2 was used. In PNR studies it is advantageous for reasons of intensity to choose substrates with a large area. Prior to film deposition, the sapphire substrate was annealed until it had a contaminant-free surface, as checked by Auger electron spectroscopy (AES). Before the growth of an epitaxial Au(1 1 1) substrate layer, an epitaxial W(1 1 0) buffer layer was deposited keeping the substrate temperature T 1200 K. A distance of 0.60 m between the evaporator and facing substrate ensures a homogeneous film thickness across the substrate area. Thickness of W was dW ¼ 10 nm, of Au dAu ¼ 5:1 nm, of Co dCo ¼ 2:1 nm and Au-cap dAu ¼ 2:8 nm. A detailed study of the deposition and growth mechanism of the layers can be found in [13].
3. Experiments X-ray absorption and XMCD spectroscopy measurements were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) using an elliptical polarized undulator EPU and the beam line 5.2 with an UHV chamber specially designed for spectroscopy experiments [14]. The EPU in combination with beam line 5.2 is capable to produce highly brilliant, linear or circular X-ray light in the soft energy range of 0.2–1.5 keV [15]. The EPU delivers nearly 100% circularly polarized light, and the beam line optics deliver a beam spot of about 1.5 mm2 on the sample. The sample was remanently magnetized by means of magnetic field pulses of 170 Oe, well beyond the in-plane coercive field of the sample (Hc 20 to 30 Oe). All absorption spectra were taken in the remanent state. To analyze the XMCD spectra a well established standard procedure was used [16] giving results on a per atom basis. All spectra were taken at an grazing angle of incidence of 10° and therefore
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
(a)
300 K
500
σ
+
σ
250
0
XMCD [a.u.]
100
(b) 500
275 K
250
σ σ
+
0 100
(c)
300 K
50 0 -50 -100 -150 760
Comparing the ml =meff values in Table 1 at difs ferent temperatures one realizes the biggest change (46%) from 300 to 275 K. Obviously it is due to the observed big change in the orbital moment component Dml of 0.057 lB . The orbital moment ml decreases by 51% in this temperature range, compared to only 8% in the decrease of the spin moment ms . The PNR measurements were carried out on the ) [20] at the BERII V6 reflectometer (k ¼ 4:66 A reactor of the Hahn-Meitner-Institute (HMI) in Berlin in the temperature range 300–90 K. Polarized neutrons with spin parallel (spin-up) and antiparallel (spin-down) with respect to the external magnetic field H were reflected off the surface of the sample at grazing incidence in order to detect the reflected intensities of the spin-up (Rþ ) and spindown (R ) neutrons, respectively. The external field H was applied parallel to the film plane and perpendicular to the incoming beam. The experimental run was started on a fully in-plane saturated sample in 0.4 T at room temperature. Then the field was switched off, and the sample was measured in the remanent state at different temperatures. However, to prevent a depolarization of the neutrons outside the sample, a small guide field of
Norm. XAS [a.u.]
Norm. XAS [a.u.]
corrected for saturation effects which are pronounced at small incidence angles [17]. An effective electron escape depth of 1.7 nm was used for the saturation correction. The values of local magnetic moments for the system Au/Co/Au were determined by XMCD according to the magnetooptical sum rules [17–19]. XMCD spectra were measured by switching the helicity of the circular polarization of the beam while the sample magnetization was fixed. In order to calculate the spin magnetic moment ms from the effective spin magnetic moment meff s , the magnetic dipole term of mT ¼ 0:0486 lB [1] was taken into account. In Fig. 1 are shown XAS and XMCD spectra for the 8.4 ML Co in Au/Co/Au(1 1 1) measured at grazing incidence at 300 and 275 K. The data from Fig. 1 indicate a qualitatively different magnetic behavior of the Co layer depending on the temperature. The magnetic splitting is the biggest at 300 K and it decreases with temperature. Finally no magnetic splitting is observed at 90 K. In Fig. 2 and Table 1 the results from the XMCD measurements are summarized. The ratio of the orbital and spin moments, the separated components of the spin and orbital moments, and the total moment decrease with decreasing temperature.
780
800
Photon Energy [eV]
820
XMCD [a.u.]
212
(d)
275 K
50 0 -50 -100 -150 760
780
800
820
Photon Energy [eV]
Fig. 1. XAS spectra of Au/8.4 ML Co/Au(1 1 1) taken with different beam helicity rþ or r at an angle of 10° with respect to the surface (grazing incidence) are presented for 300 K (a) and 275 K (b). The corresponding XMCD spectra corrected for the saturation effects are displayed in (c) and (d).
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
(a)
0.08
213
(b)
0.10 0.08
0.06
0.06 0.04
0.04
0.02
0.02
0.00
0.00
(c) 1.0
eff
ms
1.5
(d)
ms
XMCD PNR
1.0 0.5
0.5 0.0
0.0 0
100
200
300
0
100
200
300
eff Fig. 2. XMCD sum rule results for (a) ml =meff s , (b) ml and (c) ms , ms of the Co layer plotted versus temperature, for an incidence angle h ¼ 10° from the surface of the circularly polarized X-rays. In (d) ltot determined by XMCD and PNR are compared showing a good agreement.
Table 1 Magnetic parameters determined by XMCD measurements using the sum rules T (K)
ml =meff s
ml ½lB
meff s ½lB
ms ½lB
ltot ½lB
300 (rem) 275 (rem) 250 (rem) 240 (rem) 222 (rem) 90 (rem)
0.091 0.049 0.045 0.044 0.032 0
0.111 0.054 0.029 0.025 0.013 0
1.219 1.109 0.645 0.564 0.401 0
1.299 1.189 0.725 0.644 0.481 0
1.41 1.243 0.754 0.669 0.494 0
ml is the orbital moment; meff s is the effective spin moment; ms is the spin moment and ltot the total magnetic moment of the Co atom; (rem) means remanent state.
20 Oe was always present. Simulations were performed using the HMI Parrat code [21]. First the analysis of the PNR data was done for the fully saturated sample using the tabulated (bulk) densities and coherent scattering lengths of the individual layers under the assumption to stay constant. Only the magnetic moments, layer thickness and roughness parameters were varied. In the model which has been applied, the PNR best-fit thickness parameter is an average value, since it is assumed that there is a region of roughness associated with each interface. The magnetic moment of 1.76 lB was estimated during simultaneous variation of other parameters. In the simulations of the temperature dependent data the values of the layer
thickness and interface roughness were fixed and only the magnetic moment was varied until a best fit was achieved. Table 2 summarizes the ltot values of the total in-plane magnetization derived from the PNR experiment. In the remanent state the film shows a reduced magnetization compared to the saturated film at room temperature. It is a good indication that the sample consists of magnetic domains, which are all aligned in-plane at high enough fields, i.e. in the saturated sample. The key observation from PNR measurements is the decreasing values of the so called spin asymmetry (Rþ R Þ=ðRþ þ R ) with decreasing temperature from 300 to 90 K, which directly indicate the decrease of the total in-plane magnetic moment per
214
K. Removic-Langer et al. / Nucl. Instr. and Meth. in Phys. Res. B 200 (2003) 210–214
Table 2 Magnetic parameters determined by PNR T (K)
ltot ½lB
300 (sat) 300 (rem) 275 (rem) 250 (rem) 225 (rem) 200 (rem) 90 (rem)
1.76 1.64 1.18 0.82 0.61 0.49 0.12
ltot is the total in-plane magnetic moment of the Co atom; (rem) and (sat) mean remanent and saturated state, respectively.
atom in the Co layer. The Au/Co/Au(1 1 1) sample shows almost no in-plane component of magnetization at a temperature of 90 K.
4. Conclusions The change of the total in-plane magnetic moment as well as the in-plane orbital and spin magnetic moment components were studied using PNR and XMCD as complementary methods in the region of the temperature-driven SRT for the Au/Co/Au(1 1 1) system from 300 to 90 K. By lowering the temperature one expects the magnetocrystalline anisotropy near the interfaces to become larger than the shape anisotropy, inducing the magnetization (which is in-plane at high temperatures) to turn perpendicular to the film plane at and below the SRT. From PNR measurements a significant change in the total in-plane magnetization component was observed immediately below room temperature, confirmed by XMCD measurements. In addition, XMCD gives further insight into the change of the total in-plane magnetization since one can separate the contributions from the orbital and spin magnetic moments. From synchrotron hysteresis loops measured on the same sample (to be published) and results presented in [10] we estimate the SRT temperature of about 230 K. Our main result, observed for the first time, is that already above the SRT the in-plane orbital moment component changes significantly stronger than the spin com-
ponent. The strong change of the orbital magnetic moment is a precursor of the SRT. It clearly starts already above the transition and triggers the transition to the perpendicular magnetization at the SRT temperature.
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