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Layer-selective magnetization reversal in GMR layer systems
ARTICLE IN PRESS
Physica B 345 (2004) 173–176
Layer-selective magnetization reversal in GMR layer systems Michael Heckera,*, Claus M. Schneidera,1, Peter M. Oppeneera, Hans-Christoph Mertinsb a
Leibniz-Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany b BESSY GmbH, Albert-Einstein-Str. 15, D-12489 Berlin, Germany
Abstract IrMn-based spin-valve systems with CoFe and CoFe/FeNi combinations serving as hard and soft magnetic layers, respectively, were investigated by polarized soft X-ray scattering at photon energies close to the L absorption edges of the constituent elements. Structural and magnetic information was obtained making use of the XMCD (X-ray magnetic circular dichroism) technique in reflection. Hysteresis loops of the individual layers in the spin-valve system were measured element-selectively by tuning the incidence angle of the radiation, opening the possibility of magnetic depth profiling. r 2003 Elsevier B.V. All rights reserved. PACS: 75.47.De; 75.70.Cu; 75.25.+z Keywords: Spin values; X-ray reflectometry; XMCD; Hysteresis loops
1. Introduction In recent years there has been a growing interest in layer stacks composed of different ferromagnetic and antiferromagnetic layers due to their suitability for sensor and data storage technology and magnetoelectronics, as for example in the case of spin-valve systems [1–3]. In order to tailor the magnetic properties of such complex layer stacks, dedicated analytical tools are required which are able to measure the magnetic response of the individual layers in a stack. Resonant X-ray *Corresponding author. Tel.: +49-351-4659-246; fax: +49351-4659-452. E-mail address: [email protected] (M. Hecker). 1 Present address: Institute of Solid State Research, Research Centre Julich, . 52425 Julich, . Germany.
reflection performed with circularly and linearly polarized light is a perfectly suited tool for that purpose [4]. As a photon-in–photon-out technique it provides information about the magnetic state of films at varying magnetic fields, and the magnetic switching can be pursued element-selectively in a surface-sensitive scattering geometry not restricted to thin transmission foils. For the improvement of application systems, the understanding of the hysteresis phenomena in the individual layers of such complex stacks is mandatory. In the present paper the details of the magnetic switching in both the free and pinned layers of spin-valve systems are investigated. It is shown that apart from the element-selectivity of the method attained by tuning the photon energy to the absorption edges of the constituent elements, layer sensitivity can be reached also by
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.11.047
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M. Hecker et al. / Physica B 345 (2004) 173–176
tuning the incidence angle and by making use of the interference of the radiation within the layer stack. Thus, also the magnetic properties of different films consisting of the same elements can be distinguished.
2. Experimental Exchange-coupled films with a nominal structure of Si/SiO2/Ta(5 nm)/NiFe(3.5 nm)/ CoFe(0.5 nm)/Cu(2.5 nm)/CoFe(3 nm)/IrMn(8 nm)/ Ta(10 nm)/Cu(0.5 nm) have been prepared by magnetron sputtering. The antiferromagnetic IrMn film is on top of the pinned CoFe film, whereas the free layer is a double-film (0.5 nm CoFe and 3 nm NiFe) below the Cu spacer layer. The pinning direction was set by field cooling, i.e. by application of a magnetic field of 3 T at elevated temperature (280 C) and subsequently cooling down to room temperature in the applied field. The soft X-ray reflection experiments were performed at the BESSY II UE56-PGM undulator beamline using a dedicated reflectometer/polarimeter set-up [5] and circularly polarized light (Pcirc=0.9). The reflectance of the light was measured while a magnetic field was applied along the pinning direction of the spin-valve, which was aligned within the scattering plane (longitudinal geometry). Both reflectometry studies and hysteresis loop measurements were performed at photon energy close to the L absorption edges of Co, Fe and Ni, thus resulting in a strongly enhanced magnetic contrast with sensitivity to the corresponding elements. For the measurement geometry utilized here this contrast is related to the X-ray magnetic circular dichroism (XMCD), measured in reflection geometry.
3. Results and discussion Fig. 1 shows the course of the reflectivity curves measured close to the Co L3 absorption edge for two different directions of the longitudinal magnetic field. The oscillations correspond to the layer structure of the spin-valve and show a significant magnetic contribution, i.e. an intensity difference
for the two signs of the longitudinal magnetic field B. The magnetic part of the reflectometry curve, which appears due to the resonance exchangescattering [6] close to the Co L3 absorption edge (778 eV) and diminishes far from the edge, is attributed to the Co containing layers. At several fixed scattering angles y additionally hysteresis loops were measured. As shown in Fig. 2
Fig. 1. Reflectivity curve of the spin-valve measured for an applied magnetic field B of +8 and –8 mT, respectively, at a photon energy of 775 eV.
Fig. 2. Hysteresis loops measured at photon energies corresponding to the L3 absorption edges of Ni, Co and Fe. Loop 1 corresponds to the Ni, Co and Fe containing free layer, and loop 2 to the pinned layer containing only Co and Fe. The arrows represent the magnetic alignment of the pinned (pl) and free (fl) layer for the corresponding range of the magnetic field. The intensity of each loop is normalized according to 2I/ (Imin+Imax) 1 with Imin and Imax denoting the minimum and maximum intensity of each loop, respectively.
ARTICLE IN PRESS M. Hecker et al. / Physica B 345 (2004) 173–176
for different photon energies, but corresponding scattering vectors, the shape of these loops differs significantly. For the measurements at photon energies at the Co L3 and at the Fe L3 absorption edges, two separated loops can be distinguished. In the case of the investigated spin-valve system, both the free and the pinned magnetic layers contain Co and Fe. It can be assumed, that the minor subloop (loop 1) and the following part for larger negative magnetic fields (loop 2) correspond to the switching of the free and of the pinned layer, respectively. However, a clear proof of this assumption and separation of both parts requires the measurement of only one of the magnetic layers. For the present system, this has been attained by tuning the photon energy to the Ni absorption edge (852 eV), thus being sensitive to the free layer only (Fig. 2, open symbols). The loops of the free and pinned layers differ not only in amplitude, but also distinctly in shape. The complex loop obtained from the Co and Fe signals reveals the switching of the ultrathin lower CoFe layer at small fields simultaneously with the Ni film in a nearly rectangular hysteresis loop, and subsequently the switching of the CoFe pinned layer at larger fields in a more s-shaped loop. In the case of different layers containing the same elements (as here for Fe and also Co), the utilization of the element-selectivity of the XMCD reflection techniques alone may not be sufficient to separate the involved layers of a stack. The interference of the radiation within the stacks visible e.g. in the modulations of the reflectometry curve (Fig. 1) opens an additional possibility to investigate the magnetic switching of the individual layers. As a measure of the magnetic signal, Fig. 3 represents the asymmetry of the reflectometry curve measured at the Co absorption edge, i.e. the difference I+ I between the signals measured for the two signs of BL, normalized by their sum I+ + I . Analogously to the modulation of the average intensity due to the depth profile of the nonmagnetic optical constants in the stack, the magnetic parts also exhibit oscillations corresponding to the depth-profile of the magnetic part of the optical constants. Interestingly, an increase of the amplitude of the magnetic field up to 38 mT does not only increase the measured asymmetry
175
ratio, but leads in certain angular ranges also to its decrease (Fig. 3). In the angular range of such cross-overs between the asymmetry curves, hysteresis loops were measured at constant photon energy (Fig. 4). By tuning the scattering angle y, the intensities of the minor loop corresponding to the free layer and of the loop of the pinned layer can be varied separately. Fig. 4 contains three loops in the angular range around y=12 , where the height of the minor loop (loop 1) decreases from a maximum value to zero, whereas simultaneously the height of loop 2 increases from zero to a
Fig. 3. XMCD asymmetry ratio (I+ I )/(I++I ) of the reflectometry measurements given in Fig. 1 for applied magnetic fields B of 78 mT, and for enlarged fields of 738 mT.
Fig. 4. Hysteresis loops measured close to the Co L3 absorption edge (775 eV) for different scattering angles y.
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maximum value. Therefore, separate measurements of each of the magnetic layers are possible, as nearly realized in the upper and lower curve of Fig. 4. In the case of the thin CoFe film on top of the free layer, the depth resolution corresponding to its thickness is 0.5 nm. Corresponding to the amplitude changes of the two loops between y=11.7 and 12.2 , the sign of the asymmetry also changes in this range. Not only the asymmetry of the sum of both loops, but also the sign of the asymmetry of each of the individual loops can alternate, as observed in other angular ranges. Thus, these measurements contain very sensitive information on the depth-modulation of the magneto-optical constants within the stack, requiring further efforts in the modeling of the scattered intensity from complex stacks for quantitative evaluation of the magneto-optical depth profiling.
4. Conclusions The magnetic switching of an IrMn-based spinvalve system was measured element-selectively by polarized soft X-ray scattering. Thus, individual hysteresis loops attributed to the free and the pinned layer were measured. The near-edge asymmetry effects were observed over a large angular range of the reflectometry curve. Comparing the measurements for different amplitudes of the magnetic field, as a striking feature crossovers
of the asymmetry ratios for different fields were observed. A mapping of hysteresis loops in corresponding angular ranges enabled to correlate this feature with the depth-sensitive probing of the magneto-optical properties of the spin valve. The combination of the element- and depth-sensitivity of the applied scattering method offers the opportunity of magnetic depth profiling in layer stacks.
Acknowledgements We are grateful to Dr. D. Elefant for providing the samples and additional magnetic characterization.
References [1] J. Nogues, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] M.D. Stiles, R.D. Mc Michael, Phys. Rev. B 59 (1999) 3722. [3] B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. [4] O. Zaharko, P.M. Oppeneer, H. Grimmer, M. Horisberger, H.-Ch. Mertins, D. Abramsohn, F. Sch.afers, A. Bill, H.-B. Braun, Phys. Rev. B 66 (2002) 134406. [5] F. Sch.afers, H.-Ch. Mertins, A. Gaupp, W. Gudat, M. Mertin, I. Packe, F. Schmolla, S. Di Fonzo, G. Soullie, W. Jark, R. Walker, X. Le Cann, R. Nyholm, M. Eriksson, Appl. Opt. 38 (1999) 4074. [6] J.P. Hannon, G.T. Trammel, Phys. Rev. Lett. 61 (1988) 1245.
Physica B 345 (2004) 173–176
Layer-selective magnetization reversal in GMR layer systems Michael Heckera,*, Claus M. Schneidera,1, Peter M. Oppeneera, Hans-Christoph Mertinsb a
Leibniz-Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany b BESSY GmbH, Albert-Einstein-Str. 15, D-12489 Berlin, Germany
Abstract IrMn-based spin-valve systems with CoFe and CoFe/FeNi combinations serving as hard and soft magnetic layers, respectively, were investigated by polarized soft X-ray scattering at photon energies close to the L absorption edges of the constituent elements. Structural and magnetic information was obtained making use of the XMCD (X-ray magnetic circular dichroism) technique in reflection. Hysteresis loops of the individual layers in the spin-valve system were measured element-selectively by tuning the incidence angle of the radiation, opening the possibility of magnetic depth profiling. r 2003 Elsevier B.V. All rights reserved. PACS: 75.47.De; 75.70.Cu; 75.25.+z Keywords: Spin values; X-ray reflectometry; XMCD; Hysteresis loops
1. Introduction In recent years there has been a growing interest in layer stacks composed of different ferromagnetic and antiferromagnetic layers due to their suitability for sensor and data storage technology and magnetoelectronics, as for example in the case of spin-valve systems [1–3]. In order to tailor the magnetic properties of such complex layer stacks, dedicated analytical tools are required which are able to measure the magnetic response of the individual layers in a stack. Resonant X-ray *Corresponding author. Tel.: +49-351-4659-246; fax: +49351-4659-452. E-mail address: [email protected] (M. Hecker). 1 Present address: Institute of Solid State Research, Research Centre Julich, . 52425 Julich, . Germany.
reflection performed with circularly and linearly polarized light is a perfectly suited tool for that purpose [4]. As a photon-in–photon-out technique it provides information about the magnetic state of films at varying magnetic fields, and the magnetic switching can be pursued element-selectively in a surface-sensitive scattering geometry not restricted to thin transmission foils. For the improvement of application systems, the understanding of the hysteresis phenomena in the individual layers of such complex stacks is mandatory. In the present paper the details of the magnetic switching in both the free and pinned layers of spin-valve systems are investigated. It is shown that apart from the element-selectivity of the method attained by tuning the photon energy to the absorption edges of the constituent elements, layer sensitivity can be reached also by
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.11.047
ARTICLE IN PRESS 174
M. Hecker et al. / Physica B 345 (2004) 173–176
tuning the incidence angle and by making use of the interference of the radiation within the layer stack. Thus, also the magnetic properties of different films consisting of the same elements can be distinguished.
2. Experimental Exchange-coupled films with a nominal structure of Si/SiO2/Ta(5 nm)/NiFe(3.5 nm)/ CoFe(0.5 nm)/Cu(2.5 nm)/CoFe(3 nm)/IrMn(8 nm)/ Ta(10 nm)/Cu(0.5 nm) have been prepared by magnetron sputtering. The antiferromagnetic IrMn film is on top of the pinned CoFe film, whereas the free layer is a double-film (0.5 nm CoFe and 3 nm NiFe) below the Cu spacer layer. The pinning direction was set by field cooling, i.e. by application of a magnetic field of 3 T at elevated temperature (280 C) and subsequently cooling down to room temperature in the applied field. The soft X-ray reflection experiments were performed at the BESSY II UE56-PGM undulator beamline using a dedicated reflectometer/polarimeter set-up [5] and circularly polarized light (Pcirc=0.9). The reflectance of the light was measured while a magnetic field was applied along the pinning direction of the spin-valve, which was aligned within the scattering plane (longitudinal geometry). Both reflectometry studies and hysteresis loop measurements were performed at photon energy close to the L absorption edges of Co, Fe and Ni, thus resulting in a strongly enhanced magnetic contrast with sensitivity to the corresponding elements. For the measurement geometry utilized here this contrast is related to the X-ray magnetic circular dichroism (XMCD), measured in reflection geometry.
3. Results and discussion Fig. 1 shows the course of the reflectivity curves measured close to the Co L3 absorption edge for two different directions of the longitudinal magnetic field. The oscillations correspond to the layer structure of the spin-valve and show a significant magnetic contribution, i.e. an intensity difference
for the two signs of the longitudinal magnetic field B. The magnetic part of the reflectometry curve, which appears due to the resonance exchangescattering [6] close to the Co L3 absorption edge (778 eV) and diminishes far from the edge, is attributed to the Co containing layers. At several fixed scattering angles y additionally hysteresis loops were measured. As shown in Fig. 2
Fig. 1. Reflectivity curve of the spin-valve measured for an applied magnetic field B of +8 and –8 mT, respectively, at a photon energy of 775 eV.
Fig. 2. Hysteresis loops measured at photon energies corresponding to the L3 absorption edges of Ni, Co and Fe. Loop 1 corresponds to the Ni, Co and Fe containing free layer, and loop 2 to the pinned layer containing only Co and Fe. The arrows represent the magnetic alignment of the pinned (pl) and free (fl) layer for the corresponding range of the magnetic field. The intensity of each loop is normalized according to 2I/ (Imin+Imax) 1 with Imin and Imax denoting the minimum and maximum intensity of each loop, respectively.
ARTICLE IN PRESS M. Hecker et al. / Physica B 345 (2004) 173–176
for different photon energies, but corresponding scattering vectors, the shape of these loops differs significantly. For the measurements at photon energies at the Co L3 and at the Fe L3 absorption edges, two separated loops can be distinguished. In the case of the investigated spin-valve system, both the free and the pinned magnetic layers contain Co and Fe. It can be assumed, that the minor subloop (loop 1) and the following part for larger negative magnetic fields (loop 2) correspond to the switching of the free and of the pinned layer, respectively. However, a clear proof of this assumption and separation of both parts requires the measurement of only one of the magnetic layers. For the present system, this has been attained by tuning the photon energy to the Ni absorption edge (852 eV), thus being sensitive to the free layer only (Fig. 2, open symbols). The loops of the free and pinned layers differ not only in amplitude, but also distinctly in shape. The complex loop obtained from the Co and Fe signals reveals the switching of the ultrathin lower CoFe layer at small fields simultaneously with the Ni film in a nearly rectangular hysteresis loop, and subsequently the switching of the CoFe pinned layer at larger fields in a more s-shaped loop. In the case of different layers containing the same elements (as here for Fe and also Co), the utilization of the element-selectivity of the XMCD reflection techniques alone may not be sufficient to separate the involved layers of a stack. The interference of the radiation within the stacks visible e.g. in the modulations of the reflectometry curve (Fig. 1) opens an additional possibility to investigate the magnetic switching of the individual layers. As a measure of the magnetic signal, Fig. 3 represents the asymmetry of the reflectometry curve measured at the Co absorption edge, i.e. the difference I+ I between the signals measured for the two signs of BL, normalized by their sum I+ + I . Analogously to the modulation of the average intensity due to the depth profile of the nonmagnetic optical constants in the stack, the magnetic parts also exhibit oscillations corresponding to the depth-profile of the magnetic part of the optical constants. Interestingly, an increase of the amplitude of the magnetic field up to 38 mT does not only increase the measured asymmetry
175
ratio, but leads in certain angular ranges also to its decrease (Fig. 3). In the angular range of such cross-overs between the asymmetry curves, hysteresis loops were measured at constant photon energy (Fig. 4). By tuning the scattering angle y, the intensities of the minor loop corresponding to the free layer and of the loop of the pinned layer can be varied separately. Fig. 4 contains three loops in the angular range around y=12 , where the height of the minor loop (loop 1) decreases from a maximum value to zero, whereas simultaneously the height of loop 2 increases from zero to a
Fig. 3. XMCD asymmetry ratio (I+ I )/(I++I ) of the reflectometry measurements given in Fig. 1 for applied magnetic fields B of 78 mT, and for enlarged fields of 738 mT.
Fig. 4. Hysteresis loops measured close to the Co L3 absorption edge (775 eV) for different scattering angles y.
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maximum value. Therefore, separate measurements of each of the magnetic layers are possible, as nearly realized in the upper and lower curve of Fig. 4. In the case of the thin CoFe film on top of the free layer, the depth resolution corresponding to its thickness is 0.5 nm. Corresponding to the amplitude changes of the two loops between y=11.7 and 12.2 , the sign of the asymmetry also changes in this range. Not only the asymmetry of the sum of both loops, but also the sign of the asymmetry of each of the individual loops can alternate, as observed in other angular ranges. Thus, these measurements contain very sensitive information on the depth-modulation of the magneto-optical constants within the stack, requiring further efforts in the modeling of the scattered intensity from complex stacks for quantitative evaluation of the magneto-optical depth profiling.
4. Conclusions The magnetic switching of an IrMn-based spinvalve system was measured element-selectively by polarized soft X-ray scattering. Thus, individual hysteresis loops attributed to the free and the pinned layer were measured. The near-edge asymmetry effects were observed over a large angular range of the reflectometry curve. Comparing the measurements for different amplitudes of the magnetic field, as a striking feature crossovers
of the asymmetry ratios for different fields were observed. A mapping of hysteresis loops in corresponding angular ranges enabled to correlate this feature with the depth-sensitive probing of the magneto-optical properties of the spin valve. The combination of the element- and depth-sensitivity of the applied scattering method offers the opportunity of magnetic depth profiling in layer stacks.
Acknowledgements We are grateful to Dr. D. Elefant for providing the samples and additional magnetic characterization.
References [1] J. Nogues, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] M.D. Stiles, R.D. Mc Michael, Phys. Rev. B 59 (1999) 3722. [3] B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. [4] O. Zaharko, P.M. Oppeneer, H. Grimmer, M. Horisberger, H.-Ch. Mertins, D. Abramsohn, F. Sch.afers, A. Bill, H.-B. Braun, Phys. Rev. B 66 (2002) 134406. [5] F. Sch.afers, H.-Ch. Mertins, A. Gaupp, W. Gudat, M. Mertin, I. Packe, F. Schmolla, S. Di Fonzo, G. Soullie, W. Jark, R. Walker, X. Le Cann, R. Nyholm, M. Eriksson, Appl. Opt. 38 (1999) 4074. [6] J.P. Hannon, G.T. Trammel, Phys. Rev. Lett. 61 (1988) 1245.