- Email: [email protected]
Fe multilayers
Journal of Magnetism and Magnetic Materials 156 (1996) 58-60
N
ELSEVIER
~ H Journalof magnetism 4 ~ i and magnetic materials
Neutron reflectivity on Tb/Fe multilayers J. Tappert
a, *
, F. K l o s e b C h .
Rehm b, W . S .
K i m a R.A. B r a n d a H. M a l e t t a b
W. K e u n e a a Angewandte Phvsik Gerhard-Mercator-Universitiit, 47048 Duisburg, Germany b Hahn-Meitner-lnstitut Berlin, Glienicker Strasse 100, 14109 Berlin, Germany
Abstract Temperature-dependent spin-polarized neutron reflectometry and SQUID magnetization measurements have been carried out on a [26 A T b / 5 0 A Fe] X l0 multilayer, which shows perpendicular anisotropy. By comparing the results with previous M~issbauer spectroscopy data, we can develop a detailed model which can describe how the Fe spins turn out of the layer plane at lower temperatures. This magnetic reorientation process leads to a canted spin structure. The SQUID data indicate increasing Tb moments at low temperatures, which are coupled antiparallel to the Fe moments. The field dependence of the orientation of the Tb and Fe moments is discussed.
We have shown in previous experiments with M~Sssbauer spectroscopy that the perpendicular anisotropy present in T b / F e multilayers does not lead to a simple magnetic texture. This texture, as given by the average canting angle ( O ) (the angle between the Fe moment and the film normal direction), is never completely perpendicular, and it is even temperature dependent, changing from in-plane at high to at least strongly out-of-plane at low temperatures [1-3]. Partly similar behaviour was recently found in C e H J F e multilayers [4]. To get a better understanding of the temperature and field dependence of the orientations of the Fe and Tb moments, we have carried out spin-polarized neutron reflectometry with subsequent polarization analysis (SPNR) and SQUID magnetization measurements. The first method can deliver a magnetic depth profile of the multilayer, while the latter can measure the averaged magnetization with high precision. The results are compared with the previous M5ssbauer data. We have performed the experiments on a [26 ,~ T b / 5 0 A_ Fe] X 10 multilayer at the V6 neutron reflectometer at the Hahn-Meitner-Institut in Berlin. This instrument is working in a 0 / 2 0 geometry (vertical scattering plane) with 4.7 ,~ neutrons. The magnetic field is applied horizontally, transverse to the beam and the polarization of the neutrons is parallel (' + ') or antiparallel (' - ') to the field direction. In this geometry the neutrons are sensitive only to the in-plane components of the sample magnetization. If the magnetic moments in the film are aligned along the
* Corresponding author. Fax: [email protected].
+49-203-379-3163;
email:
external magnetic field, the spin state of the neutrons is conserved, but in-plane components perpendicular to Hext can lead to spin-flip processes during reflection. Thus SPNR is very sensitive to a canted spin structure if all four possible reflectivity channels are being measured (two non-spin-flip reflectivities R ++, R - - and two spin-flip reflectivities R +-, R - + ) . Out of all our temperature-dependent measurements, we present two data sets: one at high temperature representing the in-plane-oriented state of the magnetic moments; the other at lower temperature representing the out-of-plane case. Fig. la shows the non-spin-flip neutron reflectivity curves of the sample at T = 350 K in an applied field of He×t = 800 G. From the SQUID measurements (compare Fig. 2) we know that in this field the sample is completely saturated. The splitting of the reflectivity curves for parallel (R ++) and antiparallel ( R - - ) polarized neutrons (relative to Hcxt) reflects the in-plane magnetization of the sample. A fit to the data corresponds to a reduced Fe moment ( - 2 0 % ) and paramagnetic Tb. In Fig. lb the temperature is lowered to 100 K. Now the splitting of the curves is much smaller than at 350 K reflecting the out-of-plane turning of the Fe moments. Here the external field of 800 G is not sufficient to saturate the sample. By fitting the data, we can estimate the in-plane projection of the Fe moment in the direction of Hext to be l.l/x B. During cooling down, Tb has ordered ferromagnetically (compare SQUID data below) and shows a projected moment of 0.1/x B, which is antiparallel to the Fe moment. In Fig. lc the temperature dependence of the spin-flip reflectivity R + (measured by analyzing the neutron spin state after reflection) is shown. The values were taken at
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 7 8 5 - 7
J. Tappert et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 58-60 10°~ >,
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T [K] Fig. 1. Non-spin-flip neutron reflectivities R ++ and R - of the [26 A Yb/50 ,~ Fe] X 10 multilayer: (a) at 350 K; (b) at 100 K. (c) Temperature dependence of the spin-flip reflectivity R +- measured at the muhilayer Bragg peak at Q=0.088 ~,-I. The magnetic field H~×~ was 800 G during all measurements.
the maximum of the multilayer Bragg peak (Q = 0.088 ,~ ]). While we cannot observe spin-flip scattering at 350 K, the spin-flip signal strongly increases by lowering the temperature. Since the spin-flip signal is related to in-plane components of the magnetization, which points perpendicular to the external field, we have a clear indication of a canted spin structure developing at lower temperatures. Fig. 2 shows hysteresis loops of the multilayer at 350 and 100 K taken perpendicular and parallel to the sample surface. At 350 K the magnetic easy axis is in the sample plane. Here (Fig. 2a) the multilayer can be saturated in small fields of ~ 200 G in contrast to the perpendicular axis (Fig. 2b) where the saturation field is ~ 16 kG.
59
Taking into account the reduced Fe moment, this value is only slightly lower than the expected saturation field 47rM s (resulting from demagnetization energy), which can be calculated from the saturation magnetization M s . During cooling of the sample the general shape of the hysteresis loops is preserved at first, but the perpendicular saturation field tends to decrease while the in-plane value increases. This indicates that the perpendicular anisotropy becomes more important. At 200 K the shape of the loops suddenly changes: Now the available field of 55 kG is no longer high enough to saturate the multilayer neither in-plane nor perpendicular. This must be connected with the ferromagnetic ordering in the Tb layers (Tc = 220 K for bulk Tb). Since the relative Fe and Tb layer thicknesses are chosen such that. even if Tb would develop the bulk moment of 9.7/z B at low temperatures, the total Fe moment dominates the sample, and therefore the Fe sub[attice points into the field direction. Obviously, at first the Tb moments couple antiparallel to the Fe moments but get more and more field aligned with increasing external field. At lower temperatures, hysteresis effects develop, more pronounced in the perpendicular loop (Fig. 2d) than in the in-plane case (Fig. 2c). Perpendicular hysteresis loops measured with M~ssbauer spectroscopy show that the Fe moments are practically saturated in the perpendicular direction at the same field of ~ 6 kG as the closure point of the hysteresis loop in Fig. 2d. Thus the hysteresis is dominated by cycling the Fe moments with the magnetic field while the Tb moments are coupled antiparallel. The fact that the Fe moments are perpendicularly saturated at ~ 6 kG allows us to estimate the Tb moment by subtracting the magnetization at this point of the hysteresis curve from the saturation value at high temperatures (under the assumption that at 350 K the Tb moments are zero and that the Fe moments do not change during cooling down). Doing so, we obtain a drastic increase of the Tb moment, starting at 200 K and increasing up to 4/.t B at 100 K. A further increase of the field beyond the characteristic value of 6 kG (Fig. 2d) leads to a gradual turning of the Tb moments into the field direction. Note that for field values beyond 40 kG the total moment of the sample at 100 K is higher than the saturation moment at 350 K (resulting exclusively from the Fe part). This means that the average angle between Fe and Tb is already smaller than 90 ° (antiparallel coupling = 180°). The in-plane hysteresis loop (Fig. 2c) looks different. First, the hysteresis is less pronounced than in the perpendicular case, second the high field slope is not constant. Both are characteristics of reversible turning processes of the magnetic moments. Unlortunately, we have no MSssbauer data to determine at which field the Fe moments are saturated in-plane, but we can speculate that the hysteresis develops from domain wall movements which are connected with the alignment of the randomly distributed Fe moments (without changing their average canting angle of ((0} = 4 0 ° relative to the sample normal,
60
J. Tappert et al. // Journal of Magnetism and Magnetic Materials 156 (1996) 58-60 1500.
1500• 1000-
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Magnetic Field [kGauss] Fig. 2. In-plane and perpendicular hysteresis loops measured at 350 and 100 K with a SQUID magnetometer.
which was measured with MiSssbauer spectroscopy). In a second step, the increasing external field should result in a turning of the Fe moments into the layer plane. Additionally the external field destroys the antiparallel coupling of the Tb and Fe layers. Also for the in-plane case in high fields, the total moment of the sample is higher than can be put down to the Fe contribution alone. Acknowledgements: This work was supported by BMFT and Deutsche Forschungsgemeinschaft within SFB 166.
References [1] B. Scholz, R.A. Brand and W. Keune, J. Magn. Magn. Mater. 104-107 (1992) 1889. [2] B• Scholz, R.A. Brand and W. Keune, Hyperfine Interactions 68 (1991) 409. [3] B. Scholz, R.A. Brand and W. Keune, Phys. Rev. B 50 (1994) 2537. [4] O. Schulte, F. Klose and W. Felsch, Phys. Rev. B 52 (1995) 6480.
N
ELSEVIER
~ H Journalof magnetism 4 ~ i and magnetic materials
Neutron reflectivity on Tb/Fe multilayers J. Tappert
a, *
, F. K l o s e b C h .
Rehm b, W . S .
K i m a R.A. B r a n d a H. M a l e t t a b
W. K e u n e a a Angewandte Phvsik Gerhard-Mercator-Universitiit, 47048 Duisburg, Germany b Hahn-Meitner-lnstitut Berlin, Glienicker Strasse 100, 14109 Berlin, Germany
Abstract Temperature-dependent spin-polarized neutron reflectometry and SQUID magnetization measurements have been carried out on a [26 A T b / 5 0 A Fe] X l0 multilayer, which shows perpendicular anisotropy. By comparing the results with previous M~issbauer spectroscopy data, we can develop a detailed model which can describe how the Fe spins turn out of the layer plane at lower temperatures. This magnetic reorientation process leads to a canted spin structure. The SQUID data indicate increasing Tb moments at low temperatures, which are coupled antiparallel to the Fe moments. The field dependence of the orientation of the Tb and Fe moments is discussed.
We have shown in previous experiments with M~Sssbauer spectroscopy that the perpendicular anisotropy present in T b / F e multilayers does not lead to a simple magnetic texture. This texture, as given by the average canting angle ( O ) (the angle between the Fe moment and the film normal direction), is never completely perpendicular, and it is even temperature dependent, changing from in-plane at high to at least strongly out-of-plane at low temperatures [1-3]. Partly similar behaviour was recently found in C e H J F e multilayers [4]. To get a better understanding of the temperature and field dependence of the orientations of the Fe and Tb moments, we have carried out spin-polarized neutron reflectometry with subsequent polarization analysis (SPNR) and SQUID magnetization measurements. The first method can deliver a magnetic depth profile of the multilayer, while the latter can measure the averaged magnetization with high precision. The results are compared with the previous M5ssbauer data. We have performed the experiments on a [26 ,~ T b / 5 0 A_ Fe] X 10 multilayer at the V6 neutron reflectometer at the Hahn-Meitner-Institut in Berlin. This instrument is working in a 0 / 2 0 geometry (vertical scattering plane) with 4.7 ,~ neutrons. The magnetic field is applied horizontally, transverse to the beam and the polarization of the neutrons is parallel (' + ') or antiparallel (' - ') to the field direction. In this geometry the neutrons are sensitive only to the in-plane components of the sample magnetization. If the magnetic moments in the film are aligned along the
* Corresponding author. Fax: [email protected].
+49-203-379-3163;
email:
external magnetic field, the spin state of the neutrons is conserved, but in-plane components perpendicular to Hext can lead to spin-flip processes during reflection. Thus SPNR is very sensitive to a canted spin structure if all four possible reflectivity channels are being measured (two non-spin-flip reflectivities R ++, R - - and two spin-flip reflectivities R +-, R - + ) . Out of all our temperature-dependent measurements, we present two data sets: one at high temperature representing the in-plane-oriented state of the magnetic moments; the other at lower temperature representing the out-of-plane case. Fig. la shows the non-spin-flip neutron reflectivity curves of the sample at T = 350 K in an applied field of He×t = 800 G. From the SQUID measurements (compare Fig. 2) we know that in this field the sample is completely saturated. The splitting of the reflectivity curves for parallel (R ++) and antiparallel ( R - - ) polarized neutrons (relative to Hcxt) reflects the in-plane magnetization of the sample. A fit to the data corresponds to a reduced Fe moment ( - 2 0 % ) and paramagnetic Tb. In Fig. lb the temperature is lowered to 100 K. Now the splitting of the curves is much smaller than at 350 K reflecting the out-of-plane turning of the Fe moments. Here the external field of 800 G is not sufficient to saturate the sample. By fitting the data, we can estimate the in-plane projection of the Fe moment in the direction of Hext to be l.l/x B. During cooling down, Tb has ordered ferromagnetically (compare SQUID data below) and shows a projected moment of 0.1/x B, which is antiparallel to the Fe moment. In Fig. lc the temperature dependence of the spin-flip reflectivity R + (measured by analyzing the neutron spin state after reflection) is shown. The values were taken at
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 7 8 5 - 7
J. Tappert et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 58-60 10°~ >,
"5 ¢,
10-1
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(a)
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10.2 *5 Z
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,\ •
~ 2 X 1 0 3-
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2;0
210
3;0
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350
T [K] Fig. 1. Non-spin-flip neutron reflectivities R ++ and R - of the [26 A Yb/50 ,~ Fe] X 10 multilayer: (a) at 350 K; (b) at 100 K. (c) Temperature dependence of the spin-flip reflectivity R +- measured at the muhilayer Bragg peak at Q=0.088 ~,-I. The magnetic field H~×~ was 800 G during all measurements.
the maximum of the multilayer Bragg peak (Q = 0.088 ,~ ]). While we cannot observe spin-flip scattering at 350 K, the spin-flip signal strongly increases by lowering the temperature. Since the spin-flip signal is related to in-plane components of the magnetization, which points perpendicular to the external field, we have a clear indication of a canted spin structure developing at lower temperatures. Fig. 2 shows hysteresis loops of the multilayer at 350 and 100 K taken perpendicular and parallel to the sample surface. At 350 K the magnetic easy axis is in the sample plane. Here (Fig. 2a) the multilayer can be saturated in small fields of ~ 200 G in contrast to the perpendicular axis (Fig. 2b) where the saturation field is ~ 16 kG.
59
Taking into account the reduced Fe moment, this value is only slightly lower than the expected saturation field 47rM s (resulting from demagnetization energy), which can be calculated from the saturation magnetization M s . During cooling of the sample the general shape of the hysteresis loops is preserved at first, but the perpendicular saturation field tends to decrease while the in-plane value increases. This indicates that the perpendicular anisotropy becomes more important. At 200 K the shape of the loops suddenly changes: Now the available field of 55 kG is no longer high enough to saturate the multilayer neither in-plane nor perpendicular. This must be connected with the ferromagnetic ordering in the Tb layers (Tc = 220 K for bulk Tb). Since the relative Fe and Tb layer thicknesses are chosen such that. even if Tb would develop the bulk moment of 9.7/z B at low temperatures, the total Fe moment dominates the sample, and therefore the Fe sub[attice points into the field direction. Obviously, at first the Tb moments couple antiparallel to the Fe moments but get more and more field aligned with increasing external field. At lower temperatures, hysteresis effects develop, more pronounced in the perpendicular loop (Fig. 2d) than in the in-plane case (Fig. 2c). Perpendicular hysteresis loops measured with M~ssbauer spectroscopy show that the Fe moments are practically saturated in the perpendicular direction at the same field of ~ 6 kG as the closure point of the hysteresis loop in Fig. 2d. Thus the hysteresis is dominated by cycling the Fe moments with the magnetic field while the Tb moments are coupled antiparallel. The fact that the Fe moments are perpendicularly saturated at ~ 6 kG allows us to estimate the Tb moment by subtracting the magnetization at this point of the hysteresis curve from the saturation value at high temperatures (under the assumption that at 350 K the Tb moments are zero and that the Fe moments do not change during cooling down). Doing so, we obtain a drastic increase of the Tb moment, starting at 200 K and increasing up to 4/.t B at 100 K. A further increase of the field beyond the characteristic value of 6 kG (Fig. 2d) leads to a gradual turning of the Tb moments into the field direction. Note that for field values beyond 40 kG the total moment of the sample at 100 K is higher than the saturation moment at 350 K (resulting exclusively from the Fe part). This means that the average angle between Fe and Tb is already smaller than 90 ° (antiparallel coupling = 180°). The in-plane hysteresis loop (Fig. 2c) looks different. First, the hysteresis is less pronounced than in the perpendicular case, second the high field slope is not constant. Both are characteristics of reversible turning processes of the magnetic moments. Unlortunately, we have no MSssbauer data to determine at which field the Fe moments are saturated in-plane, but we can speculate that the hysteresis develops from domain wall movements which are connected with the alignment of the randomly distributed Fe moments (without changing their average canting angle of ((0} = 4 0 ° relative to the sample normal,
60
J. Tappert et al. // Journal of Magnetism and Magnetic Materials 156 (1996) 58-60 1500.
1500• 1000-
(a)
1000'
500-
500.
LL
%
0-
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Magnetic Field [kGauss] Fig. 2. In-plane and perpendicular hysteresis loops measured at 350 and 100 K with a SQUID magnetometer.
which was measured with MiSssbauer spectroscopy). In a second step, the increasing external field should result in a turning of the Fe moments into the layer plane. Additionally the external field destroys the antiparallel coupling of the Tb and Fe layers. Also for the in-plane case in high fields, the total moment of the sample is higher than can be put down to the Fe contribution alone. Acknowledgements: This work was supported by BMFT and Deutsche Forschungsgemeinschaft within SFB 166.
References [1] B. Scholz, R.A. Brand and W. Keune, J. Magn. Magn. Mater. 104-107 (1992) 1889. [2] B• Scholz, R.A. Brand and W. Keune, Hyperfine Interactions 68 (1991) 409. [3] B. Scholz, R.A. Brand and W. Keune, Phys. Rev. B 50 (1994) 2537. [4] O. Schulte, F. Klose and W. Felsch, Phys. Rev. B 52 (1995) 6480.