- Email: [email protected]
W multilayers
Journal of Magnetismand Magnetic Materials 165 (1997) 165-167
~ H Journalof magnetism and magnetic ~ H materials
ELSEVIER
Reduced magnetization in Gd/W multilayers Yi Li *, C. Polaczyk, J. Kapoor, D. Riegel ttahn-Meitner-lnstitut Berlin, Glienicker Strafle l O0, 14109 Berlin, Germany
Abstract Structural and magnetic properties of G d / W multilayers prepared by electron-beam evaporation in UHV have been studied. The thickness of the W layers has been kept constant at 18 A and the thickness of Gd layers has been varied from 8 to 85 A. Structural studies reveal the multilayers to have a well layered structure and to be polycrystalline with (0001) and (110) textures for the Gd and W layers, respectively. The magnetizations measured in fields up to 5 T yield an average magnetic moment of about 6.1 /xB/Gd atom, which is significantly smaller than 7.6 /xB known for bulk Gd. The previously observed very high magnetizations in G d / W multilayers [1] are not consistent with our results. Keywords: Magnetization;Multilayers;Magnetic moment - reduced
The magnetism of thin Gd films and multilayers is a subject of intense interest [1-5]. Recently, Heys et al. reported a very large enhancement of the saturation magnetization in G d / W multilayers [1]. According to Ref. [1], the total magnetic moment per Gd atom increases from 8 /xao for thick Gd layers to 34/z B for thin Gd layers (about 8 A), if one assumes the W layers to be paramagnetic. For an interpretation of this suppressing result Heys et al. suggested an unusual large ferromagnetic spin polarization in the W layers in order to explain the observed excess moment. We have investigated magnetic and structural properties of G d / W multilayers and found magnetizations corresponding to a reduced average Gd moment of about 6.1 /xB per Gd atom almost independent of the Gd layer thickness in the range from 8 to 85 ,~. The G d / W multilayers have been prepared by electron-beam evaporation in an ultrahigh vacuum system with a base pressure of 2 × 10 - t ° mbar. The pressure during the growth has remained below 8 × 10 -9 mbar. The multilayers have been deposited on Si(100) substrates at room temperature. A 100 ,~ buffer layer of W has been deposited first, followed by the G d / W multilayers (deposition rate around 6 A / m i n ) which finally have been covered by a 100-A, thick W layer to prevent oxidation. Seven multilayer samples have been prepared with Gd layer thicknesses of 8, 11, 19, 27, 29, 47, 85 ,~ while the thickness of the W layers has been kept constant at 18 A.
* Corresponding author. Email: [email protected]; fax: +49-308063-2523.
The film thicknesses have been monitored in situ using a quartz microbalance calibrated by X-ray reflectivity. Depending on the thickness of the Gd layers, the number of periods has been varied from 9 to 20 so that the total multilayer thicknesses range from 520 to 930 ,~. Parameters of the samples are summarized in Table 1. The structure of the multilayers has been examined in situ by Auger electron spectroscopy (AES) and ex situ by X-ray reflectometry and high-angle X-ray diffractometry. The AES studies yield no indications of contaminants (e.g., O, C, CO) or interface mixing of the G d / W multilayers. Examples of the low-angle X-ray reflection spectra, measured with Cu K,~ radiation, are shown in Fig. 1. The oscillations of the reflections arise from the interference of waves reflected from the surface and the interfaces. The well-defined peaks in the reflectivities allow an accurate determination of the thicknesses of the films, which agree well with those determined from the quartz microbalance. The low-angle reflection and high-angle X-ray diffraction studies reveal that the multilayers exhibit well layeredstructures and are polycrystalline with (0001) and (110) textures for the Gd and W layers, respectively. The magnetic properties of the multilayers have been investigated by SQUID magnetometry. Magnetic hysteresis loops ( M - H curves) have been measured at different temperatures in magnetic fields up to 5 T applied parallel and perpendicular to the film plane. A detailed discussion of the complex magnetic behavior as a function of temperature, Gd film thickness, and strength of the external magnetic field will be given elsewhere [6]; here we concentrate on the study of the saturation magnetization at low temperatures. The observed magnetic hysteresis loops indi-
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00496-9
166
Y. Li et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 165-167 Table 1 Summary of parameters of the Gd/W multilayer samples
100 10 -1 10-2 10-3 ~ 1 0 -4 >
. m
10 .5
Sample
Number of periods
GO(8 A)/W(lS A) Gd(11 ,~)/W(18 ,~) Gd(19 A)/W(18 A) Gd(27 A)/W(18 A)
20 20 20 20
Gd(29 ]k)/W(18 ,~) Gd(47 A)/W(18 A) Gd(85 A)/W(18 A)
13 10 9
10-6 cate that all samples are ferromagnetic at 5 K. From the measured magnetizations we have deduced the average magnetic moment per Gd atom, using the density of bulk Gd and assuming the W layers to be paramagnetic with no contribution to the total moment. As shown in Fig. 2 the magnetic moments per Gd atom come out to be around 6.1 /~a almost independent from the thickness of the Gd films in the range from 8 to 85 ,~. We also have found no significant changes of the magnetization and the shape of hysteresis loops for G d / W samples annealed up to 800 K. These results are not consistent with the strongly enhanced magnetization for thin Gd films in G d / W multilayers reported previously [1].
10 .7
10-a 10 -9
®(deg) Fig. 1. Examples for X-ray reflectivities observed in Gd/W multilayers. For clarity the data for the Gd(19 ,~)/W(18 ,~) multilayer are shifted on the vertical axis.
E
8.5
,..
8.0
o
G d / W Multilayers
.~
O.
o
E o
~ o.o0, 0,000
jS
7.5
-0.001
7.0
-ooo2-4o~oo'-2o~oo
'2o6oo4o6oo
magnetic field (G)
6.5
E •"o
6.0
o 5.5
<>
5.0
a
0
I
20
,
I
40
=
I
60
=
I
80
,
100
Thickness (A) Fig. 2. Average magnetic moment per Gd atom in Gd/W multilayers as a function of the Gd layer thickness. Parameters of the samples are given in Table 1. Inset: Magnetic hysteresis loop at 5 K for the Gd(19 ,~)/W(18 ,~) multilayer.
Y. Li et al. /Journal of Magnetism and Magnetic Materials 165 (1997) 165-167
i
ILl 4 X 1 0 -4 >., "10 3 x 1 0 4
o OQ. 2xlO 4 0 lx104
E o
c
0
2'0
40 60 Thickness (A)
80
100
Fig. 3. Total magnetic moment per Gd layer at 5 K as a function of Gd layer thickness.
Our above described analysis yields an averaged Gd moment in the G d / W multilayers which comes out to be reduced by about 20% compared to the moment of 7.6 /x B known in bulk Gd. Among the possible reasons for such a reduction are (i) reduced (or vanishing) Gd magnetizations at the interfaces between the Gd and W layers, (ii) an antiferromagnetic spin polarization of (parts of) the W layers (iii) a reduced density of the Gd films compared to bulk Gd [7]. Some insight into the role of the contributions (i) and (ii) can be obtained by plotting the observed total magnetic moment per Gd layer ( M t) as a function of the Gd layer thickness (Fig. 3). As shown in Fig. 3 we found a linear relation between M t and d~d which extrapolates to M t = 0 for d~d = 0. This result indicates that the observed reduction of the magnetization can not be attributed to the mechanisms (i) and (ii), since such contributions would cause an intercept at dGd > 0 for M t = 0. We note that the effect (i) has been observed in G d / Y multilayers [8] which in difference to G d / W forms interfacial alloys. Thus, at present, we cannot give a simple straightforward reasoning for the reduced magnetization in the G d / W multilayers under investigation. The atomic Gd moment (7 /z B) is hard to reduce by chemical bonds and a drastic change of the (small) spin polarized 5d band contribution (0.6/x B in bulk Gd) cannot be easily accepted. We suggest the possibility that the magnetic coupling in (small) parts of the Gd films may deviate from being ferromagnetic due to reduced dimension, strains a n d / o r dislocations. In this
167
context we note that reduced Gd magnetizations have been reported for a variety of different Gd systems. Concerning Gd films the average moment per Gd atom in G d / Y multilayers has been analyzed to be about 6 /z B per Gd atom in the interior of the Gd layer after substraction of interface effects [8]. For a number of binary Gd alloys Buschow [9] has found reduced magnetizations as well corresponding to averaged Gd moments in the range 5 - 7 /z B. Moreover, strongly reduced Gd magnetizations (corresponding partly to averaged moments of 3 iz B per Gd atom) have been reported for Gd clusters containing less than 100 Gd atoms [10]. All these results seem to provide support for our suggestion that changes of the magnetic coupling can be of relevance for an explanation of reduced magnetizations in Gd systems. Further experimental and theoretical studies are needed for a better understanding of the reduction of the average Gd moment in G d / W multilayers and other Gd systems. A c k n o w l e d g e m e n t s . We like to acknowledge the support of this work by F. Mezei, A. Weidinger, F. Klose and Ch. Rehm. This work has been supported by the BMBF. References [1] A. Heys, P.E. Donovan, A.K. Petford-Long and R. Cywinski, J. Magn. Magn. Mater. 131 (1994) 265. [2] A. Berger, A.W. Pang and H. Hopster, Phys. Rev. B 52 (1995) 1078. [3] H. Tang, D. Weller, T.G. Walker, J.C. Chappert, H. Hopster, A.W. Pang, D.S. Dessau and D.P. Pappas, Phys. Rev. Lett. 71 (1993) 444. [4] A. Berghaus, M. Farle, Yi Li and K. Baberschke, in: Magnetic Properties of Low-Dimensional Systems II, eds. B.T. Jonker et al. (Springer, Berlin, 1990) p. 61. [5] U. Paschen, C. Siirgers and H. v. LiShneysen, Z. Phys. B 90 (1993) 289. [6] Yi Li, C. Polaczyk and D. Riegel, to be published. [7] Consistent with our analyses of the X-ray reflectivitty data such a reduction of the Gd film densities can range from 0 to a few percent. Therefore such possible differences can only cause a minor part of the differences of the magnetizations observed in the multilayers compared to bulk Gd. [8] C.F. Majkrzak, J. Kwo, M. Hong, Y. Yafet, D. Gibbs, C.L. Chien and J. Bohr, Adv. Phys. 40 (1990) 99. [9] K.H.J. Buschow, Solid State Commun. 27 (1978) 275. [10] D.C. Douglass, J.P. Bucher and L.A. Bloomfield, Phys. Rev. Lett. 68 (1992) 1774.
~ H Journalof magnetism and magnetic ~ H materials
ELSEVIER
Reduced magnetization in Gd/W multilayers Yi Li *, C. Polaczyk, J. Kapoor, D. Riegel ttahn-Meitner-lnstitut Berlin, Glienicker Strafle l O0, 14109 Berlin, Germany
Abstract Structural and magnetic properties of G d / W multilayers prepared by electron-beam evaporation in UHV have been studied. The thickness of the W layers has been kept constant at 18 A and the thickness of Gd layers has been varied from 8 to 85 A. Structural studies reveal the multilayers to have a well layered structure and to be polycrystalline with (0001) and (110) textures for the Gd and W layers, respectively. The magnetizations measured in fields up to 5 T yield an average magnetic moment of about 6.1 /xB/Gd atom, which is significantly smaller than 7.6 /xB known for bulk Gd. The previously observed very high magnetizations in G d / W multilayers [1] are not consistent with our results. Keywords: Magnetization;Multilayers;Magnetic moment - reduced
The magnetism of thin Gd films and multilayers is a subject of intense interest [1-5]. Recently, Heys et al. reported a very large enhancement of the saturation magnetization in G d / W multilayers [1]. According to Ref. [1], the total magnetic moment per Gd atom increases from 8 /xao for thick Gd layers to 34/z B for thin Gd layers (about 8 A), if one assumes the W layers to be paramagnetic. For an interpretation of this suppressing result Heys et al. suggested an unusual large ferromagnetic spin polarization in the W layers in order to explain the observed excess moment. We have investigated magnetic and structural properties of G d / W multilayers and found magnetizations corresponding to a reduced average Gd moment of about 6.1 /xB per Gd atom almost independent of the Gd layer thickness in the range from 8 to 85 ,~. The G d / W multilayers have been prepared by electron-beam evaporation in an ultrahigh vacuum system with a base pressure of 2 × 10 - t ° mbar. The pressure during the growth has remained below 8 × 10 -9 mbar. The multilayers have been deposited on Si(100) substrates at room temperature. A 100 ,~ buffer layer of W has been deposited first, followed by the G d / W multilayers (deposition rate around 6 A / m i n ) which finally have been covered by a 100-A, thick W layer to prevent oxidation. Seven multilayer samples have been prepared with Gd layer thicknesses of 8, 11, 19, 27, 29, 47, 85 ,~ while the thickness of the W layers has been kept constant at 18 A.
* Corresponding author. Email: [email protected]; fax: +49-308063-2523.
The film thicknesses have been monitored in situ using a quartz microbalance calibrated by X-ray reflectivity. Depending on the thickness of the Gd layers, the number of periods has been varied from 9 to 20 so that the total multilayer thicknesses range from 520 to 930 ,~. Parameters of the samples are summarized in Table 1. The structure of the multilayers has been examined in situ by Auger electron spectroscopy (AES) and ex situ by X-ray reflectometry and high-angle X-ray diffractometry. The AES studies yield no indications of contaminants (e.g., O, C, CO) or interface mixing of the G d / W multilayers. Examples of the low-angle X-ray reflection spectra, measured with Cu K,~ radiation, are shown in Fig. 1. The oscillations of the reflections arise from the interference of waves reflected from the surface and the interfaces. The well-defined peaks in the reflectivities allow an accurate determination of the thicknesses of the films, which agree well with those determined from the quartz microbalance. The low-angle reflection and high-angle X-ray diffraction studies reveal that the multilayers exhibit well layeredstructures and are polycrystalline with (0001) and (110) textures for the Gd and W layers, respectively. The magnetic properties of the multilayers have been investigated by SQUID magnetometry. Magnetic hysteresis loops ( M - H curves) have been measured at different temperatures in magnetic fields up to 5 T applied parallel and perpendicular to the film plane. A detailed discussion of the complex magnetic behavior as a function of temperature, Gd film thickness, and strength of the external magnetic field will be given elsewhere [6]; here we concentrate on the study of the saturation magnetization at low temperatures. The observed magnetic hysteresis loops indi-
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00496-9
166
Y. Li et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 165-167 Table 1 Summary of parameters of the Gd/W multilayer samples
100 10 -1 10-2 10-3 ~ 1 0 -4 >
. m
10 .5
Sample
Number of periods
GO(8 A)/W(lS A) Gd(11 ,~)/W(18 ,~) Gd(19 A)/W(18 A) Gd(27 A)/W(18 A)
20 20 20 20
Gd(29 ]k)/W(18 ,~) Gd(47 A)/W(18 A) Gd(85 A)/W(18 A)
13 10 9
10-6 cate that all samples are ferromagnetic at 5 K. From the measured magnetizations we have deduced the average magnetic moment per Gd atom, using the density of bulk Gd and assuming the W layers to be paramagnetic with no contribution to the total moment. As shown in Fig. 2 the magnetic moments per Gd atom come out to be around 6.1 /~a almost independent from the thickness of the Gd films in the range from 8 to 85 ,~. We also have found no significant changes of the magnetization and the shape of hysteresis loops for G d / W samples annealed up to 800 K. These results are not consistent with the strongly enhanced magnetization for thin Gd films in G d / W multilayers reported previously [1].
10 .7
10-a 10 -9
®(deg) Fig. 1. Examples for X-ray reflectivities observed in Gd/W multilayers. For clarity the data for the Gd(19 ,~)/W(18 ,~) multilayer are shifted on the vertical axis.
E
8.5
,..
8.0
o
G d / W Multilayers
.~
O.
o
E o
~ o.o0, 0,000
jS
7.5
-0.001
7.0
-ooo2-4o~oo'-2o~oo
'2o6oo4o6oo
magnetic field (G)
6.5
E •"o
6.0
o 5.5
<>
5.0
a
0
I
20
,
I
40
=
I
60
=
I
80
,
100
Thickness (A) Fig. 2. Average magnetic moment per Gd atom in Gd/W multilayers as a function of the Gd layer thickness. Parameters of the samples are given in Table 1. Inset: Magnetic hysteresis loop at 5 K for the Gd(19 ,~)/W(18 ,~) multilayer.
Y. Li et al. /Journal of Magnetism and Magnetic Materials 165 (1997) 165-167
i
ILl 4 X 1 0 -4 >., "10 3 x 1 0 4
o OQ. 2xlO 4 0 lx104
E o
c
0
2'0
40 60 Thickness (A)
80
100
Fig. 3. Total magnetic moment per Gd layer at 5 K as a function of Gd layer thickness.
Our above described analysis yields an averaged Gd moment in the G d / W multilayers which comes out to be reduced by about 20% compared to the moment of 7.6 /x B known in bulk Gd. Among the possible reasons for such a reduction are (i) reduced (or vanishing) Gd magnetizations at the interfaces between the Gd and W layers, (ii) an antiferromagnetic spin polarization of (parts of) the W layers (iii) a reduced density of the Gd films compared to bulk Gd [7]. Some insight into the role of the contributions (i) and (ii) can be obtained by plotting the observed total magnetic moment per Gd layer ( M t) as a function of the Gd layer thickness (Fig. 3). As shown in Fig. 3 we found a linear relation between M t and d~d which extrapolates to M t = 0 for d~d = 0. This result indicates that the observed reduction of the magnetization can not be attributed to the mechanisms (i) and (ii), since such contributions would cause an intercept at dGd > 0 for M t = 0. We note that the effect (i) has been observed in G d / Y multilayers [8] which in difference to G d / W forms interfacial alloys. Thus, at present, we cannot give a simple straightforward reasoning for the reduced magnetization in the G d / W multilayers under investigation. The atomic Gd moment (7 /z B) is hard to reduce by chemical bonds and a drastic change of the (small) spin polarized 5d band contribution (0.6/x B in bulk Gd) cannot be easily accepted. We suggest the possibility that the magnetic coupling in (small) parts of the Gd films may deviate from being ferromagnetic due to reduced dimension, strains a n d / o r dislocations. In this
167
context we note that reduced Gd magnetizations have been reported for a variety of different Gd systems. Concerning Gd films the average moment per Gd atom in G d / Y multilayers has been analyzed to be about 6 /z B per Gd atom in the interior of the Gd layer after substraction of interface effects [8]. For a number of binary Gd alloys Buschow [9] has found reduced magnetizations as well corresponding to averaged Gd moments in the range 5 - 7 /z B. Moreover, strongly reduced Gd magnetizations (corresponding partly to averaged moments of 3 iz B per Gd atom) have been reported for Gd clusters containing less than 100 Gd atoms [10]. All these results seem to provide support for our suggestion that changes of the magnetic coupling can be of relevance for an explanation of reduced magnetizations in Gd systems. Further experimental and theoretical studies are needed for a better understanding of the reduction of the average Gd moment in G d / W multilayers and other Gd systems. A c k n o w l e d g e m e n t s . We like to acknowledge the support of this work by F. Mezei, A. Weidinger, F. Klose and Ch. Rehm. This work has been supported by the BMBF. References [1] A. Heys, P.E. Donovan, A.K. Petford-Long and R. Cywinski, J. Magn. Magn. Mater. 131 (1994) 265. [2] A. Berger, A.W. Pang and H. Hopster, Phys. Rev. B 52 (1995) 1078. [3] H. Tang, D. Weller, T.G. Walker, J.C. Chappert, H. Hopster, A.W. Pang, D.S. Dessau and D.P. Pappas, Phys. Rev. Lett. 71 (1993) 444. [4] A. Berghaus, M. Farle, Yi Li and K. Baberschke, in: Magnetic Properties of Low-Dimensional Systems II, eds. B.T. Jonker et al. (Springer, Berlin, 1990) p. 61. [5] U. Paschen, C. Siirgers and H. v. LiShneysen, Z. Phys. B 90 (1993) 289. [6] Yi Li, C. Polaczyk and D. Riegel, to be published. [7] Consistent with our analyses of the X-ray reflectivitty data such a reduction of the Gd film densities can range from 0 to a few percent. Therefore such possible differences can only cause a minor part of the differences of the magnetizations observed in the multilayers compared to bulk Gd. [8] C.F. Majkrzak, J. Kwo, M. Hong, Y. Yafet, D. Gibbs, C.L. Chien and J. Bohr, Adv. Phys. 40 (1990) 99. [9] K.H.J. Buschow, Solid State Commun. 27 (1978) 275. [10] D.C. Douglass, J.P. Bucher and L.A. Bloomfield, Phys. Rev. Lett. 68 (1992) 1774.