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Pd trilayers
Journal of Magnetism and Magnetic Materials 239 (2002) 313–315
Enhancement of magnetic anisotropy of hydrogenated Pd/Co/Pd trilayers Satoshi Okamoto*, Osamu Kitakami, Yutaka Shimada Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Abstract Magnetic and structural properties of Pd/Co/Pd trilayers in hydrogen atmosphere (pressure PH ) have been investigated. The Pd layers are well hydrogenated and fully transformed to b-Pd–H phase at PH ¼ 1 atm, but this b phase is distorted along the film normal under the restriction of substrate, and the atomic ratio of H/Pd is estimated to be 0.6. Hydrogenation at low PH gives rise to enhancement of perpendicular magnetic anisotropy K eff : For instance, ( 3 A-thick Co at PH ¼ 1:3 atm exhibits very high K eff (3.5 107 erg/cm3) being two times larger than that of as-prepared one (1.6 107 erg/cm3). Further increase in PH decreases K eff gradually, while K eff recovers to some extent after full dehydrogenation. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic anisotropy; Trilayers
( sandwiched between Ultra-thin films of Co (o10 A) Pd show perpendicular magnetic anisotropy (PMA) due to strong magnetic surface anisotropy (MSA) [1]. It is well known that Pd can absorb a large amount of hydrogen. When the atomic ratio x (=H/Pd) reaches xX0:6; a-Pd–H phase fully transforms to b-Pd–H phase in bulk Pd [2]. This transformation causes large lattice expansion up to several percent. Moreover, although pure Pd shows the largest magnetic susceptibility among 4d transition metals, it goes down to zero at xE0:6 [2]. Recently, some attempts have been carried out to control the magnetic properties of ferromagnetic materials by hydrogen absorption [3,4]. Since ultra-thin films are much more sensitive to the surface/interface state, it can be expected that magnetic properties of ultra-thin Co films will be drastically modified by hydrogenation of adjacent Pd layer. In the present study, we have investigated magnetic and structural properties of Pd/Co/Pd trilayers processed in hydrogen gas atmosphere. Samples are trilayer ( ( ( structure of Pd (50 A)/Co (d ¼ 3–115 A)/Pd (200 A) *Corresponding author. Tel.: +81-22-217-5359; fax: +8122-217-5404. E-mail address: [email protected] (S. Okamoto).
grown on Si substrates. The structural changes by hydrogen absorption were traced by X-ray diffraction (XRD) with Cu Ka line in hydrogen atmosphere. The magnetic anisotropy and magnetization were measured by using a magneto-optical Kerr effect (MOKE) system and a vibrating sample magnetometer (VSM). The samples were encapsulated in a quartz tube at fixed hydrogen gas pressure PH ranging from 1 to 3 atm. The detail of sample preparation and anisotropy measurements are described elsewhere [5,6]. By the electric resistivity measurements, it was confirmed that Pd layers in Pd/Co/Pd were well hydrogenated for the whole PH range examined in the present study, and hydrogen was found to be desorbed within several minutes after exposure in the air [6]. Fig. 1 shows the change of XRD profiles for a Pd/Co ( (5 A)/Pd trilayer in hydrogenation at PH ¼ 1 atm and dehydrogenation processes. Note that Pd (1 1 1) and (2 2 2) shift to lower angle by hydrogen absorption and then go back to near the original position after full desorption, indicating that aas-b-adehyd transformation occurs in these processes. We also found that all Pd(–H) (1 1 1) and (2 2 2) peak shapes can be fitted very well by a single Gaussian, meaning that both the top and bottom Pd layers are uniformly hydrogenated. Moreover, the diffraction intensity for the hydrogenated and
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 9 6 - 0
314
S. Okamoto et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 313–315
40
60 2θ (deg.)
80
( Fig. 1. XRD profiles of a Pd/Co (5 A)/Pd trilayer for before and after hydrogenation at PH ¼ 1 atm and after dehydrogenation. The circles and triangles indicate Pd(–H) (1 1 1) and (2 2 2) diffraction peaks, respectively. Others come from Si substrate.
Table 1 Characteristic structural parameters of aas, adehyd and b phase
( a1 1 1 (A) ( d1 1 1 (A) FWHM (deg.)
aas
adehyd
b
2.78 2.25 6.90
2.78 2.23 4.42
2.84 2.35 4.32
the dehydrogenated samples is enhanced significantly compared with that of the as-prepared one. According to rocking curve measurements of Pd(–H) (1 1 1) peak, the full width at half maximum (FWHM) is decreased by hydrogenation even after hydrogen is fully desorbed (see Table 1). This result indicates that (1 1 1) orientation is appreciably improved by hydrogenation. Since the lateral lattice expansion of Pd/Co/Pd is restrained by substrates, the structure of our b phase should be distorted along the film normal [1 1 1]. In order to characterize the structure of the b phase, (2 0 0) and (2 2 0) diffractions were observed by the XRD configuration of the sample tilted from the X-ray incident plane. By numerical fitting of each peak position, interatomic distance a1 1 1 in (1 1 1) plane and (1 1 1) spacing d1 1 1 were determined as listed in Table 1. The results clearly indicate that the b phase elongates along [1 1 1]. Moreover, the dehydrogenated adehyd slightly contracts along [1 1 1]. Assuming isovolumetric distortion of the b phase, hydrogen content x is estimated to be E0.6 from the calculated unit cell volume by using the relationship between the lattice constant and x [3], and this estimated x coincides with the hydrogen content where susceptibility vanishes [2]. It is well known that magnetization of Pd/Co/Pd trilayers increases with decreasing Co thickness d [7], probably due to polarization of interfacial Pd. Since the estimated
8
10
: as-prepared : PH = 1.3 atm : PH = 2 atm : PH = 3 atm : dehydrogenated
5 3
as-prepared
eff
hydrogenated
K (erg/cm )
Intensity (a.u.)
dehydrogenated
value of xE0:6 at PH ¼ 1 atm coincides with the composition of nonmagnetic b phase, no Pd polarization due to proximity effect is expected. However, the induced magnetization by Pd polarization of b-Pd/Co/ b-Pd decreased only by 40% compared with that of Pd/ Co/Pd [6]. The reason for this result is now under investigation. From the dependence of K eff ð¼ KU 22pMS2 Þ on Co layer thickness d in Fig. 2, K eff increases with decreasing d; indicating existence of large MSA. Three remarkable features can be noticed in this figure. First, K eff is significantly enhanced by hydrogen absorption at low ( shows very high PH : Especially, very thin Co (d ¼ 3 A) K eff (3.5 107 erg/cm3) which is two times larger than that of the as-prepared (1.6 107 emu/cm3). Second, increase of PH above 1.3 atm leads to monotonic decrease in K eff : Third, K eff recovers again after full desorption of hydrogen. In order to understand the hydrogen absorption effect on the perpendicular magnetic anisotropy (PMA) of Pd/ Co/Pd trilayers, the following three effects should be taken into account. The first is the magnetoelastic effect due to lattice expansion of Pd. Since the magnetostriction coefficient of Co is negative, in-plane tensile stress would contribute to perpendicular anisotropy. However, this effect is trivial as verified by a simple calculation. In fact, this effect cannot explain large difference in PMA between as-prepared and fully dehydrogenated samples, which possess exactly the same interatomic distance a1 1 1 shown in Table 1. The second is the change in Pd polarization due to hydrogen absorption. It is well known that magnetic susceptibility of Pd is weakened by incorporation of hydrogen atoms. Probably this effect would explain gradual decrease in PMA with PH shown in Fig. 2. The third point we have to notice is the drastic enhancement of Pd (1 1 1) orientation by hydrogen absorption (Fig. 1 and Table 1). From a lot of experiments, we found a strong correlation between K eff and the degree of Pd (1 1 1) orientation. For
2 7
10
5
3
4
5
Thickness d ( )
( Fig. 2. K eff of Pd/Co (d A)/Pd trilayer for before and after hydrogenation and after dehydrogenation as a function of d:
S. Okamoto et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 313–315
( instance, K eff of dehydrogenated Pd/Co(3 A)/Pd (solid triangles in Fig. 2) is two times larger than that of asprepared one (solid circles in Fig. 2) in the whole range of Co thickness, although the lattice parameters are almost the same. This result leads us to think that PMA is enhanced by improvement of Pd (1 1 1) orientation although the reason still remains unclear. As mentioned above, hydrogen absorption in Pd/Co/ Pd causes irreversible enhancement of Pd (1 1 1) orientation along with the significant increase in perpendicular magnetic anisotropy. The anisotropy decreases with increasing the amount of absorbed hydrogen probably due to weakening of Pd polarization. After full desorption of hydrogen the anisotropy recovers again to high value. This work is supported by the Research for the Future Program of Japan for the Promotion of Science under
315
Grant No. 97R14701, and the Storage Research Consortium in Japan.
References [1] P.F. Carcia, A.D. Meinhaldt, A. Suna, Appl. Phys. Lett. 47 (1985) 178. [2] H.C. Jamieson, F.D. Manchester, J. Phys. F 2 (1972) 323. [3] T. Manago, H. Miyajima, Y. Otani, E. Akiba, J. Phys. Soc. Japan 66 (1997) 831. . [4] D. Labergeriec, C. Sutter, H. Zabel, B. Hjorvarsson, J. Magn. Magn. Mater. 192 (1999) 238. [5] S. Okamoto, K. Nishiyama, O. Kitakami, Y. Shimada, J. Appl. Phys., to be published. [6] S. Okamoto, O. Kitakami, Y. Shimada, J. Magn. Soc. Japan 25 (2001) 839. [7] B.N. Engel, C.D. England, R.V. Leeuwen, M. Nakada, C.M. Falco, J. Appl. Phys. 69 (1991) 5643.
Enhancement of magnetic anisotropy of hydrogenated Pd/Co/Pd trilayers Satoshi Okamoto*, Osamu Kitakami, Yutaka Shimada Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Abstract Magnetic and structural properties of Pd/Co/Pd trilayers in hydrogen atmosphere (pressure PH ) have been investigated. The Pd layers are well hydrogenated and fully transformed to b-Pd–H phase at PH ¼ 1 atm, but this b phase is distorted along the film normal under the restriction of substrate, and the atomic ratio of H/Pd is estimated to be 0.6. Hydrogenation at low PH gives rise to enhancement of perpendicular magnetic anisotropy K eff : For instance, ( 3 A-thick Co at PH ¼ 1:3 atm exhibits very high K eff (3.5 107 erg/cm3) being two times larger than that of as-prepared one (1.6 107 erg/cm3). Further increase in PH decreases K eff gradually, while K eff recovers to some extent after full dehydrogenation. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic anisotropy; Trilayers
( sandwiched between Ultra-thin films of Co (o10 A) Pd show perpendicular magnetic anisotropy (PMA) due to strong magnetic surface anisotropy (MSA) [1]. It is well known that Pd can absorb a large amount of hydrogen. When the atomic ratio x (=H/Pd) reaches xX0:6; a-Pd–H phase fully transforms to b-Pd–H phase in bulk Pd [2]. This transformation causes large lattice expansion up to several percent. Moreover, although pure Pd shows the largest magnetic susceptibility among 4d transition metals, it goes down to zero at xE0:6 [2]. Recently, some attempts have been carried out to control the magnetic properties of ferromagnetic materials by hydrogen absorption [3,4]. Since ultra-thin films are much more sensitive to the surface/interface state, it can be expected that magnetic properties of ultra-thin Co films will be drastically modified by hydrogenation of adjacent Pd layer. In the present study, we have investigated magnetic and structural properties of Pd/Co/Pd trilayers processed in hydrogen gas atmosphere. Samples are trilayer ( ( ( structure of Pd (50 A)/Co (d ¼ 3–115 A)/Pd (200 A) *Corresponding author. Tel.: +81-22-217-5359; fax: +8122-217-5404. E-mail address: [email protected] (S. Okamoto).
grown on Si substrates. The structural changes by hydrogen absorption were traced by X-ray diffraction (XRD) with Cu Ka line in hydrogen atmosphere. The magnetic anisotropy and magnetization were measured by using a magneto-optical Kerr effect (MOKE) system and a vibrating sample magnetometer (VSM). The samples were encapsulated in a quartz tube at fixed hydrogen gas pressure PH ranging from 1 to 3 atm. The detail of sample preparation and anisotropy measurements are described elsewhere [5,6]. By the electric resistivity measurements, it was confirmed that Pd layers in Pd/Co/Pd were well hydrogenated for the whole PH range examined in the present study, and hydrogen was found to be desorbed within several minutes after exposure in the air [6]. Fig. 1 shows the change of XRD profiles for a Pd/Co ( (5 A)/Pd trilayer in hydrogenation at PH ¼ 1 atm and dehydrogenation processes. Note that Pd (1 1 1) and (2 2 2) shift to lower angle by hydrogen absorption and then go back to near the original position after full desorption, indicating that aas-b-adehyd transformation occurs in these processes. We also found that all Pd(–H) (1 1 1) and (2 2 2) peak shapes can be fitted very well by a single Gaussian, meaning that both the top and bottom Pd layers are uniformly hydrogenated. Moreover, the diffraction intensity for the hydrogenated and
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 9 6 - 0
314
S. Okamoto et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 313–315
40
60 2θ (deg.)
80
( Fig. 1. XRD profiles of a Pd/Co (5 A)/Pd trilayer for before and after hydrogenation at PH ¼ 1 atm and after dehydrogenation. The circles and triangles indicate Pd(–H) (1 1 1) and (2 2 2) diffraction peaks, respectively. Others come from Si substrate.
Table 1 Characteristic structural parameters of aas, adehyd and b phase
( a1 1 1 (A) ( d1 1 1 (A) FWHM (deg.)
aas
adehyd
b
2.78 2.25 6.90
2.78 2.23 4.42
2.84 2.35 4.32
the dehydrogenated samples is enhanced significantly compared with that of the as-prepared one. According to rocking curve measurements of Pd(–H) (1 1 1) peak, the full width at half maximum (FWHM) is decreased by hydrogenation even after hydrogen is fully desorbed (see Table 1). This result indicates that (1 1 1) orientation is appreciably improved by hydrogenation. Since the lateral lattice expansion of Pd/Co/Pd is restrained by substrates, the structure of our b phase should be distorted along the film normal [1 1 1]. In order to characterize the structure of the b phase, (2 0 0) and (2 2 0) diffractions were observed by the XRD configuration of the sample tilted from the X-ray incident plane. By numerical fitting of each peak position, interatomic distance a1 1 1 in (1 1 1) plane and (1 1 1) spacing d1 1 1 were determined as listed in Table 1. The results clearly indicate that the b phase elongates along [1 1 1]. Moreover, the dehydrogenated adehyd slightly contracts along [1 1 1]. Assuming isovolumetric distortion of the b phase, hydrogen content x is estimated to be E0.6 from the calculated unit cell volume by using the relationship between the lattice constant and x [3], and this estimated x coincides with the hydrogen content where susceptibility vanishes [2]. It is well known that magnetization of Pd/Co/Pd trilayers increases with decreasing Co thickness d [7], probably due to polarization of interfacial Pd. Since the estimated
8
10
: as-prepared : PH = 1.3 atm : PH = 2 atm : PH = 3 atm : dehydrogenated
5 3
as-prepared
eff
hydrogenated
K (erg/cm )
Intensity (a.u.)
dehydrogenated
value of xE0:6 at PH ¼ 1 atm coincides with the composition of nonmagnetic b phase, no Pd polarization due to proximity effect is expected. However, the induced magnetization by Pd polarization of b-Pd/Co/ b-Pd decreased only by 40% compared with that of Pd/ Co/Pd [6]. The reason for this result is now under investigation. From the dependence of K eff ð¼ KU 22pMS2 Þ on Co layer thickness d in Fig. 2, K eff increases with decreasing d; indicating existence of large MSA. Three remarkable features can be noticed in this figure. First, K eff is significantly enhanced by hydrogen absorption at low ( shows very high PH : Especially, very thin Co (d ¼ 3 A) K eff (3.5 107 erg/cm3) which is two times larger than that of the as-prepared (1.6 107 emu/cm3). Second, increase of PH above 1.3 atm leads to monotonic decrease in K eff : Third, K eff recovers again after full desorption of hydrogen. In order to understand the hydrogen absorption effect on the perpendicular magnetic anisotropy (PMA) of Pd/ Co/Pd trilayers, the following three effects should be taken into account. The first is the magnetoelastic effect due to lattice expansion of Pd. Since the magnetostriction coefficient of Co is negative, in-plane tensile stress would contribute to perpendicular anisotropy. However, this effect is trivial as verified by a simple calculation. In fact, this effect cannot explain large difference in PMA between as-prepared and fully dehydrogenated samples, which possess exactly the same interatomic distance a1 1 1 shown in Table 1. The second is the change in Pd polarization due to hydrogen absorption. It is well known that magnetic susceptibility of Pd is weakened by incorporation of hydrogen atoms. Probably this effect would explain gradual decrease in PMA with PH shown in Fig. 2. The third point we have to notice is the drastic enhancement of Pd (1 1 1) orientation by hydrogen absorption (Fig. 1 and Table 1). From a lot of experiments, we found a strong correlation between K eff and the degree of Pd (1 1 1) orientation. For
2 7
10
5
3
4
5
Thickness d ( )
( Fig. 2. K eff of Pd/Co (d A)/Pd trilayer for before and after hydrogenation and after dehydrogenation as a function of d:
S. Okamoto et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 313–315
( instance, K eff of dehydrogenated Pd/Co(3 A)/Pd (solid triangles in Fig. 2) is two times larger than that of asprepared one (solid circles in Fig. 2) in the whole range of Co thickness, although the lattice parameters are almost the same. This result leads us to think that PMA is enhanced by improvement of Pd (1 1 1) orientation although the reason still remains unclear. As mentioned above, hydrogen absorption in Pd/Co/ Pd causes irreversible enhancement of Pd (1 1 1) orientation along with the significant increase in perpendicular magnetic anisotropy. The anisotropy decreases with increasing the amount of absorbed hydrogen probably due to weakening of Pd polarization. After full desorption of hydrogen the anisotropy recovers again to high value. This work is supported by the Research for the Future Program of Japan for the Promotion of Science under
315
Grant No. 97R14701, and the Storage Research Consortium in Japan.
References [1] P.F. Carcia, A.D. Meinhaldt, A. Suna, Appl. Phys. Lett. 47 (1985) 178. [2] H.C. Jamieson, F.D. Manchester, J. Phys. F 2 (1972) 323. [3] T. Manago, H. Miyajima, Y. Otani, E. Akiba, J. Phys. Soc. Japan 66 (1997) 831. . [4] D. Labergeriec, C. Sutter, H. Zabel, B. Hjorvarsson, J. Magn. Magn. Mater. 192 (1999) 238. [5] S. Okamoto, K. Nishiyama, O. Kitakami, Y. Shimada, J. Appl. Phys., to be published. [6] S. Okamoto, O. Kitakami, Y. Shimada, J. Magn. Soc. Japan 25 (2001) 839. [7] B.N. Engel, C.D. England, R.V. Leeuwen, M. Nakada, C.M. Falco, J. Appl. Phys. 69 (1991) 5643.