- Email: [email protected]
Pt multilayers
Journal of Magnetism and Magnetic Materials 120 (1993) 236-238
M M
North-Holland
mad ~ i
Structure and magnetic anisotropy of Fe/Pt multilayers E. Devlin a, V. Psycharis ", A. Kostikas a, A. Simopoulos a, D. Niarchos a, A. Jankowski b, T. Tsakalakos c, Hong Wan d and G. Hadjipanayis a a Institute of Materials Science, NCSR "Demokritos'" Aghia Paraskevi 153 10, Greece b Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Department of Mechanics and Materials Science, Rutgers University, Piscataway, NJ 08855-0909, USA d Department of Physics and Astronomy, Unioersity of Delaware, Newark, DE 19716, USA
A series of Fe/Pt multilayers prepared by magnetron sputtering have been studied by XRD, conversion electron M6ssbauer spectroscopy (CEMS) and SQUID magnetometry. The samples with individual Fe thickness of 3 A show orientation of the magnetic moment close to perpendicular to the film plane and Curie temperatures near 350 K. Interdiffusion at the interface is manifested in a distribution of hyperfine fields.
The application of thin films to magnetic recording has led to a dramatic increase in areal density in the last decade, and has prompted extensive investigations to obtain a fundamental understanding of magnetism in these systems and the conditions required for reproducible, tailor-made properties. The observation of perpendicular magnetic anisotropy in P t / C o compositionally modulated multilayer films (CMF) [1,2] has attracted interest in their potential use for magnetooptic recording, but has also motivated several studies in order to elucidate its physical origin and its dependence on structural parameters such as the thickness of individual layers, the degree of interdiffusion at the interface, and the role of magneto-elastic effects due to coherence strains. In this paper we report on the structural and magnetic properties of a series of P t / F e CMFs prepared by magnetron sputtering. Perpendicular magnetic anisotropy in P t / F e CMFs has been reported recently by Katayama et al., who have studied the dependence of the anisotropy constant on Fe layer thickness and temperature [3]. The crossover to perpendicular anisotropy at 293 K is observed near a layer thickness of about 5 ,~ for Fe with a constant Pt thickness of 18 ~,, while the anisotropy constant changes sign at low temperatures. On the other hand, a transmission M6ssbauer study of a Pt (20 ~k)/Fe (5 ,~) CMF showed anisotropy parallel to the plane of the film, at 4.2 and 295 K [4]. In our study we have focused on the effect of varying the Pt layer thickness, in view of other recent Correspondence to: Dr. A. Kostikas, Institute of Materials Science, NCSR Demokritos, Aghia Paraskevi, 153 10 Greece. Tel.: +30-1 6522 872; telefax: +30-1 6519 430; E-mail: [email protected].
results on magnetic coupling in CMFs where magnetic layers are separated by a non-magnetic layer of varying thickness. The F e / P t multilayer samples were prepared using magnetron sputter deposition. The deposition chamber was cryogenically pumped to a base pressure of 1.3 × 10 -7 mbar. A circular array of magnetron sources was situated 20 era beneath an oxygen-free copper plate. The magnetron sources were operated in the dc mode at 300-350 V discharge. A n argon working gas pressure of 4 × 10 -3 mbar was used at a flow rate of 15 cm 3 min-1. The substrates were sequentially rotated over each source at 1.0 rev/min. The target materials were 0.9999 pure. The cleaved mica and polished Si substrates were cleaned using a standard procedure, consisting of detergent wash, deionized water and alcohol rinses, followed by N 2 gas drying, prior to deposition. The substrates remained at a temperature between 293 and 306 K during deposition. The sputter deposition rates were monitored using calibrated quartz crystals as 0.35 to 1.1 .~ s - I for Fe and 1.2-4.6 A s -1 for Pt. A list of the samples prepared and their structural parameters is given in table 1. The final thickness for all multilayer films was between 0.17 and 0.19 ~m. The structure of the films was examined by X-ray diffraction using 1.54 A Cu radiation, in both the lowand high-angle rel]ions. The low-angle pattern for the Fe (9 A ) / P t (9 A) CMF is shown in fig. 1. These results indicate a periodic structure along the normal to the film plane. The bilayer period has been calculated by fitting the low-angle patterns to all reflective orders with proper correction for refraction; the results are listed in table 1. The agreement with the crystal monitor values is excellent, supporting the accuracy of the component thicknesses of the Fe and Pt layers.
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
E. Devlin et al. / Structure and anisotropy of Fe / Pt multilayers Table 1 Structural parameters for Fe/Pt multilayers (N is the number XTCt are the thicknesses as of bilayers, dF~ec, d~xa'c and dFe/p determined by a quartz crystal monitor, and dFeX~/pt is the bilayer thickness from XRD; all spacings are listed "in ]k) Sample
N
dF~eC
dvtXTC
3-91-726 3-91-731 3-91-802 3-91-805
135 76 44 92
3.02 3.06 3.09 9•48
9•65 19.39 38.78 9.47
XTC dF¢/Pt 12•7 22.3 41•9 19.0
X~ dFc/Pt 12•8 22.3 42•4 20.1
237
1.0
0.8
0.6
0.4
0.2
0.0 6O
1.0
The high-angle results exhibit at least one satellite peak and show that the samples have ( l i d texture from the presence of only the allowed (111) reflection while the linewidths of the Bragg reflection peaks are fairly narrow, indicating .perpendicular coherence lengths of the order of 500 A. The layered structure of the samples was also verified by high-resolution transmission electron micrographs of the film cross sections, which indicate a single-phase fee structure with twinning in the growth direction. A detailed analysis of the structural results including a fitting of the X-ray patterns to obtain information about interdiffusion at the interface is in progress• Magnetization data were obtained at 10 K with a SQUID magnetometer in parallel and perpendicular orientation of the film relative to the magnetizing field. The results for the three samples with approximately constant Fe thickness of 3 A and Pt thicknesses of 9, 19 and 39 .A are shown in fig. 2. The most interesting feature with • . of these graphs. is the change in anisotropy o o increasing Pt layer thzckness. The Fe (3 A ) / P t (9 A) CMF clearly has an easy axis parallel to the plane of the film while in the Fe (3 A ) / P t (19 ~ ) and Fe (3 A / P t (39 ~,) samples the perpendicular orientation is •slightly more favourable. As suggested in a previous study of C o / P t multilayers [5], this effect may be
~'~100000 ~ ~ ' i 10000
,ooo .~
~ l.
2nd
100 L06 . . . . . .
'5"66. . . . . . '~'.66. . . . . . 2-theta
ia.b0....
Fig. 1. Low-angle XRD pattern of a Fe (9 ,~)/Pt (9 ,~) multilayer.
O.8
o 0.6
E
~0.4
y
F~PtIgA
* A
Parallel to Plane Perp• to Plane
0.2 0.0 ......
~'o' . . . . . . .
~,'d . . . . . . .
~
.......
~
.......
.~' .......
~'o
~
~'o
1•0 0.8 0.6 0.4 0•2 0.0
,,,[,,, 'll'..,,,..I,,.,,,,,,i,, ~o 20 3o
...... e (KG)
~'d . . . . . . .
.......
Fig• 2. Magnetization curves at 10 K in directions parallel and perpendicular to the film plane for three Fe/Pt CMFs with Fe layer thickness of 3 ~, and Pt thicknesses of 9, 19 and 39 A. attributed to a variation in magnetic interaction between the iron layers, which has also been demonstrated in other systems [6], or to an interdiffusion of Fe and Pt at the interface. We have also measured the temperature dependence of the saturation magnetization, and this is displayed in normalized form in fig. 3. Here it is seen that the variation depends also on the Pt layer thickness, being in all eases much faster than in bulk iron. Similar results have been reported for C o / P t multilayers [5]. The orientation of the magnetic moment in the Fe (3 A ) / P t (9 A) and Fe (9 , ~ ) / P t (9 ~,) was also studied by conversion electron M6ssbauer spectroscopy. The CEMS spectrum of the latter sample at 300 K is shown in fig. 4. The relative intensities of the magnetic hyper-
238
E. Devlin et aL / Structure and anisotropy of Fe / Pt multilayers
,oo
Fe
.
* * * * * * * * * * * . , ****
****** Fe9/Pt9A ******,
*
*
, ,
, * ,
**** * **
,
.~
* * , Fe3/PtgA *** *i
*
*
O
**
0.50
* . •
* ,Fe3/Pt19A
* " •
S Fe3/ Pt39A
* ,
*
*
.,
O.OO
i i i i I I I i I I I i i I I I I I I I 100 200
*'*****
I I i I 300
~ li
I I 400
T (K)
Fig. 3. Temperature dependence of saturation magnetization of Fe/Pt CMFs. The corresponding curve for bulk a-Fe is included for comparison. fine structure lines imply a magnetic moment perpendicular to the ,/-rays and hence in the plane of the film. In contrast, the spectrum of the Fe (3 A ) / P t (9 ,&) CMF shows a decrease in the Am = 0 lines, indicating a magnetic moment close to the normal to the film plane. The spectra were fitted with a distribution of hyperfine field and isomer shift which give average values ( B e n ) = 326 kG, ( I S ) = 0.14 m m s -1 (relative to a-Fe) for the Fe (9 A ) / P t (9 .~) sample and (Ben) = 226 kG, (IS) = 0.27 mm s -1 for the Fe (3 ,~)/Pt (9 ~k) sample. The values of the angles with respect to the normal to the film plane are 90 and 39°, respectively. These results are in qualitative agreement with recent
studies of F e / P t multilajcers [4]. The reduced effective field for the film with 3 A Fe suggests a Curie temperature slightly above 300 K, which is also shown by the magnetization results. Our results for this CMF are intermediate between the results for Fe thicknesses of 2 and 5 ,~ presented in ref. [4]. Polar Kerr rotation was measured for all the samp,les at room temperature using a H e - N e laser (6328 A) with the applied field of 15 kG perpendicular to the film plane. The Kerr angle for the samples with Fe thickness of 3 ,~ and Pt thicknesses of 9, 19 and 39 ,~ are 0.090, 0.025 and 0.011 °, respectively, showing a rapid decrease with Pt thickness while the Fe (9 A ) / P t (9 A) CMF shows a Kerr angle of 0.071. From the loops obtained by varying the field it is found that the sample Fe (3 ~k)/Pt (9 ,~,) has high perpendicular anisotrol~y, while the samples with Pt thicknesses of 19 and 39 A have no clearly defined anisotropy. On the other hand, the Fe (9 A ) / P t (9 A) CMF clearly shows in-plane anisotropy. In conclusion, the results obtained from several experimental methods on three F e / P t CMFs with constant Fe thickness of 3 ,~ and varying Pt thickness of 9, 19 and 39 ,~ demonstrate a dependence of magnetization and anisotropy on Pt thickness. The Curie temperature is found to decrease with increasing Pt layer thickness. Further study will be required to resolve the origin of this effect. The perpendicular anisotropy is also enhanced with increasing Pt layer thickness. On the other hand, with an Fe thickness of 9 A as in the sample Fe (9 ,~)/Pt (9 ,~) we have clear in-plane anisotropy and magnetization approaching that of bulk ct-Fe. This work was supported in part by the EC grant SC1 + 302.
1.00
References 0.50 LtJ
0.00
.... -10
,,.. . . . .
I, -5
........
Vel
i . . . . . . . . . 0
i, 5
, ,.l.m.
, ,l 10
(mrnls)
Fig. 4. CEMS spectrum at 300 K for the Fe (9 A)/PI (9/~) CMF. The solid line is a least-squares fit assuming several magnetic components to simulate a distribution of hyperfine fields.
[1] P.F. Garcia, J. Appl. Phys. 63 (1988) 5066. [2] W.B. Zeper, F.J.A.M. Greidamm, P.F. Garcia and C.R. Fincher, J. Appl. Phys. 65 (1989) 4971. [3] T. Katayama, Y. Suzuki, Y. Nishihara, T. Sugimoto and M. Hashimoto, J. Appl. Phys. 69 (1991) 5658. [4] R.A. Brand, O. Bohne and W. Keune, J. Magn. Magn. Mater. 104-107 (1992) 1891. [5] P.J.H. Bloemen, W.J.M. de Jonge and F.J.A. den Broeder, J. Magn. Magn. Mater. 93 (1991) 105. [6] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304.
M M
North-Holland
mad ~ i
Structure and magnetic anisotropy of Fe/Pt multilayers E. Devlin a, V. Psycharis ", A. Kostikas a, A. Simopoulos a, D. Niarchos a, A. Jankowski b, T. Tsakalakos c, Hong Wan d and G. Hadjipanayis a a Institute of Materials Science, NCSR "Demokritos'" Aghia Paraskevi 153 10, Greece b Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Department of Mechanics and Materials Science, Rutgers University, Piscataway, NJ 08855-0909, USA d Department of Physics and Astronomy, Unioersity of Delaware, Newark, DE 19716, USA
A series of Fe/Pt multilayers prepared by magnetron sputtering have been studied by XRD, conversion electron M6ssbauer spectroscopy (CEMS) and SQUID magnetometry. The samples with individual Fe thickness of 3 A show orientation of the magnetic moment close to perpendicular to the film plane and Curie temperatures near 350 K. Interdiffusion at the interface is manifested in a distribution of hyperfine fields.
The application of thin films to magnetic recording has led to a dramatic increase in areal density in the last decade, and has prompted extensive investigations to obtain a fundamental understanding of magnetism in these systems and the conditions required for reproducible, tailor-made properties. The observation of perpendicular magnetic anisotropy in P t / C o compositionally modulated multilayer films (CMF) [1,2] has attracted interest in their potential use for magnetooptic recording, but has also motivated several studies in order to elucidate its physical origin and its dependence on structural parameters such as the thickness of individual layers, the degree of interdiffusion at the interface, and the role of magneto-elastic effects due to coherence strains. In this paper we report on the structural and magnetic properties of a series of P t / F e CMFs prepared by magnetron sputtering. Perpendicular magnetic anisotropy in P t / F e CMFs has been reported recently by Katayama et al., who have studied the dependence of the anisotropy constant on Fe layer thickness and temperature [3]. The crossover to perpendicular anisotropy at 293 K is observed near a layer thickness of about 5 ,~ for Fe with a constant Pt thickness of 18 ~,, while the anisotropy constant changes sign at low temperatures. On the other hand, a transmission M6ssbauer study of a Pt (20 ~k)/Fe (5 ,~) CMF showed anisotropy parallel to the plane of the film, at 4.2 and 295 K [4]. In our study we have focused on the effect of varying the Pt layer thickness, in view of other recent Correspondence to: Dr. A. Kostikas, Institute of Materials Science, NCSR Demokritos, Aghia Paraskevi, 153 10 Greece. Tel.: +30-1 6522 872; telefax: +30-1 6519 430; E-mail: [email protected].
results on magnetic coupling in CMFs where magnetic layers are separated by a non-magnetic layer of varying thickness. The F e / P t multilayer samples were prepared using magnetron sputter deposition. The deposition chamber was cryogenically pumped to a base pressure of 1.3 × 10 -7 mbar. A circular array of magnetron sources was situated 20 era beneath an oxygen-free copper plate. The magnetron sources were operated in the dc mode at 300-350 V discharge. A n argon working gas pressure of 4 × 10 -3 mbar was used at a flow rate of 15 cm 3 min-1. The substrates were sequentially rotated over each source at 1.0 rev/min. The target materials were 0.9999 pure. The cleaved mica and polished Si substrates were cleaned using a standard procedure, consisting of detergent wash, deionized water and alcohol rinses, followed by N 2 gas drying, prior to deposition. The substrates remained at a temperature between 293 and 306 K during deposition. The sputter deposition rates were monitored using calibrated quartz crystals as 0.35 to 1.1 .~ s - I for Fe and 1.2-4.6 A s -1 for Pt. A list of the samples prepared and their structural parameters is given in table 1. The final thickness for all multilayer films was between 0.17 and 0.19 ~m. The structure of the films was examined by X-ray diffraction using 1.54 A Cu radiation, in both the lowand high-angle rel]ions. The low-angle pattern for the Fe (9 A ) / P t (9 A) CMF is shown in fig. 1. These results indicate a periodic structure along the normal to the film plane. The bilayer period has been calculated by fitting the low-angle patterns to all reflective orders with proper correction for refraction; the results are listed in table 1. The agreement with the crystal monitor values is excellent, supporting the accuracy of the component thicknesses of the Fe and Pt layers.
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
E. Devlin et al. / Structure and anisotropy of Fe / Pt multilayers Table 1 Structural parameters for Fe/Pt multilayers (N is the number XTCt are the thicknesses as of bilayers, dF~ec, d~xa'c and dFe/p determined by a quartz crystal monitor, and dFeX~/pt is the bilayer thickness from XRD; all spacings are listed "in ]k) Sample
N
dF~eC
dvtXTC
3-91-726 3-91-731 3-91-802 3-91-805
135 76 44 92
3.02 3.06 3.09 9•48
9•65 19.39 38.78 9.47
XTC dF¢/Pt 12•7 22.3 41•9 19.0
X~ dFc/Pt 12•8 22.3 42•4 20.1
237
1.0
0.8
0.6
0.4
0.2
0.0 6O
1.0
The high-angle results exhibit at least one satellite peak and show that the samples have ( l i d texture from the presence of only the allowed (111) reflection while the linewidths of the Bragg reflection peaks are fairly narrow, indicating .perpendicular coherence lengths of the order of 500 A. The layered structure of the samples was also verified by high-resolution transmission electron micrographs of the film cross sections, which indicate a single-phase fee structure with twinning in the growth direction. A detailed analysis of the structural results including a fitting of the X-ray patterns to obtain information about interdiffusion at the interface is in progress• Magnetization data were obtained at 10 K with a SQUID magnetometer in parallel and perpendicular orientation of the film relative to the magnetizing field. The results for the three samples with approximately constant Fe thickness of 3 A and Pt thicknesses of 9, 19 and 39 .A are shown in fig. 2. The most interesting feature with • . of these graphs. is the change in anisotropy o o increasing Pt layer thzckness. The Fe (3 A ) / P t (9 A) CMF clearly has an easy axis parallel to the plane of the film while in the Fe (3 A ) / P t (19 ~ ) and Fe (3 A / P t (39 ~,) samples the perpendicular orientation is •slightly more favourable. As suggested in a previous study of C o / P t multilayers [5], this effect may be
~'~100000 ~ ~ ' i 10000
,ooo .~
~ l.
2nd
100 L06 . . . . . .
'5"66. . . . . . '~'.66. . . . . . 2-theta
ia.b0....
Fig. 1. Low-angle XRD pattern of a Fe (9 ,~)/Pt (9 ,~) multilayer.
O.8
o 0.6
E
~0.4
y
F~PtIgA
* A
Parallel to Plane Perp• to Plane
0.2 0.0 ......
~'o' . . . . . . .
~,'d . . . . . . .
~
.......
~
.......
.~' .......
~'o
~
~'o
1•0 0.8 0.6 0.4 0•2 0.0
,,,[,,, 'll'..,,,..I,,.,,,,,,i,, ~o 20 3o
...... e (KG)
~'d . . . . . . .
.......
Fig• 2. Magnetization curves at 10 K in directions parallel and perpendicular to the film plane for three Fe/Pt CMFs with Fe layer thickness of 3 ~, and Pt thicknesses of 9, 19 and 39 A. attributed to a variation in magnetic interaction between the iron layers, which has also been demonstrated in other systems [6], or to an interdiffusion of Fe and Pt at the interface. We have also measured the temperature dependence of the saturation magnetization, and this is displayed in normalized form in fig. 3. Here it is seen that the variation depends also on the Pt layer thickness, being in all eases much faster than in bulk iron. Similar results have been reported for C o / P t multilayers [5]. The orientation of the magnetic moment in the Fe (3 A ) / P t (9 A) and Fe (9 , ~ ) / P t (9 ~,) was also studied by conversion electron M6ssbauer spectroscopy. The CEMS spectrum of the latter sample at 300 K is shown in fig. 4. The relative intensities of the magnetic hyper-
238
E. Devlin et aL / Structure and anisotropy of Fe / Pt multilayers
,oo
Fe
.
* * * * * * * * * * * . , ****
****** Fe9/Pt9A ******,
*
*
, ,
, * ,
**** * **
,
.~
* * , Fe3/PtgA *** *i
*
*
O
**
0.50
* . •
* ,Fe3/Pt19A
* " •
S Fe3/ Pt39A
* ,
*
*
.,
O.OO
i i i i I I I i I I I i i I I I I I I I 100 200
*'*****
I I i I 300
~ li
I I 400
T (K)
Fig. 3. Temperature dependence of saturation magnetization of Fe/Pt CMFs. The corresponding curve for bulk a-Fe is included for comparison. fine structure lines imply a magnetic moment perpendicular to the ,/-rays and hence in the plane of the film. In contrast, the spectrum of the Fe (3 A ) / P t (9 ,&) CMF shows a decrease in the Am = 0 lines, indicating a magnetic moment close to the normal to the film plane. The spectra were fitted with a distribution of hyperfine field and isomer shift which give average values ( B e n ) = 326 kG, ( I S ) = 0.14 m m s -1 (relative to a-Fe) for the Fe (9 A ) / P t (9 .~) sample and (Ben) = 226 kG, (IS) = 0.27 mm s -1 for the Fe (3 ,~)/Pt (9 ~k) sample. The values of the angles with respect to the normal to the film plane are 90 and 39°, respectively. These results are in qualitative agreement with recent
studies of F e / P t multilajcers [4]. The reduced effective field for the film with 3 A Fe suggests a Curie temperature slightly above 300 K, which is also shown by the magnetization results. Our results for this CMF are intermediate between the results for Fe thicknesses of 2 and 5 ,~ presented in ref. [4]. Polar Kerr rotation was measured for all the samp,les at room temperature using a H e - N e laser (6328 A) with the applied field of 15 kG perpendicular to the film plane. The Kerr angle for the samples with Fe thickness of 3 ,~ and Pt thicknesses of 9, 19 and 39 ,~ are 0.090, 0.025 and 0.011 °, respectively, showing a rapid decrease with Pt thickness while the Fe (9 A ) / P t (9 A) CMF shows a Kerr angle of 0.071. From the loops obtained by varying the field it is found that the sample Fe (3 ~k)/Pt (9 ,~,) has high perpendicular anisotrol~y, while the samples with Pt thicknesses of 19 and 39 A have no clearly defined anisotropy. On the other hand, the Fe (9 A ) / P t (9 A) CMF clearly shows in-plane anisotropy. In conclusion, the results obtained from several experimental methods on three F e / P t CMFs with constant Fe thickness of 3 ,~ and varying Pt thickness of 9, 19 and 39 ,~ demonstrate a dependence of magnetization and anisotropy on Pt thickness. The Curie temperature is found to decrease with increasing Pt layer thickness. Further study will be required to resolve the origin of this effect. The perpendicular anisotropy is also enhanced with increasing Pt layer thickness. On the other hand, with an Fe thickness of 9 A as in the sample Fe (9 ,~)/Pt (9 ,~) we have clear in-plane anisotropy and magnetization approaching that of bulk ct-Fe. This work was supported in part by the EC grant SC1 + 302.
1.00
References 0.50 LtJ
0.00
.... -10
,,.. . . . .
I, -5
........
Vel
i . . . . . . . . . 0
i, 5
, ,.l.m.
, ,l 10
(mrnls)
Fig. 4. CEMS spectrum at 300 K for the Fe (9 A)/PI (9/~) CMF. The solid line is a least-squares fit assuming several magnetic components to simulate a distribution of hyperfine fields.
[1] P.F. Garcia, J. Appl. Phys. 63 (1988) 5066. [2] W.B. Zeper, F.J.A.M. Greidamm, P.F. Garcia and C.R. Fincher, J. Appl. Phys. 65 (1989) 4971. [3] T. Katayama, Y. Suzuki, Y. Nishihara, T. Sugimoto and M. Hashimoto, J. Appl. Phys. 69 (1991) 5658. [4] R.A. Brand, O. Bohne and W. Keune, J. Magn. Magn. Mater. 104-107 (1992) 1891. [5] P.J.H. Bloemen, W.J.M. de Jonge and F.J.A. den Broeder, J. Magn. Magn. Mater. 93 (1991) 105. [6] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304.