592 Journal of Magnetism and Magnetic Materials 93 (1991) 592-596 North-Holland

Molecular beam epitaxial growth and magnetic properties of Co-Pt superlattices oriented along the [001], [110] and [111] axes of Pt*

C.H. Lee , R.F.C. Far row, B.D. H e r m s m e i e r , R.F. Marks, W.R. Benne t t , C.J. Lin, E.E. M a r i n e r o

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA

P.D. Kirchne r

IBM Research DiL,ision, T.J. Watson Research Center, Yorktown Heights, NY 10598, USA

and C.J. Chien

Department of Materials Science and Engineering, Stanford Unil ersity, Stanford, CA 94305, USA

The magnetic and structural properties of epitaxial Co/Pt superlattices, oriented along the [001], [110] and [111] directions of Pt, are presented. We find that the magnetic properties of the superlattices are reproducible and distinctly different for the different orientations. For structures with 3.2A Co/16.7A Pt, the easy axis of magnetization is perpendicular to the film plane for growth along Pt[lll] but in the film plane for growth along [001]. The [110] oriented superlanices have intermediate behaviour. X-ray and TEM analysis of the superlattices reveals significant differences in interface quality and defect structure for the different orientations. These effects may play a more important role in the origin of magnetic anisotropy than previously assumed.

1. Introduction

Artificially-layered magnetic metal structures, including superlattices, are a topic of intense current interest because of their unusual proper- ties and potential device applications. Indeed, these structures have, in the past few years, demonstrated [1] a variety of novel phenomena such as perpendicular anisotropy, giant magneto- resistance and magnetic exchange coupling. As has been seen in several systems, Co -Au [2], Co-Cu [3], F e - A g [4], Co-Pd [5] and Co-P t [6], a perpendicular easy axis of magnetization can be achieved when the magnetic film thickness is smaller than a few monolayers. However, the

*This work was supported in part by ONR.

origin of this anisotropy remains unclear. Al- though there are indications that the magnetic and structural properties are closely related, it is often difficult to obtain a well-defined single crystal structure. MBE provides a route to such structures since it permits in situ structural char- acterization at all stages of film growth, For ex- ample, using MBE growth techniques, Den Broederet al. [6] have demonstrated that the growth axis of C o / P d superlattices strongly in- fluences the magnetic, anisotropy and conclude that coherency strain in Co is a key factor in this effect.

In this paper we report the MBE growth of C o / P t along three major directions of Pt, se- lected by growth on suitably prepared substrates. We find that the magnetic properties of the su-

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C.H. Lee et al. / Growth and properties of Co-Pt superlattices 593

perlattices are reproducible and dependent on superlattice growth orientation.

Selected Orientation of F'pitaxial Co/Pt Superlaffices

[111] Pt, Ag [O01]Pt, Ag [110] Pt, Ag

t t t 2. Experimental techniques

The C o - P t superlattices reported in this paper were prepared in a VG 80-M MBE system (VG Semicon Ltd.). The Co and Pt layers were grown from e-gun sources at rates of =0.15 and

o

= 0.25 A / s , respectively. The deposition rates and beam-interrupt shutters were computer con- trolled a using Sentinel III deposition control system (Leybold Heraus Inc.). Fluctuations in film thickness were less than 2%. Ag layers were grown at a rate of 0 . 4 A / s from a Knudsen cell held at = 1050°C. The background pressure prior to film growth was approximately = 2 x 10 ~ mbar, while the pressure during superlat- tice growth was = 2 x 10-10 mbar or better.

3. Superlattice growth and structure

The key to successful C o - P t growth in the three different orientations hinges on initial epi- taxy of Ag. The initial Ag film seeds parallel epitaxy of the isostructural Pt in each orientation and controls the orientation of the entire super- lattice.

Fig. 1 shows a schematic diagram of the se- lected-orientation epitaxial superlattices. The [111] orientation of C o - P t superlattices was se- lected by first depositing = 200,& of Ag onto a clean GaAs(1--i-1) 1 x 1 substrate at a substrate temperature of 100°C. R H E E D and LEED stud- ies showed that the Ag grew with its (111) plane parallel to the G a A s ( l l l ) plane. The Ag film consisted of two orientations of Ag related by a 180 ° rotation about the Ag[ l l l ] axis. The C o / P t superlattice was then grown with the Pt film deposited first. The superlattices contained 7 or 15 periods and were capped with a 16,~ Pt film to stabilize the surface against atmospheric oxida-

P t ~

c

Fig. 1. Schematic diagram of the selected-orientation epitax- ial C o / P t superlattices. In each case the superlattices were grown on a Ag film but the orientation of this film was selected by different growth procedures.

tion. The entire superlattice followed the rota- tional twinning of the initial Ag film:

Ag, P t ( l l l ) , [011]llGaAs(l~]-), [011],

Ag, Pt(111), [110111GaAs(1-i]-), [0111.

The [110] oriented C o - P t superlattices were pre- pared by first heating the GaAs(001) wafer to about =500°C to remove the As cap from a pre-grown homoepitaxial GaAs film. The sub- strate was then cooled to = 100°C and = 200,~ of Ag deposited. C o - P t superlattices were then grown with the substrate held at 100°C, starting with Pt. The epitaxial relations determined from the R H E E D and LEED data were:

Ag(110), [110] II GaAs(001 ), [ 100].

The [100] oriented superlattices were produced by deposition on Ag grown onto a thin ( = 3 -9 ml) Co seed film grown onto the GaAs(001) surface. This seeds the epitaxial growth of Ag along the [001] axis. The Co prelayer adopted [8] a body centered tetragonal structure, commensurate in- plane with the GaAs substrate. The Ag film grew

594 C.H. Lee et al. / Growth and properties o f Co-Pt superlattices

106 ~--L I [111 ]

A ~ o

105 ~ ~

104 c ~ ©

eJ U ~ +

L , I A

l ' i [100]

v

C~l C

,03 : I ,,

30 40 50 60

2O

Fig. 2. High angle X-ray diffraction 0-20 scan for [111] and [001] oriented superlattices comprising: (a) GaAs(111)/200,g A g ( l l l ) / [ 1 6 . 7 o , ~ P t - 3 . 2 ,~ Co]15/16 .7 A Pt: (b) GaAs(00l)/ol0 A Co/200 ,~ Ag(001)/[16,7 ,~ Pt-3 .7 ,~ Co]15/16.7A Pt.

~>, 102 u3 c

c

~ 1°6 I cr x

10 5

epitaxially on the Co rotated in plane by 45°:

Ag(001), [lOO]11Co(OO1), [110111

x GaAs(001), [110].

A Pt seed film was also effective in seeding the Ag(001) epitaxy. The C o - P t superlattice was then grown with the Pt film deposited first.

X-ray diffraction measurements were carried out on a double-crystal diffractometer with a G a A s ( l l l ) plane monochromator crystal operat- ing in the (111) Bragg diffraction. The C u K ~ 1 line was selected by a slit. High and low angle 0 - 2 0 scans were made in the symmetric reflec- tion geometry and are shown in figs. 2 and 3, respectively, for the [111] and [001] oriented su-

10000

4 0 0 0

2 0 0 0

1000

400

c 10000

i x

4 0 0 0

f J } [111)

_- c

f [ 1 0 0 ] 1, ._

I

/ O4

2000 =

1000 ~ I ~ II

400

I 4 8 12 16

20

Fig. 3. Low angle X-ray diffraction 0-20 scan for same su- pedattices as fig. 2.

perlattices. The n = 0 superlattice peak, which occurs at the Bragg angle corresponding to the average lattice spacing for one period, was found at 20 values of 40.24, 47.32 and 68,7 ° for the [111], [001] and [110] oriented superlattices. These angles are in agreement with the predicted peak positions for these orientations and the orienta- tions were mutually exclusive. The superlatticc period determined from the superlattice peaks agreed within 2% of the period determined from the Co and Pt thickness calibrations. It can be seen from fig. 2 that the [001] oriented superlat- tice has a slower fall-off in intensity of the higher order satellite peaks than for the [111] oriented superlattice. This suggests that the [001] superlat- tice has better-defined interfaces than the [111] case. This view is supported by the low-angle X-ray scattering data shown in fig. 3. It is clear

C.H. Lee et al. / Growth and properties of Co-Pt superlattices 595

that the [111] structure has a much more rapid fall-off of satellite intensities. In fact the third order satellite is almost negligible for this orien- tation. Modelling of the low-angle X-ray re- flectance using an X-ray multilayer code [10] shows that the data for [111] could be caused by interface compositional mixing. A linear grading length of -- 3,~ gives a close fit to the data. This would suggest that the superlattice does not con- tain pure Co but is rather a compositionally mod- ulated structure comprising graded transitions between a Co-P t alloy and Pt. On the other hand, the data for the [001] structure is well fitted by an abrupt step profile with no grading.

High-resolution transmission electron mi- croscopy (HRTEM) studies have been carried out on the superlattices to determine the microstruc- ture of the samples. These results will be dis- cussed in detail elsewhere [10]. The data confirm the epitaxial relations determined from the RHEED and X-ray diffraction studies. They also reveal structural differences between the differ-

ent orientations. For all three orientations the stacking within the superlattice is fcc. This is expected from the fcc structure of Pt and the large P t / C o ratio. Additionally, stacking faults on the (111) planes were a common defect for the [111] oriented superlattices but were not observed for the other orientations. Furthermore, a high density of steps in the (111) planes of Pt were indicated [11] by the existence of symmetry-for- bidden Bragg peaks such as 3(422).

4. Magnetic measurements

Room temperature magnetic measurements were made on the superlattices using the polar Kerr effect (MOKE, 633nm), vibrating sample magnetometry and torque magnetometry. Fig. 4 shows the results of the MOKE measurements for three different orientation, 15-period super- lattices, each of the composition = 3 A Co-16.7 Pt. The [111] sample exhibits a highly square loop

[3~, Co/16~, Pt]l 5

Tsu b = 100°C / ;=~---

I Hper p v///~/,-',-'rl

J

#MBE255 [111]

f

-0.1

i I i I , I , , I , I , I , I -16 -12 -8 -4 0 4 8 12 16

H (KOe)

0k(deg)

Fig. 4. Magnetic hysteresis loops recorded at 20°C by Kerr rotation for oriented C o / P t superlattices with ( ~ 3,~ Co-16.7,& Pt) × 15 periods. Wavelength of incident laser 0.633 nm. The magnetic field was applied normal to the sample surface, i.e. along the directions indicated.

596 C.H. Lee et al. / Growth and properties of Co-Pt superlattices

with a coercivity of 3.2kOe. This sample has a

r e m a n e n t magnet iza t ion perpendicu la r to the film

plane. On the other hand, the [001] sample has

no r emanence and cannot be sa turated at the

highest field (16kOe) we have used. This sample

clearly has strong in-plane anisotropy. The [110]

sample has in te rmedia te behaviour with both in-

plane and out-of-plane remanence .

5. Discussion and conclusions

In summary, we have demons t ra ted single crys-

tal growth of C o / P t superlatt ices along three

or ienta t ions and observed distinctly different

magnet ic anisotropy. In addit ion, our X-ray and

TEM analysis indicates differences between the

or ienta t ions in interface abruptness and defect

structure. Therefore it is apparen t that the origin

of the perpendicu la r anisotropy observed in these

superlatt ices cannot be accounted for by surface

anisotropy and broke-symmetry a rguments alone.

However, epitaxy along these different or ienta-

tions can clearly induce defect s tructures and

local lattice distort ions that may result in differ-

ent values of the magnetocrystal l ine anisotropy.

These effects may play a more impor tant role in

the origin of magnet ic anisotropy than previously

assumed.

References

[1] For a comprehensive current review of the field of mag- netism in artificially layered magnetic films see Appl. Phys. A49 (5&6) (1989).

[2] (a)C. Chappert, K. Le Dang, P. Beauvillain, H. Hurdequint and D. Renard, Phys. Rev. B 34 (1986) 3192. (b) C.H. Lee, Hui He, F. Lamelas, W. Vavra, C. Uher and R. Clarke, Phys. Rev. Lett. 62 (1989) 653.

[3] (a) D. Pescia, G. Zampieri, M. Stampanoni, G.L. Bona, R.F. Willis and F. Meier, Phys. Rev. Lett. 58 (1987) 933. (b) F.J. Lamelas, C.H. Lee, Hui He, W. Vavra and Roy Clarke, Phys. Rev. B 40 (1989) 8.

[4] J.R. Dutcher, J.F. Cochran, B. Heinrich and A.S. Arrott, J. Appl. Phys. 64 (1988) 6095

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[7] S. Hashimoto, Y. Ochia and K. Aso, J. Appl. Phys. 67 (1990) 2136.

[8] High-resolution, cross-section transmission electron mi- croscopy of the Co-GaAs interface region confirmed in-plane coherency between the GaAs, Co and Ag lat- tices but also revealed a tetragonal distortion of the Co lattice.

[9] S. Mrowka, Oxford Research Group. [10] C.J. Chien, J.C. Bravman, C.H. Lee and R.F.C. Farrow,

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