STRUCTURE AND ANISOTROPY OF [001] Co/Pd ARTIFICIAL SUPERLATTICES

(1)

HAL Id: jpa-00229002

https://hal.archives-ouvertes.fr/jpa-00229002

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

STRUCTURE AND ANISOTROPY OF [001] Co/Pd

ARTIFICIAL SUPERLATTICES

F. den Broeder, D. Kuiper, H. Donkersloot

To cite this version:

(2)

JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, decembre 1988

STRUCTURE AND ANISOTROPY OF

[OOl]

Co/Pd ARTIFICLAL SUPERLATTICES

F. J. A. den Broeder, D. Kuiper and H. C. Donkersloot

Philips Research Laboratories, 5600 J A Eindhoven, The Netherlands

Abstract. - Single crystalline [001] fcc Co/Pd artificial superlattices were prepared by vapour deposition in UHV. X-ray diffraction revealed periodic structures even for films containing only one atomic layer Co and three atomic layers Pd per period. Perpendicular anisotropy, which is found only for superlattices containing Co monolayers, increases sharply with substrate temperature, indicating a strong dependence on atomic layer roughness.

It has recently been found that polycrystalline [ I l l ] textured fcc Co/Pd multilayers, acquire, by the existence of interface anisotropy, a perpendicular easy axis below a Co layer thickness tco of 8

A

[I, 2, 31. To study the effect of another crystallographic orienta- I tion, this paper deals with the structure and anisotropy of single-crystalline [OOl] fcc Co/Pd artificial superlat-

t

tices.

They were prepared by e-beam evaporation in UHV onto cleaved [OOl] NaCl a t substrate temperature T,

after deposition of a 1000

a

[001] epitaxial Pd base layer at 300 OC. Shutters were used t o alternate the constituents, while deposition rates ( w 1 A/s) were

monitored by quartz oscillators. ₁₅ _{2 0} _{2 5} ₃₀ The samples are designated by ComPdn, in which m -L d ( A )

and n are numbers of (001)-monolayers per modulation Fig. 1.

-

X-ray diffractogram (CuKa) of a [001] ColPdlo period, assuming for Co and Pd an fcc structure with superlattice deposited at T, = 520 OC on NaCl (001); d = lattice constants ac, = 3.55

A

and spa = 3.89

A.

Total sin

'.

numbers of bilayers were chosen to obtain a total Co thickness of about 700

A.

The following ComPd, series were prepared: a) n = 10 with m = 1,2,

...,

6; T. = 50 OC; b ) m = l w i t h n = 1 , 2 , ..., 5; T , = 5 0 ° C ;

c) m = 1 with n = 1,2;

T,

= 100 OC, 150 OC, 200 OC. For the whole series a and for series b with n

>

2, X- ray diffraction (XRD) showed a periodic structure. As an example, figure 1 is an XRD pattern for ColPdlo. Although the Co layers are nominally one monolayer thick, the pattern displays six satellites around the main 200 reflection, indicating a well-defined superlattice. When the substrate had been rotated during layer deposition, the XRD rocking curve of the strongest superlattice reflection was symmetrical, showing a mo- saic spread of only 0.32 deg around [OOl]. However, when no rotation had been applied, the rocking curve _{- -} _.+ was asymmetrical, indicating that the mean [001] axis

Fig. 2. - Plan-view electron diffraction pattern of Co3Pdlo was somewhat inclined towards the original position _{superlattice.}

of the Pd source. This points to shadowing effects,

-

caused by limited surface diffusion at T,

=

50 OC.

For all films plan-view transmission electron diffrac- single, circular spots from planes which are perpendic- tion (TED), showed a single crystalline fcc structure, ular t o the film plane were observed. This means that as shown in figure 2 for Co3Pd10. Despite the 9 % dif- the layers are structurally coherent and consequently ference in interatomic distance for Co and Pd, only tetragonally deformed.

(3)

C8 - 1664 JOURNAL DE PHYSIQUE

For the series a with n = 10, the saturation magnetization

Is

per unit volume Co, measured with a vi- brating sample magnetometer, was found to be higher than for bulk Co

(2

= 1.76 T) and to increase with lower m up to about 2,65 T for m = 1. Such an en- hancement has already been explained by polarization of Pd-interfacial atoms [3].

Of all the multilayers in series a, only those containing Co monolayers (m = 1) had a perpendicular anisotropy. As an example, figure 3 shows the

Fig. 3. - Magnetic hysteresis curves measured in fields parallel

([I)

and perpendicular (I) to the film plane of a) Co2Pd10 and b) ColPdlo superlattices.

highly different hysteresis curves for Co2Pd10 (in-plane anisotropy) and ColPdlo (perpendicular anisotropy). This result differs from [ I l l ] multilayers, for which the anisotropy was perpendicular up to tco = 8 [2, 31. In the latter films magnetocrystalline anisotropy, probably originating from a [00.1] hcp-Co stacking, contributed significantly t o the anisotropy. In the present [OOl] fcc multilayers the thicker Co layers have a cubic stacking, for which magnetocrystalline anisotropy is probably absent. Also anisotropy in- duced by coherency stresses may play a role. The ori-

gin of the perpendicular anisotropy of the Co monolayers is thought to be interface anisotropy or magnetocrystalline anisotropy associated with the specific atomic arrangement.

From the area between the magnetization curves, measured in fields perpendicular and parallel to the film, the uniaxial anisotropy constant K,, per unit vol-

ume Co was determined. Figure 4a shows for the case of Co monolayers (series b) that Ku increases with n.

This may indicate a magnetic infiuence of the P d thickness layers on Ku. However, since for n = 1 and n = 2 XRD did p o t show clear superlattice reflections, the increase of K, may also originate from a better defi- nition of the superlattice structure with larger n. This prompted us to investigate ColPdl and ColPd2 made at higher T, (series c).

Fig. 4. - Uniaxial anisotropy energy Ku of a) ColPdn multilayers prepared at Ts = 50 OC. b) ColPdl and C01Pd2 multilayers prepared at increasing substrate temperature rn

Figure 4b shows that there is no systematic change of Ku for ColPdl, but for ColPdz Ku rises sharply with T,. For the latter films XRD gave weak superlattice reflections whose intensity increased with T,. Evi-

dently a t higher T,, increased surface diffusion during growth makes the layers smoother while their perpendicular anisotropy increases. It is also noted that for ColPdl, prepared a t 100 OC and 200 OC, for which Ku is also positive, TED gave (110) superlattice spots.

If the perpendicular anisotropy is due to interface anisotropy, the latter thus appears to be very sensitive to layer roughness. For the smoothest layers prepared at

T

s

= 200 "C, we obtain from Ku=3.28 M J / ~ ~ , after correcting for the demagnetization energy of pure Co, an interface anisotropy constant Ks = 0.39 m ~ / m ~ . This higher value compared to [ I l l ] Co/Pd (K, = 0.26 m3;/m2, Ref. [2]) may reflect an orientation dependence of K., but it is more likely caused by a greater smoothness. This agrees with a recent geo- metric model based on Co-Co and Co-Pd pair interac- tions which predicts a strong dependence of K, on interface diffuseness [4].

In conclusion, we succesfully prepared [001] fcc Co/Pd artificial syperlattices. Only with Co monolayers they show perpendicular anisotropy, which increases when the Co layers become smoother.

[I] Carcia, P. I?., Meinholdt, A. D. and Suna, A.,

A p p l . Phys. Lett. 47 (1985) 178.

[2] Draaisma, H. J. G., den Broeder, F. J. A. and de Jonge, W. J. M., J. Magn. Magn. Muter. 66 (1987) 351.

[3] Den Broeder, F. J. A., Donkersloot, H. C., Draaisma, H. J. G. and de Jonge, W. J. M., J.

A p p l . Phys. 61 (1987) 4317.