Magnetic and magneto-optical properties of Pd/Au/Pd/Co and Pd/Cu/Pd/Co multilayers

Journal of Magnetism and Magnetic Materials 128 (1993) 237-246

North-Holland

Magnetic and magneto-optical properties of Pd/Au/Pd/Co and Pd/Cu/Pd/Co multilayers

M. Sakurai ’ and T. Shinjo

Institute for Chemical Research, Kyoto University, Uji Kyoto-fu 611, Japan

Received 16 December 1992, in revised form 19 February 1993

To study the role of interfaces on magnetic properties, the magnetic anisotropy was systematically investigated for evaporated [Pd(t &>/X(z &/Pd(t &)/Cof y &]rs (X = Au, Cu, Pd) multilayers. The magnetic anisotropy for theOmultilay- ers ch$nges wit! vatyingOan interfacial Pd layer thickness. The anisotropy energy iook a maximum value for [Pd(t A)/Au(25 - 2.f A)/PdQ A)/Co($ A&, multilayers with Pd layer thickness around 5 A. The anisotropy for [Pd(t &/Cu(25- 2t A)/Pd(t A)/Co(6 AIlls multilayers increased with increasing Pd layer thickness. The difference is due to the interface anisotropy. The saturation magnetization for multilayers shows a dependence on the Pd layer thickness. The magnetostatic energy for multilayers is discussed and it is found that change of magnetization at the interface contributes to the interface anisotropy. Enhancement of Kerr rotation was observed, which corresponds to the plasma edge of the intermediate layer.

1. Introduction

Studies of multilayers have attracted consider- able attention from the viewpoints of applications and also fundamental materials research. A perpendicular magnetic anisotropy was observed for Pd/Co [l], Pt/Co [2], Au/Co [3] and Ru/Co [41 multilayers when the magnetic layer thickness is smaller than a few monolayers. In particular, Pt/Co and Pd/Co multilayers seem to have po- tential for future applications as high-perfor- mance magneto-optical media [5]. The large perpendicular magnetic anisotropy has generally been attributed to inter-facial properties. The role of the interface anisotropy, K,, was confirmed by Draaisma et al. [6]. Den Broeder et al. first investigated the dependence of the anisotropy on the epitaxial orientation [7]. The studies by Engel et al. [81 showed that the interface anisotropy

Correspondence to: Dr T. Shinjo, Institute for Chemical Re- search, Kyoto University, Uji Kyoto-fu 611, Japan. ’ Permanent address: Advanced Materials Research Labora-

tory, Tosoh Corporation, Hayakawa 2743-1, Ayase, Kana- gawa 252, Japan.

of Pd/Co multilayers was independent of the epitaxial orientations. Recently, the magnetic anisotropy energy has been calculated on the basis of local spin density approximation [9,10]. These calculations showed that the large spin- orbit coupling on the Pd site contributed to the anisotropy energy by introducing large splittings in the strongly hybridized Co and Pd d bands. However, the origin of the interface anisotropy has not been resolved completely.

The purpose of this paper is to clarify the role of interfaces on the magnetic anisotropy. We try to answer the following questions.

(1) If the perpendicular magnetic properties of Pd/Co multilayers are due to an interface effect, what Pd layer thickness contributes to this property at the interface?

(2) Does the stress from the Au or Cu layer give rise to the change in magnetic anisotropy for the Pd/Co system?

(3) Does the hysteresis loop change with increasing interfacial Pd layer thickness?

(4) How does the Kerr rotation of multilayers change according to the plasma edge of the Au or Cu layer?

0304-8853/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

238 M. Sakurai, T. Shinjo / Properties of Pd /Au /Pd / Co and Pd / Cu / Pd / Co multilayers

The magne$c anisotropy andomagnetic properties of [Pd(t A)/X(z A)/Pd(t A)/Co(y A)],, (X = Au, Cu, Pd) multilayers were systematically investigated as functions of the thicknesses of the Pd and Co layers. Hereafter the Cu or Au layers are denoted as ‘intermediate layers’. The magnetostatic energy for multilayers and the contribution of magnetostatic energy to the interface anisotropy are discussed in section 4.

2. Experimental

The multilayers for the present study were deposited by e-beam evaporation in an ultrahigh vacuum system. The base pressure was less than 1 X 10v9 Torr. The typical pressure during deposition was 2-4 X 10e9 Torr. A 500 fi Au buffer layer was grown at 20 ’ C onto glass or polyimide films and then annealed at 150 “C for 1 h. The buffer layer was a (111) textured polycr;ystalline and the crystallite size was around 2000 A. Multi- layers were deposited onto the buffer layer at about 35 o C. The designed structure for a multilayer is iilustrated in fig. 1. The deposition rate was 0.3 A/s. The thickness was monitored by a quartz crystal sensor. The number of repetitions was fixed to 15. Finally a protective Pd/Au/Pd

Pd (xA)

Au(25-2xA) 1 protective

Pd(xA) layer

Co(yA)

: .

Pd(xA)

Co(yA)

Au(2.S2xA) 1

Pd(xA) period

Pd(xA) .

:

Au(25-2xA)

WW

Au (500A) I

glass or polyimid

Fig. 1. Schematic diagram of multilayer preparation.

cap was grown on the top Co layer of the multilayer. Pd/Cu/Pd/Co and Pd/Co multilayers were produced under similar conditions.

The periodic compositional modulations and crystallographic structures of these multilayers were examined by small- and high-angle X-ray diffraction (XRD) using CuKa. Cross-section transmission electron microscopy (TEM) was used to confirm the periodic structure of these multilayer films.

Magnetization at room temperature was measured using a vibrating-sample magnetometer (VSM), and the perpendicular magnetic anisotropy was determined using a torque magnetometer with applied fields up to 20 kOe. Magnetic properties at low temperatures were investigated by SQUID magnetometer. Magneto-optical properties were measured from the fihn side in the wavelength range 400-800 mn, using a polar Kerr rotation measurement system.

2 8 (deg.)

2 0 (deg.)

Fig. 2. X-ray dif’fractograms at (a) high and (b) low angles for [Pd(5 &)/A~(15 &/Pd(5 &/Co@ &II5 multilayers grown on 500 A (111) textured Au. The calculated intensity from the

step model is shown below.

Information Storage: Basic and Applied

M. Sakurai, T. Shinjo / Properties of Pd /Au / Pd / Co and Pd / Cu / Pd / Co multilayers 239

3. Results

3.1. Crystal structure

A typical small-angle0 XRD pattern of [Pd(5 &/Au05 A)/Pd(S A)/Co(6 A& multilayers is shown in fig. 2(b). The modulation period is deduced from the positions of the Bragg peaks. The high-angle XRD pattern (fig. 2(a)) shows the main (111) peak n = 0 and the satellite peaks n = -2, - 1, + 1, + 2 for the artificial su-

perstructure. We compared these peaks with those of the step model calculation. An agree- ment with the theoretical results indicates that the multilayered structures were prepared as designed.

Figure 3(a) shows the cross-sectional TEM view of [PdG .&)/Au05 A)/Pd(5 &/Co(22 A>],, multilayers, which shows a stripe pattern due to the artificial periodic structure. From the bending profile of the stripes, the sizes of columns are roughly estimated. The average grain size was

Fig. 3. Cross-sectional ‘EM micrograph of (a) [Pd(S &)/Au05 &/Pd(5 &)/Cd18 %11~ Cd12 &I,, multilayers.

and (b) [Pd(8 &)/Cu(9 &/Pd(8 A)/

240 M. Sakurai, T. Shinjo / Properties of Pd /Au / Pd / Co and Pd / Cu / Pd / Co multilayers

200-500 A, considerably smaller than that of the Au buffer layer. The dark bands corresponds to Au layers and the light ones to Co layers. The contrast corresponding to Pd-rich layer was not clearly distinguished. Sotripe patt$rns were $so observtd for [Pd@ A)/ C&9 A)/ Pd(8 A)/ CM12 A)],, multilayers in fig. 3(b), in which the dark bands correspond to Pd layers and the light bands correspond to Co and Cu layers.

3.2. Magnetic anzkotropy

Figure 4 shows the anisotropy energy per unit Co volume of Pd/Au/Pd/Co and Pd/Cu/ Pd/ Co multilayers with varying the interfacial Pd layer thickness. The thickness of the nonmagnetic layer, Pd/Au/Pd or P$/Cu/Pd, and Co layer were fixed at 25 and 6 A, respectively. The magnetic anisotropy of each multilayer system increased with increasing Pd layer thickness up to a few angstroms, because the coating of the Co layer by Pd is not complete to tl$s thickness. For Pd layer thicknesses beyond 5 A, the anisotropy in the Pd/Au/Pd/Cu multilayers decreased slightly. In Pd/Cu/Pd/Co multilayers, on the other hand, a slight inzrease was observed for Pd layer thicker than 7 A. There exists some stress from intermediate layer to Pd/Co/Pd layer, because the lattice constant of Pd is smaller than

x 106 20

[Pd(x)/Au(25-2x)/Pd(x)/Co(6)115

I I I I

0 Pd T:ICKNESS(&

10

Fig. 4. Dependence of the magnetic anisotropy energy, K,e, per Co volume on the interfacial Pd layer thickness for [Pd(t &/X(25 - 2t &/Pd(t &/Co(6 &II5 (X = Au, Cu)

multilayers.

x IO6 L”

[Pd(5)lAu(x)lPd(5)/Co(6)1 ,5 - w l

A .

Y [Pd(5)ICu(x)/Pd(5)/Co(6)1 ,5 AA

T=300K

I I I I I I I 0 10 20 30 40

INTERMEDIATE LAYER THICKNESS(A)

Fig. 5. The dependence of magnetic anisootropy e!ergy on Jhe intermediate layer0 thickness for [Pd(5 A)/X(t A)/Pd(S A)/

CM6 A& (X = Au, Cu) multilayers.

that of Au and is a little larger than that of Cu. Hence, the change in stress may contribute to the change in anisotropy.

To study the contribution from the intermtdi- ate layer, the magnetic anisotropy for [Pd(5 A)/ X0 &/Pd(5 &/Co(6 -8’)]r5 (X = Au, Cu) multilayers was measured with varying the intermediate layer thickness. Figure 5 shows that the anisotropy increased for Pd/Au/Pd/Co multilayers and decreased for Pd/Cu/Pd/Co multilayers. The anisotropy changed for both multilay- e,‘s with the intermediate layer thickness up to 5 A. This suggests that the contribution from the intermediate layer does not change for each system with the intermediate layer thickness beyond 5 A.

To display the effect of the interface more clearly, we systematically oinvestigatFd the mOag- netic anisotropy of [Pd(5 A)/X(15 A)/Pd(5 A)/ Co(y &],, (X = Au, Cu or Pd) multilayers when the Co layer thickness was varied. The anisotropy energy, K,n, can be written as

K,, = K, + 2K,/ki,,

where K, is the volume anisotropy energy and KS is the interface anisotropy energy [6]. The number of interfaces per Co thickness is 2/t,. In fig. 6 a plot of t&K,, versus t, shows a linear dependence. This relation suggests that the

information Storage: Basic and Applied

M. Sakurai, T. Shinjo / Properties of Pd /Au / Pd / Co and Pd / Cu / Pd /Co mulrilayers 241

(a) (W

0 10 20

Co THICKNESS(A) Fig. 6. Dependence of K, t, on Co layer thickness, &,, for [Pd(5 &X(15 &)/PdU %),co(y A& (X = Au, Cu, Pd)

interfacial effect is independent of the Co layer thickness. From the linear fits to the data, KS and K, are given as listed in table 1. The results of Au/Co and Cu/Co multilayers are shown for comparison. The volume anisotropy energy is almost the same except for Au/Co multilayer. On the other hand, the interface anisotropy is different for each multilayer. These results indicate that the difference in the anisotropy is due mainly to the interface anisotropy.

3.3. Magnetization

Room-temperature hysteresis curves of [Pd(t &/A~(25 - 2 t A)/ Pd(t &/ Co(6 &I and [Pd(t A)/&(25 - 2t &/Pd(t &/Co(6 A)] multilayers are shown in fig. 7. The field was applied perpendicular to the film surface. The shapes of the hysteresis loops changed for Pd/Au/Fd/Co multilayers with Pd layer thickness of 2 A. The

Table 1 Interface and volume anisotropy for multilayers

KS (erg/cm2) K, (erg/cm31

Pd(5 .%)/Au05 &/

Pd(5 &)/Cd Y & 0.80 - 9.1 x 106

Pd(5 z&/Cu(I5 &/

Pd(5 &)/Co( y & 0.59 - 9.5 x 106

Pd(25 &)/Co( y & 0.74 - 9.6 x lo6

Au(25 &)/Co( y & 0.55 -5.2x106

Cu(25 &)/C&y & 0.28 - 8.5 x 10”

TX- Pd=5A

-!T Pd=lO

g-i-

-4 t-l&e)

4 -4 u 4

H(kOe)

Fig. 7. Hysteresis loops for (a) [Pd(t &)/A~(25 -2t &/Pd(r &)/CM6 .&&s and (b) [Pd(r &/Cu(25 - 2t &)/Pd(t &)/CM6 &]15 multilayers with varying Pd layer thickness measured at room temperature. The Pd layer thickness is shown in the picture. The field was applied perpendicular to the film

surface.

nucleation field at which the magnetization rever- sal starts was al?ost equal for Pd layer thicknesses beyond 2 A. The anisotropy field at which magnetization saturates increased with the Pd layer thickness. These results indicate that hysteresis loop for Pd/A_u/Pd/Co multilayers is rather sensitive to the Pd layer thickness.

The coercive force and nucleation field for Pd/Cu/Pd/Co multilayers increased with Pd layer thickness.OFor the multilayers with Pd layers thicker than 5 A, the loops were almost the same. The shapes of the Pd/Cu/Pd/Co multilayers can be accounted for by the increase in the magnetic anisotropy energy.

The dependence of the magnetization per unit Co volume on the Pd layer thickness at 5 K is shown in fig. 8. The magnetization saturates for Pd/Au/Pd/Co multilayers with around 5 %i Pd layer thickness. The increase in magnetization is related to the amount of the polarized Pd. Mag- netization in Pd/Cu/Pd/Co muliilayers increased monotonically up to 12.5 A Pd layer thickness. In Cu/Co multilayers, there may exist a decrease in magnetization at the interface.

242 M. Sakurai, T. Shinjo / Properties of Pd /Au / Pd / Co and Pd / Cu /Pd / Co multilayers

[Pd(x)/Cu(25-2x)/Pd(x)/Co(G)]

i :;::lj T=5K

I I I I 0

Pd T&KNESS(/i) 10

Fig. 8; Saturation Omagnetiz$ion pef Co volume for [Pd(t AI/X(25 - 2t A)/Pd(t A)/Co(6 A)],, (X = Au, Cu)

multilayers with varying Pd layer thickness measured at 5 K.

Hence, the change in magnetization is due to the interfacial Co layer and polarized Pd.

Figure 9 is a plot of saturation magnetization versus Co layer thickness. The saturation magnetization per unit area, Msd,, obeys the relation:

M&c, = K&c, + 2P,, , (2)

where d, is the Co layer thickness, P,, is the magnetization from polarized Pd per unit area at each interface and the factor 2 arises from the two interfaces of each Co layer. This method can be applied to other systems. A positive value of the intercept corresponds to an increase in the magnetization at the interface. A negative value indicates the existence of a dead layer at the interface.

As shown in fig. 9, the slopes are almost the same, and are identical to the quoted value for the bulk hcp Co, 1422 emu/cc. From the inter-

x 10-d

T=5K

I 0 10 20

Co THICKNESS(A) Fig. 10. Relative magnetization for [Pd(S &)/X(15 &/ Fig. 9. Saturation magnetization per unit area versus CQ Pd(5 &)/Co(6 I%)],, (X = Au, Cu, Pd) multilayers versus tem-

thickness. perature.

cept of the linear fits to the data, the amount of Pd polarization can be estimated. Linear rela- tions suggest that the interfacial effect is independent of the Co layer Jhickness. yalf of t$e intercep\value for [Pd(5 A)/Au(15 A)/Pd(5 A) /CJo(y A)],, multilayers was 1.5 x 10e5 emu/ cm2, which is almost equal to that for [Pd(25+)/ Co( y A)],, multilayers. The values for [Pd(5 A)/ Ml5 &/Pd(5 &/cO(y 8,>115 and [CM25 A)/ Co(y &I,, multilayers were 0.7 x 10m5 and -0.7 x lo-’ emu/cm2, respectively. This suggests that there exists a dead layer at the interface in Cu/Co and a polarized layer in Pd/Co, Pd/Au/Pd/Co and Pd/Cu/Pd/Co multilayers. The polarized layer of Pd/ Cu/Pd/ Co multilayers is a little smaller than that of Pd/Co multilayers. With this method, however, we can not know how the distribution of polarized layer is changed by the different intermediate layers.

The temperature dependence of magnetization for [Pd(5 &/X(15 &/Pd(5 AI/M6 &]i5 (X = Au, Cu, Pd) multilayers was measured by a SQUID with applied field 20 kOe perpendicular to the film plane. Figure 10 shows the relative magnetization versus temperature. The changes in magnetization for Pd/Au/Pd/Co and Pd/ Cu/ Pd/ Co multilayers were smaller than that of Pd/Co multilayer, and the relative magnetization for these multilayers exhibits a T312 dependence. Theseoresults sttggest thaj a magnetic coupling of Pd(5 A)/&(6 A)/Pd(S A) parts across an intermediate layer is a little stronger for Au and Cu layers than for a Pd layer.

. [Pd(S)/Au(l5)/Pd(5)/Co(6)1i5 - A [Pd(S)/Cu(l5)/Pd(5)/Co(6)115

l PdPWW6)115 0.9 I I I I

100 200

T(K)

ktformation Storage: Basic and Appkd

M. Sakurai, T. Shinjo / Properties of Pd /Au / Pd / Co and Pd / Cu / Pd / Co multilayers 243

3.4. Magneto-optical properties

Figure 11 shows the polar Kerr spectra of [Pd(t &/X(25 - 2t &/Pd(t A)/Co(6 A& (X = Au, Cu) multilayers at room temperature in the wavelength range 400-800 nm. An enhancement of the Kerr rotation was observed because of the plasma edge of the intermediate Au and Cu layers. Such an enhancement has already been observed in the Cu/Fe and Au/Co multilayers [14]. For Pd/Au/Prl/Co multilayers with Pd layer thicknesses of 2-7 A, the Kerr rotation was larger than that of [Pd(25 &/Co(6 A&, and [Au@5 A)/Co(6 A& multilayers in the wavelength range 400-800 nm. The increase was caused partly by the plasma edge of Au at around 480 nm and partly by the intrinsic properties of PdCo.

These enhancements were also observed for Pd/Cu/Pd/Co multilayers. The plasma edge of Cu is around 560 nm. The source of the enhancement can, therefore, be distinguished. The increase around 400 run may be due to the PdCo property. The increase in Kerr rotation of Pd/Co multilayers at shorter wavelengths (= 400 run) has generally been attributed to exchange-induced polarization of Pd 4d levels [15]. The increase at 400 nm was also observed for

(a) (b) Pd=OA

Pd=Oi+

-7 -7

$ Pd=2A z

n 0

Pd=2A

g Pd=5A g

2

~~

5 Pd=5?!

k? u

Pd=7A I 0.03-

B E

Y I O.Od iii

Pd=lOA

Pd=12sA

400 600 800

400 600 800 WAVE LENGTH(nm)

WAVE LENGTH(nm)

Fig. 11. Pd layer thickness degendeces of polarOKerr spectra for (a) [Pd(t A))/Au(25 - 2t A)/Pd(r &)/CM6 AllIS and (b) [Pd(r &)/CM25 - 2r &/Pd(t &)/C4$6 A)l15 multilayers measured from the film plane. The Pd layer thickness is shown in the picture. The peaks at around 560 and 4.80 nm correspond

to plasma edges of the Cu and Au layers, respectively.

[Pd(2 &/Cot21 A)/Pd(2 &/Co@ A)] multilayers. This indicates that there exists a polarization of 2 A interfacial Pd layer thickness, and this layer contributes to the increase in Kerr rotation.

4. Discussion

Magnetic anisotropy can be generally clarified into volume anisotropy, Kv, and interface anisotropy, KS, from eq. (1). The volume anisotropy contains contributions from magnetocrystalline, magnetostatic and magnetoelastic anisotropy. The origin of the interface anisotropy has not been solved yet. There may be contributions of electronic structure, magnetoelastic and magnetostatic anisotropy. We now study some of these contributions in detail.

The magnetocrystalline anisotropy of Co layer is closely related to the hcp structure. When the structure is fee, this anisotropy is negligible. Re- cent studies showed that the stacking of Co layers was fee for Pd/Co [ll] and Cu/Co [12] multilayers, and hcp for Au/Co [13] multilayers. Hence, there is the contribution to the volume anisotropy from the magnetocrystalline anisotropy for Au/Co multilayer. From the value of table 1, the volume anisotropy for Au/Co multilayer was greater than those of the other multilayers. The difference is almost equal to the magnetocrystalline anisotropy energy of bulk hcp Co: 4.53 X

lo6 erg/cc. The structure of Co for other systems is expected to be fee from the value of the volume anisotropy.

If the distribution of a magnetization is known, the magnetostatic anisotropy for a multilayer can be estimated as follows. We assume that the x-y plane is parallel to the film plane, the z-plane is perpendicular to the plane and is the direction of easy magnetization. By taking an average over the x-y plane, all variables are functions of the z-axis. The magnetic charge, a(z), per unit area is:

dNz)m(z) g(z)= - dz 7 (3)

where N(z) is the number of magnetic moments per unit area, and m(z) is the magnetic moment.

fnfocmation Storage: Basic and Applied


The product of N(z) and m(z) corresponds to the magnetization per unit area. If we make an approximation for an infinite plane, demagnetizing field, H,(z), can be written:

where L is the total thickness. We use CGS units for the calculations. The magnetostatic anisotropy energy per unit area can be obtained as:

EM= -;/N(z)m(z) xZZ,(z) dz. (5)

Figure 12(a) shows the case of uniformly distributed magnetization only in the magnetic layer. The magnetic charge is confined to the interface and can be expressed by a d-function as shown in the figure. The demagnetizing fields inside and outside a magnetic layer are -4pZ, and 0, respectively. Dividing E, by the thickness, it is obvious that the magnetostatic energy is 2~1:.

Figure 12(b) shows the case of strongly en- hanced magnetization such as Pd/Co multilayer. The distribution of magnetization can be sepa- rated into two parts, which is expressed by the broken line in the figure. One is the contribution

from the magnetic layer; as shown in fig. 9, this is 2&f&. The other is that from the polarized layer. The area shadowed in the second from the top of the fig. 12(b) corresponds to the intercept in fig. 9. Since the polarized part does not change with varying Co layer thickness (as evidenced in fig. 91, the contribution should not be involved in the volume anisotropy but in the interface anisotropy. This is because the value depends on the number of interfaces. The product of the areas shadowed in the second from the top and in the bottom leads to magnetostatic contributions to the interface anisotropy. This value is negative. If we assume that the distribution of the polarization changes linearly with the Pd layer thickness, the polarized layer for Pd/Au/Pd/Co multilayers causes the decrease in interface anisotropy by 0.08 erg/cm2. The intercept in fig. 9 shows that the contribution to the interface anisotropy from the magnetostatic energy is larger for Pd/Au/ Pd/Co than for Pd/Cu/Pd/Co multilayers.

This method can be applied to Pd/Co or Pt/Co multilayers. The magnetic anisotropy for the multilayer decreases with decreasing Pd or Pt layer thickness [22]. This may be due to the decrease of the interface anisotropy, which is caused by the increase of the polarized Pt layer.

(a) Dtstnbution of magnetization

(b) Dlstrlbution of magnetization Cc) Distribution of magnetization

N(z)m(z) / N(z)m(z)

Fig. 12. The top of each figure shows illustrations of distributed magnetization for a multilayer. The second from the top shows the magnetization per unit area, N(z)m(z). The third from the top shows the magnetic charge, a(z), calculated using eq. (3). The

bottom shows the demagnetization field, E&(z), calculated using eq. (4).

hformatlon Storage: Basic and Applied

M. Sakurai, T. Shinjo / Properties of Pd /Au / Pd / Co and Pd / Cu / Pd /Co multilayers 245

For a film with interfacial roughness, the magnetic charge, a(z), changes from a b-function to a smoothly varying function at the interface. This leads to a reduction of the magnetostatic energy and gives rises to an increase in the interface anisotropy, a result that has already been discussed by Bruno [16]. Figure 12(c) shows the case of a dead layer at the interface such as Cu/Co and Ru/Co multilayers. The area shadowed in the second from the top corresponds to the intercept in fig. 9. The product of the areas in the second and in the bottom leads to magnetostatic contributions. If there exists a dead layer with two monolayers at the interface, a virtual increase in the interface anisotropy by 0.24 erg/cm2 should be induced. These results suggest that the interface anisotropy obtained from eq. (1) contains a large uirtual magnetostatic contribution. Hence, we should be careful not to overestimate the interface anisotropy by use of eq. (1).

Then we find that the contribution of the magnetostatic anisotropy to the volume anisotropy is 2rrM&. The value does not change as far as the M, in eq. (2) is equal to the bulk Co. The change in magnetization at the interface gives rise to the interface anisotropy. This magnetostatic contribution can be distinguished from the other origin by use of eqs. (3)-(5).

The strain of a magnetic layer induces a magnetoelastic anisotropy. For a cubic magnetic layer, the uniform expansion of the lattice within the (111) plane causes the anisotropy:

E 6C,(C11+ 2C12) SU

MW11) = +‘h 4C, + Cl, + 2C,, - a

X(ala,+ a2a3+Cf3al), (6)

where A,,, is the magnetostriction constant for the (111) planes, Cll, Cl2 and C, are the elastic stiffness constants for a cubic material, al, a2 and a3 are direction cosines of the magnetization, and &z/a is the strain within the (111) plane. For a lattice expanded within the (001) plane, the anisotropy is given by:

E Q (Cl1 - cl2)(cll+ 2Cl2) h

ME(100) = 2 100 -

Cl1 U

x(& $), (7)

where &z/u is the strain within the (001) plane. The anisotropy contributes to the volume anisotropy, as has been observed in Cu/Ni/ Cu(OO1) sandwiches [17] and Pd/Co(OOl) multilayers [7,8]. In contrast, high-resolution TEM showed the existence of misfit dislocations at interfaces for Au/Co(lll) and Pd/Co(lll) multilayers [18]. Purcell showed that the Co layer on the (111)Pd layer had a lattice spacing near the bulk value [19]. As for the multilayers studied here, they are all (Ill)-oriented multilayers, and there is no magnetoelastic contribution to the volume anisotropy as expressed in eq. (6). Hence the difference between the volume anisotropies for the present multilayers can be explained by the magnetocrystalline anisotropy and 27~M&,.

The interaction between ferromagnetic layers across a nonmagnetic metal layer was observed in Fe/Cr [20] and Co/Cu [21] multilayers. The strength of the antiferromagnetic coupling reveals an oscillatory behavior with a period of about 10 monolayers. The coupling is expected to reduce the interface anisotropy, because this interaction can be expressed as -J cos 8, where the sign of coupling constant J is minus and 8 is the angle of magnetization across a nonmagnetic layer. As for Pd/Au/Pd/Co and Pd/Cu/Pd/Co multilayers, the interaction across Au or Cu layers is thought to decrease because of the Pd layer. Hence, there is small contribution to the interface anisotropy from this interaction. The results of fig. 5 show that there is no such interaction.

As for the interface anisotropy, we can not explain results in table 1. There are contributions from electronic structure and strain at the interface. Further investigations are necessary to re- solve this problem.

5. Conclusion

The magnetic anisotropy for the Pd/X/Pd/Co multilayers (X = Au, Cu, Pd) was found to change when the interfacial Pd layer thickness0 was varied. The magnttic anisot;opy of [Pd(t A)/Au(25 - 2t A)/Pd(t A)/Co(6 A)],, multilayers took a maximum when the Pd layer thickness was around

lnfcmnation Storage: Basic and Applied


5 A. On the other hand, the magnetic anisotropy for [Pd(t A)/Cu(25 - 2t A)/Pd(t A)/Co(6 A& multilayers increased with Pd layer thickness. The change in magnetic anisotropy is due mainly to the interface anisptropy. pe anisootropy didonot change for [Pd(5 A)/X(t A)/Pd(S A)/Co(6 Allis (X = Au, Cu) multilayers with an intermediate layer thickness beyond 5 A. The difference between the volume anisotropies for the present multilayers can be explained by the magnetocrystalline anisotropy and 27riI4& The difference between the interface anisotropies can not be explained.

The saturation magnetization of Pd/ Au/ Pd/ Co and Pd/Cu/Pd/Co multilayers changed with varying Pd layer thickness. The intercept and slope obtained by the equation M,d, = M,d,, + 2P,, show that this difference is due to the change in magnetization at the interface.

The shapes of hysteresis lvops for [Pd(t A>/ X(25 - 2 t A)/Pd(t A)/Co(6 A& (X = Au and Cu> multilayers changed with varying Pd layer thickness.

An enhancement of the Kerr rotation was observed because of the plasma edge of the Au or Cu layer. An increase in the Kerr rotation at 400 nm was observed for films with 2 A Pd layer thickness.

The magnetostatic anisotropy energy for a multilayer can be calculated. The change in magnetization at the interface contributes to the interface anisotropy. The contribution of the magnetostatic anisotropy to the volume anisotropy is 27rM& which is constant as long as the magnetization of a magnetic layer, it4,, is not changed.

Acknowledgements

We gratefully acknowledge Y. Ishikawa of Tosoh Corporation for the TEM measurements,

and Dr T. Takahata and Dr I. Moritani of Tosoh Corporation for useful discussions.

References

111 P.F. Garcia, AD. Meinhaldt and A. Suna, Appl. Phys. Lett. 47 (1985) 178.

[2] P.F. Garcia, J. Appl. Phys. 63 (1988) 5066. [3] F.J.A. den Broeder, D. Kuiper, A.P. van der Mosselaer

and W. Hoving, Phys. Rev. Lett. 60 (1988) 2769. [4] M. Sakurai, T. Takahata and I. Moritani, J. Magn. Sot.

Jpn. 15 (1991) 411. [S] S. Hashimoto and Y. Ochiai, J. Magn. Magn. Mater. 88

(1990) 211. [6] H.J.G. Draaisma, F.J.A. den Broeder and W.J.M. de

Jonge, J. Magn. Magn. Mater. 66 (1987) 351. [7] F.J.A. den Broeder, D. Kuiper, H.C. Dinkersloot and W.

Having, Appl. Phys. A 49 (1989) 507. 181 B.N. Engel, C.D. England, R.A. Van Leeuwen, M.H.

Wiedmann and C.M. Falcon, Phys. Rev. Lett. 67 (1991) 1910.

[9] G.H.O. Daalderop, P.J. Kelly and M.F.H. Schuurmans, Phys. Rev. B 42 (1990) 7270.

[lo] G.H.O. Daalderop, P.J. Kelly and F.J.A. den Broeder, Phys. Rev. Lett. 68 (1992) 682.

[ll] H.A.M. de Gronckel, C.H.W. Swuste, K. Kopinga and W.J.M. de Jonge, Appl. Phys. A 49 (1989) 467.

[12] C. Mery, P. Panissod and R. Lo&e, Phys. Rev. B 45 (1992) 12269.

[13] K. Le Dang, P. Veillet, C. Chappert, P. Beauvillain and D. Renard, J. Phys. F: Metal Phys. 16 (1986) L109.

[14] T. Katayama, Y. Suzuki, H. Awano, Y. Nishihara and N. Koshizukam, Phys. Rev. Lett. 60 (1988) 1426.

[U] D. Weller, W. Reim, K. Spiirl and H. Bandle, J. Magn. Magn. Mater. 93 (1991) 183.

[16] P. Bruno, J. Appt. Phys. 64 (1988) 3153. [17] Chin-An Chang, J. Appl. Phys. 68 (1990) 4873. [18] F. Hakkens, W. Coene and F.J.A. den Broeder, Mater.

Res. Sot. Symp. Proc. 231 (1992) 397. [19] S.T. Purcell, H.W. van Kesteren, E.C. Cosman and W.

Hoving, J. Magn. Magn. Mater. 93 (1991) 25. [20] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen van Dau,

F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472.

[21] D.H. Mosca, F. Petroff, A. Fert, P.A. Schroeder, W.P. Pratt and R. Loloee, J. Magn. Magn. Mater. 94 (1991) 1.

[22] 2. Zhang, P.E. Wigen and S.S.P. Parkin, J. Appl. Phys. 69 (1991) 5649.

Documents

Magnetic and magneto-optical properties of Pd/Au/Pd/Co and Pd/Cu/Pd/Co multilayers