Magnetic and structural properties of Co/Pt multilayers

194 Journal of Magnetism and Magnetic Materials 93 (1991) 194 211~ North-t tolland

Invi ted paper

Magnetic and structural properties of Co/Pt multilayers

C.-J. Lin, G.L. G o r m a n , C.H. Lee , R.F.C. Farrow, E.E. Mar ine ro , H.V. Do, H. Notarys

IBM Research Dicision, Almaden Research Center, San Jose, CM 95120-6099, USA

and

C.J. Chien

Department of Materials Science and Engineering, Stan[~)rd Unit'ersity, Stanfi;rd, ('/t 94305, US, M

The magnetic properties of C o / P t multilayers, in particular anisotropy and coercivity, are very sensitive to the ( 'o layer thickness and less dependent on the Pt layer thickness. Such dependence is illustrated and discussed for e-beam cwlporatcd C o / P t multilayers of various Co and Pt thicknesses. It is shown that the largest perpendicular anisotropy occurs fl)r ( 'o layer thickness of 1 2 monolayers. The magnetic anisotropy of C o / P t multilayers also strongly depends on the crystallographic orientation. The largest perpendicular magnetic anisotropy was obtained for C o / P t multilayers with P t ( l l I ) parallel to the film plane. This was demonstra ted in both evaporated and sputtered polycD'stalline multilayers, as well as in molecular beam epitaxy (MBE) grown C o / P t ( l 11) superlattices. By contrast, MBE grown 3.7 A Co/16 .8 /k Pt(100) multilayers shov~ in-plane anisotropy, and 3.7,~ C o / 1 6 . S A Pt(110) multilayers exhibit very strong anisotropy within the film plane. These observations suggest that the magneto-crystalline anisotropy plays a key role in the magnetic anisotropy of these structures. Significantly enhanced effective magnetization was observed for C o / P t multilayers with ultrathin Co layer.

1. Introduction

Research in artificial magnetic multilayers of C o / P d [1-9], C o / P t [6-12] and C o / A u [13-16] has recently become very active. These multilayers can have very large perpendicular magnetic anisotropy and coercivity when the Co layer thickness is only a few monolayers. These multilayers are therefore potentially useful as magneto-optical recording or perpendicular magnetic recording media. The large perpendicular ma- netie anisotropy has generally been attributed to the reduced symmetry at the interface between magnetic and non-magnetic layers. The formation of ordered Co-P t alloy phases at the interface has also been considered as a possible explana- tion for the observed anisotropy in C o / P t multilayers [6]. The exact origin, however, remains unclear.

Recently Den Broeder et al. [5] studied the effect of crystalline orientation on the magnetic anisotropy of C o / P d multilayers. They compared (111) textured (i.e., P d ( l l l ) parallel to the tilm plane) polycrystalline mult.ilayers with single crystal Co/Pd(100) multilayers epitaxially grown on NaCI, and found that the cross-over Co layer thickness was orientation-dependent. The cross- over Co layer thickness refers to the Co layer thickness where the preferred magnetization di- rection changes from in-plane to perpendicular. It is about 2-4 and 8 -1 2 A for Co/Pd(100) and C o / P d ( l l l ) , respectively. Their results are in- terpreted as due to a larger negative volume contribution to the anisotropy in Co/Pd(100) multilayers than in Co/Pd(111) multilayers. They attributed the easy-in-plane volume anisotropy term to the magneto-elastic anisotropy arising from stretching of the Co layer.

0304-8853/91/$1)3.50 (c 1991 Elsevier Science Publishers B.V. (North-Itolland)

C.-J. Lin et al. / Properties of Co / Pt multilayers 195

We have systematically investigated the dependence of the magnetic anisotropy of e-beam evaporated Co/Pt multilayers on the Co layer and Pt layer thicknesses. We also examined the anisotropy dependence on the crystallographic orientation of Co/Pt multilayers prepared by evaporation, sputtering and molecular beam epitaxy (MBE). We used (111) textured Pt underlayer to promote dramatically the (111) texture of Co/Pt multilayers in both evaporated and sputtered films. Such results were also cross-checked by MBE grown Co /P t ( l l l ) superlattices. Fur- thermore, we have used MBE techniques to grow Co/Pt multilayers of other orientations, namely Co/Pt(100) and Co/Pt(l l0), to investigate the orientation dependence of magnetic anisotropy [17]. These observations shed light on the origin of the magnetic anisotropy in Co/Pt multilayers.

2. Experimental

Most Co/Pt multilayers studied here, unless specified, were deposited by e-beam evaporation from Co and Pt sources onto room temperature glass, fused quartz and Si coupons. The base pressure was 3-8 x 10 -8 Torr. The typical pressure during deposition was 3-6 x 10 - 7 Torr. The substrate holder was placed 42cm above the sources and rotated at 21rpm. The deposition rates and thickness of Co and Pt layers were monitored independently by two crystal oscilla- tors. The deposition rates were kept low, typically 0.15]~/s for Co and 0.30A/s for Pt, to obtain good control of the layer thickness.

Co/Pt multilayers were also prepared by sputtering from Co and Pt targets at various deposition rates and argon pressures. In order to further investigate the orientation dependence of the manetic anisotropy, seeded molecular beam epitaxy techniques were used to grow Co/Pt superlattices along three different crystallographic orientations of Pt on 100°C single crystal GaAs substrates: Co/Pt(111) on GaAs(111), Co/Pt(110) and Co/Pt(100) both on GaAs(100). Details of

preparation and structural characterizations of these MBE superlattices are reported separately [18, 19].

The periodicity of the compositional modulation and the crystallographic structures of these multilayers were investigated by small and high angle X-ray diffraction (XRD) using Cu Ka radi- ation in a geometry with the scattering vector normal to the film plane. The compositions of the multilayer films were measured by both X-ray fluorescence (XRF) and Rutherford backscatter- ing spectroscopy (RBS). The thicknesses of Co and Pt layers were then calculated from the measured composition and modulation period, using bulk densities of Co and Pt. Cross-section and plane-view transmission electron microscopy (TEM) were also used to examine the structure of these multilayer films.

The Kerr rotation angle and the perpendicular coercivity were obtained from polar Kerr hysteresis loops. The magnetization at room temperature was measured using a vibrating sample magne- tometer (VSM), and the perpendicular magnetic anisotropy was determined using a torque magne- tometer with applied fields up to 20 kOe.

3. Results and discussion

3.1. Structure

The small angle XRD pattern shown in fig. l(a) confirms the layered structure in the evaporated Co/Pt multilayers. The modulation period is deduced from the positions of the Bragg peaks. The high angle XRD pattern (fig. l(b)) shows the mean (111) peak n =0 and the satellite peaks n = - 1, + 1 for the artificial superlattice. The position of the mean (111) peak is between the fcc Pt( l l l ) and fcc Co(Il l) reflections, and depends on the amount of Co in the multilayer. The pronounced (111) texture of these Co/Pt multilayers is inferred from the absence of distinct (200) reflection peaks (cannot be separated from the n = + 1 peak). The compositions determined

196 C.-J. Lin et al. / Properties ()/' ( ' ( ) /Pt rnultilayers"

106 I I I I I I ] 1 I

(a) n = l

g 105

+_,

104

103 .E

102 I L I 6 10 14 18 22

1 0 4 , E = ~ - - (b)

n=O

• - ~ I o 103 ~

m - ] ~- + I

• ~ lo 2 c

_c

101 ] ] ~ L ~ J 30 35 40 45 50 55

20 (degree)

Fig. 1. X-ray diffractograms at (a) low angle and (b) high angle for a 17 × Co(8.1 ~.)/Pt(10.3,~) multilayer.

by both XRF and RBS are in very good agree- ment. The deduced modulation period and the Co and Pt layer thicknesses in general are within 5% of nominal values.

The plane-view TEM micrograph and diffraction pattern in fig. 2(a) show the typical fcc polycrystalline structure of evaporated C o / P t multilayers, with an average grain size of about 100 ]k. The results of XRD (with scattering vector perpendicu[ar to the film plane) and plane-view TEM diffraction (with scattering vector in the film plane) indicate that these multilayers have a fcc crystal structure with a lattice constant in between those of fcc Pt and fcc Co. The cross-section TEM micrograph shown in fig. 2(b) clearly shows the lattice coherency between Co and Pt layers,

We have also grown optimally ( l l ] ) - textured C o / P t multilayers at room temperature by using

20(1,~ ( l l l ) - tex tured Pt underlayers. Highly (111)-textured Pt underlayers were obtained by evaporating Pt onto substrates at high tempera- tures, e.g., 200°C. The XRD patterns in fig. 3(a) and (b) show that the (111) texture of multilayers was greatly enhanced by the presence of the Pt underlayer. The dramatic improvement in perpendicular loop coercivity is also depicted. In fig. 4(a), the plane-view TEM micrograph shows the microstructure of the evaporated C o / P t multilayer grown on a Pt underlayer. An average grain size of about 300/k is deduced from the figure. The diffraction pattern shows two distinctive sets of rings corresponding to the fcc Pt underlayer and the fcc C o / P t multilayer. Note that the layered structure of the multilayer is discernible in the grain at the upper right hand side of fig. 4(b). The lattice coherency between the C o / P t multilayer and the Ptunderlayer can bc seen by their closely correlated textures exhibited in the diffraction pattern (fig. 4(a)) and also by the cross-section lattice image in fig. 4(b). Similar results have also been obtained for sputtered C o / P t multilayer. For example, fig. 5 shows that the better (111) textured multilayer (fig. 5(b)) results in a larger perpendicular magnetic anisotropy and loop coercivity (6.5kOe). The MBE grown Co/Pt(111) multilayer exhibits also very large perpendicular magnetic anisotropy,

similar to that of the (111) textured multilayer evaporated on a (111) Pt underlayer. These observations suggest that (111) film plane is required for obtaining very large perpendicular magnetic anisotropy K~,.

3.2. Magnetic anisotropy

3.2.1. Dependence on Co layer thickness

The perpendicular magnetic anisotropy of C o / P t multilayers is very sensitive to the Co layer thickness, as clearly shown by the three sets of polar Kerr hysteresis loops in figs. 6-8. The best perpendicular hysteresis loops are obtained when the Co layer thickness is in the range of 2-4,~, corresponding to 1-2 monolayers. Comparing fig.

C.-J. Lin et al. / Properties of Co / Pt muhilayers 197

Fig. 2. (a) Plane-view TEM micrograph and diffraction Co(3.1 A)/Pt (102 ,~) multilayer.

8 to fig. 6, one can easily see how the (111) textured Pt underlayer has dramatically increased the perpendicular coercivity and the cross-over Co layer thickness. In fig. 9, the effective perpendicular magnetic anisotropy Kef f iS plotted against the Co layer thickness tco for the three sets of

pattern and (b) cross-section TEM micrograph of a 23 ×

multilayers shown in figs. 6-8. Ket- f is obtained by dividing the anisotropy energy of a multilayer by the Co volume on the film. The largest Kea, occurs for Co layer thickness of about 1-2 monolayers. The 195°C-deposited (111) textured Pt underlayer enhances the peak value of Kej,f by

198 ('.-J. ldn et al. / Properties of ('o / Pt multiho'er~

7500

o

> -

c m

(al

o o

> -

c 0~

c

/ U -

l Co/Pt (111) n=O

-1 Co/Pt (200) I I I I l I l l I l l I I I I I f - I I I I

30 35 40 45 50 55

15300q

(b} Pt(111

f Co/Pt(111) ~kOe d

n=O

[ i i i f ~ i - - " ~ i E i [

30 35 40 45 50 55 2// (degree)

Fig. 3. X-ray difl'ractograrns of (a) a 23 × Co(3.1 e~,)/ Pt(10.2A) mulfila.ver and (b) a 15 × Co(3.1 ,~)/Pt(10.3,~) mul- lilayer grown on 21"1(},~ (111) 1.exlured PI.

almost a factor of 4 (from about 1.2 × 10 7 to 4.(1x 107 erg/cm~), and greatly increases the cross-ovcr Co layer thickness. The cross-over Co layer thicknesses are 5.2, 7.5 and 16.5,~ for the three sets of multilayers shown in figs. 6-8. We have observed a K~.f value as high as 5.5 × 10Vcrg/cm 3 for 1 6 × C o ( 2 . 5 A ) / P t ( 1 0 A ) g r o w n at room temperature on 200,A 300°C-deposited Pt underlayer. The increase in Kdj is attributed to a better (111) orientation. In fig. 10, tc,,K~, . was plotted against too for the above three sets of multilayers. They obey the following relation [3]:

to, , Kd- ) = 2 K s + t{,,, K , , l)

where K, is the slope of the fitted straight line,

representing the volume anisotropy energy constant, and K~ represents an interfacial anisotropy energy density. For Co layer thickness less than

o

about 3 A, values of t c o K ~ lie below the straight lines of eq. ( 1 ). This may be due to large decrease in the Curie temperature, or because it is no hmger appropriate to discuss such thin Co layers in terms of K~ and K,. The deduced values of K~ (in erg /cm- ' ) and K, (in 10~'erg/cm 3) for the

three sets of multilayers labeled (A), (B) and (C) in fig. 10 are: (A) K~ = 0.31, K v = 10; (B) K~ = 0.37, K, .= -9 .1 ; and (C) K~ = 0.76, K,~ = -9 .2 . The K~ values for these three sets of multilayers are similar, all smaller than the shape anisotropy energy density ( - 1.27 × 10Verg/cm ~) of Co layer, indicating contributions of magneto-crystalline and magneto-elastic anisotropy energy. The interfacial anisotropy energy density K~ clearly depends strongly on the crystallographic orientation of the multilayer. The K~ value of 0.76 e r g / c m 2 for (C) is close to that of MBE grown single crystal C o / P t ( l 11) multilayers (see section 3.2.4). If we assume the difference in K, values of (A) and (C) is because many grains in (A) arc not oriented with C o / P t ( l l l ) parallel to the film plane, the very little difference in K~ values of (A) and (C) suggests that different crystallographic orientations give rise to large difference primarily in K~ but not K v. The orientation dependence of magnetic anisotropy is further investigated using MBE grown superlattices in section 3.2,4.

3.2.2. Dependence on Pt layer thickness

Figure l l(a) shows how the Pt layer thickness affects the perpendicular magnetic hysteresis loop behavior of evaporated C o / P t multilayers. The corresponding variations in KL, jj are shown in fig. l l(b). Although the hysteresis loop behavior shows significant variation fl)r Pt layer thickness beyond 10,~, the K~,t~ wduc shows very little change beyond 8 ,~, This suggests that for Pt layer exceeding about 4 monolayers in thickness the


Fig. 4. (a) Plane-view TEM micrograph and diffraction pattern and (b) cross-section TEM micrograph of a Co(3.1A)/Pt(10.2,~) multilayer grown on 200A (111) textured Pt underlayer.

15×

exchange coupling between adjacent Co layers becomes insignificant. This of course could be affected by the degree of interdiffusion, which depends on the preparation conditions of the multilayers.

3.2.3. Dependence on total film thickness

It has been reported that the perpendicular hysteresis loop behavior of a Co/Pt multilayer depends on the total thickness of the multilayer,

2[){} (7.-J. Lin et al. / Properties of (70/Pt muhiho'ers

18300-

C

O v :>-

C

185700

O

>.

C m

Pt(111) Co/Pt ( 111 )

n=0 (a)

j ~ . _ m

n : - I Pt(200) ~ I , ~ F 7 I , i I t I , i i F [ 30 35 40 45 50 55 60

l P t ( l l l )~

(b) IIO°/--PO(I~ ) , l

i 6 kOe

30 35 40 45 50 55 60 20 (degree)

Fig. 5. X-ray diffractograms of sputtered 13xCo(1.7,~)/ Pt(7.8,~) multilayers, grown on = 250A of Pt underlayer with different degrees of (111) texture, with much better (111) texture in (b) than (a).

with better squareness of thinner multilayer [8]. Figure 12(a) shows the hysteresis loop dependence on the total thicknes of evaporated C o / P t multilayers. Perfectly square loops were obtained for multilayers of Co(3A)/Pt (10 ,~) with thickness up to about 10 periods. The Kef f value (fig. 12(b)), however, increases with the film thickness, in accord with the change in coercivity. This could be due to improvement in the (111) texture as the multilayer grows. Better loop squareness in thinner multilayers could be due to smaller shape anisotropy energy resulting from the substrate roughness. Note that for multilayers shown in fig. 12(a), Kerr loops from both film side and substrate side are no different for film thicknes up to 38 periods. The Kerr loop of the 77 period multilayer exhibits a much lager saturation field at the film side (fig. 12(a)), while the substrate side has a

~300A N[Co(xA)/Pt{10A)]

X = 633 nm x = 1 2

N = 27 x=1.7

x=2.1

N=25 x=2.5

N=24 x=3.1

x=4.1

x= 5.2

x = 6.0

N=19 ~ x=8.1

N=171 I [ I I -10 -5 0 5 10

kOe

Fig; 6. Polar Kerr hysteresis loops for Co/Pt multilayers with 10A Pt layer and varying Co layer thickness.

saturation field similar to that of the 38 period multilayer. We believe this is due to columnar microstructure which becomes more pronounced as film thickness increases.

3.2. 4. Dependence on crystallographic orientat ions

Co(3.7 A ) / P t ( -~ 16.8,~) superlattices have been grown on GaAs single crystal substrates along three different crystallographic orientations of Pt, namely [111] (on GaAs(111)), [110] and [100] (both on GaAs(100)). Figure 13 shows the perpendicular Kerr hysteresis loops for these three superlat- rices. Clearly the C o / P t ( l l 1) superlattice has a large perpendicular magnetic anisotropy. The measured K~,rr is 3.5 x 107erg/cm 3, which is about equal or slightly larger than that of the

C.-J. Lin et a l. / Properties of Co / Pt multilayers 201

-300~, N[Co(x~,)/Pt(20/k)]

X = 633 nm x=2

N=14 /7

x = 3 N: 3 J f

X=4

X = 5

X = 6

X = 8

N=11 J x=10

N=IO J x=15 / -

N =9 ~ / - - x = 20

N = 8 ~ I I 0"4 °

lO I I I I

-10 -5 0 5 kOe

Fig;, 7. Polar Kerr hysteresis loops for C o / P t multilayers with 20A Pt layer and varying Co layer thickness.

Substrate//200A Pt/-300,& N[Co(xA)/Pt(10/k)] / 195°C / R.T.

X = 633 nm x= 1.5

x=2

x=3

x = 4

x=5

X = 6

X = 8

I I I I I ]. ~10 -5 0 5 10

kOe

Fig. 8. Polar Kerr hysteresis loops for C o / P t multilayers with 10,~ Pt layer and varying Co layer thicknes, grown at room temperature on 200A of Pt, which is grown at 195°C.

polycrystalline evaporated multilayer grown on (111) textured Pt underlayer. Assuming the same K~ of - 9 . 2 × 10 6 e rg / cm 3, one obtains K~ = 0.82 e r g / c m 2.

The Co/Pt(100) superlattice clearly shows that the film normal is the hard axis. VSM and torque magnetometry measurements reveal that the film plane is the easy plane with Keff = - 2 × 107 e r g / c m 3. An anisotropy also exists within the film plane, but with a relatively much smaller energy density 2.8 × 105erg /cm 3. Our prelimi- nary results show an increasing tendency toward perpendicular anisotropy for Co/Pt(100) superlattices with Co layer thickness decreasing to about 1 monolayer. However, it becomes no longer appropriate to discuss such thin Co layers

in terms of K S and K v. If we assume a non-negative K~ value, the K v value will be more negative than - 2 × 107erg/cm 3. This indicates that in

addition to the shape anisotropy there exist other easy-in-plane anisotropy contributions, either interfacial magneto-crystalline or magneto-elastic, similar to the results in Co/Pd(100) reported by Den Broeder et al. [5]. However, as mentioned earlier, the results of studies in (111) textured evaporated C o / P t multilayers suggest an orientation dependence of K~, but not Kv. More work needs to be done in order to resolve this issue.

The perpendicular Kerr hysteresis loop of Co/Pt(110) superlattice shown in fig. 13 suggests this is an intermediate case between C o / P t ( l l l ) and Co/Pt(100). However, the field that is needed

202 C.-J. Lin et al. / Properties of Co / Pt multilayers

E

0

X v

"6

8

7 -

6 -

5

4-

3

2

1

0

- 1 -

-2 -

I I I [ 1 i 1 I I I

o A: Co(x~)/Pt(lO~)

[] B: Co(x/~)/Pt(20~)

Pt

I

I I I I I I I I I I I 2 4- 6 8 10 12 14 16 18 20 22

tco (k) 24

Fig. 9. D e p e n d e n c e of effective Kef f on Co layer th ickness tco for three sets of C o / P t mult i layers depos i ted at room tempera tu re :

(A) Co(x ,~) /Pt(10A,) , (B) Co(x ,~) /P t (20A,) and (C) Co(x A,) /P t (10 ,~) on 200A, (111) tex tured Pt.

2.0 I I I I I I I I I I I

- o A: _

"" "'- J~)/Pt(20/!t.) 1 . 2 - ~ ~ [ ] B : C ° ( ( x , ) / P t ( l O , ) . -

.._.. o.8 • C: Cox on PL_ - 5 E -.

~ 0.4 - ~" ~

~ o.o

"-~ - 0 . 4

"- ' -0 .8

-1 .2

-1 .6 - -

- 2 . 0 I I I I ! I I I I I L 0 2 4 6 8 10 12 14 16 18 20 22 24

tco (k) Fig. 10. Effective anisot ropy energy K d r t imes Co layer th ickness , too, versus Co th ickness for the same three sets of C o / P t mult i layers shown in fig. 9.


(a )

N = 50

N = 33

N = 27

N = 23

N = 1 7

N = 1 3

I - 1 0

-300 , &, NlCo(3,&)/Pt(x,&)]

X = 633 nm

/ /

,t f

I I I - 5 0 5

kOe

x = 3

x = 6

x = 8

x = 1 0

x = 1 5

x = 20

i [ 0.4o

10

to saturate this film is far less than the demagne- tizing field, indicating the existence of a fairly strong perpendicular anisotropy. In fact, VSM and torque magnetometry measurements reveal that there is a hard axis in the film plane, and perpendicular to that hard axis is an easy plane that contains the film normal. The anisotropy energy constant between the hard axis and the easy plane amounts to 3.6 × 107erg/cm 3. This is nearly the same anisotropy strength as observed in the Co/Pt(111) superlattice. The implication of such a good match is being investigated. Within the easy plane there exists a relatively much smaller anisotropy of energy constant 2.2 x 10 5 e rg / cm 3. Details will be published elsewhere

[201.

3.2.5. On the origin o f magnetic anisotropy

The fact that eq. (1) can fit the data very well does mean that phenomenologically there exists an interfacial anisotropy term, as predicted originally by N6el [21] for a free surface. The dramatic orientation dependence of the interfacial mag-

4 " E

p~ O

3.0

2 . 5 -

2 . 0 -

1 . 5 -

1 .0- -

0 .5 - -

0 .0 - -

-0.5 - -

- 1 . 0 - -

- 1 . 5 --

-2.0 0

I I I I I I I I I I

Co(3A)/Pt(X )

I (b)

I I I I f I l I I I I 2 4 6 8 10 12 14 16 18 20 22 24

tpt (/~) Fig. ] 1. Polar Kerr hysteresis loops (a) and K~f r (b) for Co /£ t multilayers with 3A. Co layer and varying Pt layer thickness.

2114 C.-J. Lin et al. / Properties or" C o / P t multilayel:s'

(a)

N = 4

N = 8

N[Co(3/~)/Pt(10/~,)l

I I ~. = 633 nm

I I

N = 1 5 - -

N = 23

N = 38

N=77

I -10

/ /

I I I -5 0 5

kOe

~'4° I

10

netic anisotropy which we have observed in MBE grown superlattices sheds light on the origin of interracial magnetic anisotropy in multilayers. All three C o / P t superlattices of different orientations exhibit uniaxial anisotropy behavior. How- ever, the symmetry axis is not always along the film normal, as usually thought. It could also lie in the film plane, as observed in the [110] case. The fact that the interfacial magnetic anisotropy in multilayers could be orientation dependent even within the film plane suggests that K s, as also originally pointed out by Ndel [21], should not be treated as a scalar quantity along the film normal. The C o / P t ( l l 0 ) superlattice has only 2-fold symmetry within he film plane, and therefore makes possible the observed large anisotropy within the film plane. This clearly shows that the effect of interracial magneto-crystalline anisotropy can be much larger than that resulting from the reduced symmetry at an interface. Generally speaking, one could say that the origin of the interfacial magnetic anisotropy is basically magneto-crystalline in nature, with the effect of atomic distance included.

,.t.0

2.5

2.0

1.5

"• 1.0

% o.5

~x 0.0

2 -o.5

-1.0

- 1 . 5 -

- 2 . 0

I I I I I I I I I (b)

N X Co(5~,)/Pt(lO/~)

I I I J I I I I J 10 20 30 40 50 60 70 80 90

Number of Period (N)

m

m

m

m

100

Fig. 12. Polar Kerr hysteresis loops (a) and K~, u (b) for Co(3,~)/Pt(ll)/~) muhilayers of different total thickness.

C.-J. Lin et aL / Properties of Co/Pt muhilayers 205

Tsu b = 100°C ( "

i

J

i [111]

- - - ' [ i l O ;

I , I , I , I , , I , I , I , I

-16 -12 -8 -4 0 4 8 12 16

H(kOe)

0.1

0 0k(deg )

-0.1

Fig. 13. Polar Kerr hysteresis loops for three Co(3.7A,)/Pt(16.8/k) superlattices MBE grown along three different crystallographic orientations of Pt: [111] (on GaAs(l I 1)), [110] and [100] (both on GaAs(100)).

E

E ..._..,

2300

2200

2100

2000

1900

1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

I I I I I

0 [ ]

I I I I i I

o A: Co(xA)/Pt(lO/~)

B: Co(xA)/Pt(2OA)

• C: Co(x/~)//Pt(lOA) on Pt

I I I I I I I I I 1 2 4 6 8 10 12 14 16 18 20 22

too 24

Fig. 14. Effective magnetization M~q r versus Co thickness tco for the same three sets of C o / P t multilayers shown in fig. 9.

3.3. Magnetization

In fig. 14 the room temperature effective saturation magnetization of C o / P t multilayers, Men-,

is plotted against the Co layer thickness. Men is obtained by dividing the saturation magnetization of a multilayer by the Co volume in the film. For Co layer thicknes less than one monolayer, Melt .

2(16 C.-J. Lin et al. / Properties ~" C o / P t multilayerg

drops below the magnetization (1420emu/cm ~) of bulk Co at room temperature, presumably due to large decreases in Curie temperature. For Co layer thickness larger than one monolayer, M~f~ is larger than the magnetization of bulk Co. For example, at room temperature the Co(3 A ) / P t ( 1 0 A ) multilayer exhibits an M~I t of 1850emu/cm ~, which is about 30% larger than the magnetization of bulk Co. Such an enhancement at room temperature is rather impressive given that the Curie temperature of this multilayer is only about 330°C, much less than that

3 ° (11~ 1 C) of bulk Co. The enhancement of M~f~ over the magnetization of Co is attributed primarily to the magnetization contributed by the polarized Pt atoms [22, 23] next to the Co layer.

4. Summary and conclusions

By comparing two sets of evaporated multilayers, both grown at room temperature but with one set having a much better (111) texture, we conclude that a (111) film plane is required for obtaining very large uniaxial perpendicular magnetic anisotropy K~.j. The largest K~t t value we have obtained is 5.5 × 107erg/cm 3. The dramatic orientation dependence of magnetic anisotropy exhibited in MBE grown C o / P t multilayers along three different crystallographic orientations of Pt lattice, [111], [110] and [100], indicates that interfacial magneto-crystalline anisotropy is the basic origin of the observed interfacial magnetic anisotropy. The interfacial magnetic anisotropy could be orientation-dependent within the film plane and therefore should not be treated as a scalar. Significantly enhanced effective saturation magnetization of Co was observed at room temperature. It is attributed to the magnetization contributed by the polarized Pt atoms at the interface.

Acknowledgements

The authors gratefully thank Noa More for the XRD work, A Kellog for the RBS work, and Z. Li and D. Smith for the TEM efforts.

References

[1] P.F. Carcia, A.D. Meinhaldt and A. Suna, Appl. Phys. Left. 47 (1985) 178.

[2] F.J.A. den Broeder, H.C. Donkersloot, H.J.G. Draaisma and W.J.M. de Jonge. J. App. Phys. 61 (1987) 4317.

[3] It.J.G. Draaisma, W.J.M. de Jonge and F.J.A. den Broeder, J. Magn. Magn. Mat. 66 (1987) 351.

[4] H.J.G. Draaisma, F.J.A. den Broeder and W.J.M. dc Jonge, J. Appl. Phys. 63 (1988) 3479.

[5] F.J.A. den Broedcr, D. Kuiper, H.C. Donkersloot and W. Hoving, Appl. Phys. A49 (1989) 507.

[6] P.F. Carcia, J. Appl. Phys. 63 (1988) 51166. [7] N. Sato, J. App. Phys. 64 (1988) 6424. [8] Y. Ochiai, S. Hashimoto and K. Aso, IEEE Trans. Magn.

MAG-25 (1989) 3755. [9] S. Hashimoto, H. Matsuda and Y. Ochiai, App. Phys.

Len. 56 (1990) 1069. [10] W.B. Zeper, F.J.A.M. Greidanus, P.F. Carcia and C.R.

Fincher, J, Appl. Phys. 65 (1989) 4971. [ l l ] W.B. Zeper, F.J.A.M. Greidanus and P.F. Carcia, IEEE

Trans. Magn. MAG-25 (1989)3767. [12] C.-J. Lin and tt.V. Do, UK, IEEE Trans. Magn. MAG-26

(1990) 1700. [13] F.J.A. den Broeder, D. Kuiper, A.P. van de Mosselaer

and W. Hoving, Phys. Rev. Left. 60 (1988) 2769. [14] S. Araki, T. Takahata and T. Shinjo, J. Magn. Soc. Jpn.

13 (1989) 339. [15] C.H. Lee, llui He, F. Lamelas, W. Vavra, ('. Uher and

Roy Clarke, Phys. Rev. Lett. 62 (1989) 653. [16] J. Ferrc, G. Penissard, ( . Marliere, D, Renard,

P. Beauvillain and J.P. Renard, Appl. Phys. LeU. 56 (199[)) 1588.

[17] C.II. Lee, R.F.C. Farrow, C.-J. Lin, E.E. Marmero and C.J. Chien, Phys. Rev. B (submitted).

[18] C.tt. Lee, R.F.C. Farrow, B.D. t termsmeier , R.F. Marks, W.R. Bennett , C.-J. Lin, E.E. Marinero, P.D. Kirchner and C.J. Chien, J. Magn. Magn. Mat. 93 (1991) 592.

[19] C.J. Chien, R.F.C. Farrow, C.tt. Lee, C.J. Lin and E.E. Marinero, J. Magn. Magn. Mal. 93 11991) 47.

[20] C-J. Lin, C.[I. Lee, E. Marincro and R.F.C. Farrow, to be published.

[21] L. Ndel, J. Phys. Radium, 15 (1954) 225. [22] T.R. McGuire, J.A. Aboaf and E. Klokholm, J. Appl.

Phys. 55 (1984) 1951. [23] G. Schutz. R. Wienke, W. Wilhelm, W.B. Zeper,

1t. Ebcrt and K. Sporl, J, Appl. Phys. 67 (1990) 4456.

Magnetic and structural properties of Co/Pt multilayers

Documents