Structure and magnetism in Co/Pt multilayers

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Magnetic Multilayers II David J. Larson, Chairman

Structure and magnetism in Co Õ Pt multilayers

J. C. A. Huang^a)

Physics Department, National Cheng-Kung University, Tainan, 70101, Taiwan C. H. Lee and K. L. Yu

Department of Engineering and System Science, National Tsing-Hua University, Hsinchu, 30043, Taiwan The structure of

关Co(t

Co)/Pt(10 Å)

兴

30 multilayers has been studied by x-ray absorption spectroscopy and x-ray diffraction using in-plane and out- of-plane polarized synchrotron radiation.

The x-ray absorption near-edge spectroscopy shows that the Co layer is like a fcc pseudomorphic structure for t_Coless than 4 Å. For t_Co

⫽10 Å, the spectra in both directions are separated and look

closer to the bulk hcp Co. The extended x-ray absorption fine structure studies reveal that the Co–Pt interface of the multilayers fitted with a sharp boundary better than an interdiffusion model. The in-plane x-ray diffraction shows that the Pt layer in the multilayer possesses a 2%–3.5%

compressible strain. Although the saturation magnetization M_sdoes not depend on t_Coor interfacial roughness in any simple form, the M_s values scale quite linearly with the in-plane Pt strain. We conclude that the interfacial strain is important for the perpendicular magnetization in the Co/Pt multilayers. © 2001 American Institute of Physics.

关DOI: 10.1063/1.1354580兴

The structure and magnetism of magnetic multilayers

共MLs兲 composed of modulated ferromagnetic-nonferro-

magnetic

共FNF兲 layers have attracted much attention in re-

cent years. For instance, Co/Pt MLs with large perpendicular magnetic anisotropy

共PMA兲 and Kerr rotations have received

considerable interest for basic research and application for high density data storage.^1–5The PMA effect can exist in the magnetic multilayer as a consequence of symmetry breaking at the FNF interface. Therefore understanding of the interfacial structure is crucial for the PMA effect. Co/Pt films have been studied by the MCD effect, where a fcc pseudomorphic layer and PMA effect was found for Co less than 5 monolayers.^6,7 Previously we also reported the epitaxial growth and the magnetic and magneto-optical properties of the Co/Pt MLs.^8,9 We found that the magnetization of the Co/Pt MLs was perpendicular to the film surface for a Co layer thickness (t_Co) less than about 6 Å. In addition, it has also been predicted¹⁰ that the PMA for the multilayer might be strain and interfacial roughness related. We were thus motivated to probe the structure difference of Co/Pt MLs with distinct ultrathin Co layer thickness. The

共111兲 oriented 关Co(t

Co)/Pt(10 Å)

兴

30 (t_Co

⫽1.6– 10 Å) MLs studied here

were synthesized by the molecular beam epitaxy

共MBE兲

technique. The details of the MBE system and sample prepa- ration procedures have been described elsewhere.^8,9

To probe the local structure of the ultrathin Co layer, both the in-plane and out-of-plane x-ray absorption spectroscopy were performed in a Wiggler beamline of Taiwan Light Source

共TLS兲. The absorption spectra were performed using

the fluorescence mode with sample surfaces rotated at an angle either parallel to or nearly perpendicular to the polar-

ization of the synchrotron light. For comparison, the referencing spectra of hcp-Co foil, and CoPt and CoPt₃ alloys were also taken. The energy resolution is about 1 eV in the x-ray absorption near edge spectroscopy

共XANES兲 region

and 3 eV in the extended x-ray absorption fine structure

共EX-

AFS

兲 region. The crystal orientation, strain, and interfacial

roughness of the Co/Pt MLs were also measured using x-ray diffraction

共XRD兲 and x-ray reflectivity. The magnetization

values and hysteresis loops of the Co/Pt MLs were measured by a vibrating sample magnetometer

共VSM兲.

Figure 1 shows the derivative spectra of Co K-edge XANES for the Co/Pt MLs and the referencing samples.

Note that the XANES spectra of the in- plane and out-of- plane directions are quite similar for samples with t_Co less than 4 Å. This result suggests that the Co layer is more like a fcc pseudomorphic structure at a thickness of less than 4 Å because the symmetries along the in-plane and out-of-plane directions are much the same. For t_Co increased to 10 Å, in contrast, the XANES spectra in both directions are separated and look very similar to the bulk hcp Co. It is possible that a fcc to hcp structural transition occurs at t_Coof about 3 to 4 Å.

Note that the x-ray absorption spectroscopy data of the Co/Pt multilayer with Co layer thickness of about 1 monolayer is quite similar to the ordered CoPt alloy.¹¹ Both cases reveal good PMA effect.

Table I summarizes the EXAFS fitting parameters in- cluding the calculated neighboring distance, coordination number of surrounding Pt atoms, and the Debye–Waller factor. These parameters are obtained by the fitting procedure where the number of Co neighbors has been fixed

共to 12兲.

For a t_Colayer of 10 Å, the bonding distance of the first shell is 2.5 Å, which is very close to the Co–Co distance in the bulk hcp Co. On the other hand, for a Co layer of less than 3 Å, the in-plane first shell distance of Co is expanded by

a兲Electronic mail: [email protected]

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7059

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about 3% to 4%. To understand the interfacial morphology in the Co/Pt MLs, we compare the EXAFS results with two models, a sharp boundary model and a total interdiffusion model. Figure 2 plots the EXAFS fitting parameter of Pt coordination numbers around the Co atom for the Co/Pt MLs and alloys as well as those predicted by the two models. It appears that both in-plane and perpendicular data from the Co/Pt MLs fits the sharp boundary model much better than the interdiffusion model. Note that the interfacial roughness determined from the x-ray reflectivity is about 3–5 Å for all the Co/Pt MLs. The relatively large interfacial roughness de- duced from the x-ray reflectivity does not contradict the sharp boundary model because distinct length scales are probed between the x-ray reflectivity and the EXAFS mea- surements. The x-ray reflectivity takes a lateral average over a length scale of several microns, but the EXAFS probes only local environment on the angstrom scale. So the sharp boundary indicates that the Co/Pt interface is a jagged one instead of an interdiffused one. The result suggests that significant interdiffusion in the Co/Pt interface did not occur

FIG. 1. The derivative spectra of Co K near-edge x-ray absorption for Co/Pt multilayers and alloys with the synchrotron light polarization perpendicular

共solid curves兲 or parallel 共dashed curves兲 to the film surface.

FIG. 2. The Pt coordination numbers around the Co atom for the Co/Pt MLs and alloys obtained from experiments

共circle and square, respectively兲 and

EXAFS fitting. The prediction from sharp boundary and interdiffusion models

共triangle兲 are also plotted for comparison.

TABLE I. The EXAFS fitting parameters for the out-of-plane

共upper兲 and

in-plane

共bottom兲 polarization for Co/Pt multilayers and alloys. Here R is

the first shell bonding distance,␴²is the Debye–Waller factor, and nPtis the Pt coordination numbers around the Co atom.

Sample R

共Å兲

␴²⁽¹⁰^⫺3^Å²⁾ ⁿPt

Out-of-plane

Cohcp 2.50

⫾0.025

4.9⫾2 0

关Co/Pt兴

30

tCo

⫽1.63

2.64

⫾0.026

11.7⫾2 7.3⫾2

tCo

⫽1.8

2.62⫾0.026 12.2

⫾2

5.9

⫾2

tCo

⫽1.9

2.64

⫾0.026

11.0⫾2 7.6⫾2

tCo

⫽4.0

2.56

⫾0.026

11.4

⫾2

3.8

⫾2

tCo

⫽4.3

2.56

⫾0.025

11.0⫾2 2.1⫾2

tCo

⫽10.7

2.53

⫾0.025

9.9

⫾2

0.3

⫾2

Alloy

Co3Pt1 2.55⫾0.025 7.9

⫾2

2.3

⫾2

Co1Pt1 2.62

⫾0.026

9.3⫾2 6.7⫾2

In-plane

Cohcp 2.50

⫾0.025

5.9⫾2 0

关Co/Pt兴

30

tCo

⫽1.6

2.61

⫾0.026

9.5⫾2 7.7⫾2

tCo

⫽1.8

2.60⫾0.026 8.6

⫾2

5.6

⫾2

tCo

⫽1.9

2.61

⫾0.026

9.3⫾2 7.4⫾2

tCo

⫽4.0

2.55⫾0.025 9.7

⫾2

4.3

⫾2

tCo

⫽4.3

2.57

⫾0.025

7.5⫾2 6.2⫾2

tCo

⫽10.7

2.52

⫾0.025

8.2

⫾2

0.3

⫾2

Alloy

Co3Pt1 2.53

⫾0.025

7.5⫾2 2.3⫾2

Co1Pt1 2.62⫾0.026 9.1

⫾2

7.0

⫾2

7060 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Huang, Lee, and Yu

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even at a growth temperature of 200 °C. This is in good agreement with the recent Auger experiments^12,13where the interdiffusion of Co on Pt was found to be significant only at temperatures higher than about 280 °C.

The nearest atom distances follow the same trend for both in-plane and out-of-plane directions. This implies that the Co/Pt multilayers might not be Poisson materials in which the typical Poisson ratio for the bulk material is appli- cable. The Debye–Waller factors fitted from the EXAFS data taken are also shown in Table I. The results indicate that the Debye–Waller factors obtained from the out-of-plane is larger than that from in-plane direction

共and larger than that

of CoPt alloys

兲, even though the nearest neighbor distances

along the plane normal are slightly larger than that along the in-plane direction. This might suggest that the crystal growth along the plane normal consists of more defects or stacking faults than the in-plane direction. However, in contrast to the in-plane strain of the Pt layer, the Co near shell distance is not correlated with the in-plane strain of Pt

共see Tables I and

II

兲. Indeed, a maximum strain occurs at a structural phase

transition thickness for Co layer thickness of about 4 Å.

The XRD results indicate that the Pt layer in the multilayer possesses a compressible strain of about 2%–

3.5% along the Pt

关1-10兴 direction. Samples with large in-

plane mosaic

共about 2° – 10°) and small coherence length 共about 20–28 Å兲 were measured, indicating a plastic de-

formed of the Pt layer resulting from the lattice misfit

共10%兲

between the Co and Pt layer. The structural parameters obtained from the x-ray diffraction and x-ray reflectivity for the Co/Pt MLs are summarized in Table II. Table II also lists the

共VSM measured兲 magnetic parameters, the saturation mag-

netization M_s, polar coercivity H_c, and loop squareness.

Note that the change of H_c and squareness reveals a transition of the PMA effect at a Co thickness of about 4 to 5 Å.

Interestingly enough, although the saturation magnetization M_s does not depend on t_Co or interfacial roughness in any simple form, the M_s values scale quite linearly with the in- plane Pt strain

共except for the non-PMA sample with t

Co

⫽10 Å), as illustrated in Fig. 3. This result leads to a specu-

lation that a larger strain of the Pt layer might imply a less strained Co layer, which in turn might give rise to a higher value toward the bulk Co value

共1420 emu/cc兲. On the other

hand, the polar coercivity does not scale in any simple rela- tion with the Pt strain or any other structural parameters, indicating that the coercivity is more likely a magnetic do- main related parameter. We conclude that the interfacial strain is important for the perpendicular magnetization in the Co/Pt multilayers.

The authors are grateful for the financial support by the ROC NSC under Grant Nos. NSC 89-2112-M-006-037

共J.C.A.H.兲 and 88-2112-M-007-035 共C.H.L.兲.

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FIG. 3. The saturation magnetization Msof the Co/Pt multilayers plotted as a function of the in-plane Pt strain. The Ms values for PMA samples

共square兲 scale quite linearly with the in-plane Pt strain except for the non-

PMA sample

共circle兲 with t

Co

⫽10 Å.

TABLE II. The determined structural parameters

共by x-ray diffraction and reflectivity兲 together with the

magnetic properties

共by VSM兲 as functions of Co thickness for the Co/Pt multilayers.

Cobalt thickness

共Å兲

Interface roughness

共Å兲

In-plane strain of Pt

共2–20兲

共%兲

Rocking curve width of Pt

共2–20兲

共deg兲

Ms

共emu/cc兲

Hc

共kOe兲

Squareness of hysteresis

loops

1.6 3.0

⫺2.04

10.5 317 4.1 0.963

1.8 3.5

⫺2.34

3.1 281 4.8 0.998

1.9 3.0

⫺2.47

4.8 187 5.1 0.976

4.0 4.5

⫺3.52

4.6 755 1.6 0.969

4.3 4.5

⫺2.73

2.3 426 1.3 0.949

10.7 3.0

⫺2.51

3.1 689 0.4 0.107

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J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Huang, Lee, and Yu