Photon Factory Activity Report 2014 #32 (2015) B

BL-16A/2014G159

Resonant soft-x-ray scattering study of Co/Pt multilayers
Goro Shibata1,*, Keisuke Ishigami1, Yukio Takahashi1, Shoya Sakamoto1, Yosuke Nonaka1,Atsushi Fujimori1, and Tsuneharu Koide2
1
Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan2
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research
Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
Transport and magnetic properties of metal andtransition-metal-oxide thin films and multilayers havedrawn much attention for years. Especially, novel
physical properties induced by low dimensionality,interfacial effects, and epitaxial strain have been
intensively studied [1]. In order to understand the originof these physical properties specific to thin films andmultilayers, it is of great importance to clarify the depth
distribution of atoms, valences, and magnetic moments as
fundamental information. So far, photoemission
spectroscopy, x-ray absorption spectroscopy, and x-ray
magnetic circular dichroism have been widely used to
study depth-resolved electronic states by utilizing the factthat the escape depth of photoelectrons depends on theelectron emission angle [2,3]. However, the spatial
resolution along the depth direction and the probing depth
are severely limited in these methods because of the small
escape depth of the electrons (a few nm). Recently, analternative method, in which the incident-angle
dependence of the x-ray scattering intensity is measured,
has been developed as a probe for the depth-dependent
electronic structure [4]. We have been developing anapparatus for such experiments, namely resonant soft xray scattering (RSXS). In this paper, we report on thepreliminary RSXS experiments of [Co/Pt (111)]nmultilayers, which are known to show perpendicularmagnetic anisotropy with decreasing Co thickness [5].
2 Experiment[Co/Pt (111)]n multilayer samples with a total thicknessof ~100 nm were grown on the glass substrate by the RF
sputtering method. Prior to the growth of the Co/Pt
multilayer, a 15-nm-thick Pt (111) buffer layer wasdeposited and the samples were annealed at 350 oC for 1h. After the deposition of the Pt buffer layer, 4-monolayer(ML)-thick Co and 10-ML-thick Pt layers werealternatingly deposited, terminating with the Pt layer inorder to protect the surface. X-ray diffraction confirmedthat the periodicity of the superlattice almost agreed withthe designed values. Details of the sample preparation aredescribed in Ref. 6.
RSXS experiments have been performed at BL-16A ofKEK-PF. The photon energy (h) was adjusted to the Co
L3 absorption edge. Scattering profiles were taken atroom temperature. A magnetic field of ~ 0.1 T wasapplied parallel to the incident x rays using permanent
magnets. Signal was collected using a silicon diodedetector. A slit of ~0.1 mm width was attached in front of
the detector to achieve the angular resolution of ~ 0.05o.
3 Results and Discussion
Figure 1 shows the scattering profile of the [Co (4ML)/Pt (10 ML)]n multilayer at the Co L3 absorption edge(h = 775.6 eV) with a fixed sample angle (sample =16.15o) and a varying detector angle (det). A sharp peaklocated at det =32.3o and a broader peak located arounddet =37o are observed. The sharp peak is identified as thespecular reflection peak because the scattering angles
nearly satisfy the -2 relation (det ≃ 2sample). On theother hand, the broader peak is identified as the first-ordersuperlattice reflection from the [Co/Pt]n multilayer. The
Bragg condition for the first-order reflection is given by
𝑞𝑧 = 2𝜋/𝐿 , where 𝑞𝑧 = 𝑘(sin 𝜃sample − sin(𝜃det −𝜃sample )) is the momentum transfer and L is thesuperlattice period. From the scattering profile in Fig. 1,the momentum transfer for the broad peak is calculated tobe 𝑞𝑧 ~2.5 nm−1, yielding L ~ 2.5 nm. On the other hand,
the theoretical superlattice period of the [Co (4 ML)/Pt(10 ML)]n multilayer is estimated to be 2.590 nm, using
the lattice parameters of hcp (0001) Co (1 ML = 0.2053nm) and fcc (111) Pt (1 ML = 0.1769 nm). The
experimental superlattice period is, therefore, consistent
with the theoretical value. Furthermore, the peak width(half width at the half maximum) is estimated to be qz ~0.1 nm-1, corresponding to the total superlattice thicknessof 2/qz ~ 60 nm. This is also consistent with thedesigned total thickness of ~ 100 nm, within theexperimental errors due to the estimation of qz. TheseFig. 1: Scattering profile of the [Co (4 ML)/Pt (10ML)]n multilayer at the Co L3 absorption edge (h =775.6 eV) with fixed the sample angle (sample =16.15o) and the varying detector angle (det).

Photon Factory Activity Report 2014 #32 (2015) B

estimations justify the assignment that the broad peakaround det =37o is the first-order superlattice reflection.
Thus we have succeeded in the observation ofscattering peaks not only of the specular reflection but
also of the superlattice reflection from the [Co/Pt]nmultilayer. In the future, we are going to investigate theangular and photon-energy dependencies of the reflection
intensity in order to probe the depth-resolved electronicand magnetic states.
References[1] H. Y. Hwang et al., Nature Mater. 11, 103 (2012).
[2] M. Sing et al., Phys. Rev. Lett. 102, 176805 (2009).
[3] K. Amemiya et al., Phys. Rev. B 72, 201404(R)(2005).
[4] E. Benckiser et al., Nature Materials 10, 189 (2011).
[5] P. F. Carcia, J. Appl. Phys. 63, 5066 (1988).
[6] N. Nakajima et al., Phys. Rev. Lett. 81, 5229 (1998).
* shibata@wyvern.phys.s.u-tokyo.ac.jp