Photon Factory Activity Report 2014 #32 (2015) B
BL-7A, BL-16A/2013G195

Observation of perpendicular magnetization in Cu layer inserted between Co andPt layers revealed by x-ray magnetic circular dichroism
Jun Okabayashi1* , Tomohiro Koyama2, and Daichi Chiba2
1

Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan2
Department of Applied Physics, The University of Tokyo, 113-8656 Tokyo, Japan

Perpendicular magnetic anisotropy (PMA) inmagnetic multilayers is strongly desired for thedevelopments of high-density magnetic recording media.
Co/Pt layers become one of the candidates possessing thePMA using the large spin-orbit interaction of Pt atoms
and spin moments of Co atoms, which collaborates toenhance the PMA through the proximity effects at theinterfaces. Recently, ultrathin Co/Pt layers have been
also studied for the electronic-field modulations ofmagnetic properties [1]. On the other hand, the Co/Cu
multilayer structures possessing the in-plane anisotropy
derived from the shape anisotropy have been investigatedextensively for the giant magneto-resistance devices [2].
The Cu spacer layers bring the RKKY-like oscillatory
behavior in the magnetoresistance through the exchange
coupling between the Co layers [3]. Combining theseprevious works, our objectives consist of the following
two ideas; (i) the insertion of the Cu layer as an inserted
layer between Co and Pt in order to separate theproximity between them and (ii) the induction of PMA in
Cu spacer layer. In order to achieve these properties, weconsider the stacked structures of Co/Cu/Pt thin films
with different Cu layer thicknesses. In this paper, weinvestigate the element-specific spin and orbital
moments of Co and Cu in the Co/Cu/Pt stacked
structures using XMCD with the element-specifichysteresis curves in order to discuss the origin of theproximity-effect-driven PMA in the Cu inserted layer.
Samples prepared by rf-sputtering method consistof the stacked structures: MgO (2 nm)/Co (0.4 nm)/Cu (tnm)/Pt (2 nm)/Ta buffer layer deposited on the Si
substrates with various Cu layer thicknesses (t). Samplesof t=0.4 and 2.0 nm show the PMA and in-planeanisotropy, respectively. X-ray absorption spectroscopy(XAS) and XMCD measurements for Co and Cu L-edges
were performed at BL-7A and 16A in Photon Factory(KEK). Photon helicity was fixed and a magnetic field of±1.2 T was applied along the incident polarized soft
x-ray. All measurements were carried out at room
temperature in the total electron yield mode.
By inserting the Cu layer between Co and Pt layers,
remanent magnetization (mr) decreases rapidly at thethreshold of 0.6-nm-thick Cu layer, suggesting that theproximity effects for PMA are weakened by the insertionof Cu layers. For less than 0.6 nm Cu thicknesses, the mrslightly decreases. Near the threshold thickness, squareshapes become broad and mr decreases. For more than
the threshold layer thickness, in-plane magnetization ofCo derived from the demagnetization field in the films
contributes to the magnetic anisotropy. These systematicchanges confirm the abrupt interface formations without
the intermixing.

Figure 1 displays the XAS and XMCD of Co and
Cu L-edges in Co /Cu /Pt stacked sample showing PMA.
In Co L-edge spectra, clear metallic XAS line shapes
and XMCD can be observed. Quite small XMCD
intensities in the Cu L-edges are detected as shown inFig. 1 (b), which suggests that the XMCD signals areinduced from the magnetic Co atoms with the same
sign directions corresponding to the parallelalignments. Broad satellite structures in Cu L 2,3 edges
are also observed corresponding to the transition from2p to 4s states.
The element-specific hysteresis curves taken at thephoton energies of Co and Cu L3-edges clearly display thePMA (not shown). Here, we emphasize that the hysteresis
curve of Cu displays the out-of-plane magnetization.
PMA in the Cu layer is clearly observed which is inducedby the proximity effects from both Co and Pt. In case of
Co/Cu multilayers [3], in-plane induced magnetization in
Cu is observed. Therefore, the PMA in Cu is derived not
only from Co but also Pt spin-orbit interactions.

Fig. 1, XAS and XMCD of Co and Cu L-edges inCo /Cu /Pt stacked structure.

[1] D. Chiba et al, Nature Materials 10, 853 (2011).
[2] S.S.P. Perkin et al, Phys. Rev. Lett. 66, 2152 (1991).
[3] M. G. Samant et al, Phys. Rev. Lett. 72, 1112 (1994).
*jun@chem.s.u-tokyo.ac.jp