Photon Factory Activity Report 2019 #37 (2020)

BL-16A/2016G066

Co thickness dependence of spin and orbital magnetic moments in W/Co/Pt
heterostructures studied by x-ray magnetic circular dichroism
Zhendong Chi1,*, Yong-Chang Lau1,2, Goro Shibata1, Shoya Sakamoto1, Yosuke Nonaka1,Keisuke Ikeda1, Yuxuan Wan1, Masahiro Suzuki1, Masako Suzuki-Sakamaki3, Kenta Amemiya3,
Masamitsu Hayashi1,2 and Atsushi Fujimori1
1
Department of Physics, The University of Tokyo, Bunkyo, Tokyo, 113-0033, Japan2
National Institute for Materials Science, Tsukuba, 305-0047, Japan3
Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan
1 Introduction
Ultrathin film heterostructures that consist of aferromagnetic metal (FM) and a non-magnetic heavy metal(HM) layer are attracting great interest because of novel
phenomena that originate from strong spin-orbit couplingin bulk and at the interfaces originating from the HM.
Efficient current-induced magnetization reversal and fast
motion of magnetic domain walls in heterostructures withperpendicular magnetic anisotropy (PMA) in the FM
layers have been observed and contributed to spin-orbit
coupling-induced effects [1, 2]. To stabilize chiralmagnetic structures, which are essential for magnetization
switching and magnetic domain wall motion based on
FM/HM heterostructures, strong Dzyaloshinskii-Moriya
interaction (DMI) [3, 4] is necessary [5]. In order todevelop systems with strong DMI, a deeper understandingof these phenomena, in particular at FM/HM interfaces, isrequired. The microscopic origin of interfacial DMI has
been discussed in relation to the orbital moment anisotropy(OMA) [6] and magnetic dipole (𝑚! ) [7] in the FM layer.
In order to investigate the OMA of the FM in FM/HM
heterostructures with different environments, here, wereport the Co layer thickness dependence of spin andorbital magnetic moments of Co in W/Co/Pt
heterostructures, which have in-plane magnetic anisotropy,
studied by using x-ray magnetic circular dichroism(XMCD) spectroscopy.
2 Experiment
Co thin films sandwiched by W and Pt, 3 W/𝑡"# Co/1
Pt/1 Ru (the numbers denote the nominal thicknesses in nm,
stack order from button to top) were grown on 10×10 mm2
thermally oxidized Si substrates (SiO2/Si) by magnetron
sputtering at room temperature in a base pressure better
than 5×10-7 Pa (See Fig. 1 for the detail of sample). The
top Ru layer is used to protect the heterostructures fromoxidization. The thicknesses of Co ( 𝑡"# ) in the samples
were changed from 0.6 to 1.19 nm. X-ray absorption
spectroscopy (XAS) and XMCD measurements at the Co
L3,2 edges were performed using soft x rays at the helicalundulator beamline BL-16A of Photon Factory, HighEnergy Accelerator Research Organization (KEK-PF). Thespectra were measured in the total electron yield (TEY)mode. The measurements were performed at room
temperature in an ultra-high vacuum (UHV) lower than

5×10-7 Pa. The magnitude of the external magnetic field(Hext) was set as 5T and the field was applied parallel to theincident x rays in all the measurements (See Fig. 1). In
order to obtain the out-of-plane (OP) and in-plane (IP)magnetization configurations, Hext was applied to thesample with an incident angle, 𝜃, of 90° and 30° to thesample surface, defined as out-of-plane and “in-plane”
magnetic field.

Fig. 1: Schematic figure of the sample stacking and theexperimental set-up. 𝑡"# is the thickness of the Co layer.
3 Results and Discussion
Figure 2 illustrates the XAS and XMCD spectra at theCo L3,2 edge of the W/Co/Pt heterostructures measured
under a magnetic field of 5 T. No obvious peak shift orspectral line-shape change is found in both the XAS andXMCD spectra, suggesting that there is no significant
changes in the chemical state of Co, i.e., the oxidation of
Co is negligibly small. The solid and dashed curves in Fig.
2 represent spectra measured when out-of-plane and “inplane” magnetic field are applied, respectively. Theseresults indicate that the magnetic moments of Co in theW/Co/Pt heterostructures are anisotropic.
The spin magnetic moments (𝑚$%&' ) and 𝑚! of Co atom
are estimated using XMCD sum rules [8]. The out-of-plane
and in-plane components of the magnetic moments areobtained from the integrated XAS/XMCD spectra
measured under out-of-plane and “in-plane” magnetic
fields, respectively. Here, 𝑚$%&' is considered to bePhoton Factory Activity Report 2019 #37 (2020)

isotropic, but 𝑚! possesses an angular dependence as𝑚! (𝜃) = − 1⁄2 𝑚! (1 − 3 sin( 𝜃) [9]. The estimated values of𝑚$%&' and 𝑚! of Co of the W/Co/Pt heterostructures areshown in Fig. 3 as a function of magnetization effective
thickness (𝑡)** ). 𝑡)** is defined as 𝑡)** = 𝑡"# − 𝑡+ , where 𝑡+ isthe magnetic dead layer thickness in W/Co/Pt and its value
is obtained by using a vibrating sample magnetometer.
Here, the magnetic moments are presented in units of Bohrmagnetron per hole. 𝑚$%&' decreases with decreasing 𝑡)**from the bulk value (a dashed horizontal line). Suchvariation of 𝑚$%&' with magnetic layer thickness has also
been observed in similar systems [10]. 𝑚! , which isconsiderably small than 𝑚$%&' , represents the anisotropic
spin-density distribution and its strength characterizes theanisotropy of the charge/spin distribution of the d orbitals.
The value of 𝑚! is positive, which means that the spin
density distribution is expanded in the in-plane direction.
Although it has been reported that 𝑚! is related to theemergence of DMI [7], here it is below the accuracy limit
and we cannot determine its dependence on 𝑡)** .

Fig. 2: XAS and XMCD spectra at the Co L3,2 edges for theW/Co/Pt heterostructures with different Co thin
thicknesses (𝑡"# ). Spectra after background subtraction areshown. The spectra obtained under out-of-plane and “inplane” magnetic field are plotted by solid and dashedcurves, respectively.
The orbital magnetic moment (𝑚#,- ) of Co atom is also
estimated using the XMCD sum rule [8]. The out-of-plane.
, is estimated from the XAS andcomponent of 𝑚#,- , 𝑚#,XMCD spectra measured under the out-of-plane magnetic∥, is obtained using thefield. The in-plane component, 𝑚#,spectra measured under the out-of-plane and “in-plane”
fields according to the relationship,∥.
𝑚#,- (𝜃) = 𝑚#,sin( 𝜃 + 𝑚#,cos( 𝜃,with 𝜃 is 30°. The 𝑡)** dependence of 𝑚#,- is plotted in Fig.
∥.
and 𝑚#,decrease with decreasing 𝑡)**4. Both of 𝑚#,∥from the bulk value. We find the decrease of 𝑚#,with 𝑡)**.
is stronger than that of 𝑚#,- . The difference leads to theOMA which is shown by the black squares in Fig. 4, where

one can see that the OMA of Co increases with decreasing𝑡)** and its slope changes abruptly at 𝑡)** ~ 0.9 nm.

Fig. 3: Magnetization effective thickness (𝑡)** ) dependenceof spin magnetic moment (𝑚$%&' , red circles) and magnetic
dipole (𝑚! , blue squares) in the W/Co/Pt heterostructures.
The spin magnetic moment value of bulk hcp Co [10] isshown by a red horizontal line.

Fig. 4: Magnetization effective thickness (𝑡)** ) dependenceof the orbital magnetic moment ( 𝑚#,- ) for the different
magnetization directions in the W/Co/Pt heterostructures.
The orbital magnetic moment value of bulk hcp Co [10] isshown as the red dashed horizontal line.
Although all the samples have in-plane magnetic
anisotropy, the positive OMA and 𝑚! of Co indicates thatthe magnetocrystalline anisotropy (MCA) favors
perpendicular magnetization based on the formulas driven
by Bruno [12] and van der Lann [13]. We conclude that theorigin of the in-plane magnetic anisotropy in our W/Co/Ptsamples is attributed to the shape anisotropy energy
exceeding the small MCA. The abrupt slope change of theOMA of Co at 𝑡)** ~ 0.9 nm indicates the complicatedinterfacial environment in the W/Co/Pt heterostructureswith ultrathin Co layer. The growth of hcp/fcc Co on βphase (or amorphous) W results in the degradation of the(111) texture compared to the structures where Co is grown
on fcc based materials (e.g. Pt and Ru). We suggest thatsuch degradation contributes to intermixing at the W/Cointerfaces. Furthermore, the interfacial DMI is a property

Photon Factory Activity Report 2019 #37 (2020)

which is sensitive to the interface condition. Therefore, weinfer that such interfacial environment should result in thecomplicated behavior of DMI in the W/Co/Pt
heterostructures.
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* z.chi@qspin.phys.s.u-tokyo.ac.jp