Photon Factory Activity Report 2016 #34 (2017)

BL-16A/2016S2-005

Orbital moment anisotropy in Co-heavy metal heterostructures studied by x-ray
magnetic circular dichroism
Zhendong Chi1,*, Goro Shibata1, Shoya Sakamoto1, Yosuke Nonaka1, Keisuke Ikeda1, Masako
Sakamaki2, Kenta Amemiya2, Masamitsu Hayashi1,3 and Atsushi Fujimori1
1
Department of Physics, The University of Tokyo, Bunkyo, Tokyo, 113-0033, Japan1
Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan
3National Institute for Materials Science, Tsukuba, 305-0047, Japan
1 Introduction
Fast domain wall (DW) motion controlled by electricalcurrent in ferromagnet (FM)/5d heavy metal (HM)heterostructures with perpendicular magnetic anisotropy(PMA) may be a promising playground for realizing novel
spintronics devices, such as current-controlled and low
energy-consumption
magnetic
memorydevices.
Dzyaloshinskii-Moriya interaction (DMI) [1, 2] isconsidered to play an essential role in stabilizing Néel typeDWs at the interface of FM/HM heterostructures [3]. Such
Néel type DW can be moved by electric current induced
spin orbit torque [4-6]. However, the origin of suchinterfacial DMI is still unclear, becoming a large obstacle
to design practical devices. Although micromagnetics or
perturbation theory introduces the DMI term to predictspin texture of FM/HM interfaces and try to understand theorigin of DMI in these systems, the observation of nontrivial thickness-dependent interfacial DMI [7] suggeststhat the mechanism behind it is much more complicated.
It has been proposed that the microscopic origin of theinterfacial DMI may be attributed to anisotropies of theorbital magnetic moment and magnetic dipole moment ofthe ferromagnetic metal [8,9]. However, it has beentheoretically found that both the FM and HM elements in
FM/HM heterostructures will have orbital magnetic
moment anisotropy (OMA) [10], which may be closely
correlated with the strength and sign of the DMI. In order
to investigate the relationship between the DMI strengthand the OMA of FM in FM/HM systems, we have studied
the spin and orbital magnetic moments of Co in different
HM/Co/HM sandwich heterostructures by x-ray magnetic
circular dichroism (XMCD) spectroscopy. We have foundthat the magnitude of the OMA of Co indeed has an HM
dependence similar to that of the DMI strength estimatedfrom the DW motion velocity under a pinning magnetic
field [11]. This gives a direct experimental evidence for thecorrelation between the OMA of Co and the DMI strength.

Sub./1Ta/3Pt/tCo/1Pt/1Ru samples were 0.6, 1, 1.5 and 3(nm). X-ray absorption spectroscopy (XAS) and XMCD
measurements were performed at the helical undulator
beamline BL-16A1 of KEK-PF. The spectra were
measured in the total electron yield (TEY) mode, as arelatively surface sensitive measurement mode. All themeasurements were performed at room temperature in anvacuum of ~5×10-9 Torr. The magnitude of the magnetic
field was set to 5 T and it is applied parallel to the incident
x-ray in all the measurements. The angle between themagnetic field and the sample surface was set to 30o for thein-plane magnetization configuration.

2 Experiment
Co ultrathin films sandwiched by different HMs,
Sub./1Ta/3Pt/tCo/1Pt/1Ru and Sub./1Ta/3W/0.6Co/1Pt/
1Ru (the numbers denote the nominal thicknesses in nm),together with a reference Sub./1Ta/3Cu/0.6Co/1Cu/1Ru
sample, were grown on 10×10 mm2 thermally oxidized Si
substrates by the magnetron sputtering method at roomtemperature. The thicknesses of Co (t) in theFig. 1: Co L2,3-edge XAS and XMCD spectra of (a)3W/tCo/1Pt samples with different Co thin thicknessesand (b) 0.6Co sandwiched by different elements.

3 Results and Discussion

Figure 1 illustrates the XAS and XMCD spectra at theCo L2,3 edges in different sandwich heterostructures. Thespectral line shapes of all the samples are similar to that ofmetallic Co, suggesting that Co is protected from oxidation

Photon Factory Activity Report 2016 #34 (2017)

Fig. 2: Spin (a) and orbital magnetic moments (b) of Co
in the 3W/tCo/1Pt heterostructures with different Co
thicknesses.
by the Ru cap well. From these spectra, we have estimatedthe spin and orbital magnetic moments of Co by using theXMCD sum rules [12]. Here the hole number of Co has
been assumed to be 2.45 [13]. For the in-planemagnetization configuration in our experiments, thedirections of the magnetic field is different from the sample
surface. Therefore, the in-plane orbital magnetic moment
should be corrected by using 𝑚 = 𝑚⊥ sin2 𝜃 + 𝑚∥ cos 2 𝜃(here 𝜃 is 30°), where 𝑚⊥ is the out-of-plane magnetic
moment and can be obtained from the sum rules directly.
Fig. 2(a) and 2(b) show the Co-thickness dependencies ofthe effective spin and orbital magnetic moments,
respectively, for the W/Co/Pt heterostructures. Both theeffective spin and orbital magnetic moments are enhancedwith increasing Co thickness, which agrees with theprevious result of CoFeB/HM heterostructures [14]. The
anisotropy of the effective spin magnetic moment is almost
zero for all the thicknesses, suggesting that Co ismagnetically saturated under external magnetic field of 5T. However, the OMA of Co reduces with increasing Cothickness. When the Co layer thickness exceeds around 1nm, the OMA becomes very small and does not depend on
the thickness.
In order to see the element dependence of the OMA, wehave estimated the magnetic moments of theheterostructures with Co thickness of 0.6 nm, which is
expected to show the largest OMA among all thethicknesses studied. The effective spin and orbital
magnetic moments of 0.6 nm-thick Co in different HM/CoFig. 3: Spin (a) and orbital magnetic moments (b) of Co
sandwiched with different elements.
/HM heterostructures are shown in Figs. 3(a) and 3(b),respectively. As a reference, the data for the Cu/Co/Cu
heterostructure is also plotted, which is considered that
there should be almost no DMI at the Cu/Co interface dueto the weak spin-orbit interaction (SOI) of Cu. The
absolute value of the effective spin moment of Co in
W/Co/Pt is smaller than those in the Pt/Co/Pt andCu/Co/Cu samples. This may be attributed to a magnetic
dead layer formed at the Co/W interface [15]. The
anisotropy of the spin magnetic moment is almost zero for
all the heterostructures, suggesting that Co is magneticallystatured for both the out-of-plane and in-planemagnetization configurations. It is interesting to find that
the OMA of Co shows a strong correlation with the HMelements. Although the absolute value of the orbital
moments in W/Co/Pt is the smallest, the OMA is thelargest in all the three systems. For Pt/Co/Pt, the OMA of
Co is smaller than that of W/Co/Pt. For Cu/Co/Cu, Co has
almost no anisotropy of orbital magnetic moments.

Fig. 4: DMI enhancement and cancellation in asymmetricand symmetric HM/Co/HM sandwich structures.

Photon Factory Activity Report 2016 #34 (2017)

The correlation between the OMA of Co and the DMI
can be understood as follows. As illustrated in Fig. 4, there
are two Co/HM interfaces in the sandwich heterostructures,and the spin chirality of Co is determined by the strengthand sign of the DMI at both the interfaces. When Co issandwiched by HMs of the same element, the chiralmagnetic structure induced by the DMI will be cancelled
out, making the DMI to be zero as a whole. However, if
the upper and lower interfaces have the opposite signs ofDMI, it should be enhanced as a whole. It has beenreported that the signs of the DMI for Co/Pt and Co/W are
opposite [16,17]. Therefore, the DMI of W/Co/Pt shouldbe the largest among the heterostructures studied. The
small but finite OMA in the Pt/Co/Pt heterostructure may
be attributed to the difference in the thicknesses ofoverlayer and underlayer Pt, which results in the breakingof the inversion symmetry. As mentioned above, there
should be no DMI in Cu/Co/Cu due to the weak SOI ofCu. Thus, we conclude that the OMA of Co in HM/Co/HM
has the same HM dependence as the DMI strength. Our
results may give a new understanding of microscopic
origin of interfacial DMI.
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* chi@wyvern.phys.s.u-tokyo.ac.jp