Photon Factory Activity Report 2022 #40 (2023)

BL-7A/2021G069
Magnetic structures at Co/oxide interface for voltage-controlled magnetic
anisotropy eﬀect studied by XMCDJun Okabayashi1∗ and Tomohiro Nozaki2
12

Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo113-0033, Japan
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan

Electric-field control of magnetism has attracted
significant attention because it can realize ultralowpower spintronics devices. The voltage-controlled
magnetic anisotropy (VCMA) eﬀect is a high-speed
spin manipulation technique with excellent compatibility with magnetoresistive random access memory.
VCMA eﬀects in bcc-Fe-(001)-based stacks have been
investigated extensively and a VCMA coeﬃcient of350 fJ/Vm has been achieved using high-quality epitaxial films [1]. However, further improvement in theVCMA coeﬃcient is demanded. The VCMA eﬀect
was also investigated for fcc-Co-(111)-based stacks because of a large VCMA coeﬃcient of 230 fJ/Vm [2].
However, in fcc-Co-based stacks, systematic studies ofthe VCMA eﬀect have not been conducted and theunderstanding is still in its infancy compared withthat of bcc-Fe-based stacks. In fcc-Co-based stacks,a particularly large VCMA eﬀect, including voltagecontrolled coercivity (Hc ) (VCC), has been reported
at interfaces with surface oxidation of Co, suggesting the significant role of surface oxidation [3]. However, the reasons for the improvement in VCC by postannealing remain unclear.
Pt/Co/AlOx trilayers have long been focused on thestudies of large interface PMA, strong Rashba eﬀect,and Dzyaloshinskii-Moriya interaction. The eﬀect ofthe oxidation conditions on the interface PMA was examined in detail using Pt/Co/post-oxidised Al trilayers [4]. The PMA at the Co/oxide interface was maximised under appropriate oxidation conditions with
the Co-O bonds at the interface and enhanced by postannealing. To investigate the interfacial oxidation andchemical states, we employed x-ray magnetic circular dichroism (XMCD) to probe the spin and orbital
magnetic moments (morb ). In this study, to understand VCMA eﬀect at fcc-Co (111)/oxide interfaces,
we systematically investigated the dependence of theVCC in the Pt/Ru/Co/CoO/TiOx structure on Co
film thickness and annealing temperature.
The samples were grown by molecular beam epitaxy and sputtering. The sample structures consistof SiOx sub./Ta (5 nm)/Ru (10 nm)/Ta (5 nm)/Pt(10 nm)/Ru (0.2 nm)/Co wedge (tCo nm)/TiOx (2nm) as shown in Fig. 1a. XAS and XMCD were performed at BL-7A in the Photon Factory (KEK-PF).
For XAS and XMCD measurements, the photon helicity was fixed, and a magnetic field of ±1.2 T wasapplied parallel along the incident polarised soft X-ray
beam to obtain signals defined as µ+ and µ- spectra.
The total-electron-yield mode was adopted and all themeasurements were performed at room temperature.
The Co L-edge XAS and XMCD spectra of asdeposited and 350◦ C-annealed samples are shown inFig. 1. The XAS signals comprised metallic Co (777.0eV) and chemically shifted CoO (Co2+ ) components(778.0 eV). Metallic Co components of the XAS andXMCD signals increased with an increasing tCo . Notethat while the spectra after annealing appeared as asingle peak of metallic Co component, the spectra
contain the CoO component; the peak at 777.0 eV
broadened compared with that of metallic Co, suggesting the partial reduction of the CoO layer owing
to post-annealing. Despite the reduction in CoO, nosignificant increase in XMCD signals was observed;
the sample with tCo =2.0 nm showed an XMCD intensity comparable to that of the as-deposited sample.
The sample with tCo = 1.5 nm showed an XMCD intensity smaller than that of the as-deposited sample.
These can be attributed to the decrease in Co owing
to the diﬀusion of Co atoms into the Pt layer. By using the magneto-optical sum rules, the post-annealing
induced increase in morb /mspin , which corresponds tothe increase in the Hc and VCC and may occurs atthe Co/oxide upper interface.

Fig. 1, (a) Sample structures, (b-e) XAS and XMCD of Co
L-edge for as-deposited and 350◦C-annealed samples for
tCo=1.5 and 2.0 nm.
[1]T. Nozaki et al. APL Mater. 8, 011108 (2020).
[2]T. Hirai et al., Appl. Phys. Lett. 112, 122408 (2018).
[3]T. Nozaki et al., Sci. Rep. 11, 21448 (2021).
[4]C. Nistor et al. Phys. Rev. B 84, 054464 (2011).
*e-mail: jun@chem.s.u-tokyo.ac.jp