Photon Factory Activity Report 2017 #35 (2018)
BL-16A/2016S2-005

Voltage-dependent magnetic properties of Co/BiFeO3 heterostructure: x-ray magnetic
circular dichroism study
Zhendong CHI1,*, Hiroyuki YAMADA2, Goro SHIBATA1, Shoya SAKAMOTO1,Yosuke NONAKA1, Keisuke IKEDA1, Masahiro SUZUKI1, Yuxuan WAN1, Masako SAKAMAKI3,Kenta AMEMIYA3, Akihito SAWA2 and Atsushi FUJIMORI1
1
Department of Physics, The University of Tokyo, Bunkyo, Tokyo, 113-0033, Japan
2
National Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8565, Japan3
Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan
1 Introduction
Aiming to realize low-energy consumption through theapplication of spintronics, i.e., controlling of spin states byelectric current or voltage, a variety of methods have been
proposed using spin accumulation or spin transfer byelectric current, e.g., current-induced domain-wall motion
and magnetization reversal [1,2] and current control ofmagnetization in ferromagnetic semiconductors [3].
However, the current densities required to transfer the spin
moment in such systems are very large, still resulting in alarge loss of energy. Another approach to control
magnetization is to use electric voltage. A promising seriesof materials in the application of voltage control ofmagnetization are magnetoelectric multiferroics [4,5].
Bismuth ferrite (BiFeO3, BFO) is the most studied
multiferroic material as it is the only material showing
multiferroic properties at room temperature so far. BFO
has a nearly G-type antiferromagnetic order with finite
incommensurability and the electrical polarization of BFO
is along the (111) orientation. Due to the spin canting
induced by Dzyaloshinskii-Moriya interaction [6,7], weakferromagnetism also exists in BFO [8] and its orientation
can be modulated by an electric field [9].
Because of the weak magnetization of BFO,
ferromagnet (FM)/BFO thin film heterostructures are often
used to realize the voltage control of magnetization in suchsystems. As usual, oxides, e.g., CrO2, La0.7Sr0.3MnO3, andLa0.5Ca0.5MnO3, are used for the FM layer [10-13].
However, the Tc of the magnetic oxide is often lower thanroom temperature, which prohibits their practicalapplications. By domain architecture utilizing the strain ofsubstrates, it has been found that stripe-like ferroelectricdomains of BFO can be formed on a (110)-oriented
DyScO3 (DSO) substrate [14]. Then by depositing CoFealloy onto the BFO thin film with the magnetic domain of
the same pattern, the 180° reversal of magnetization atroom temperature has been realized [15].
Though the realization of electrical-field control ofmagnetization in CoFe/BFO heterostructure at room
temperature, the nature of the magnetic couplingmechanism between the metallic alloy CoFe and theperovskite oxide BFO is still unknown. It has been known
that the polarization and the Dzyaloshinskii-Moriya vectorof BFO can be reversed after applying a voltage along thec-axis, resulting in the 180° reversal of weak moment of Fe[9]. It is theoretically shown that the magnetic moment ofFe in BFO is enhanced at the CoFe/BFO interface andcoupled with the magnetic moment of CoFe [16,17]. As aresult, the magnetic moment of CoFe should be also varied
when modulating the magnetic moment of Fe in BFO.
However, direct evidences for the magnetic moment of Fein BFO and the coupling mechanism between BFO andCoFe have still remained uncertain so far. In this report, byusing element-specific x-ray magnetic circular dichroism(XMCD), the magnetic properties and their voltage
dependence of Fe and Co in a Co/BFO heterostructure has
been studied. The magnetic moment of Fe in BFO at room
temperature was confirmed by XMCD. Furthermore, themagnetic moments of both Co and Fe have also been foundto vary under different electric voltages.
2 Experiment

Figure 1. Setup for the voltage-dependent XAS andXMCD measurements.
An 80 nm BFO (001)/SrRuO3 (SRO) thin film wasgrown on an (110)-oriented DSO substrate by the pulsed
laser deposition (PLD) method. SRO thin film was used as
an electrode to apply voltage. 1nm Co was deposited on
BFO by electron-beam evaporation method, followed by a
2 nm Pt thin film used as a capping layer and an electrode.
X-ray absorption spectroscopy (XAS) and XMCD
measurements were performed at the helical undulator
beamline BL-16A1 of KEK-PF. The spectra were collectedboth in the total electron yield (TEY) mode and the total

Photon Factory Activity Report 2017 #35 (2018)
fluorescence mode (TFY). The measurements were all
performed at room temperature in an ultrahigh vacuum
below 4×10-7 Torr. The magnetic field was set at 1.2 T andparallel to the incident x rays. There was an angle of 60°
between the magnetic field and the sample normal during
the measurement. The projection of the magnetic field on
the ab plane of the film was along the (1-10) direction ofthe DSO substrate. A voltage was applied between Pt andSRO before each measurement. The experimental setup isillustrated in Fig. 1.
3 Results and Discussion
Figure 2 shows the XMCD spectra at the Fe and Co L3,2edges of Co/BFO heterostructure measured at roomtemperature. In Fig. 2(a)-(c), it can be found that there areclear XMCD signals of Fe after adding different external
voltages. The spectral line shapes are similar to what havebeen observed on the ferromagnetic oxide/BFO interface
at low temperature so far [12,13]. From Fig. 2(a)-(c), one
finds that the XMCD intensity changed after the polingwith different voltage direction. Although the XMCD
spectra under different orientations of magnetic field show
different intensities, we note that the background is rather
strong here and further studies are needed to confirm thedifferences. Figure 2(d)-(f) illustrate the XMCD spectra of
Co in Co/BFO heterostructure under different directions ofmagnetic field and after different poling voltages anddirections. For comparison of the XMCD intensity, theXMCD spectra measured under a -500 Oe magnetic field
have been reversed in all the three panels. In all the XASspectra of Co, a shoulder exists on the high energy side,suggesting a partially oxidized state of Co in theheterostructure. Similar to the results of the Fe L3,2 edge,
the XMCD intensities of Co are also asymmetric andchanged after the poling of voltage with orientation
reversal.

Figure 3. XAS spectra at the O K edge collected in the TEY
mode (a) and TFY mode (b).
In order to understand the coupling mechanism between
Co and BFO, XAS at the oxygen K edge was also measured
using x rays with linear polarization. Figures 3(a) and (b)illustrate the O K-edge XAS collected using the TEY andTFY modes, respectively. The line shape of O K-edge XAS
after the poling is much different from that at the as-grownFigure 2. XAS and XMCD spectra at the Fe and Co L3,2-edges of Co/BFO heterostructure. (a) Fe L3,2 edges at theas-grown state, (b) After a +7 V voltage poling, (c) After -20 V voltage poling. (b)-(d) Co L3,2 edges under the same
condition as Fe.

Photon Factory Activity Report 2017 #35 (2018)state in the TEY mode but is almost the same in the TFY
mode. This may be due to the diffusion of oxygen atoms
across the Co/BFO interface. However, we cannot exclude
the possibility of the removal of adsorbed oxygen atoms on
the surface of the sample.

Figure 4. XAS and XMCD spectra at the Co L3,2 edge after
a +20 V voltage poling.
After the -20 V poling, another +20 V voltage was thenapplied to the Co/BFO heterostructure to check thereproducibility of the voltage dependence. XAS andXMCD spectra at the Co L3,2 edges are illustrated in Fig. 4.
The asymmetry of the XMCD intensity under the magnetic
field reversal is the same as that after the -20 V poling. This
may indicate that the BFO thin film was electrically broken
down after the application of the too high voltage of thefirst -20 V poling. Although the reproducibility of thevoltage dependence could not be confirmed, our results
still provide direct evidence for the existence of themagnetic moment of Fe in BFO at room temperature andthere should be a magnetic coupling between Co metal and
Fe in BFO at the Co/BFO interface. Further studies areneeded to confirm the magnetic field and voltage
dependence of Fe and Co L3,2-edges XMCD andinvestigate the coupling mechanism of Co and Fe at theCo/BFO interface.
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* chi@wyvern.phys.s.u-tokyo.ac.jp