Photon Factory Activity Report 2022 #40 (2023)

BL-7A/2021G069
Comparison between conductive and insulating cobalt-ferrite thin films
with perpendicular magnetic anisotropy studied by XMCDJun Okabayashi1∗ , Masaya Morishita2 , Masaaki A. Tanaka2 , and Ko Mibu2
1

Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo113-0033, Japan
2
Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Aichi 466-8555, Japan

Spinel-type cobalt ferrites [CoFe2 O4 (CFO)] havebeen investigated for a long time for magnetic applications as magnetic insulators in the forms of bulk,powder, and thin films. They have an inverse spinel
structure in AB2 O4 chemical formulation, with Fe3+
at the tetragonal A-site (Td ) and Fe3+ and Co2+ at
the octahedral B-site (Oh ). Recent developments inspintronics boost the application of thin CFO films
stacked with other nonmagnetic and magnetic layers,which are, for example, utilized as spin-filtering tunnel
barriers. There are some reports that CFO (001) films
exhibit perpendicular magnetic anisotropy (PMA) under tensile epitaxial strain [1]. The PMA in CFO films
is thought to originate from Co2+ (3d7 ) at the octahedral B-site (Oh ) in the tetragonally distorted inverse
spinel structure. For the charge neutrality, the formal
valence states are recognized as Co2+ (Oh ), Fe3+ (Oh ),and Fe3+ (Td ) in stoichiometric CFO films. However,the substitution of Co2+ by Fe2+ promotes the formation of Fe2+ (Oh ) sites, which induces the conductive
properties in CFO films. The conductivity in CFO(C-CFO) films would arise depending on the compositions due to the formation of Fe2+ states throughCo2+ → Fe2+ substitution, which is regarded as anapproach to magnetite Fe3 O4 . On the other hand,insulative CFO (I-CFO) can be grown by Co2+ →Fe3+2/3 □1/3 (□= cation vacancies) substitution, which
is an approach to insulative maghemite γ-Fe2 O3 .
In spite of applications of both C-CFO and I-CFO
films in the spintronics researches, the characterizations of both CFOs have not been performed until
now [2,3]. Here, we aim to establish the selective
film growth methods between I-CFO and C-CFO films
with PMA and discuss the origin of conductive properties from the results of transport measurements andsite-specific magnetic spectroscopies of XMCD.
Samples were grown on MgO (001) substrates bya pulsed laser deposition (PLD) method using source
materials with diﬀerent powder compositions. The
source materials for I-CFO were prepared by mixing
CoFe2 O4 and Fe3 O4 powder, and those for C-CFO,
were by mixing CoFe2 O4 and α-Fe powder.
X-ray absorption spectroscopy (XAS) and XMCD
were performed at BL-7A in the Photon Factory atthe High Energy Accelerator Research Organization(KEK-PF). For the XAS and XMCD measurements,
the photon helicity was fixed, and a magnetic field of±1.2 T was applied parallel along the incident polarized soft x-ray beam, to obtain signals defined as m+and m-spectra. The total electron yield (TEY) mode
was adopted, and all measurements were performedat room temperature.
Figure 1 shows the XAS and XMCD of C-CFO andI-CFO for Fe and Co L-edges with diﬀerent Co compositions. The Fe and Co intensities are plotted inthe same vertical scales because XAS roughly provides the Fe and Co compositions. XAS and XMCDline shapes for the Fe L-edges show distinctive features with diﬀerential line shapes due to the threekinds of Fe states (Fe3+ in Oh , Fe3+ in Td , and Fe2+in Oh ). For the Fe L-edges, although the diﬀerence
in XAS is small, clear diﬀerential XMCD line shapes
are detected. The Fe3+ state with Td symmetry exhibits the opposite XMCD sign, which is common forthe spinel ferrite compounds. The Fe2+ component at708.0 eV decreases in high Co compositions. In thecase of Fe3 O4 , the Fe2+ component is more enhanced.
Therefore, the conductive properties are related to theamounts of Fe2+ states, whose quantitative analysis isperformed by using the ligand-field-multiplet calculation [4].

Fig. 1, XAS and XMCD of (a) conductive Co0.23 Fe2.77 O4
and (b) insulative Co0.23Fe2.77O4.
[1]K. Naruse et al., J. Magn. Magn. Mater. 475, 721 (2019).
[2]M.A. Tanaka et al,, Appl. Phys. Lett. 122, 042401 (2023).
[3]J. Okabayashi et al,, Phys. Rev. B 105, 134416 (2022).
[4]M. Morishita et al., Phys. Rev. Mater. 7, 054402 (2023).
*e-mail: jun@chem.s.u-tokyo.ac.jp