Photon Factory Activity Report 2018 #36 (2019)

BL-16A/2017G597

Probing the Antiferromagnetic-Ferromagnetic Phase Transition in GdBaCo2O5.5 Thin
Films by Soft X-ray Magnetic Circular Dichroism
Yujun ZHANG1,2*, Keisuke IKEDA2, Tsukasa KATAYAMA3, Akira CHIKAMATSU3, Tetsuya
HASEGAWA3, and Hiroki WADATI1,2
1Graduate School of Materials Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun,
Hyogo 678-1297, Japan
2
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Chiba 277-8581, Japan3
Department of Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan

1 Introduction
Entanglement of charge, spin, lattice and orbital
degrees of freedom in transition metal oxide (TMO)
systems have attracted much attention recently [1]. The
electric and magnetic properties of TMOs, which are
dominated by the partially filled d orbitals, could bestrongly affected by the crystal field effect from thesurrounding lattice and corresponding d-orbital
occupation. With a strongly localized feature of delectrons in TMO systems (especially 3d TMO systems),
variety of ordering phenomena like magnetic ordering,
charge ordering and ferroelectricity can coexist andsignificantly couple with each other. Tantalizing physicalphenomena with prospective applications, such as high-Tc
superconductivity and colossal magnetoresistance, haveemerged during the investigation of strongly correlated
TMO systems.
Among 3d TMO systems, except the famous maganites
and cuprates, cobaltites have also drawn much researchattention due to their specific spin-state transition of Co
ions. In many cobaltite systems, for instance the mostsimple case, perovskite LaCoO3, the competition between
crystal field splitting and Hund coupling can lead tovarious spin states of Co ions[2]. Take Co3+ as an
example, with a strong octahedral crystal field, low-spin(LS, t2g6eg0, S=0) state is energetically preferable andhigh-spin (HS, t2g4eg2, S=2) state with occupied eg orbitalis preferred when crystal field is weak. In spite of LS and
HS states, there is also debate that intermediate-spin (IS,t2g5eg1, S=1) state can also be realized in various cobaltites.
The spin state of Co ions is very sensitive to the external
condition such as temperature, pressure, magnetic fieldand substrate strain, etc. It is intensely reported that themagnetic properties of cobatites are dominated by thespin state of Co ions and complicated magnetic structures
could be observed in quite a few cobaltite systems.
Recently, REBaCo2Ox(5<x<6, RE is rare earth element)
systems have been intensely investigated [3-5] for theirintriguing physical properties such as high oxygen
conductivity, metal-insulator transiton, giant magnetoresistance and spin-state ordering, etc.. The oxygenconcentration in these systems is always variable, whichleads to the high oxygen mobility within the lattice. Whenx=6, the cystal structure is standard double perovskite
with RE and Ba sitting at A site, while Co sitting at B site.
The nominal valence state of Co is +3.5. Due to thedifferent ionic size of RE and Ba ions, A site is often
ordered into layers with alternating RE and Ba layers.
When x=5, a structure with ordered oxygen vacancy
layers can be formed. When x=5.5, the oxygen vacancies
order into columns along a axis, resulting in 4 nonequivalent Co sites coordinated by either normal oxygenoctahedra or oxygen pyramids. With this oxygen
concentration, the nominal valence state of Co is +3. In
this case spin-state ordering is frequently observed. For
example it is reported that in PrBaCo2O5.5+x samples HS,
IS and LS coexist and contribute to a complicatedmagnetic structure and magnetic phase transition [4].
On the other hand, phase transition with large
resistivity
changeentangledwith
antiferromagnetic(AFM)-ferromagnetic (FM) transitionand spin-state transition were also explored inGdBaCo2O5.5-x system[5]. Magnetic ordering with anindex of (0 0 1/2) was clearly observed. However,systematic investigation of Co moments and spin-states,
as well as how they changes across the AFM-FMtransition (~230 K) is still absent. To achieve this, soft xray magnetic circular dichroism (XMCD) characterizationis necessary.
Despite the report of rough XMCD measurement inRef. [5], systematic XMCD characterization of this AFMFM transition is still absent for GdBaCo2O5.5 system, andeven absent for all the similar materials systems. Detailedtemperature/field dependent XMCD characterization will
be greatly helpful to understand the complicated spinstate configuration and magnetic structure. For thispurpose, we prepared high-quality epitaxial GdBaCo2O5.5(GBCO) thin film sample on SrTiO3(001) (STO) andLaSrAlO4 (LSAO) substrates and conducted the soft xray XMCD measurement to investigate the phasetransition behavior.
2 Experiment
We measured XAS/XMCD spectra for GdBaCo2O5.5
thin film samples at BL-16A of Photon Factory. Since thesamples have in-plane magnetic anisotropy, a 30o

Photon Factory Activity Report 2018 #36 (2019)

3 Results and Discussion
Fig. 1 shows the full Co L edge XAS of GBCO thinfilms. Intense Ba M edge signal can also be observed near
the Co absorption edges.
Fig. 2 shows expanded Co L edge XAS of GBCO thinfilms. One can clearly see the different spectral shape ofsamples on different substrates at Co L3 edge, whichshould originate from the different electronic structure of
Co 3d band induced by different substrate strain.
Fig. 3 shows the Co L edge XMCD spectra of GBCOthin films. One can easily observe a larger XMCD signal
at 250 K than that of 200 K for both samples, whichshows clear sign of the AFM-FM phase transition. Inaddition, in the FM phase, film on STO has larger XMCD
than the film on LSAO. This difference may originate
from the different spin state and local environment ofmagnetic Co ions.
Although we successfully conducted the XMCD
measurement for GBCO samples, it is hard to conduct
sum-rules analysis for these data due to the overlap
between Co L edge and Ba M edge.
Our experimental results confirmed the AFM-FM
phase transition behavior in these GBCO thin film
samples. These results are important for discussion about
the electronic structure and substrate strain effect inGBCO thin films.

LSAO_200K
LSAO_250K
STO_200K
STO_250K

XAS (arb. units)

1.0
Ba M5

0.8
0.6

Ba M4

0.4
Co L3
0.2

Co L2

0.0
770780790

800810820

Photon Energy (eV)Fig. 1: Full Co L edge XAS of GBCO thin films.
0.35
LSAO_200K
LSAO_250K
STO_200K
STO_250K

XAS (arb. units)

0.30
0.25
0.20
0.15
0.100.05
0.00
775780

785

790

795

800

Photon Energy (eV)Fig. 2: Expanded Co L edge XAS of GBCO thin films.

0.03
LSAO_200K
LSAO_250K
STO_200K
STO_250K

0.02

XMCD (arb. units)

incidence of Co L edge soft x-ray was used. A magnetic
field of 5 T was applied during the measurement and total
electron yield was detected to obtain the XAS spectra.

0.01
0.00
-0.01
-0.02
-0.03

770780

790

800

810

820

Photon Energy (eV)Fig. 3: Co L edge XMCD spectra of GBCO thin films.

References[1] E. Dagotto. Science 309: 257-262 (2005).
[2] M. W. Haverkort, et al. Phys. Rev. Lett. 97: 176405(2006).
[3] T. Katayama, et al. J. Mater. Chem. C 6: 3445 (2018).
[4] P. Miao, et al. Phys. Rev. B 95: 125123 (2017).
[5] M. García-Fernández, et al. Phys. Rev. B 78: 054424(2008).
[6] A. A. Taskin, et al. Phys. Rev. Lett. 90: 227201 (2003).
* zhangyujun@issp.u-tokyo.ac.jp