UVSOR-II, BL5U

Temperature Dependent Angle-Resolved Photoemission Spectroscopy
on Ferromagnetic EuO Thin Films
H. Miyazaki1, T. Ito2 H. J. Im3, S. Yagi2, M. Kato2, K. Soda2 and S. Kimura1, 4
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Department of Advanced Physics, Hirosaki University, Hirosaki 036-8560, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki444-8585, Japan
Europium monoxide (EuO) is a ferromagneticsemiconductor with the Curié temperature (TC) ataround 70 K [1, 2]. In the electron doping case by theEu excess or substitute Gd3+ or La3+ from Eu2+ ion, theTC increases up to 150 K and the electrical resistivity
drops twelve-order of magnitude below the TC
originating in a metal-insulator transition (MIT) [2, 3].
To reveal the origin of these physical properties ofEuO, it is important to clarify the electronic structure.
Three dimensional angle-resolved photoemission
spectroscopy (3D-ARPES) using a synchrotron
radiation source is the most powerful technique to
directly determine the electronic band structure. Using
this technique we observed the change in the Eu 4f andO 2p states across TC.
Single-crystalline EuO thin films with a thickness ofabout 50 nm were fabricated by the molecular beam
epitaxy (MBE) [4]. Epitaxial growth of thesingle-crystalline EuO thin films with the 1 x 1 EuO(100) patterns was confirmed with low energy
electron diffraction (LEED) and reflection highenergy electron diffraction (RHEED) methods. TheTC measured with a superconducting quantum
interference device (SQUID) magnetometer was 71 K.
The 3D-ARPES measurements were performed at thebeam line 5U of UVSOR-II combined with the MBEsystem.
Figures 1(g1) [1(g2)] and 1(x1) [1(x2)] show thetemperature dependence of the Eu 4f [O 2p] band at
the Γ and X points, respectively. The circles andbright area correspond to the peak positions of thesecond-derivative energy distribution curves (EDCs).
With decreasing temperature across TC, all of thebands shift by 0.2–0.3 eV to the lower binding energy
side. The overall energy shift is in good agreementwith the magnetic red shift estimated from the opticalabsorption spectra [5]. This indicates that the overall
energy shift originates from the changing of thebottom energy of the Eu 5d conduction band at the X
point due to the energy gain of the Eu 5dmajority-spin state after band splitting caused byferromagnetic ordering. In addition, the top of the Eu
4f states is shifted away from the main 4f states at theΓ and X points. In contrast, the O 2p state is spitted
into two bands below TC. The bands at the higher andlower binding energies are attributed to the majority
and minority spin states, respectively. Since the Eu 4f
state is fully polarized, the 4f state mainly hybridizes

with the majority spin state of O 2p and Eu 5d.
Therefore, the band shits can be attributed to thehybridization effect between the Eu 4f and O 2p states(superexchange interaction) and between the Eu 4f
and 5d sates (indirect exchange interaction). The
observed temperature dependent energy shift of the 4f
state is the essential origin of the ferromagnetic phasetransition of EuO.

Fig. 1 Temperature dependence of the photoemission
peak energies derived from the second-derivativeEDCs of the Eu 4f [(g1), (x1)] and O 2p [(g2), (x2)]states at the Γ and X points. The open circles indicate
the peaks of the second derivative EDCs.
[1] N. Tsuda et al., Electronic Conduction in Oxides(Springers College) (1976).
[2] A. Mauger et al., J. Phys. (paris) 39, 1125
(1978).
[3] Y. Shapira, S. Foner, and T. B. Reed, Phys.
Rev. B 8, 2299(1973); 8, 2316 (1973).
[4] H. Miyazaki et al., Jpn. J. Appl. Phys. 48, 055504(2009).
[5] S. Kimura et al., Phys. Rev. B 78, 052409 (2008).