Photon Factory Activity Report 201 #34 (2017) B

AR-NE1A/ 2014G577

Magnetic structure of Fe3O4 (111) surfaces

Kanta Asakawa1,Taizo Kawauchi1, Xiao Wei Zhang2, and Katsuyuki Fukutani1
1
Institute of Industrial Science, University of Tokyo, Japan2
High Energy Accelerator Research Organization (KEK), Japan
Abstract: The magnetic structure of the Fe3O4(111) surface was investigated by conversion electron Mossbauer
spectroscopy (CEMS) and nuclear resonant X-ray scattering (NRS). An 57Fe3O4 layer of about 1 nm was formed on asingle-crystal Fe3O4(111) in an ultrahigh vacuum, and the magnetization at the surface was probed by CEMS and NRS.
Both CEMS and NRS revealed resonance signals corresponding to the Fe A and B sites. From the analysis of the data,
we discuss that the magnetization direction is near in-plane at the surface.

1 Introduction
Magnetite (Fe3O4) has attracted interest due to its
fascinating characters such as catalytic activity [1,2] andpredicted half-metallicity [3,4]. It also exhibits a metal-toinsulator transition at a transition temperature (Tv) of 120
K called the Verwey transition [5]. This is observed as a
discontinuous drop in the electrical conductivity. Inambient conditions, Fe3O4 has the inverse-spinel cubic
structure with space group Fd¯3m. Its unit cell contains
32 O atoms and 24 Fe atoms. The 24 Fe atoms consist of
8 tetrahedrally coordinated Fe(A) sites that are occupied
by Fe3+ ions and 16 octahedral Fe(B) occupied by equal
numbers of Fe2+ and Fe3+ ions [6-8]. Above 122 K, theelectron hopping between Fe2+ and Fe3+ of B site gives
rise to the electrical conductivity [9]. The decrease ofelectrical conductivity by the Verwey transition isexplained by formation of charge ordering of B-site Fe2+
and Fe3+. However, this mechanism is not experimentallyconfirmed despite intensive research in the last decades.
There are, on the other hand, some reports claiming thatthe surface of Fe3O4 has an insulating character even atroom temperature [3,9,10]. This might suggest that thesurface Verwey transition occurs at a much higher
temperature than 122 K [11,12]. Across the Verwey
temperature, the crystallographic structure also changes
from cubic (T > Tv) to monoclinic (T < Tv) [13,14].
Simultaneously, the magnetic easy axis changes from111 to the monoclinic c axis[15-17]. In this study, themagnetization of the Fe3O4 (111) surface was investigatedat room temperature using conversion electron Mossbauer
spectroscopy (CEMS) and nuclear resonant X-ray
scattering (NRS).
2 Experiment
Before CEMS and NRS experiments, an 57Fe3O4 layer
was deposited on the surface in order to enhance thesurface signal intensity. The deposition was performed inan ultrahigh vacuum (UHV) chamber with a basepressure of 1×10−10 mbar. The (111) surface of Fe3O4
single crystal was cleaned by Ar+ sputtering, annealing at1000 K and annealing in oxygen atmosphere of 5×10−6mbar at 1000 K [18]. After cleaning, 1 nm of 57Fe3O4
epitaxial layer was grown by depositing 57Fe in an oxygenatmosphere with a partial pressure of 8 × 10−6 mbar. Theevaporation rate was approximately 0.2 nm/min and thesample temperature was kept at 550 K. After deposition,
the sample was annealed in UHV at 800 K for 10 min[19]. The surface crystallinity was checked by reflection
high-energy electron diffraction (RHEED) before andafter deposition. NRS experiments were performed at theNE1 beam line of the accumulation ring (AR) of HighEnergy Accelerator Research Organization (KEK). The
linearly polarized SR beam with an energy of 14.4 keVand energy width of 6.5 meV irradiated the sample with
its electric polarization parallel to the surface. The
reflected beam was detected by avalanche photodiodes(APD) set in the specular reflection direction [20]. During
the experiment, the sample was kept at room temperature
in a UHV chamber with a base pressure of 8 × 10−10 mbar.
After installing the sample, NRS spectra were taken in six
incident directions in order to estimate the in-planemagnetic anisotropy. After observing the azimuthal-angle
dependence, the sample was cleaned by Ar+ sputtering,annealing at 1000 K and annealing in an oxygen
atmosphere of 5 × 10−6 mbar at 1000 K. Finally, the NRS
spectrum of the clean surface was observed with anincident angle parallel to the [1-10] axis. After the NRS
experiments, the CEMS spectrum was observed in air atroom temperature using a 57Co source with the incidence
angle parallel to the surface normal. A sample withnatural abundance of 57Fe was also used for comparison
3 Results and Discussion
Figure 1(a) and (b) shows the CEMS of natural and 57Feenriched samples. The incident direction was parallel tothe [111] axis. The six lines correspond to the ΔM = 0;±1
excitations between the ground and excited states. The
experimental data also shows two sites with different
Zeeman splits. These sites correspond to the Fe(A) andFe(B) sites [21]. The estimated internal magnetic fields
were 48.6 and 45.5 T for the Fe(A) and Fe(B) sites,
respectively. Also, the line intensity ratios weresignificantly different from that of the non-magnetized
powder sample shown in figure 1(c). This is due to themagnetic anisotropy specific to the (111) surface. The
averaged angle  formed by the surface magnetization
axis and the incident -ray direction was calculated using
equations (1)-(3) [22]. For the 57Fe-enriched sample, theangles were 104◦ (A-site) and 101◦ (B-site). For thePhoton Factory Activity Report 2016 #34 (2017) B

natural sample,  was 110◦(Asite) and and 106◦(B-site).
These values are close to the angle formed by four 111
axes, which means that only the in-plane ( 111 ,111 and 111 ) magnetization axes are dominant on the(111) surface. The decrease in the angle after deposition
might indicate that the magnetization is tilted even moreparallel to the surface at the near-surface region.

FIG. 2: Relation of the magnetization direction(m) with SR polarization and quantum-beatfrequencies.

FIG. 1: Conversion electron Mossbauer spectrum(CEMS) of (a) natural Fe3O4 (111) surface, (b) 1 nm
57
Fe3O4 on Fe3O4 (111) surface and (c) powder Fe3O4.
The lines show the spectral components deduced fromthe ftting result. The background signal represented by
the red line is derived from the sample holder.
W1 = W6 = 3(1 + cos2 θ)=2 (1)W3 = W4 = (1 + cos2θ)=2 (2)W2 = W5 = 2 sin2θ (3)W1−6 : relative line intensities
Figure 3(a) and (b) shows the time spectra of nuclear
resonant X-ray and the corresponding frequency spectra
for air-exposed surface taken at various azimuthal angles.
The relationship between the frequency component andthe magnetization axis is shown in figure 2. The
intensities of the peaks at 170 and 182 MHz reflect themagnetization parallel to the electric field of the incident
X-ray [23-25]. The intensities of the peaks at 170 and 182
MHz have weak dependence on the azimuthal angle,
which suggests that the domains magnetized along three
in-plane magnetization axes ( 111 , 111 and 111 ) areequally dominant on the surface. Figure 4 shows the time
spectrum taken after the cleaning procedure with anincident direction of [10 1 ]. The solid curve shows afitting result. The fitting function is written asI tFIG. 3: Time spectra of nuclear resonant X-ray(a) and corresponding frequency spectra (b)observed at,,,,/s : Fe sites (Fe(A),Fe(B)): Magnetization axis 111 , 111 , 111 and 111
Here,

,

is the Fourier transformation of nuclear

is the timescattering length matrix element andis the fitting parameter, which isconstant.
,proportional to the number of atoms of site s withmagnetization axis
. Also, from the absence ofazimuthal-angle dependence of the time spectra, weassume,,,Table I shows the parameter , obtained by fitting. This
result indicates that the perpendicularly magnetizeddomain is absent in the near-surface region. Also, theFe(B)/Fe(A) intensity ratio was 1.87, which is consistent
with the bulk composition. These results also show thatthe perpendicularly magnetized domain is absent on
Fe3O4 (111) surface. The most possible cause is themagnetostatic effect, which tend to rotate themagnetization parallel to the surface to form closuredomains [26-27]. The decrease in angle  after deposition

Photon Factory Activity Report 2016 #34 (2017) B

TABLE I: Parameter , obtained by fitting the time
spectrum of clean surface.

might indicate that magnetization even rotates towards
the surface-parallel direction at the near-surface region.
This may be due to the magnetostatic energy, which tend
to rotate the magnetization to the surface-parallel
direction. The size of this closure domain is several tensof micrometers [27], which is much larger than theprobing depth of CEMS (about 100 nm).
4 Conclusion
We have studied the magnetization of the Fe3O4 (111)
surface with CEMS and NRS. The results show that theperpendicularly magnetized domain is absent on theFe3O4 (111) surface. The absence of azimuthal-angle
dependence in NRS spectra indicates that domains
magnetized along the other three magnetization axes
( 111 , 111 and 111 ) exist equally. At the nearsurface region, magnetization rotates from the original
111 axes to the surface-parallel direction. These can beattributed to the effect of magnetostatic energy.
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FIG. 4: Experimental data and the ftting
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