PHYSICAL REVIEW B 97, 195405 (2018)

Spin polarization of graphene and h-BN on Co(0001) and Ni(111) observedby spin-polarized surface positronium spectroscopy
A. Miyashita, M. Maekawa, K. Wada, A. Kawasuso,* T. Watanabe, S. Entani, and S. Sakai
National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan(Received 1 February 2018; revised manuscript received 24 April 2018; published 4 May 2018)
In spin-polarized surface positronium annihilation measurements, the spin polarizations of graphene and h-BN
on Co(0001) were higher than those on Ni(111), while no significant differences were seen between grapheneand h-BN on the same metal. The obtained spin polarizations agreed with those expected from first-principles
calculations considering the positron wave function and the electron density of states from the first surface layer
to the vacuum region. The higher spin polarizations of graphene and h-BN on Co(0001) as compared to Ni(111)
simply reflect the spin polarizations of these metals. The comparable spin polarizations of graphene and h-BN on
the same metal are attributed to the creation of similar electronic states due to the strong influence of the metals:
the Dirac cone of graphene and the band gap of h-BN disappear as a consequence of d-π hybridization.
DOI: 10.1103/PhysRevB.97.195405
I. INTRODUCTION

Graphene is a semimetal with an extremely high carrier
mobility and a long spin relaxation time. Hence graphene is
expected to be a good spin conductor [1]. An ordinary way
to inject excess spins into graphene is to place it in contact
with a ferromagnet. The induced spin polarization is called
the proximity effect. Its physical origin is the modulationof the electronic state of graphene due to hybridization withthe ferromagnet. An alternative method is tunneling injection
through barrier materials. Since single layer hexagonal boron
nitride (h-BN) is an insulator with a band gap of 5–6 eV,
inserting it between the ferromagnet and the graphene may
make tunneling spin injection feasible [2].
Previous work based on spin-polarized metastable-atom
deexcitation spectroscopy (SPMDS) has shown the appearanceof a new electronic state in graphene on Ni(111) that exhibits
spin-polarized metallic conduction [3]. The ferromagnetismof graphene decorated with Co atoms is explained in termsof charge transfer from the Co atoms to the graphene [4].
Co-intercalated graphene shows an out-of-plane spin polarization [5]. SPMDS studies have shown that h-BN on Ni(111)also exhibits spin-polarized metallic conduction arising fromthe in-gap states [6]. A theoretical study has suggested thath-BN on Co(0001) may be metallic, while h-BN on Ni(111) ishalf-metallic [7]. If this is correct, then h-BN attached to metals
would hardly maintain its insulating properties, which would
inhibit tunneling spin injection. It is, therefore, important toinvestigate the spin polarization of graphene and h-BN on
Co(0001) and Ni(111) more systematically.
In this research, we employed spin-polarized surface
positronium annihilation spectroscopy. This method is assured to detect the electron spin polarization of the firstsurface layer [8–11]. The results show that, in both grapheneand h-BN on both Co(0001) and Ni(111), electrons have a*

Corresponding author: kawasuso.atsuo@qst.go.jp

2469-9950/2018/97(19)/195405(5)similar positive spin polarization. First-principles calculations
explain that this similarity between graphene and h-BN isdue to the appearance of similar electronic structures ingraphene and h-BN when they are attached to Co(0001)
and Ni(111).
II. EXPERIMENTIn the sample preparation, first, 30 nm thick films ofCo(0001) and Ni(111) were grown on Al2 O3 (0001) using anelectron beam evaporator. Subsequently, graphene and h-BN
layers were grown on the metal films by ultrahigh vacuum
chemical vapor deposition (CVD) [6,12,13]. As references,graphene and h-BN layers on paramagnetic Ru(0001) werealso prepared in a similar manner. After the growth of grapheneand h-BN, the flatness, morphology, and chemical composition
were checked using reflection high-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy, respectively. The samples were transferred throughair to a vacuum chamber (base pressure: 1 × 10−7 Pa) equippedwith a positron beam apparatus. The samples were cleaned byheating at 1000 K for 30 min. In this treatment, most of theadsorbates, mainly carbon hydrides and oxides, were removed.
Figure 1(a) shows the schematics of the measurement
system. Transversely spin-polarized positrons with an energy
of 200 eV were injected into the sample. Some of these
positrons return to the surface and are emitted into the vacuum
as positronium (an electron-positron bound state) by picking
up surface electrons. Spin-singlet (S = 0) positronium annihilates into two photons, while spin-triplet (S = 1) positronium
annihilates into three photons. Two-photon events create asharp peak at 511 keV (=m0 c2 , where m0 is the electron rest
mass and c is the speed of light). Three-photon events giverise to a continuous spectrum below 511 keV. Figure 1(b)shows the typical annihilation photon energy spectra for 0%(mica at room temperature) and 100% (Ge at 1000 K) positronium formation. The spectrum intensities are normalized to
the 511 keV peak. Denoting the intensities of the shaded

195405-1©2018 American Physical Society

A. MIYASHITA et al.

PHYSICAL REVIEW B 97, 195405 (2018)

12keV e+ (a)

HV

R
R
HPGe

200eV

Coil

Counting rate (arb. units)Reight spectra were acquired: four spectra were recorded forthe positive direction and four for the negative direction. In
each spectrum, total counts of 4 × 105 were accumulated.
Subsequently, the electron spin polarization was obtained using
Eq. (2). For each sample, the above protocol was repeated morethan four times. The final spin polarization was determined byaveraging these measurements.

(b)

IPs=100% Ge@1000K
Intensity P

IPs=0%
Mica@RT

Intensity R

III. THEORETICAL CALCULATION

SampleA

300

350 400 450 500
Photon energy (keV)

FIG. 1. (a) Schematic diagram of the experimental setup. A
positron beam with an energy of 12 keV is decelerated to 200 eV
and injected into the sample. Some of the positrons reach thesample surface and are emitted as positronium into the vacuum.
Annihilation photons are observed by a Ge detector. The positron
beam is transversely spin polarized with P+ = 0.3. The sample isin-plane magnetized by a Helmholtz coil with a field strength of
30 mT. (b) Energy spectra of annihilation photons for 0% (mica atroom temperature) and 100% (Ge at 1000 K) positronium formation.
The intensities are normalized to the 511 keV peak intensity. The
shaded areas define the intensities R and P in Eq. (1).

areas in Fig. 1(b) as R and P , the positronium intensity
is given by
P100% R100% − R −1
IP s = 1 +,(1)
P0% R − R0%
where the subscript of 100% or 0% denotes positronium
intensity [14–16]. Though IP s is between zero and 1, the actual
positronium three-photon intensity is given, for instance, bymultiplying by 3/4 for a spin-compensated system. The threephoton intensity is higher (lower) when positron and electron
spins are parallel (antiparallel). Based on this, measuring thefield-reversal asymmetry of positronium intensity gives thesurface spin polarization (P− ) asP− =1 IP s (−M) − IP s (+M),
αP+ IP s (−M) + IP s (+M)(2)

where P+ (=0.3) is the positron spin polarization, α (= 0.6)
is the coefficient determined from the spin-dependent detection
efficiency of annihilation photons [17], and ±M indicates
whether the magnetization direction is parallel (+) or antiparallel (−) to the positron spin polarization. From Eq. (2), positive(negative) P− indicates that the number of majority (minority)
spin electrons is more than that of minority (majority) spinelectrons.
The ferromagnetic layers were in-plane magnetized by amagnetic field of 30 mT. All the measurements were conducted
in remanence. To avoid any artificial effects of the magnetic
field, the direction of the magnetic field was always fixed(in the positive or negative direction) and the magnetization
direction with respect to the positron spin polarization direction
was changed by rotating the sample. The sample holder was
made of only nonferromagnetic materials. In each experiment,

To interpret the experimental data quantitatively, density
functional theory (DFT) calculations were carried out using
ABINIT code [18] with the projector-augmented-wave method[19] within the generalized gradient approximation (GGA)[20]. The initial valence electron configurations were assumed
to be 3s 2 3p6 3d 7 4s 2 (Co), 3s 2 3p6 3d 8 4s 2 (Ni), 2s 2 2p2 (C),2s 2 2p1 (B), and 2s 2 2p3 (N). The Co(0001) and Ni(111) films
were composed of 7 monolayers stacked in the surface normal
direction and the primitive cell was aligned parallel to thesurface. For graphene, C atoms were distributed to on-top andfcc positions of the Co(0001) and Ni(111) surfaces. For h-BN,
B and N atoms were distributed to fcc and on-top positions,
respectively, of the Co(0001) and Ni(111) surfaces. The backsurfaces were treated in the same way to avoid any artificial
effects arising from asymmetry. The van der Waals potential
was also considered [21]. The vacuum layer was initiallyassumed to be 20 Å. For the electron calculation, the number ofk points sampled was 9 × 9 × 1. For the positron calculation
only the  point was considered. Full structural optimization
was carried out in all the calculations.
IV. RESULTS AND DISCUSSION
A. Experiment

In all samples, the positronium intensity was found toincrease from ∼30% before heat treatment to more than
60% after heat treatment. Also, no spin polarizations wereobserved without heat treatment. Figure 2 shows the spinpolarizations obtained for all the samples. Here, Eq. (2) isrewritten as P− = P− (+M) − P− (+M) and the values ofP s (−M)+IP s (+M)]/2
P− (±M) = αP1 + IP s (±M)−[Iare plotted in the
IP s (−M)+IP s (+M)
figure. The average spin polarizations are shown below eachpanel. After heat treatment, finite spin polarizations appear
for graphene and h-BN on Co(0001) (+3–+4%) and Ni(111)(+1–+2%). No spin polarization is seen for either graphene orh-BN on paramagnetic Ru(0001). Thus the spin polarization
is induced from Co and Ni to graphene and h-BN after heat
treatment, even though the samples have been once exposed toair. In addition, the induced spin polarizations were persistently
observed after heat treatment for at least 4 days. This meansthat Co and Ni surfaces covered by graphene and h-BN arequite inert.
Figure 3 shows the temperature dependence of spin polarization for graphene and h-BN on Co(0001) and Ni(111).
Spin polarizations for graphene and h-BN on Co(0001) areunchanged up to 700 K, while those for graphene and h-BN
on Ni(111) disappear at 700 K. Thus the spin polarizations ofgraphene and h-BN are lost above the Curie temperature forthe ferro- to para-magnetic transition.

195405-2

SPIN POLARIZATION OF GRAPHENE AND h-BN ON …

4

Graphene/Co(0001)/Al2O3(0001)

Graphene/Ni(111) Graphene/Ru(0001)/Al2O3(0001)/Al2O3(0001)

Heat-treatedmeas.@RT

No heat-treated

3 meas.@RT

PHYSICAL REVIEW B 97, 195405 (2018)

Heat-treatedmeas.@RT

Heat-treatedmeas.@RT

P- (%)

2
1
0

-1
-2
-3
-4

-M

+M

-M

+M

-M

+M

-M

+M

P-= +0.5(7)% P-= +3.4(7)% P-= +1.4(3)% P-= +0.1(2)%43h-BN/Co(0001)/Al2O3(0001)
No heat-treatedmeas.@RTh-BN/Ni(111)/Al2O3(0001)
Heat-treatedmeas.@RT

Heat-treatedmeas.@RTh-BN/Ru(0001)/Al2O3(0001)
Heat-treatedmeas.@RT

positronium are positively spin polarized (majority spins are
more abundant), (iii) the spin polarizations of graphene and hBN on Co(0001) are higher than those on Ni(111), and (iv) the
spin polarizations of graphene and h-BN are nearly the sameon the same metal. From (i) we conclude that the magnetic
proximity effect may exist. In (ii), the obtained signs of the spinpolarization seem to be consistent with those in the SPMDS
experiments [3,6]. Positrons may pick up electrons located not
only at the Fermi level but also at deeper levels, i.e., fromEF + P s to EF , where P s is the positronium work function(∼ − 3 eV for Co and Ni). Hence the spin polarization obtainedby the present experiment is expected to be the average in thisenergy range. From (iii) and (iv) it is inferred that the electronic
states of graphene and h-BN are strongly dominated by themetal characters. To confirm these speculations, we made thefollowing first-principles calculations.

P- (%)

2
1

B. Theoretical calculations

0800

After structural optimization, the interlayer distance between the graphene and Co(0001) or Ni(111) layer was 2.10 Å
or 2.09 Å. The buckling between two inequivalent C atoms was
0.019 Å on Co(0001) and 0.007 Å on Ni(111). The interlayer
distances between the h-BN and Co(0001) layers was 2.07 Å
and that between the h-BN and Ni(111) layers was 2.06 Å.
The buckling between B and N atoms was 0.12 Å on Co(0001)
and 0.11 Å on Ni(111). N and B atoms are shifted outward
and inward, respectively. These values are consistent with theprevious report [22].
Figure 4 shows the electron density of states [denoted by
DOS or D(E)] calculated through the ABINIT computation
itself. For isolated graphene, the DOS profile reflects the Diraccone at the K point. A band gap of 4.6 eV was obtained for
isolated h-BN, although underestimation is a common problem
in DFT calculations. In graphene on Co(0001) and Ni(111), theDirac conelike DOS profile remains above the Fermi level, atleast partially, but the profile below the Fermi level is strongly
modulated due to d-π hybridization. As for h-BN on Co(0001)
and Ni(111), the band gap completely disappears and the
profile of the valence band is also significantly changed, again
due to d-π hybridization. Thus h-BN attached to Co or Ni is
expected to become metallic.
As mentioned above, when positronium is formed via thework function mechanism, positrons can capture electrons
located from EF to EF + P s . Although the electron pick-up
probability may be estimated in terms of the golden rule approach considering the wave functions of electrons, positrons,
and positronium and also the effective Coulomb potential, we
first assume that it is proportional to the density of statessince the positronium time-of-flight (energy) spectrum may
be approximated as the surface density of states [23]. In thiscase, the spin polarization would be given by ↑{D (E) − D ↓ (E)}dE
P− =  ↑,(3){D (E) + D ↓ (E)}dE

FIG. 3. Temperature dependence of spin polarizations obtainedfor graphene and h-BN on Co(0001) and Ni(111). The broken lines
are guides for the eye.

where ↑ (↓) denotes the majority (minority) spin channel
and the integration range may be from EF + P s up to
EF . However, the spin polarizations obtained using the DOS
profiles shown in Fig. 4 did not give results that systematically

-1
-2
-3
-4

-M

+M

P-= -0.1(6)%

-M

+M

-M

+M

-M

+M

P-= +4.0(3)% P-= +2.1(2)% P-= -0.0(2)%FIG. 2. Spin polarizations obtained for all the samples. The
vertical axis is the shift of spin polarizations for positive (+M) andnegative (−M) magnetizations (see text). The net spin polarizations[P− (−M) − P− (+M)] are indicated below each panel.

The above results can be summarized as (i) the observed
spin polarizations are induced from the ferromagnetic Co
and Ni to graphene and h-BN, (ii) electrons detected as

8Spin polarization, P - (%)

7
6

Graphene/Co(0001)/Al2O3(0001)
h-BN/Co(0001)/Al2O3(0001)
Graphene/Ni(111)/Al2O3(0001)
h-BN/Ni(111)/Al2O3(0001)

5
4
3
2
1
0
-1

300

400
500
600
700
Measurement temperature (K)195405-3

A. MIYASHITA et al.

0.3

PHYSICAL REVIEW B 97, 195405 (2018)

Graphene(a) 0.04 Graphene/Co(0001)

h-BN

e-p DOS (electrons/positron/eV)

0.2
0.1
0.0
-0.1
-0.3
0.3

Graphene/Co(0001)

h-BN/Co(0001)

0.2
0.1
0.0
-0.1
-0.2

0.02
0.00
-0.02
-0.04
0.04

h-BN/Ni(111)

0.00
-0.02
-0.04

-0.3
0.3

Graphene/Ni(111)

0.02

-4

Graphene/Ni(111)h-BN/Ni(111)

0.1
0.0
-0.1
-0.2
-0.3
-4

-2

0

2

4 -4

-2

0

2

-3

-2

(b)

0.2

4

E-EF (eV)Spin polarization, P- (%)

DOS(electrons/atom/eV)-0.2

h-BN/Co(0001)

explain the experimental data. The density of states of Fig. 4 issampled only around atoms in spheres with typical diametersof 0.5 to 1 Å. For a better approximation, we calculated
the electron-positron density of states (e−p DOS), which isdefined here as the integration of the product of the electron

FIG. 5. Cross-sectional electron-positron density distributions
calculated for Co, graphene/Co, and h-BN/Co on the (11̄00) plane
and Ni, graphene/Ni, and h-BN/Ni on the (112̄) plane. The unit of thecolor bar is aB−6 . White circles are Co and Ni atoms and gray and blue
circles are C and N atoms, respectively. B atoms are not shown on
these planes.

0 -4 -3
E-EF (eV)

-2

-1

0

Graphene/Co(0001)
h-BN/Co(0001)
Graphene/Ni(111)h-BN/Ni(111)

50

0
10

-50

5
Ps: Co0-4.0

FIG. 4. Electron density of states (DOS) profiles calculated forgraphene and h-BN in isolated form, on Co(0001) and on Ni(111).

-1

-3.5

Ps: Ni

-3.0

-2.5

-4
-3
-2
-1
0
Lower energy of DOS integration from EF (eV)FIG. 6. (a) Electron-positron density of states (e−p DOS) calculated for graphene/Co(0001), h-BN/Co(0001), graphene/Ni(111), andh-BN/Ni(111). (b) Electron spin polarization calculated from Eq. (3)and the above electron-positron densities of states as a function of thelower energy of integration. The inset is a magnification of the region
around the positronium work function.

density of states and the positron density at arbitrary positionsin the total space, as follows.
The electron density of states at arbitrary positions wascomputed from the ABINIT outputs. In the calculation of
positron density, any surface potentials and the enhancement
factor based on the weighted density approximation [24]were not assumed, since in the work function mechanism
positronium will be formed when positrons propagate in thevacuum side of the surface. That is, the positron state may bedescribed as a plane wave or a scattering state in the surface
potential, but not in the surface state, while the electron density
decreases to less than ∼1/100 from the first surface layer to∼2 Å vacuum side. Therefore, most simply, it may be enoughto obtain the positron density which varies slowly but reflects
the atomic configuration in such a thin layer. Here, we used thepositron wave function in the vacuum region calculated using
the updated parameter-free GGA enhancement factor [25].

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PHYSICAL REVIEW B 97, 195405 (2018)

Figure 5 shows the calculated electron-positron densities.
(The positron density was renormalized to the integrated density in the region where the electron-positron density is above
1% of the maximum.) From this, electrons from the first surface
layer to 1–2 Å vacuum side are sampled by positrons. This
picture is basically the same as that expressed by the g function
which was introduced for evaluating the matrix element andhas a peak in the vacuum side of the surface [23]. Fromthe viewpoint that positronium is not formed inside metals
due to the screening of high density electrons, the present
calculation may not give rise to serious problems. But, innerelectrons will also be able to hop onto positrons in the vacuum
region to form positronium when the Coulomb interaction acts
effectively. If this process occurs with high enough probability,
the present treatment underestimates the contribution frominner electrons. The evaluation of this process remains for the
future. Nevertheless, the experimental spin polarizations arewell reproduced, as shown below.
Figure 6(a) shows e−p DOS profiles for graphene andh-BN on Co(0001) and Ni(111). Since positrons mainly exist
in the vacuum region, the DOS profiles are reduced comparedwith the conventional DOS profiles shown in Fig. 4. Figure 6(b)shows the spin polarizations obtained using Eq. (3) and the
DOS profiles of Fig. 6(a) as a function of the lower energy ofintegration from the Fermi level. Above −3 eV, the calculated
spin polarization varies between positive and negative values
and the magnitude is much greater than the experimental
values. Hence, only considering such an energy region,
the experimental data are hardly explained. In the present

calculations, P s = −3.6 eV for Co and P s = −3.2 eV for
Ni. Spin polarizations at around these energies are +6–+7%
for graphene and h-BN on Co(0001) and +4–+5% forgraphene and h-BN on Ni(111). These values are only a few% larger than the experimental values in Fig. 2 and hence
explain the experimental magnitude of the spin polarization.
The other two experimental facts—that the spin polarizationsof graphene and h-BN on Co(0001) are higher than those on
Ni(111) and that the spin polarizations of graphene and h-BNon the same metals are comparable—are also explained by
the above calculation.

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V. SUMMARY

The spin polarizations for graphene and h-BN on Co(0001)
and Ni(111) observed by spin-polarized surface positronium
annihilation measurement were well explained by the present
DFT calculation. Conversely, a series of features predicted in
the calculation, such as the destruction of the Dirac cone ofgraphene and the disappearance of the band gap of h-BN due
to hybridization with the metals, may be justified. Grapheneand h-BN are spin polarized by contacting them to Co and Ni,
but instead their original characteristics are lost significantly.
If this is a problem when realizing graphene-based devices,
some means to avoid it needs to be established.
ACKNOWLEDGMENT

This work was financially supported by JSPS KAKENHI
under Grant No. 17K19061.

195405-5