Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgOstudied by angular-dependent x-ray magnetic circular dichroism
J. Okabayashi, J. W. Koo, H. Sukegawa, S. Mitani, Y. Takagi, and T. Yokoyama
Citation: Applied Physics Letters 105, 122408 (2014); doi: 10.1063/1.4896290
View online: http://dx.doi.org/10.1063/1.4896290
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APPLIED PHYSICS LETTERS 105, 122408 (2014)

Perpendicular magnetic anisotropy at the interface between ultrathin Fe filmand MgO studied by angular-dependent x-ray magnetic circular dichroism
J. Okabayashi,1 J. W. Koo,2,3 H. Sukegawa,2 S. Mitani,2,3 Y. Takagi,4 and T. Yokoyama4
1

Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan
3
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
4
Institute of Molecular Science, Okazaki, Aichi 444-8585, Japan
2(Received 20 July 2014; accepted 4 September 2014; published online 26 September 2014)
Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigatedusing angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic
orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies,
whose values depend on the annealing temperature. The large PMA energies determined frommagnetization measurements are related to those estimated from the XMCD and the anisotropic orbital
magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital
magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896290]
V
Perpendicular magnetic anisotropy (PMA) is an important topic in spintronics research because of the advantagesof thermal stability enhancement and the low current magnetization switching, which allow the development of lowpower-consumption operation in spintronics devices such as
magnetoresistive random access memories (MRAMs). Since
the discovery of large tunnel magnetoresistance in singlecrystal MgO-based magnetic tunnel junctions,1 the interfaces
between MgO and magnetic layers have been investigatedthoroughly. Recently, large PMA energies of 0.21 MJ/m3
have been developed by utilizing the interfaces between
MgO and CoFeB transition metal alloys2 that are comparable
to the PMA in Co/Pt multilayers.3 This finding has initiatedthe development of high-performance MRAM using theinterface PMA. For a fundamental understanding of theinduction of PMA at the interface between a ferromagnetic
layer and a MgO barrier layer, the electronic and magnetic
structure of the interface between an ultrathin Fe layer andMgO must be clarified explicitly from the viewpoint of band
dispersions.4,5 This would also provide an explanation forthe origin of PMA at the interface between FeCo-based
alloys and MgO.6 Density functional theory (DFT) calculations revealed that a larger PMA appears at the interface
between Fe and MgO than that of Co/MgO interfaces
because of the difference in 3d orbital occupancies.7 It has
also been reported that the oxygen stoichiometry at the interface between Fe and MgO strongly depends on the PMA
energies; namely, the over- and under-oxidization at theinterface reduces the PMA. Experimentally, Koo et al.8
found that the PMA energies depend on the interface conditions and are controllable by the post-annealing process at
the ultrathin 0.7-nm-thick Fe/MgO interface. There areseveral experimental reports about the amplitude of PMA
energies,8–10 and the maximum interfacial PMA is reported
as 2.0 mJ/m2.8 The values of PMA are of the same order ofmagnitude as those estimated by theoretical calculations. In
order to investigate the PMA energy (Keff [unit in J/m3]), it
is necessary to evaluate microscopically the orbital magnetic
moments along parallel and perpendicular directions to the0003-6951/2014/105(12)/122408/5/$30.00

surface. Spin-orbit coupling is an essential factor concerning
the origin of the interface PMA. The model proposed by
Bruno for the quantitative estimation of the PMA energies
through the second-order perturbation for the spin-orbit
interaction is related to the anisotropic orbital magnetic
moments in parallel and perpendicular to the surface.11,12
Detailed investigations of the interface PMA in Fe/MgO systems from the viewpoint of the orbital magnetic moments
are required for the understanding of the fundamental
physics of the interface PMA. Furthermore, the interpretationof recent extensive investigations of electric-field-dependent
modulations in PMA theoretically13–16 and experimentally17
necessitates an explicit clarification of the origin of PMA in
Fe/MgO interfaces.
In order to investigate the orbital magnetic moments,
x-ray magnetic circular dichroism (XMCD), associated with
spectral analysis using magneto-optical sum rules,18–20 constitutes a powerful technique. XMCD studies for Fe/MgO
systems have been performed in the past.21,22 However, theorigin of the large PMA in Fe/MgO has not yet been established because of the necessity of a large saturation magnetic
field along the hard magnetization axis. Angular-dependent
XMCD enables us to deduce the anisotropic orbital magnetic
moments and investigate the large PMA at the Fe/MgO interface. In particular, Fe/MgO systems with different annealing
conditions, which give rise to different PMA values, can provide the interpretation of the relationship between anisotropic orbital magnetic moments and PMA energies.
In this paper, we report the anisotropic interface orbital
magnetic moments of ultrathin Fe facing a MgO layer byusing angular-dependent XMCD. Additionally, we discuss the
PMA energies deduced from the orbital magnetic moments
and the interfacial electronic and magnetic structures from theviewpoint of orbital magnetic moments.
Samples were grown by using an ultra-high vacuum
electron-beam evaporation on MgO (001) substrates. The
sample structures are shown in Fig. 1(a). After cleaning the
MgO (001) substrate at 1000  C, a 5-nm-thick MgO layer
was deposited on the substrate at 450  C and a 30-nm-thick

105, 122408-1

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Appl. Phys. Lett. 105, 122408 (2014)

FIG. 1. (a) Sample structure with thefilm thicknesses. (b) and (c) The M-Hloops measured by SQUID-VSM after
the post-annealing at (b) 300  C and(c) 450  C.

Cr buffer layer at 150  C. Subsequently, the annealing process was performed at 800  C in order to prepare the flat
surface. A 0.7-nm-thick Fe layer, which corresponds to 5
monolayers (MLs), was deposited on the Cr buffer layer at
150  C and a MgO layer was also grown on the Fe layer at
150  C. After the deposition of the MgO capping layer, twokinds of samples prepared with the above conditions were
post-annealed separately at 300 or 450  C in order to enhance
the PMA. Magnetization curves were measured as functionsof the magnetic fields (M-H) using superconducting quantum
interference devices with a vibrating sample magnetometer(SQUID-VSM) at room temperature. Details of the surface
and interface conditions and the fabrication procedures arereported in Ref. 8.
X-ray absorption spectroscopy (XAS) and XMCD
measurements for Fe L-edges were performed at the
UVSOR BL-4B beamline, Institute of Molecular Science,
Japan, under conditions of 5 K.23 Magnetic fields (Hext) of
65 T were applied using a superconducting magnet along
the incident polarized soft X-rays in order to sufficiently
saturate the magnetization along the direction of the magnetically hard axis. The total electron yield mode wasadopted by detecting the drain currents from the samples.
We changed the magnetic field directions in order to obtainright- and left-hand-side polarized X-rays while fixing
the polarization direction of the incident X-ray. Angulardependent XMCD was performed by rotating the angle
between the incident beam and the direction of the sample’s
surface normal from the surface normal to 60 ; thesegeometries are defined as normal incidence (NI) and grazing incidence (GI), respectively. In the case of the NI
configuration, where both the photon helicity and the magnetic field directions are normal to the surface, the X-ray
absorption processes involve the normal direction components of the orbital angular momentum (morb?). The GI

configuration mainly allows the detection of only the inplane orbital momentum components (morbk).
Figures 1(b) and 1(c) show the M-H curves measured by
SQUID-VSM at room temperature with respect to theapplied magnetic field with the directions parallel and perpendicular to the sample surface. After post-annealing at 300
or 450  C, distinct PMA characteristics were observed withdifferent anisotropic fields in both cases. The saturation magnetization (l0Ms) of 2 T, which corresponds to 2.2 lB for Fe
atom is consistent with the value of bulk Fe. In order to saturate the magnetization along the hard axis, magnetic fields of
62 T are necessary. A coercive field of 0.13 T was observedin the 450  C annealing case. Since the area of the M-Hcurves surrounded between the out-of-plane and in-plane
directions corresponds to the PMA energy, the PMA in thecase of the 450  C annealing was estimated to be larger thanthat at 300  C. For the annealing temperatures of 300 and
450  C, the crystalline magnetic anisotropy energies (Keff [J/m3]) were estimated to be 0.38 and 1.4 MJ/m3, respectively.
Interface anisotropy energies (Ki [J/m2]) are related by thefollowing relationship: Keff ¼ Kv þ Ki/t, where Kv is the volume anisotropy corresponding to the diamagnetic contribution ((l0/2)Ms2) and t [nm] denotes the layer thickness.
Using this relation, the values of Ki were deduced as 1.15
and 2.1 mJ/m2 for 0.7-nm-thick Fe/MgO structures annealed
at 300 and 450  C, respectively. These differences correspond to the interface conditions. The annealing at 450  C is
the optimum condition to obtain large PMA energy.8 It must
be noted that, after the annealing at 500  C, the easy magnetization axis was turned out to the direction parallel to thefilm plane, which might result from the intermixing at theother interface between Fe and Cr layers.
Figure 2(a) shows the XAS of a 0.7-nm-thick Fe/MgO
interface after the annealing at 450  C. The XMCD taken at
the NI and GI geometries and the integrals of the Fe L2,3This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 157.82.8.220
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FIG. 2. (a) X-ray absorption spectra of 0.7-nm-thick Fe/MgO structures foran annealing temperature of 450  C, measured in the NI geometry. (b)XMCD spectra of the NI and GI setups. (c) Integrated XMCD spectra of the
NI and GI setups. Inset of (b) shows the magnetic field dependence at L3
peak in the NI geometry.

absorption edges XMCD spectra are also shown in Figs. 2(b)and 2(c). Distinct metallic peaks are evident in the XAS of
the Fe L2,3 edges, which indicates that no atomically mixed
layer formation with oxygen atoms occurred at the interface,even after the 450  C annealing process. There are clear differences in the XAS spectra between right- and left-handside polarized X-rays and they reveal the XMCD signals.
Since the XAS spectra obtained from the NI and GI setups
were identical, only the XAS spectra in the NI configuration
are shown. In Fig. 2(b), the XMCD spectra in the NI and GI
setups display a distinct difference in the intensity between
the L3 edges while the L2 edges show almost similar intensity. The measured XMCD signal for the Fe L3 edge fromthe NI geometry was larger than that from the GI geometry,
which suggests that the large orbital magnetic moments are
induced when the Hext is perpendicular to the film plane. The
magneto-optical sum rule indicates that the integrated areasof both negative L3 and positive L2 peaks are proportional tothe orbital magnetic moments.18 The residuals of the integrals of both L3 and L2 edges in the XMCD spectra are larger
in the NI configuration than in the GI one, indicating that thelarge orbital magnetic moments remain in the NI setup.
Figure 2(c) shows the integrated XMCD signals of the Fe
L-edges for both NI and GI setups. A difference can beclearly observed in the residuals of the integrals for both L3and L2 peaks. These integrated XMCD spectra indicate that

Appl. Phys. Lett. 105, 122408 (2014)

the large orbital magnetic moments are enhanced in the NI
geometry compared with those in GI one. This is reasonable
for a Fe/MgO interface with a PMA related to the orbital
magnetic moments. In addition, the magnetic field dependence of XMCD at L3 peak from the NI geometry, shown in
the inset of Fig. 2, reveals the large PMA of Fe.
Using the magneto-optical sum rules for the estimationof orbital and spin magnetic moments, we list the results forboth the 300 and 450  C annealing cases with the NI and GI
geometries in Table I. The effective spin magnetic moments(mspin) values were determined only from the GI geometry
because the magic angle geometry of 57.3 from the surface
normal can theoretically neglect the magnetic dipole terms.24
For the application of the sum rules, we assumed the holenumbers of the Fe 3d states to be 3.4 as a standard value of
Fe bulk.25 For the 450  C annealing case, the results listed inTable I show that the orbital magnetic moments with morb?
of 0.30 and morbk of 0.21 lB were calculated. In contrast, forthe 300  C annealing case, the difference in the orbital magnetic moments (Dmorb ¼ morb?  morbk) is reduced comparedto that in the case of 450  C. Next, considering the Bruno
relationship: K ’ (n/4) a Dmorb, where n is the spin-orbit
coupling constant, and a is the band-structure parameter, thePMA energies are proportional to Dmorb. For the 450  Ccase, we obtained K ¼ 130 leV/atom, which corresponds to a
PMA value of 1.48 mJ/m2, assuming a Fe lattice constant of
0.287 nm with a body-centered-cubic structure facing the
MgO at the interface. For the 300  C annealing case, the difference in orbital magnetic moments is smaller than that in
the case of 450  C annealing. In the former case, a K of
77 leV/atom was estimated, which is smaller than that calculated in the latter case. These values are comparable to theresults obtained by SQUID-VSM, shown in Fig. 1, although
the magnetization measurements were performed at room
temperature. It must be noted that magnetization measurements at low temperatures are difficult owing to the strong
diamagnetic contribution from the large volume of the MgOsubstrates, which interrupts the intrinsic magnetic contribution from the thin Fe layers. Therefore, the element-specific
XMCD measurements in the Fe L-edges constitute a unique
technique for probing the magnetism of ultrathin Fe layers
on MgO. The VSM results suggesting that the 450  C annealing process enhances the PMA at the 0.7-nm-thick Fe/MgO
interface are consistent with those obtained by XMCD andthe subsequent analysis, which indicates that the contribution

TABLE I. Spin and orbital magnetic moments estimated from the XMCD
sum rules for Fe at the Fe/MgO interface with annealing at 300 and 450  C.
The in-plane (morbk) and out-of-plane (morb?) components are listed in units
of lB. The PMA amplitudes obtained from XMCD in units of mJ/m2 arealso shown with those from SQUID-VSM.8
300  C

Annealing temperaturemspin [lB]morb [lB]Ki XMCD (KXMCD)Ki VSM450  C

m?[lB]

mk[lB]

m?[lB]

mk[lB]

…
0.24

2.07
0.19

…
0.30

2.08
0.21

0.86 mJ/m2 (77 leV/Fe) 1.48 mJ/m2 (132 leV/Fe)
1.19 mJ/m2
2.01 mJ/m2

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Okabayashi et al.

of the anisotropic orbital magnetic moments is essential forthe appearance of PMA at the Fe/MgO interface.
Here, we discuss the origin of the PMA at the interfaceof Fe/MgO. Since the excitation processes in XMCD are
regarded to be the atomic excitations from the core to unoccupied states, one can estimate the magnetic anisotropy
energy per atom through the Bruno’s relation by using
the element-specific orbital magnetic moments. Since thediamagnetic and shape-anisotropic components of the magnetic anisotropy energy do not depend on the anisotropic orbital magnetic moments, the interface PMA values in unitsof J/m2 were directly estimated. The contribution of Keffshould be calculated by using the diamagnetic components
and the ultrathin Fe layer thickness, which is the same process
as the estimation of PMA values using DFT calculations.
Next, we discuss the PMA values obtained by XMCD
and the comparison with other experimental and theoreticalworks reported in the literature. The DFT calculations
determine a magneto-crystalline anisotropy energy of
0.2 meV/atom in a free-standing Fe with 1 ML in thickness,a value of 0.9 meV/atom at the Fe (1 ML)/MgO interface,and 1.5 meV/atom at the MgO/Fe (1 ML)/MgO sandwiched
structures.16 These results suggest that the PMA of the Fe/
MgO interface is enhanced with a double-facing interface.
Considering that the obtained PMA value of 0.132 meV/(Fe atom) through the XMCD measurement for Fe (5 MLs)/
MgO structure after annealing at 450  C, the result issomewhat smaller than the estimated value from the DFT
calculation (0.2 meV/Fe atom).16 It is noted that the value
for morbk can be underestimated, because the angle for the
X-ray incident in the NI configuration was not exactly parallel to the film plane, i.e., 60 .Therefore, the PMA values
can be estimated to be small relative to those from DFT calculations. Nevertheless, the estimated value of PMA
through angular dependent XMCD for Fe (5 MLs)/MgO(001) structure, without an applied external electric field,
was turned out to be comparable to that from the literatureon the modulation of PMA by an electric field.12–17
Considering the above results, the origin of PMA at the
Fe/MgO interface can be described by the anisotropic orbital magnetic moments induced by the spin-orbit interaction at the interface. The anisotropic orbital magnetic
moments, resulting from the modulation of the occupanciesof the Fe 3d states at the interface, were obtained throughthe sum-rule analysis of the XMCD spectra. Figure 3 displays the schematic energy diagram of the Fe 3d states at

FIG. 3. Schematic diagram of the Fe 3d states with the crystal field, surface
field, and spin-orbit interaction.

Appl. Phys. Lett. 105, 122408 (2014)

the Fe/MgO interface. First, the Fe 3d levels split into eg
and t2g states due to the crystal field. At the surface or theinterface accompanied with the symmetry breaking, thedegenerated eg states of the dx2y2 and dz2 orbitals split further because of the surface field and the hybridization with
the O 2pz orbital. The spin-orbit interaction induces further
splitting depending on the direction of the Hext and themagnetic quantum number m (0, 61, and 62). The dyz anddzx orbitals corresponding to m ¼ 1 consist of the complex
orbitals denoted as the dyzþizx and dyzizx states.18 The anisotropy of charge occupancy between these complex states
results in the anisotropic orbital magnetic moments. As was
also indicated in the DFT calculations, the Fe 3dz2 states arepushed up above the Fermi level through the hybridization
with the O 2pz orbital and the charge occupancies are
modulated.7 In addition, only when the Hext perpendicular
to the film plane, the modulation in the electron occupancies results in the enhanced morb?. Therefore, due to theenhancement of morb? resulted from the spin orbit interaction and the hybridization between Fe 3dz2 and O 2pz orbitals, the large PMA can be expected for the Fe/MgO
interface. It is well coincided with our XMCD results for
the NI configuration, which showed enhanced morb?.
In conclusion, we have studied the interface PMA inultrathin Fe/MgO (001) using angular-dependent XMCD. We
found that the anisotropic orbital magnetic moments determined from the analysis of XMCD contribute to the large
PMA energy, whose values depend on the annealing temperature. The large PMA energies deduced from the magnetization
measurements are almost consistent with those estimatedfrom the anisotropic orbital magnetic moments through thespin-orbit interaction. The enhancement of orbital magnetic
moments can be explained by the hybridization between the
Fe 3dz2 and O 2pz states at the Fe/MgO interface.
This work has been performed under the approval of the
UVSOR facility programs at the Institute of Molecular
Science, Japan. This work was partly supported by
Nanotechnology Platform Program (Molecule and Material
Synthesis) and a Grant-in-Aid for Young Scientists (A)(Grant No. 24686007) of Japan Society for the Promotion of
Science (JSPS).
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