PHYSICAL REVIEW B 78, 212409 共2008兲

Nonlinear optical detection of a ferromagnetic state at the single interface of an antiferromagnetic
LaMnO3 Õ SrMnO3 double layer
Naoki Ogawa*Research Center for Advanced Science and Technology (RCAST), The University of Tokyo,
Komaba, Meguro-ku, Tokyo 153-8904, Japan

Takuya Satoh
Institute of Industrial Science (IIS), The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8904, Japan

Yasushi Ogimoto† and Kenjiro Miyano
Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8904, Japanand CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
共Received 8 November 2008; published 23 December 2008兲
Emergence of a ferromagnetic state at the single interface of two antiferromagnetic perovskite manganites,
LaMnO3 and SrMnO3, has been detected with nonlinear magneto-optical Kerr rotation. The samples arecomposed of approximately four unit cells of each material, with reversed stacking sequence. The existence
and orientations of the interface dipole or that of so-called toroidal moment were determined through phasesensitive second-harmonic detection. Characterization of multiferroic states at the single interfaces, and nonnegligible effect of surfaces on the Kerr rotation are demonstrated.
DOI: 10.1103/PhysRevB.78.212409

PACS number共s兲: 75.70.Cn, 78.20.Ls, 42.65.Ky

Surfaces and interfaces have physical properties distinct
from those of bulk due to their broken symmetries. Engineering and/or spontaneous emergence of unique material properties at interfaces of perovskite oxides became considerably
important in modern electronics. Atomically sharp interfacesof these oxides can be readily fabricated by state-of-the-art
thin-film growth techniques. In these artificial materials,
electronic 共orbital兲, magnetic, and atomic reconstructions
with a scale of single atomic layer are driven by chargetransfer, exchange interactions, and strain effects, leading to
exotic electronic states bound to the interface. Recent examples are highly conducting and superconducting layers between insulating oxides LaAlO3 / SrTiO3 共Refs. 1 and 2兲 andemergence of ferromagnetism at the interface of antiferromagnetic 共AF兲 insulators LaMnO3 / SrMnO3 共Ref. 3兲 or at theinterface of AF insulator CaMnO3 and paramagnetic metal
CaRuO3.4 Besides, magnetism at the interfaces has anotherimportant aspect as an essential ingredient to achieve functional multiferroic properties.5
Optical second-harmonic generation 共SHG兲 is a versatileand nondestructive probe to study buried interfaces.6 Usually, magnetization alone does not break the space-inversion
symmetry. However it can lower the symmetry of the interface, leading to the modification of the nonlinear susceptibility tensor.7 For example, magnetization-induced secondharmonic generation 共MSHG兲 has been utilized to study
surface and interface magnetism.8,9 In addition to the interface sensitivity, MSHG shows large nonlinear Kerr
rotation,10,11 whose origin has been ascribed to the simultaneous breaking of inversion 共at interface兲 and time-reversal
共by magnetization兲 symmetries. Recently, SHG is also successfully applied for detecting toroidal moments in multiferroic materials,12,13 proving its high sensitivity to electronic
and magnetic symmetries.
There have been several reports on the detection of elec1098-0121/2008/78共21兲/212409共4兲

tronic reconstructions at the interface of manganese oxides
through linear14,15 and nonlinear13,16 optics. To enhance signal intensity, however, “tricolor” superlattices have been employed in most of the cases. The unavoidable presence of thetwo additional interfaces in these samples, whose effects
cannot be separately measured, makes unambiguous interpretation difficult. A simpler characterization of a single isolated interface has been awaited.
In this Brief Report, we demonstrate the detection of ferromagnetism or breaking of time-reversal symmetry at thesingle interfaces of AF insulators LaMnO3 共LMO兲 and
SrMnO3 共SMO兲 by means of nonlinear magneto-optical Kerr
effect 共NMOKE兲. These compounds are the end materials ofa prototypical strongly correlated electron system,
La1−xSrxMnO3 共LSMO兲.17,18 Here LMO has one eg electron
per Mn site 共d4兲, which is expected to form alternating alignment of d3x2−r2 and d3y2−r2 orbitals, while SMO has no eg
electron 共d3兲. In terms of magnetism, LMO and SMO are
A-type and G-type AF insulators, respectively. At these heterointerfaces, charge transfer, due to the difference in thechemical potential, is correlated with the spin and orbital
degrees of freedom, resulting in the formation of twodimensional ferromagnetic layers as have been reported bymany groups.3,14,19–23 Since space-inversion and timereversal symmetries are broken simultaneously, we have additional second-harmonic 共SH兲 components induced by toroidal moment.12,24 We also show that the direction of chargetransfer or the orientations of the interface dipole and toroidal moments can be inferred from the measurements of relative phase of the nonlinear susceptibilities.
We prepared two samples having the same heterointerfaces,25 with reversed stacking sequence, LMO on SMO
and SMO on LMO, on 共LaAlO3兲0.3共SrAl0.5Ta0.5O3兲0.7
共LSAT兲共001兲 substrates by pulsed laser deposition 共for details, see Ref. 26兲. Both materials are approximately four unit

212409-1©2008 The American Physical Society

PHYSICAL REVIEW B 78, 212409 共2008兲

BRIEF REPORTS(a)(b)
SMO/LMO/LSAT
1

P

10 K
30 K

-1

0

0.1

(b)

0.2

Magnetic field (T)

1.0

LMO/SMO/LSAT x 10

0.5

p-in - p-out
0
0

100

SMO/LMO/LSAT

200

-B

+B

LMO/SMO/LSAT
-0.1

T
M

300

Temperature (K)FIG. 1. 共Color online兲 共a兲 A schematic of spin and orbital configurations at the interface of antiferromagnetic LaMnO3 andSrMnO3. At the heterointerface, a ferromagnetic state is expected.
共b兲 In-plane M-H loops for the fabricated LMO/SMO and SMO/LMO interfaces at 10 and 30 K. 共c兲 Temperature dependence of the
SH intensity for pin-pout geometry. The data from LMO/SMO are
multiplied by 10.

cells 共⬃1.6 nm兲 in thickness, as controlled by monitoring
the intensity oscillation of reflection high-energy electron
diffraction 共RHEED兲. These samples were mounted in an
ultrahigh vacuum 共UHV兲 cryostat, and SHG was measured
with 800 nm fundamental light 共150 fs duration at 1 kHz
repetition rate兲 incident through a ␭ / 2 plate with 90° reflection geometry. The generated SH was directed to a glanprism, color filters, and a monochromator and detected witha photomultiplier tube. The SH signal was normalized bythat of a reference KDP crystal and accumulated more than
104 times at each polarization configuration. The magnetic
field was applied in plane 共关100兴兲 of the samples with permanent magnets 共up to ⬃0.1 T 关Fig. 2共a兲兴兲.
The spin and orbital reconstructions at the interface of
LMO and SMO are illustrated in Fig. 1共a兲.14 At the heterointerface, orbital-liquid and ferromagnetic spin alignment are
expected. In fact, both fabricated samples undergo ferromagnetic transition at ⬃150 K and show clear and almost identical hysteresis in M-H curves at low temperature 关Fig. 1共b兲兴,
indicating that both samples have flat interfaces with identical structure except for the stacking sequence.27 The transition temperature and magnetic moment 共⬃1.5␮B / Mn site at
low temperature兲 are consistent with those of previous reports on superlattice samples.14
At the heterointerface of cubic perovskites, 4mm symmetry is expected, which has three independent componentsddd= ␹dyzy, ␹zxx
= ␹dzyy, and ␹zzz
兲 in nonlinear optical suscepti共␹xzx
共2兲
bility tensor ␹ . In our setup, the SH signal is dominated by
the interface dipole, which yields mainly p-polarized light.
For p-polarization incidence and p-polarization reflection geometry 共pin-pout兲, SMO/LMO has about ten times larger SH
intensity than LMO/SMO, but both samples show similar
temperature dependence 关Fig. 1共c兲兴. We performed polarization analysis28 by assuming the refractive index of the non-

LMO/SMO/LSAT

0.6

-B

SMO/LMO/LSAT

90(s-out)

-B

0

0.6

90(s-out)0(p-out)

SH intensity (arb. units)y0(p-out)

e
te
rfa
c

z

0

(c)

In

x

s

30 K
10 K

-2
-0.2

SrMnO3(G-type AF)p2

SH intensity (arb. units)

LaMnO3(A-type AF)

Magnetization (µB/Mn)(a)+B 270(s-out)
10+B 270(s-out)
1

1

0

1

FIG. 2. 共Color online兲 共a兲 Experimental geometry and coordination for the NMOKE measurement. s-polarized fundamental lightincidents on the sample and polarization of the output SH is analyzed. The expected interface dipole 共P兲, magnetic 共M兲, and toroidal 共T兲 moments are shown by arrows. 共b兲 Detected polarization
rotation for LMO/SMO and SMO/LMO samples at 30 K under thein-plane magnetic field of 0.1 T 共indicated with ⫾B兲. By reversing
the external magnetic field, the direction of polarization rotation
changes.

linear layer to be that of an LSMO film.29 We confirmed
4mm symmetry at both interfaces, as expected, and foundthat ␹zzz are more than 1 order of magnitude larger than otherelements, indicating the formation of relatively large interface dipoles 共P兲. This is consistent with the previous reporton the same sample systems investigated with the Maker
fringe technique.26
Figure 2共a兲 shows the optical setup for the NMOKE measurement. The samples were illuminated with s-polarized
fundamental light, and the polarization of the reflected SH
signal was analyzed. With the presence of magnetic moment
共M兲 at the interface, now we have toroidal moment 共T = P⫻ M兲 acting on the electrons as vector potential,17 which
induces s-polarized SH 共␹myyy兲. As a result, the polarization of
the output SH will rotate 共angle ␾兲 depending on the direction of magnetic 共or toroidal兲 moment, according totan ␾ ⬃ Re␤␹myyy,␣共␹dzyy + ␹mzyy兲

共1兲

where ␣ and ␤ are Fresnel factors and ␹dzyy, ␹myyy, and ␹mzyy arethe elements of second-order nonlinear susceptibility induced by electric dipole 共d兲 and magnetic 共m兲 moments.
Here the SH rotation induced by ␹myyy is directly related to thetoroidal moment, and the contribution from ␹mzyy is in general
negligible compared to ␹dzyy.
Figure 2共b兲 shows the NMOKE measured at 30 K underin-plane magnetic field of 0.1 T. By reversing the direction of
external magnetic field, we can clearly observe the change inthe direction of rotation in the SH polarization. The rotation
angle is larger for LMO/SMO, whose origin will be given
later.

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PHYSICAL REVIEW B 78, 212409 共2008兲

BRIEF REPORTS
(a)

0

2

+B
270

0.5

1

0
50

100
150 200
Temperature (K)

250

300

2
SH interference(arb. units)

1.5
1

1.5

LMO/SMO

1
SMO/LMO00.5

20 40 60 80
Distance (mm)0

SMO/LMO/LSAT
050

100 150200
Temperature (K)

0.5

250

Kerr rotation angle (deg)00I2␻共d兲 = I20␻ + I21␻共d兲cos共4␲⌬nd/␭ + ␾0兲,0(b)Magnetization (µB/Mn)

3

Kerr rotation angle (deg)

1

4

-B
p-out

1.5

sion of air, we have cosinelike interferogram after the SH
interference,31

90 s-out

LMO/SMO/LSAT
Kerr rotation 2φ

Magnetization (µB/Mn)

2300

FIG. 3. 共Color online兲 Temperature dependence of the Kerr rotation angle 共filled circles with error bars兲 and magnetization 共solidlines兲 for 共a兲 LMO/SMO and 共b兲 SMO/LMO, respectively. The
magnetization was measured with a field cooling run under a magnetic field of 0.1 T. Inset in 共a兲 shows a schematic of the Kerr
rotations under opposite external magnetic fields. Inset in 共b兲 shows
the result of SH phase measurements, indicating that the generated
SH signals from LMO/SMO and SMO/LMO are in the opposite
phase 共⬃␲ different兲. The solid lines are fits to Eq. 共2兲.

Temperature dependence of the Kerr rotation angle, together with the magnetization, is shown in Fig. 3. For both
samples, the rotation angles, which are proportional to theamplitude of the toroidal moment, closely follow the evolutions of the magnetization measured separately with a superconducting quantum interference device. We emphasize thatthese signals are from the single interfaces in ⬃3.2-nm-thicksamples, which prove high sensitivity of the nonlinear optical method. We also fabricated single SMO and LMO films
on LSAT substrates as control samples, and confirmed thatthe single SMO film does not have spontaneous magnetization. Single LMO film shows weak magnetic moment at low
temperature3 but does not show Kerr rotation within a current resolution.
To depict a clear picture on the multiferroic alignment of
the moments, we performed the phase-sensitive SHG
measurements30 and determined the relative orientation of
the dipole moments of both samples. For this measurement
the samples were placed in the ambient air. A reference␣-quartz crystal was placed after the sample and displaced
共distance d兲 along the optical path to have a path-lengthdependent interference of SH fields from the sample and thereference crystal; the latter is generated with the reflectedfundamental light on the sample surface. Due to the disper-

共2兲

where ⌬n is the dispersion of the air, ␭ is the fundamental
wavelength, and ␾0 is the initial phase difference. In our
setup, 2␲ phase shift corresponds to d ⬃ 50 mm.32 We include slight misalignment of the optical path in the interference amplitude I21␻共d兲.
As shown in the inset of Fig. 3共b兲, the relative phases for
LMO/SMO and SMO/LMO are nearly ␲ different, indicating that the interface dipole moments are pointing in the
opposite directions. This result also shows that the interface
dipoles are well established, and the fabricated samples are
not like a solid solution, which is consistent with the previous reports in that the charge transfer between LMO andSMO is expected to extend two to three unit cells from theinterface.20,22 Now we have a complete picture of the orientations of dipole, magnetic, and toroidal moments. For example, the upper side should correspond to LMO in the schematic of Fig. 2共a兲.
In Fig. 2共b兲, the SH polarization rotates in the same direction under the reversal of magnetic field for both samples,irrespective of the orientation of interface dipoles. This isbecause of the parities of the tensor elements7 in Eq. 共1兲,
where ␹myyy 共␹d,mzyy 兲 is odd 共even兲 function of time-reversal operation. Therefore, for the reversal of the magnetic moment,
only ␹myyy changes its sign, which appears as the change in the
direction of Kerr rotation. However, all the elements in Eq.
共1兲 are odd function for space inversion; under the reversal of
dipole moment, the overall sign, i.e., the direction of Kerr
rotation, is left unaffected. Thus we have to measure theorientation of the dipole moments separately, as has been
done in the above, to determine the multiferroic alignmentsof the respective moments.
In Figs. 3共a兲 and 3共b兲, the Kerr rotation angles differ by afactor of 2.4, although both interfaces have similar magnetization at low temperature 共⬃1.5␮B / Mn site兲. This is also
related to Eq. 共1兲, which shows that the rotation angle depends not only on ␹myyy but also on the intensity of thep-polarized SH. In the real situation, the p-polarized SH hascontributions from interfaces of LMO and SMO and also
from the surfaces, where the space-inversion symmetry is
broken. As shown in Fig. 1共c兲, SMO/LMO has approximately ten times larger p-polarized SH intensity, which has
been explained with constructive 共for SMO/LMO兲 and destructive 共LMO/SMO兲 interferences of SH fields from theheterointerfaces and surfaces26 共here the signals from film/
substrate interfaces are negligible兲. After polarization analysis, we found approximately five times larger apparent ␹dzyy
for SMO/LMO than LMO/SMO, which should result in fivetimes smaller Kerr rotation for SMO/LMO. Although the expected difference in the rotation angle is larger than that ofour observation, the apparent magnitude of the susceptibility
tensor elements sensitively depends on the complex refractive index of the nonlinear layer 共i.e., Fresnel factors兲,28
which may differ from our assumption. This result indicatesthe possibility of increasing Kerr rotation angle by modifying the surface while film preparation.

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PHYSICAL REVIEW B 78, 212409 共2008兲

BRIEF REPORTS

We note in passing that ␹myyy is imaginary by symmetry
consideration.7 The Kerr rotation, therefore, vanishes if all
the Fresnel factors and ␹dzyy are real in Eq. 共1兲: some of themmust be complex. In the present experiment, the fundamental
wavelength is above the optical gap of both LMO and SMO
thin films, thus, providing phase shift in the optical response.
dd, ␹zxx,
Although we do not know the absolute phases of ␹xzxdand ␹zzz at this moment, it is important to choose the fundamental wavelength suitable for the sample system to utilize
the complex optical response to have larger Kerr rotation. It
has been pointed out that the incident angle of the fundamental light is also essential for the optimal Kerr rotation.16
In summary, we have detected the multiferroic state, resulting from the electronic and magnetic reconstructions at

the single interfaces of LaMnO3 and SrMnO3 thin films
through NMOKE and phase-sensitive SH techniques. We
demonstrated that we have the sensitivity of magnetic moment down to single interfaces within the ⬃3.2-nm-thicksamples, with the complete assignment of the orientations ofdipole, magnetic, and toroidal moments. The large interface
dipole enhances the magnetic contribution to the nonlinear
susceptibility, enabling us to observe sufficient Kerr rotation.
We are grateful to H. Tamaru for technical assistance.
This work has been supported by JSPS KAKENHI 共Grant
No. 19840014兲 and MEXT TOKUTEI 共Grant No.
16076207兲.

Phys. Rev. B 66, 094410 共2002兲.

*ogawa@myn.rcast.u-tokyo.ac.jp†Also at Fuji Elec. Adv. Tech., Hino, Tokyo 191-8502, Japan.

19 Ş. Smadici, P. Abbamonte, A. Bhattacharya, X. Zhai, B. Jiang,

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