RAPID COMMUNICATIONSPHYSICAL REVIEW B 80, 241104共R兲 共2009兲

Half-metallic spin dynamics at a single LaMnO3 Õ SrMnO3 interface studied with nonlinear
magneto-optical Kerr effectNaoki Ogawa,1,2,* Takuya Satoh,3 Yasushi Ogimoto,1,2,† and Kenjiro Miyano1,2
1Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8904, Japan
2

CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

3Institute of Industrial Science (IIS), The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8904, Japan

共Received 18 September 2009; published 10 December 2009兲
Electronic and magnetic dynamics at a single interface of antiferromagnetic manganites, LaMnO3 / SrMnO3,
were studied with time-resolved second-harmonic generation and time-resolved nonlinear magneto-optical
Kerr effect. The temporal evolution of the respective signals shows clear differences; an optical pump makes
an instantaneous response of the electrons and, in contrast, induces slow demagnetization, which points to apossibility of two-dimensional half-metallic ferromagnetism for this heterointerface. Detailed analysis withvarious pump and probe polarizations reveals possible pathways of the optical charge-transfer excitation
between Mn d orbitals, leading to a subsequent perturbation to the interface spin system.
DOI: 10.1103/PhysRevB.80.241104

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

1098-0121/2009/80共24兲/241104共4兲

sensitive SHG, have been proven to meet with thesedemands.11,21
In the present study, we investigated ultrafast electron andspin dynamics at the single heterointerface of LMO/SMO
with TR-SHG and time-resolved nonlinear MOKE 共TRNMOKE兲. The interface electron and spin systems show distinct time evolution after optical excitation. The observed
fast electron response and slow demagnetization are a common feature of half-metallic ferromagnets with a weakelectron-spin coupling. This temporal demagnetizationshows clear pump polarization dependence, whereas TRSHG is least affected by the pump polarization, but varies
depending on the probe polarization, i.e., elements of thenonlinear optical susceptibility tensor. We found that the orbital alignment and resultant transition dipoles are responsible for the anisotropy in the electronic excitation and relaxation pathways.
(a)interface

3y2-r2

SrMnO3

(b)

2 2

3y -r

LaMnO3yxzSH

3y2-r2

T

mp

pu

be

M

pro

As invaluable magnetic materials for spintronic applications, half-metallic ferromagnets have been attracting muchattention.1 The perovskite manganites, such as
La1−xSrxMnO3 共LSMO兲, have been shown to possess halfmetallic nature with rich physics of charge and orbital degrees of freedom2 and are now at the stage of designing newelectronic phases by forming heterointerfaces with the helpof state-of-the-art thin-film growth techniques. For example,
at the interface of LaMnO3 共LMO兲 and SrMnO3 共SMO兲,both are insulating and antiferromagnetic in the bulk form
共Fig. 1兲;3,4 the charge redistribution modifies the electronic
and magnetic properties, leading to ferromagnetic and, insome cases, conducting states.5–12 This interface electronic
state is predicted to be a fully spin-polarized twodimensional electron gas.13,14
The dynamic aspects of the half-metallic ferromagnets arealso of particular importance in terms of ultrafast manipulation of spin systems. In many optical pump-probe experiments, thermalization of quasiparticles was found to takeplace without a loss of magnetization.15,16 The subsequentslow demagnetization, usually detected through timeresolved 共TR兲 magneto-optical Kerr effect 共MOKE兲, manifests small electron-spin coupling and has been ascribed tothe thermalization of the spin system with spin-latticeinteraction.16,17 The time constants for this spin dynamics are
determined by the spin-orbit interaction, i.e., magnetocrystalline anisotropy.18 A ferromagnetic state formed at the heterointerface can be viewed as ferrotoroidic with the induced
interface dipole. In the TR second-harmonic generation
共SHG兲 experiments on a bulk multiferroic GaFeO3, time
evolutions of the crystallographic polarization 共P兲 and thetoroidal moment 共T兲 have been detected separately.19 However, the spin dynamics 共magnetic moment M兲 was inferredindirectly from the combination with P 共T = P ⫻ M兲.
Up to now, most of the experimental works on the interface of LMO/SMO, including TR-MOKE,20 have been performed on the superlattice structures, presumably to amplify
the signal intensity. However, to obtain intrinsic informationand for future applications, single interfaces should be examined. Nonlinear optical techniques, especially interface-

P

(c)

y2-z2

FIG. 1. 共Color online兲 共a兲 Experimental coordinates and schematic illustration of the interface of LaMnO3 / SrMnO3 double layer.
LaMnO3 共d4兲 and SrMnO3 共d3兲 are anticipated to be A- and G-type
antiferromagnetic insulators, respectively 共Ref. 4兲. The alternatingalignment of d3x2−r2 and d3y2−r2 orbitals in LaMnO3 and an orbitalmixed ferromagnetic state at the heterointerface are expected. 共b兲
and 共c兲 Illustrations for the interlayer and intralayer opticaltransitions.

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OGAWA et al.

0.1

p-in

90 (s-out)

0.1

p-out

0.13

90 (s-in)(a)p-in/p-out

p-pumps-pump

0

0.08
0.06
(b)

s-in/p-out(c)s-in/45deg.-out(d)45deg.-in/s-out

0.05

0.1

270 (s-out)

s-in
0.017

90 (s-out)

0.04

270 (s-in)

s-out

90 (s-in)00 (p-in)

0.02

0
0.02

0.017
0.04

0

270 (s-out)

0.04

0.04

0.03
0.022

0.013
0.04

270 (s-in)

0.02
0.02

FIG. 2. 共Color online兲 SH polarization diagrams for the
LaMnO3 / SrMnO3 heterointerface at 30 K. Solid 共dotted兲 lines indicate the signals under the in-plane magnetic field B = 0.1
共−0.1兲 T. The nonlinear magneto-optical Kerr rotation can be seenin s-in geometry 共magnified in the vertical axis for clarity兲. Openmarks show the polarization configurations used for time-resolved
measurements in Fig. 3.

A double layer, LMO on SMO, was prepared on a
共LaAlO3兲0.3共SrAl0.5Ta0.5O3兲0.7 共LSAT兲 共001兲 substrate bypulsed laser deposition 共for details, see Refs. 22 and 23兲.
Both materials are approximately four unit cells 共⬃1.6 nm兲
in thickness. SHG was measured in a UHV cryostat with 800
nm probe light 共150 fs duration at 1 kHz repetition rate兲 with90° reflection geometry. A pump light pulse from the same
source was applied on the sample, with a 3° offset from theprobe, after a delay line. A magnetic field was applied inplane 共x direction in Fig. 1兲 with permanent magnets. TRNMOKE was measured by analyzing the SH polarization
with s-polarized incident light 共Fig. 2兲. As reported previously, the sample undergoes a ferromagnetic transition at⬃150 K,6 and the rotation angle of NMOKE closely follows
the magnetization of the sample down to low temperatures.11
In addition, confined electronic states were confirmed bymeasuring the directions of interface dipoles. The observed
interface magnetism has been explained with the competition
between antiferromagnetic superexchange of core spins andferromagnetic double exchange mediated by the itinerant eg
electrons.13 We note that the SHG from the LSAT substrate is
negligible. The pump and probe intensities are 3 and1.25 ␮J on 150 and 100 ␮m ␾ spots, respectively, whichwere finely chosen to avoid a damage on the sample and to
induce detectable SHG. The fluence seems to be relatively
large. However, the overall temperature rise must be negligible judging from the near coincidence of the temperaturedependence of the Kerr rotation angle with that of the sample
magnetization under the similar experimental setup.11
An example of SH polarization diagram at 30 K is shownin Fig. 2 under the external magnetic field of ⫾0.1 T. The
heterointerface of LMO/SMO shows 4mm symmetry, as exd= ␹dyzy,pected, which has three independent elements 共␹xzx(e)Relative
SH intensity

0.1

Relative SH intensity

0.05

0 (p-out)

Relative SH intensity00 (p-in)

0.05

0 (p-out)

0.05

0.04

s-in/s-out

s-pump

0.02
0

0

2

4

6

8

Pump-probe delay (ps)

0
0

50

100

150

200

Pump-probe delay (ps)FIG. 3. 共Color online兲 共a兲–共e兲 Time-resolved SH signals for eachpolarization configuration at 30 K and under B = −0.1 T. Here 45°means mixed polarization. Solid 共dotted兲 lines show the time evolution after p- 共s-兲 polarized pump excitation. Thin horizontal lines
indicate the SH intensity without a pump. Inset in 共e兲 shows an
instantaneous response around 0 ps for s-in and s-out configurations. Each signal contains the information of the following tensordddelements: 共a兲 p-in/ p-out 共␹xzx, ␹zxx, ␹zzz兲, 共b兲 s-in/ p-out 共␹dzyy兲, 共c兲
s-in/ 45° -out 共␹dzyy , ␹m
兲,
共d兲
45°
-in/
s-out
共␹dyzy , ␹myyyyyy 兲, and 共e兲ms-in/ s-out 共␹yyy兲.
dd␹zxx= ␹dzyy, and ␹zzz
兲 in nonlinear optical susceptibility tensor
共2兲␹ 共lattice dipole origin: d兲. The charge redistribution at theinterface from LMO to SMO is within one to two unit
cells,14,24 and the lattice is polarized to partly screen the potential difference,13,24 as has been predicted for
LaTiO3 / SrTiO3.25 With the presence of magnetic moment at
the interface, five additional tensor elements appear.26 The
NMOKE can be induced by ␹myyy, which is proportional to thetoroidal moment,19 as is clearly observed as a polarization
rotation in the s-in configuration 共Fig. 2兲, even from thissingle interface.11
Figure 3 shows the time evolutions of the SH intensity
after the pump excitation at 30 K. The corresponding polarization configurations are indicated in Fig. 2. It was foundthat there is almost no pump polarization dependence except
for at time zero. However, the relaxation characteristics show
clear variations with probe polarization. The SH intensity
shows faster relaxation 共20–50 ps兲 for the s-polarized probe
关Figs. 3共b兲 and 3共c兲兴, whereas Figs. 3共a兲 and 3共d兲 reveal slow
共⬎200 ps兲 recovery with minima around 30–40 ps for p andmixed probe polarization. The reversal of the magnetic field
共⫾0.1 T兲 did not make qualitative differences 共not shown兲.

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HALF-METALLIC SPIN DYNAMICS AT A SINGLE LaMnO…

4.5

Kerr rotation angle (deg)Note that it was difficult to directly observe the dynamics ofthe toroidal moment 关Fig. 3共e兲兴 due to the small signal intensity for the s-in/ s-out geometry.
First, we discuss the absence of the pump polarization
dependence in TR-SHG. There are several pathways for thepump excitation in this heterostructure; optical transitions in
LMO, across the interface, inside the interface, and in SMO.
The latter two processes are not relevant because SMO hasrelatively small optical conductivity at the pump energy,27
and the interface with the orbital-liquid state is presumably
isotropic within the interfacial plane. From first-principles
calculations,14,28,29 the optical charge-transfer 共CT兲 transition
would proceed between neighboring eg orbitals for LMO andacross the interface. Here, possible effects of p-d
hybridization30 are neglected for simplicity. According to theprevious pump-probe spectroscopy on a LMO film,31 the absence of the pump polarization dependence can be explainedwith the coincident final state after the optical excitation; if
we start with an occupied Mn d3y2−r2 orbital in LMO 关Figs.
1共b兲 and 1共c兲兴, the spin-conserving CT transition is to theinterplane d3y2−r2 orbital 共for p pump兲 and to the in-planeneighboring dy2−z2 共for s pump兲, where originally the former
is half occupied by the majority spin and the latter is empty,
and both are 1–2 eV above the ground state.29 The successive
in-plane transfer is prohibited due to the small transfer integral. The energy of the both excited states is nearly degenerate with that of the available orbitals in the neighboring
planes, thus forming a conduction band along the c axis irrespective of the initial pump polarization. Accordingly, theexcited electrons are mobile in the c direction even with theorbital- and spin-ordered ground states, which affects the interface electronic state and leads to the large anisotropy inthe subsequent SHG.
After the pump irradiation, the SH intensity temporally
decreases due to the bleaching of the CT excitation, increasein the lattice temperature, and screening of lattice polarization by the excited quasiparticles32 共Fig. 3兲. The relaxation
proceeds with the recovery of the orbital order, which will bein the time scale of 10 ps as reported for a LMO film.31 Thus,
the time evolution for s-in probe geometry 关Figs. 3共b兲 and3共c兲兴 can be ascribed to this orbital motion. The slow relaxation observed for the p-in probe can be explained by theinherent large anisotropy at the heterointerface and existenceof the pump-induced conduction band along the c axis,
which would help one to screen the interface with large lattice motion.13
Figure 4 shows the demagnetization dynamics at 30 and
150 K. This TR-NMOKE can be resolved only at lower temperatures 共below TC ⬃ 150 K兲 and shows a clear difference
between p- and s-pump excitations, which is distinct fromthe case of the TR-SHG. For each pump polarization, themagnetic moment at the interface starts to decrease in a similar manner up to ⬃40 ps, has a minimum 共⬃120 ps for p
pump and ⬃75 ps for s pump兲, and shows a recovery of
about 1000 ps. The time constant on the order of 100 ps forthis demagnetization is common for half-metallic
ferromagnets,16 and a similar value has been reported for abulk LSMO.17
The decrease in the Kerr rotation angle, i.e., magnetic
moment M at the interface, is caused by the thermalization ofs-pump3.5

2.5
p-pump30 K

1.5
2
p-pump

1
0
-1

150 K

-2
0

100

200

300

400

500

Pump-probe delay (ps)FIG. 4. 共Color online兲 Time-resolved nonlinear magneto-optical
Kerr rotation for the single LaMnO3 / SrMnO3 interface at 30 and
150 K. The shaded lines are the guides to the eye. The error bars areomitted from the upper panel for clarity.

the spin system. The observed difference between p- ands-pump excitations in the TR-NMOKE signal indicates that,
although the electronic final state is the same, the initial CTtransition of the minority spin toward the interface in the
p-pump configuration disturbs the spin system more efficiently. Since TR-SHG reflects the dynamics of electron
and/or lattice system, the observed clear difference in thetime evolution between TR-SHG and TR-NMOKE manifests
weak couplings for electron-spin, and even for lattice-spin
interaction. This weak electron-spin coupling indicates thehalf-metallic nature of this interface.16 We note that, in thecurrent experiment, the TR-NMOKE can reveal nearly purespin dynamics. Since the Kerr rotation angle is expressed astan ␪ ⬃ 共T / P兲 for the s-in probe configuration, the modulation after the pump excitation is ⌬共T / P兲 ⬃ ⌬M even when
⌬T contains contributions both from ⌬P and ⌬M; this is adifferent situation from detecting only the toroidally induced
component.19
We also note that the spikelike components around 0 ps
have been observed commonly in transient reflectivity andsometimes in TR-MOKE signals for many materials, whichwere discussed in connection with instantaneous electronic
and magnetic excitations.16 Here, we observed similar features both in TR-SH and TR-NMOKE signals 关Fig. 3共e兲 inset兴. However, these involve other nonlinear effects due tothe temporal overlap of pump and probe fields, which needfurther investigation. We observed almost the identical
TR-SHG for a double layer with the reversed stacking order,i.e., SMO/LMO/LSAT. However, it was difficult to resolve
TR-NMOKE for this sample due to its smaller Kerr
rotation.11
In summary, time-resolved nonlinear optical techniques
were applied on the single interface of LMO/SMO. It was
found that the electronic and magnetic signals show distinct

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OGAWA et al.

time evolution and pump-probe polarization dependenceseven at this single interface; the reflection TR-SHG shows nopump polarization dependence, whereas TR-NMOKE presents clear pump polarization dependence, which are explained by the selective optical excitation on the spin- andorbital-ordered ground states. The observed slow demagneti-*ogawa@myn.rcast.u-tokyo.ac.jp†Also at Fuji Elec. Hold., Shinagawa, Tokyo 141-0032, Japan.
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This work was partly supported by JSPS KAKENHI
共Contract No. 21740243兲 and GCOE for Physical Sciences
Frontier, MEXT, Japan.

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