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Ultrafast Manipulation of Antiferromagnetism of NiO
N. P. Duong,1 T. Satoh,1,2 and M. Fiebig1,*
1

2

Max-Born-Institut, Max-Born-Straße 2A, 12489 Berlin, Germany
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan(Received 12 May 2004; published 10 September 2004)

Photoexcitation of antiferromagnetic NiO leads to ultrafast reorientation of Ni2 spins due to changeof the magnetic anisotropy. Recovery of the magnetic ground state occurs as coherent oscillation of theantiferromagnetic order parameter between hard- and easy-axis states manifesting itself as quantum
beating. The coherence time is 1 ns with the beating frequency being determined by the anisotropy
energy.
DOI: 10.1103/PhysRevLett.93.117402

PACS numbers: 78.47.+p, 42.65.Ky, 75.30.Gw, 75.50.Ee

The question of how fast the magnetic state of a medium can be changed has far-reaching consequences forthe future of data storage. Ultrafast control of the magnetic order parameter is therefore a key issue of therapidly evolving field of spin electronics. Specificallydefined magnetic-field pulses allow repeated magnetic
switching on the sub-ns time scale [1]. In 1996 opticalexperiments surmounted the time scale of spin-lattice
relaxation by revealing magnetization dynamics below
1 ps, at which electrons, spins, and lattice are regarded
as partly decoupled systems with individual temperatures[2]. Although a wealth of data on ultrafast spin dynamics
of magnetically doped semiconductors and metals has
been accumulated up to date [1,3,4], studies on antiferromagnetic (AFM) systems are restricted to solitary cases[5–8]. Yet, AFM systems offer inherent advantages forultrafast magnetization dynamics: The absence of a macroscopic magnetization makes AFM compounds candidates for extremely rapid spin manipulation sinceconservation of angular momentum does not affect the
dynamical properties [9,10]. Further, many AFM compounds are insulators with a characteristic sequence of
discrete states. The associated long coherence times andlocalization of magnetic moments are favorable attributes
in quantum computing. Practical interest in AFM compounds based on their use as pinning or spacer layer inmodern electronic devices and the close relation between
antiferromagnetism and high-correlation phenomena likecolossal magnetoresistance and high-temperature superconductivity supplement the contemporary interest in the
AFM state [11] and make a better understanding of its
dynamical properties indispensable.
In this Letter we report the magnetization dynamics of
AFM NiO after excitation with an intense optical 100 fs
pump pulse using second and third harmonic generation
as probes for the AFM and crystallographic sublattices,
respectively. An increase of spin temperature and reorientation of the AFM order parameter ‘~ in the presence ofthe pump pulse is observed. Recovery of the magnetic
ground state can be described as oscillation of ‘~ between
the hard- and easy-axis states, their interference leading

to pronounced quantum beating with a coherence time of
1 ns. Whereas in many previous experiments on antiferromagnets [5–7] the quenching dynamics of the AFM
state was investigated and probed indirectly via an accompanying ferromagnetic component or a modificationof band structure, the present work demonstrates manipulation of the order parameter of a pure antiferromagnet
observed by a magneto-optical process with direct coupling to the order parameter.
Recent interest in NiO was triggered by its attempted
use as exchange-bias compound in magnetic read-write
heads which is favored by the high Néel temperature
TN  523 K. The optical properties of NiO were attributed to transitions within the 3d8 levels as sketched inFig. 1(a) [12 –14]. Paramagnetic NiO possesses NaCl-type
cubic structure (point group m3m). Below TN the Ni2
spins are ordered ferromagnetically in f111g planes where
they lie along h112i axes [15]. The in-plane magnetization
alternates along the stacking direction so that the crystal
is AFM with 24 possible domains. Magnetostrictive deformation of the unit cell along the stacking and spin
directions reduce the magnetic symmetry to 2=m [12].
The Ni2 spins are fixed in the h112i direction by mag-

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 2004 The American Physical Society

0031-9007=04=93(11)=117402(4)$22.50(a)(b) 〈111〉

〈112〉
t<0

ωp
0<t<100 fs
t~100 ps

FIG. 1. (a) Electronic states of Ni2 3d8 for NiO spins alongh112i and h111i axes and possible routes for second harmonic
generation (SHG). (b) Schematic evolution of the dynamic
antiferromagnetic phase transition triggered by the pump pulse
at h!p (see the text).

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netic anisotropy which is parametrized by the anisotropy
constants hD1  0:10 0:01 meV for the h111i axes
and hD2  0:005 meV for the h110i axes which makes
NiO an easy-plane system [15,16].
We studied the magnetization dynamics of NiO by
optical pump-and-probe techniques which offer the highest temporal resolution of any experiment. During the last
decade nonlinear optical processes were developed into
an exceptionally powerful tool for investigation of themagnetic properties of matter [3,17]. When light waves
with frequencies !i are incident on a crystal, harmonic~ i andgeneration from the electromagnetic fields E!
~H!i can induce a polarization at any frequency which
is a linear combination of the frequencies of the incident
waves. While linear optical effects like Faraday and Kerr
rotation measure the absolute magnetization and are
therefore unsuitable for probing AFM compounds, thecoupling of nonlinear optical processes to the AFM orderparameter is especially effective [18–20]. For NiO it was
shown that magnetic-dipole-type SHG [14,21,22]
2(1)

is present below TN . Figure 1(a) shows that the SHG
process is made up by transitions jgi ! jii, jii ! jfi,
and jfi ! jgi which are all resonant. The nonlinear sus2
ceptibility ^ 2 ‘~ mediates the coupling between theincident light at frequency ! and the induced polarization
at frequency 2!. It couples quadratically to the AFM
order parameter, so that SHG constitutes a direct probeof the AFM state [14].
Contributions to SHG from ^ 2 0 were not observed[14]. Therefore electric-dipole-type third harmonic generation (THG) [21,22]
Pi 3!  0 3
ijkl 0 Ej ! Ek ! El !

THG light was separated from the incident beams by
optical filters and detected by a GaAs photomultipliertube. Spectroscopy data were normalized with respect
to the intensity of the incident probe beam.
In Fig. 2 we compare SHG transmission and reflection
spectra in order to identify the electronic states probed by
SHG. The transmission spectrum exhibits a peak at
2.05 eV which is due to the two-photon transition fromthe 3 2 ground state to the 1 3 =3 4 excited state of theNi2 3d8 ion which is resonance enhanced by the 3 2 !
3
5 single-photon transition as depicted in Fig. 1(a) [14].
The reflection spectrum displays the same transitions as
the transmission spectrum while new surface-bound transitions predicted for the 1.7 eV range [10] were not observed. The magnetic SHG signal thus probes exclusively
bulk spin dynamics while being insensitive to surfacebound magnetization processes.
Figure 3 shows the temporal evolution of the change ofreflected intensities of SHG and THG after excitation
with the pump pulse. Based on Fig. 2 the photon energyof the probe beam was fixed at h!  1:03 eV. Within<1 ps a steplike decrease of magnetic SHG by hIi ’
10% is observed. It is followed by a pronounced oscillation which persists for >100 ps and is observed in thewhole investigated temperature range with only its frequency decreasing by 5% between 5 and 300 K.
Independent of polarization the steplike decrease andoscillation of SHG were observed for all SHG tensor~2components 2
ijk ‘ which was verified by probing different domains on a sample with large domains. The universality confirms that manipulation of the magnetic order parameter, which enters all components of ^ 2 in thesame way, forms the basis of the observed effects [23].

(2)is employed as a probe of the ‘‘spinless’’ crystallographic
lattice.
For the experiment (111)-oriented polished bulk
samples were prepared from single crystals grown by
the flame-fusion method. Brownish samples with domains in the order of 1 m and greenish samples
with domains in the order of 100 m produced byoxygen annealing were used. Samples were investigatedin the 5–300 K range. They were excited with amplified
100 fs pump-light pulses at h!p  1:55 eV emitted froma Ti:sapphire laser. Harmonic generation with 100 fs
pulses (h!  0:82 1:06 eV), which were emitted
from an optical parametric amplifier pumped by theTi:sapphire laser, was employed for probing spin andlattice dynamics. Pump and probe beams irradiated thesame (111) face of the sample at near-normal incidence.
Wave plates and polarizers were applied to set the polarization of the incoming light and analyze the polarizationof the reflected signal light at 2! or 3!. The SHG and(a)NiO (111)

Transmission

SHG intensity~
Pi 2!  0 2
ijk ‘ Ej ! Hk !

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0(b)
Reflection

0

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

SHG energy (eV)FIG. 2. Normalized SHG spectrum of NiO measured at room
temperature with light incident along the 111 axis. (a) Transmission data taken with the 3 ns laser system described inRef. [14]. (b) Reflection data taken with 100 fs laser pulses.

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80

100

0

THG ~ lattice

-5

SHG

-10

spins

-15 (a)Amplitude (arb.)

(b)

0.5

(c)

2

0.0
-0.5

1

-1.0

THG
0
0.0

0.1

0.2

0.3

Frequency (meV/h)

0

-1.5

20 40 60 80 100

Delay (ps)FIG. 3. (a) Change of reflected SHG and THG intensities independence of delay t between pump beam (h!p  1:55 eV)and probe beam (h!  1:03 eV) at 6 K for a photonic excitation density of 1:02  1020 cm 3 . (b) Fourier transform of the
SHG data after subtraction of the steplike decrease at t  0.
Dashed and straight lines: fitted spectral contributions andenvelope. (c) THG signal from (a).

Because of the large thickness of samples interference
effects from multiple reflection of acoustic phonons asobserved in magnetic heterostructures [24] cannot explain the remarkable oscillation of ‘‘magnetic’’ SHG in
Fig. 3. For the same reason unpinning effects [9,25] are
ruled out. Typical phonon and magnon frequencies differ
by 1 order of magnitude [26]. Since the change of linear
reflectivity by the pump pulse was found to be of the orderof 10 3 trivial modulation of the SHG signal by variationof the refractive index at ! or 2! is also excluded.
hIi is 1 order of magnitude too large to account for
demagnetization as a consequence of lattice heating fromthe pump pulse, which amounts to  10 K at 80 K [12].
We therefore conclude that hIi originates in an increaseof spin temperature which occurs independent of the(presumably fast) increase of lattice temperature. Thisis corroborated by the evolution of the ‘‘nonmagnetic’’
THG signal which reproduces none of the dynamic properties of the SHG signal. THG simply displays a small
continuous decrease which may be due to magnetostrictive modification of ^ 3 0 [12].
Magnetic anisotropy is determined by dipolar andquadrupolar interactions between the Ni2 spins [27,28].
It is therefore easily modified by the shift of 3d orbital
wave functions accompanying the excitation of d dtransitions by the pump pulse. According to our model
this leads to a change of easy direction from h112i (‘‘112
117402-3phase’’) to h111i (‘‘111 phase’’) as a consequence of
which a photoinduced reorientation of the AFM orderparameter ‘~ occurs. A photoinduced change of magnetic
anisotropy or structure at a similar excitation density was
already reported for various transition-metal compounds[5,7,8,29]. For NiO manipulation of ‘~ within 10 fs by anintense laser field was also predicted [10], and a staticchange of the easy axis was already observed [30]. The
proposed spin reorientation cannot be directly observed
since with quadratic coupling of SHG to the AFM orderparameter selection rules are the same for the 112 and111 phases. However, as will be seen our model of a
dynamic AFM phase transition is in excellent agreementwith the characteristic oscillation of the SHG signal in
Fig. 3.
For recovery of the 112 phase the subpicosecond spinreorientation processes mediated by the laser field [10] are
no longer available so that we should be able to temporally
resolve the rotation of Ni2 spins from the 111 phase backto the 112 ground state. Quantum mechanically the rotation corresponds to a superposition of the coupled (seebelow) 112 and 111 eigenstates. Physically their coherent
interference corresponds to the oscillation of the system
between the 112 and 111 states. The beating frequency of
the interference is determined by the energy separating
the eigenstates, i.e., the anisotropy energy hD1  0:10
0:01 meV. Figure 1(a) depicts that SHG in NiO withpopulated 111 and 112 states can lead to emission ofcoherent light waves at 2!  D1 , 2!, and 2! D1 , theinterference of which would lead to quantum beats at
frequencies D1 and 2D1 . The model is in outstanding
agreement with the observation of spectral components
at energies 0.116 meV and 2  0:107 meV in the Fourier

3.0

15

(b)

2.5

10

〈∆I〉 (%)

60

Amplitude of oscillation (%)

40

Change of intensity (%)

20~

Change of intensity (%)

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2.0

5

1.5
1.0

0
0.0 0.2 0.4 0.6 0.8
1.0
20
-3

Exc. density (10 cm )

0.5

(a)
0.0
0.0

0.2

0.4

0.6

0.8

20

-3

1.0

Excitation density (10 cm )FIG. 4. Dependence of antiferromagnetic spin dynamics on
photonic excitation density at 80 K. The data points were
measured in random sequence in order to exclude systematic
errors. (a) Amplitude of the coherent oscillation in the change
I of magnetic SHG in Fig. 3(a) fitted in the 50 –100 ps
interval. (b) Average drop hIi of magnetic SHG in Fig. 3(a).
Lines are guides to the eye.

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PHYSICA L R EVIEW LET T ERS

transform of It in Fig. 3(b). After 100 ps damping of
the interference is not yet noticeable, implying a coherence time of 1 ns in good agreement with the decoupledbehavior of the lattice-sensitive THG signal in Fig. 3(c).
Presumably coupling of the 112 and 111 states in Fig. 1(a)is mediated by spin-orbit interaction for which lifetimesof *100 ps are a typical value. Note that in the case of‘‘classical’’ quantum beats, as, e.g., in semiconductor
quantum wells [31] the beating states decay simultaneously and are no longer populated after the system
has returned to its ground state. In the present case oneof the beating states (the 112 state) is the ground state,
and the recovery occurs as a transfer of averaged population density between the beating states due to theircoupling.
The most striking evidence for a photoinduced AFM
phase transition is provided by Fig. 4 which shows thedependence of the photoinduced change of magnetic SHG
on the photonic excitation density. The steplike decrease
and the oscillation of I display an entirely different
behavior which immediately confirms their different
origin. hIi increases continuously with the excitation
density which reflects the continuous increase of spintemperature with laser fluence. However, the amplitudeof the oscillation exhibits a pronounced threshold behavior. Only if the photon fluence of the pump beam exceeds
a value of 0:52  1020 cm 3 is the oscillation of I
observed. The threshold value marks the fluence at which
the photoinduced modification of magnetic anisotropy
exceeds the critical value where a new global minimum
for h111i orientation of the Ni2 spins is formed. The
threshold is indicative of a collective process in theform of a photoinduced phase transition with ultrafast
reorientation of antiferromagnetic order in NiO.
In summary, ultrafast manipulation of AFM order in
NiO was observed using SHG as a probe for the dynamics
of the AFM order parameter. d d transitions excited bya 100 fs pump pulse modify the magnetic anisotropy, thus
leading to photoinduced reorientation of the Ni2 spinsfrom the h112i into the h111i direction. The process isevidenced by subsequent recovery of the 112 ground state
via coherent oscillation of the Ni2 spins between the 112
and 111 eigenstates with a lifetime in the nanosecond
range. The coupling is presumably guided by spin-orbit
interaction and leads to pronounced quantum beating
between the eigenstates. With an appropriately designed
sequence of pump pulses repetitive switching of Ni spins
between the 112 and 111 phases as discussed in Ref. [10] is
realistic. With application of such a kind of experiment to
Ni=NiO heterostructures ultrafast manipulation of exchange bias becomes possible which opens up interesting
opportunities for device construction.
Subsidy from the SPP 1133 of the Deutsche Forschungsgemeinschaft and discussions with K. Reimann,
M. Wörner, and W. Hübner are appreciated. M. F. thanks
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T. Elsässer for continuous support.

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