PHYSICAL REVIEW B 82, 174431 共2010兲

Influence of laser pulse shaping on the ultrafast dynamics in antiferromagnetic NiO
Andrea Rubano,1 Takuya Satoh,2,3 Alexey Kimel,4 Andrey Kirilyuk,4 Theo Rasing,4 and Manfred Fiebig1
1HISKP, Universität Bonn, Nussallee 14-16, 53115 Bonn, Germany
2Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan
3PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
4Radboud University Nijmegen, Institute for Molecules and Materials, 6525 AJ Nijmegen, The Netherlands

共Received 13 July 2010; published 23 November 2010兲
Femtosecond laser excitation of antiferromagnetic NiO leads to an oscillatory temporal behavior of opticalsecond-harmonic generation in this material. Depending on laser pulse length, these oscillations have frequencies of either 55 or 1 GHz, which correspond to the magnetic anisotropy energy separating the ground statewith spins along the 具112̄典 axis from the excited states with spins along the 具111典 or 具11̄0典 axis, respectively.
The oscillations can be explained as beatings between the ground and the respective excited state triggered byan instantaneous laser-induced flip of the Ni2+ spins which can thus be controlled three dimensionally by pulse
shaping.
DOI: 10.1103/PhysRevB.82.174431

PACS number共s兲: 78.47.⫺p, 42.65.Ky, 75.30.Gw, 75.50.Ee

I. INTRODUCTION

One of the emerging approaches of magnetic memory applications is ultrafast optical manipulation of spins in magnetically ordered materials.1 Over the last 20 years considerable progress has been achieved in understanding thefundamental time scales for the speed of magnetization reversal in ferromagnet and ferrimagnet.2 Although magnetization reversal in these materials at a subpicosecond time scale
seems to be feasible,3,4 it has also been realized that spin
reorientation in antiferromagnets may occur at a much fastertime scale than that in ferromagnets,5–7 because of the direct
involvement of the exchange interaction between the sublattices making up the antiferromagnetic 共AFM兲 state.
Experimental observation of spin reorientation over 90° in
an antiferromagnet has been claimed in only three compounds thus far: NiO,8 TmFeO3,9 and HoFeO3.10 In TmFeO3
and HoFeO3 the reorientation occurs between two equilibrium states of the materials. The transition is observed via
linear magneto-optical effects that couple directly to the
AFM order parameter. The spin reorientation is followed byspin precession with the frequency of the magnetic resonance. The manifestation of the photoinduced spin reorientation in NiO is quite different. After the photoexcitation by a⬃100 fs laser pulse, the second-harmonic generation 共SHG兲
probe signal showed a 55 GHz oscillation that, however, did
not match any of the known collective spin excitations of thematerial and was explained as interference effect.
In this paper we aim to advance our understanding of theultrafast SHG response of photoexcited NiO. We show thatthe temporal behavior of SHG from photoexcited NiO can becontrolled by varying the pulse length of the excitation laser.
In particular, a fine tuning of the pulse width leads to an
abrupt and dramatic change in the oscillation frequency.
These observations can be explained by assuming that thelaser pulse causes an instantaneous flip of the Ni2+ spinsfrom the 具112̄典 ground state into the 具111典 hard axis or into
the 具11̄0典 direction within the easy plane. Due to beatings,
the SHG signal shows oscillations with the frequencies determined by the energy separation of either the 具112̄典 and
1098-0121/2010/82共17兲/174431共5兲

具111典 states 共55 GHz兲 or the 具112̄典 and 具11̄0典 states 共1 GHz兲.
The measurements in the time domain also provide an approach for probing the corresponding magnetic anisotropy
energies with unprecedented resolution.
II. PROBING AFM NiO BY SHG

NiO is an insulating material with a compensated AFM
structure. Because of its high Néel temperature of TN= 523 K,11 it is a promising exchange-bias material. In theparamagnetic state, NiO has a NaCl-like cubic structure with
m3m point symmetry. At 523 K the Ni2+ spins become confined to 兵111其 planes, where they are ordered ferromagnetically along 具112̄典 axes. Opposite spin orientation in neighboring 兵111其 planes makes the compound AFM. NiO is aneasy-plane system. The hard axis is 具111典, i.e., the stacking
direction of the ferromagnetic planes. It is separated by ananisotropy energy បDhard = 100⫾ 10 ␮eV from the easy
plane in which the much weaker anisotropy energy បDeasy= 5 ⫾ 4 ␮eV distinguishes the 具112̄典 and the 具11̄0典 axes anddetermines the spin direction.12–14
SHG has been demonstrated to be an effective tool for
probing the magnetic state in ferromagnetic15 and, in particular, in AFM materials.16 SHG is a second-order optical process in which an incident light field at frequency ␻ interacts
with the material to generate a polarization at frequency 2␻.
The coupling term is the second-order susceptibility tensor
␹ˆ 共2兲 which, in the case of NiO, depends on the square of the
AFM order parameter.17 This coupling was employed to investigate the electronic transitions, the AFM domain structure, and the magnetization dynamics of this compound.8,17
In the present experiment, polished 共111兲-cut NiO
samples were prepared from single crystals grown by theflame-fusion method and annealed in oxygen in order to obtain a domain size of ⬃100 ␮m. Pump pulses of about 100
fs were generated by an amplified Ti:sapphire laser with aphoton energy of 1.55 eV. Probe pulses were generated by an
optical parametric amplifier at a photon energy of 1.03 eV
where the SHG yield reaches a spectral maximum.17 The

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NiO (111)

0

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3

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Energy (µeV)
456Fouriertransform

SHG intensity change (%)

RUBANO et al.

0

2

4

6

8

10

12

Delay (ns)FIG. 1. Temporal evolution of the SHG intensity of a NiO 共111兲
single crystal induced by optical pumping. The change in SHG
intensity is shown in dependence of the delay to the photoexcitation
at t = 0. Data were taken at room temperature. The gray thick line
and the envelope 共black lines兲 were derived by fitting Eq. 共1兲. The
inset shows the Fourier-transformed SHG data, after deconvolutionof the exponential decay.

probe pulse was sent through a mechanical delay line forgenerating a retardation of up to 12 ns. It was focused onto asingle domain of the sample with a focus diameter of about
50 ␮m. Pump and probe pulses differed in energy by a factor 20 and in diameter by a factor 3. SHG was generated innear-normal-incidence reflection and detected as described
elsewhere.8,17
III. PUMP-AND-PROBE SHG EXPERIMENTS ON NiO
A. Probe-signal beating

In Fig. 1 the relative change in the SHG yield upon exciting the NiO sample with the pump beam is shown as afunction of the delay between the arrival of the pump and theprobe pulse. Two features characterize the evolution of the
SHG signal at t ⬎ 0. First, a drop of the average SHG intensity by about 7% occurs within the duration of the pumppulse. Second, the SHG signal exhibits a pronounced oscillation with a frequency of about 1 GHz. The amplitude of theoscillation is modulated by beating and an exponential decay.
It was verified that neither the drop nor the modulated oscillation of the SHG signal is reproduced by the simultaneously
measured reflectivity at frequency ␻.
The evolution of the SHG intensity change ⌬I共t兲 at t ⬎ 0
can be described as follows:⌬I共t兲 = ⌬I0 + A exp共− t/␶兲cos共⍀t兲sin共⌬⍀t兲.

共1兲

Here, ␶ represents the decay time of the SHG oscillation
while ⍀ and ⌬⍀ denote its mean frequency and the doublet
splitting, respectively, according to the Fourier transformation in the inset of Fig. 1. A fit reveals ␶ = 9.7⫾ 0.2 ns, ប⍀= 4.39⫾ 0.08 ␮eV, and ប⌬⍀ = 0.172⫾ 0.002 ␮eV, a valuethat lies almost seven orders of magnitude below the excitation photon energy used in this experiment. Surprisingly, theoscillation frequency of the SHG intensity triggered by thephotoexcitation does not match any known collective excitation of spins or lattice in NiO. It is also clear that opticaldecoherence processes are operating at a subpicosecond timescale and cannot explain oscillations of an optical signal in
the nanosecond regime.
A similar situation was reported in Ref. 8. There it was
proposed that optical pumping leads to a transient change inthe magnetic anisotropy of the Ni2+共3d8兲 ion so that the Ni2+
spins flip collectively and instantaneously from the 具112̄典 tothe 具111典 direction while maintaining their AFM order. Thissituation is illustrated by the light gray curve in Fig. 3共a兲. It
visualizes a situation where pronounced photoexcitation initiates a transient change in the magnetic easy direction from
具112̄典 to 具111典 so that a proposed spin flip can occur. As
further detailed in Ref. 8, subsequent SHG on the 具112̄典 and
具111典 states lead to three SHG contributions at 2␻ and2␻ ⫾ Dhard. Interference of these contributions leads to beating in the intensity of the SHG signal at 2បDhard 共with asmall admixture at បDhard兲.
The SHG oscillation in Fig. 1 is reminiscent of thisbehavior but distinct discrepancies to the earlier result are
obvious: 共i兲 the additional modulation of the SHG oscillation and the exponential decay were not found in the earlier
work. 共ii兲 The period ⍀−1 of the oscillation is more thanan order of magnitude longer than the one obtained inRef. 8, whereas the former SHG oscillation is not observed.
Yet, the value ប⍀ = 4.39⫾ 0.08 ␮eV is of the same orderas 2បDeasy with បDeasy = 5 ⫾ 4 ␮eV as anisotropy energy
separating the spin directions 具112̄典 and 具11̄0典 according toinelastic neutron scattering.12 The coincidence between⍀ / 2 and Deasy becomes even more striking in the viewof recent magnon excitation experiments.18 They reveal a
ratio of magnetic in-plane and out-of-plane anisotropies of
Deasy / Dhard = 0.017⫾ 0.002 while we obtain 共⍀ / 2兲 / Dhard= 0.019⫾ 0.002 in the present experiment. The question is
why, under similar conditions as in Ref. 8, such striking differences are obtained in the ultrafast response of photoexcited NiO in the present experiment and how they can be
explained.
B. Pump-pulse tuning

Figure 2 shows how the temporal evolution of the photoinduced change in the SHG intensity depends on the durationof the pump pulse. A pulse length of 130 fs 共top兲 and 110 fs
共bottom兲 was set by slightly tuning the pulse compression.
However, for all plots shown in Fig. 2 the energy of thepump pulse and, hence, the laser fluence on the sample, were
kept constant in order to prevent any effects related to thelaser pulse energy. Figure 2 reveals that the relatively smallchange in the pump-pulse parameters has a dramatic effecton the observed dynamics. With the 130 fs pulse the same
long-term oscillation as in Fig. 1 is obtained, and the shortterm oscillation of Ref. 8 is absent. In contrast, the 110 fs
pulse reveals the short-term oscillation while the long-term
oscillation is not present anymore.
First of all, this behavior is consistent with the model of aphotoinduced instantaneous reorientation of the AFM spins.

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PHYSICAL REVIEW B 82, 174431 共2010兲

(a)

0

(b) 0
(a)

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(c)

0

(d) 0

-5

<111>

<112>

<110>

Pot. energy

SHG intensity change (%)

INFLUENCE OF LASER PULSE SHAPING ON THE…

ħDhardħDeasy-10
Pump power:

-10

-20
0.0 0.5 1.0 1.5 2.0

(ns)

Delay

0 20 40 60 80 100

(b)

(ps)

n
dn
= ␣G共t兲 − ,
dt
␶e

共2兲

where ␣ is the optical absorption and ␶e the electron relaxation time. G共t兲 = 共G0 / 冑2␲␴2兲exp关−共t / ␴兲2兴 describes theGaussian pump pulse with ␴ as pulse length and G0 proportional to the pulse energy. This differential equation is solved
byn共t兲 =

␣2
G0e共␴/2␶兲
2

冉冋 冉
erf

冊 册冊

t
␴
−
+1
␴ 2␶

e−t/␶ ,

共3兲

where erf共x兲 = 共2 / 冑␲兲兰x0exp共−y 2兲dy is the Gaussian error
function. In Fig. 3共b兲, the behavior of n共t兲 for two extreme

x 0.25Threshold

FIG. 2. Temporal evolution of the SHG signal at fixed laser
fluence and different pump pulse durations ␶ P. 关共a兲 and 共b兲兴
␶ P = 130 fs; 关共c兲 and 共d兲兴 ␶ P = 110 fs. Data were taken at 80 K.

Such an excitation can flip the spins into either direction:
within or out of the easy plane, but not both. This is illustrated by Fig. 3共a兲. Depending on the strength of the photoexcitation, either the 具11̄0典 共dark gray curve兲 or the 具111典
共light gray curve兲 direction can become the transient easyaxis. Consequently, only one of the two SHG oscillations canbe present at a time. Second, since the experiment was done
at a constant laser fluence and the pulse lengths were far
below the spin-lattice relaxation time, the data shown in Fig.
2 cannot be understood in terms of laser-induced heating of
NiO. The mechanism responsible for the excitation of theoscillations therefore must have an entirely nonthermal origin.
For understanding the behavior illustrated by Fig. 2, wehave to recall that the observed phenomenon is a thresholdprocess.8 As Fig. 3共a兲 shows, the photoexcitation has to surpass the point where the magnetic anisotropy acquires a new,transient, easy direction as a prerequisite for the flip of theNi2+ spins8 共dark or light gray curve兲. Exceeding this threshold is a matter of balance between the number of electrons
displaced by the pump pulse and the number of carriers returning to the ground state via electron relaxation.
This is described by the simulation shown in Fig. 3共b兲.
The simulation is based on a rate equation for the density ofphotoexcited electrons n共t兲,n(t)τp=110 fs

τe=20 fsτe=2 psτp=130 fs

-100

0

100
Delay (fs)

τe=20 fsτe=2 ps200

FIG. 3. Model for the photoinduced processes observed in NiO.
共a兲 Sketch of the proposed change in magnetic anisotropy from its
ground state 共black curve兲 by the presence of the optical pumppulse. The energy of the 具112̄典 state was taken as reference value.
Once the excitation becomes sufficiently strong, 具11̄0典 共dark graycurve兲 or 具111典 共light gray curve兲 may become the transient neweasy direction for the Ni2+ spins and a collective spin flip occurs.
共b兲 Simulation based on Eq. 共3兲 highlighting the influence of theelectron-electron relaxation time ␶e and the pump pulse duration ␶ P
on the photoexcited density of carriers n共t兲. Solid and dashed lines
refer to ␶e = 20 fs and ␶e = 2 ps, respectively; black and gray lines
refer to ␶ P = 110 fs and ␶ P = 130 fs, respectively. The threshold wasarbitrarily entered in order to illustrate the experimental observation: with ␶ P = 110 fs the density for a spin flip into the 具111典 direction is reached while it is missed for ␶ P = 130 fs.

cases is shown in order to illustrate the role of the electron
relaxation time in our experiment. We assume an electron
relaxation time that is much shorter 共20 fs, solid lines兲 ormuch longer 共2 ps, dashed lines兲 than the laser pulse length.
In the fast-decay scenario, the pulse duration plays a criticalrole in overcoming the threshold for the hard-axis reorientation while in the slow-decay scenario, the influence of thepulse duration is negligible. Comparison of Fig. 3共b兲 with
Fig. 2 clearly points to the former scenario, consistent with
an electron relaxation time Ⰶ100 fs for NiO, as was suggested in Ref. 8. With respect to Fig. 2, we can therefore
conclude that the energy we provide to the system using the
110 fs pump pulse is sufficient for overcoming the thresholdfor spin reorientation out of the easy plane and along the
具111典 axis and, thus, the corresponding beating is seen in
Fig. 2共d兲 and gives បDhard = 114.8⫾ 0.3 ␮eV as value of theanisotropy energy. By increasing the pulse duration to 130 fs,

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RUBANO et al.

the electronic relaxation gains weight and even though thepulse energy is not changed the threshold for a spin flip outof the easy plane is no longer reached. Instead, SHG beating
as in Fig. 2共a兲 is obtained, which points to a spin flip within
the easy plane from the 具112̄典 to the 具11̄0典 axis. With
Dhard / Deasy ⯝ 50, the threshold for the latter process will be
relatively low. Figure 2共a兲 gives បDeasy = 2.20⫾ 0.04 ␮eV as
corresponding value of the anisotropy energy in agreement
but with a much smaller error than in Ref. 12.
Note that the change in frequency of the SHG oscillationin Fig. 2 can also be triggered by tuning the laser fluence
while keeping the width of the laser pulse unchanged. However, this would disguise the independence of heating effects
that we want to emphasize. In addition, the SHG oscillation
is more sensitive to tuning the pulse width than it is to achange in fluence which indicates that pump-dump effects inthe stretched laser pulse19 may also affect the density of photoexcited electrons n共t兲.
IV. DISCUSSION OF MICROSCOPIC MECHANISMS

The oscillatory SHG response of NiO subsequent to photoexcitation is thus consistent with an ultrafast reorientationof the AFM spins, where the details of the reorientationstrongly depend on the length of the pump pulse according to
Fig. 3. A microscopic justification of this effect must relate
the optical excitation of the Ni2+共3d8兲 electrons to a modification of the ground state of the electronic system by achange in the magnetic anisotropy as sketched in Fig. 3共a兲.
The optical excitation shifts electrons between the 3d statesof the Ni2+ that are split by the ligand field, spin-orbit interaction, and exchange effects.17 We have to assume that: 共i兲the repopulation modifies the anisotropy to such an extent
that a transient change in the easy axis occurs which provokes the 90° spin flip; 共ii兲 to see the beatings, the spin
reorientation itself must occur within the duration of thepump-laser pulse. Assumption 共i兲 can be justified. Though
the exact mechanism of this modification with the electronic
processes on the atomic level of the Ni2+ ion is not yet understood it is known that the easy axis of NiO can bechanged relatively easily by a magnetic field,20 by Co
deposition,21 or by pressure.22 In addition the holes generated
by the photoinduced excitation of electrons may lead to theformation of Zheng-Rice-type singlet states that were shown
to modify the spin direction while retaining the antiferromagnetic order.23 Regarding assumption 共ii兲, reorientation of
an AFM order parameter in the presence of photoexcitation
was discussed in Refs. 5–7. Note that we do not pump the

1 B. Koopmans, in Spin Dynamics in Confined Magnetic Struc-tures II, Topics in Applied Physics Vol. 87, edited by B. Hillebrands and K. Ounadjela 共Springer, Berlin, 2003兲, pp. 256–323.
2
J. Stöhr and H. C. Siegmann, Magnetism: From Fundamentals to
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P. Parkin, H. C. Siegmann, and J. Stöhr, Phys. Rev. Lett. 102,

NiO resonantly and an influence of the pump polarization is
not observed so that a different scenario than in the former
theoretical work has to be considered in our case.
The pronounced and long lasting beating effects observedin the experiment indicate that the interactions destroying
this coherence are weak. The impressively long coherencetime, following from Fig. 1, shows that the oscillations cannot be due to the optical coherence between the pump andprobe light fields.24 In contrast, the spin system with its weakcoupling to the lattice can remain sufficiently unperturbed
for maintaining the long coherence times obtained here. Extending this rationale, we may conclude that the beating in
Fig. 1 with the splitting revealed in the inset must also be
spin related. Possible mechanisms falling in the same energy
range as ប⌬⍀ are, for instance, hyperfine interactions,domain-wall motion, or the Zeeman splitting caused by perturbations in the order of the terrestrial magnetic field. The
fact that we observe such beatings at room temperature corroborates a collective behavior of large number of spins.
Otherwise temperature fluctuations 共⬃25 meV兲 would easily destroy the coherence. However, more detailed theory is
required to substantiate these suggestions, as well as to findthe explicit microscopic mechanism of the instantaneous spin
flip induced by the femtosecond pump pulse.
V. CONCLUSION

In summary, we have observed an oscillatory opticalsecond-harmonic generation from NiO excited by femtosecond laser pulses. Depending on the laser pulse at fixed laser
fluence oscillation frequencies of 55 or 1 GHz are observedthat are related to the out-of-plane or the in-plane anisotropy
energy for a 具112̄典 → 具111典 or 具112̄典 → 具11̄0典 flip, respectively, of the Ni2+ spins. These results point to a nonthermal
three-dimensional reorientation of the AFM spins of NiO by
optical pumping with tailored femtosecond laser pulses.
They also provide a rather sensitive method to probe thesevery small differences in the anisotropy energy revealingបDhard = 114.8⫾ 0.3 ␮eV and បDeasy = 2.20⫾ 0.04 ␮eV in
the present case.
ACKNOWLEDGMENTS

The research leading to these results has received funding
from de Nederlandse Organizatie voor Wetenschappelijk
Onderzoek 共NWO兲, de Stichting voor Fundamenteel Onderzoek der Materie 共FOM兲, and the Deutsche Forschungsgemeinschaft 共DFG, through the priority program SPP1133兲.
T.S. was supported by the Foundation for the Promotion ofIndustrial Science.

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