PHYSICAL REVIEW B 74, 012404 共2006兲

Coherent control of antiferromagnetism in NiO
Takuya Satoh,1,2 Nguyen Phuc Duong,1,3 and Manfred Fiebig1,4,*
1Max-Born-Institut, Max-Born-Straße 2A, 12489 Berlin, Germany
2Department of Basic Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
3

International Training Institute for Materials Science, Hanoi University of Technology, 1 Dai Co Viet Road, Hanoi, Vietnam
4Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Nussallee 14-16, 53115 Bonn, Germany
共Received 10 January 2006; published 17 July 2006兲
Coherent control of the antiferromagnetic 共AFM兲 order parameter of NiO is achieved by sequences of
optical pump pulses modifying the magnetic anisotropy. The modification allows reversible switching of the
AFM order parameter between hard- and easy-axis states. The switching frequency is determined by themagnetic anisotropy only and can therefore be higher than in the case of ferromagnetic spin precession. For
NiO frequencies up to 100 GHz are achieved. Possibilities for device application are discussed.
DOI: 10.1103/PhysRevB.74.012404

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

The potential of exploiting the spin of the electron in addition to its charge for electronic device applications leads to
the rapidly evolving field of spintronics.1 The tunneling magnetoresistance effect and its application in magnetoresistive
random access memory 共MRAM兲 is an example of a powerful spintronic effect. Future MRAM devices require spin manipulation rates much greater than 1 GHz so that rapid reversible switching of a magnetic state has become a major
challenge.2 So far, four methods for rapid manipulation of
the magnetization of ferromagnets have been described. 共i兲
Specifically designed magnetic-field pulses applied perpendicular to the direction of magnetization initialize spin
precession.2 共ii兲 Spin-polarized dc currents can change thespin orientation of a ferromagnet via an exchange of spin
angular momentum.3 共iii兲 Irradiation with laser pulses
quenches the magnetization by laser-induced heating above
the Curie temperature.4 共iv兲 Laser pulses modify the magnetic anisotropy, thus inducing the reorientation and/or precession of spins.5–7 Up to now, reversible manipulation of theferromagnetic order parameter has been achieved with methods 共i兲, 共ii兲, and 共iv兲. The time needed for the reversible
manipulation of magnetization is determined by the Larmor
frequency of spin precession and is thus limited to ⱖ100 ps.
In spite of the important role antiferromagnetism plays inmany spintronics applications not much is known about themagnetization dynamics of antiferromagnetic 共AFM兲 compounds. This is mostly due to the absence of a macroscopic
magnetization which inhibits direct access to the AFM orderparameter. Thus far, time-resolved optically induced spin reorientation or incoherent demagnetization has been observed
for the AFM order in FeBO3,8 orthoferrites,9 and NiO.10,11
However, “AFM switching” in the sense of the reversible
manipulation of an AFM order parameter has not been reported yet.
In this Brief Report we describe ultrafast all-optical control of the AFM order parameter of NiO by sequential 100 fs
laser pulses using optical second-harmonic generation 共SHG兲
as a probe for the AFM state. The first pulse triggers a collective reorientation of Ni2+ spins because of photoinduced
modification of the magnetic anisotropy. The second pulse
enhances or reverses the reorientation, depending on the timelag between the two pump pulses. Since our effect is basedon a transient phase transition with collective reorientation of1098-0121/2006/74共1兲/012404共4兲

spins, but not on the magnon-related precession and reversalof spins as in the case of the experiments on ferromagnets,
spin manipulation in NiO allows us to achieve repetition
rates as high as 100 GHz.
The high Néel temperature 共TN = 523 K兲 makes NiO an
ideal compound for device applications. An anisotropy
energy12,13 of D1 = 0.10± 0.01 meV confines the Ni2+ spins to
the 兵111其 planes where they are ordered ferromagneticallyalong the 具112̄典 axes. Opposite spin orientation in neighboring 兵111其 planes makes the compound AFM.
The magnetization dynamics of NiO was studied by alloptical pump-and-probe techniques which offer the highest
temporal resolution of any experiment. Furthermore, excitation with laser pulses with photon energies of ⬃1 eV canmodify the electronic and the magnetic system more easily
than magnetic field pulses which introduce the equivalent of
only ⬃0.1 meV/ T. Optical SHG described by the relation2
Pi共2␻兲 = ␧0␹共2兲
ijk 共ᐉ 兲E j共␻兲Hk共␻兲

共1兲

was used as ultrafast probe for the evolution of the AFMជ 共␻兲 and H
ជ 共␻兲 denote the electricorder parameter.14 Here E
and magnetic fields of the incident light wave at frequency ␻ជ 共2␻兲 denotes the nonlinear polarization induced atwhereas Pជ 共2␻兲 to the AFM order2␻. Because of the coupling of P
parameter ᐉ parametrized by the nonlinear susceptibilityជ 共2␻兲兩2 of the SHG light can be used␹ˆ 共ᐉ2兲, the intensity I ⬀ 兩P
as a probe for the AFM state of our NiO samples.
The bulk samples were pumped at a repetition rate of
1 kHz with amplified 100 fs light pulses at ប␻pu = 1.55 eVemitted from a Ti:sapphire laser. The pump pulses possess anenergy on the order of 10 ␮J and were focused onto an areawith a diameter of ⬃200 ␮m to produce excitation densities
of up to 1020 cm−3 in NiO. They heat the sample by less than
10 K at low temperature and by less than 1 K at room temperature, which can be neglected for the experiment. The
probe pulses were generated by an optical parametric amplifier pumped by the Ti:sapphire laser. The photon energy ofthe probe pulses was tuned to the d-d transitions of the
Ni2+共3d8兲 ion at ប␻pr = 1.03 eV in order to achieve large
SHG intensities.14 Pump and probe beams were incident in

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PHYSICAL REVIEW B 74, 012404 共2006兲

BRIEF REPORTS

FIG. 1. Change of linear and nonlinear optical responses after
photoexcitation with 1.55 eV light pulses at t = 0 of a 共111兲-oriented
NiO bulk sample at 80 K. 共a兲, 共b兲 Temporal evolution of the intensity change of reflected SHG for two different photon excitation
densities F. The inset shows the dependence of the amplitude of theoscillation in 共a兲 and 共b兲 on the excitation density. The line is a
guide to the eye. 共c兲 Temporal evolution of the linear reflectivity at␻ and 2␻ with ប␻ = 1.03 eV as photon energy of the probe pulses.

near-normal incidence onto a polished 共111兲 face of the NiO
crystal. The diameter of the pump pulses exceeded that of theprobe pulses by a factor of 3. The overlap of pump and probe
pulses was verified by maximizing the three-photon difference frequency signal between the probe pulse and at 2␻pu
− ␻pr at zero time delay. This process mixes pump and probewaves and therefore requires their overlap to be observed.
The reflected SHG of the probe beam was separated from theother light waves by optical filters and detected by a GaAs
photomultiplier tube. Initially, experiments were performed
below 80 K in a cryostat. However, after no major difference
was observed in experiments at room temperature, we refrained from using the cryostat for simplicity and to approach realistic conditions for device applications.
Figure 1 shows the intensity of the magnetic SHG signal

as a function of the delay between the pump pulse and theprobe pulse for two different photon excitation densities F of
the pump beam. In Fig. 1共a兲 a decrease of the average intensity of the SHG by 7% within 1 ps is observed. It is superimposed by a pronounced oscillation with an amplitude of±3%. The origin of the oscillation is discussed in detail in
Ref. 11 and briefly summarized here for comprehensiveness:
The pump pulse changes the magnetic anisotropy with atransient change of the easy direction from 具112̄典 to 具111典.
共In supplement to what was said in Ref. 11 we note that thephotoinduced modification of anisotropy is plausible because
shifts of 3d orbital wave functions accompanying the excitation of d-d transitions modify the dipolar and quadrupolarinteractions between the Ni2+ spins which determine the anisotropy. Further, the same change of the easy direction canbe achieved by a magnetic field, which is a relatively weakperturbation.16兲 The transient change of the easy direction
leads to a rapid photomediated reorientation of Ni2+ spins15
from the local 关112̄兴 direction 共“112̄ state”兲 to the local 关111兴
direction 共“111 state”兲. Subsequently, the 112̄ and 111 statesinterfere coherently. In the absence of the pump pulse, thespin-orbit interaction mediates the—in terms of a classicalpicture—slow reversion of the magnetic system to the 112̄
ground state. The interference is described by the wave function ␺ = a␺112¯ + b␺111exp共iD1t / ប 兲 with 兩a兩2 + 兩b兩2 = 1 andlimt→⬁b = 0. Thus, the beating frequency is given by the anisotropy energy. Since the SHG process probes the 112̄ and111 states it reproduces their coherent interference in theform of the oscillation of SHG intensity in Fig. 1共a兲.11
Figure 1共b兲 shows that reduction of the excitation density
by 30% quenches the oscillation of the SHG signal. According to the inset of Fig. 1 the threshold F0 for occurrence ofthe oscillation is near 5.4⫻ 1019 cm−3. The threshold valuemarks the excitation density at which the modification of theanisotropy by the pump pulse lets the energy of the 111 state
fall below that of the 112̄ state so that the collective rotationof the Ni2+ spins and, thus, the oscillation of the SHG signal,
are initiated. Because of the transient nature of the modification the reorientation process is not completed so that theamplitude of the oscillation is larger at higher excitation densities from the pump laser.
Figure 1共c兲 shows the temporal evolution of the linear
reflectivity of the probe beam tuned to the photon energies ofthe fundamental and the SHG waves. At 1.03 eV the excitation by the pump pulse leads to an increase of reflectivity by⬃0.3% whereas at 2.06 eV a change of reflectivity is notdetected. Along with the threshold behavior and the resultsof nonlinear optical experiments discussed in Ref. 11, Fig.
1共c兲 confirms that an emission of phonons cannot be responsible for the periodic modulation of SHG. Their presence
usually becomes manifest in an oscillatory modulation of thedielectric function 共and thus of the reflectivity17兲 the frequency of which, in the case of acoustic modes, scales linearly with the photon energy. However, none of this behavior
is observed in Fig. 1共c兲.
Note, that the collective reorientation of Ni2+ spins cannot
be understood as conventional generation of magnons. Magnons describe the coherent oscillation of spins around the012404-2

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FIG. 2. Change of the intensity of the reflected SHG of a 共111兲oriented NiO bulk sample at room temperature for two pump pulses
incident at t = 0 共1兲 and ⌬t 共2兲. ⌬t = 共a兲 19 and 共b兲 9.5 ps.

ground-state orientation. In terms of a classical picture thiscorresponds to demagnetization but not to a reorientation of
the order parameter as in the present case.
Contrary to the periodic modulation of SHG intensity thedecrease of the average SHG intensity in Figs. 1共a兲 and 1共b兲does not show any threshold behavior. This decrease is due
to fluctuations of the Ni2+ spins around the 112̄ ground state,i.e., the generation of magnons which disperse the excessenergy introduced by the pump laser. This is an incoherent
process corresponding to an increase of the spin temperature
for which the absence of the threshold behavior is natural.
In the classical picture the coherent superposition of thecoupled 111 and 112̄ states with a gradual transfer of amplitude from the former to the latter corresponds to the rotationof the sublattice magnetization back toward the 关112̄兴 direction, a process which according to the persistence of theoscillation in Fig. 1共a兲 clearly exceeds 100 ps. Figures 1共a兲and 1共b兲 thus display an example of active, ultrafast
共⬍1 ps兲 manipulation of an AFM state followed by passive,
slow 共100 ps兲 recovery of the original state. However, reversible switching requires active ultrafast control not onlyof the transfer to the excited spin state but also of the returnto the ground state.
In Fig. 2 the temporal evolution of magnetic SHG after
illumination with two pump pulses is shown for different
time lags between the pump pulses. Both pump pulses wereoverlapped by the aforementioned procedure. Therefore theyalso overlapped with each other, exciting the same region on
the sample. As mentioned, the experiments in Fig. 2 were
performed at room temperature in order to approach realistic
conditions for device applications. Just as in the case of the
low-temperature measurements a single pump pulse 共“pump1” incident at t = 0 or “pump 2” incident at t = ⌬t兲 with an
excitation density above the threshold F0 induces a drop of
the average SHG intensity and the pronounced oscillation
which is characteristic for the photoinduced 112̄ → 111 reorientation of spins.
A comparison of Figs. 1共a兲 and 2 reveals that the envelopeof the SHG oscillation up to a delay of 110 ps is increasingin the former and decreasing in the latter case. The increase
in Fig. 1共a兲 is not reproducible. It may be related to a small
increase of the pump intensity in the progress of data acquisition to which the SHG response is particularly sensitive
near the threshold fluence F0. A survey of all data sets gained
at F ⬎ F0 at ⱕ80 K shows no increase or decrease of theenvelope of the SHG oscillation up to a delay of 110 pswithin error limits. However, the decrease of the envelope in
Fig. 2 is reproducible and points to a coherence time of the
SHG interference at room temperature on the order of
100 ps.
When the time lag separating the two pump pulses is ⌬t= 19 ps both the average drop of the SHG intensity as well as
the amplitude of the oscillation are approximately doubled.
The two excitations with the pump wave correspond to independent events during each of which the magnetic anisotropy
is modified to the same extent. This case is different fromdoubling the energy of a single pump pulse. It therefore leads
to a dependence of the amplitude of the oscillation on the
total excitation energy which is different from that shown in
the inset of Fig. 1.
When the time lag between the two pump pulses is reduced to ⌬t = 9.5 ps the oscillation initiated by the first pulse
is stopped by the second pulse. The similarity in the temporaldependence of SHG in Fig. 2共b兲 after arrival of the second
pump pulse and in Fig. 1共b兲 after arrival of a single pumppulse at an excitation density F ⬍ F0 indicates that thequenching of the oscillation is related to the ultrafast reversalof the Ni2+ spins to the 112̄ ground state. The reversal occurs
because the coherent interference of the 112̄ and 111 statesof the Ni2+ spins 共expressed by ␺兲 adds up destructively forthe two pump pulses due to the phase shift introduced by the
9.5 ps delay. A similar optical two-pulse manipulation of amagnetic order parameter was reported for ferromagnetic
compounds.6 However, contrary to these previous experiments no remanent oscillation is observed in Fig. 2共b兲 since
the lifetime of the 112̄ → 111 excitation is much longer than
the time needed for one 112̄ → 111→ 112̄ switching cycle
共⬃100 ps versus ⬃10 ps兲.
Independent of the value of ⌬t the average drop of SHG
intensity is enhanced by the arrival of the second pumppulse. This corroborates the idea that the decrease of average
SHG intensity is related to incoherent deviations of the Ni2+
spins from the 112̄ ground state corresponding to a magnonrelated decrease of 共sublattice兲 magnetization whereas theoscillation of the SHG intensity is related to coherent devia-

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tions of the Ni2+ spins from the 112̄ ground state corresponding to a reorientation of the AFM order parameter.
The time needed for a switching cycle of the AFM orderparameter is only determined by the energy separating themagnetic ground state and the photoinduced excited magnetic state, i.e., by the magnetic anisotropy. In NiO this energy corresponds to a manipulation time of 9.5 ps so that inprinciple AFM switching in this compound can occur at arepetition rate as high as 100 GHz. This is an order of magnitude faster than in related experiments on ferromagnetic
systems2,6 in which the cycle time for the switching process
is restricted by the Larmor precession. Note that precession
frequencies in AFM compounds are in general much higher
than in ferromagnetic compounds9,18 so that precessional
AFM spin reversal can in principle be even faster than theanisotropy-related switching reported here. However, thishas not been realized thus far. Further, photomagnetic effects
allow one to reorient spins in ferromagnetic systems persistently within much less than 100 ps, but only if the reorientation is small 共⬍1 ° 兲, and reversible switching has not been
demonstrated yet.7
At present the number of repetitions in our AFM switching experiment is limited by the rate at which the excessenergy introduced by the pump pulse can be dissipated. We
found that the relation between the amplitude of the coherent
SHG oscillation and the incoherent drop of SHG intensity isvery sensitive to fine tuning of the photon energy and thepulse shape of the pump wave. Therefore, pulse shaping19
and frequency tuning of the pump wave in a two-color mixing experiment with two optical parametric oscillators may*Electronic address: fiebig@hiskp.uni-bonn.de
1

S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S.
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help to minimize the incoherent part of the excitation. Use ofNiO thin films or heterostructures instead of bulk crystals
will further modify the performance under repeated switching of the order parameter.
In summary, reversible switching of the AFM order parameter of NiO between hard- and easy-axis states has beenachieved by a succession of pump pulses modifying the magnetic anisotropy. Sequences with a limited number of pumppulses can be used to coherently control the orientation of
the AFM order parameter at a repetition rate which can behigher than that of ferromagnetic compounds and reaches
100 GHz in NiO.
Experimentally, the AFM state is less accessible than themacroscopic magnetization characterizing any ferromagnetic
state. However, the orientation of the AFM order parameter
determines many other macroscopic properties such as exchange bias20,21 or magnetoresistance22 which are easily detectable. In such systems the higher repetition rates attainablein AFM structures because of the absence of a macroscopic
magnetization can be employed to construct spintronics applications with shorter magnetic manipulation times. Moreover, transient photoinduced modification of the magnetic
anisotropy with subsequent quantum mechanical interference
共quantum beating兲 is a convenient way to measure the anisotropy energy.
Support from the SPP 1133 of the Deutsche Forschungsgemeinschaft and discussion with B. B. Van Aken are appreciated. M.F. thanks T. Elsässer for continuous support.

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