１

Photo-Induced Magnetism in La0.5Pr0.5CrO3 with Canted
Antiferromagnetic Spin Order Studied by Electron Spin Resonance.
Osami Yanagisawa*, Masato Arai**, Takanobu Odagiri**,
Takahiro Nakayama** and Mitsuru Izumi**
柳沢
中山

修実*・荒井 雅人**・小田切貴伸**
貴博**・和泉充**

Photo-magnetic properties on the distorted perovskite chromates, La0.5Pr0.5CrO3 (Space group :Pnma at 300 K) was studied by X-band (9.0 GHz) Electron Spin Resonance (ESR). The La0.5Pr0.5CrO3shows the canted antiferromagnetic (CAF) transition at TCAF 〜 261 K and the magnetic domains withrandom orientation of ferromagnetic component in the CAF spin order below the TCAF. A YAG laser
with photon energy h ＝ 1.17 eV (wavelength λ ＝ 10640 A) which is near infrared region was
employed for a photo injection. During the photo injection, the ESR intensity increases dramaticallyaround the TCAF. The observed photo-induced ESR profiles retain Lorentzian curvature with center
resonance magnetic field, 320 mT. The photo-induced ESR intensity increase gradually with aincrease in temperature up to the TCAF i.e. the CAF state and grow up dramatically above the TCAF i.e.
the paramagnetic state. This high TCAF allows us to provide photo-magnetic functional devices
operated at room temperature. This transient magnetization was with the characteristic thermal
activation energy of 130 meV. The present results open an intriguing possibility of collective photoinduced magnetism in the distorted perovskite chromates.

Keywords : Magnetic materials., Chromates., Photo-induced effect., Electron paramagnetic resonance., Near-infrared light.
1. INTRODUCTION
To meet a recent industrial demand such as
photo-magnetic hybrid device, with which one cancontrol the electric property by the photo injection and/or the magnetic field, photo-induced phenomena areintensively studied. The photo-induced phenomena
also has been attracted great academic interests as afrontier for post superconductivity in the strongly
correlate system. There are tow types of approach tothe photo-induced phenomena. One is compound
mixed with ferromagnet and ferrimagnet [1] and otheris with the canted antiferromagnetic (CAF) spin order.
Both type of compounds have mach frustration in spin
system and are easily achieved phase transition byvarious external stimulation such as light, magnetic
field, electric field, pressure and etc.
In Prussian blue analog with mixed ferromagnet
and ferrimagnet, photo-induced effects have beenreported [2, 3]. The Prussian blue are molecular magnets with a relative high Curie temperature. For (M2＋
x
3＋
2＋
Mn2＋
or Fe2＋), depending on1−x)1.5Cr (CN)6 (M ＝ Ni
the mixing rate x, the magnetic pole can be inverted
by the light illumination. This is Optimized tuning
ferromagnetic and ferrimagnetic exchange interactions
induced by the light illumination.
In the distorted perovskite manganese, La1−xSrx
MnO3−d, there are tow types of photo-induced effects.
One is persistent photoconductivity and other istransient one[4]. La1−xSrxMnO3−d (0.2x0.5) with
3＋
4＋
mixed Mn and Mn shows ferromagnetic metal transition and so-call colossal magnetoresistance (CMR)
around this transition temperature. At small oxygen
deficiency, it shows metallic behavior in dark and is
induced persistent photoconductivity under a whitelight illumination. On the country, at large oxygen
deficiency, the film is semiconductor in dark and thetransient photoconductivity is induced by the light ＊Maritime Technology Department, Yuge National College of Maritime Technology.
平成１４年９月３０日受理＊＊Laboratory of Applied Physics, Tokyo University of Mercantile Marine.（東京商船大）２弓削商船高等専門学校 紀要 第２５号（平成１５年）

illumination.
Recent progress on the survey of photo-induced
effect in Pr0.65Ca0.35MnO3[5, 6] is reported with respect tothe electron spin resonance (ESR)[7, 8] and the powder x-ray diffraction[9, 10].
This photo-induced effect isreported in the charge-ordered (CO) state formed by
the alternation of Mn3＋ and Mn4＋ ions associated with
the CAF spin order. First, the paramagnetic resonance, which is diminished at the onset of the CAF
spin order, revives under the photo injection by nearinfrared cw-YAG laser with a photon energy, h ＝1.17 eV. Second, the superlattice reflections, which
are from the CO structure not suppressed by only Xray irradiated for the diffraction study, are suppressed
under the photo injection, leading to the structuralevidence of the collapse of the CO state by the photo
injection. The present results indicate that the photoinduced paramagnetic resonance is associated with themelting of the CO structure.
In this communication, we focus on chromates, La0.5
Pr0.5CrO3 as a new candidate for the photo-magnetic
material from the distorted perovskite type transition
metal oxide. The Cr3＋ (3d 3) in La0.5Pr0.5CrO3 is very
similar to Mn4＋ (3d 3) in the manganese, Pr0.65Ca0.35
MnO3 where either spin is S ＝ 3/2 in the t2g state.
The La0.5Pr0.5CrO3 shows the CAF transition at TCAF 〜
261 K and the magnetic domains with randomorientation of ferromagnetic component in the CAF
spin order below the TCAF [11, 12]. This high TCAF may
allows us to provide photo-magnetic functional devices
operated at room temperature. The magnetic domain
is easily suppressed by the external stimulation. The
photo-induced effect in La0.5Pr0.5CrO3 was investigatedby the X-band (9.0 GHz) ESR under the photo
injection with unique photon energy h ＝ 1.17 eV(wavelength λ ＝ 10640 A) which is near infrared
region.



2. EXPERIMENTThe powder samples of La0.5Pr0.5CrO3 were carefully prepared by calcining the mixture of prescribedamount of Pr6O11 (99.9%), Cr2O3 (99.98%) and La2O3(99.99%). The mixed powder was made into a pellet
and precalcined in air atmosphere at 1100 ℃ for 24 h.
Then the pellet was ground, made into a pellet again
and calcined in oxygen atmosphere at 1350 ℃ for 24h[11, 12, 13].
The powder x-ray diffraction measurement was
carried out to verify the existence of single La0.5Pr0.5

CrO3 phase and to obtain the lattice constants. In the
powder x-ray diffraction measurement, powder samples were attached to copper sample holder. Data
were collected using powder diffractometer (Rigaku,
RAD-IIB) with Cu Ka radiation (λ ＝ 1.5418 A)
equipped with a rotating anode generator operated at
40 kV and 200 mA. The 2 − step scan was usedwith step width ∆ ＝ 0.01 − 0.02 °, accumulation
time 10 − 100 s/step. The calculation of the observed
structure factor together with both profile fitting
process and the refinement of crystal structure basedon the powder x-ray diffraction data was performed by
Rietveld method with a program RIETAN[14].
D.c. magnetization was measured by SQUID magnetometer (Quantum Design, 7T-MPMS) to obtaincomplementary magnetic information. The d.c. magnetization was measured under the magnetic field 0.01
T under the sequence in the field cooling run (FC) andthe zero field cooling run (ZFC).
In the ZFC
measurements were performed after sample wascooled down to a prescribed temperature under zeromagnetic field, then a magnetic field was raised to
0.01 T. Magnetization vs magnetic field (M-H) curve
was measured in the ZFC with magnetic field up to 7
T at 5 K.
The ESR measurement for the electron spin on
3＋
Cr ionic sites in La0.5Pr0.5CrO3 was done using the Xband spectrometer (JEOL, JES-RE1X) operated at 9.0
GHz with a 100 kHz field modulation. The resonanceabsorption of the ESR measurement was observed as a
derivative signal curve. The sample was mounted in aliq. He continuous-flow type cryostat and was cooled
from 300 K down to 10 K. The sample temperature
was monitored by thermometer located near sample
and was controlled by varying the flow rate of liq. He.
Mn2＋ diluted in MgO was employed for a reference.
To investigate a photo-induced effect, a cw NdYAG laser with unique photon energy h ＝ 1.17 eV
were employed for the photo injection. The laser light
was introduced into the sample situated in a cavity
resonator through a double shielded quartz tubes
belonging to the continuous flow He cryostat. The
laser power was adjusted with the Kapton films whichwere 5 mW (4 sheets), 93 mW (3 sheets), 107 mW (2sheets), 120 mW (1 sheet) and 175 mW (without
sheet). The laser power of 175 mW is equivalent to
the injection of 9×1017 photons/sec. It is noted thatthe described value of the monitored laser power does
not indicate the precise value injected into the samples
due to the existence of the tow quartz tubes asPhoto-Induced Magnetism in La0.5Pr0.5CrO3 with Canted Antiferromagnetic
Spin Order Studied by Electron Spin Resonance.（柳沢・荒井・小田切・中山 ・和泉）

mentioned above. The laser spot size was about 1
mm2. The penetration depth of the laser light isestimated to be about 0.2 mm for the present sample.
The ESR measurement was done under the sequencein which we measure the ESR profile without thephoto injection, then under the photo injection andfinally without the photo injection again in turn tocheck if any kinds of damage to the sample occur.

３

to the creation of rather broad bands.
In the antiferromagnetic spin order, each spinsare arranged antiparallel like spin up, spin down, spinup due to the large antiferromagnetic exchangeinteraction. If there is frustration in the system, thespins arranged antiparallel are canted from antiparallel position. This is so-called CAF spin order. In the

3. RESULTS AND DISCUSSION
Figure 1 shows the x-ray powder diffraction
profile for La0.5Pr0.5CrO3 at 290 K with dot marks andthe result of the profile fit by Rietveld analysis withsolid lines. The x-ray powder diffraction analysis
indicated that the present powder sample was insingle phase with the distorted perovskite structureand symmetry is orthorhombic with the space group
Pnma with lattice constants a ＝ 5.475 A, b ＝ 7.741
A and c ＝ 5.480 A, respectively at 290 K.
Figure 2 shows the d.c. magnetization as afunction of temperature in La0.5Pr0.5CrO3 under magnetic field 0.01 T in the FC and ZFC run with □ and◯ marks. The magnetization indicates a magnetictransition at TCAF 〜 261 K. A irreversible hysteresis
between the FC and ZFC appears at the TCAF anddevelops with decreasing temperature. This transitionis attributed to the CAF transition. The magnetization in the FC is accompanied by a hump at 240 K. It
decreases remarkably with decreasing temperature
below 240 K. It passes through a magnetization
value, M ＝ 0 at 200 K, and sin of magnetization
change from positive to negative in which is magnetized opposite to applied magnetic field. It continues todecrease up to minimum value＝ -250 emu/mol at 5 K.
On the contrary, the magnetization in the FC remains
at positive and its absolute value is relatively small.
In the La0.5Pr0.5CrO3, since La and Pr are trivalentrare earth element, all of Cr ions are Cr3＋. The Cr 3dlevels are separated by the oxygen octahedral crystal
field to a lower energy t2g orbital (triplet) and a higher
energy eg orbital (doublet). The Cr3＋ ion in the La0.5
Pr0.5CrO3 has a 3d 3 electron spin configuration t32g.
Hund’s rule makes all spins align with each other on
the atomic site by a large intra-atomic exchange. The
t2g orbital hybridize with O2p orbital much more weakly
than the eg orbital, and is regarded as forming the
localized spin (S ＝ 3/2). On the contrary, the eg
orbital have lobes directed to the counter oxygen
atoms and hybridize strongly with O2p orbital, leading

Figure 1 The x-ray powder diffraction profile for La0.5
Pr0.5CrO3 at 290 K with dot marks and theresult of the profile fit by Rietveld analysis
with solid lines.

Figure 2 The d.c. magnetization as a function oftemperature in La0.5Pr0.5CrO3 under magnetic field 0.01 T in the field cooling run(FC) and the zero field cooling run (ZFC)
with □ and ◯ marks.

４

弓削商船高等専門学校 紀要 第２５号（平成１５年）

Figure 3 The ESR profiles for the La0.5Pr0.5CrO3 withand without (under dark) the photon injection by Nd-YAG laser (h ＝ 1.17 eV) withsolid curves and broken curves.

Fig. 4 (a)CAF, there are relatively small ferromagnetic components. The present observation is a typical magnetic
behavior in case of the existence of the spin-glass
state. It attributes to the magnetic domains withrandom orientation of ferromagnetic component in the
CAF spin order.
Figure 3 shows the ESR profiles for La0.5Pr0.5CrO3
without the photon injection (dark) as broken curves.
Several sharp dips between 300 mT and 330 mT in
the ESR profiles are coming from the Mn2＋ as areference. No ESR signal was observed from room
temperature down to 10 K.
Figure 3 shows the ESR profiles for La0.5Pr0.5CrO3
with the photon injection (h ＝ 1.17 eV) as solid
curves. No photo-induced ESR was observed below
210 K. During the photo injection, the ESR intensity
increase dramatically around the TCAF. The observed
photo-induced ESR profiles retain Lorentzian curvature with center resonance magnetic field ＝ 320 mT.
The photo-induced ESR intensity increase graduallywith a increase in temperature up to TCAF i.e. the CAF
state and grow up dramatically above TCAF i.e. theparamagnetic state. The penetration depth of thelaser light has been estimated to be about 0.2 µm forthe present sample.

Fig. 4 (b)Figure 4 The ESR profiles for end compound of
La1−xPrxCrO3, (0 < x < 1), LaCrO3 (a) and
PrCrO3 (b) with and without the photon
injection (h ＝ 1.17 eV) with solid curves
and broken curves at 290 K.

Photo-Induced Magnetism in La0.5Pr0.5CrO3 with Canted Antiferromagnetic
Spin Order Studied by Electron Spin Resonance.（柳沢・荒井・小田切・中山 ・和泉）One might suspect that this transition is simply
driven by the laser heating. The evidence against thelaser heating is provided by the temperature dependence of ESR profiles. There is no difference among
those profiles at low temperature where is expected to
be sensitive for this kind of heating. This transition
was reproducible, not permanent and also not due toany damage to the sample by laser irradiation. In
fact, the ESR profile after the post-measurement withthe photo injection shows almost identical with thatobtained prior the measurement under the photo
injection.
Figure 4 shows the ESR profiles for end compoundof La1−xPrxCrO3, (0 < x < 1), LaCrO3 (a) and PrCrO3 (b)with and without the photon injection (h ＝ 1.17 eV)
as solid curves and broken curves at 290 K. No
remarkable change were observed in these ESR
profiles with the photon injection. The observed photo-induced ESR intensity in La0.5Pr0.5CrO3 is attributed toarise from the solid solution of La1−xPrxCrO3.
Figure 5 shows temperature dependence of thephoto-induced ESR intensity for La0.5Pr0.5CrO3 with thephoton injection (h ＝ 1.17 eV). Below the TCAF 〜
261 K, it decrease with decreasing temperature and

５





Fig. 6 (a)Fig. 6 (b)Figure 5 Temperature dependence of the photoinduced ESR intensity for La0.5Pr0.5CrO3
with the photon injection (h ＝ 1.17 eV).



Figure 6 The ESR profiles for various incident laser
power (a) and incident laser power dependence of the photo-induced ESR intensity forLa0.5Pr0.5CrO3 with the photon injection (h ＝1.17 eV) at 290 K.

６弓削商船高等専門学校 紀要 第２５号（平成１５年）

follows the thermal activation type with characteristicenergy of 130 meV. Above TCAF, it is enhanced dramatically and deviating from the thermal activation
type.
Figure 6 shows the ESR profiles for various
incident laser power (a) and incident laser power
dependence of the photo-induced ESR intensity forLa0.5Pr0.5CrO3 with the photon injection (h = 1.17 eV)
at 290 K. There is obvious change in the ESR profiles
and the effective spin susceptibility increases with aincreasing the incident laser power. It is noted thatthe described value of the laser power does not mean
the exact value injected to the samples due to the
double shielded He flow guide made from quartz glass
as mentioned above.
Time dependence of the photo-induced ESR intensity under the sequence of before, during and after thephoton injection was also measured. It is worth tonote that the relaxation of photo-induced ESR intensity is quite rapid.
The precise mechanism of the photo-induced
magnetism in La0.5Pr0.5CrO3 is not clear in this present
stage. However it may be important that the Cr3＋ (3d 3) in La0.5Pr0.5CrO3 is very similar to the Mn4＋ (3d 3) inmanganese, Pr0.65Ca0.35MnO3 where either spin is S ＝
3/2 in the t2g state. The photo-induced effect in Pr0.65
Ca0.35MnO3 is caused by the photo injection only withphoton energy 〜 1.2 eV which is near-infrared region.
This energy value is characteristic in the optical
spectra in these manganese. For example, there is abroad peak in the photoconductivity function centeredaround 1.2 eV in similar compound, La0.7Ca0.3MnO3[6]and Nd0.7Ca0.3MnO3[15]. It is assigned as a chargetransfer excitation of an electron from the lower JahnTeller split eg of Mn3＋ to the eg of adjacent Mn4＋ ion,
which exhibits the promotion of the dipole active
photoionization of the Jahn-Teller small polaron[6]. Ourpresent result suggests that the photon injection
enhances the vibronic state and eventually releases
the cooperative Jahn-Teller distortion associated with
the CO state. The Mn3＋ and Mn4＋ in Pr0.65Ca0.35MnO3
have spin 3d electron configuration t32ge1g and t32g,
respectively. The possible optical charge transitions
are from the spin up eg levels in the Mn3＋ to the spinup eg level of Mn4＋. The spin up eg levels in the Mn3＋
ions are split by EJT due to the Jahn-Teller effect. In
the Mn4＋ the spin up eg states are shifted upwards(relative to the unsplit Mn3＋ ion spin up eg levels) by
EB due to the oxygen breathing mode distortions. The
spin down eg level of Mn4＋ is higher by Hund’s

coupling energy EJ ＝ JHSC relative to the spin up eg
level of Mn4＋, where JH is the exchange constant ofHund’s coupling, SC is the Mn ion core spin (SC ＝ 3/2).
The charge transition energy from the spin up eg levelsin the Mn3＋ to the spin up eg level of Mn4＋ is about 1.2
eV[9].
This process leads in promoting the hopping of anelectron from a Mn3＋ to a neighboring Mn4＋ with thetransfer energy, t and mixing species of Mn3＋ and
Mn4＋ ion site. It corresponds to the transition fromthe CO state to the CD state with the metallic
property. In other word, it is the photoionization of
the Jahn-Teller small polaron. The large JH meansthat the hopping of an outer-shell electron between
two Mn sites is affected by the relative alignment of
the core spins, being maximal when the core spins areparallel and minimal when they are antiparallel. This
results in the double-exchange interaction between thelocalized spins, the core t2g orbital (S ＝3/2) mediatedby the hopping eg orbital electron[16, 17, 18]. The insulating sate is antiferromagnetic and the metallic state is
ferromagnetic. Highly polarized state of Mn3＋ and
Mn4＋ spins as a ferromagnetic order is necessary toachieve the d.c. conductivity based on the collapsed
CO state.
By analogy with the Pr0.65Ca0.35MnO3, electrons at
the 3d -t2g state is excited to the 3d -eg state by thephoton injection (h ＝ 1.17 eV). Above the TCAF, thisphoto-excited electrons (photo-excited spin state) contribute to the ESR. Below the TCAF, the activationenergy 130 meV is required to get a photo-excited spinstate due to some kind of collective mechanism. The
present results open an intriguing possibility ofcollective photo-induced magnetism in the distorted
perovskite chromates.



4. SUMMARY
The photo-induced effect in the distorted perovskite chromates, La0.5Pr0.5CrO3 was investigated by
the X-band (9.0 GHz) ESR under the photo injection
with photon energy h ＝ 1.17 eV which is nearinfrared region. The La0.5Pr0.5CrO3 shows the CAF
transition at TCAF 〜 261 K and the magnetic domains
with random orientation of ferromagnetic component
in the CAF spin order below the TCAF. During thephoto injection, the ESR intensity increases dramatically around the TCAF. The observed photo-induced
ESR profiles retain Lorentzian curvature with center
resonance magnetic field ＝ 320 mT. The photo-Photo-Induced Magnetism in La0.5Pr0.5CrO3 with Canted Antiferromagnetic
Spin Order Studied by Electron Spin Resonance.（柳沢・荒井・小田切・中山 ・和泉）

induced ESR intensity increase gradually with aincrease in temperature up to 270 K i.e. the CAF
state and grow up dramatically above the TCAF i.e. theparamagnetic state. This high TCAF allows us to provide
photo-magnetic functional devices operated at room
temperature. This transient magnetization was withthe characteristic thermal activation energy of 130
meV. By analogy with the Pr0.65Ca0.35MnO3, electrons
at the 3d -t2g state is excited to the 3d -eg state by thephoton injection (h ＝ 1.17 eV). Above the TCAF, thisphoto-excited electrons (photo-excited spin state) contribute to the ESR. Below the TCAF, the activationenergy 130 meV is required to get a photo-excited spinstate due to some kind of collective mechanism.



5. ACKNOWLEDGMENTS
This study has been supported by the Science andTechnology Research Grant from Ministry of Education, Culture, Science and Technology in Japanthrough grant No. Youth B 14750562 and Sprout
13875121. The authors thank the Materials Design
and Characterization Laboratory, Institute of Solid
State Physics, University of Tokyo for the d.c.
magnetization measurement facilities.
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