PHYSICAL REVIEW B, VOLUME 65, 125103

Irreversible photoinduced insulator-metal transitionin the Na-doped manganite Pr0.75 Na0.25 MnO3
Takuya Satoh* and Yoshinao Kikuchi
Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan

Kenjiro Miyano
Research Center for Advanced Science and Technology (RCAST), University of Tokyo, Tokyo 153-8904, Japan

Emil Pollert, Jiri Hejtmánek, and Zdenek JirákInstitute of Physics of ASCR, Cukrovarnicka 10, 16253 Prague 6, Czech Republic
共Received 1 August 2001; published 11 March 2002兲
Persistent photoconductivity has been observed in a perovskite manganite doped with an alkali metal element, Pr0.75Na0.25MnO3 . Considering the shallow penetration depth (⬃0.1 ␮ m) of visible light, the increase of
the conductance is estimated to be of the order of 104 to 105 , which implies an irreversible photoinduced
insulator-metal transition. Analysis of the dc- and ac-susceptibility data shows that the system is in a glassy
state below 45 K. The system is therefore viewed as being in a highly nonuniform and metastable state, inwhich ferromagnetic metallic clusters exist in insulating matrix. The photoexcitation helps the clusters todislodge from a local energy minimum and to seek more stable configuration. The percolative conduction canwell explain the observed large increase of the conductivity. We argue that metastability is due to the large
charge mismatch between the monovalent Na⫹ ion and Pr3⫹ or the incommensurability of the Mn valencyfrom 3.5⫹.
DOI: 10.1103/PhysRevB.65.125103

PACS number共s兲: 71.27.⫹a, 71.30.⫹h, 71.45.⫺d, 78.20.⫺eI. INTRODUCTION

Phase control by a photon such as photoinduced insulatormetal transition 共PIMT兲 has recently attracted much attention. The photon triggers a collective change of the electronicstate. A material that has a mixed valence with localized
electrons, and strong electron-lattice coupling can be a goodcandidate for the PIMT. Photons delocalize the electrons andthe excited electrons cause a lattice relaxation, resulting in acollapse of charge gap. Perovskite manganites with generalformula R 1⫺x A x MnO3 , where R is a rare-earth ion and A is amonovalent alkaline ion or a divalent alkaline-earth ion, areamong the candidates, because they have mixed valent Mn3⫹
and Mn4⫹ , and strong electron-lattice coupling due to theJahn-Teller effect.
Pr0.7Ca0.3MnO3 is a charge ordered 共CO兲 insulator below
220 K with Mn3⫹ and Mn4⫹ in a checkerboard array due tothe Coulomb repulsion and the Jahn-Teller interaction. A
first-order phase transition to a ferromagnetic metallic 共FM兲phase occurs under a magnetic field giving rise to the colossal magnetoresistance 共CMR兲.1 The transition is accompanied by a relaxation of Jahn-Teller distortions and the metallic state is sustained below 60 K after the removal of themagnetic field.
In a single crystal Pr0.7Ca0.3MnO3 , insulator-metal transition by IR/visible light was reported by Miyano et al.2,3 The
photoinduced metallic state is not stable and goes back toinsulating state when applied electric field is turned off.
Therefore, it can be regarded as an avalanche effect or adielectric breakdown triggered by light illumination.
On the other hand, irreversible insulator-metal transition
was observed under x-ray illumination below 40 K in the0163-1829/2002/65共12兲/125103共4兲/$20.00

same compound.4 It is later argued that ferromagnetic clusters appear below 120 K and the x-ray irradiation leads to thecluster coalescence and phase segregation of the FM state.5
Transport is thus governed by the percolation.6
Persistent photoconductivity 共PPC兲 by visible light was
first observed in oxygen deficient La0.7Cax Ba1⫺x MnO3 thinfilm by Cauro et al.7 The mechanism is suggested as follows.
First, a photon creates an electron-hole pair. The electron istrapped in an oxygen vacancy while the hole cannot recombine with the electron due to the lattice distortion. Thus thenumber of the hole carriers increases and the system, initiallyin the low-doped insulating region in the electronic phase
diagram, moves into the high-doped metallic state.8
Persistent photoconductivity via yet another route has
been recently demonstrated in Cr-doped Pr0.5Ca0.5MnO3 thinfilms.9 The Cr acts as a random field that disrupts otherwise
stable CO state. As a result, FM cluster appears as the COstate grows. The light illumination connects the clusters resulting in the PPC through the percolative transport. Theeffect is, however, an increase of conductivity by a modest
factor of 2 and no sign of volumetric increase of the FM stateis suggested.
Pr1⫺x Nax MnO3 is a manganite doped with monovalent
alkaline metal Na and a CO insulator in the range 0.2⭐x⭐0.25.10 Relatively low magnetic field (⬃1.5 T) induces aninsulator-metal transition. This is even lower than the wellstudied CMR material, Pr0.7Ca0.3MnO3 .1 Therefore, one can
expect that the energy barrier between antiferromagnetic共AF兲 CO and FM states is small and hope for a significant
decrease of resistance by light illumination.
In this paper, we will show that Pr1⫺x Nax MnO3 is in cluster glass 共CG兲 state below 45 K based on the resistance anddc/ac magnetization measurements. Furthermore, we have

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PHYSICAL REVIEW B 65 125103

FIG. 1. Temperature dependence of dc magnetization in zerofield- cooled 共ZFC兲 and field-cooled 共FC兲 runs under 50 Oe and 500
Oe.

observed insulator-metal transition by photon in the energy
range 0.1–2.3 eV, and ascribe it to an increase of FM clusterin the CO matrix.
II. EXPERIMENTAL

Polycrystalline sample was prepared using a solid state
reaction with a detailed description published elsewhere.11
The sample is oxygen-deficient–actual-valency Mn3.39⫹ bychemical analysis. dc magnetization and ac susceptibility
were measured with a SQUID magnetometer 共Quantum Design MPMS兲 in the temperature range of 5–300 K. In the dc
measurement, the sample was cooled under either magnetic
field 共FC兲 or zero magnetic field 共ZFC兲, and the magnetization was measured during the subsequent warming. The ac
susceptibility was measured at 0.1, 1, 10, and 100 Hz as afunction of temperature. The resistance was measured by
two-wire method with a gap of 50 ␮ m in a constant current
mode. The samples were placed inside a He flow type cryostat in which the temperature is controlled within 1 K. We
used three types of light sources for illuminating the sample.
A second-harmonic light with photon energy of 2.33 eV (␭⫽532 nm) pumped with a cw Nd:YVO4 laser light was focused with a cylindrical lens on the gap of the electrodes. Alaser light of an optical parametric amplifier 共OPA兲 in thephoton energy range of 0.25 eV (␭⫽5 ␮ m)⬃1.55 eV (␭⫽800 nm), pumped with a mode locked Ti:sapphire laser
light with a pulse width of 200 fs and at a repetition rate of
1 kHz, was focused in the same way as above. A CO2 laser
with photon energy of 0.12 eV (␭⫽10.6 ␮ m) was illuminated to the sample without focusing. Since these three measurements were performed under different conditions, one
cannot make a quantitative comparison among them.

FIG. 2. Temperature dependence of in-phase component关 ␹ ⬘ac(T) 兴 of ac susceptibility in an ac field of 5 G for the frequenciesof 0.1, 1, 10, and 100 Hz, respectively. The inset shows a magnification of the cusp region.

temperature dependence of in-phase component 关 ␹ ⬘ (T) 兴 ofac-susceptibility in an ac field of 5 G for the frequencies 0.1,
⬘ presents cusps around T f ⫽43 K 共see
1, 10, and 100 Hz. ␹ ac
inset兲, which is close to T irr . The cusps shift toward a higher
temperature with increasing frequency, which is typical of aglassy state. However, the frequency dependence of the shift⬘ , i.e., p⫽ ␦ T f /(T f ␦ log10 f )⬃10⫺4 , is muchof the cusp in ␹ acsmaller than the typical values for canonical spin glass systems in which p ranges from 0.0045 to 0.28.12 Therefore, theresult suggests the system is CG rather than SG.
Magnetic measurements under various conditions arecomplied to construct a magnetic phase diagram ofPr0.75Na0.25MnO3 shown in Fig. 3. The critical magnetic field

III. RESULTS AND DISCUSSIONSWe show in Fig. 1 temperature dependence of dc magnetization in ZFC and FC runs under 50 and 500 Oe. The cuspsat T CO⫽220 K and T N ⫽170 K are the charge ordering transition temperature and the Néel temperature, respectively.
The difference between ZFC and FC appearing below T irr⫽45 K suggests the spin glass 共SG兲 state.
For further investigation into the glassy state, we performed ac susceptibility measurements. Figure 2 shows theFIG. 3. Magnetic phase diagram of Pr0.75Na0.25MnO3 . The critical magnetic field was determined by M -H loops at each temperature. T CO under each magnetic field was estimated with a temperature dependence of the magnetization. The PI, COAF, FM, and CG
denote the paramagnetic insulating, charge ordered antiferromagnetic, ferromagnetic metallic, and cluster glass states, respectively.
The T CO denotes the charge ordering transition temperature. Shaded
portions above and below 50 K are hysteresis regions between
COAF and FM, and between CG and FM, respectively.

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IRREVERSIBLE PHOTOINDUCED INSULATOR-METAL . . .

PHYSICAL REVIEW B 65 125103

FIG. 4. Resistance as a function of time at 20 K. The sample
was irradiated with a cw laser (h ␯ ⫽2.33 eV) at the power densityof 10 mW/0.01 cm2 .

was determined by M -H loops at each temperature. T CO under each magnetic field was estimated with a temperaturedependence of the magnetization. A striking feature is that
AFCO-FM transition is induced by relatively low magnetic
field (⬃1.5 T) at 50 K. Below 50 K, the FM state is metastable even without magnetic field, suggesting that FM clusters exist in the CO insulating matrix.
Figure 4 shows the transport response to the light illumination at 2.33 eV at 20 K and Fig. 5 at 60 K. The initialresistance was about 5⫻1010 ⍀ at 20 K and 1.1⫻107 ⍀ at60 K. Just after the light illumination starts, the resistancedecreases abruptly. While the resistance at 60 K recovered tothe initial value when light is turned off, the resistance at 20
K remains low, i.e., persistent photoconductivity. It lasts aslong as the sample is kept below 50 K.
To investigate both pulse width dependence and photon
energy dependence, we have utilized a tunable pulse laser
共200 fs pulse width, 1 kHz repetition rate兲 in photon energy
range of 0.25 eV (␭⫽5 ␮ m)⬃1.55 eV (800 nm). Typicalresults are shown in Figs. 6 and 7. These results obtained at
20 K indicate that even very short pulses can induce the PPC
in the photon energy range from 0.25 eV although the excitation with 0.25 eV is less efficient. A comparison between
Figs. 4 and 6 shows that the effect depends on the integrated
photon fluence and that there is no significant nonlinear process involved. It is worth noting that judged from the penetration depth (⬃0.1 ␮ m) of the light used, the increase of
the conductance in Fig. 6 implies the conductivity increase

FIG. 5. Resistance as a function of time at 60 K. The sample
was irradiated with a cw laser (h ␯ ⫽2.33 eV) at the power densityof 10 mW/0.01 cm2 .

FIG. 6. Resistance as a function of time at 20 K. The sample
was irradiated with a pulse laser (h ␯ ⫽1.55 eV,200 fs,1 kHz兲 at theaverage power density of 10, 1, and 5 mW/0.01 cm2 , respectively.

of the order of 104 to 105 in the photoexcited portion of thesample.
The photon energy dependence of PPC was further investigated. We used CO2 laser with 0.12 eV photon energy. As
shown in Fig. 8, no PPC was observed at 20 K. The decrease
in resistance during the light illumination is probably due tothe heating effect.
In a perovskite manganite R 0.5A 0.5MnO3 , in which A site
cation radius is small or the mismatch of the cation radii islarge, the system has strong randomness. Randomness promotes a glassy state by bringing frustration into the competition between the double exchange ferromagnetic interaction
and the superexchange antiferromagnetic interaction. Therandomness also induces phase separation.13 In the case ofPr0.7Ca0.3MnO3 , the system is spin glass state due to theincommensurability of Mn valency from 3.5⫹. The averagevalency of Mn in Pr0.75Na0.25MnO3 is ⫹3.39, i.e., the sameas in Pr0.61Ca0.39MnO3 . Therefore, we ascribe the CG state in
Pr0.75Na0.25MnO3 below 45 K to the randomness of the large
charge mismatch between Pr3⫹ and Na⫹ or the incommensurability of Mn valency from 3.5⫹.
Although in the case of Pr0.61Ca0.39MnO3 , the CO state isstable and robust against the magnetic field, the cation charge
mismatch or incommensurability in Pr0.75Na0.25MnO3 appears to be large enough to disrupt the CO state and to bringthe critical field down to 1.5 T. The instability of the COstate is such that, once it is destroyed by the screening by thephotocarriers, at least part of it does not recover. Hence, theFIG. 7. Resistance as a function of time at 20 K. The sample
was irradiated with a pulse laser (h ␯ ⫽0.25 mW,200 fs,1 kHz兲 at
the average power density of 2 mW/0.01 cm2 .

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PHYSICAL REVIEW B 65 125103

that in La0.7Ca0.3MnO3⫺ ␦ , we doubt that our PPC is due tothe oxygen deficiency. Pr0.75Na0.25MnO3 is a CO insulator
and no metallic state can appear with increasing hole concentration x contrary to La0.7Ca0.3MnO3⫺ ␦ . It should be
noted that PPC occurs with pulse laser in Pr0.75Na0.25MnO3
while it does not in oxygen deficient material such as
YBa2 Cu3 Ox . 15
IV. CONCLUSIONS
FIG. 8. Resistance as a function of time at 20 K. The sample
was irradiated with a cw laser (h ␯ ⫽0.12 eV) at the power densityof 2 mW/0.1 cm2 .

metastable FM clusters grow, leading to PPC with percolative conduction. Note that the large conductivity increase of
the order of 104 to 105 can only be explained in terms of net
growth of the FM state rather than rearrangement of existing
FM clusters.9
In the case of Pr0.69Ca0.31MnO3 , in which detailed opticalmeasurements are available,14 the CO gap is 0.2 eV. Because
the mechanism for the gap formation should be the same inPr0.75Na0.25MnO3 , the CO gap must be of a similar magnitude as well. The fact that the PPC was observed not at 0.12
eV but at 0.25 eV is therefore quite reasonable because asingle particle excitation beyond the CO gap muse be a prerequisite for the destruction of the charge order.
Although the observed PPC with the cw laser is similar toACKNOWLEDGMENTS

The authors are grateful to M. Izumi, H. Tamaru, Y. Tomioka, Y. Taguchi, K. Miyagawa, and T. Ogasawara for technical assistances and fruitful discussions. This work is supported by a Grant-in-Aid for COE Research from theMinistry of Education, Culture, Sports, Science and Technology of Japan. Three of us acknowledge the financial supportof the Grant Agency of Academy of Sciences, Czech Republic 共Grant No. A1010004/00兲
8*Electronic address: tsatoh@myn.rcast.u-tokyo.ac.jp
1

Pr0.75Na0.25MnO3 is a charge ordered insulator whichshows colossal magnetoresistance under relatively low magnetic field 共1.5 T兲. Below 45 K, the system is in a highly
nonuniform and metastable state, in which ferromagnetic
metallic clusters exist in insulating matrix. Light illumination
makes the charge ordered gap collapse, and the clusters expand. The persistent photoconductivity was observed due tothe percolation conduction.

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