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Role of photoinduced heating in transient photoconductivity in CMR materials
ARTICLE IN PRESS
Physica B 363 (2005) 76–81 www.elsevier.com/locate/physb
Role of photoinduced heating in transient photoconductivity in CMR materials N. Noginova, C.E. Bonner Center for Materials Research, Norfolk State University, 700 Park Ave, Norfolk, VA 23504, USA Received 11 January 2004; accepted 18 February 2005
Abstract The temperature dependence of the photoconductivity of La1xSrxMnO3 colossal magnetoresistance (CMR) films and single crystals has been studied using visible pulsed radiation. The responses in films and crystals have similar appearance but are in different time scales. Critical reduction of the relaxation rate was observed in vicinity of the transition temperature. The experimental results are explained with a simple thermodynamic model accounting for the latent heat of ferromagnetic transition as well as heat conduction and redistribution processes. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30V; 75.30K; 73.50P; 72.40 Keywords: Colossal magnetoresistance materials; Photoresponse; Magnetic phase transitions
1. Introduction As known, colossal magnetoresistance (CMR) materials [1] exhibit a sharp transition from a paramagnetic insulator to a ferromagnetic metal at room temperature range, which makes them promising for applications in light sensors, in particular, in room-temperature microbolometers [2–4]. Under cw light illumination, CMR materials demonstrate significant change in conductivity Corresponding author. Tel.: +1 757 823 8047;
fax: +1 757 823 9054. E-mail address: [email protected] (N. Noginova).
[2,5]. As it was shown in these experiments, the response has mostly bolometric origin and is due to heating by the cw laser radiation. In a number of publications, it was argued that the responses to the pulsed laser radiation in DC or optical conductivity as well as light-induced transient thermoelectrical (TTE) effects in CMR materials with relaxation times in nanosecond and microsecond scales have non-bolometric origin and are related to the photoionization of Jahn– Teller (JT) polarons and spin–flip processes due to the direct excitation of Mn centers [6–9]. It was reported that such transient effects demonstrate significant slowdown of the relaxation rate when
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.02.034
ARTICLE IN PRESS N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
the temperature approaches the phase transition temperature, Tc, from below. The slowdown was observed in the different materials and in different time scales. The photoinduced effects in reflectivity observed below the transition in layered perovskite manganite, La0.5Sr1.5MnO4 [6] have the evolution times changing from 2 to 8 ns as temperature approaches Tc. Critical slowdown of the relaxation rate was observed in photoinduced transmission in La0.7Ca0.3MnO3 thin film with relaxation times changing from 20 ns to more than 7 ms [7]U Similar growth of the relaxation time (from 0.01 to 0.3 s) was observed in time-resolved thermoelectrical (TTE) measurements in Nd0.67Sr0.37MnO3d [8]. Changes in temporal behavior in microsecond scale were observed in light-induced TTE profile [9] in Pr0.7Ca0.3MnO3. Such critical behavior observed in the vicinity of the transition in CMR thin films was ascribed to JT effects and charge ordering fluctuations [7–10]. It should be noted as well that oxygen-deficient CMR materials demonstrate an additional contribution to the response, with very long relaxation times, t41 s; related to the increase of the carrier concentration due to photoionization of electron–hole pairs and trapping of electrons in oxygen vacancies [11].
2. Experiment To obtain more information on the temperature dependence and to clarify the origin of the photoresponse and slowdown of its relaxation rate in CMR materials, we have studied the photoinduced changes in conductivity on exposure to pulsed laser light radiation in La1xSrxMnO3 thin films and single crystals. The film of La0.6Sr0.4MnO3 (with thickness of 0.6 mm) on sapphire substrate was fabricated by MOCVD technique. The bulk single crystal of La0.8Sr0.2MnO3 was grown by the floating zone technique with radiation heating. In the experiment, we used the crystal in the shape of a thin plate with thickness of 0.5 mm. The resistance vs. temperature dependences were measured by the standard four-point probe technique. The transition temperatures are 230 K for the film and 306 K for the crystal.
77
To study the photoresponse, the sample was mounted in a special holder with two electrical contacts and placed in the optical liquid helium continuous flow cryostat. A non-heating DC current was applied in the constant current mode. The sample surface between the contacts was illuminated with optical pulses at 532 nm, 3 ns in duration, and repetition rate of 10 Hz. The area of laser spot was about 1 mm2. The energy of the pulse used in experiments with the film and the crystal was about 0.05 and 1 mJ/pulse, respectively. A response to the illumination as a change in the resistance was recorded with 500 MHz digital oscilloscope. Typical responses to the pulsed illumination in electrical resistance in the film and crystal are shown in Fig. 1a and b, respectively. As one can see, they have different time scales but similar features. The responses are negative in the high temperature range (decrease in the resistance due to the laser illumination), and positive at temperatures below transition temperature, Tc (increase in the resistance). The response in the film (Fig. 1a) can be described as a sum of two components: a fast component with a very short rise time and a relaxation time of about 100 ns, and an additional slow component observed as a step function on this time scale. The relaxation of the slow component does not depend on the temperature significantly. In our opinion, this contribution to the relaxation is due to the heat conduction from the film substrate to the holder and will not be discussed here. On the other hand, the relaxation of the fast component is strongly dependent on the temperature, demonstrating a kind of critical behavior at Tc. At temperatures below Tc, the relaxation of the ‘‘positive’’ response slows down with increasing temperature, demonstrating sharp decrease of the relaxation rate in vicinity Tc. The ‘‘negative’’ photoresponse observed at T4T c demonstrates acceleration of the relaxation rate as T is approaching Tc, see Fig. 2. The photoconductive response recorded in the CMR single crystal is shown in Fig. 1b. One can see that the time scale of the relaxation changes dramatically from the 100 ns for the film
ARTICLE IN PRESS N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
78
1.5 R, kΩ
225 K
1 response, a.u.
to 200 ms for the crystal. In the crystal, the relaxation is hardly described with a single or a sum of two exponentials. Similar to the film, significant slowdown of the relaxation is observed as temperature increases from below to Tc. (Compare curves at 260 K and 282 K, Fig. 1b.) However, at T4T c ; the relaxation time in the crystal practically does not depend on temperature.
20 10 0 100
0.5
300 T, K
205 K
0
228 K 260 K
-0.5 -50
3. Discussion 150 time, ns
(a)
100
response, a.u.
R,Ω
260K
350
50
0 282K 200
0.5
300 T, K
400
296K
316K -0.5 -0.1
0.4
0.9
1.4
time, ms
(b)
Fig. 1. Transient photoconductivity in La1xSrxMnO3 CMR film (a) and crystal (b) at different temperatures. Insets: temperature dependence of the resistance.
250
τ, ns
200
We believe the fact that the response in both single crystals and thin films demonstrates common features and similar dependences on temperature but different relaxation times points to that the relaxation is determined mostly by thermal conduction processes. To explain the critical reduction of the relaxation rate near Tc in our experiments, let us consider a simple model. The absorption of a laser pulse results in the change of the effective temperature of the sample from T0 to T þ DT; where T0 is the temperature before illumination, and DT is the change induced by light absorption. Let us assume that the change in the temperature due to the heating by the laser pulse can be high and in the range of tens of K. During the cooling following the absorption of the pulse, the resistance of the sample changes from RðT 0 þ DTÞ back to R(T0) following the R(T) curve as the temperature of the sample relaxes to T0. In the case of a single exponential relaxation of temperature, after the laser pulse absorption the response in resistance depends on time as DR ¼ RðT 0 þ DT expðt=tÞÞ RðT 0 Þ,
150 100 50 0 0
100
200
300
T, K Fig. 2. The dependence of the relaxation time on the temperature in the film (in the approximation of the relaxation with a single exponential).
(1)
where t is the relaxation time constant, determined by the heat conduction process. Using our experimental data on the temperature dependence of the resistance (see insets in Fig. 1) we fit the relaxation profiles observed in our samples with Eq. (1) using t and DT as fitting parameters. As one can see, our experimental data (Fig. 1) can be well fit with the model, see Fig. 3. The time constants are 280 ns and 1400 ms for the film and the crystal, respectively. The fitting parameter, temperature change, DT, the initial
ARTICLE IN PRESS N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
Table 1 The fitting parameter, DT; and the temperatures before and after pulse absorption in La0.6Sr0.4MnO3 film
response, a.u.
1.5 A 225K B
1
La0.6Sr0.4MnO3 film 0.5
Type of response (K) Positive Positive Positive Negative
205K
0
DT T0 T1
228K D C 260K
-0.5 0
100
200
25 205 230
14 215 229
3 225 228
30 260 290
300
τ, ns
(a)
Table 2 The fitting parameter, DT; and the temperatures before and after pulse absorption in La0.8Sr0.2MnO3 crystal
260K response, a.u.
79
La0.8Sr0.2MnO3 crystal
282K
0.5
Type of response (K) Positive Positive Positive Negative A
DT T0 T1
B 296K C
46 260 306
24 282 306
9 296 305
50 316 366
D 316K
-0.5 0
(b)
0.5
1
1.5
time, ms
Fig. 3. Results of the simulation: (a) Film—traces A and C present experimental responses at T ¼ 225 and 260 K, traces B and D are simulations for the corresponding temperatures. (b) Crystal—traces A and C present experimental responses at T ¼ 296 and 316 K, traces B and D are simulations for the corresponding temperatures.
temperature T0, and the temperature obtained after the pulse absorption, T 1 ¼ ðT 0 þ DTÞ; are shown in Tables 1 and 2 for the film and crystal, respectively. As one can see from the Table 1, in all the cases, when the initial temperature is below Tc, the effective temperature of the sample after the pulse absorption T 1 ¼ T 0 þ DT is exactly equal to the temperature of the transition. This result is quite remarkable and provides strong support for our model. Indeed, if the initial temperature, T0, is much lower than transition temperature and T 1 T c ; the temperature range with a strong change of R(T) is being passed during the very beginning of the relaxation process. In this case, the transient
signal DRðtÞ ¼ RðTÞ RðT 0 Þ has an observable amplitude at a very short time just after the laser pulse absorption while at the rest of the relaxation process, DRðtÞ 0: If the initial temperature, T0, is in the range of a strong dependence R(T) (that is sufficiently close to the transition temperature), the increment DT caused by the heating becomes limited and the observed dependence of R on time during the relaxation of temperature from T 1 T c to T0 lasts much longer. The change of the temperature due to the absorption of the laser pulse could be high in the case of the film. Assuming a heat capacity, cp, of about 0.3 J/g K [8,12], the estimated change of the temperature in the film can be in the order of tens of K. The relaxation time constant of about 200 ns for the fast component of the response is determined by heat conduction from the film to the substrate and seems to be reasonable according to the estimations of that from the data on thermal conductivity [8]. However, for the bulk crystal, similar calculations yield the estimation of the temperature change of about 1 K, which is much lower, and
ARTICLE IN PRESS 80
N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
the relaxation time constant of 0.2 s which is much slower then those from our fitting. The discrepancy can be explained by taking into account that the laser pulse delivers energy faster than it can be removed from the illuminated volume through heat conduction and redistribution in the bulk crystal, so not the whole crystal but only a thin surface layer is heated due to the pulse absorption. The relatively low amplitude of the response DR=R of 0.001–0.01 confirms this assumption. The non-uniform heating of the sample explains additional contributions observed in some experiments as well. That the surface layer is heated to the transition temperature but not higher is due to the fact that the transition consumes additional heat without changing the temperature. During the laser pulse, energy is absorbed by the electron system, and transferred to the lattice phonons as well to the magnetic subsystem. The rate of the energy exchange between the spin and phonon subsystems is too fast to be observed on this time scale. It is of the order of 1–3 ns [13] and comparable with the duration of the laser pulses used. Thus, the increase of the temperature, T, during the laser pulse can be approximately described as dT ¼ dQ=C,
(2)
where C is the total heat capacity of the lattice and magnetic subsystem, and Q is the energy received due to the laser pulse absorption. As T approaches Tc, the energy is used for the phase transition, and the temperature remains constant. If T0, is close enough to the transition, the temperature T1 at the end of the pulse can exceed the transition temperature (for a given laser power), and contributions from both negative and positive responses are observed, see Fig. 1a, the relaxation at 228 K. In this case, however, the thermal relaxation cannot be described by a single exponential, and the fit at 228 K (Fig. 3a) does not reproduce all the details of the experimental curve. Detailed analysis of such complex photoresponses observed at high-energy pulses in the range of the phase transition was presented in Ref. [12]. It was found that the temporal profile of the response is well described with heating and heat conduction processes. The response demonstrates
a negative component at the initial stage of the relaxation (when the sample is cooling down to Tc), a flat segment (at T ¼ T c ), and a positive component at the final stage of the relaxation when temperature relaxes to the initial temperature below Tc. The presence of such a flat segment in the temporal profile of the response can be explained by the energy release during phase transition. The relaxation of the response is determined by the heat conduction from illuminated volume. Note that in some manganites, the heat conductivity demonstrates a peak close to the transition temperature [14], thus the heat realized during the transition back to the ferromagnetic state is removed faster than that during the following relaxation of the temperature. In this case, the temporal profile of the relaxation is mostly determined by the relaxation of the temperature and can be fairly described with Eq. (1). In conclusion, we explain our results on photoresponses, including slowdown of the relaxation in vicinity of Tc using only a simple model accounting for thermodynamic processes in the illuminated volume.
Acknowledgments This work was supported by the NASA NRA99-OEOP-4 Grant, and NSF CREST Project HRD-9805059. Authors would like to thank E.S. Gillman for the providing the CMR thin films, A. Balbashov for providing the CMR single crystals, and A. Verevkin for useful discussions. References [1] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413; J.H. Hao, X.T. Zeng, H.K. Wong, J. Appl. Phys. 79 (1996) 1810. [2] J.H. Hao, X.T. Zeng, H.K. Wong, J. Appl. Phys. 79 (1996) 1810. [3] A. Lisauskas, S.I. Khartsev, A. Grishin, Appl. Phys. 77 (2000) 756. [4] A.A. Verevkin, N. Noginova, E.S. Gillman, A.A. Verevkin, N. Noginova, E.S. Gillman, Prospects of CMR thin-film-based microbolometers, in: Magnetoresistive
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[5]
[6] [7] [8]
[9]
Oxides and Related Materials, MRS Symposium Proceedings, vol. 602, 2000, pp. 357–361. Y.P. Gousev, A.A. Verevkin, H.K. Olsson, N. Noginova, E.S. Gillman (P), K.-H. Dahmen, J. Low Temp. Phys. 117 (5–6) (1999) 1665. T. Ogasawara, T. Kise, T. Ishikara, M. Kuwata-Gonokami, Y. Tokura, J. Lumin. 87–89 (2000) 639. X.J. Liu, Y. Moritomo, A. Nakamura, H. Tanaka, T. Kawai, Phys. Rev. B 64 (2001) 1000401. N.V. Kasper, A. Kattwinkel, N. Hamad, K. Ba¨rner, I.O. Troyanchuk, D.D. Khalyavin, M. Dressel, E. Gmelin, E. Schmitt, Physica B 292 (2000) 54. M. Fiebig, K. Miyano, Y. Tomioka, Y. Tokura, J. Lumin. 87–89 (2000) 82.
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[10] Y.G. Zhao, J.J. Li, R. Shreekala, H.D. Drew, C.L. Chen, W.L. Cao, C.H. Lee, M. Rajeswari, S.B. Ogale, R. Ramesh, G. Baskaran, T. Venkatesan, Phys. Rev. Lett. 81 (1998) 1310. [11] R. Cauro, A. Gilabert, J.P. Contour, R. Lyonnet, M.-G. Medici, J.-C. Grenet, G. Leighton, Ivan Schuller, Phys. Rev. B. 63 (2001) 174423. [12] N. Noginova, E. Arthur, R. Ulysse, E.S. Gillman, C.E. Bonner, in: Spintronics, MRS Proceedings, vol. 690, 2002, p. 49. [13] V.A. Atsarkin, V.V. Demidov, G.A. Vasneva, K. Conder, Phys. Rev. B 63 (2001) 92405. [14] J. Liebe, H. Kang, L. Haupt, P. Mandal, I.V. Medvedeva, K. Barner, A. Poddar, E. Gmelin, Physica B 239 (1997) 322.
Physica B 363 (2005) 76–81 www.elsevier.com/locate/physb
Role of photoinduced heating in transient photoconductivity in CMR materials N. Noginova, C.E. Bonner Center for Materials Research, Norfolk State University, 700 Park Ave, Norfolk, VA 23504, USA Received 11 January 2004; accepted 18 February 2005
Abstract The temperature dependence of the photoconductivity of La1xSrxMnO3 colossal magnetoresistance (CMR) films and single crystals has been studied using visible pulsed radiation. The responses in films and crystals have similar appearance but are in different time scales. Critical reduction of the relaxation rate was observed in vicinity of the transition temperature. The experimental results are explained with a simple thermodynamic model accounting for the latent heat of ferromagnetic transition as well as heat conduction and redistribution processes. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30V; 75.30K; 73.50P; 72.40 Keywords: Colossal magnetoresistance materials; Photoresponse; Magnetic phase transitions
1. Introduction As known, colossal magnetoresistance (CMR) materials [1] exhibit a sharp transition from a paramagnetic insulator to a ferromagnetic metal at room temperature range, which makes them promising for applications in light sensors, in particular, in room-temperature microbolometers [2–4]. Under cw light illumination, CMR materials demonstrate significant change in conductivity Corresponding author. Tel.: +1 757 823 8047;
fax: +1 757 823 9054. E-mail address: [email protected] (N. Noginova).
[2,5]. As it was shown in these experiments, the response has mostly bolometric origin and is due to heating by the cw laser radiation. In a number of publications, it was argued that the responses to the pulsed laser radiation in DC or optical conductivity as well as light-induced transient thermoelectrical (TTE) effects in CMR materials with relaxation times in nanosecond and microsecond scales have non-bolometric origin and are related to the photoionization of Jahn– Teller (JT) polarons and spin–flip processes due to the direct excitation of Mn centers [6–9]. It was reported that such transient effects demonstrate significant slowdown of the relaxation rate when
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.02.034
ARTICLE IN PRESS N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
the temperature approaches the phase transition temperature, Tc, from below. The slowdown was observed in the different materials and in different time scales. The photoinduced effects in reflectivity observed below the transition in layered perovskite manganite, La0.5Sr1.5MnO4 [6] have the evolution times changing from 2 to 8 ns as temperature approaches Tc. Critical slowdown of the relaxation rate was observed in photoinduced transmission in La0.7Ca0.3MnO3 thin film with relaxation times changing from 20 ns to more than 7 ms [7]U Similar growth of the relaxation time (from 0.01 to 0.3 s) was observed in time-resolved thermoelectrical (TTE) measurements in Nd0.67Sr0.37MnO3d [8]. Changes in temporal behavior in microsecond scale were observed in light-induced TTE profile [9] in Pr0.7Ca0.3MnO3. Such critical behavior observed in the vicinity of the transition in CMR thin films was ascribed to JT effects and charge ordering fluctuations [7–10]. It should be noted as well that oxygen-deficient CMR materials demonstrate an additional contribution to the response, with very long relaxation times, t41 s; related to the increase of the carrier concentration due to photoionization of electron–hole pairs and trapping of electrons in oxygen vacancies [11].
2. Experiment To obtain more information on the temperature dependence and to clarify the origin of the photoresponse and slowdown of its relaxation rate in CMR materials, we have studied the photoinduced changes in conductivity on exposure to pulsed laser light radiation in La1xSrxMnO3 thin films and single crystals. The film of La0.6Sr0.4MnO3 (with thickness of 0.6 mm) on sapphire substrate was fabricated by MOCVD technique. The bulk single crystal of La0.8Sr0.2MnO3 was grown by the floating zone technique with radiation heating. In the experiment, we used the crystal in the shape of a thin plate with thickness of 0.5 mm. The resistance vs. temperature dependences were measured by the standard four-point probe technique. The transition temperatures are 230 K for the film and 306 K for the crystal.
77
To study the photoresponse, the sample was mounted in a special holder with two electrical contacts and placed in the optical liquid helium continuous flow cryostat. A non-heating DC current was applied in the constant current mode. The sample surface between the contacts was illuminated with optical pulses at 532 nm, 3 ns in duration, and repetition rate of 10 Hz. The area of laser spot was about 1 mm2. The energy of the pulse used in experiments with the film and the crystal was about 0.05 and 1 mJ/pulse, respectively. A response to the illumination as a change in the resistance was recorded with 500 MHz digital oscilloscope. Typical responses to the pulsed illumination in electrical resistance in the film and crystal are shown in Fig. 1a and b, respectively. As one can see, they have different time scales but similar features. The responses are negative in the high temperature range (decrease in the resistance due to the laser illumination), and positive at temperatures below transition temperature, Tc (increase in the resistance). The response in the film (Fig. 1a) can be described as a sum of two components: a fast component with a very short rise time and a relaxation time of about 100 ns, and an additional slow component observed as a step function on this time scale. The relaxation of the slow component does not depend on the temperature significantly. In our opinion, this contribution to the relaxation is due to the heat conduction from the film substrate to the holder and will not be discussed here. On the other hand, the relaxation of the fast component is strongly dependent on the temperature, demonstrating a kind of critical behavior at Tc. At temperatures below Tc, the relaxation of the ‘‘positive’’ response slows down with increasing temperature, demonstrating sharp decrease of the relaxation rate in vicinity Tc. The ‘‘negative’’ photoresponse observed at T4T c demonstrates acceleration of the relaxation rate as T is approaching Tc, see Fig. 2. The photoconductive response recorded in the CMR single crystal is shown in Fig. 1b. One can see that the time scale of the relaxation changes dramatically from the 100 ns for the film
ARTICLE IN PRESS N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
78
1.5 R, kΩ
225 K
1 response, a.u.
to 200 ms for the crystal. In the crystal, the relaxation is hardly described with a single or a sum of two exponentials. Similar to the film, significant slowdown of the relaxation is observed as temperature increases from below to Tc. (Compare curves at 260 K and 282 K, Fig. 1b.) However, at T4T c ; the relaxation time in the crystal practically does not depend on temperature.
20 10 0 100
0.5
300 T, K
205 K
0
228 K 260 K
-0.5 -50
3. Discussion 150 time, ns
(a)
100
response, a.u.
R,Ω
260K
350
50
0 282K 200
0.5
300 T, K
400
296K
316K -0.5 -0.1
0.4
0.9
1.4
time, ms
(b)
Fig. 1. Transient photoconductivity in La1xSrxMnO3 CMR film (a) and crystal (b) at different temperatures. Insets: temperature dependence of the resistance.
250
τ, ns
200
We believe the fact that the response in both single crystals and thin films demonstrates common features and similar dependences on temperature but different relaxation times points to that the relaxation is determined mostly by thermal conduction processes. To explain the critical reduction of the relaxation rate near Tc in our experiments, let us consider a simple model. The absorption of a laser pulse results in the change of the effective temperature of the sample from T0 to T þ DT; where T0 is the temperature before illumination, and DT is the change induced by light absorption. Let us assume that the change in the temperature due to the heating by the laser pulse can be high and in the range of tens of K. During the cooling following the absorption of the pulse, the resistance of the sample changes from RðT 0 þ DTÞ back to R(T0) following the R(T) curve as the temperature of the sample relaxes to T0. In the case of a single exponential relaxation of temperature, after the laser pulse absorption the response in resistance depends on time as DR ¼ RðT 0 þ DT expðt=tÞÞ RðT 0 Þ,
150 100 50 0 0
100
200
300
T, K Fig. 2. The dependence of the relaxation time on the temperature in the film (in the approximation of the relaxation with a single exponential).
(1)
where t is the relaxation time constant, determined by the heat conduction process. Using our experimental data on the temperature dependence of the resistance (see insets in Fig. 1) we fit the relaxation profiles observed in our samples with Eq. (1) using t and DT as fitting parameters. As one can see, our experimental data (Fig. 1) can be well fit with the model, see Fig. 3. The time constants are 280 ns and 1400 ms for the film and the crystal, respectively. The fitting parameter, temperature change, DT, the initial
ARTICLE IN PRESS N. Noginova, C.E. Bonner / Physica B 363 (2005) 76–81
Table 1 The fitting parameter, DT; and the temperatures before and after pulse absorption in La0.6Sr0.4MnO3 film
response, a.u.
1.5 A 225K B
1
La0.6Sr0.4MnO3 film 0.5
Type of response (K) Positive Positive Positive Negative
205K
0
DT T0 T1
228K D C 260K
-0.5 0
100
200
25 205 230
14 215 229
3 225 228
30 260 290
300
τ, ns
(a)
Table 2 The fitting parameter, DT; and the temperatures before and after pulse absorption in La0.8Sr0.2MnO3 crystal
260K response, a.u.
79
La0.8Sr0.2MnO3 crystal
282K
0.5
Type of response (K) Positive Positive Positive Negative A
DT T0 T1
B 296K C
46 260 306
24 282 306
9 296 305
50 316 366
D 316K
-0.5 0
(b)
0.5
1
1.5
time, ms
Fig. 3. Results of the simulation: (a) Film—traces A and C present experimental responses at T ¼ 225 and 260 K, traces B and D are simulations for the corresponding temperatures. (b) Crystal—traces A and C present experimental responses at T ¼ 296 and 316 K, traces B and D are simulations for the corresponding temperatures.
temperature T0, and the temperature obtained after the pulse absorption, T 1 ¼ ðT 0 þ DTÞ; are shown in Tables 1 and 2 for the film and crystal, respectively. As one can see from the Table 1, in all the cases, when the initial temperature is below Tc, the effective temperature of the sample after the pulse absorption T 1 ¼ T 0 þ DT is exactly equal to the temperature of the transition. This result is quite remarkable and provides strong support for our model. Indeed, if the initial temperature, T0, is much lower than transition temperature and T 1 T c ; the temperature range with a strong change of R(T) is being passed during the very beginning of the relaxation process. In this case, the transient
signal DRðtÞ ¼ RðTÞ RðT 0 Þ has an observable amplitude at a very short time just after the laser pulse absorption while at the rest of the relaxation process, DRðtÞ 0: If the initial temperature, T0, is in the range of a strong dependence R(T) (that is sufficiently close to the transition temperature), the increment DT caused by the heating becomes limited and the observed dependence of R on time during the relaxation of temperature from T 1 T c to T0 lasts much longer. The change of the temperature due to the absorption of the laser pulse could be high in the case of the film. Assuming a heat capacity, cp, of about 0.3 J/g K [8,12], the estimated change of the temperature in the film can be in the order of tens of K. The relaxation time constant of about 200 ns for the fast component of the response is determined by heat conduction from the film to the substrate and seems to be reasonable according to the estimations of that from the data on thermal conductivity [8]. However, for the bulk crystal, similar calculations yield the estimation of the temperature change of about 1 K, which is much lower, and
ARTICLE IN PRESS 80
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the relaxation time constant of 0.2 s which is much slower then those from our fitting. The discrepancy can be explained by taking into account that the laser pulse delivers energy faster than it can be removed from the illuminated volume through heat conduction and redistribution in the bulk crystal, so not the whole crystal but only a thin surface layer is heated due to the pulse absorption. The relatively low amplitude of the response DR=R of 0.001–0.01 confirms this assumption. The non-uniform heating of the sample explains additional contributions observed in some experiments as well. That the surface layer is heated to the transition temperature but not higher is due to the fact that the transition consumes additional heat without changing the temperature. During the laser pulse, energy is absorbed by the electron system, and transferred to the lattice phonons as well to the magnetic subsystem. The rate of the energy exchange between the spin and phonon subsystems is too fast to be observed on this time scale. It is of the order of 1–3 ns [13] and comparable with the duration of the laser pulses used. Thus, the increase of the temperature, T, during the laser pulse can be approximately described as dT ¼ dQ=C,
(2)
where C is the total heat capacity of the lattice and magnetic subsystem, and Q is the energy received due to the laser pulse absorption. As T approaches Tc, the energy is used for the phase transition, and the temperature remains constant. If T0, is close enough to the transition, the temperature T1 at the end of the pulse can exceed the transition temperature (for a given laser power), and contributions from both negative and positive responses are observed, see Fig. 1a, the relaxation at 228 K. In this case, however, the thermal relaxation cannot be described by a single exponential, and the fit at 228 K (Fig. 3a) does not reproduce all the details of the experimental curve. Detailed analysis of such complex photoresponses observed at high-energy pulses in the range of the phase transition was presented in Ref. [12]. It was found that the temporal profile of the response is well described with heating and heat conduction processes. The response demonstrates
a negative component at the initial stage of the relaxation (when the sample is cooling down to Tc), a flat segment (at T ¼ T c ), and a positive component at the final stage of the relaxation when temperature relaxes to the initial temperature below Tc. The presence of such a flat segment in the temporal profile of the response can be explained by the energy release during phase transition. The relaxation of the response is determined by the heat conduction from illuminated volume. Note that in some manganites, the heat conductivity demonstrates a peak close to the transition temperature [14], thus the heat realized during the transition back to the ferromagnetic state is removed faster than that during the following relaxation of the temperature. In this case, the temporal profile of the relaxation is mostly determined by the relaxation of the temperature and can be fairly described with Eq. (1). In conclusion, we explain our results on photoresponses, including slowdown of the relaxation in vicinity of Tc using only a simple model accounting for thermodynamic processes in the illuminated volume.
Acknowledgments This work was supported by the NASA NRA99-OEOP-4 Grant, and NSF CREST Project HRD-9805059. Authors would like to thank E.S. Gillman for the providing the CMR thin films, A. Balbashov for providing the CMR single crystals, and A. Verevkin for useful discussions. References [1] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413; J.H. Hao, X.T. Zeng, H.K. Wong, J. Appl. Phys. 79 (1996) 1810. [2] J.H. Hao, X.T. Zeng, H.K. Wong, J. Appl. Phys. 79 (1996) 1810. [3] A. Lisauskas, S.I. Khartsev, A. Grishin, Appl. Phys. 77 (2000) 756. [4] A.A. Verevkin, N. Noginova, E.S. Gillman, A.A. Verevkin, N. Noginova, E.S. Gillman, Prospects of CMR thin-film-based microbolometers, in: Magnetoresistive
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