- Email: [email protected]
Local magnetization and transport properties in phase-separated (La,Pr)1−xCaxMnO3 in the vicinity of the phase transition
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e289–e290
Local magnetization and transport properties in phase-separated ðLa; PrÞ1xCaxMnO3 in the vicinity of the phase transition M. Tokunaga*, Y. Tokunaga, T. Tamegai Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract Local magnetization has been studied in a phase-separated manganite ðLa1y Pry Þ0:7 Ca0:3 MnO3 ðy ¼ 0:7Þ by a magneto-optical imaging technique simultaneously with resistivity measurements. The observed image demonstrates a region in which magnetization changes corresponding to a telegraph noise in resistive fluctuation. This observation is the first direct evidence for a switching domain in phase-separated manganites. r 2003 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 72.70.+m; 75.30.Vn Keywords: Manganite; Phase separation; Telegraph noise
102 101
1. Introduction
*Corresponding author. Tel.: +81-3-5841-6848; fax: +81-35841-6848. E-mail address: [email protected] (M. Tokunaga).
ρ (Ω cm)
Manganites with the perovskite-type structure are famous for the colossal magneto-resistance effect in the vicinity of a transition between ferromagnetic metal (FMM) and antiferromagnetic (or paramagnetic) insulator (AFMI (or PMI)). In some manganites, competition between phases realizes co-existence of two phases within a sample (phase separation) [1]. Indeed, such coexisting states have been directly observed by microscopic experiments [2,3]. In this state, insulator–metal transition is no more homogeneous but occurs in a percolative way [4]. Consequently, a transition in a small domain can cause significant change in the resistivity of a macroscopic sample by cutting (or connecting) a conduction path. Actually, several groups observed random telegraph noise in resistivity fluctuation and ascribed it to the existence of a switching domain as a result of thermodynamic discussion [5–7]. Up to now,
(a)
100 10-1 10-2 10-3 10-4 0
50
100
150
200
250
300
T (K)
(b)
T = 91 K
∆R
1%
0
20
40 time (s)
60
80
Fig. 1. (a) Temperature dependence of resistivity in LPCMO in cooling and heating processes. (b) Temporal variation of resistivity at 91 K: The dashed line represents the threshold value we set to distinguish high and low resistivity states.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.400
ARTICLE IN PRESS e290
M. Tokunaga et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e289–e290
(a)
(b)
50
µm
Fig. 2. (a) An MO image at 91 K: (b) A differential MO image between high and low resistivity states at 91 K in the same region as (a).
however, no direct observation has been reported for the existence of the switching domain to the best of our knowledge. In this study, we performed magneto-optical (MO) imaging on ðLa1y Pry Þ0:7 Ca0:3 MnO3 with y ¼ 0:7 (LPCMO) with simultaneous transport measurements in which a phase separation into FMM and AFMI has been reported below about 130 K [8].
2. Experimental results and discussion Crystals of LPCMO were grown by the floating-zone method. The sample surface was polished by 0:5 mm diamond slurry and scraped by Ar-ion milling to remove strain in the surface. Magnetic induction normal to the surface was detected through Faraday rotation in a garnet film mounted on the sample and visualized by a polarizing microscope. The MO images were captured by a cooled CCD-camera. Details of our MO imaging technique are described in Ref. [9]. Fig. 1(a) shows temperature dependence of resistivity in LPCMO in cooling and heating processes. Resistivity of the sample shows metallic behavior below about 120 K; which coincides with a ferromagnetic transition determined by a bulk magnetization measurement. Below this temperature, we observed telegraph noise in resistivity as shown in Fig. 1(b). Such a stepwise change between high and low resistivity states has been ascribed to the presence of a switching domain. The MO image at 91 K is inhomogeneous as shown in Fig. 2(a). Such inhomogeneities are hardly visible above the insulator–metal transition temperature. To clarify the change in local magnetization, we took a differential
image between states with higher and lower resistivity triggered by the resistivity data. Fig. 2(b) shows thus obtained differential MO image at 91 K in the same region as Fig. 2(a). We can see a white spot pointed by an arrow at the left bottom in Fig. 2(b). The observed spot looks too large compared with the typical size ðo100 nmÞ expected from thermodynamic discussion. A switching domain deep in the sample can only be detected as much larger domain in the MO image than the real one because of the diffusion of magnetic flux before reaching the garnet film. Taking into consideration this factor, the observed domain can be ascribed to the switching domain visualized for the first time.
Acknowledgements This work is supported by Grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
E. Dagotto, Phys. Rep. 344 (2001) 1 and references therein. M. Uehara, et al., Nature 399 (1999) 560. M. Fath, et al., Science 285 (1999) 1540. L. Zhang, et al., Science 298 (2002) 805. R.D. Merithew, et al., Phys. Rev. B 84 (2000) 3442. B. Raquet, et al., Phys. Rev. Lett. 84 (2000) 4485. V. Podzorov, et al., Phys. Rev. B 64 (2001) 115113. A.M. Balagurov, et al., Phys. Rev. B 64 (2001) 024420. M. Yasugaki, et al., Phys. Rev. B 65 (2002) 212502.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e289–e290
Local magnetization and transport properties in phase-separated ðLa; PrÞ1xCaxMnO3 in the vicinity of the phase transition M. Tokunaga*, Y. Tokunaga, T. Tamegai Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract Local magnetization has been studied in a phase-separated manganite ðLa1y Pry Þ0:7 Ca0:3 MnO3 ðy ¼ 0:7Þ by a magneto-optical imaging technique simultaneously with resistivity measurements. The observed image demonstrates a region in which magnetization changes corresponding to a telegraph noise in resistive fluctuation. This observation is the first direct evidence for a switching domain in phase-separated manganites. r 2003 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 72.70.+m; 75.30.Vn Keywords: Manganite; Phase separation; Telegraph noise
102 101
1. Introduction
*Corresponding author. Tel.: +81-3-5841-6848; fax: +81-35841-6848. E-mail address: [email protected] (M. Tokunaga).
ρ (Ω cm)
Manganites with the perovskite-type structure are famous for the colossal magneto-resistance effect in the vicinity of a transition between ferromagnetic metal (FMM) and antiferromagnetic (or paramagnetic) insulator (AFMI (or PMI)). In some manganites, competition between phases realizes co-existence of two phases within a sample (phase separation) [1]. Indeed, such coexisting states have been directly observed by microscopic experiments [2,3]. In this state, insulator–metal transition is no more homogeneous but occurs in a percolative way [4]. Consequently, a transition in a small domain can cause significant change in the resistivity of a macroscopic sample by cutting (or connecting) a conduction path. Actually, several groups observed random telegraph noise in resistivity fluctuation and ascribed it to the existence of a switching domain as a result of thermodynamic discussion [5–7]. Up to now,
(a)
100 10-1 10-2 10-3 10-4 0
50
100
150
200
250
300
T (K)
(b)
T = 91 K
∆R
1%
0
20
40 time (s)
60
80
Fig. 1. (a) Temperature dependence of resistivity in LPCMO in cooling and heating processes. (b) Temporal variation of resistivity at 91 K: The dashed line represents the threshold value we set to distinguish high and low resistivity states.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.400
ARTICLE IN PRESS e290
M. Tokunaga et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e289–e290
(a)
(b)
50
µm
Fig. 2. (a) An MO image at 91 K: (b) A differential MO image between high and low resistivity states at 91 K in the same region as (a).
however, no direct observation has been reported for the existence of the switching domain to the best of our knowledge. In this study, we performed magneto-optical (MO) imaging on ðLa1y Pry Þ0:7 Ca0:3 MnO3 with y ¼ 0:7 (LPCMO) with simultaneous transport measurements in which a phase separation into FMM and AFMI has been reported below about 130 K [8].
2. Experimental results and discussion Crystals of LPCMO were grown by the floating-zone method. The sample surface was polished by 0:5 mm diamond slurry and scraped by Ar-ion milling to remove strain in the surface. Magnetic induction normal to the surface was detected through Faraday rotation in a garnet film mounted on the sample and visualized by a polarizing microscope. The MO images were captured by a cooled CCD-camera. Details of our MO imaging technique are described in Ref. [9]. Fig. 1(a) shows temperature dependence of resistivity in LPCMO in cooling and heating processes. Resistivity of the sample shows metallic behavior below about 120 K; which coincides with a ferromagnetic transition determined by a bulk magnetization measurement. Below this temperature, we observed telegraph noise in resistivity as shown in Fig. 1(b). Such a stepwise change between high and low resistivity states has been ascribed to the presence of a switching domain. The MO image at 91 K is inhomogeneous as shown in Fig. 2(a). Such inhomogeneities are hardly visible above the insulator–metal transition temperature. To clarify the change in local magnetization, we took a differential
image between states with higher and lower resistivity triggered by the resistivity data. Fig. 2(b) shows thus obtained differential MO image at 91 K in the same region as Fig. 2(a). We can see a white spot pointed by an arrow at the left bottom in Fig. 2(b). The observed spot looks too large compared with the typical size ðo100 nmÞ expected from thermodynamic discussion. A switching domain deep in the sample can only be detected as much larger domain in the MO image than the real one because of the diffusion of magnetic flux before reaching the garnet film. Taking into consideration this factor, the observed domain can be ascribed to the switching domain visualized for the first time.
Acknowledgements This work is supported by Grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
E. Dagotto, Phys. Rep. 344 (2001) 1 and references therein. M. Uehara, et al., Nature 399 (1999) 560. M. Fath, et al., Science 285 (1999) 1540. L. Zhang, et al., Science 298 (2002) 805. R.D. Merithew, et al., Phys. Rev. B 84 (2000) 3442. B. Raquet, et al., Phys. Rev. Lett. 84 (2000) 4485. V. Podzorov, et al., Phys. Rev. B 64 (2001) 115113. A.M. Balagurov, et al., Phys. Rev. B 64 (2001) 024420. M. Yasugaki, et al., Phys. Rev. B 65 (2002) 212502.