Giant 1/f Noise in Low- T c CMR Manganites: Evidence of the Percolation Threshold
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ABSTRACT We observed a dramatic peak in the 1/f noise at the metal-insulator transition (MIT) in low-T, manganites. This many-orders-of-magnitude noise enhancement is observed for both polycrystalline and single-crystal samples of La 5 /8-. Pr, Ca 3/ 8 MnO 3 (y = 0.35 - 0.4) and Prj-x Ca.Mn0 3 (x = 0.35 - 0.5). This observation strongly suggests that the microscopic phase separation in the low-T, manganites causes formation of a percolation network, and that the observed MIT is a percolation threshold. It is shown that the scale of phase separation in polycrystalline samples is much smaller than that in single crystals. INTRODUCTION It is well known that the electronic phase diagrams of perovskite manganites are very complex; they exhibit numerous ground states and phase transitions as the carrier concentration is varied [1]. The phase diagram for the system LajiCa.Mn0 3 in the plane of the doping concentration x and temperature T is shown in Fig. 1. The concentration of charge carriers in this system is proportional to the Ca doping level. At high temperatures, the system is in a paramagnetic insulating phase. Recent resistivity and X-ray studies revealed that this paramagnetic phase comprises three different lattice types [2]. In the temperature range T - 200 - 800K, the lattice is orthorhombic (Jahn-Teller distorted at low x, and octahedron rotated at higher doping levels); at higher temperatures, it is rhombohedral. In the range x 0.3-0.5, the system undergoes a transition into the ferromagnetically-ordered state at T 200 - 250K; this magnetic transition is accompanied by the metal-insulator transition (MIT) [3, 4, 5]. A very high sensitivity of this MIT to the external magnetic field results in the so-called colossal magnetoresistance (CMR) [6]. The change of the resistivity across the transition and, correspondingly, the CMR are more dramatic in compounds with a lower transition temperature T,. The MIT in low-T, manganites bears many features which are intrinsic to the first-order transitions, including a strong thermal hysteresis of the resistivity p and magnetization M [7, 8]. There is a growing theoretical and experimental evidence that transport properties of the insulating state above T, are dominated by small polarons or magnetic polarons, and that the band-like carriers become important below T, [9, 10, 11]. However, the details of the MIT remain unclear. The experimental data suggest that the phase separation occurs gradually with decreasing temperature [7, 8]. Appearance of small ferromagnetic (FM) regions in the charge-ordered (CO) phase has been reported at T >> T,, well beyond the conventional fluctuation regime [12]. A very large and temperature-independent resistivity in the FM state, much greater than the Ioffe-Regel limit for a uniform metallic system, also indicates 113 Mat. Res. Soc. Symp. Proc. Vol. 602 0 2000 Materials Research Society
1000
800 S600I.-
0
400
3/8 4/8
5/8
o . 200
x=1/8
0 0.0
7/8
CO
0.2
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0.8
1.0
Ca X Figure 1: The phase diagram for Laj_,Ca.Mn0 3 [2].
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