Effect of strain and electric field on the electronic soft matter in manganite thin films

Abstract

We have studied the effect of substrate-induced strain on the properties of the hole-doped manganite (La1−yPry)0.67Ca0.33MnO3 (y = 0.4, 0.5 and 0.6) in order to distinguish between the roles played by long-range strain interactions and quenched atomic disorder in forming the micrometerscale phase separated state. We show that a fluid phase separated (FPS) state is formed at intermediate temperatures similar to the strain-liquid state in bulk compounds, which can be converted to a metallic state by applying an external electric field. In contrast to bulk compounds, at low temperatures a strain stabilized ferromagnetic metallic (FMM) state is formed in the y = 0.4 and 0.5 samples. However, in the y = 0.6 sample a static phase separated (SPS) state is formed similar to the strain-glass phase in bulk compounds. Hence, we show that long-range strain interaction plays a dominant role in forming the micrometer-scale phase separated state in manganite thin films.PACS numbers: 75.47. Lx, 73.50.Fq, 75.47.Gk, Multiphase coexistence in hole-doped manganites is a result of the competition between phases of different electronic, magnetic and structural orders [1,2]. This competition leads to large changes in the physical properties of manganites due to small perturbations e.g. colossal negative magnetoresistance (CMR). At low temperatures the two competing phases are the ferromagnetic metallic (FMM) and charge-ordered insulating (COI) phases. In manganites with greater average A-site cation radii ( r A ) and consequently a larger effective one-electron bandwidth (W ) (e.g. La 1−x Ca x MnO 3 , 0.2 < x < 0.5), the pseudocubic FMM phase is favored at low temperatures [3]. When smaller ions such as Pr are substituted at the A-site, r A and W are reduced. In these compounds the double-exchange mechanism is suppressed and hence the pseudotetragonal (distorted) COI phase has a comparable free energy to the FMM phase, resulting in micrometer scale phase separation [4]. It was shown that in the presence of quenched disorder introduced by the ions of different radii, the similarity of the free energies leads to coexistence of the two competing phases [2]. However, the observation of martensitic strain accommodation in manganites [5] and fluid-like growth of the FMM phase observed in magnetic force microscopy (MFM) images of phase separated manganites [6], suggests that the phases are not pinned. In fact, due to this behavior the phase separated state in manganites has been described as an "electronic soft matter" state [2,7]. These observations can be explained by an alternative model which shows that the different crystal structures of the FMM and COI phases generate long range strain interactions leading to an intrinsic elastic energy landscape, which leads to micrometer scale phase separation even without quenched disorder [1]. To understand the underlying mechanism for micrometer scale phase separation in manganites, it is essential to distinguish between the roles played by quenched disorder and long range strain interactions. We ...

Electric field driven dynamic percolation in electronically phase separated (La0.4Pr0.6)Jeen1, Biswas2 2013Phys. Rev. BSelf Cite321310View full textAdd to dashboardCite

“…An important feature of the resistance anisotropy is that it is observed only during the cooling cycle. It has been shown that the FPS state in which the FMM regions behave in a dynamic, fluid-like fashion, occurs only during the cooling cycle [7,12]. Such dynamic behavior in the phase coexistence region has also been observed using magnetization, noise, and neutron diffraction measurements [25][26][27].…”

“…In addition, theoretical calculations have also shown that an electric field can lead to anisotropic resistivity and CER in the phase separated state of manganites [16,19]. We observed the DEP-like behavior only in the fluid phase separated (FPS) region of the (La 0.4 Pr 0.6 ) 0.67 Ca 0.33 MnO 3 (LPCMO) phase diagram which corresponds to the same region of the phase diagram where strong electric field effects have been observed before [12,13,20]. On the other hand, in the glassy, static phase separated (SPS) region where the electric field effects are weak, DEP-like behavior is not observed.…”

“…The high resistance of the microstructure necessitated a two-probe, constant voltage resistance measurement as described in ref. [12].…”

“…Electric field induced resistance changes are of particular interest and when resistance drops of several orders of magnitude were observed on the application of an electric field (colossal electroresistance or CER) in (La 1−y Pr y ) 1−x Ca x MnO 3 [12][13][14][15], it was suggested that the effect was due to growth of the FMM regions which, if true, would be a unique form of ME coupling. However, magnetization measurements in the presence of an electric field ruled out such a scenario [13].…”

“…The details of the measurement scheme is given in ref. [12]. The resistance in the orthogonal in-plane direction was measured by applying a small ac voltage (V ac ) of 1 V rms in that direction.…”