The sensitivity of transport in colossal magnetoresistance (CMR) manganites to external electric and magnetic fields is examined using field effect configurations with La(0.7)Ca(0.3)MnO(3) (LCMO), Na(0.7)Sr(0.3)MnO(3), La(0.7)Ba(0.3)MnO(3), and La(0.5)Ca(0.5)MnO(3) (0.5-doped LCMO) channels, and ferroelectric PbZr(0.2)Ti(0.8)O(3) (PZT) or dielectric (SrTiO(3)) gates. A large electroresistance (ER) of approximately 76% at 4 x 10(5) V/cm is found in LCMO with PZT-ferroelectric gate, but the magnitude of the effect is much smaller (a few percent) in the other three channels. The ER and CMR effects are remarkably complimentary. The size and systematics of the effect strongly favor a percolative phase separation picture.

Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3 start to grow and merge at the metal-insulator transition temperature TP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2 higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials.

We present a study of the effect of biaxial strain on the electrical and magnetic properties of thin films of manganites. We observe that manganite films grown under biaxial compressive strain exhibit island growth morphology which leads to a non-uniform distribution of the strain. Transport and magnetic properties of these films suggest the coexistence of two different phases, a metallic ferromagnet and an insulating antiferromagnet. We suggest that the high strain regions are insulating while the low strain regions are metallic. In such non-uniformly strained samples, we observe a large magnetoresistance and a field-induced insulator to metal transition.Comment: 5 pages ReVTeX, 5 figures included, Figures 3, 4 and 5 low resolution, high resolution figures available on request from authors, submitted to Phys. Rev.

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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 ...

We present evidence for the coexistence of ferromagnetic metallic and charge ordered insulating phases in strained thin films of La0.67Ca0.33MnO3 at low temperatures. Such a phase separated state is confirmed using low temperature magnetic force microscopy and magnetotransport measurements. This phase separated state is not observed in the bulk form of this compound and is caused by the structural inhomogeneities due to the non-uniform distribution of strain in the film. The strain weakens the low temperature ferromagnetic metallic state and a charge ordered insulator is formed at the high strain regions. The slow dynamics of the transport properties of the mixed phase is illustrated by measurements of the long time scale relaxation of the electrical resistance.

Ceramics of A2 FeReO6 double perovskites have been prepared and studied for A = Ba and Ca. Ba2 FeReO6 has a cubic structure (Fm3m) with a 8.0854(1) Å whereas Ca2 FeReO6 has a monoclinic symmetry with a 5.396(1) Å, b 5.522(1) Å, c 7.688(2) Å and = 90.4° (P21/n) . The barium compound is metallic from 5 K to 385 K, i.e. no metal - insulator transition has been seen up to 385 K, and the calcium compound is semiconducting from 5 K to 385 K. At 5 K, we observed a negative magnetoresistance of 10% in a magnetic field of 50 kOe for Ba2 FeReO6 . Magnetization measurements show a ferrimagnetic behaviour for both materials, with Tc 315 K for Ba2 FeReO6 and above 385 K for Ca2 FeReO6 . A specific heat measurement on the barium compound gave an electron density of states at the Fermi level, N( EF ) , equal to 5.9 × 1024 eV-1 mol-1 . Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2 Fe(Mo,Re)O6 .

The temperature, doping, and field dependences of the magnetoresistance (MR) in Pr2-xCexCuO4-delta films are reported. We distinguish between orbital MR, found when the magnetic field is applied perpendicular to the ab planes, and the nearly isotropic spin MR. The latter, the major MR effect in the superconducting samples, appears in the region of the doping-temperature phase diagram where drho/dT<0, or an upturn in the resistivity appears. We conclude that the upturn originates from spin scattering processes.