Showing papers on "Colossal magnetoresistance published in 2003"

CMR manganites: physics, thin films and devices

Abstract: The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

Open Questions in CMR Manganites, Relevance of Clustered States, and Analogies with other Compounds

Abstract: This is an informal paper that contains a list of ``things we know'' and ``things we do not know'' in manganites. It is adapted from the conclusions chapter of a recent book by the author, {\it Nanoscale Phase Separation and Colossal Magnetoresistance. The Physics of Manganites and Related Compounds}, Springer-Verlag, Berlin, November 2002. The main new result of recent manganite investigations is the discovery of tendencies toward inhomogeneous states, both in experiments and in simulations of models. The colossal magnetoresistance effect appears to be closely linked to these mixed-phase tendencies, although considerably more work is needed to fully confirm these ideas. The paper also includes information on cuprates, diluted magnetic semiconductors, relaxor ferroelectrics, cobaltites, and organic and heavy fermion superconductors. These materials potentially share some common phenomenology with the manganites, such as a temperature scale $T^*$ above the ordering temperature where anomalous behavior starts. Many of these materials also present low-temperature phase competition. The possibility of colossal-like effects in compounds that do not involve ferromagnets is briefly discussed. Overall, it is concluded that inhomogeneous ``clustered'' states should be considered a new paradigm in condensed matter physics, since their presence appears to be far more common than previously anticipated.

Structural phase transition at the percolation threshold in epitaxial (La0.7Ca0.3MnO3)1-x:(MgO)x nanocomposite films.

Abstract: 'Colossal magnetoresistance' in perovskite manganites such as La0.7Ca0.3MnO3 (LCMO), is caused by the interplay of ferro-paramagnetic, metal-insulator and structural phase transitions. Moreover, different electronic phases can coexist on a very fine scale resulting in percolative electron transport. Here we report on (LCMO)1-x:(MgO)x (0 < x < or = 0.8) epitaxial nano-composite films in which the structure and magnetotransport properties of the manganite nanoclusters can be tuned by the tensile stress originating from the MgO second phase. With increasing x, the lattice of LCMO was found to expand, yielding a bulk tensile strain. The largest colossal magnetoresistance of 10(5)% was observed at the percolation threshold in the conductivity at xc 0.3, which is coupled to a structural phase transition from orthorhombic (0 < x < or 0.1) to rhombohedral R3c structure (0.33 < or = x < or = 0.8). An increase of the Curie temperature for the Rc phase was observed. These results may provide a general method for controlling the magnetotransport properties of manganite-based composite films by appropriate choice of the second phase.

High-pressure x-ray scattering of oxides with a nanoscale local structure: Application to Na 1 / 2 Bi 1 / 2 TiO 3

Abstract: Many of the outstanding properties in oxides are related to materials with an intrinsic nanoscaled local structure, where the different regions are characterized by competing chemical, structural, and/or physical properties. One of the major challenges in the analysis of the nanoscaled oxides is experimental access to the local properties, which is often at best a difficult task, a fact that often inhibits the understanding of properties such as colossal magnetoresistance, giant piezoelectricity, and high-temperature superconductivity. Here we present an investigation of the relaxor ferroelectric ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3},$ considered as a model type of nanostructured oxide, by combining the parameter high-pressure with x-ray diffuse scattering. We show that nanoscaled characteristics can be investigated in detail by combining such measurements with simulation of different diffuse scattering models. We observe two distinct structural disorders, one of which is characterized by the observation of asymmetric diffuse scattering in perovskites due to planar polar defects.

Observation of minority spin character of the new electron doped manganite La0.7Ce0.3MnO3 from tunneling magnetoresistance

Abstract: We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction built of an electron doped manganite (La0.7Ce0.3MnO3) and a hole doped manganite (La0.7Ca0.3MnO3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T(C) the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La0.7Ce0.3MnO3 is a minority spin carrier ferromagnet with a high degree of spin polarization, i.e., a transport half-metal.

Antisites and electron-doping effects on the magnetic transition of Sr 2 FeMoO 6 double perovskite

Abstract: The effect of antisite (AS) defects and electron doping on the ferromagnetic-paramagnetic (FM-PM) transition of Sr 2 FeMoO 6 (SFMO) double perovskite has been studied. From a detailed analysis of the magnetization curves across the FM-PM transition we conclude that AS defects decrease the average strength of the magnetic interactions, although, remarkably enough, some of them are actually enhanced. It follows that the mean-field Curie temperature lowers whereas the onset of magnetization occurs at somewhat higher temperatures. The FM-PM transition has also been analyzed in electron-doped La x Sr 2 - x FeMoO 6 , where the AS defects concentration has been found to increase upon La doping. It turns out that in spite of the presence of AS, the mean-field Curie temperature significantly rises upon La and electron doping. This experimental finding contrasts with some recent predictions and emphasizes the role of itinerant electrons in the ferromagnetic coupling in these oxides. Moreover, our results indicate that disordered (i.e., with AS) double perovskite materials should be described as systems with random magnetic anisotropy.

Ferroelectric-field-induced tuning of magnetism in the colossal magnetoresistive oxide La 1 − x Sr x MnO 3

Abstract: A ferroelectric field effect approach is presented for modulating magnetism in the colossal magnetoresistive oxide ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ (LSMO). The ferromagnetic Curie temperature of ultrathin LSMO films was shifted by 35 K reversibly using the polarization field of the ferroelectric oxide $\mathrm{Pb}({\mathrm{Zr}}_{x}{\mathrm{Ti}}_{1\ensuremath{-}x}){\mathrm{O}}_{3}$ in a field effect structure. This shift was also observed in magnetoresistance measurements, with the maximum magnetoresistance ratio at 6 T increasing from 64% to 77%. This model system approach does not introduce substitutional disorder or structural distortion, demonstrating that regulating the carrier concentration alone changes the magnetic phase transition temperature and leads to colossal effects.

Magnetic and magnetotransport properties of La1−xBaxMnO3−x/2 perovskite manganites

Abstract: Crystal structure as well as magnetization and electrical transport vs. temperature and field for the La3+1−xBa2+xMn3+O2−3−x/2 (0 ≤ x ≤ 0.50) anion-deficient manganites have been studied. It is established that the samples in the region 0 ≤ x ≤ 0.05 are O/-orthorhombic perovskites and are rhombohedric with 0.10 ≤ x ≤ 0.25, whereas with 0.27 ≤ x ≤ 0.50 they are cubic. As the doping level increases the samples undergo a transition from a weak ferromagnetic (x = 0) to an inhomogeneous ferromagnetic (x ≥ 0.03) state. At x ≥ 0.12 samples show cluster spin glass properties with the temperature of magnetic moment freezing ∼45 K. All the reduced samples are semiconductors and show considerable magnetoresistance over a wide temperature range. The largest magnetoresistance ratio is observed for the x = 0.30 sample. Concentration dependences of spontaneous magnetization and the magnetic ordering temperature for the reduced La3+1−xBa2+xMn3+O2−3−x/2 manganites have been established by magnetic measurements and compared with those for stoichiometric ones. Magnetic data for the reduced La3+1−xBa2+xMn3+O2−3−x/2 manganites have been compared with those for the stoichiometric La3+1−xBa2+xMn3+1−xMn4+xO2−3. The magnetic state of the anion-deficient samples is interpreted on the basis of the superexchange interaction model.

Structural transformation induced by magnetic field and "colossal-like" magnetoresistance response above 313 K in MnAs.

Abstract: MnAs is a commercially available material, intensively studied, both theoretically and experimentally, since the beginning of the last century. Interest in this compound could come up again as a consequence of new ideas and conjectures formulated during the last decade in connection with the study of the colossal magnetoresistance (CMR) response in Mn perovskites. Among these ideas is the invocation of a phase separation scenario for CMR manganese oxides and related materials that might be of particular relevance in systems, like MnAs, where first-order phase transitions occur.

Magnetization distribution in the mixed-phase state of hole-doped manganites

Abstract: The effect of 'colossal magnetoresistance' (CMR) in hole-doped manganites--an abnormal decrease of resistivity when a magnetic field is applied--has attracted significant interest from researchers in the past decade. But the underlying mechanism for the CMR phenomenon is not yet fully understood. It has become clear that a phase-separated state, where magnetic and non-magnetic phases coexist, is important, but the detailed magnetic microstructure of this mixed-phase state is so far unclear. Here we use electron microscopy to study the magnetic microstructure and development of ferromagnetic domains in the mixed-phase state of La(1-x)Sr(x)MnO3 (x = 0.54, 0.56). Our measurements show that, in the absence of a magnetic field, the magnetic flux is closed within ferromagnetic regions, indicating a negligible magnetic interaction between separated ferromagnetic domains. However, we also find that the domains start to combine with only very small changes in temperature. We propose that the delicate nature of the magnetic microstructure in the mixed-phase state of hole-doped manganites is responsible for the CMR effect, in which significant conduction paths form between the ferromagnetic domains upon application of a magnetic field.

Enhanced magnetoresistance in the complex perovskite LaCu3Mn4O12

Abstract: Moderate-pressure techniques (P=2 GPa) have been used to prepare the complex LaCu3Mn4O12 perovskite. It has been characterized by neutron powder diffraction, magnetic, and magnetotransport measurements. This material is ferrimagnetic below TC=361 K. The magnetoresistance (MR) is enhanced with respect to that of CaCu3Mn4O12 due to the effective electronic injection that dramatically reduces the bulk resistivity, thus promoting the grain-boundary contribution to the electrical resistance. Values of low-field MR close to 3% at room temperature are achieved for magnetic fields of 1 T.

Competing orders and disorder-induced insulator to metal transition in manganites.

Abstract: Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remains the charge/lattice fluctuations instead which are enhanced toward the transition temperature.

Ground-state properties of ferromagnetic metal/conjugated polymer interfaces

Abstract: We theoretically investigate the ground-state properties of ferromagnetic metal/conjugated polymer interfaces. The work was partially motivated by recent experiments in which injection of spin-polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a half-metallic ferromagnetic colossal magnetoresistance (CMR) manganite that has explicit structural degrees of freedom. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. We find that there can be spin density polarization in the polymer near the interface. The spin-density oscillates and decays into the polymer with a decay length of about six times the lattice constant of the polymer. We find an expansion of the end bonds of the CMR manganite segment and a contraction of the polymer bonds near the interface. By adjusting the relative chemical potential of the contact and the polymer, electrons can be transferred into the polymer from the magnetic layer through the interfacial coupling. We calculate the density of states (DOS) before and after coupling for cases in which electrons are transferred and are not transferred to the polymer. The DOS has important consequences for spin injection under electrical bias: polarized spin injection is possible when the Fermi level of the ferromagnet lies below the the bipolaron level of the polymer. However, if the Fermi level of the CMR manganite lies above the bipolaron level of the polymer, the transferred electrons form bipolarons, which have no spin, and there is no spin density in the bulk of the polymer.

Theoretical study of half-doped models for manganites: Fragility of CE phase with disorder, two types of colossal magnetoresistance, and charge-ordered states for electron-doped materials

Abstract: A comprehensive analysis of half-doped manganites is presented using Monte Carlo simulations applied to the double-exchange model with cooperative Jahn-Teller lattice distortions in two dimensions. A variety of results are reported. In particular (i) The phase diagram is established in the λ-J A F plane, with λ the electron-phonon coupling and J A F the antiferromagnetic exchange between classical t 2 g spins. The results include standard phases, such as the CE-insulating and FM-metallic regimes, but they also include states, such as a ferromagnetic charge-ordered (CO) orbital-ordered phase originally predicted by Hotta et al. This state is compatible with recent experimental results by Loudon et al. (ii) For realistic couplings, it was observed that the charge disproportionation δ of the CO phase is far from the widely accepted extreme limit δ=0.5 of a 3 +/4+ charge separation. A far smaller δ appears more realistic, in agreement with recent experiments by Garcia et al. and Daoud-Aladine et al. (iii) Colossal magnetoresistance (CMR) effects are found in calculations of cluster resistances using the Landauer formalism. This occurs near the ubiquitous first-order phase transitions between the insulating and metallic states. The present result reinforces the previous conjecture that CMR phenomenology exists in two forms: the low-temperature CMR addressed here and the more standard CMR above the Curie temperature. (iv) The CE state is found to be very sensitive to disorder since its long-range order rapidly disappears when quenched disorder is introduced, contrary to the FM stale which is more robust. This is also in qualitative agreement with recent experiments by Akahoshi et al. and Nakajima et al. (v) The phase diagram in the half-doped electron doping regime is briefly discussed as well. A charge-ordered state is found, which is the analog of the x=0.5 CE phase. It contains a 3 +/2+ charge arrangement at large λ. Numerical results suggest that an approximate symmetry exists between the hole- and electron-doped systems in the large Hund coupling limit.

Searching for quantum interference effects in La 0.7 Ca 0.3 MnO 3 films on SrTiO 3

Abstract: The resistivity of epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ films deposited on ${\mathrm{SrTiO}}_{3}$ shows an upturn at low temperatures that is enhanced by the application of a strong magnetic field. This can be understood within the theory of quantum interference effects taking both the interaction and the weak localization correction into account.

Magnetic phase transitions in the anion-deficient La1−xBaxMnO3−x/2 (0 ≤ x ≤ 0.50) manganites

Abstract: The crystal structure, magnetization and electrical resistivity properties of the anion-deficient La1−xBaxMnO3−x/2 (0 ≤ x ≤ 0.50) perovskite manganites without Mn4+ ions have been investigated. It is established the reduced samples in the region 0 ≤ x ≤ 0.05 are O'-orthorhombic perovskites, in 0.10 ≤ x ≤ 0.25 they are rhombohedric whereas in 0.27 ≤ x ≤ 0.50 they are cubic. It is found that, as the x doping level increases, the samples undergo a transition from a weak ferromagnetic state (x = 0) to an inhomogeneous ferromagnetic one, (x ≥ 0.03) being a mixture of antiferromagnetic and ferromagnetic phases. At x ≥ 0.12 competition between antiferromagnetic and ferromagnetic interactions leads to a cluster spin glass state appearance with a magnetic moment freezing temperature of ~ 45 K. The dominant magnetic phase for x ≥ 0.22 is supposed to be antiferromagnetic. All the reduced samples are semiconductors and show considerable magnetoresistance over a wide temperature range in a magnetically ordered state. The largest magnetoresistance (~ 34% in a 9 kOe field at liquid nitrogen temperatures) is observed for an x = 0.30 sample. The magnetic phase diagram of La1−x3+Ba x2+Mn3+O 3−x/22− manganites has been established by combining the results of magnetic and electrical measurements. The results obtained could be understood in terms of the phase separation and the 180° superexchange Mn3+–O–Mn3+ interaction model.

Colossal magnetoresistance behavior and ESR studies of La1-xTexMnO3 (0.04 <= x <= 0.2)

Abstract: In this paper, we report the magnetic and electrical properties of the Te-doped lanthanum manganite perovskite compound La 1 - x Te x MnO 3 and the related electron spin resonance (ESR) study. This is a material with rhombohedral structure and it shows good colossal magnetoresistance behavior. The magnetoresistance ratio MR=[ρ(0)-ρ(H)]/ρ(0)is about 63% for x=0.04 in an applied magnetic field of 40 kOe. The resistivity of the compounds is affected by temperature and the amount of Te doping. The temperature dependence of the resistivity showed that the conductivity above the Curie temperature was dominated by the hopping of the small polarons, and the resistance was attributed to the electron-phonon and magnon scattering mechanism below the metal-insulator transition temperature. The ESR investigation indicated that the compound was in the paramagnetic phase above 230 K, and in the single ferromagnetic state below 170 K. In the temperature range of 230 to 170 K, the compound showed phase separation phenomena.

Origin of metal–insulator transition temperature enhancement in underdoped lanthanum manganite films

Abstract: The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal–insulator transitions. Their transition temperatures TMI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the TMI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that TMI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films.

Epitaxial colossal magnetoresistive La0.67(Sr,Ca)0.33MnO3 films on Si

Abstract: La067(Sr,Ca)033MnO3 (LSCMO) films have been grown by a pulsed-laser deposition technique on Si(001) substrates buffered with Bi4Ti3O12/CeO2/yttrium-stabilized zirconia (YSZ) heteroepitaxial layers X-ray diffraction has revealed cube-on-cube growth of an epitaxial Bi4Ti3O12/CeO2/YSZ/Si heterostructure whereas the LSCMO layer grows in the “diagonal-on-side” manner on top of the Bi4Ti3O12 (BTO) template The maximum temperature coefficient of resistivity (TCR)=44% K−1 and colossal magnetoresistance (CMR) Δρ/ρ∼29% kOe−1 have been reached at 294 K This was achieved due to the successive improvement of c-axis orientation of the layers: Full widths at half-maximum 065°, 058°, 065°, 113°, and 018° in LSCMO/BTO/CeO2/YSZ/Si stack, respectively As a prototype of an uncooled bolometer, heteroepitaxial CMR structure on Si demonstrates, at 294 K, the noise equivalent temperature difference of 12 μK/√Hz@30 Hz

Colossal magnetoresistance in manganites as a multicritical phenomenon.

Abstract: The colossal magnetoresistance in manganites $A{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ is studied from the viewpoint of multicritical phenomena. To understand the complicated interplay of various phases, we study the Ginzburg-Landau theory in terms of both the mean-field approximation and the renormalization-group analysis for comparison with the observed phase diagram. Several novel features, such as the first-order ferromagnetic transition and the dip in the transition temperature near the multicritical point, can be understood as being driven by enhanced fluctuations near the multicritical point. Furthermore, we obtain a universal scaling relation for the $H/M$ versus ${M}^{2}$ plot (Arrott plot), which fits rather well with the experimental data, providing further evidence for the enhanced fluctuation.

Dynamically tuning properties of epitaxial colossal magnetoresistance thin films

Abstract: The strain state of epitaxial La0.5Sr0.5MnO3 thin films on BaTiO3 are dynamically tuned by temperature and substrate bias. The resistivity of the La0.5Sr0.5MnO3 thin films is particularly sensitive to changes in structure. Fractional changes in magnetization and resistivity as a function of temperature reveal a direct correlation with fractional changes in the structure, as measured by out-of-plane x-ray diffraction. Fractional changes in resistivity, as large as 30%, are observed for strain induced by the structural phase transitions of the BaTiO3 substrate, and a 12% change is induced by an inverse piezoelectric effect at room temperature.

Thin films of double perovskite Sr2FeMoO6: Growth, optimization, and study of the physical and magnetotransport properties of films grown on single-crystalline and polycrystalline SrTiO3 substrates

Abstract: Thin films of Sr2FeMoO6 have been deposited on single-crystalline and polycrystalline SrTiO3 substrates by pulsed laser deposition from a stoichiometric target. In order to obtain high-quality films, the deposition parameters were systematically optimized. The films grown under optimized conditions show properties comparable to those of bulk Sr2FeMoO6 single crystals. Surprisingly, polycrystalline films obtained by depositing on polycrystalline SrTiO3 substrates do not show any significant low-field magnetoresistance (MR), contrary to expectations for a spin-polarized material. However, after controlled annealing-induced alteration of the grain-boundary properties, a development of the low-field MR feature is clearly observed. The occurrence of the low-field MR is accompanied by nonlinearity in the current-voltage characteristics. These results are analyzed in the light of the prevalent understanding of the properties of this double perovskite material.

Structural, electric and magnetic properties of the electron-doped manganese oxide: La1-xTexMnO3 (x=0.1, 0.15)

Abstract: In this study, the electrical and magnetic properties of La1−xTexMnO3 (x=0.1, 0.15), which is a new material and shows good colossal magnetoresistance behavior, have been investigated. These compounds have rhombohedral structure. In this material, Te replaced a part of La ions, which induced the lattice cell constriction and Mn–O–Mn bond angle widening. X-ray photoemission spectroscopy measurement revealed that the Te ions were in the tetravalent state and the manganese ions could be considered as in a mixture state of Mn2+ and Mn3+. Thus the material could be viewed as an electron-doped compound. The Curie temperature (Tc) was about 240 and 255 K for x=0.1, 0.15, respectively. The maximum magnetoresistance ratio MR=[ρ(0)−ρ(H)]/ρ(0) was about 51% at 200 K and in the applied magnetic field of 40 kOe.

Large magnetoresistance in (La1−xCaxMnO3)1−y:ZrO2 composite

Abstract: Colossal magnetoresistance (CMR) composite materials have been synthesized to explore the possibility of improving magneto-transport and structural properties in CMR systems. In this work we describe (La1−xCaxMnO3)1−y (LCMO) (ZrO2)y (x≈0.3 and 0.0⩽y⩽0.40 mole %) composites that have been synthesized using a modified (non Pechini type) sol–gel technique. Magnetoresistivity of the composites was evaluated at 5 T field and in the temperature range 5–300 K. The composites show higher magnitude of MR compared to pure LCMO. The MR rises from a base value 76%, for the case y=0, to a maximum value of 93.8%, obtained at y=0.05. dc susceptibility measurements show a distinct ferromagnetic to paramagnetic transition in all composites. The ferromagnetic transition temperature (TC) drops from 225 K in pure LCMO (y=0) to 121 K in y=0.05 and then slowly rises to 157 K as y increases. The plots of zero field cooled susceptibility χZFC (T) and field cooled susceptibility χFC (T) diverge clearly below TC, indicating magnet...

Surface-induced phase separation in manganites: A microscopic origin for powder magnetoresistance

Abstract: Through the analysis of the magnetic properties and of the nuclear magnetic resonance response of La2/3Ca1/3MnO3 ceramics with different grain sizes, we have found that poorly conducting regions, some ferromagnetic and some weakly magnetic, are located at the surface of the grains. We state that these regions constitute the tunnel barrier responsible for the low-field magnetoresistance usually observed in powders of half-metallic oxides. In addition, the spin disorder accompanying the coexistence of phases with different magnetoelectronic character could contribute to the large high-field magnetoresistance also typical of such ceramic samples. From a more general perspective, these findings can be of relevance to understand the microscopic origin of phase separation in manganites.