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The influence of magnetic field on the crystal structure of colossal magnetoresistance (CMR) material La0.5Sr0.5MnO3+delta has been investigated. Low temperature high-magnetic field powder X-ray diffraction (XRD) measurements were carried out on La0.5Sr0.5MnO3+delta in temperature range from 4.2 to 300 K and magnetic field range from 0 to 8 T. A first order structural phase transition from tetragonal I4/mcm to orthorhombic Fmmm phase, coupled with reported electronic and magnetic phase transition from ferromagnetic metal to antiferromagnetic insulator state has been started at similar to 200 K and completed at similar to 100 K. On the application of 8 T magnetic field this magneto-structural phase transition shifted to lower temperature (similar to 162 K) with phase coexistence regime down to 4.2 K. Isothermal X-ray diffraction measurements at 150 K infer the evidence of magnetic field driven first order structural phase transition from Fmmm to I4/mcm. These results consistent with the resistivity and magnetoresistance results; present the microscopic evidence of strong spin-lattice coupling and reveal the magnetic field driven structural phase transition as the origin of observed colossal magnetoresistance in this material. (C) 2016 Published by Elsevier B.V.

Manganites with the formula La1−x Ca x MnO3 for 0.2

The magnetic and transport properties of the manganites with the perovskite structure are mainly characterized by a competition between ferromagnetism and antiferromagnetism, and between a metallic like and an insulating behavior. Charge and orbital ordering, and phase separation play a prominent role in the appearance of such properties, since they can be modified in a spectacular manner by external factors, making the different physical properties metastable. There we describe two effects that deeply modify those properties, the doping of Mn sites and the thermal cycling under a magnetic field. The doping of Mn sites by various magnetic cations-Cr, Co, Ni, Ru, Rh, Ir-destroys charge/orbital-ordering and induces ferromagnetism and metallicity in the antiferromagnetic matrix of the manganites. The magnetic phase diagram of the systems Ln(1-x)Ca(x)MnO(3) is considerably modified by such doping. The metastability of the magnetic states is explained in terms of models based on the electronic structure of the doping elements in connection with a possible valence fluctuation. The thermal cycling is also a spectacular effect, observed in chromium doped manganites. For instance, an increase of resistivity by several orders of magnitude can be observed by thermal cycling under a magnetic field, whereas no effect is obtained in the corresponding undoped material. This behavior is interpreted in terms of strains induced charge localization, at the interface between ferromagnetic/antiferromagnetic domains in the antiferromagnetic matrix. (C) 2002 Elsevier Science Ltd. All rights reserved.

It is shown that ferromagnetism and insulator to metal transitions in small A site cation manganites Pr1−xCaxMnO3 are induced by rhodium doping. Colossal magnetoresistance properties are evidenced for a large compositional range (0.35⩽x

We analyze the interplay between charge ordering, magnetic ordering, and the tendency toward phase separation and its importance for the physics of manganites. A simple model of charge ordering is considered. It takes into account both the Coulomb repulsion at neighboring sites responsible for charge ordering and the essential magnetic interactions. It is shown explicitly that at any deviation from half-filling (n≠1/2) the system is unstable with respect to phase separation into charge-ordered regions with n=1/2 and metallic regions with smaller electron or hole density. A possible structure of this phase-separated state (metallic droplets in a charge-ordered matrix) is discussed. We construct the phase diagram reproducing the main features observed in real manganites. Based on the same physical picture, we also derive explicit expressions for the dc conductivity and noise power in the phase-separated state. It is shown that the noise spectrum has a nearly 1/f form in the low-frequency range.

MnKβ x-ray emission spectra provide a direct method to probe the effective spin state and charge density on the Mn atom and is used in an experimental study of a class of Mn oxides. Specifically, the MnKβ line positions and detailed spectral shapes depend on the oxidation and the spin state of the Mn sites as well as the degree of d covalency/itinerancy. Theoretical calculations including atomic charge and multiplet effects, as well as crystal-field splittings and covalency effects, are used as a guide to the experimental results. Direct comparison of the ionic system MnF2 and the covalent system MnO reveals significant changes due to the degree of covalency of Mn within atomic-type MnKβ simulations. Moreover, comparisons of measurement with calculations support the assumed high spin state of Mn in all of the systems studied. The detailed shape and energy shift of the spectra for the perovskite compounds, LaMnO3 and CaMnO3, are, respectively, found to be very similar to the covalent Mn3+−Mn2O3 and Mn4+−MnO2 compounds thereby supporting the identical Mn-state assignments. Comparison to the theoretical modeling emphasizes the strong covalency in these materials. Detailed MnKβ x-ray emission results on the La1−xCaxMnO3 system can be well fit by linear superpositions of the end member spectra, consistent with a mixed-valent character for the intermediate compositions. However, an arrested Mn-valence response to the doping in the x0.3 range is found. No evidence for Mn2+ is observed at any x values seemingly ruling out proposals regarding Mn3+ disproportionation.

The LaMnO3.08 manganite system was studied as a function of temperature ranging from 10 up to 292K using the perturbed angular correlation local probe technique at ISOLDE-CERN facility. The electrical field gradient (EFG) measurements performed across the different temperatures revealed a single EFG distribution, corresponding to a unique probe local environment. In contrast with the small-polaron picture, no fully Jahn-Teller distorted Mn3+O6 octahedra was observed in this insulator sample. A local single-phase scenario with uniform Mn charge distribution is proposed. (C) 2013 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density.

Charge ordering phenomena in overdoped La(1-x)Ca(x)MnO(3) (LCMO) manganites with x >= 0: 5 are generally believed to be associated with the formation of charge stripes composed of alternating Mn(3+) and Mn(4+) charges. However, a number of recent experiments indicate that instead of stripes the charge in these systems is spatially organized in a uniform charge density wave. At the same time theory predicts that the ground state is modulated by an incommensurate (IC) orbital and charge soliton lattice. Here, by using nuclear magnetic resonance we provide the first direct evidence that the spin ground state in overdoped LCMO manganites is IC modulated with phase solitons. At higher temperatures the solitonic superstructure is replaced by a uniform spin-density wave, subjected to coherent slow fluctuations, showing a striking similarity with slow fluctuations in the striped phase of high T(c) cuprates and nickelates. Phys Rev Lett

The coupling of the manganite stripe phase to the lattice and to strain has been investigated via transmission electron microscopy studies of polycrystalline and thin film manganites. In polycrystalline \PCMOfiftwo a lockin to $q/a^*=0.5$ in a sample with $x>0.5$ has been observed for the first time. Such a lockin has been predicted as a key part of the Landau CDW theory of the stripe phase. Thus it is possible to constrain the size of the electron-phonon coupling in the CDW Landau theory to between 0.04% and 0.05% of the electron-electron coupling term. In the thin film samples, films of the same thickness grown on two different substrates exhibited different wavevectors. The different strains present in the films on the two substrates can be related to the wavevector observed via Landau theory. It is demonstrated that the the elastic term which favours an incommensurate modulation has a similar size to the coupling between the strain and the wavevector, meaning that the coupling of strain to the superlattice is unexpectedly strong., 6 pages, 7 figures

(Pr, Ca)MnO3 is the archetypal charge-ordered manganite, but in Pr0.48Ca0.52MnO3, we find (using convergent-beam electron diffraction and dark-field images) that the superlattice period is locally incommensurate with respect to the parent lattice and that the superlattice orientation possesses significant local variations. This suggests that local electron-lattice coupling never overwhelmingly dominates the rich physics of manganites, even in the most extreme scenarios that produce the largest colossal magnetoresistance effects.

A detailed comparative Mossbauer study of La(0.67)Ca(0.33)MnO(3) and La(0.67)Sr(0.33)MnO(3) samples doped with 1% Sn is presented. The spectra at 5 K can be analyzed by a single ferromagnetic component. However, at higher temperatures two separate ferromagnetic components and a paramagnetic one are required to fit the spectra. The ferromagnetic component with the lower hyperfine field is attributed to clusters of weakened double-exchange interactions due to competition by localization mechanisms. Clear differences between the two samples in regard to the critical behavior arise: (i) The hyperfine field of La(0.67)Ca(0.33)Mn(0.99)Sn(0.01)O(3) disappears abruptly at T(c) in contrast to that of La(0.67)Sr(0.33)Mn(0.99)Sn(0.01)O(3) which varies continuously and goes to zero smoothly at the critical temperature. (ii) The second ferromagnetic phase appears above 0.35T(c) in the Ca sample but only above 0.83T(c) in the Sr sample. The paramagnetic component appears for T > 0.67T(c) in the former compared to T > 0.9T(c) in the latter. The results support recent studies that show differences in the polaronic character, and critical behavior thereof, between the two materials. Physical Review B

A detailed comparative Mossbauer study of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 samples doped with 1% Sn is presented. The spectra at 5 K can be analyzed by a single ferromagnetic component. However, at higher temperatures two separate ferromagnetic components and a paramagnetic one are required to fit the spectra. The ferromagnetic component with the lower hyperfine field is attributed to clusters of weakened double-exchange interactions due to competition by localization mechanisms. Clear differences between the two samples in regard to the critical behavior arise: (i) The hyperfine field of La0.67Ca0.33Mn0.99Sn0.01O3 disappears abruptly at T-c in contrast to that of La0.67Sr0.33Mn0.99Sn0.01O3 which varies continuously and goes to zero smoothly at the critical temperature. (ii) The second ferromagnetic phase appears above 0.35T(c) in the Ca sample but only above 0.83T(c) in the Sr sample. The paramagnetic component appears for T > 0.67T(c) in the former compared to T > 0.9T(c) in the latter. The results support recent studies that show differences in the polaronic character, and critical behavior thereof, between the two materials. Physical Review B

International audience; The origin of the colossal magnetoresistance (CMR) observed in La1-xCaxMnO3, at x approximate to 0.3, is still largely debated. Actually, the precursor phase of the ferromagnetic ( F) metallic state, defined as the concentration ranges 0.125

Nanocrystalline La0.67Ca0.33MnO3 powders were prepared from organic precursors using a microwave synthesis technique. A variation of the precursor solution pH and annealing temperature was found to change the effective Mn4+ concentration which leads to variations in electronic behavior. In all the cases however, a single phase microstructure was observed showing the stability of orthorhombic manganite. The nanocrystalline manganite prepared from a solution with a pH of 10.5 has the lowest Mn4+ and accordingly low electrical and magnetic transition temperatures. The electrical transport behavior of powders prepared from precursor solutions with pH of 11.5 and 12.5 could be modeled using an effective medium approach across the whole temperature range, 30 K to 300 K. The magnetoresistance in these nanocrystalline manganites increases with decreasing temperature, an effect of increased grain boundaries. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The magnetic and transport properties of the manganites with the perovskite structure are mainly characterized by a competition between ferromagnetism and antiferromagnetism, and between a metallic like and an insulating behavior. Charge and orbital ordering, and phase separation play a prominent role in the appearance of such properties, since they can be modified in a spectacular manner by external factors, making the different physical properties metastable. There we describe two effects that deeply modify those properties, the doping of Mn sites and the thermal cycling under a magnetic field. The doping of Mn sites by various magnetic cations-Cr, Co, Ni, Ru, Rh, Ir-destroys charge/orbital-ordering and induces ferromagnetism and metallicity in the antiferromagnetic matrix of the manganites. The magnetic phase diagram of the systems Ln(1-x)Ca(x)MnO(3) is considerably modified by such doping. The metastability of the magnetic states is explained in terms of models based on the electronic structure of the doping elements in connection with a possible valence fluctuation. The thermal cycling is also a spectacular effect, observed in chromium doped manganites. For instance, an increase of resistivity by several orders of magnitude can be observed by thermal cycling under a magnetic field, whereas no effect is obtained in the corresponding undoped material. This behavior is interpreted in terms of strains induced charge localization, at the interface between ferromagnetic/antiferromagnetic domains in the antiferromagnetic matrix. (C) 2002 Elsevier Science Ltd. All rights reserved.

It is shown that ferromagnetism and insulator to metal transitions in small A site cation manganites Pr1−xCaxMnO3 are induced by rhodium doping. Colossal magnetoresistance properties are evidenced for a large compositional range (0.35⩽x

We present ab initio quantum chemical embedded cluster calculations of Mn core-valence and d−d transitions in LaMnO3. The results are also important for the analysis of recent x-ray absorption and x-ray scattering experiments at the Mn K edge in LaMnO3. We find that the first two peaks of the pre-edge region correspond to majority-spin and minority-spin eg(3d) states on adjacent Mn ions. For on-site 1s to 4p transitions we find that the ordering of the p components is due to the Jahn-Teller distortion. We also show that the 1→s4p transitions are split due to 3d−4p exchange interactions.

A simple model of charge ordering is considered. It is shown explicitly that at any deviation from half-filling ($n \neq 1/2$) the system is unstable with respect to phase separation into charge ordered regions with $n = 1/2$ and metallic regions with smaller electron or hole density. Possible structure of this phase-separated state (metallic droplets in a charge-ordered matrix)is discussed. The model is extended to account for the strong Hund-rule onsite coupling and the weaker intersite antiferromagnetic exchange. An analysis of this extended model allows us to determine the magnetic structure of the phase-separated state and to reveal the characteristic features of manganites and other substances with charge ordering., Comment: 9 pages, revtex

We present ab initio quantum chemical embedded cluster calculations of Mn core-valence and d−d transitions in LaMnO3. The results are also important for the analysis of recent x-ray absorption and x-ray scattering experiments at the Mn K edge in LaMnO3. We find that the first two peaks of the pre-edge region correspond to majority-spin and minority-spin eg(3d) states on adjacent Mn ions. For on-site 1s to 4p transitions we find that the ordering of the p components is due to the Jahn-Teller distortion. We also show that the 1→s4p transitions are split due to 3d−4p exchange interactions.

Compositionally modulated structures consisting of La2/3Ca1/3MnO3 ferromagnetic (FM) layers and La1/3Ca2/3MnO3 antiferromagnetic (AF) layers, with layer thickness 1.5 nm less than or equal to t(FM),t(AF)less than or equal to 6 nm, were grown on (100)LaAlO3 by pulsed laser deposition. Thermomagnetic and isothermal magnetic measurements reveal an exchange-biasing mechanism with the same blocking temperature T(B)approximate to 70 K for all the examined combinations of t(FM) and t(AF). The ratio of colossal magnetoresistance increases in multilayers with larger t(FM), whereas the exchange-biasing field increases in samples with larger t(AF). The independence of T-B on t(FM) and t(AF) shows that the artificially induced La1-xCaxMnO3 (x approximate to 0.5) compositional modulation at the interfaces is of critical importance for exchange biasing in La-Ca-Mn-O based multilayers. (C) 2000 American Institute of Physics. [S0021-8979(00)01708-4]. Journal of Applied Physics

Mn K β x-ray emission spectra provide a direct method to probe the effective spin state and charge density on the Mn atom and is used in an experimental study of a class of Mn oxides. Specifically, the Mn K β line positions and detailed spectral shapes depend on the oxidation and the spin state of the Mn sites as well as the degree of d covalency/itinerancy. Theoretical calculations including atomic charge and multiplet effects, as well as crystal-field splittings and covalency effects, are used as a guide to the experimental results. Direct comparison of the ionic system MnF 2 and the covalent system MnO reveals significant changes due to the degree of covalency of Mn within atomic-type Mn K β simulations. Moreover, comparisons of measurement with calculations support the assumed high spin state of Mn in all of the systems studied. The detailed shape and energy shift of the spectra for the perovskite compounds, LaMnO 3 and CaMnO 3 , are, respectively, found to be very similar to the covalent Mn 3 + − M n 2 O 3 and Mn 4 + − M n O 2 compounds thereby supporting the identical Mn-state assignments. Comparison to the theoretical modeling emphasizes the strong covalency in these materials. Detailed Mn K β x-ray emission results on the La 1 − x Ca x MnO 3 system can be well fit by linear superpositions of the end member spectra, consistent with a mixed-valent character for the intermediate compositions. However, an arrested Mn-valence response to the doping in the x 0.3 range is found. No evidence for Mn 2 + is observed at any x values seemingly ruling out proposals regarding Mn 3 + disproportionation.

An explanation is given for the charge order, orbital order and insulating state observed in half-doped manganese oxides, such as Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$. The competition between the kinetic energy of the electrons and the magnetic exchange energy drives the formation of effectively one-dimensional ferromagnetic zig-zag chains. Due to a topological phase factor in the hopping, the chains are intrinsically insulating and orbital-ordered. Most surprisingly, the strong Coulomb interaction between electrons on the same Mn-ion leads to the experimentally observed charge ordering. For doping less than 1/2 the system is unstable towards phase separation into a ferromagnetic metallic and charge-ordered insulating phase., To appear in Phys. Rev. Lett., 4 pages, 4 figures

Low-temperature La-139 and Mn-55 NMR line shape measurements in La1-xCaxMnO3 for 0.125 less than or equal to x less than or equal to 0.5, and La-0.67(Ca1-yBay)(0.33)MnO3 for 0 less than or equal to y less than or equal to 1 are shown to directly reflect changes of the structure and of the magnetic state at a local level. For low x values in La1-xCaxMnO3 evidence is given about the presence of polaronic lattice distortions. By increasing; x, structurally undistorted conductive ferromagnetic (FM) regions are formed indicating a percolative metal to insulator phase transition, whereas for x = 0.5 the system is shown to break into FM and antiferromagnetic regions in accordance with the predictions of the electronic phase separation model. [S0163-1829(99)04609-3]. Physical Review B

A series of [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3](15) multilayers, with bilayer thicknesses between 2 and 32 nm, has been prepared by pulsed laser deposition. The study of their magnetic and magnetotransport properties reveals, for the first time in this category of materials, the presence of an exchange biasing mechanism at low temperatures. Zero-field-cooling and field-cooling magnetic measurements reveal a blocking temperature around 70 K that is independent of the bilayer thickness, whereas the average film magnetization becomes zero at 250 K. (C) 1999 American Institute of Physics. [S0021-8979(99)27408-7]. Journal of Applied Physics

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density.

We present a temperature-dependent single-crystal x-ray-diffraction study of twinned orthorhombic perovskites La1-xCaxMnO3, for x=0.16 and x=0.25. These data show the evolution of the crystal structure from the ferromagnetic insulating state to the ferromagnetic metallic state. The data are modeled in space group Pnma with twin relations based on a distribution of the b axis over three perpendicular cubic axes. The twin model allows full structure determination in the presence of up to six twin fractions using the single-crystal x-ray-diffraction data.