Horacio Aliaga - Academia.edu (original) (raw)

Papers by Horacio Aliaga

Research paper thumbnail of Structural Phase Diagram of Ca1−xYxMnO3: Characterization of Phases

Journal of Solid State Chemistry, 2001

To help the understanding of the physical behavior of Ca 1؊x Y x MnO 3 , its phase diagram in the... more To help the understanding of the physical behavior of Ca 1؊x Y x MnO 3 , its phase diagram in the whole x concentration range was investigated taking into account the stability of phases and the possible coexistence of di4erent structural phases. By careful analysis of powder X-ray di4raction (XRD) patterns, we were able to observe the following phase diagram: (i) Orthorhombic phases were detected both in the region of 04x40.25 (O type phase with Ca site twelve fold coordinated) and in the region of 0.54x(0.75 (O type phase with Ca site ninefold coordinated). (ii) Phase segregation for 0.254x40.5 and for x50.75 that have not been reported previously, hexagonal YMnO 3 segregates as a separate phase for x'0.75, and for 0.254x40.5 the coexistence of Ca 0.75 Y 0.25 MnO 3 (O) and Ca 0.5 Y 0.5 MnO 3 (O) have to be included in the re5nement for it to converge.

Research paper thumbnail of Structural Phase Diagram of Ca

Journal of Solid State Chemistry, 2001

To help the understanding of the physical behavior of Ca{sub 1-x}YâMnOâ, its phase diagram in the... more To help the understanding of the physical behavior of Ca{sub 1-x}YâMnOâ, its phase diagram in the whole x concentration range was investigated taking into account the stability of phases and the possible coexistence of different structural phases. By careful analysis of powder X-ray diffraction (XRD) patterns, we were able to observe the following phase diagram: (i) Orthorhombic phases were detected

Research paper thumbnail of Phase diagram of electron-doped Mn perovskites

Journal of Magnetism and Magnetic Materials, 1999

We study the competition between ferromagnetic double exchange and antiferromagnetic superexchang... more We study the competition between ferromagnetic double exchange and antiferromagnetic superexchange present in Mn perovskites. The model represents each Mn 4+ ion by a spin ½, on which an electron can be added to produce Mn 3 We include a hopping energy t, a strong intratomic interaction exchange J {in the limit J/t-+ ,JJ), and an interatomic antiferromagnetic interaction K between the local spins. Using the renormalized perturbation expansion and a mean-field approximation we calculate the free energy from which the stability of the antiferromagnetic, canted, ferromagnetic and 'pseudo-spin-glass' phases can be determined. The phase diagram can be drawn in terms of temperature and doping x, for each value of K/t. We discuss the presence of new 'pseudo-spin-glass' phases. ,

Research paper thumbnail of Phase diagram for Ca1−xYxMnO3 type crystals

Solid State Communications, 1999

We present a simple model to study the electron doped manganese perovskites. The model considers ... more We present a simple model to study the electron doped manganese perovskites. The model considers the competition between double exchange mechanism for itinerant electrons and antiferromagnetic superexchange interaction for localized electrons. It represents each Mn 4ϩ ion by a spin 1/2, on which an electron can be added to produce Mn 3ϩ ; we include a hopping energy t, a strong intratomic ferromagnetic interaction exchange J (in the limit J=t 3 ∞), and an interatomic antiferromagnetic interaction K between the local spins. Using the Renormalized Perturbation Expansion and a Mean Field Approximation on the hopping terms and on the superexchange interaction, we calculate the free energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic, and novel spin glass phases can be determined as functions of the parameters characterizing the system. The model results can be expressed in terms of t and K for each value of the doping x in phase diagrams. The magnetization m and canting angle u can also be calculated as fuctions of temperature for fixed values of doping and model parameters.

Research paper thumbnail of Resistivity peak in the one-band double-exchange model with cooperative phonons

ABSTRACT We present the results of Monte Carlo simulations of a single-band double-exchange model... more ABSTRACT We present the results of Monte Carlo simulations of a single-band double-exchange model with cooperative phonons, both with and without quenched disorder, at a carrier density n=0.3. Both in two and three dimensions, the simulations reveal a peak in the resistivity at TCurie, in agreement with previous studies by other groups. The peak gets destroyed upon the application of an external field, resulting in a large magnetoresistance. The results are in good agreement with experiments involving CMR manganites. Studies at other densities are also presented, and an intuitive picture for the presence of the resistivity peak is discussed.

Research paper thumbnail of Time-dependent local Green’s operator and its applications to manganites

Physical Review B, 2005

An algorithm is presented to calculate the electronic local time-dependent Green's operator for m... more An algorithm is presented to calculate the electronic local time-dependent Green's operator for manganites-related hamiltonians. This algorithm is proved to scale with the number of states N in the Hilbert-space to the 1.55 power, is able of parallel implementation, and outperforms computationally the Exact Diagonalization (ED) method for clusters larger than 64 sites (using parallelization). This method together with the Monte Carlo (MC) technique is used to derive new results for the manganites phase diagram for the spatial dimension D=3 and half-filling on a 12x12x12 cluster (3456 orbitals). We obtain as a function of an insulating parameter, the sequence of ground states given by: ferromagnetic (FM), antiferromagnetic AF-type A, AF-type CE, dimer and AF-type G, which are in remarkable agreement with experimental results.

Research paper thumbnail of Time-dependent local Green's operator and its applications to manganites

Physical review. B, Condensed matter

An algorithm is presented to calculate the electronic local time-dependent Green's operator f... more An algorithm is presented to calculate the electronic local time-dependent Green's operator for manganites-related hamiltonians. This algorithm is proved to scale with the number of states NNN in the Hilbert-space to the 1.55 power, is able of parallel implementation, and outperforms computationally the Exact Diagonalization (ED) method for clusters larger than 64 sites (using parallelization). This method together with the Monte Carlo (MC) technique is used to derive new results for the manganites phase diagram for the spatial dimension D=3 and half-filling on a 12x12x12 cluster (3456 orbitals). We obtain as a function of an insulating parameter, the sequence of ground states given by: ferromagnetic (FM), antiferromagnetic AF-type A, AF-type CE, dimer and AF-type G, which are in remarkable agreement with experimental results. Comment: 9 pages, 11 figures

Research paper thumbnail of Fragility of the A -type AF and CE phases of manganites: Insulator-to-metal transition induced by quenched disorder

Physical Review B Condensed Matter and Materials Physics, Jun 1, 2006

Using Monte Carlo simulations and the two eg -orbital model for manganites, the stability of the ... more Using Monte Carlo simulations and the two eg -orbital model for manganites, the stability of the CE and A -type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller) phonon coupling, the previously hinted ``fragility'' of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it probably occurs with striped states in the cuprates.

Research paper thumbnail of Influence of Disorder in the Fm-Ce Competition of Half-Doped Manganites

Using Monte Carlo techniques, we study the two orbitals model with cooperative Jahn-Teller phonon... more Using Monte Carlo techniques, we study the two orbitals model with cooperative Jahn-Teller phonons for the case of half-doped manganites, where ferromagnetic (FM) and CE states are in competition. The phase diagram is obtained, which is shown to include standard as well as novel phases. Quenched disorder in the coupling that move us from the FM to the CE phases is also included in the simulations. It is shown that the CE state is highly sensitive to disorder, as found in recent experiments, while the FM is more robust. The effects of magnetic fields are also discussed. Transport properties are analyzed by calculating the conductance of the cluster considered as coupled to ideal leads.

Research paper thumbnail of Resistivity peak in the one-band double-exchange model with cooperative phonons

We present the results of Monte Carlo simulations of a single-band double-exchange model with coo... more We present the results of Monte Carlo simulations of a single-band double-exchange model with cooperative phonons, both with and without quenched disorder, at a carrier density n=0.3. Both in two and three dimensions, the simulations reveal a peak in the resistivity at TCurie, in agreement with previous studies by other groups. The peak gets destroyed upon the application of an external field, resulting in a large magnetoresistance. The results are in good agreement with experiments involving CMR manganites. Studies at other densities are also presented, and an intuitive picture for the presence of the resistivity peak is discussed.

Research paper thumbnail of 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

Physical Review B, 2003

A comprehensive analysis of half-doped manganites is presented using Monte Carlo simulations appl... more 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 lambda-JAF plane, with lambda the electron-phonon coupling and JAF the antiferromagnetic exchange between classical t2g spins. The results include

Research paper thumbnail of Island phases and charge order in two-dimensional manganites

Physical Review B, 2001

The ferromagnetic Kondo lattice model with an antiferromagnetic interaction between localized spi... more The ferromagnetic Kondo lattice model with an antiferromagnetic interaction between localized spins is a minimal description of the competing kinetic (t) and magnetic (K) energy terms which generate the rich physics of manganite systems. Motivated by the discovery in one dimension of homogeneous "island phases", we consider the possibility of analogous phases in higher dimensions. We characterize the phases present at commensurate fillings, and consider in detail the effects of phase separation in all filling and parameter regimes. We deduce that island and flux phases are stable for intermediate values of K/t at the commensurate fillings n = 1/4, 1/3, 3/8, and 1/2. We discuss the connection of these results to the charge and magnetic ordering observed in a wide variety of manganite compounds.

Research paper thumbnail of Colossal magnetoresistance observed in Monte Carlo simulations of the one- and two-orbital models for manganites

Physical Review B, 2006

The one-and two-orbital double-exchange models for manganites are studied using Monte Carlo compu... more The one-and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling ͑but neglecting the antiferromagnetic exchange J AF between the localized spins͒. The focus in this effort is on the analysis of charge transport. Our results for the one-orbital case confirm and extend previous recent investigations that showed the presence of robust peaks in the resistivity versus temperature curves for this model. Quenched disorder substantially enhances the magnitude of the effect, while magnetic fields drastically reduce the resistivity. A simple picture for the origin of these results is presented. It is also shown that even for the case of just one electron, the resistance curves present metallic and insulating regions by varying the temperature, as it occurs at finite electronic density. Moreover, in the present study these investigations are extended to the more realistic two-orbital model for manganites. The transport results for this model show large peaks in the resistivity versus temperature curves, located at approximately the Curie temperature, and with associated large magnetoresistance factors. Overall, the magnitude and shape of the effects discussed here resemble experiments for materials such as La 0.70 Ca 0.30 MnO 3 , and they are in agreement with the current predominant theoretical view that competition between a metal and an insulator, enhanced by quenched disorder, is crucial to understanding the colossal magnetoresistance ͑CMR͒ phenomenon. However, it is argued that further work is still needed to fully grasp the experimentally observed CMR effect, since in several other Mn oxides an antiferromagnetic chargeordered orbital-ordered state is the actual competitor of the ferromagnetic metal.

Research paper thumbnail of Fragility of the A-type AF and CE phases of manganites: Insulator-to-metal transition induced by quenched disorder

Physical Review B, 2006

Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the C... more Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller)phonon coupling, the previously hinted "fragility" of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. It is argued that the effect unveiled here has unique properties that may define a new class of giant effects in complex oxides. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it occurs with striped states in the cuprates.

Research paper thumbnail of Magnetic polarons in Ca1−xYxMnO3

Physica B: Condensed Matter, 2002

Experimental evidence shows that in the magnetoresistive manganite Ca 1Àx Y x MnO 3 ; ferromagnet... more Experimental evidence shows that in the magnetoresistive manganite Ca 1Àx Y x MnO 3 ; ferromagnetic (FM) polarons arises in an antiferromagnetic (AF) background, as a result of the doping with yttrium. This hypothesis is supported in this work by classical Monte Carlo calculations performed on a model where FM double exchange and AF superexchange compete. r

Research paper thumbnail of Ca1−xYxMnO3 manganites: synthesis and ESR characterization

Physica B: Condensed Matter, 2002

Ceramic samples of Ca1−xYxMnO3 were synthesized by a liquid-mix method obtaining single phase mat... more Ceramic samples of Ca1−xYxMnO3 were synthesized by a liquid-mix method obtaining single phase materials, for 0.1⩽x⩽1, with orthorhombic structure. The cell volume increases with x indicating that changes in the Mn electronic state overcomes the progressive diminishing of the (Ca, Y) cationic radius, rCa>rY. We observed a continuous broadening of the electronic spin resonance line width with x. This is

Research paper thumbnail of Charge ordering in the electron doped Ca1−xYxMnO3 manganites

Physica B: Condensed Matter, 2002

Structural studies on the electron doped Ca1−xYxMnO3 are presented. At 300K, orthorhombic O-phase... more Structural studies on the electron doped Ca1−xYxMnO3 are presented. At 300K, orthorhombic O-phase was observed in all cases, associated to low electric resistivity and high Curie–Weiss temperature. For samples with x>0.07, structural phase transitions to more distorted orthorhombic and monoclinic phases were found at T<170K. In these phases only weak ferromagnetic interactions were observed.

Research paper thumbnail of Double exchange model for RuSr2(Eu,Gd)Cu2O8

Physica B: Condensed Matter, 2002

We propose a double exchange model to describe the RuO2 planes of RuSr2(Eu,Gd)Cu2O8. The Ru +5 io... more We propose a double exchange model to describe the RuO2 planes of RuSr2(Eu,Gd)Cu2O8. The Ru +5 ions are described by localized spins, and additional electrons provided by the superconducting CuO2 planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru +5 spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.

Research paper thumbnail of High temperature susceptibility in electron doped Ca 1  x Y x MnO 3 : double exchange versus superexchange

Journal of Physics: Condensed Matter, 2003

We present a study of the magnetic properties of the electron doped manganites Ca 1-x Y x MnO 3 (... more We present a study of the magnetic properties of the electron doped manganites Ca 1-x Y x MnO 3 (for 0 ≤ x ≤ 0.25) in the paramagnetic regime. For the less doped samples (x ≤ 0.1) the magnetic susceptibility, χ(T), follows a Curie-Weiss (CW) law only for T > 450 K and, below this temperature, χ -1 (T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn 3+ and Mn 4+ ions, competes with classical superexchange. For higher levels of doping (x ≥ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (T mo ) and a better description of χ(T) was obtained by assuming full delocalization of the e g electrons. In order to explore the degree of delocalization as a function of T and x, we analyzed the problem through Montecarlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when T mo is approached, they form magnetic polarons of large spin that cause the observed curvature in χ -1 (T) for x ≤ 0.1.

Research paper thumbnail of Structural, electric, and magnetic study of Y0.5Ca0.5MnO3

Journal of Magnetism and Magnetic Materials, 2004

Correlations between magnetic, structural and transport properties were found in the Y-Ca half-do... more Correlations between magnetic, structural and transport properties were found in the Y-Ca half-doped manganite. At To500 K the crystalline structure changes from an orthorhombic O-phase to a more distorted O 0 -phase. The electron spin resonance behaviour allow us to determine the transition temperatures T CO ¼ 290 and T N ¼ 130 K. r

Research paper thumbnail of Structural Phase Diagram of Ca1−xYxMnO3: Characterization of Phases

Journal of Solid State Chemistry, 2001

To help the understanding of the physical behavior of Ca 1؊x Y x MnO 3 , its phase diagram in the... more To help the understanding of the physical behavior of Ca 1؊x Y x MnO 3 , its phase diagram in the whole x concentration range was investigated taking into account the stability of phases and the possible coexistence of di4erent structural phases. By careful analysis of powder X-ray di4raction (XRD) patterns, we were able to observe the following phase diagram: (i) Orthorhombic phases were detected both in the region of 04x40.25 (O type phase with Ca site twelve fold coordinated) and in the region of 0.54x(0.75 (O type phase with Ca site ninefold coordinated). (ii) Phase segregation for 0.254x40.5 and for x50.75 that have not been reported previously, hexagonal YMnO 3 segregates as a separate phase for x'0.75, and for 0.254x40.5 the coexistence of Ca 0.75 Y 0.25 MnO 3 (O) and Ca 0.5 Y 0.5 MnO 3 (O) have to be included in the re5nement for it to converge.

Research paper thumbnail of Structural Phase Diagram of Ca

Journal of Solid State Chemistry, 2001

To help the understanding of the physical behavior of Ca{sub 1-x}YâMnOâ, its phase diagram in the... more To help the understanding of the physical behavior of Ca{sub 1-x}YâMnOâ, its phase diagram in the whole x concentration range was investigated taking into account the stability of phases and the possible coexistence of different structural phases. By careful analysis of powder X-ray diffraction (XRD) patterns, we were able to observe the following phase diagram: (i) Orthorhombic phases were detected

Research paper thumbnail of Phase diagram of electron-doped Mn perovskites

Journal of Magnetism and Magnetic Materials, 1999

We study the competition between ferromagnetic double exchange and antiferromagnetic superexchang... more We study the competition between ferromagnetic double exchange and antiferromagnetic superexchange present in Mn perovskites. The model represents each Mn 4+ ion by a spin ½, on which an electron can be added to produce Mn 3 We include a hopping energy t, a strong intratomic interaction exchange J {in the limit J/t-+ ,JJ), and an interatomic antiferromagnetic interaction K between the local spins. Using the renormalized perturbation expansion and a mean-field approximation we calculate the free energy from which the stability of the antiferromagnetic, canted, ferromagnetic and 'pseudo-spin-glass' phases can be determined. The phase diagram can be drawn in terms of temperature and doping x, for each value of K/t. We discuss the presence of new 'pseudo-spin-glass' phases. ,

Research paper thumbnail of Phase diagram for Ca1−xYxMnO3 type crystals

Solid State Communications, 1999

We present a simple model to study the electron doped manganese perovskites. The model considers ... more We present a simple model to study the electron doped manganese perovskites. The model considers the competition between double exchange mechanism for itinerant electrons and antiferromagnetic superexchange interaction for localized electrons. It represents each Mn 4ϩ ion by a spin 1/2, on which an electron can be added to produce Mn 3ϩ ; we include a hopping energy t, a strong intratomic ferromagnetic interaction exchange J (in the limit J=t 3 ∞), and an interatomic antiferromagnetic interaction K between the local spins. Using the Renormalized Perturbation Expansion and a Mean Field Approximation on the hopping terms and on the superexchange interaction, we calculate the free energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic, and novel spin glass phases can be determined as functions of the parameters characterizing the system. The model results can be expressed in terms of t and K for each value of the doping x in phase diagrams. The magnetization m and canting angle u can also be calculated as fuctions of temperature for fixed values of doping and model parameters.

Research paper thumbnail of Resistivity peak in the one-band double-exchange model with cooperative phonons

ABSTRACT We present the results of Monte Carlo simulations of a single-band double-exchange model... more ABSTRACT We present the results of Monte Carlo simulations of a single-band double-exchange model with cooperative phonons, both with and without quenched disorder, at a carrier density n=0.3. Both in two and three dimensions, the simulations reveal a peak in the resistivity at TCurie, in agreement with previous studies by other groups. The peak gets destroyed upon the application of an external field, resulting in a large magnetoresistance. The results are in good agreement with experiments involving CMR manganites. Studies at other densities are also presented, and an intuitive picture for the presence of the resistivity peak is discussed.

Research paper thumbnail of Time-dependent local Green’s operator and its applications to manganites

Physical Review B, 2005

An algorithm is presented to calculate the electronic local time-dependent Green's operator for m... more An algorithm is presented to calculate the electronic local time-dependent Green's operator for manganites-related hamiltonians. This algorithm is proved to scale with the number of states N in the Hilbert-space to the 1.55 power, is able of parallel implementation, and outperforms computationally the Exact Diagonalization (ED) method for clusters larger than 64 sites (using parallelization). This method together with the Monte Carlo (MC) technique is used to derive new results for the manganites phase diagram for the spatial dimension D=3 and half-filling on a 12x12x12 cluster (3456 orbitals). We obtain as a function of an insulating parameter, the sequence of ground states given by: ferromagnetic (FM), antiferromagnetic AF-type A, AF-type CE, dimer and AF-type G, which are in remarkable agreement with experimental results.

Research paper thumbnail of Time-dependent local Green's operator and its applications to manganites

Physical review. B, Condensed matter

An algorithm is presented to calculate the electronic local time-dependent Green's operator f... more An algorithm is presented to calculate the electronic local time-dependent Green's operator for manganites-related hamiltonians. This algorithm is proved to scale with the number of states NNN in the Hilbert-space to the 1.55 power, is able of parallel implementation, and outperforms computationally the Exact Diagonalization (ED) method for clusters larger than 64 sites (using parallelization). This method together with the Monte Carlo (MC) technique is used to derive new results for the manganites phase diagram for the spatial dimension D=3 and half-filling on a 12x12x12 cluster (3456 orbitals). We obtain as a function of an insulating parameter, the sequence of ground states given by: ferromagnetic (FM), antiferromagnetic AF-type A, AF-type CE, dimer and AF-type G, which are in remarkable agreement with experimental results. Comment: 9 pages, 11 figures

Research paper thumbnail of Fragility of the A -type AF and CE phases of manganites: Insulator-to-metal transition induced by quenched disorder

Physical Review B Condensed Matter and Materials Physics, Jun 1, 2006

Using Monte Carlo simulations and the two eg -orbital model for manganites, the stability of the ... more Using Monte Carlo simulations and the two eg -orbital model for manganites, the stability of the CE and A -type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller) phonon coupling, the previously hinted ``fragility'' of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it probably occurs with striped states in the cuprates.

Research paper thumbnail of Influence of Disorder in the Fm-Ce Competition of Half-Doped Manganites

Using Monte Carlo techniques, we study the two orbitals model with cooperative Jahn-Teller phonon... more Using Monte Carlo techniques, we study the two orbitals model with cooperative Jahn-Teller phonons for the case of half-doped manganites, where ferromagnetic (FM) and CE states are in competition. The phase diagram is obtained, which is shown to include standard as well as novel phases. Quenched disorder in the coupling that move us from the FM to the CE phases is also included in the simulations. It is shown that the CE state is highly sensitive to disorder, as found in recent experiments, while the FM is more robust. The effects of magnetic fields are also discussed. Transport properties are analyzed by calculating the conductance of the cluster considered as coupled to ideal leads.

Research paper thumbnail of Resistivity peak in the one-band double-exchange model with cooperative phonons

We present the results of Monte Carlo simulations of a single-band double-exchange model with coo... more We present the results of Monte Carlo simulations of a single-band double-exchange model with cooperative phonons, both with and without quenched disorder, at a carrier density n=0.3. Both in two and three dimensions, the simulations reveal a peak in the resistivity at TCurie, in agreement with previous studies by other groups. The peak gets destroyed upon the application of an external field, resulting in a large magnetoresistance. The results are in good agreement with experiments involving CMR manganites. Studies at other densities are also presented, and an intuitive picture for the presence of the resistivity peak is discussed.

Research paper thumbnail of 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

Physical Review B, 2003

A comprehensive analysis of half-doped manganites is presented using Monte Carlo simulations appl... more 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 lambda-JAF plane, with lambda the electron-phonon coupling and JAF the antiferromagnetic exchange between classical t2g spins. The results include

Research paper thumbnail of Island phases and charge order in two-dimensional manganites

Physical Review B, 2001

The ferromagnetic Kondo lattice model with an antiferromagnetic interaction between localized spi... more The ferromagnetic Kondo lattice model with an antiferromagnetic interaction between localized spins is a minimal description of the competing kinetic (t) and magnetic (K) energy terms which generate the rich physics of manganite systems. Motivated by the discovery in one dimension of homogeneous "island phases", we consider the possibility of analogous phases in higher dimensions. We characterize the phases present at commensurate fillings, and consider in detail the effects of phase separation in all filling and parameter regimes. We deduce that island and flux phases are stable for intermediate values of K/t at the commensurate fillings n = 1/4, 1/3, 3/8, and 1/2. We discuss the connection of these results to the charge and magnetic ordering observed in a wide variety of manganite compounds.

Research paper thumbnail of Colossal magnetoresistance observed in Monte Carlo simulations of the one- and two-orbital models for manganites

Physical Review B, 2006

The one-and two-orbital double-exchange models for manganites are studied using Monte Carlo compu... more The one-and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling ͑but neglecting the antiferromagnetic exchange J AF between the localized spins͒. The focus in this effort is on the analysis of charge transport. Our results for the one-orbital case confirm and extend previous recent investigations that showed the presence of robust peaks in the resistivity versus temperature curves for this model. Quenched disorder substantially enhances the magnitude of the effect, while magnetic fields drastically reduce the resistivity. A simple picture for the origin of these results is presented. It is also shown that even for the case of just one electron, the resistance curves present metallic and insulating regions by varying the temperature, as it occurs at finite electronic density. Moreover, in the present study these investigations are extended to the more realistic two-orbital model for manganites. The transport results for this model show large peaks in the resistivity versus temperature curves, located at approximately the Curie temperature, and with associated large magnetoresistance factors. Overall, the magnitude and shape of the effects discussed here resemble experiments for materials such as La 0.70 Ca 0.30 MnO 3 , and they are in agreement with the current predominant theoretical view that competition between a metal and an insulator, enhanced by quenched disorder, is crucial to understanding the colossal magnetoresistance ͑CMR͒ phenomenon. However, it is argued that further work is still needed to fully grasp the experimentally observed CMR effect, since in several other Mn oxides an antiferromagnetic chargeordered orbital-ordered state is the actual competitor of the ferromagnetic metal.

Research paper thumbnail of Fragility of the A-type AF and CE phases of manganites: Insulator-to-metal transition induced by quenched disorder

Physical Review B, 2006

Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the C... more Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller)phonon coupling, the previously hinted "fragility" of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. It is argued that the effect unveiled here has unique properties that may define a new class of giant effects in complex oxides. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it occurs with striped states in the cuprates.

Research paper thumbnail of Magnetic polarons in Ca1−xYxMnO3

Physica B: Condensed Matter, 2002

Experimental evidence shows that in the magnetoresistive manganite Ca 1Àx Y x MnO 3 ; ferromagnet... more Experimental evidence shows that in the magnetoresistive manganite Ca 1Àx Y x MnO 3 ; ferromagnetic (FM) polarons arises in an antiferromagnetic (AF) background, as a result of the doping with yttrium. This hypothesis is supported in this work by classical Monte Carlo calculations performed on a model where FM double exchange and AF superexchange compete. r

Research paper thumbnail of Ca1−xYxMnO3 manganites: synthesis and ESR characterization

Physica B: Condensed Matter, 2002

Ceramic samples of Ca1−xYxMnO3 were synthesized by a liquid-mix method obtaining single phase mat... more Ceramic samples of Ca1−xYxMnO3 were synthesized by a liquid-mix method obtaining single phase materials, for 0.1⩽x⩽1, with orthorhombic structure. The cell volume increases with x indicating that changes in the Mn electronic state overcomes the progressive diminishing of the (Ca, Y) cationic radius, rCa>rY. We observed a continuous broadening of the electronic spin resonance line width with x. This is

Research paper thumbnail of Charge ordering in the electron doped Ca1−xYxMnO3 manganites

Physica B: Condensed Matter, 2002

Structural studies on the electron doped Ca1−xYxMnO3 are presented. At 300K, orthorhombic O-phase... more Structural studies on the electron doped Ca1−xYxMnO3 are presented. At 300K, orthorhombic O-phase was observed in all cases, associated to low electric resistivity and high Curie–Weiss temperature. For samples with x>0.07, structural phase transitions to more distorted orthorhombic and monoclinic phases were found at T<170K. In these phases only weak ferromagnetic interactions were observed.

Research paper thumbnail of Double exchange model for RuSr2(Eu,Gd)Cu2O8

Physica B: Condensed Matter, 2002

We propose a double exchange model to describe the RuO2 planes of RuSr2(Eu,Gd)Cu2O8. The Ru +5 io... more We propose a double exchange model to describe the RuO2 planes of RuSr2(Eu,Gd)Cu2O8. The Ru +5 ions are described by localized spins, and additional electrons provided by the superconducting CuO2 planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru +5 spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.

Research paper thumbnail of High temperature susceptibility in electron doped Ca 1  x Y x MnO 3 : double exchange versus superexchange

Journal of Physics: Condensed Matter, 2003

We present a study of the magnetic properties of the electron doped manganites Ca 1-x Y x MnO 3 (... more We present a study of the magnetic properties of the electron doped manganites Ca 1-x Y x MnO 3 (for 0 ≤ x ≤ 0.25) in the paramagnetic regime. For the less doped samples (x ≤ 0.1) the magnetic susceptibility, χ(T), follows a Curie-Weiss (CW) law only for T > 450 K and, below this temperature, χ -1 (T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn 3+ and Mn 4+ ions, competes with classical superexchange. For higher levels of doping (x ≥ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (T mo ) and a better description of χ(T) was obtained by assuming full delocalization of the e g electrons. In order to explore the degree of delocalization as a function of T and x, we analyzed the problem through Montecarlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when T mo is approached, they form magnetic polarons of large spin that cause the observed curvature in χ -1 (T) for x ≤ 0.1.

Research paper thumbnail of Structural, electric, and magnetic study of Y0.5Ca0.5MnO3

Journal of Magnetism and Magnetic Materials, 2004

Correlations between magnetic, structural and transport properties were found in the Y-Ca half-do... more Correlations between magnetic, structural and transport properties were found in the Y-Ca half-doped manganite. At To500 K the crystalline structure changes from an orthorhombic O-phase to a more distorted O 0 -phase. The electron spin resonance behaviour allow us to determine the transition temperatures T CO ¼ 290 and T N ¼ 130 K. r