Search

Manganitesof La0.7Ca0.3−xSrxMnO3 have been produced using the hydrothermal method. The influence of the concentration of La, Ca, and Sr on the structural, morphological, and magnetic properties was studied and analyzed. The x-ray diffraction studies showed the occurrence of orthorombic and trigonal symmetries in the crystalline structure, depending on the stoichiometry. X-ray photoelectron spectroscopy studies were performed, showing the presence of the elements La3d, Ca2p, Sr3d, Mn2p, and O1s. In all samples, a deficiency of oxygen was observed and ascribed to hydroxide formation. The FTIR spectra shows the Mn–O–Mn bonds in the stretching vibration mode; on the other hand, a great relative amount of Mn4+ ions that are not included in the manganite phase was observed. Furthermore, morphology analyses developed using scanning electron microscopy showed cauliflower-type microstructures in all samples. The dependence of magnetization on magnetic field was also studied where a low coercivity and a high-saturation magnetization were evidenced. Values of the squareness ratio coefficient ranged between 0.0587 and 0.0867, indicating that the individual particles exhibit a strong random anisotropy. [ABSTRACT FROM AUTHOR]

AC magnetic susceptibility of LaCaMnO ( x = 0.25, 0.33, 04, 0.5) thin films was obtained using Monte Carlo simulations combined with Fourier transform. The material stoichiometry was varied depending on the combination of Mn and Mn ions. Hamiltonian used includes the Heisenberg interaction, magnetocrystalline anisotropy, and Zeeman effect. For determining AC susceptibility, a time-dependent external magnetic field h( t) was applied. The study was conducted for various angular frequencies and temperatures. The simulations were obtained using a graphical interface that allowed the online monitoring of the results. According to the results, the AC susceptibility decreases not only with the temperature but also with the frequency. [ABSTRACT FROM AUTHOR]

The Metropolis algorithm and the classical Heisenberg approximation were implemented by the Monte Carlo method to design a computational approach to the magnetization and resistivity of La 2/3 Ca 1/3 MnO 3 , which depends on the Mn ion vacancies as the external magnetic field increases. This compound is ferromagnetic, and it exhibits the colossal magnetoresistance (CMR) effect. The monolayer was built with L × L × d dimensions, and it had L =30 umc (units of magnetic cells) for its dimension in the x – y plane and was d =12 umc in thickness. The Hamiltonian that was used contains interactions between first neighbors, the magnetocrystalline anisotropy effect and the external applied magnetic field response. The system that was considered contains mixed-valence bonds: Mn 3+eg’ –O–Mn 3+eg , Mn 3+eg –O–Mn 4+d3 and Mn 3+eg’ –O–Mn 4+d3 . The vacancies were placed randomly in the sample, replacing any type of Mn ion. The main result shows that without vacancies, the transitions T C (Curie temperature) and T MI (metal–insulator temperature) are similar, whereas with the increase in the vacancy percentage, T MI presented lower values than T C . This situation is caused by the competition between the external magnetic field, the vacancy percentage and the magnetocrystalline anisotropy, which favors the magnetoresistive effect at temperatures below T MI . Resistivity loops were also observed, which shows a direct correlation with the hysteresis loops of magnetization at temperatures below T C . [ABSTRACT FROM AUTHOR]

Abstract: In this work we address the study of the magnetic and magnetotransport properties of ferromagnetic (FM)/antiferromagnetic (AFM) La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 manganites bilayers by means of the standard Monte Carlo method. Simulations, implemented with a single spin flip Metropolis dynamics, were performed in the framework of a three-dimensional classical Heisenberg model. Four different superexchange interactions, with the two different types of ions Mn3+ and Mn4+ involved and the orbital differences accounting for the Mn3+ ions, were considered in the Hamiltonian. Likewise, terms dealing with magnetocrystalline anisotropy and the influence of an external applied magnetic field were also included. Samples were simulated having dimensions L×L×(d FM +d AFM ) where L, measured in lattice parameter units, stands for the linear and lateral dimension along which periodic boundary conditions were implemented, whereas d FM and d AFM stand for the thicknesses of FM and AFM layers respectively with free boundary conditions. Different degrees of roughness at the interface were also simulated in order to analyze its influence on the magnetic and magnetotransport properties of the system. In particular, low-temperature results reveal a trend of the coercive field to increase with roughness contrary to the exchange bias field behavior, which tends to decrease. Both quantities are modulated by oscillations ascribed to the atomic disorder at the interface. Finally, the roughness effect upon the bilayer resistivity and on the metal–insulator temperature is also presented and discussed. [Copyright &y& Elsevier]

Abstract: The magnetocaloric effect (MCE) of La1−x Ca x MnO3 was analyzed when varying the stoichiometry (x=0.2, 0.33, 0.4 and 0.5) and the external applied magnetic field. Simulations were carried out using the Monte Carlo method having a configuration ascribed by the set S z of all-site spin projections, under the Metropolis algorithm. La1−x Ca x MnO3 is characterized for three types of magnetic ions corresponding to Mn4+ (S=3/2), which are bonded to Ca2+, and Mn3+(e g ) and Mn3+(e g′, S=2), which are both bonded to La3+. Different interactions must be considered depending on the type of interacting ions. The entropy change ΔS in an isothermal process as well as the temperature change ΔT in an adiabatic process was determined with this model. Both ΔS and ΔT show stoichiometry-dependent maximums near the paramagnetic–ferromagnetic transition temperature (T C ), confirming the order–disorder change at this temperature. The strong dependence in La1−x Ca x MnO3 of the MCE on the stoichiometry and the external magnetic field makes this phenomenon important for different technological applications, particularly in refrigeration. [Copyright &y& Elsevier]

This study presents the magnetic properties of manganite fine particles using Monte Carlo simulations in the framework of a core-shell model. A single-spin movement Metropolis dynamics was implemented to compute equilibrium averages. Calculations were performed on the basis of a three-dimensional classical Heisenberg Hamiltonian, involving the presence of Mn ( $\mathrm{Mn}^{3+e_{g}}$ and $\mathrm{Mn}^{3+e_{g}'}$) and Mn ( $\mathrm{Mn}^{4 + d^{3}}$) cations, and their nearest neighbor interaction. The Hamiltonian includes a surface anisotropy term applied to surface ions, and cubic anisotropy for ions belonging to the core. Different diameters were considered in order to figure out different off-stoichiometric scenarios and the influence on the magnetic properties. Results reveal a well-defined linear particle size inverse dependence of the Curie temperature. No evidence for surface spin disorder was detected. Finally, susceptibility data reveal that the ferromagnetic-to-paramagnetic transition occurs in a gradual fashion ascribed to a differentiated behavior between the core and surface. Initially, the surface contribution to magnetic properties is high; as the nanoparticle size increases, the core contribution becomes stronger. [ABSTRACT FROM AUTHOR]

Abstract: This work presents a critical temperature study of La1−x Ca x MnO3 manganites in bulk by means of Monte Carlo method thermal activated magnetic properties. The analysis was carried out for stoichiometries in the range of 0≤x≤1. The model is based on a three-dimensional classical Heisenberg–Hamiltonian involving the presence of Mn3+eg , Mn3+eg′ and Mn4+ ions, and their nearest neighbor interaction. For this modeling, simple cubic lattice samples of size L 3, with L=6, 15 and 30 were used. The values of exchange parameters were determined by using LaMnO3 (x=0), La0.5Ca0.5MnO3 and CaMnO3 (x=1) phases. Relationships between exchange parameters and anisotropy constants for different hole densities were found. Results of transition temperatures for each phase showed good agreement with experimental reports, especially for L=30 and L→∞. [Copyright &y& Elsevier]

In this work, the role of the geometrical shape, using the Minkowski's metrics, upon the temperature dependence of the magnetic and electrical transport properties of LaCaMnO nanoparticles was carried out. We considered a set of small particles with different shapes containing approximately the same amount of Mn ions (~6500) and distributed as a simple cubic structure in agreement with the manganites perovskite structure. The model is based on the standard Monte Carlo-Metropolis method and the classical Heisenberg Hamiltonian involving nearest magnetic neighbors interactions. Results dealing with the dependence of the Curie temperature for nanoparticles is lower than material in bulk. Moreover, magnetization and magnetic susceptibility as a function of the metric for some field cooled values. As the field cooled increased the transition temperature T increased. On the other hand, low temperature coercive force and resistivity with the shape of the nanoparticles, having different metric parameters, are present and discussed. [ABSTRACT FROM AUTHOR]

Abstract: Critical exponents offer important information concerning the interaction mechanisms near the paramagnetic to ferromagnetic transition. In this work a Monte Carlo–Metropolis simulation of the critical behavior in La2/3Ca1/3MnO3 thin films is addressed. Canonical ensemble averages for magnetization per site, magnetic susceptibility and specific heat of stoichiometric manganite within a three-dimensional classical Heisenberg model with nearest magnetic neighbor interactions are computed. The La2/3Ca1/3MnO3 thin films were simulated addressing the thickness influence and thermal dependence. In the model, Mn magnetic ions are distributed on a simple cubic lattice according to the perovskite structure of this manganite. Ferromagnetic coupling for the bonds Mn3+–Mn3+(e g –e g ′), Mn3+–Mn4+(e g –d 3) and Mn3+–Mn4+(e g ′–d 3) were taken into account. On the basis of finite-size scaling theory, our best estimates of critical exponents, linked to the ferromagnetic to paramagnetic transition, for the correlation length, specific heat, magnetization and susceptibility are, respectively: v=0.56±0.01, α=0.16±0.03, β=0.34±0.04γ and γ=1.17±0.05. These theoretical results are consistent with the Rushbrooke equalitiy α+2β+γ=2. [ABSTRACT FROM AUTHOR]

In this study we emphasize on the role of the geometry upon the magnetic and critical properties of ferromagnetic manganite nanoparticles having stoichiometry La2/3 Ca1/3MnO3. To do that, we consider a set of fine particles of different shapes (spherical and polyhedral) containing approximately the same amount of ions (~8000 magnetic Mn ions) and distributed according to a simple cubic structure. The method is based on the standard Monte Carlo-Metropolis technique. Our theoretical framework employs a classical Heisenberg-like Hamiltonian, where different magnetic nearest neighbor superexchange integrals, preserving orbital ordering for this composition, have been considered. Results showed the specific behavior of the Curie temperature and low temperature coercive force. The studies were done depending on the nanoparticle shapes. [ABSTRACT FROM AUTHOR]

This work presents a study of dynamic phase transitions in samples of perovskite type manganites of L a 2 3 C a 1 3 Mn O 3 , when they were submitted to the influence of an external magnetic field that includes both a time dependent and a time independent magnetic fields. This study was carried out based on Monte Carlo simulations and using the Metropolis algorithm and the mixed spin Ising model. Exchange parameters are proposed to simulate this material. These exchange parameters were obtained in order to reproduce the critical temperature for L a 2 3 C a 1 3 Mn O 3 . Results showed that it exists a competition between different factors that generate order or disorder in the magnetic behavior of that material. It was observed that, as the time independent field increases, the critical temperature increases. Moreover, simulations showed that as the amplitude or the period of the time dependent magnetic field increases, the critical temperature decreases. This behavior allows to assert that an time dependent magnetic field induced disorder while a time independent field induces order. [ABSTRACT FROM AUTHOR]

The graphics interface is useful in specifying and examining chemical structures and physical properties. In this scientific and technological research paper, the SIMMON (MMSP - Manganite magnetic simulation properties) software is presented, as a visualization program, that allows to observe, the magnetization, susceptibility, energy, specific heat, the hysteresis loops and the resistivity of magnetic materials. This properties can be obtained from the simulation of different manganites Re2/3A1/3MnO3, where Re is a rare earth metal ions as La, Nd and Pr with valence 3+, which are bonded with Mn3+–3d4 ions and A is a 2+ alkaline earth ion such as Ba, Ca, Sr, among others, bonded with Mn4+–3d3. The based method for the simulations is Monte Carlo combined with Metropolis algorithm and Heisenberg model. [ABSTRACT FROM AUTHOR]

Samples of La 0.5 Ca 0.5 Mn 1−x Fe x O 3+δ (0≤x≤0.5) were synthesized using a solid-state reaction method involving a milling process and thermal treatment up to 1200 °C in an air atmosphere. Samples were characterized structurally with X-ray diffraction analysis and Rietveld refinement with morphology characterization using scanning electron microscopy. Magnetic properties were investigated using a physical property measurement system to obtain zero field cooling and the associated curves to plot hysteresis loops. Our results revealed the interplay between the structural and magnetic properties as Fe ions attached to the crystalline structure. A mechanism based on the substitution of Mn 3+ and Mn 4+ by low-spin Fe 3+ and Fe 4+ ions, respectively, was hypothesized to interpret the experimental data. More specifically, the temperature at which the transition from ferromagnetic to paramagnetic occurred increased with increasing Fe content as a result of a greater density of oxygen-mediated ferromagnetic bonds. Conversely, the magnetization weakened because the t 2g electrons were distributed in the respective d orbitals by adopting a low-spin configuration. Such a configuration is preferred as a result of the unit cell distortion in the milling process where the greater ionic radius of the Fe 4+ ions leads to an elongated c -axis tetragonal symmetry and a greater unit cell volume. Finally, low-temperature magnetic behavior revealed the occurrence of a reentrant spin-glass type state within the ferromagnetic matrix favored by a milling-driven structural disorder and the existence of competitive superexchange interactions. [ABSTRACT FROM AUTHOR]