Emergent phenomena in manganites under spatial confinement

It is becoming increasingly clear that the exotic properties displayed by correlated electronic materials such as high Tc superconductivity in cuprates,colossal magnetoresistance(CMR) in manganites,and heavy-fermion compounds are intimately related to the coexistence of competing nearly degenerate states which couple simultaneously active degrees of freedom-charge,lattice,orbital,and spin states.The striking phenomena associated with these materials are due in a large part to spatial electronic inhomogeneities,or electronic phase separation(EPS).In many of these hard materials,the functionality is a result of the soft electronic component that leads to self-organization.In this paper,we review our recent work on a novel spatial confinement technique that has led to some fascinating new discoveries about the role of EPS in manganites.Using lithographic techniques to confine manganite thin films to length scales of the EPS domains that reside within them,it is possible to simultaneously probe EPS domains with different electronic states.This method allows for a much more complete view of the phases residing in a material and gives vital information on phase formation,movement,and fluctuation.Pushing this trend to its limit,we propose to control the formation process of the EPS using external local fields,which include magnetic exchange field,strain field,and electric field.We term the ability to pattern EPS "electronic nanofabrication." This method allows us to control the global physical properties of the system at a very fundamental level,and greatly enhances the potential for realizing true oxide electronics.     

High pressure-high temperature synthesis of distorted perovskite-type Cu-based oxides

Abstract

We have synthesized the rhombohedrally distorted perovskite phase of LaCuO₃ by reacting mixtures of La₂O₃ and CuO in an oxygen rich atmosphere at 1500°C and 6.5 GPa. We find this phase to be metastable; at 410°C and ambient pressure, it undergoes an irreversible transition to a tetragonal structure. By selective replacement of some or all of the La or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr, over 150 different alloys have been formed. Magnetic susceptibility measurements have failed to reveal the presence of superconductivity in any of these new polymorphs.     

Critical behavior studies in Ti-substituted lanthanum bismuth perovskite manganites

Perovskite manganite La_0.4Bi_0.6Mn_1−xTi_xO₃ (x = 0.05 and 0.1) synthesized using conventional solid state route method give rise to critical phenomenon in their magnetic interactions due to the substitution of non magnetic Ti ions. The critical behavior is observed near paramagnetic–ferromagnetic transition and is studied by magnetization measurements. Various techniques like Modified Arrott plot, Kouvel–Fisher method, scaling equation of state analysis and the critical magnetization isotherm were used to analyze the magnetization data on magnetic phase transition. The values of critical exponents β and γ obtained using different techniques are in good agreement. The obtained critical exponents are found to follow scaling equation with the magnetization data scaled into two different curves below and above the transition temperature, T_C. This confirms that the critical exponents and T_C are reasonably accurate. The obtained critical exponents for both the samples deviates from mean-field model and do not completely follow the static long range ferromagnetic ordering. This behavior is consistent with non magnetic nature of Ti substituted at Mn site and can be associated with Griffiths phase like phenomenon.     

Low-field magnetoresistance of Ag-substituted perovskite-type manganites

A series of bulk polycrystalline Nd_1−xAg_xMnO₃ samples were prepared by conventional solid-state reaction processing, for x between 0.1 and 0.5. The structure, magnetism, and magnetoresistance (MR) of the samples are investigated. The X-ray diffraction patterns show that the x=0.1 sample is a single perovskite structure, while x0.167, samples consist of a ferromagnetic perovskite phase and two nonmagnetic phases, Ag and Mn₃O₄. The MR of Nd_1−xAg_xMnO₃ is enhanced by increasing the composition of Ag. Compared with the sample of x=0.1, the x=0.5 sample has a significantly larger value of low-field MR. The enhanced low-field MR is related to the spin-dependent scattering of spin-polarized electrons at the interfaces between the perovskite grains and silver granules.     

物理学中的演生现象

在物理学过去的发展历史中,还原论的观点一直是物理学工作者进行研究的最基本的指导原则.它对整个学科的发展起到了巨大的推动作用,并取得了辉煌的成就.但是,以还原论为基础来研究和讨论复杂系统的合作现象时,却遇到了前所未有的挑战,从而使演生论的思想孕育而生,并成为当今物理学研究的重要指导原则.文章详细介绍了凝聚物理学中典型的演生现象的形成和发展的历史过程,主要的研究内容和研究方法,以及所取得的重要进展.     

物理学中的演生现象

在物理学过去的发展历史中,还原论的观点一直是物理学工作者进行研究的最基本的指导原则.它对整个学科的发展起到了巨大的推动作用,并取得了辉煌的成就.但是,以还原论为基础来研究和讨论复杂系统的合作现象时,却遇到了前所未有的挑战,从而使演生论的思想孕育而生,并成为当今物理学研究的重要指导原则.文章详细介绍了凝聚物理学中典型的演生现象的形成和发展的历史过程,主要的研究内容和研究方法,以及所取得的重要进展.     

Status of PRESS-MAG-O: The experimental apparatus to probe materials and phenomena under extreme conditions at Frascati

In this contribution we up-to-date the status of the PRESS-MAG-O device, a new instrument under commissioning at the INFN designed to perform magnetic and spectroscopic experiments on samples under extreme conditions. The system has been designed to work at SINBAD, the IR synchrotron radiation beamline operational at DAΦNE. The instrument, that is the result of a significant R&D, will allow performing concurrent high harmonic ac magnetic susceptibility measurements and magneto-optic experiments on a sample under high pressure, with a variable DC magnetic field in a wide temperature range. The vacuum vessel has been designed with four crossing windows to allow optical measurements in the transmission geometry on the sample loaded inside a Diamond Anvil Cell. A new superconducting miniaturized micro-SQUID gradiometer has been also developed to detect the low magnetic signal of the sample and a customized optical system has also been designed to perform IR synchrotron radiation experiments.     

PRESS-MAG-O: A new facility to probe materials and phenomena under extreme conditions

In the recent years, several experiments performed under high magnetic fields (HMFs), at high pressure (HP) and/or at low temperature (LT) have led to spectacular discoveries in condensed matter. In many new systems, although challenging, it is strategic to perform a magneto-optical analysis, to investigate the phonon behavior in the far infrared (IR) domain. By combining HMF and HP in a wide temperature (T) range to perform concurrently IR magneto-optics and ac-magneto-dynamic experiments, it will be possible to achieve unique information on systems and/or new phenomena, almost impossible to obtain with standard spectroscopic methods. Here we present PRESS-MAG-O, a new facility under construction that will perform HP experiments under HMF in a wide T range. The system is expected to be operational by the end of 2008 and will be tested at SINBAD, the IR synchrotron radiation beamline operational since 2001 at DAΦNE (Double Annular Φ-factory for Nice Experiments), the storage ring of the INFN Frascati National Laboratory (LNF). While for IR experiments an interferometer will be used, for the magneto-dynamic experiments a SQUID magnetometer in the 10 Hz-2 KHz frequency range will be utilized. HP will be applied to samples by a Cu-Be diamond anvil cell (DAC), so that the device will be able to collect FTIR spectra and high harmonic ac susceptibility data in a dc magnetic field up to 8 T and to about 20 GPa in a wide temperature range (4.2-200 K).     

Tunable two-phase coexistence in half-doped manganites

We discuss our very interesting experimental observation that the low-temperature two-phase coexistence in half-doped manganites is multi-valued (at any field) in that we can tune the coexisting antiferromagnetic-insulating (AF-I) and the ferromagnetic-metallic (FM-M) phase fractions by following different paths in (H, T) space. We have shown experimentally that the phase fraction, in this two-phase coexistence, can take continuous infinity of values. All but one of these are metastable, and two-phase coexistence is not an equilibrium state.      

Curie temperature and frustrated phase separation in manganites

There is an increasing experimental evidence that phase separation between insulating and metallic phase plays an important role in the physics of manganites. On general grounds one can argue that the electronic density in the metallic and insulating region will be generally different. This implies that phase separation of the ordinary “Maxwell construction” type is frustrated by the long-range Coulomb interaction. We present a generalization of Maxwell construction to this situation. The system is assumed to separate in islands of one phases hosted by the other. The size of the islands is determined minimizing a free energy that takes into account both surface energy and Coulomb effects. We discuss the peculiarities of this kind of phase separation and the consequences for the manganites. In particular, we present an explanation for the non-monotonous behavior of the Curie temperature as a function of doping.     

Emergent reaction-diffusion phenomena in capillary tubes

Pattern formation in the Belousov-Zhabotinsky reaction experiments carried out by filling capillary glass tubes with catalyst-immobilized gel for the reaction is reported. Under unperturbed and oscillatory conditions, helicoidal waves appear spontaneously. Quantitative structural data of those helices are obtained by devising an optical tomography technique for extracting rotationally symmetric structures from time-lapse data. Space-time representation of the catalyst oxidation reveals wave transmission phenomenon that is studied further by numerical simulations of a reduced spatial model.     

Effect of pressure on magnetic phases of (Fe1-x Ni x )49Rh51

The effect of high pressure and magnetic field on the stability of magnetic phase transitions in (Fe_1?x Ni _x )₄₉Rh₅₁ has been studied. It has been shown that the increase in the Ni content suppresses the stability of the low-temperature antiferromagnetic phase and leads to its disappearance. Pressure on the other hand restores the stability to the antiferromagnetic order. These findings are explained using first-principles band-structure calculations.     

Thermal stability of A-site ordered PrBaMn2O6 manganites

The crystal structure and electromagnetic properties as well as thermal stability of the A-site ordered PrBaMn₂O₆ manganites have been investigated. These samples have been prepared by using ‘two-steps’ synthesis mode. They have tetragonal structure with no tilt of MnO₆ octahedra and show ferromagnetic metal to paramagnetic semiconductor transition. The most significant structural feature of the A-site ordered manganites is that the MnO₂ sublattice is sandwiched by two types of rock-salt layers PrO and BaO. The different degree of Pr and Ba ions in the A-sublattice is revealed. The A-site ordered PrBaMn₂O₆ sample with maximum degree of the A-site order demonstrates ferromagnetic metallic to paramagnetic insulating transition with the Curie point ∼320 K. The A-site disordered Pr_0.50Ba_0.50MnO₃ sample is ferromagnetic metal below T_C≈140 K. The cation order in these compounds is stable in air up to 1300 °C. For the partly A-site ordered samples the magnetic and electronic phase separation is observed. The magnetotransport properties of the A-site ordered manganites treated under different conditions are discussed in terms of the superexchange interactions and A-site order degree.     

Phase separation, percolation and giant isotope effect in manganites

Phase separation and a tendency to form inhomogeneous structures seems to be a generic property of systems with strongly correlated electrons. After shortly summarising the existing theoretical results in this direction, I concentrate on the phenomena in doped manganites. I discuss general theoretical results on the phase separation at small doping and close to the doping x=0.5. The “global” phase diagram in this region is constructed. These general results are illustrated on the example of the particular system with rich and complicated properties — (LaPr)_1−xCa_xMnO₃, in which there exist a ferromagnetic metallic (FM) phase and a charge ordered (CO) insulating one. The experimental situation in this system is discussed and the interpretation is given in the framework of the model with competition of FM and CO, and the indications of phase separation and percolative nature of this system are given. The giant isotope effect observed in this situation is shortly discussed.     

Emergent ferroelectricity in disordered tri-color multilayer structure comprised of ferromagnetic manganites

Multiferroic materials,showing the coexistence and coupling of ferroelectric and magnetic orders,are of great technological and fundamental importance.However,the limitation of single phase multiferroics with robust magnetization and polarization hinders the magnetoelectric effect from being applied practically.Magnetic frustration,which can induce ferroelectricity,gives rise to multiferroic behavior.In this paper,we attempt to construct an artificial magnetically frustrated structure comprised of manganites to induce ferroelectricity.A disordered stacking of manganites is expected to result in frustration at interfaces.We report here that a tri-color multilayer structure comprised of non-ferroelectric La_(0.9)Ca_(0.1)MnO_3(A)/Pr_(0.85)Ca_(0.15)MnO_3(B)/Pr_(0.85)Sr_(0.15)MnO_3(C) layers with the disordered arrangement of ABC-ACBCAB-CBA-BAC-BCA is prepared to form magnetoelectric multiferroics.The multilayer film exhibits evidence of ferroelectricity at room temperature,thus presenting a candidate for multiferroics.     

Phase transition of a second kind in nickel as observed through optical reflection under pressure

The magnetic permeability of materials at optical frequencies is usually suggested in the literature to be $\mu =1$. In this case one cannot expect to measure the magnetic second order phase transition at optical frequencies. The main novel idea of this paper is that the magnetic permeability μ is not equal to 1 for optical frequency and a phase transition of magnetism was measured experimentally with an optical frequency. In particular, this work presents the detection of a magnetic second order phase transition in nickel with temperature and at different pressures, by reflectivity measurements at an optical frequency. Based on our experiments the magnetic permeability is calculated as a function of temperature for pressures of 0.3, 5 and 10 GPa attained in a diamond anvil cell (DAC).     

Electrical switching and memory phenomena ino-tolidine DDQ under pressure

Electrical switching has been observed ino-tolidine-DDQ at pressures of 7.66 GPa and fields ∼3×10⁵ V/m withσ _ON/σ _OFF≈10³ at a temperature of 300 K. The switching is found to be of the memory type and the sample can be driven back to the low conducting state by applying ac pulses of sufficient magnitude but independent of frequency.     

Magnetic inhomogeneity of lanthanum manganites single crystals

The magnetic inhomogeneity of the single crystals of La_1−xD_xMnO₃, where D=Ca, Sr, Ba, is studied. The Curie temperature distribution function is found. The form of this function is shown to depend on whether the mean value of the divalent element concentration is close to the “optimal” concentration (≈1/3) or not. The standard deviation for Curie temperature is evaluated. The correlation between the distribution coefficient for divalent elements and the standard deviation is established.     

High-pressure Raman and X-ray studies of barite,BaSO4

High-pressure Raman spectroscopic and X-ray diffraction experiments of barite, BaSO₄, were carried out in a diamond anvil cell up to 25?GPa at room temperature. On the basis of the changes in the diffraction patterns and the variation of lattice parameters with pressure, it is inferred that barite undergoes a phase transformation at 10?GPa. The phase transition accompanies the change in the force constant of vibrational modes in barite. Further compression beyond the phase transition causes the distortion of SO₄ tetrahedron as indicated by the splitting in the SO₄ stretching modes. Both X-ray and Raman data support that the phase transition in BaSO₄ is reversible. The compression data yield a bulk modulus of 63?±?2?GPa for barite. Barite shows anisotropic compressibility along three crystallographic axes with c being the most compressible axis.