Session N43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides II

The Relationship between Local Structure Changes and Magnetization as a Function of Hole Concentration in Doped Perovskite Manganites

We present X-ray Absorption Fine Structure (XAFS) measurements on several perovskite manganite samples, La$_{1-x}$Ca$_{x}$MnO$_{3}$, in the CMR region (0.2 $\le \quad x \quad \ge $ 0.5) as a function of temperature and applied magnetic field. These results indicate that polaron-induced changes in the local structure depend only on magnetization for a given sample, irrespective of whether the sample magnetization, M, is achieved through a decrease in temperature or an applied magnetic field. Furthermore, the relationship between changes in local structure and magnetization is clearly a function of hole concentration, $x$, demonstrated by a change in slope at roughly $\textstyle{M \over {M_0 }}\approx 2x$ (M$_{0}$ is the saturation magnetization at low T). These results lead to a proposed model for the magnetization process, in which the magnetization initially develops via Mn pairs throughout the sample. This model is similar to the cluster and phase separated models proposed by others but the clusters are at the nanoscale. NSF DMR0301971.   

Complex interactions between structural, magnetic and electronic properties of epitaxial thin films of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

The bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ resides within the Ruddlesden-Popper (RP) family of materials and consists of interleaved blocks of two metallic/ferromagnetic (La,Sr)MnO$_{3}$ layers and one insulating (La,Sr)O layer. We have grown epitaxial thin films on (110)-oriented SrTiO$_{3}$ substrates by pulsed laser deposition with the $c$-axis aligned in the plane of the film. These in-plane aligned films are a model system for probing the anisotropic magnetic and electronic properties of this bilayer manganite. The films display similar properties to their bulk counterparts with the easy direction lying within the $a-b$ planes and coincident metal/insulator and ferromagnet/paramagnet transitions occurring at a suppressed $T_{c}\sim $90K (120K for bulk). While the magnetic properties are robust to the presence of defects, the electronic properties are highly sensitive to these defects, which include amorphous regions, antiphase boundaries and trace amounts of other RP phases.   

Anisotropic Local Distortion of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ Through the Ferromagnetic Transition Temperature

In previous temperature-dependent EXAFS studies of the quasi-cubic manganite, La$_{1-x}$Ca$_{x}$MnO$_{3}$, excess broadening of the Mn-O pair-distribution function has been observed near and above the Curie temperature, T$_{c}$=120, attributed to the appearance of Jahn-Teller polarons. A similar, yet anisotropic, broadening is expected in the bilayer system, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$. We report a temperature-dependent, polarized EXAFS study at the Mn-K edge for the x=0.4 sample. This analysis shows similar broadening for Mn-O pairs in both the ab-plane and the c-axis, but the magnitude of the distortion in the ab-plane is about 4 times greater than along the c-axis. The data also show a small excess of broadening below T$_{c }$ as well, indicating that polaron distortions are present even at 100K where the magnetization becomes constant. Analysis of further Mn neighbors is currently underway. Details of the analysis and a discussion of the implications for CMR in the bilayer manganites will be presented. Support: NSF DMR 0301971.   

Multi-scale Phase Competition and Coexistence in Strongly Correlated Materials

The intriguing interplay between charge/spin/lattice/orbital degrees of freedom gives rise to various competing phases as ground states in strongly correlated materials such as manganites. The delicate balance between these ground states leads to new metastable ground state which is characterized by the coexistence of phases with distinct electronic/magnetic ground state which is characterized by the coexistence of phases with electronic/magnetic properties such as the insulating charge ordered phase and the metallic ferromagnetic phase. The length scale of the phase separated system was found to vary from micrometer to nanometer in various manganites and under different situation. The CMR is closely related to the coexistence of the phase separated inhomogenieties. Lattice strain apparently plays a very significant role affecting the length scale of these phase separated systems. Nanoscale phase separation is often observed at temperatures above the characteristic temperature associated with formation of each particular phase. For example, we have observed the formation of nano-scale ferromagnetic droplets in a well defined temperature window preceding the long-range ferromagnetic ordering. The complexity of the electronic phase separation in different length scales will be discussed. This work is now doing in collaboration with C.H.Chen, S-W.Cheong, T.Asaka, Y.Horibe, Y.Matsui, T.Katsufuji, M.Uehara, Y.Moritomo, H.Kuwahara and their team members.   

Epitaxial strain and phase separation in La$_{0.7}$Ca$_{0.3}$MnO$_3$ manganite

Epitaxial strain has been explored as a method of tailoring structural distortions to examine their influence on physical properties of thin films of colossal magnetoresistance manganites. However depressed saturation magnetizations have been found in nanometer thick samples both for tensile and compressive in plane strains, and a clear picture of the effect of strain on the phase separation picture has not emerged yet. In this paper we present results of ultrathin La$_{0.67}$Ca$_{0.33}$MnO$_{3 }$films grown on SrLaAlO$_{4}$ (a=0.375 nm) under high in plane compressive strain (-3.1 {\%}). We show 2D dimensional epitaxial growth below the critical thickness what allows exploring the effect of lattice distortions on the PS in highly ordered films with thickness in the 2 - 6.5 nm range. Aberration corrected scanning transmission electron microscopy (STEM) with atomic resolution combined with energy loss spectroscopy are used to show evidence for strain induced nanoscale phase separation.   

Current images and anomalous transport properties in phase-separated manganites

We have observed inhomogeneous current distributions in the phase-separated manganite crystals by using magneto-optical imaging technique. Increase of the current causes a change of conduction paths from inhomogeneous to homogeneous concomitantly with sharp increase in resistivity. Using this anomalous current-voltage relation, we demonstrate the possibility of low- field colossal magnetoresistance effects. In addition, we found oscillations of the current at a constant voltage condition. These anomalous transport properties are reasonably explained by taking into account the effects of Joule heating.   

Charge, orbital and magnetic ordering in the bandwidth-control manganites $Pr_0.55(Ca_{1-y}Sr_{y})_0.45MnO_3$ ($y=0.15,0.20$)

$\rm Pr_{0.55}(Ca_{1-y}Sr_{y})_{0.45}MnO_3$ has been reported to display bicritical features near y=0.25. We have utilized neutron and synchrotron x-ray scattering techniques to study the interactions between FM, AFM and charge/orbital ordering (CO-OO) for samples with y=0.15 and y=0.20 at H=0 and in a magnetic field up to 7 Tesla. In the absence of magnetic field, both samples display insulating AFM ordered ground states. The CO-OO shows an incommensurate-commensurate transition as the temperature is lowered and the correlation lengths of the CO-OO remain much shorter than those for the AFM ordering at all temperatures. With the application of a magnetic field there is a sharp first-order like transition from the CO-OO insulating phase to a metallic ferromagnetic state. This work was supported by the US DOE under Contract No. DE-AC05- 00OR22725 with UT-Batelle, LLC and by U.S. NSF DMR-0139882.   

Strain-induced metal-insulator phase coexistence in perovskite manganites

The observed coexistence of distinct metallic and insulating electronic phases within the same sample of a perovskite manganite, such as La$_{1-x-y} $Pr$_y$Ca$_x$MnO$_3$, has been a puzzle to both theorists and experimentalists. In particular, colossal magnetoresistance in these materials is considered to be closely related to the texture owing to nanometer- and micrometer-scale heterogeneities. In this talk, we show that such texturing can be due to the intrinsic complexity of a system with strong coupling between the electronic and elastic degrees of freedom. More specifically, we demonstrate, using an atomic scale description of lattice distortions, that the presence of multiple local energy minimum states with different lattice distortions and different electronic properties, and the long-range interaction between strain fields provide a natural mechanism for such self-organized multiphase coexistence within the same material. This framework provides a basis for engineering nanoscale patterns of metallic and insulating phases, and for understanding other novel features observed in manganites, such as: precursor short range ordering and quasielastic scattering near the phase transition temperature; hysteretic and glassy dynamics; metastability; and photoinduced insulator-metal transition.   

Reentrant Charge Ordering In Manganites as Experimental Evidence for a ``Strain Glass"

A reentrant charge-ordering transition occurs within the micron scale phase separated manganite (La,Pr)$_{5/8}$Ca$_{3/8}$MnO$_{3}$. This low temperature state, in which charge-ordered and ferromagnetic-metallic phases coexist, accompanies spin glass-like magnetism. Furthermore, thermal conductivity measurements reveal an irreversibility characteristic of a freezing transition in the lattice degrees of freedom, strongly suggesting the presence of inhomogeneous long-range strain. Our results point to a unique phase transition from a ``strain liquid'' to a ``strain glass'' state where phase-separated regions strongly interact via martensitic accommodation strain resulting in a cooperative freezing of the combined charge/spin/strain degrees of freedom.   

Charge ordering structure and Mn valence in La0.33Ca0.67MnO3

Charge ordering (CO) and its structure is one of the long standing questions about manganites. Previously, X-ray, neutron and electron diffraction had been used to explore the crystallographic structure in charge ordered La1-xCaxMnO3 [2-3]. Two possible models, ``Wigner-crystal'' model and ``Bi-stripe'' model of the CO structure were proposed and supported by different evidences. Convergent beam electron diffraction (CBED) has unique capability on crystallographic symmetry determination and for probing crystal charge density [1]. The CBED patterns are sensitive to the charge states of atoms. The experimental CBED patterns show clearly the symmetry against the ``Bi-stripe'' model. The experimental CBED patterns are further compared with simulation using different charge valence models. The results show a surprisingly small difference between Mn(3+) and Mn(4+). The significance of this work will be discussed. Reference: [1] J. M. Zuo, Rep. Prog. Phys, 67, 2053 (2004) [2] P. G. Radaelli et al., Phys. Rev. B 55, 3015 (1997) [3] S. Mori, C. H. Chen, and S.-W. Cheong, Nature 392, 473 (1998)   

Energetics of reversible and non-reversible charge disordering effect in manganites induced by external beam irradiation

The reversible electron beam-induced melting and reentrant behavior of charge-ordered state in (Bi,Ca)MnO$_{3 }$phenomenon is in sharp contrast with the non-reversible effect in (Pr,Ca)MnO$_{3}$ observed under similar experimental conditions. In the Pr-manganites the charge disordering effect depends upon the total energy deposited on the sample by the incident beam, whereas in the Bi-manganites the reversible effect is a function of the power input from the incident beam. The energetics of the electron beam-induced effect in these two systems can be understood in the frame work of lifetime of the charge disordered clusters induced by the external irradiation. .