Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B20: Focus Session: Phase Competition and Separation in Pervoskite Oxides |
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Sponsoring Units: GMAG DMP Chair: Chris Leighton, University of Minnesota Room: Baltimore Convention Center 317 |
Monday, March 13, 2006 11:15AM - 11:51AM |
B20.00001: Phase Competition and Magnetotransport Phenomena in Manganite Films and Mesoscopic Structures Invited Speaker: The importance of competition between ferromagnetic metallic (FMM) and charge-ordered insulating (COI) phases in the physics of bulk manganites has been established through a wide variety of techniques. One exotic consequence of this phase competition is step-like features in magnetotransport observed in bulk and single-crystals of Pr$_{0.65}$(Ca$_{y}$Sr$_{1-y})_{0.35}$MnO$_{3}$ (PCSMO) with 0.7$\le $y$\le $0.8. The length-scale of the phase coexistence is $\sim $1 micron, motivating a study of structures with dimensions similar to this natural length scale where phenomenology distinct from that of bulk counterparts is expected. Toward that end, we have synthesized films and laterally confined mesoscopic bridges of PCSMO and studied their magnetotransport properties. In particular, we observed: (1) \textit{Intrinsic} ultrasharp magnetization steps below 5 K in both bulk and film samples and their dependence on the \textit{extrinsic} measurement protocols; (2) Spontaneous jumps of resistance during both the ramping of magnetic field and the relaxation after the field cycle; (3) I-V curves exhibiting negative differential resistance (NDR) in certain ranges of temperature and magnetic field. All of these phenomena can be explained in the context of interconversion between the COI phase and the FMM phase. As expected, this interconversion can be triggered by external magnetic field, as found in the case of the magnetization and resistance steps. Alternatively, in the mesoscopic structures with dimensions similar to the size of the competing FMM and COI domains, a local Joule heating-induced annihilation of conducting filaments causes the anomalous NDR. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B20.00002: Mapping nanometer-sized phase separation in La1-xCaxMnO3 Jing Tao, M. Varela, S.J. Pennycook, D. Niebieskikwiat, M.B. Salamon, J.M. Zuo, W.D. Luo, S.T. Pantelides It is well known that three phases dominate over a wide doping range in La1-xCaxMnO3: the paramagnetic (PM) phase is favored for all x at high temperatures, while at low temperatures, the ferromagnetic (FM) phase is favored for x less than 0.5, and the charge ordered (CO) phase for x greater than 0.5. The transitions between these phases are not simple in this system and the mechanism is still not fully understood [1-2]. Here we report direct observation of the formation and melting of CO droplets in the PM-FM regime (x less than 0.5) using in-situ electron microscopy. The distributions of the CO droplets are directly mapped in real space and show maximum density near the PM-FM transition temperature. This behavior contrasts strongly with the behavior in the PM-CO (x greater than 0.5) regime, where the CO clusters are found to nucleate as nanophase domains and percolate on cooling through the transition temperature [3]. We attribute the nanoscale phase separation observed in this system to the competition between electron mobility and localization. Free energies of the competing phases will be estimated using density-functional calculations to understand the nature of the observed phase separation phenomena. [1]. G.C. Milward, M.J. Calderon and P.B. Littlewood, Nature 433, 607 (2004) [2]. J.M. Zuo and J. Tao, PRB 63, 060407 (2001) [3]. J. Tao and J.M. Zuo, PRB 69, 180404 (2004) [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B20.00003: Implications of a dimeron magnetic pair model for CMR manganites. Frank Bridges, Lisa Downward Recently, based on extensive EXAFS data, we have proposed a magnetic pair model for clusters that exist above the ferromagnetic transition temperature, Tc [1]. The magnetic clusters must contain equal numbers of hole and electron Mn sites and have a reduced distortion compared to the remaining Jahn-Teller distorted electron sites. The smallest such unit (one electron and one hole site) we call a dimeron - it is an electron delocalized over two Mn sites. Here we consider why such a model might be stable, possible configurations of the local distortions for the site pair, and the implications for filamentary clusters throughout the sample when it is only partially magnetized. We also address electrical transport via hopping dimerons and include the coulomb interactions between dimerons and the Ca(+2) dopants, which provides a quenched-in disorder. Several open questions such as the possible interactions between dimerons will also be discussed. \newline \newline [1] L. Downward et. al., Phys Rev Lett. 95, 106401 (2005). [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B20.00004: Microscopic Ferromagnetic and Antiferromagnetic Clusters in Pr${0.7}$Ca$_{0.3}$MnO$_{3}$ Hao Sha, Jiandi Zhang, Feng Ye, Jaime Fernandez-Baca, Pengcheng Dai, Y. Tomioka, Y. Tokura ``Colossal'' magnetorestive manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO30) is an ideal system to test the microscopic phase separation scenario because it has an inhomogeneous low-temperature insulating metastable state where ferromagnetic (FM), antiferromagnetic (AF), and charge/orbital (CO-OO) phases coexist. On cooling from room temperature, a CO-OO state occurs below T$_{CO-OO }\sim $200 K, followed by AF ordering below T$_{N }\sim $140 K. Below T$_{C }\sim $110 K, the magnetic structure develops a FM component coexisting with AF ordering. We have used neutron scattering to study FM, AF and CO-OO phase transitions in a single-crystal PCMO30. The diffuse scattering of FM component demonstrates the presence of short-range ferromagnetic clusters both above and below T$_{C}$, while no diffuse component in the CO-OO scattering peaks has been observed near T$_{CO-OO}$. Interestingly, the short-range AF correlations associated with Mn$^{4+}$ sites but not with Mn$^{3+}$ sites are observed for both above and below T$_{N}$, indicating that the local AFM clustering is directly associated with doped holes in this system. The work was supported by NSF-DMR0453804, NSF-DMR0346826, DE-FG02-05ER46202, and DOE DE-FG02-04ER46125. ORNL is managed by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-00OR22725. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B20.00005: Phase separation in magnetic spinels induced by Jahn-Teller distortions Sunmog Yeo, Chenglin Zhang, Yoichi Horibe, Sabya Guha, Sang-Wook Cheong We found a strong tendency of chemical/structural phase separation in spinels with Jahn-Teller ions, and this phase separation and the associated microstructure are sensitive on how specimens are cooled down from high temperatures. From our comprehensive study of magnetic and structural properties of magnetic spinels with Jahn-Teller ions with various cooling rates, we have found close relationship between bulk magnetic properties and microstructure. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B20.00006: Photoinduced phase-coexistence in Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin films. Vera Smolyaninova, R. Kennedy, E. Talanova, L. Aldaco, R.M. Kolagani, M. Overby Doped manganites exhibit a wide variety of physical phenomena due to complex interplay of electronic, magnetic, orbital, and structural degrees of freedom. One of the most intriguing properties of manganites is coexistence of several distinct electronic phases. A photoinduced insulator to conductor transition in charge-ordered (CO) manganites is especially interesting from the point of view of creating photonic devices [1]. We have observed a photoinduced sub-micron phase coexistence of CO insulating phase and conducting phase via optical contrast in Near-field Scanning Optical Microscope images. Such phase coexistence is possible because of the presence of two local energy minima corresponding to CO insulating and charge-disordered conducting phases in the energy landscape [2]. To better understand the physics of phase coexistence in manganites we studied the dynamics of photoinduced conductivity changes. The temperature dependence of this process will be presented. The energy barrier separating the CO insulating and conducting states will be discussed. This work is supported by NSF grants DMR-0348939, DMR-04221141. [1] V. N. Smolyaninova at al., APL 86, 071922 (2005). [2] K. H. Ahn at el., Nature 428, 401 (2004). [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B20.00007: Electron-correlation induced phase separation in Li$_x$FePO$_4$ Gerbrand Ceder, Fei Zhou Li$_x$FePO$_4$ is a promising rechargeable Li-ion battery cathode material. The Li$_x$FePO$_4$ system is known experimentally to phase separate into FePO$_4$ end LiFePO$_4$ up to $\sim$ 400 K, and form solid solutions above $\sim$ 600 K. Phase separation in this system is surprising as one would expect Li$^+$ ions to repel each other and form ordered compounds at compositions between those of the end members FePO$_4$ and LiFePO$_4$. This is exactly what is found in the local density or generalized gradient approximations to DFT, though it is in stark disagreement with experiments. The fact that the LDA+U method corrects this qualitative error points at the role of electron correlation in inducing phase separation in this material. We have thoroughly studied the LixFeO4 phase diagram with LDA+U. We found that the charge ordering of Fe$^{2+}$/Fe$^{3+}$ on the iron sub-lattice, induced by $d$ electron localization, couples delicately with Li/vacancy ordering on the lithium sub-lattice. Although the repulsion within each sub-lattice favors compound formation, the Li-e$^-$ attraction favors phase separation. It is the balance of interactions within and between the two sub-lattices that gives rise to the unexpected phase behavior of Li$_x$FePO$_4$. Possible relevance of this novel phase transition mechanism to other alkali metal-intercalated materials will be discussed. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B20.00008: The electric field effect on the electronic soft matter in the manganite (L$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3 }(y$=0.4, 0.5 and 0.6) Tara Dhakal, Jacob Tosado, SungHee Yun, Amlan Biswas Hole-doped manganites are known for their colossal magnetoresistive (CMR) property and insulator to metal phase transition near their curie temperatures. For certain compositions of manganites a coexistence of a charge ordered insulating (COI) and ferromagnetic metallic (FMM) phases has been observed. Changing the temperature, magnetic field or strain modifies the spatial arrangement and relative proportions of these two phases, in this mixed phase state. This property suggests the existence of electronic soft matter in manganites. We have studied the properties of the electronic soft matter in one such material (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3 }$(LPCMO, $y$=0.4, 0.5 and 0.6). We have grown thin films of LPCMO using pulsed laser deposition. To understand the dynamics of the local phase, we have experimentally mapped the temperature-magnetic field ($T-H)$ phase diagram of this material. The phase diagram shows 4 distinct regions, namely the COI state, the fluid phase separated state (FPS), the static phase separated state (SPS), and the FMM state. We have measured the effect of an electric field on this phase diagram and observed that the electric field can significantly modify the FPS state. We will explain the reason behind the electronic soft matter like behavior in the FPS state and the origin of the electric field effect in manganites. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B20.00009: High Temperature EXAFS Studies of La$_{1-x}$Ca$_x$MnO$_3$: New Evidence for Magnetic Dimers L. Downward, F. Bridges, T.A. O'Brien, J.J. Neumeier We present Extended X-ray Absorption Fine Structure (EXAFS) data on the CMR perovskite manganites La$_{1-x}$Ca$_x$MnO$_3$ ($0.2 < x < 0.5$) to high-temperature (300-600 K) We have recently reported[1] changes in the Mn-O distortion (measured as a broadening, $\sigma$ of the Mn-O pair distribution function) as a function of magnetic field, temperature and Ca-concentration, $x$. Extensive data indicate that there is a universal relationship between changes in the local distortion ($\Delta \sigma^2$) and the sample magnetization. These data suggest that small magnetic clusters containing equal numbers of electron and hole sites must exist just above T$_c$. These clusters have little Jahn-Teller distortion and must form at some higher temperature; the smallest such unit would be an electron/hole site pair, which we call a dimeron. The new high temperature data presented here shows another significant step in the Mn-O distortion near 450 K, which we associate with the dimeron formation temperature, T*. This second step occurs at the same temperature for which other authors have observed a break in the inverse magnetic susceptibility. [1] L. Downward \emph{et al.}, Phys Rev Lett {\bf 95}, 106401 (2005). [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B20.00010: Origin of the non-thermal photoresponse in thin films of two-phase manganites Anthony Davidson III, Mason Overby, Rajeh Mundle, Grace Yong, David Cox, Elena Talanova, Vera Smolyaninova, David Schaefer, Rajeswari M. Kolagani Our studies of light-induced resistance changes (photoresponse) in thin films the colossal magnetoresistive manganite material (La,Pr)$_{0.67}$Ca$_{0.33}$MnO$_{3 }$experiments have revealed a non-thermal component of the light-induced resistance change .This non-thermal component is also observed in thin films of oxygen deficient La$_{0.67}$Ca$_{0.33}$MnO$_{3. }$The common feature of both these material systems is the co-existence of metallic and insulating phases. Our results indicate that this component may be associated with the light-induced resistance decrease in the insulating regions through an electronic mechanism. Previous studies have shown insulator-metal transitions induced by magnetic fields as well as electric fields in these materials. We will present our studies of the correlation of the observed non-thermal photoresponse with magnetoresistance as well as current-voltage characteristics. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B20.00011: Charge Separation and Pre-percolation Regime in Manganites Zsolt Marton, Takeshi Egami, Rinat Mamin Complex manganites have been intensively studied in the last decade since they are model objects for investigations of the collosal magnetoresistance. Temperature dependence of La$_{0.875}$Sr$_{0.125}$MnO$_{3}$ single crystals' dielectric and magnetic properties are investigated in the wide range of temperature and external magnetic field. In order to clarify the nature of the charge, spin and orbital ordering as well as the nanoscale inhomogeneity dynamics, we studied their low-frequency capacitances. It is found that giant dielectric permittivity arises in this manganite. The origin of this enormous dielectric response is believed to be connected with the fact that in the pre-percolation regime of charge separation there are very thin regions of insulator phase with large net surface embedded in metallic matrix. The features of the temperature and field dependence of these dielectric properties are explained by the nanoscale dynamic inhomogeneties appearing due to charge and spin ordering. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B20.00012: Magnetic Phase Diagram of (NaMn$_3$)Mn$_4$O$_{12}$ A. Prodi, F. Bolzoni, F. Licci, E. Gilioli, M. Marezio, M. Affronte, A. Gauzzi The double pervoskite (NaMn$_3$)Mn$_4$O$_{12}$ represents a model system to explore ordering phenomena usually sought after in half-doped manganites thanks to its unique crystal structure characterized by the absence of quenched disorder. We present here magnetization, transport and specific heat measurements performed under magnetic field aimed at characterizing the two distinct paramagnetic and two antiferromagnetic phases observed in (NaMn$_3$)Mn$_4$O$_{12}$ upon cooling. In particular, we investigated the change of paramagnetic regime associated with the structural phase transition at 176 K, the stability of the resulting charge ordered state as well as the peculiar magnetic response of the CE spin structure which orders below 125 K. Analysis of the corresponding specific heat anomalies confirms the nature of the transitions while at low temperatures specific heat data evidences a large Sommerfeld coefficient, unexpected for the insulating state, and reveals the presence of extra entropy with origin in nonmagnetic excitations. [Preview Abstract] |
Monday, March 13, 2006 2:03PM - 2:15PM |
B20.00013: Magnetic ordering of rare-earth compounds: first-principles studies Chun-gang Duan, R. F. Sabirianov, L Liu, W.N. Mei, P.A. Dowben, E.Y. Tsymbal We report a systematic theoretical study on the magnetic ordering in heavy rare-earth compounds with face-centered cubic structure. Based on first-principles total energy calculations of Gd monopnictides, we deduced the exchange interaction parameters of these systems from fitting the total energies of different magnetic configurations to those computed from the Heisenberg model. Then we demonstrated the formation of different magnetic structures in these compounds by using the Monte Carlo simulations. The so obtained Curie (N\'{e}el) temperatures agreed well with experiments. Detailed analysis on the trend of exchange parameters changing with anion sizes and distances between neighboring magnetic sites clearly demonstrate the co- existence of RKKY type metallic exchange interactions and anti- ferromagnetic superexchange interactions. We then propose a Bethe-Slater type curve which can qualitatively explain the behaviors of the exchange parameters in Gd monopnictides. [Preview Abstract] |
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