In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped $\mathrm{La_{0.5}Ca_{0.5}MnO_3}$(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15nm. It was observed that the size reduction leads to change in crystal structure and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples. The structural change mainly affects the orthorhombic distortion of the lattice. By making comparison with observed crystal structure data under hydrostatic pressure it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction. This not only changes the structure but also causes the room temperature structure to freeze-in. The size reduction also does not allow the long supercell modulation needed for the Charge Ordering, characteristic of this half-doped manganite, to set-in. The magnetic and transport measurements also show that the Charge Ordering (CO) does not occur when the size is reduced below a critical size. Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic type conductivity. Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals.
We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in Pr0.67Ca0.33MnO3(PCMO) with the help of x ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the positive up negative down method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron(ZP) ordered P21nm and disordered Pbnm structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroye…
The hole-doped manganite La0.7Ca0.3MnO3 and the electron-doped manganite La0.7Ce0.3MnO3 undergo an insulator to metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have successfully fabricated an epitaxial trilayer (La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3), where SrTiO3 is an insulator. At room temperature, i.e. in the semiconducting regime, it exhibits asymmetric current-voltage (I-V) characteristics akin to a p-n diode. The observed asymmetry in the I-V characteristics disappears at low temperatures where both the manganite layers are metallic. To the best…
The low-temperature spin dynamics of doped manganites have been analyzed within a tight-binding model, the parameters of which are estimated by mapping the results of ab initio density functional calculations onto the model. This approach is found to provide a good description of the spin dynamics of the doped manganites, observed earlier within the ab initio calculations. Our analysis not only provides some insight into the roles of the eg and the t2g states but also indicates that the oxygen p states play an important role in the spin dynamics. This may cast doubt on the adaptability of the …
The order parameters of the magnetic, charge and structural orders at half-doped manganites are identified. A corresponding Landau theory of the phase transitions is formulated. Many structural and thermodynamical behaviors are accounted for and clarified within the framework. In particular, the theory provides a unified picture for the scenario of the phase transitions and their nature with respect to the variation of the tolerance factor of the manganites. It also accounts for the origin of the incommensurate nature of the orbital order and its subsequently accompanying antiferromagnetic ord…
The electrical resistivity, Hall coefficient, and thermopower as functions of temperature are reported for lightly electron-doped Ca(1-x)La(x)MnO(3)(0 <= x <= 0.10). Unlike the case of hole-doped ferromagnetic manganites, the magnitude and temperature dependence of the Hall mobility for these compounds is found to be inconsistent with small-polaron theory. The transport data are better described by the Feynman polaron theory and imply intermediate coupling (alpha \~ 5.4) with a band effective mass, m*~4.3 m_0, and a polaron mass, m_p ~ 10 m_0.
The phase diagram of quarter-hole-doped perovskite manganites is investigated using the doubleexchange model. An exotic striped type-II multiferroic phase, where 25% of the nearest-neighbor spin couplings are orthogonal to each other, is found in the narrow-bandwidth region. Comparing with the spiral-spin ordering phase of undoped manganites, the multiferroic Curie temperature of the new phase is estimated to be ~4 times higher, while the ferroelectric polarization is similar in magnitude. Our study provides a path for noncollinear spin multiferroics based on electronic self-organization, diff…
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.
We propose that a specific spatial configuration of lattice sites that energetically favor {\it 3+} or {\it 4+} Mn ions in moderately doped manganites constitutes approximately a spatially random two-energy-level system. Such an effect results in a mechanism of metal-insulator transitions that appears to be different from both the Anderson transition and the Mott-Hubbard transition. Correspondingly, a disordered Kondo lattice model is put forward, whose dynamical mean-field solution agrees reasonably with experiments.
The possibility to induce ferromagnetism and insulator to metal transitions in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown for the first time. Colossal magnetoresistance (CMR) properties are evidenced for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to induce such properties is compared to the results obtained by chromium and ruthenium doping. Models are proposed to explain this behavior.
The phase diagram and low temperature properties of the doped manganites A1-xBxMnO3 are discussed for the concentrations x < 0.4. The transition from insulating antiferromagnetic to metallic ferromagnetic state at x_cr = 0.16 is treated by means of percolation theory. The unifying description of insulating and metallic states is presented. The undoped manganite is a band insulator consisting of ferromagnetic layers, which are coupled antiferromagnetically along the c direction with a low Neel temperature. The metallic phase can be described by the two-band Fermi liquid picture. The behavior of…
We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance (CMR) itself, the corresponding magnetooptical effect is explained by the dissociation of localized bipolarons into mobile polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the …
We show that specific heat C$_p$ has non-Debye behavior for glassy states in half-doped manganites. Irrespective of the magnetic order or electronic states, these magnetic glasses have higher C$_p$ compared to their equilibrium counterparts. The excess C$_p$ contributed by the glassy state varies linearly with temperature similar to conventional glasses indicating tunneling in the two-level systems. These glassy states show signature of zero point entropy. Magnetic glasses can be produced simply by different field cooling protocols and may be considered ideal magnetic counterpart of the conven…
We employ time-resolved resonant x-ray diffraction to study the melting of charge order and the associated insulator-metal transition in the doped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ after resonant excitation of a high-frequency infrared-active lattice mode. We find that the charge order reduces promptly and highly nonlinearly as function of excitation fluence. Density functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drive these dynamics, highlighting a new avenue of nonlinear phonon control.
A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped charge-transfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pairs formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse d…
The problem of adequate electronic model for doped manganites like La_{1-x}Sr_{x}MnO_3 remains controversial. In such a situation we argue a necessity to discuss all possible candidate states with different valent structure of manganese and oxygen atoms, as well as different valent states of octahedral MnO_{6} centers. In frames of quantum-chemical approach, crystal field and the ligand field models we address different types of MnO_6 centers, different types of d-d, and charge transfer transitions. We draw special attention to the so-called charge transfer states related to strong intra-cente…
Ultra-thin films of the electron doped manganite La$_{0.8}$Ce$_{0.2}$MnO$_3$ were grown in a layer-by-layer growth mode on SrTiO$_3$ (001) substrates by pulsed laser interval deposition. High structural quality and surface morphology was confirmed by a combination of synchrotron based x-ray diffraction and atomic force microscopy. Resonant X-ray absorption spectroscopy measurements confirm the presence of Ce$^{+4}$ and Mn$^{+2}$ ions. In addition, the electron doping signature was corroborated by Hall effect measurements. All grown films show ferromagnetic ground state as revealed by both XMCD…
We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction is much stronger than any relevant magnetic interaction. A polaron shift due to the Fröhlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. An opposite conclusion, recently suggested in the literature, is shown to be incorrect. The frequency and temperature dependence of the optical co…
The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory. As these materials are cooled below the Curie temperature, the colossal magnetoressitance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by exchange interaction with the localized Mn spins during the transition to the low temperature ferromagne…
In a manganite film without quenched disorder, we show texturing in the form of insulating and metallic stripes above and below Curie temperature (Tc), respectively, by high resolution scanning tunneling microscopy/spectroscopy (STM/STS). The formation of these stripes involves competing orbital and charge orders, and are an outcome of overlapping electron wave-functions mediated by long-range lattice strain. Contrary to popular perception, electronically homogeneous stripe phase underlines the efficacy of the lattice strain in bringing about charge density modulation and in impeding the cross…
The effects of dc excitation current on the current-voltage curves of polycrystalline samples of Nd0.5Ca0.5Mn0.96Cr0.04O3 were investigated. The experimental results show that an abrupt jump of the voltage is concomitant with a huge increase in the temperature of the sample. A simple model and estimates for Joule self-heating effects support the experimental data. Moreover, the data strongly suggest that both the current localization in the metallic paths and local Joule self-heating effects are essential ingredients to understand the current-induced phase transition in phase-separated mangani…
We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular we study RMnO3 (R= La, Pr, Nd, Tb, Ho, Er, Y and Ca) and the different phases of Ca or Sr doped RMnO3 compounds as well as cation deficient RMnO3. The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO6 octahedra and independently of the average structure obtained by diffraction techniques. The main peaks of compounds with regular MnO6 octahedra, as CaMnO3, highly Ca …
Magnetic field and temperature dependence of the Terahertz conductivity and permittivity of the colossal magnetoresistance manganite Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the metal-to-insulator transition (MIT) from the insulating side. In the charge-ordered state of PCSMO both conductivity and dielectric permittivity increase as function of magnetic field and temperature. Universal scaling relationships between the changes in permittivity and conductivity are observed in a broad range of temperatures and magnetic fields. Similar scaling is also seen in La_{1-x}S…
We report measurements of in-plane $ρ_{ab}$ and out-of-plane $ρ_{c}$ resistivities on a single crystal of the half-doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$. In the temperature $T$ range 220 to 300 K, the resistive anisotropy $ρ_{c}/ρ_{ab}=A+B/T$ ($A$ and $B$ constants), which provides evidence for the variable-range-hopping conduction in the presence of a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic field $H$ and $\sin^{2}ϕ$ dependences of the negative magnetoresistivity $\ln [ρ_{i}(T,H,ϕ)/ρ_{i}(T,H=0)]$ ($i=ab,c$), where $ϕ$ is the in-plane angle betwe…
The complicated electronic, magnetic, and colossal magnetoresistant (CMR) properties of Sr and Ca doped lanthanum manganites can be understood by spin-polarized first-principles calculations. The electronic properties can be attributed to a detailed balancing between Sr and Ca induced metal-like O 2p and majority-spin (majority-spin) Mn eg delocalized states and the insulator-like minority-spin (minority-spin) Mn t2g band near the Fermi level (EF). The magnetic properties can be attributed to a detailed balancing between O mediated antiferromagnetic superexchange and delocalized majority-spin …
An effective Hamiltonian is derived in the case of the strong Hund coupling and on-site Coulomb interaction by means of a projective perturbation approach. A physical mechanism for charge ordering in half-doped manganites (R_{0.5}X_{0.5}MnO_3) is proposed. The virtual process of electron hopping results in antiferromagnetic superexchange and a repulsive interaction, which may drive electrons to form a Wigner lattice. The phase diagram of the ground state of the model is presented at half doping. In the case of formation of Wigner lattice, we prove that spins of electrons are aligned ferromagne…
A giant low-frequency, in-plane dielectric constant, eps~10^6, for epitaxial thin films of Ca(1-x)La(x)MnO(3) (x<=0.03) was observed over a broad temperature range, 4K <= T 300K. This phenomenon is attributed to an internal barrier-layer capacitor (IBLC) structure, with insulating boundaries between semiconducting grains. The room-temperature eps increases substantially with electron (La) doping. The measured values of eps exceed those of conventional two-phase IBLC materials based on (Ba,Sr)TiO(3) as well as recently discovered CaCu(3)Ti(4)O(12) and (Li,Ti) doped NiO.
A unified theory is developed to explain various types of electronic collective behaviors in doped manganites R$_{1-x}$X$_x$MnO$_3$ (R = La, Pr,Nd etc. and X = Ca, Sr, Ba etc.). Starting from a realistic electronic model, we derive an effective Hamiltonianis by ultilizing the projection perturbation techniques and develop a spin-charge-orbital coherent state theory, in which the Jahn-Teller effect and the orbital degeneracy of e$_g$ electrons in Mn ions are taken into account. Physically, the experimentally observed charge ordering state and electronic phase separation are two macroscopic quan…
In their Letter (Phys. Rev. Lett. 83, 5118 (1999)), den Brink, Khaliullin, and Khomskii proposed theoretically that the one-dimensional ferromagnetic zigzag chains in CE phase in half-doped manganites play an essential role in forming the orbital ordering, and, more surprisingly, the on-site Coulomb interaction U between electrons with different orbitals leads to experimentally observed charge ordering. In this Comment, I point out that the strong U will destroy the stability of CE-type phase, which is stable in a very narrow regime in the parameter space for electronic model.To solve this iss…
Some recent experimental studies show the invisibility of antiferromagnetic transition in the cases of manganites when their particle size is reduced to nanometer scale. In complete contrast to these cases, we have observed the signature of antiferromagnetic transition in the magnetocaloric properties of nanocrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$ of average particle size 70 and 60 nm similar to its polycrystalline bulk form. The system exhibit inverse magnetocaloric effect in its polycrystalline and nanocrystalline form. An extra ferromagnetic phase is stabilized at low temperature for …
Theoretical and computational results are presented clarifying the role of long-ranged strain interactions in determining the charge and orbital ordering in colossal magnetoresistance manganites. The strain energy contribution is found to be of order 20-30 meV/Mn and in particular stabilizes the anomalous 'zig-zag chain' order observed in many half-doped manganites.
We report precise measurements and quantitative data analysis on the low-temperature resistivity of several ferromagnetic manganite films. We clearly show that there exists a T^{4.5} term in low-temperature resistivity, and that this term is in quantitative agreement with the quantum theory of two-magnon scattering for half metallic ferromagnets. Our present results provide the first bulk experimental evidence of half-metallic ferromagnetism in doped manganites.
Deterministic oscillations of current-induced metastable resistivity in changing voltage have been detected in La$_{0.82}$Ca$_{0.18}$MnO$_3$ single crystals. At low temperatures, below the Curie point, application of specific bias procedures switches the crystal into metastable resistivity state characterized by appearance of pronounced reproducible and random structures in the voltage dependence of the differential conductivity. In certain bias range equally spaced broad conductivity peaks have been observed. The oscillating conductivity has been tentatively ascribed to resonances in a quantu…
The effect of Cu-doping at Mn-site on structural, magnetic and transport properties in electron-doped manganites La0.85Te0.15Mn1-xCuxO3 has been investigated. Based on the analysis of structural parameter variations, the valence state of the Cu ion in Cu-doped manganites is suggested to be +2. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature decreases and the transition becomes broader with increasing Cu-doping level, in contrast, the magnetization magnitude of Cu-doping samples at low temperatures increase as x \leq 0.15. The insulator-metal (…
We deduce a model relevant for the anomalous metallic state of Sr doped manganites at low temperatures within the ferromagnetic phase. It provides a natural explanation to several anomalous features observed experimentally, such as the vanishing Drude contribution in optical conductivity, the pseudo-gap in the density of states, an the unusual dispersion observed in photoemission.
Bulk samples of La2.3-xYxCa0.7Mn2O7, x=0,0.3,0.5, with layered perovskite structure have been synthesized and investigated with respect to their electrical, electronic and magnetic properties. It is found that La1.8Y0.5Ca0.7Mn2O7 has tetragonal structure and is a metallic ferromagnet with a magnetic transition temperature of 170 K. The compound shows metallic behavior below 140 K and has a large magnetoresistance (MR) Delta-rho/rho(0)~94% at 100 K at 34 kOe. For x=0 and 0.3 the structure is monoclinic with a suppression of metallicity. For x=0 the material is an ferromagnetic insulator. We obs…
The magnetic nature of the B-site dopants controls the magnetic phases in B-site doped manganites RE$_{1-x}$AE$_{x}$Mn$_{1-η}$B$_η$O$_3$. Different B-site dopants of equal valence, doped into the same reference manganite, lead to different magnetic phases at low temperature, which can not be explained using the valence change scenario. We focus on trivalent B-site dopants in CE-CO-OO-I manganites at half-filling $x=0.50$ to study the role of magnetic interactions between the B-site dopants and the neighboring Mn-sites by using a two-orbital double-exchange model including super-exchange intera…
We show that metal-insulator transition in Pr0.5Ca0.5Mn1-xCrxO3 (x = 0.015-0.025) is unstable against thermal cycling. Insulator-metal transition shifts down and low temperature resistivity increases each time when the sample is cycled between a starting temperature TS and a final temperature TF. The effect is dramatic lower is x. Insulator-metal transition in x = 0.015 can be completely destroyed by thermal cycling in absence of magnetic field as well as under H = 2 T. Magnetic measurements suggest that ferromagnetic phase fraction decreases with thermal cycling. We suggest that increase in s…
We performed a systematic analysis of the tunneling magnetoresistance (TMR) effect in single grain boundary junctions formed in epitaxial La(2/3)Ca(1/3)MnO(3) films deposited on SrTiO(3) bicrystals. For magnetic fields H applied parallel to the grain boundary barrier, an ideal two-level resistance switching behavior with sharp transitions is observed with a TMR effect of up to 300% at 4.2 K and still above 100% at 77 K. Varying the angle between H and the grain boundary results in differently shaped resistance vs H curves. The observed behavior is explained within a model of magnetic domain pi…
We introduce a general model for understanding the effect of quenched disorder on charge ordering in half-doped Manganese- and Cobalt-Oxides with different crystal structures. Current experimental results (Table-1) are discussed in light of the global phase diagram of the model (Fig. 2).
We report on the influence of the lattice degrees of freedom on charge, orbital and spin correlations in colossal magnetoresistance (CMR) manganites. For the weakly doped compounds we demonstrate that the electron-phonon coupling promotes the trapping of charge carriers, the disappearance of the orbital polaron pattern and the breakdown of ferromagnetism at the CMR transition. The role of different superexchange interactions is explored.
Low as well as high-temperature electron and x-ray diffraction studies have been carried out on a rare-earth free B-site disordered electron-doped manganite SrMn0.875.Mo0.125O3-δ in the temperature range of 83K to 637K. These studies reveal the occurrence of strong charge ordering (CO) at room temperature in a pseudo tetragonally distorted perovskite phase with space-group Pmmm. Non integral modulation vector of 8.95 times along [-110] indicates a charge density wave type modulation. The CO phase with basic perovskite structure Pmmm transforms to a charge disorder cubic phase through a first o…
The exchange interaction of polaronic carriers with localized spins leads to a ferromagnetic/paramagnetic transition in doped charge-transfer insulators with strong electron-phonon coupling. The relative strength of the exchange and electron-phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of local bound pair (bipolaron) formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. Below the critical temperature of the transition, $T_c$, the b…
Doped and undoped manganites are modeled by the coupling between itinerant $e_g$ electrons and static $t_{2g}$ spins, the Jahn-Teller and breathing phonon modes, and the Coulomb interaction. We provide for a careful estimate of all parameters and solve the corresponding Hamiltonian by dynamical mean field theory. Our results for the one-electron spectrum, the optical conductivity, the dynamic and static lattice distortion, as well as the Curie temperature show the importance of all of the above ingredients for a realistic calculation as well as for describing the unusual dynamical properties o…
We have demonstrated the effect of hydrostatic pressure on magnetic and transport properties, and thermal transport properties in electron-doped manganites CaMn$_{1-x}$Sb$_{x}$O$_{3}$. The substitution of Sb$^{5+}$ ion for Mn $^{4+}$site of the parent matrix causes one-electron doping with the chemical formula CaMn$^{4+}_{1-2x}$Mn$^{3+}_{x}$Sb$^{5+}_{x}$O$_{3}$ accompanied by a monotonous increase in unit cell volume as a function of $x$. Upon increasing the doping level of Sb, the magnitudes of both electrical resistivity and negative Seebeck coefficient are suppressed at high temperatures, i…
Novel ground-state spin structures in undoped and lightly-doped manganites are here investigated based on the orbital-degenerate double-exchange model, by using mean-field and numerical techniques. In undoped manganites, a new antiferromagnetic (AFM) state, called the E-type phase, is found adjacent in parameter space to the A-type AFM phase. Its structure is in agreement with recent experimental results. This insulating E-AFM state is also competing with a ferromagnetic metallic phase as well, suggesting that large magneto-resistant effects could exist even in undoped Mn oxides. For doped lay…
We have verified that the variational mean field theory approach suggested by Narimanov and Varma (preprint cond-mat/0002191) being applied to the realistic two-band model provides a good agreement with experimental data for the Curie temperature in doped manganites A$_{1-x}$B$_x$MnO$_3$ ($x\simeq{0.3}$). We have also considered the problem of an interplay between the ferromagnetic and antiferromagnetic interactions by using the same approach.
The observation of unusually large ferromagnetism in the nanoparticles of doped oxides and enhanced ferromagnetic tendencies in manganite nanoparticles have been in focus recently. For the transition metal-doped oxide nanoparticles a phenomenological `charge transfer ferromagnetism' model is recently proposed by Coey et al. From a microscopic calculation with charge transfer between the defect band and mixed valent dopants, acting as reservoir, we show how the unusually high ferromagnetic response develops. The puzzle of nanosize-induced ferromagnetic tendencies in manganites is also addressed…
The coupling of multiple degrees of freedom - charge, spin, and lattice - in manganites has mostly been considered at the microscopic level. However, on larger length scales, these correlations may be affected by strain and disorder, which can lead to short range order in these phases and affect the coupling between them. To better understand these effects, we explore the dynamics of orbitally ordered domains in a half-doped manganite near the orbital ordering phase transition. Our results suggest that the domains are largely static, and exhibit only slow fluctuations near domain boundaries.
The role of lattice distortion in the charge, orbital and spin ordering in half doped manganites has been investigated. For fixed magnetic ordering, we show that the cooperative lattice distortion stabilize the experimentally observed ordering even when the strong on-site electronic correlation is taken into account. Furthermore, without invoking the magnetic interactions, the cooperative lattice distortion alone may lead to the correct charge and orbital ordering including the charge stacking effect, and the magnetic ordering can be the consequence of such a charge and orbital ordering. We pr…
We investigate the possibility of controlling the magnetic phase transition of the heterointerface between a half-doped manganite La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and a multiferroic BiFeO$_{3}$ through magnetoelectric coupling. Using macroscopic magnetometry and element-selective x-ray magnetic circular dichroism at the Mn and Fe \emph{L}-edges, we discover that the ferroelectric polarization of BFO controls simultaneously the magnetization of BFO and LCMO. X-ray absorption spectra at oxygen \emph{K}-edge and linear dichroism at Mn \emph{L}-edge suggest that the interfacial coupling is mainly der…
Colossal magneto-resistance manganites are characterised by a complex interplay of charge, spin, orbital and lattice degrees of freedom. Formulating microscopic models for these compounds aims at meeting to conflicting objectives: sufficient simplification without excessive restrictions on the phase space. We give a detailed introduction to the electronic structure of manganites and derive a microscopic model for their low energy physics. Focussing on short range electron-lattice and spin-orbital correlations we supplement the modelling with numerical simulations.
We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction build of an electron doped manganite (La_0.7Ce_0.3MnO_3) and a hole doped manganite (La_0.7Ca_0.3MnO_3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T_C the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La_0.7Ce_0.3MnO_3 is a minority spin carrier ferromagnet with a high degree of spin pola…
Results of LSDA and LSDA+U calculations of the electronic structure and magnetic configurations of the 50% hole-doped pseudocubic perovskite La0.5Sr0.5MnO3 and double layered LaSr2Mn2O7 are presented. We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons has a very different influence on the (i) band formations, (ii) magnetic ground states, (iii) interlayer exchange interactions, and (iv) anisotropy of the electrical transport in these two manganites. A possible reason why the LSDA failures in predicting observed magnetic and transport properties of the double layered compo…
The magnetic phase diagram of the colossal magnetoresistance (CMR) manganites is determined based on the Hamiltonian incorporating the double-exchange (DE) interaction between degenerate Mn $e_g$ orbitals and the antiferromagnetic (AF) superexchange interaction between Mn $t_{2g}$ spins. We have employed the rigorous quantum mechanical formalism and obtained the finite temperature phase diagram which describes well the commonly observed features in CMR manganites. We have also shown that the CE-type AF structure cannot be stabilized at $x$=0.5 in this model.
We analyze effective Hamiltonian of ferromagnetic half metal phase of doped manganites and find a latent d-wave spin triplet superconductivity. Spin triplet state for a d-wave pair is enabled by orbital degrees of freedom. This high Tc superconductivity is, however, kept dormant by some intrinsic strong cooper pair breaking processes. Low T anomalies such as i) long distance superconducting proximity effects into manganites, ii) pseudogaps in tunneling, ARPES and iii) nodal quasi particles and absence of bilayer splitting in ARPES in bilayer manganite get natural and qualitative explanation. S…
The complex lanthanide doping of electron manganites results in enhancement of various phase separation effects in physical properties of these compounds. Selecting Ca0.8Sm0.16Nd0.04MnO3 as a model case we show that the first order structural phase transition from paramagnetic semi-metallic phase into anti-ferromagnetic semi-metallic phase at TS ~ 158 +- 4 K is marked by an abrupt decrease in magnetization, a step like anomaly DL/L = 10-4 in thermal expansion and large latent heat DQ = 610 J/mol. In a certain temperature range below TS, the high field magnetization exhibits hysteretic metamagn…
We present the effective lagrangian for low energy and momentum spin waves in canted phases at next to leading order in the derivative expansion. The symmetry breaking pattern SU(2) --> 1 of the internal spin group and that of the crystallographic space group imply that there is one ferromagnetic and one antiferromagnetic spin wave. The interaction of the spin waves with the charge carriers is also discussed for canted, ferromagnetic and antiferromagnetic phases. All this together allows us to write the doping dependence of the dispersion relation parameters for doped manganites. We point out …
The 'two-electron' doped rare earth mangnites Ca_1-x Ce_x MnO_3 (x = 0.1,0.2) are probed using resistivity, ac susceptibility and electron paramagnetic resonance (EPR) measurements across their respective charge ordering (CO) temperatures T_CO = 173 K and 250 K. The EPR 'g' factor and intensity as well as the transport and magnetic behaviours of the two compositions are qualitatively similar and are as expected for CO systems. However, the EPR linewidth, reflective of the spin dynamics, for x = 0.1, shows a strongly anomalous temperature dependence, decreasing with decreasing temperature below…
Computational studies show the generation of large coexisting metallic and insulating clusters with equal electronic density in models for manganites. The clusters are induced by disorder on exchange and hopping amplitudes near first-order transitions of the non-disordered strongly coupled system. The random-field Ising model is used to explain the qualitative aspects of our results. Percolative characteristics are natural in this context. Our results explain the recently experimentally discovered micrometer size inhomogeneities in manganites. The conclusions are general and apply to a variety…
We investigate magnetic excitations in half-doped colossal magnetoresistance manganites. In particular, we focus on spin excitations in the CE phase originally proposed by Goodenough (Phys. Rev. 100, 564 (1955)). Using a localized spin model we calculated magnons for 3D-perovskite compounds such as La$_{1-x}$M$_x$MnO$_3$, where M=Ca,Sr,Ba, and for their 2D-laminar counterparts. We compared them with predictions for the spin excitations corresponding to other phases proposed. For the laminar half-doped manganite La$_{0.5}$Sr$_{1.5}$MnO$_4$, for which magnon measurements by inelastic neutron sca…
