Magnetic-field-induced metal-insulator phenomena in Pr1-xCaxMnO3 with controlled charge-ordering instability

Effects of V and Gd doping on novel positive colossal electroresistance and quantum transport in PbPdO2 thin films with (002) preferred orientation

In this work, PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 thin films with (002) preferred orientation were prepared using a pulsed laser deposition technique. The temperature dependence of resistivities ρI(T) was investigated under various applied DC currents. Colossal electroresistance (CER) effects were found in PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2. It was found that the positive CER values of PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 reach 3816% and 154% for I = 1.00 μA at 10 K, respectively. In addition, the ρI(T) cycle curves of PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 thin films showed a critical temperature similar to that of PbPdO2 (Tc = 260 K). Particularly, charge transfer between O1- and O2- was confirmed by in situ XPS. Additionally, based on first-principles calculations and internal electric field models, the CER and magnetic sources in PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 can be well explained. Finally, it was found that thin film samples doped with V and G ions exhibit weak localization (WL) and weak anti-localization (WAL) quantum transport properties. Ion doping leads to a transition from WAL to WL. The study results indicate that PbPdO2, one of the few oxide topological insulators, can exhibit novel quantum transport behavior after ion doping.

Tailoring of magnetism & electron transport of manganate thin films by controlling the Mn-O-Mn bond angles via strain engineering

Strain engineering beyond substrate limitation of colossal magnetoresistant thin (La0.6Pr0.4)0.7Ca0.3MnO3 (LPCMO) films on LaAlO3-buffered SrTiO3 (LAO/STO) substrates has been demonstrated using metalorganic aerosol deposition technique. By growing partially relaxed 7-27 nm thick heteroepitaxial LAO buffer layers on STO a perfect lattice matching to the LPCMO has been achieved. As a result, strain-free heteroepitaxial 10-20 nm thick LPCMO/LAO/STO films with bulk-like ferromagnetic metallic ground state were obtained. Without buffer the coherently strained thin LPCMO/STO and LPCMO/LAO films were insulating and weakly magnetic. The reason for the optimized magnetotransport in strain-free LPCMO films was found to be a large octahedral Mn-O-Mn bond angle φOOR ~ 166-168° as compared to the significantly smaller one of φOOR ~ 152-156° determined for the tensile (LPCMO/STO) and compressively (LPCMO/LAO) strained films.

Interfacial interaction driven enhancement in the colossal magnetoresistance property of ultra-thin heterostructure of Pr0.6Sr0.4MnO3 in proximity with Pr0.5Ca0.5MnO3

The ultra-thin heterostructure of Pr_0.6Sr_0.4MnO₃(15 nm)/Pr_0.5Ca_0.5MnO₃(15 nm)/SrTiO₃ fabricated using pulsed laser deposition technique exhibits the phase-segregated nature wherein the ferromagnetism of Pr_0.6Sr_0.4MnO₃, and the antiferromagnetic state of Pr_0.5Ca_0.5MnO₃ coexist in proximity. The observation of two exciting phenomena in the grown ultra-thin heterostructure, namely, the kinetic arrest and training effect, confirms its phase-segregated nature. The melting of the antiferromagnetic state in Pr_0.5Ca_0.5MnO₃ into a ferromagnetic state due to the interfacial interaction arising from the magnetic proximity of the ferromagnetic clusters of Pr_0.6Sr_0.4MnO₃ have been observed. A metal-insulator transition (T_MIT) found at 215 K, close to its Curie temperature (T_Curie) observed at 230 K, reveals a strong correlation between the electrical transport and the magnetization of the ultra-thin heterostructure. The electrical conduction in the high-temperature regime is explained in terms of the adiabatic small polaron hopping model. While the resistance in the metallic regime for temperatures above 100 K is contributed by the inelastic scattering due to the two-magnons, in the metallic regime below 100 K, the one-magnon inelastic scattering contribution is prevalent. An enhanced colossal magnetoresistance property near room temperature is obtained in the ultra-thin heterostructure arising from the proximity-driven interfacial interaction, making it a suitable candidate for technological applications near room temperature.

Freestanding complex-oxide membranes

Complex oxides show a vast range of functional responses, unparalleled within the inorganic solids realm, making them promising materials for applications as varied as next-generation field-effect transistors, spintronic devices, electro-optic modulators, pyroelectric detectors, or oxygen reduction catalysts. Their stability in ambient conditions, chemical versatility, and large susceptibility to minute structural and electronic modifications make them ideal subjects of study to discover emergent phenomena and to generate novel functionalities for next-generation devices. Recent advances in the synthesis of single-crystal, freestanding complex oxide membranes provide an unprecedented opportunity to study these materials in a nearly-ideal system (e.g. free of mechanical/thermal interaction with substrates) as well as expanding the range of tools for tweaking their order parameters (i.e. (anti-)ferromagnetic, (anti-)ferroelectric, ferroelastic), and increasing the possibility of achieving novel heterointegration approaches (including interfacing dissimilar materials) by avoiding the chemical, structural, or thermal constraints in synthesis processes. Here, we review the recent developments in the fabrication and characterization of complex-oxide membranes and discuss their potential for unraveling novel physicochemical phenomena at the nanoscale and for further exploiting their functionalities in technologically relevant devices.

Magnetic and magnetocaloric effect study of a polycrystalline Gd0.5Sr0.5-xCaxMnO3 compound

The magnetic and magnetocaloric properties of polycrystalline Gd_0.5Sr_0.5-xCa_xMnO₃ (x = 0.0, 0.2, 0.3, 0.4 and 0.5) compounds have been investigated. Depending upon the Ca and Sr proportions, fascinating magnetic ground states were observed in the Gd_0.5Sr_0.5-xCa_xMnO₃ compounds. Here, the dominating nature of the canted magnetic state (for the Gd_0.5Ca_0.5MnO₃ compound) and glassy (disordered ferromagnetic) magnetic state (for the Gd_0.5Sr_0.5MnO₃ compound) are observed. However, for the intermediate doped samples (x = 0.2, 0.3, 0.4), a competing nature is found in their magnetic and exchange bias properties. Additionally, in the low temperature region, a significantly large magnetocaloric effect is observed for all the samples. At a 70 kOe external magnetic field, the highest observed value of the magnetocaloric entropy change is 21.58 J kg^-1 K^-1 (for the Gd_0.5Ca_0.5MnO₃ sample) and the lowest is 10.15 J kg^-1 K^-1 (for the Gd_0.5Sr_0.5MnO₃ sample).

Coexisting commensurate and incommensurate charge ordered phases in CoO

The subtle interplay of strong electronic correlations in a distorted crystal lattice often leads to the evolution of novel emergent functionalities in the strongly correlated materials (SCM). Here, we unravel such unprecedented commensurate (COM) and incommensurate (ICOM) charge ordered (CO) phases at room temperature in a simple transition-metal mono-oxide, namely CoO. The electron diffraction pattern unveils a COM ([Formula: see text]=[Formula: see text] and ICOM ([Formula: see text]) periodic lattice distortion. Transmission electron microscopy (TEM) captures unidirectional and bidirectional stripe patterns of charge density modulations. The widespread phase singularities in the phase-field of the order parameter (OP) affirms the abundant topological disorder. Using, density functional theory (DFT) calculations, we demystify the underlying electronic mechanism. The DFT study shows that a cation disordering ([Formula: see text]) stabilizes Jahn-Teller (JT) distortion and localized aliovalent [Formula: see text] states in CoO. Therefore, the lattice distortion accompanied with mixed valence states ([Formula: see text]) states introduces CO in CoO. Our findings offer an electronic paradigm to engineer CO to exploit the associated electronic functionalities in widely available transition-metal mono-oxides.

Theory for Magnetic-Field-Driven 3D Metal-Insulator Transitions in the Quantum Limit

Metal-insulator transitions driven by magnetic fields have been extensively studied in 2D, but a 3D theory is still lacking. Motivated by recent experiments, we develop a scaling theory for the metal-insulator transitions in the strong-magnetic-field quantum limit of a 3D system. By using a renormalization-group calculation to treat electron-electron interactions, electron-phonon interactions, and disorder on the same footing, we obtain the critical exponent that characterizes the scaling relations of the resistivity to temperature and magnetic field. By comparing the critical exponent with those in a recent experiment [F. Tang et al., Nature (London) 569, 537 (2019)NATUAS0028-083610.1038/s41586-019-1180-9], we conclude that the insulating ground state was not only a charge-density wave driven by electron-phonon interactions but also coexisting with strong electron-electron interactions and backscattering disorder. We also propose a current-scaling experiment for further verification. Our theory will be helpful for exploring the emergent territory of 3D metal-insulator transitions under strong magnetic fields.

Electric field control of magnetism through modulating phase separation in (011)-Nd0.5Sr0.5MnO3/PMN-PT heterostructures

Large and non-volatile electric field control of magnetization is promising to develop memory devices with reduced energy consumption. Herein, we report the electric field control of magnetization with a non-volatile memory effect in an intermediate band Nd0.5Sr0.5MnO3 film grown on a (011)-cut 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystal. Applying an electric field across the ferroelectric PMN-PT increases the magnetization of the Nd0.5Sr0.5MnO3 film along both in-plane [100] and [011[combining macron]] directions. Moreover, the magnetization does not recover to its original state after withdrawal of the electric field at temperatures below 70 K, demonstrating a non-volatile memory effect. Detailed investigation showed that (011)-PMN-PT exhibits an anisotropic in-plane strain due to an electric field-induced rhombohedral to orthorhombic phase transition. This electric field-induced anisotropic strain can dynamically transfer to Nd0.5Sr0.5MnO3 film and modulate the magnetization of the Nd0.5Sr0.5MnO3 film through adjusting its phase balance between ferromagnetic (FM) and charge-orbital ordered antiferromagnetic (COO AFM) phases. The non-volatile memory effect can be ascribed to the competition of thermal energy and energy barriers between the FM and COO AFM phases at low temperatures. This work broadens the knowledge of electric field control of magnetism in the intermediate band-manganite ferromagnetic/ferroelectric multiferroic heterostructures, and may also pave a way for the control of antiferromagnetism and to design antiferromagnet-based memories.

Magnetotransport studies of Fe vacancy-ordered Fe4+δSe5 nanowires

We studied the electrical transport of Fe4+δSe5 single-crystal nanowires exhibiting √5 × √5 Fe-vacancy order and mixed valence of Fe. Fe4+δSe5 compound has been identified as the parent phase of FeSe superconductor. A first-order metal-insulator (MI) transition of transition temperature T MI ∼ 28 K is observed at zero magnetic fields (B). Colossal positive magnetoresistance emerges, resulting from the magnetic field-dependent MI transition. T MI demonstrates anisotropic magnetic field dependence with the preferred orientation along the c axis. At temperature T < ∼17 K, the state of near-magnetic field-independent resistance, which is due to spin polarized even at zero fields, preserves under magnetic fields up to B = 9 T. The Arrhenius law shift of the transition on the source-drain frequency dependence reveals that it is a nonoxide compound with the Verwey-like electronic correlation. The observation of the magnetic field-independent magnetoresistance at low temperature suggests it is in a charge-ordered state below T ∼ 17 K. The results of the field orientation measurements indicate that the spin-orbital coupling is crucial in √5 × √5 Fe vacancy-ordered Fe4+δSe5 at low temperatures. Our findings provide valuable information to better understand the orbital nature and the interplay between the MI transition and superconductivity in FeSe-based materials.

Pressure-induced semiconductor-to-metal phase transition of a charge-ordered indium halide perovskite

Phase transitions in halide perovskites triggered by external stimuli generate significantly different material properties, providing a great opportunity for broad applications. Here, we demonstrate an In-based, charge-ordered (In⁺/In³⁺) inorganic halide perovskite with the composition of Cs₂In(I)In(III)Cl₆ in which a pressure-driven semiconductor-to-metal phase transition exists. The single crystals, synthesized via a solid-state reaction method, crystallize in a distorted perovskite structure with space group I4/m with a = 17.2604(12) Å, c = 11.0113(16) Å if both the strong reflections and superstructures are considered. The supercell was further confirmed by rotation electron diffraction measurement. The pressure-induced semiconductor-to-metal phase transition was demonstrated by high-pressure Raman and absorbance spectroscopies and was consistent with theoretical modeling. This type of charge-ordered inorganic halide perovskite with a pressure-induced semiconductor-to-metal phase transition may inspire a range of potential applications.

Room-Temperature Low-Field Colossal Magnetoresistance in Double-Perovskite Manganite

We have discovered room-temperature low-field colossal magnetoresistance (CMR) in an A-site ordered NdBaMn_{2}O_{6} crystal. The resistance changes more than 2 orders of magnitude at a magnetic field lower than 2 T near 300 K. When the temperature and magnetic field sweep from an insulating (metallic) phase to a metallic (insulating) phase, the insulating (metallic) conduction changes to the metallic (insulating) conduction within 1 K and 0.5 T, respectively. The CMR is ascribed to the melting of the charge and orbital ordering. The entropy change which is estimated from the B-T phase diagram is smaller than what is expected for the charge and orbital ordering. The suppression of the entropy change is attributable to the loss of the short-range ferromagnetic fluctuation of Mn spin moments, which is an important key of the high temperature and low magnetic field CMR effect.

Manifestation of charge/orbital order and charge transfer in temperature-dependent optical conductivity of single-layered Pr0.5Ca1.5MnO4

Half-doped single-layered manganite, [Formula: see text] has shown a charge/orbital order of the e _g electrons and CE-type spin order of the t _2g electrons of the Mn ions. A previous experimental study on that system, supported by a simple modelling, has suggested that the charge/orbital ordering play an important role in governing the temperature dependence of optical conductivity of a broad peak around 0.7-0.8 eV. In addition, another peak around 3.5 eV, which is less sensitive to temperature, has been attributed to the charge transfer from O-p  to Mn-e _g orbitals. Nevertheless, the theoretical explanation was incomplete as the role of O-p  orbitals was not considered in the model. In this paper, we propose to improve the model by incorporating both Mn-e _g and O-p  orbitals. We assume the existence of charge/orbital ordering and investigate how this ordering as well as the charge-transfer phenomenon control the temperature dependence of the optical conductivity. Our results reveal the charge/orbital-ordering peak in the region 0.7-1.2 eV, which is blue-shifted with decreasing temperature, and the charge-transfer peak around 3.5 eV, which is less sensitive to temperature. The capability of our model to capture the general profile and temperature dependence of the optical conductivity suggests the validity of our theory.

Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn2Bi2

In situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn₂Bi₂ undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. Puckered Mn-Mn honeycomb layer converts to quasi-one-dimensional (1D) zigzag chains above the phase transition pressure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic 2 orders of magnitude drop of resistivity. Although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as surprisingly, are two resistivity anomalies with opposite pressure dependences, while one of them could be a magnetic transition and the other originates from Fermi surface instability. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn₂Bi₂ indicates that ferrimagnetism is thermodynamically favored.

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr₂MnNiO₆ was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+  and 2+  states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni²⁺ and Mn⁴⁺ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U  -  J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p  -  d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.

Evolution of Magneto-Orbital order Upon B-Site Electron Doping in Na_{1-x}Ca_{x}Mn_{7}O_{12} Quadruple Perovskite Manganites

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na_{1-x}Ca_{x}Mn_{7}O_{12} quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic, and orbital ordering across a wide range of compounds.

Field-induced magnetic phase transitions and memory effect in bilayer ruthenate Ca3Ru2O7 with Fe substitution

Bilayer ruthenate Ca3(Ru1-x Fe x )2O7 (x  =  0.05) exhibits an incommensurate magnetic soliton lattice driven by the Dzyaloshinskii-Moriya interaction. Here we report complex field-induced magnetic phase transitions and memory effect in this system via single-crystal neutron diffraction and magnetotransport measurements. We observe first-order incommensurate-to-commensurate magnetic transitions upon applying the magnetic field both along and perpendicular to the propagation axis of the incommensurate spin structure. Furthermore, we find that the metastable states formed upon decreasing the magnetic field depend on the temperature and the applied field orientation. We suggest that the observed field-induced metastability may be ascribable to the quenched kinetics at low temperature.

Bending and breaking of stripes in a charge ordered manganite

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal-insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi_0.35Sr_0.18Ca_0.47MnO₃ using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.

Mechanisms of photoinduced magnetization in Pr0.6Ca0.4MnO3 studied above and below charge-ordering transition temperature

We report the effect of photonic field on the electronic and magnetic structure of a low bandwidth manganite [Formula: see text] [Formula: see text]MnO₃ (PCMO) thin film. In particular, the present study confirmed a mechanism that was recently proposed to explain how optical excitation can bias or directly activate the metamagnetic transition associated with the colossal magnetoresistance (CMR) effect of PCMO. The transition is characterized by a shift in the dynamic equilibrium between ferromagnetic (FM) and antiferromagnetic clusters, explaining how it can be suddenly triggered by a sufficient external magnetic field. The film was always found to support some population of FM-clusters, the proportional size of which could be adjusted by the magnetic field and, especially in the vicinity of a thermomagnetic irreversibility, by optical excitation. The double exchange mechanism couples the magnetic degrees of freedom of manganites to their electronic structure, which is further coupled to the ion lattice via the Jahn-Teller mechanism. In accordance, it was found that producing optical phonons into the lattice could lower the free energy of the FM phase enough to significantly bias the CMR effect.

Continuously Varying Critical Exponents Beyond Weak Universality

Renormalization group theory does not restrict the form of continuous variation of critical exponents which occurs in presence of a marginal operator. However, the continuous variation of critical exponents, observed in different contexts, usually follows a weak universality scenario where some of the exponents (e.g., β, γ, ν) vary keeping others (e.g., δ, η) fixed. Here we report ferromagnetic phase transition in (Sm_1−yNd_y)_0.52Sr_0.48MnO₃ (0.5 ≤ y ≤ 1) single crystals where all three exponents β, γ, δ vary with Nd concentration y. Such a variation clearly violates both universality and weak universality hypothesis. We propose a new scaling theory that explains the present experimental results, reduces to the weak universality as a special case, and provides a generic route leading to continuous variation of critical exponents and multi-criticality.

Non-equilibrium control of complex solids by nonlinear phononics

We review some recent advances in the use of optical fields at terahertz frequencies to drive the lattice of complex materials. We will focus on the control of low energy collective properties of solids, which emerge on average when a high frequency vibration is driven and a new crystal structure induced. We first discuss the fundamentals of these lattice rearrangements, based on how anharmonic mode coupling transforms an oscillatory motion into a quasi-static deformation of the crystal structure. We then discuss experiments, in which selectively changing a bond angle turns an insulator into a metal, accompanied by changes in charge, orbital and magnetic order. We then address the case of light induced non-equilibrium superconductivity, a mysterious phenomenon observed in some cuprates and molecular materials when certain lattice vibrations are driven. Finally, we show that the dynamics of electronic and magnetic phase transitions in complex-oxide heterostructures follow distinctly new physical pathways in case of the resonant excitation of a substrate vibrational mode.

Enhancement of Photoinduced Charge-Order Melting via Anisotropy Control by Double-Pulse Excitation in Perovskite Manganites: Pr_{0.6}Ca_{0.4}MnO_{3}

To control the efficiency of photoinduced charge-order melting in perovskite manganites, we performed femtosecond pump-probe spectroscopy using double-pulse excitation on Pr_{0.6}Ca_{0.4}MnO_{3}. The results revealed that the transfer of the spectral weight from the near-infrared to infrared region by the second pump pulse is considerably enhanced by the first pump pulse and that the suppression of crystal anisotropy, that is, the decrease of long-range lattice deformations due to the charge order by the first pump pulse is a key factor to enhance the charge-order melting. This double-pulse excitation method can be applied to various photoinduced transitions in complex materials with electronic and structural instabilities.

Anderson localization of electrons in single crystals: Li (x) Fe(7)Se(8)

Anderson (disorder-induced) localization, proposed more than half a century ago, has inspired numerous efforts to explore the absence of wave diffusions in disordered media. However, the proposed disorder-induced metal-insulator transition (MIT), associated with the nonpropagative electron waves, has hardly been observed in three-dimensional (3D) crystalline materials, let alone single crystals. We report the observation of an MIT in centimeter-size single crystals of Li x Fe7Se8 induced by lattice disorder. Both specific heat and infrared reflectance measurements reveal the presence of considerable electronic states in the vicinity of the Fermi level when the MIT occurs, suggesting that the transition is not due to Coulomb repulsion mechanism. The 3D variable range hopping regime evidenced by electrical transport measurements at low temperatures indicates the localized nature of the electronic states on the Fermi level. Quantitative analyses of carrier concentration, carrier mobility, and simulated density of states (DOS) fully support that Li x Fe7Se8 is an Anderson insulator. On the basis of these results, we provide a unified DOS picture to explain all the experimental results, and a schematic diagram for finding other potential Anderson insulators. This material will thus serve as a rich playground for both theoretical and experimental investigations on MITs and disorder-induced phenomena.

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

Static strain in complex oxide heterostructures has been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically. Here, by exciting large-amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.

Identification of Giant Mott Phase Transition of Single Electric Nanodomain in Manganite Nanowall Wire

In the scaling down of electronic devices, functional oxides with strongly correlated electron system provide advantages to conventional semiconductors, namely, huge switching owing to their phase transition and high carrier density, which guarantee their rich functionalities even at the 10 nm scale. However, understanding how their functionalities behave at a scale of 10 nm order is still a challenging issue. Here, we report the construction of the well-defined (La,Pr,Ca)MnO3 epitaxial oxide nanowall wire by combination of nanolithography and subsequent thin-film growth, which allows the direct investigation of its insulator-metal transition (IMT) at the single domain scale. We show that the width of a (La,Pr,Ca)MnO3 nanowall sample can be reduced to 50 nm, which is smaller than the observed 70-200 nm-size electronic domains, and that a single electronic nanodomain in (La,Pr,Ca)MnO3 exhibited an intrinsic first-order IMT with an unusually steep single-step change in its magnetoresistance and temperature-induced resistance due to the domains arrangement in series. A simple model of the first-order transition for single electric domains satisfactorily illustrates the IMT behavior from macroscale down to the nanoscale.

Structural, transport and optical properties of (La0.6Pr0.4)0.65Ca0.35MnO3 nanocrystals: a wide band-gap magnetic semiconductor

(La0.6Pr0.4)0.65Ca0.35MnO3 system has been synthesized via a sol-gel route at different sintering temperatures. Structural, transport and optical measurements have been carried out to investigate (La0.6Pr0.4)0.65Ca0.35MnO3 nanoparticles. Raman spectra show that Jahn-Teller distortion has been decreased due to the presence of Ca and Pr in A-site. Magnetic measurements provide a Curie temperature around 200 K and saturation magnetization (MS) of about 3.43μB/Mn at 5 K. X-ray photoemission spectroscopy study suggests that Mn exists in a dual oxidation state (Mn(3+) and Mn(4+)). Resistivity measurements suggest that charge-ordered states of Mn(3+) and Mn(4+), which might be influenced by the presence of Pr, have enhanced insulating behavior in (La0.6Pr0.4)0.65Ca0.35MnO3. Band gap estimated from UV-Vis spectroscopy measurements comes in the range of wide band gap semiconductors (∼3.5 eV); this makes (La0.6Pr0.4)0.65Ca0.35MnO3 a potential candidate for device application.

Mode-selective control of the crystal lattice

CONSPECTUS: Driving phase changes by selective optical excitation of specific vibrational modes in molecular and condensed phase systems has long been a grand goal for laser science. However, phase control has to date primarily been achieved by using coherent light fields generated by femtosecond pulsed lasers at near-infrared or visible wavelengths. This field is now being advanced by progress in generating intense femtosecond pulses in the mid-infrared, which can be tuned into resonance with infrared-active crystal lattice modes of a solid. Selective vibrational excitation is particularly interesting in complex oxides with strong electronic correlations, where even subtle modulations of the crystallographic structure can lead to colossal changes of the electronic and magnetic properties. In this Account, we summarize recent efforts to control the collective phase state in solids through mode-selective lattice excitation. The key aspect of the underlying physics is the nonlinear coupling of the resonantly driven phonon to other (Raman-active) modes due to lattice anharmonicities, theoretically discussed as ionic Raman scattering in the 1970s. Such nonlinear phononic excitation leads to rectification of a directly excited infrared-active mode and to a net displacement of the crystal along the coordinate of all anharmonically coupled modes. We present the theoretical basis and the experimental demonstration of this phenomenon, using femtosecond optical spectroscopy and ultrafast X-ray diffraction at a free electron laser. The observed nonlinear lattice dynamics is shown to drive electronic and magnetic phase transitions in many complex oxides, including insulator-metal transitions, charge/orbital order melting and magnetic switching in manganites. Furthermore, we show that the selective vibrational excitation can drive high-TC cuprates into a transient structure with enhanced superconductivity. The combination of nonlinear phononics with ultrafast crystallography at X-ray free electron lasers may provide new design rules for the development of materials that exhibit these exotic behaviors also at equilibrium.

Switching mechanism of Al/La1−xSrxMnO3 resistance random access memory. I. Oxygen vacancy formation in perovskites

The oxygen vacancy formation in half-metallic perovskite LSMO itself plays an interesting role in the resistive switching.

Band filling effect on polaron localization in La1-x(Ca(y)Sr1-y)xMnO3 manganites

We report on a ìSR and 55 Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La(5/8)(CaySr(1.y))(3/8)MnO3 and in the underdoped La1.xSrxMnO3 and La1.xCaxMnO3 metallic manganite families. The study is aimed at unravelling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping x on the Curie temperature TC and the order of the magnetic transition. At optimal doping, the transitions are second order at all y values, including the y = 1 (La(5/8)Ca(3/8)MnO3) end member. By contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including La(0.75)Sr(0.25)MnO3 whose TC is much higher than that of La(5/8)Ca(3/8)MnO3. The order parameter curves, on the other hand, exhibit a very minor dependence on x, if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or over-doping. A phase diagram, separating in the x.y plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the La(1.x)(CaySr(1.y))xMnO3 system.

Glass-like recovery of antiferromagnetic spin ordering in a photo-excited manganite Pr₀.₇Ca₀.₃MnO₃

Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward undertsanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr₀.₇Ca₀.₃MnO₃ following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.

Local bias induced ferroelectricity in manganites with competing charge and orbital order states

Perovskite-type manganites, such as Pr_1−xCa_xMnO₃, La_1−xCa_xMnO₃ and La_1−xSr_xMnO₃ solid solutions, are set forth as a case study of ferroelectricity formation mechanisms associated with the appearance of site- and bond-centered orbital ordering which breaks structural inversion symmetry.

The metal-insulator transition in nanocrystalline Pr0.67Ca0.33MnO3: the correlation between supercooling and kinetic arrest

The transition and hysteresis widths of a disorder broadened first order magnetic transition vary in H-T space which influences the co-existing phase fraction at low temperature arising due to kinetic arrest of the first order transition. We explored the role of change in the relative width of the supercooling/superheating band and kinetic arrest band for a ferromagnetic metallic to antiferromagnetic insulating transition. It is shown that for a correlated kinetic arrest and supercooling bands, the topology of the devitrification curves (or transformation across the (H(K),T(K)) band during warming) changes with the change in the relative width of these two bands. In addition to this, for a broader kinetic arrest band, the transformation temperature across the superheating band under constant H now depends on the arrested phase fraction. These predictions have been tested on nanocrystalline Pr(0.67)Ca(0.33)MnO(3), which is known to show a large variation in hysteresis width in H-T space. This is the first report where correlation between the kinetic arrest band and the supercooling band has been shown experimentally, in contrast to the universal observation of anticorrelation reported so far.

Imaging the first-order magnetic transition in La0.35Pr0.275Ca0.375MnO3

The nature of the ferromagnetic, charge, orbital, and antiferromagnetic order in La0.35Pr0.275Ca0.375MnO3 on the nano- and microscale was investigated by photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (RSXS). The structure of the ferromagnetic domains around the Curie temperature T(C) indicates that they nucleate under a high degree of lattice strain, which is brought about by the charge, orbital, and antiferromagnetic order. The combined temperature-dependent PEEM and RSXS measurements suggest that the lattice distortions associated with charge and orbital order are glassy in nature and that phase separation is driven by the interplay between it and the more itinerant charge carriers associated with ferromagnetic metallic order, even well below T(C).

Time-resolved near-edge x-ray absorption fine structure spectroscopy on photo-induced phase transitions using a tabletop soft-x-ray spectrometer

We present a table-top soft-x-ray spectrometer for the wavelength range λ = 1-5 nm based on a stable laser-driven x-ray source, making use of a gas-puff target. With this setup, optical light-pump/soft-x-ray probe near-edge x-ray absorption fine structure (NEXAFS) experiments with a temporal resolution of about 230 ps are feasible. Pump-probe NEXAFS measurements were carried out in the "water-window" region (2.28 nm-4.36 nm) on the manganite Pr(0.7)Ca(0.3)MnO(3), investigating diminutive changes of the oxygen K edge that derive from an optically induced phase transition. The results show the practicability of the table-top soft-x-ray spectrometer on demanding investigations so far exclusively conducted at synchrotron radiation sources.

The effect of band Jahn-Teller distortion on the magnetoresistivity of manganites: a model study

We present a model study of magnetoresistance through the interplay of magnetisation, structural distortion and external magnetic field for the manganite systems. The manganite system is described by the Hamiltonian which consists of the s-d type double exchange interaction, Heisenberg spin-spin interaction among the core electrons, and the static and dynamic band Jahn-Teller (JT) interaction in the e(g) band. The relaxation time of the e(g) electron is found from the imaginary part of the Green's function using the total Hamiltonian consisting of the interactions due to the electron and phonon. The calculated resistivity exhibits a peak in the pure JT distorted insulating phase separating the low temperature metallic ferromagnetic phase and the high temperature paramagnetic phase. The resistivity is suppressed with the increase of the external magnetic field. The e(g) electron band splitting and its effect on magnetoresistivity is reported here.

Ferromagnetic enhancement of CE-type spin ordering in (Pr,Ca)MnO3

We present resonant soft x-ray scattering results from small bandwidth manganites (Pr,Ca)MnO(3), which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic state and CE-type ordering.

Cation Size and Disorder Effects in Conducting Perovskite Oxides

The electronic and magnetic properties of ATO3 and related perovskites (A is a mixture of lanthanide Ln3+ and alkaline earth M2+ cations; T is a transition metal) are very sensitive to the A site composition. The importance of doping effects controlled by the Ln3+/M2+ ratio is well-known, but the other lattice effects controlled by the sizes of these cations are less well understood. A simple approach making use of the mean (first moment) and the variance (second moment) in the A cation distribution has been applied to the metal-insulator transition temperature in colossal magnetoresistance AMnO3 perovskites and to the critical temperature in A2CuO4 and LnBa2Cu3O7-δ superconductors. Series of compositions prepared with a constant doping level and mean A cation radius show a linear decrease of the transition temperature Tt with the A cation size variance σ2. The rate of decrease -dTt/dσ2 is found to lie in the range 1,000-30,000 KÅ-2. The orthorhombic-tetra onal structural transition in the A2CuO4 materials is found to show a linear increase with σ2. A pair of quadratic relationships for the mean size and size variance effects are proposed to be the result of changing strain energies that give rise to these effects.

Relationship between magnetic domain configuration and crystallographic orientation in a colossal magnetoresistive material

We investigated the relationship between the magnetic domain (MD) configuration and crystallographic orientation in a colossal magnetoresistive (CMR) material La(0.69)Ca(0.31)MnO(3) in which anisotropic magnetoresistance (AMR) was observed as well. It was observed that the MD structure with a micrometre scale in the (001) plane collapses when a modulated structure with a nanometre scale emerges near the Curie temperature (T(c)). On the other hand, twin boundaries were observed to develop in the (110) plate, and they pin the MD walls. Like the pinning effect on MD walls, the emergence of vortex-like tadpole closure MDs upon the application of external magnetic field may be an origin of the AMR in La(0.69)Ca(0.31)MnO(3).

Electronic self-organization in the single-layer manganite Pr1-xCa1+xMnO4

We use neutron scattering to investigate the doping evolution of the magnetic correlations in the single-layer manganite Pr1-xCa1+xMnO4, away from the x=0.5 composition where the CE-type commensurate antiferromagnetic (AF) structure is stable. We find that short-range incommensurate spin correlations develop as the system is electron doped (x<0.5), which coexist with the CE-type AF order. This suggests that electron doping in this system induces an inhomogeneous electronic self-organization, where commensurate AF patches with x=0.5 are separated by electron-rich domain walls with short-range magnetic correlations. This behavior is strikingly different than for the perovskite Pr1-xCaxMnO3, where the long-range CE-type commensurate AF structure is stable over a wide range of electron or hole doping around x=0.5.

The evolution of Griffiths-phase-like features and colossal magnetoresistance in La(1-x)Ca(x)MnO(3) (0.18 ≤ x ≤ 0.27) across the compositional metal-insulator boundary

Detailed measurements of the magnetic and transport properties of single crystals of La(1-x)Ca(x)MnO(3) (0.18 ≤ x ≤ 0.27) are summarized, and lead to the following conclusions. While temperature-dependent (magneto-) resistance measurements narrow the compositionally modulated metal-insulator (M-I) transition to lie between 0.19 ≤ x(c) ≤ 0.20 in the series studied, comparisons between the latter magnetic data provide the first unequivocal demonstration that (i) the presence of Griffiths-phase-like (GP) features do not guarantee colossal magnetoresistance (CMR), while confirming (ii) that neither are the appearance of such features a prerequisite for CMR. These data also reveal that (iii) whereas continuous magnetic transitions occur for 0.18 ≤ x ≤ 0.25, the universality class of these transitions belongs to that of a nearest-neighbour 3D Heisenberg model only for x≤0.20, beyond which complications due to GP-like behaviour occur. The implications of the variation (or lack thereof) in critical exponents and particularly critical amplitudes and temperatures across the compositionally mediated M-I transition support the assertion that the dominant mechanism underlying ferromagnetism across the M-I transition changes from ferromagnetic super-exchange (SE) stabilized by orbital ordering in the insulating phase to double-exchange (DE) in the orbitally disordered metallic regime. The variations in the acoustic spin-wave stiffness, D, and the coercive field, H(C), support this conclusion. These SE and DE interaction mechanisms are demonstrated to not only belong to the same universality class but are also characterized by comparable coupling strengths. Nevertheless, their percolation thresholds are manifestly different in this system.

Microscopic theory of longitudinal sound velocity in charge ordered manganites

A microscopic theory of longitudinal sound velocity in a manganite system is reported here. The manganite system is described by a model Hamiltonian consisting of charge density wave (CDW) interaction in the e(g) band, an exchange interaction between spins of the itinerant e(g) band electrons and the core t(2g) electrons, and the Heisenberg interaction of the core level spins. The magnetization and the CDW order parameters are considered within mean-field approximations. The phonon Green's function was calculated by Zubarev's technique and hence the longitudinal velocity of sound was finally calculated for the manganite system. The results show that the elastic spring involved in the velocity of sound exhibits strong stiffening in the CDW phase with a decrease in temperature as observed in experiments.

A micro-Raman study of a Pr(0.5)Ca(0.5)MnO(3) single crystal and thin films

We present a micro-Raman study of a high quality Pr(0.5)Ca(0.5)MnO(3) single crystal and thin films on SrTiO(3) and LaAlO(3) substrates. Ten A(g) and seven B(2g) Raman-active modes (simplified symmetry: Pmma space group) have been observed and correlated to charge ordering around T(co) = 240 K and antiferromagnetic spin-orbital ordering at T(N)∼ 170 K. Our data reveal that coherent Jahn-Teller MnO(6) distortions prevail at T<T(N). Moreover, the temperature dependence of the frequency and linewidth of the A(g) mode corresponding to in-plane oxygen atom rotation (∼268 cm(-1)) is analyzed within a polaron-like scheme that reflects a strong charge ordering to lattice coupling. The close similarities between the single crystal and the thin films' phonon evolution confirm that the thin films growth conditions have been optimized.