Charge-ordered Manganites Research Articles

Investigation of the electronic structure of the charge-ordered phase in epitaxial and polycrystallineLa1−xCaxMnO3(x=0.55,0.67)perovskite manganites

In this work, the charge transport properties of charge ordered (CO) La_(1−x)Ca_(x)MnO_(3) (x=0.55,0.67) epitaxial thin films and polycrystals are discussed following the recent controversy of localized electron states versus weakly or delocalized charge-density wave states in CO manganites. The transport properties were investigated by current versus voltage, direct current resistivity versus temperature, local activation energy versus temperature, magnetoresistance, and alternating current admittance spectroscopy measurements, which all indicated a localized electronic structure in the single CO phase. Delocalized charge anomalies previously observed may be restricted to phase separated materials.

Limited local electron-lattice coupling in manganites: An electron diffraction study

$(\mathrm{Pr},\mathrm{Ca})\mathrm{Mn}{\mathrm{O}}_{3}$ is the archetypal charge-ordered manganite, but in ${\mathrm{Pr}}_{0.48}{\mathrm{Ca}}_{0.52}\mathrm{Mn}{\mathrm{O}}_{3}$, we find (using convergent-beam electron diffraction and dark-field images) that the superlattice period is locally incommensurate with respect to the parent lattice and that the superlattice orientation possesses significant local variations. This suggests that local electron-lattice coupling never overwhelmingly dominates the rich physics of manganites, even in the most extreme scenarios that produce the largest colossal magnetoresistance effects.

Charge-order breaking and ferromagnetism in La0.4Ca0.6MnO3 nanoparticles

La 0.4 Ca 0.6 Mn O 3 nanoparticles of grain size as small as ∼20nm are prepared and their magnetic behaviors are investigated in order to understand the size effect of the charge ordering in manganites. The highly stable charge-ordered state can be significantly suppressed upon reduction of the grain size down to nanometer scale, while the ferromagnetism is enhanced. The magnetic phase separation due to the competition between ferromagnetic state and charge-ordered state as well as the surface spin disordering is responsible for the spin-glass-like state at low temperature.

Low temperature neutron diffraction studies showing evidence for charge-exchange-type magnetic ordering in Mn doped SrRuO3

Sr Ru O 3 is a 4d transition metal oxide having a metallic and itinerant ferromagnetic character with TC∼165K. Mn doping in SrRuO3 induces antiferromagnetic interaction, with the equiatomic composition SrRu0.5Mn0.5O3 showing TC∼125K. Temperature dependent zero field cooled and field cooled magnetization data, resistivity data, and neutron diffraction studies on SrRu0.5Mn0.5O3 show the signature of the presence of two ferromagnetic zigzag chains coupled antiferromagnetically, called charge exchange (CE)-type antiferromagnetic ordering; this is contrary to the role of Ru+4, which is reported to remove charge ordering in manganites. The presence of significant amount of Mn+3 ions plays an important role in stabilizing the CE-type antiferromagnetic ordering.

First principle calculations of charge ordering in manganites

Electronic structure calculations were performed on the compound La 0.5 Ca 0.5 MnO 3 to study the relationship between the magnetic ordering, the charge ordering and the geometry of the compound. Charge ordering is intimately related to the magnetic ordering. An antiferromagnetic ordering induces charge disproportionation via a Jahn–Teller distortion. A full disproportionation in Mn 3 + / Mn 4 + occurs for the experimental geometry and allows to predict the experimentally found antiferromagnetic insulating state.

Surface phase separation in nanosized charge-ordered manganites

Recent experiments showed that the robust charge ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted.

I-V Characteristics of Resistive Oxides: DC versus Pulsed Measurements

We investigated pulsed and DC I−V characteristics of a variety of resistive oxides. Examples of pulsed, compared with DC characteristics, are shown for samples of La1.2Sr1.8Mn2O7 (a double layer manganite), Pr2/3Ca1/3MnO3 and Bi1/2Sr1/2MnO3 (charge-ordered manganites), Sr2FeMoO6, Sr2CrWO6 and Ba2MnReO6 (double perovskites). For pulsed measurements, single pulses in the ms range were applied. For short-rise-time square pulses, the Joule heating is negligible, as long as the response remains independent of time. The DC I−V characteristics were measured up to current runaway in the negative resistance regime; ∆T never exceeded a fraction of a degree. In most cases the DC characteristics mask a perfectly ohmic or moderately non-ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used DC I−V measurements in the high current regime are frequently misleading.

Rotation of orbital stripes and the consequent charge-polarized state in bilayer manganites

Nanoscale self-organization of electrons is ubiquitously observed in correlated electron systems such as complex oxides of transition metals. The phenomenon of charge ordering (CO) or the formation of charge stripes, as observed for layered-structure cuprates and nickelates, is one such example. Among them, CO in manganites is closely tied to the orbital degree of freedom of 3d electrons, leading to staggered orbital ordering or the formation of orbital stripes in the layered structure. Here, we describe the phenomena of thermally induced rotation of the orbital stripes by 90( composite function) for bilayered manganite crystals with half hole doping, that is, a 1:1 ratio of Mn3+/Mn4+. The rotation of orbital stripes and the consequent CO coupled with the underlying lattice distortion were found to produce the charge-polarized state, as also shown by its optical second-harmonic generation activity.

Effect of Cr-doping on charge ordering stability in [formula omitted

The effect of Cr-doping on electrical and magnetic properties of high-temperature charge-ordered manganites ( Bi 1 - x Sr x Mn 1 - y Cr y O 3 , where x = 0.25 , 0.3 , 0.4 ; 0 ⩽ y ⩽ 0.1 ) was investigated. Without Cr-doping, a charge disorder-to-order transition ( T co ) and antiferromagnetic phase transition ( T N ) were observed near 500 and 150 K, respectively. The charge-order phase transition broadened, but did not disappear upon Cr-doping up to y = 0.1 . The antiferromagnetic phase disappeared upon 10% Cr-doping, transforming into a paramagnetic phase, rather than into a double exchange ferromagnetic metal, in contrast to the case of Pr 1 - x Ca x Mn 1 - y Cr y O 3 .

Orbital and Charge Ordering in Manganites

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Pulsed versus dc I-V characteristics of resistive manganites

We report on pulsed and dc I-V characteristics of polycrystalline samples of three charge-ordered manganites, Pr2∕3Ca1∕3MnO3, Pr1∕2Ca1∕2MnO3, and Bi1∕2Sr1∕2MnO3, and of a double perovskite, Sr2MnReO6, in a temperature range where their Ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation [J. Phys. C 8, L49 (1975)]. For all samples, the dc I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly Ohmic or a moderately non-Ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used dc I-V measurements are usually misleading.

Anomalous spin dynamics in the charge-ordered two-electron doped manganiteCa0.9Ce0.1MnO3: Possibility of a spin-liquid phase

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 T_CO in contrast with the sample with x = 0.2 and other CO systems. Keeping in view the evidence for magnetic frustration in the system, we propose that the anomalous temperature dependence of the linewidth is the signature of the occurrence of a disorder driven spin liquid phase, present along with charge ordering.

The frustrated magnetic structure of charge-ordered manganite La 0.333Ca 0.667MnO 3

The paper is devoted to superexchange interaction in charge-ordered manganites. The charge and orbital superstructures of La 0.333Ca 0.667MnO 3 cause the frustrated magnetic structure. Within the framework of strong electron–lattice coupling, we can evaluate an orbital structure of the compound. The orbital and crystal structure parameters allow estimating the values of superexchange parameters and Neel temperature. The estimated superexchange parameters confirm the qualitative conclusions of experimental works about frustrated magnetic structure with magnetic supercell (3 a× b×2 c).

Weakening of charge order and antiferromagnetic to ferromagnetic switch over in Pr0.5Ca0.5MnO3 nanowires

We have prepared crystalline nanowires (diameter ∼50nm, length ∼a few microns) of the charge-ordering manganite Pr0.5Ca0.5MnO3 using a low reaction temperature hydrothermal method and characterized them using x-ray diffraction, transmission electron microscopy, superconducting quantum interference device (SQUID) magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ∼105K. Further, the antiferromagnetic transition disappears and the charge ordering transition is suppressed. This result is particularly significant since the charge order in Pr0.5Ca0.5MnO3 is known to be very robust, magnetic fields as high as 27 T being needed to melt it.

Charge ordered oxygen ions and bi- and tri-Mn polarons in La0.5Ca0.5MnO3

Charge ordering (CO) in manganites is widely believed to consist of ordering of Mn ions as Mn3+ and Mn4+. Unrestricted Hartree–Fock calculations show that CO in manganites is actually ordering of O ions as O− and O2− on sites adjacent to Jahn–Teller distorted Mn ions, along the direction of long Mn–O bonds. The energetics of CO of O ions is investigated by calculating the UHF total energies of all unique charge orderings and the lowest energy CO state is found to be one where all Mn ions are bound into bi-Mn polarons. The energetics of several spin states of these polarons is investigated using configuration interaction calculations on a cluster. Bi-Mn polarons are elongated along their axes which results in transverse Mn ion displacements within the perovskite unit cell and this may explain the lattice modulations observed in manganites by transmission electron microscopy.

Ultrafast carrier dynamics of charge-ordered manganite Pr 0.7Ca 0.3MnO 3 studied by pump-probe terahertz emission spectroscopy

Pump-probe terahertz emission spectroscopy (PPTS) of charge-ordered-type manganite Pr 0.7Ca 0.3MnO 3 thin films has been carried out to investigate the ultrafast charge dynamics. The PPTS characteristic of Pr 0.7Ca 0.3MnO 3 indicates that the irradiation of femtosecond laser pulses triggers a melting of a local charge-ordered state within subpicosecond time scale and a reorganization of the charge-ordering starts about 1 ps after this melting process.

Sharp step-like metamagnetic transition in the charge-ordered manganite compound(La0.3Eu0.2)(Ca0.3Sr0.2)MnO3

We have performed transport, magnetization and specific heat measurements on theABO3-type half-doped(La0.3Eu0.2)(Ca0.3Sr0.2)MnO3 (LECSMO)manganite. At T = 2 K, a sharp step-like metamagnetic transition in the magnetization with a critical field of3.3 T is observed. The transition is much sharper than that seen at higher temperatures ( K). Such step-like transitions have previously been reported only in Pr-based manganites,but the present data show that the step-like metamagnetism can also be found in othermanganite compounds.

Effect of Lattice Distortion on Charge Order inManganites at Doping x = 0.5

In the present paper, we continue our investigationon the antiferromagnetic origin of the charge orderobserved in the half-doped manganese.By introducing a Su–Schrieffer–Heeger (SSH) type of perturbationinteraction to the double-exchange Hamiltonian, we calculateagain its ground-state phase diagram at filling x = 0.5by the unrestricted real-space Hartree–Fock approximation method.We find that, as the SSH electron-phonon interaction increases,the charge order parameter decreases to zero rapidly but the CE-typeantiferromagnetic order becomes more stable. In other words,the charge order is much more fragilethan the CE-type or the Neel-type antiferromagnetic ordersunder the electron-phonon perturbation.These results support the proposed theory in the recent publicationsthat the charge order in these systems is inducedby the antiferromagnetic correlations.

Dynamic Isomer Shift in Charge-Ordering Manganite Y0.5Ca0.5MnO3: Mössbauer Spectroscopy Study

We report the Mössbauer spectroscopy study on Fe-doped charge-ordering manganite Y(0.5)Ca(0.5)MnO(3). The dynamic isomer shift is observed for charge-ordering manganite, and its origin may be due to strong Jahn-Teller distortions in Y(0.5)Ca(0.5)MnO(3), causing electron-phonon coupling. The evolution of Mössbauer spectroscopy as a function of temperature shows two different phases with significantly different quadrupole splitting values below the charge-ordering transition temperature. This confirms that there exist two different Mn sites (i.e., Mn(3+) and Mn(4+) ions), which can be identified by the microscopic method of Mössbauer spectroscopy.

Hyperfine Interaction in Charge-Ordered Manganites

Hyperfine Interaction in Charge-Ordered Manganites