Half-doped Manganites Research Articles

Two Types of Charge Ordering in the Half-Doped Manganites of Bi0.5(Ca,Sr)0.5MnO3

From our study of the evolution of charge ordering in Bi 0.5 Ca 0.5- x Sr x MnO 3 ( x = 0–0.5) with chemical pressure, we found the detailed nature of two distinct charge ordering schemes, associated with the structural change from P n m a - to I m m a -type symmetry at x = 0.2–0.25. Charge ordering for x = 0–0.2 ( P n m a ) is consistent with the checkerboard-type pattern, commonly observed in half-doped manganites. On the other hand, a large tolerance factor for x = 0.25–0.5 ( I m m a ) stabilizes a double-stripe charge ordering scheme. These findings indicate the bicritical nature of two competing charge ordered states and provide further insight towards understanding the dominant charge ordering mechanism in half-doped manganites.

NonmagneticB-site impurity-induced ferromagnetic tendency in CE-type manganites

Using a two-orbital model and Monte Carlo simulations, we investigate the effect of nonmagnetic $B$-site substitution on half-doped CE-type manganites. The lattice defects induced by this substitution destabilize the CE phase, which transforms into (1) the ferromagnetic (FM) metallic competing state, (2) a regime with short-range FM clusters, or (3) a spin-glass state, depending on couplings and on the valence of the $B$-site substitution. While a $C$-type antiferromagnetic state is usually associated with an average ${e}_{g}$ charge density of less than 0.5, the nonmagnetic $B$-site substitution that lowers the ${e}_{g}$ charge density is still found to enhance the FM tendency in our simulations. The present calculations are in qualitative agreement with experiments and provide a rationalization for the complex role of nonmagnetic $B$-site substitution in modulating the phase transitions in manganites.

Size-dependent exchange bias in half-doped manganite nanoparticles

Magnetic properties of the nanosized half-doped manganite of Sm0.5Ca0.5MnO3 with different particle sizes prepared by a sol-gel method are investigated. Exchange-bias phenomena are observed in the field-cooled magnetic hysteresis loops for these nanoparticles. The values of the exchange field, coercivity, remanence asymmetry, and remanent magnetization are found to depend strongly on the particle size. Particularly, as the particle size decreases, the exchange field shows a nonmonotonic variation with a maximum at ∼120 nm. These magnetic behaviors have been explained in terms of size effects on the charge ordered and antiferromagnetic manganite nanoparticles.

Crystal structure and physical properties of half-doped manganite nanocrystals of less than 100-nm size

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped ${\text{La}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_{3}(\text{LCMO})$ synthesized by chemical route, having particle size down to an average diameter of 15 nm. 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 to 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. 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.

The checkerboard pattern of Bi0.63Sr0.37MnO3 determined using resonant x-ray scattering at the Mn K edge

A study using resonant x-ray scattering at the Mn K edge has been carried out on aBi0.63Sr0.37MnO3 single crystal. This compound undergoes a metal–insulator phase transition to theso-called charge ordered (CO) state at about 530 K. Strong resonance signalswere observed at room temperature as the energy was tuned through the MnK edge for several superstructures of the CO phase. The energy, polarizationand azimuth angle dependences agree with a checkerboard ordering in theab plane of manganese atoms of two types, in terms of their different local geometricalstructures. One of the sites is anisotropic—a tetragonal distorted oxygenoctahedron—and the other is isotropic—a nearly undistorted one, as observed forBi0.5Sr0.5MnO3 and other half-doped manganites. This result indicates that thecheckerboard pattern is strongly stable and extends to doping concentrationsx<0.5. No superstructures corresponding to the doubling of thec axis were detected. Intermediate valence states lower than 3.5, according to thefractional charge segregation, were deduced for the two non-equivalent Mn atoms,i.e. Mn3.30+ and Mn3.44+.

Orbital domain dynamics in a doped manganite

We explore a number of novel effects near the orbital-order phase transition in a half-doped manganite, Pr0.5Ca0.5MnO3. To probe the unusual short-range orbital order in this system, we have performed coherent soft x-ray resonant scattering measurements in a Bragg geometry to measure dynamics. Near the transition temperature, we observe a small fluctuating component in the scattered signal that is correlated with three effects: a rapidly decreasing total signal and orbital domain size, as well as an abrupt onset of a broad background intensity that we attribute to the thermal production of correlated polarons. Our speckle results suggest that the transition is characterized by a competition between a pinned orbital domain topology that remains static and mobile domain boundaries that exhibit slow, temporal fluctuations.

Strain-driven inverse thermal hysteresis behaviour in half-doped manganites

In this paper, we report unusual thermal hysteresis behaviour in half-doped manganites Nd0.5Sr0.5−xCaxMnO3 (0.0 ⩽ x ⩽ 0.50). For 0.05 ⩽ x < 0.25 samples, the magnetization curves show a counterclockwise hysteresis loop. For 0.25 < x ⩽ 0.40 samples, however, they exhibit a clockwise loop. The inverse hysteretic behaviour stems from the competition between two kinds of strain, namely, the compressive strain and the tensile strain. The former produces a counterclockwise thermal hysteresis while the latter leads to a clockwise one. In the meantime, a magnetic instability is observed and the strain relaxation should be the main reason for it.

Charge ordering and dielectric properties in the near half-doped Pr 0.79Na 0.21MnO 3 perovskite

The Pr 0.79Na 0.21MnO 3 perovskite has been prepared in polycrystalline form by a ceramic method. We have carried out its structural characterization by synchrotron X-ray powder diffraction (SXRPD) at room temperature and at 150 K ( T< T CO). In the charge ordering (CO) state, the SXRPD data can be refined both on the basis of the site-centred model and the bond-centred model proposed for half-doped manganites, without a clear advantage of one model over the other. From the dielectric point of view, at the CO temperature this manganite shows a maximum in the dielectric constant whose origin is intrinsic, and cannot be attributed to the presence of extrinsic factors. We relate this dielectric behaviour to the formation of polar entities at the temperature of charge condensation, due to an asymmetric charge distribution intermediate between site-centred and bond-centred type.

Visualization of charge ordering in a half-doped manganite by an electrostatic potential analysis

We have experimentally investigated the charge density distribution and the electrostatic potential for a charge-ordering doped manganite, ${\mathrm{Nd}}_{1∕2}{\mathrm{Sr}}_{1∕2}{\mathrm{MnO}}_{3}$. In the charge-ordered state, we observed a zigzag pattern of the charge density within the (010) plane. Judging from the ${\mathrm{MnO}}_{6}$ distortion, we ascribed the pattern to the bonding electron in the ${p}_{x(y)}\ensuremath{-}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ hybridized state, not to the so-called ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbital ordering. We further calculated the electrostatic potential $U(r)$ from the charge density and the atomic positions, which clearly indicates the checkerboard-type charge disproportionation within the (010) plane.

Versatile and competing spin-charge-orbital orders in the bilayered manganite systemPr(Sr1−yCay)2Mn2O7

Electronic-phase competition has been investigated for single crystals of half-doped bilayer-structure manganites, $\mathrm{Pr}{({\mathrm{Sr}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y})}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(0\ensuremath{\leqslant}y\ensuremath{\leqslant}1.0)$, with controlled one-electron bandwidth $(W)$. For $y\ensuremath{\leqslant}0.4$, the A-type antiferromagnetic (AF) state with ${x}^{2}\ensuremath{-}{y}^{2}$ orbital order shows up as the ground state while accompanying the short-range vertical charge order with the periodicity of $\ensuremath{\approx}4{a}_{0}$ $({a}_{0}\ensuremath{\approx}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. For $y&gt;0.4$, with reduced $W$, the CE-type AF state emerges with the ordered staggered orbitals $(3{x}^{2}\ensuremath{-}{r}^{2}∕3{y}^{2}\ensuremath{-}{r}^{2})$ whose diagonal stripes undergo the 90\ifmmode^\circ\else\textdegree\fi{} rotation with decreasing temperature, perhaps coupled with the evolution of spin correlation.

Combined Effect of Bond and Potential Disorder in Half-Doped Manganites

We analyze the effects of both bond and potential disorder in the vicinity of a first-order metal insulator transition in a two-band model for manganites using a real-space Monte Carlo method. Our results reveal a novel charge-ordered state coexisting with spin-glass behavior. We provide the basis for understanding the phase diagrams of half-doped manganites, and contrast the effects of bond and potential disorder and the combination of both.

Ab initiostudy of the CE magnetic phase in half-doped manganites: Purely magnetic versus double exchange description

The leading electronic interactions governing the local physics of the CE phase of half-doped manganites are extracted from correlated ab initio calculations performed on an embedded cluster. The electronic structure of the low-energy states is dominated by double exchange configurations and $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2{p}_{\ensuremath{\sigma}}$ to $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ charge transfer configurations. The model spectra of both a purely magnetic nonsymmetric Heisenberg Hamiltonian involving a magnetic oxygen and two nonsymmetric double exchange models are compared to the ab initio one. While a satisfactory agreement between the Heisenberg spectrum and the calculated one is obtained, the best description is provided by a double exchange model involving excited non-Hund atomic states. The energy eigenvalues of this refined double exchange model are derived as a function of the local lattice distortions, and it is shown that in the absence of non-Hund states, an incorrect ground state is obtained.

Steplike magnetization and resistivity transition in the half-doped manganite compound Pr 0.75Na 0.25MnO 3

We report on the magnetic and electrical transport properties of a half-doped manganite Pr 0.75Na 0.25MnO 3. At low temperatures, a field-induced steplike magnetization and resistivity transition are observed. The step transitions of magnetization and resistivity are shifted to higher fields as a result of field cooling, and transformed to a smooth broad one when the cooling field is more than 20 kOe. Moreover, in a magnetic field slightly below the critical field, the magnetic and resistive relaxation exhibits a spontaneous step after a long incubation time when both the temperature and magnetic field are constant. Such steplike transitions are discussed in terms of a martensitic-like transformation associated with phase separation.

Tunable two-phase coexistence in half-doped manganites

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.

Optical detection of the rotation of orbital stripes in a charge–orbital ordered bilayer manganite at half-doping

The CE magnetic order of charge–orbital ordering pattern in half-doped manganites is characterized by the formation of orbital stripe as well as the checkerboard-type charge ordering. The quasi-one-dimensionality in the orbital arrangement results in the in-plane anisotropy of physical properties, e.g., optical spectra with light polarization along the orbital stripe exhibits the larger energy-gap state than those along the orbital chain. It has been recently reported that the spontaneous rotation of orbital stripe with variation of temperature ( T ) occurs in a bilayer manganite, Pr ( Sr 0.1 Ca 0.9 ) 2 Mn 2 O 7 . To confirm this peculiar behavior, we performed optical measurements of a single domain sample with polarization dependence. While the in-plane optical anisotropy is identified together with the initial formation of the charge–orbital ordering, it is reversed at a lower T , where the orbital stripe is rotated by 90 ∘ .

Optical Probe of the Control of Charge-Orbital Ordering Correlation by Using the Cation Configuration in Half-Doped Layered-Structure Manganites

Optical Probe of the Control of Charge-Orbital Ordering Correlation by Using the Cation Configuration in Half-Doped Layered-Structure Manganites

Interrelation of the spin, orbital, and charge degrees of freedom in half-doped manganites

Based on the double-exchange model in the tight-binding approximation, self-consistent energy-band-structure calculations are carried out for the first time for the orbital and charge ordering in La0.5Ca0.5MnO3-type manganites with 16 Mn ions in the unit cell under the assumption of a linear dependence of the Jahn-Teller splitting of the eg level on the occupancy of each ion. The equilibrium magnetic and orbital configurations are determined by minimizing the total energy with respect to the direction of the local magnetic moments and the orbital states of the Mn ions. The dependence of the splitting on the occupancy favors the stabilization of phases with charge separation. This separation is an important factor determining the magnetic structure of the ground state. The ground state can be an insulating antiferromagnetic structure of the CE or G type or a new ferrimagnetic structure with a magnetization lower than that of a saturated ferromagnet by a factor of 2. The existence and the width of the bandgap in the electronic energy spectrum of the CE phase depend on the ratio between the values of the Hund exchange and the splitting. When the splitting is sufficiently large, the Jahn-Teller effect stabilizes the insulating state. The finite value of the Hund coupling also favors the stabilization of the CE phase for the realistic values of the interionic exchange and the hopping integral of itinerant electrons.

Effect of antiferromagnetic spin correlations on lattice distortion and charge ordering in Pr0.5Ca1.5MnO4.

We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr(0.5)Ca(1.5)MnO(4). On cooling from high temperature, the system first becomes charge-and orbital-ordered (CO/OO) near T(CO) = 300 K and then develops checkerboard-like antiferromagnetic (AF) order below T(N) = 130 K. At temperatures above T(N) but below T(CO) (T(N)<T<T(CO)), the appearance of short-range AF spin correlations suppresses the CO/OO-induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.

Interplay of spin, orbital, and charge degrees of freedom in half-doped manganites

Energy-band-structure calculations of the ground-state orbital and charge orders for various magnetic structures have been performed in the tight-binding approximation for manganites of the ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ type with 16 Mn ions in the unit cell. The arising inhomogeneous charge distribution was accounted for self-consistently by assuming a linear dependence of Jahn-Teller (JT) splitting of the ${e}_{g}$ level on its occupation for each of the ions. This dependence promotes the phases with charge separation and the charge degree of freedom becomes an essential factor responsible for stabilization of such phases as a ground state. Among them are the $CE$ structure and also a peculiar ferrimagnetic structure with magnetization twice as low as that for the saturated ferromagnet. The existence and the width of the gap in the electron spectrum of the $CE$ phase depend on the relation between Hund exchange and JT splitting, so that the JT effect stabilizes the insulating state. A finite character of Hund coupling also favors stabilization of the $CE$ phase in the region of reasonable values of the interatomic exchange and hopping integrals.

Change in optical anisotropy with rotation of orbital stripe in a half-doped bilayer manganite

We report an optical study of a half-doped bilayer manganite, $\mathrm{Pr}{({\mathrm{Sr}}_{0.1}{\mathrm{Ca}}_{0.9})}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, which shows spontaneous rotation of the orbital stripe with variation of temperature $(T)$ in a charge-orbital ordered phase. While the in-plane optical anisotropy is identified together with the initial formation of the charge-orbital ordering, it is reversed at a lower $T$, where the orbital stripe is rotated by 90\ifmmode^\circ\else\textdegree\fi{}. The change in optical anisotropy leads to anomalous $T$ dependence of electrodynamic parameters while keeping the orthorhombic lattice axes intact.