Half-doped Manganites Research Articles

Magnetic phase diagram of nanosized half-doped manganites: role of size reduction

The magnetic properties of ~40 nm Nd(0.5)Ca(0.5)MnO(3) and Sm(0.5)Sr(0.5)MnO(3) nanoparticles are investigated by magnetometry and electronic spin resonance (ESR) spectroscopy. It is found that although their bulk counterparts have quite different magnetic properties at low temperatures, both the nanoparticles exhibit very similar magnetic behaviors, where the charge ordered transitions disappear and weak ferromagnetism emerges below about 100 K. A detailed analysis on the magnetic susceptibilities and the ESR linewidths reveals that for the two compounds the size reduction weakens both the ferromagnetic and antiferromagnetic interactions, and converts the long-range charge orderings to short-range ones. Moreover, the strength of the charge ordered correlations is observed to be not much affected by the size reduction. Based on the present results and the previous studies on various nanosized half-doped manganites, the magnetic phase diagram of the half-doped manganites with the particle sizes of ~25-40 nm is established. We find that this diagram is very similar to those for the bulk near half-doped manganites with large quenched disorder, which allows us to propose that the reported exotic phenomena in the nanosized half-doped manganites should be mainly ascribed to surface disorder effect. These results may provide a deeper insight into the role of size reduction on the physics of half-doped manganites.

Size Control of Charge-Orbital Order in Half-Doped ManganiteLa0.5Ca0.5MnO3

Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La(0.5)Ca(0.5)MnO(3), using the combination of density-functional theory (DFT) and dynamical mean-field theory (DMFT). We find that upon size reduction the charge-ordered antiferromagnetic phase, observed in bulk, is destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on a defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study additionally reports the correlation-driven stability of the charge-orbitally ordered state in bulk La(0.5) Ca(0.5) MnO(3), even in the absence of long-range magnetic order.

Spin-wave excitations in the ferromagnetic metallic and in the charge-, orbital-, and spin-ordered states in Nd1−xSrxMnO3withx≈0.5

Inelastic neutron scattering experiments have been performed on single crystals of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5. Colossal magnetoresistance (CMR) in the manganites arises from the interplay between a ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating state. Therefore, it appears important to compare these phases concerning their underlying magnetic interaction parameters. Our investigations of the spin-wave disperion in the AFM ordered state of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half-doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. Furthermore, magnetic excitations in the ferromagnetic metallic state of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x=0.49 and x=0.50 exhibit nearly the same magnon dispersion which can be described with a Heisenberg model including nearest-neighbor interactions.

Fragile Magnetic Ground State in Half-DopedLaSr2Mn2O7

We investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La(2-2x)Sr(1+2x)Mn2O7 (x ≃ 0.5) by using Mn L(2,3)-edge resonant soft x-ray scattering. Resonant soft x-ray scattering reveals the CE-type orbital order below T(oo) ≃ 220 K, which shows partial melting behavior below T(m) ≃ 165 K. We also found coexistence CE- and A-type antiferromagnetic orders. Both orders involve the CE-type orbital order with nearly the same orbital character and are coupled with each other. These results manifest that the ground state with the CE-type antiferromagnetic order is easily susceptible to destabilization into the A-type one even with a small fluctuation of the doping level, as suggested by the extremely narrow magnetic phase boundaries at x ≃ 0.5±0.005.

Buckling-induced Zener polaron instability in half-doped manganites

By calculating the electronic, orbital, magnetic, and lattice structure of half-doped manganites we establish the central role of oxygen buckling caused by the tilting of the MnO${}_{6}$ octahedra, which is, in essence, a steric effect. The buckling itself does not change the Mn-Mn bonds. Instead, it drives the system toward an instability where these bonds can dimerize. In the presence of electron-electron interactions, this instability can fully develop, and beyond a critical buckling a Zener polaron ground state with dimerized spins, lattice, and orbitals forms spontaneously, resulting in a multiferroic state.

Intricacies of strain and magnetic field induced charge order melting in Pr 0.5Ca 0.5MnO 3 thin films

Thin films of the half-doped manganite Pr 0.5Ca 0.5MnO 3 were grown on (1 0 0) oriented MgO substrates by pulsed laser deposition technique. In order to study the effect of strain on the magnetic field induced charge order melting, films of different thicknesses were prepared and their properties were studied by X-ray diffraction, electrical resistivity and magnetoresistance measurements. A field induced charge order melting is observed for films with very small thicknesses. The charge order transition temperature and the magnetic field induced charge order melting are observed to depend on the nature of strain that is experienced by the film.

A-site ion-size effect on the transport and magnetic properties of Ce doping Pr0.3Ce0.2CaxSr0.5−xMnO3 (0≤x≤0.25)

The effects of A-site ion-size 〈rA〉 on the crystal structures, transport and magnetic properties of the perovskite manganese oxide Pr0.3Ce0.2CaxSr0.5-xMnO (0 ≤ x ≤ 0.25) have been investigated. In those compounds, when 0≤x≤0.125, the temperatures (Tmax) of the resistivity maximums below Curie Temperature TC are correlated with the Kondo-like scattering of Ce3+ and the onset of antiferromagnetic ordering of Ce3+ with respect to the Mn-sublattice moments. The decrease of 〈rA〉 causes the anomalous increase of lattice parameters b, c and unit cell volume, the decrease of the differences of TC and Tmax, the weakening of the Kondo-like scattering and magnetic order of Ce3+, and the enhancement of saturation magnetic moment, which give the evidences of the valence enhancing of Ce ions from + 3 toward + 4 with 〈rA〉 decreasing. Although the Ca doping is expected to drive the system toward the antiferromagnetic ground state, the increase of valence of Ce enhances the content of Mn3+ in the system, which drives the system to the ferromagnetic ground state. The changes of ion-size 〈rA〉 and the valence of Ce are co-operating on the transport and magnetic properties of the half-doped manganites.

Suppression of Charge and Antiferromagnetic Ordering in La0.5Ca0.5MnO3 Nanoparticles

The intermediate band-width manganite La0.5Ca0.5MnO3 has a CE type charge, orbital and antiferromagnetic order below 195 K owing to strong electronic correlation and commensurability effect. We have critically investigated in details all the possible mechanisms behind the destabilization of charge/orbital ordering in La0.5Ca0.5MnO3 with the reduction of grain size in the nanometric regime as evidenced from our various experimental results. We have tried to found out a universal scenario to explain such behavior in nanometric manganites in general. This study shows that the room temperature Pnma perovskite structure, correlated with the lattice distortion, Mn-O distances and octahedral tilts, is practically unaffected with the reduction of particle size. However, in these nanocrystalline La0.5Ca0.5MnO3, the stable charge ordered state with antiferromagnetic ordering becomes significantly suppressed and FM ordering spontaneously appears below T(c) = 260 K as compared to its bulk counterpart. From strong experimental evidences on nano and bulk manganites, we revisit this issue and also discuss the absence of correlation between size reduction and pressure effects on manganites. We argue from a phenomenological analysis that the above observations could be best attributed to enhanced surface disorder in the nanosized half-doped manganite grains which is likely to be general in all nano size charge ordered manganites.

Particle Size Effects on Charge and Spin Correlations in Nd0.5Ca0.5MnO3 Nanoparticles

The particle size effects on the charge and spin correlations in half-doped manganite Nd0.5Ca0.5MnO3, which exhibits a charge-ordered (CO) transition at 250 K in the bulk counterpart, have been investigated by magnetometry and electron spin resonance (ESR). Magnetic measurements show that reducing the particle size weakens the long-range CO transition, which completely disappears when the particle size is reduced down to 40 nm. Meanwhile, a weak ferromagnetic (FM) behavior appears at low temperatures and is gradually enhanced with the decrease of the particle size. The ESR intensities of the nanoparticles reproduce well these features. However, the temperature dependences of the ESR g-factor and line width exhibit the typical characteristics of the CO states in all the nanoparticles, even in the 40 nm sample, which suggests that, even though the long-range CO transition is completely suppressed by the size reduction, the CO state is still present in the short-range ordering form. Moreover, it is found that the onset temperatures of the CO states in all the nanoparticles are almost the same as that of the bulk, which strongly indicates that the strength of the CO correlations in this compound is not influenced much by the particle size. A detailed analysis on the magnetic susceptibilities and the ESR line width further reveals that both the antiferromagnetic (AFM) and the FM spin interactions are weakened by the size reduction, which suggests that the enhanced FM behavior in the nanoparticles is not due to the enhancement of the double-exchange FM interactions. We propose that, although the FM interactions are weakened, they gradually dominate over the AFM ones at low temperatures with the decrease of the particle size due to the more significant weakening of the latter by the size reduction, which, hence, gives rise to the development of the FM behaviors in the nanoparticles.

Relationship between orbital structure and lattice distortions in Jahn-Teller systems

Analyzing the orbital structure and lattice distortions in the CE phase of half-doped manganites, we demonstrate that the usual approach directly relating the orbital occupation of Jahn-Teller ions to the displacements of neighboring ligands may be misleading. For the correct identification of orbital structure, it is necessary to take self-consistently into account the electron-lattice interactions, kinetic energy of charge carriers, and crystal-field effects. In certain situations, e.g. in the CE phase of single-layered manganite La$_{0.5}$Sr$_{1.5}$MnO$_4$, the type of orbital ordering strongly deviates from that, which one would deduce from the local lattice distortions.

Effect of strain on the orbital and magnetic ordering of manganite thin films and their interface with an insulator

We study the effect of uniform uniaxial strain on the ground state electronic configuration of a thin film manganite. Our model Hamiltonian includes the double-exchange, the Jahn-Teller electron-lattice coupling, and the antiferromagnetic superexchange. The strain arises due to the lattice mismatch between an insulating substrate and a manganite which produces a tetragonal distortion. This is included in the model via a modification of the hopping amplitude and the introduction of an energy splitting between the Mn e_g levels. We analyze the bulk properties of half-doped manganites and the electronic reconstruction at the interface between a ferromagnetic and metallic manganite and the insulating substrate. The strain drives an orbital selection modifying the electronic properties and the magnetic ordering of manganites and their interfaces.

Size-Induced Griffiths Phase and Second-Order Ferromagnetic Transition in Sm0.5Sr0.5MnO3 Nanoparticles

Recent studies on nanosized near half-doped manganites reported that the charge-ordered antiferromagnetic states present in their bulk counterparts are strongly suppressed and that ferromagnetic behaviors appear at low temperatures as the particle size is reduced to nanometer scale. In this paper, the effects of size reduction on the magnetic properties of Sm0.5Sr0.5MnO3, which shows a sharp first-order ferromagnetic transition and an inhomogeneous phase dominated by a charge-ordered antiferromagnetic state above TC in the bulk form, have been investigated. The Sm0.5Sr0.5MnO3 nanoparticles of ∼40 nm were prepared by a sol−gel method, and their magnetic properties were studied by magnetometry. It is found that similar suppression of the charge-ordered antiferromagnetic state by size reduction is also present in this system. Most interestingly, two novel phenomena associated with this suppression are observed in the nanoparticles. One is that the inhomogeneous phase above TC is dominated by the ferromagnetic interactions in place of the antiferromagnetic ones and can be well described as a Griffiths phase. The other is that the nature of the ferromagnetic transition changes from first-order to second-order. These results may provide a deeper insight into the physics of the effects of size reduction on the half-doped manganites.

Spin Superstructures in Thin Films of Correlated Electron Systems

Realizing an electronic phase competition in a material is a promising approach to induce a large electronic change by a small external field. One of typical materials showing the phase competition is the half-doped perovskite manganite, in which the ground state drastically changes between the antiferromagnetic-insulating (AFM-I) and ferromagnetic-metallic (FM-M) states depending on the slight difference in the band width. Near the phase boundary, the two phases coexist and compete with each other, giving rise to a gigantic field response like the colossal magnetoresistance. We show in this paper the observation of a self-organized anisotropic phase separation with use of a microwave impedance microscope for a thin film of Nd0.5Sr0.5MnO3, which locates on the verge of the phase boundary. We also show an artificially tailored phase separation and its magnetic control realized in superlattices composed of Pr0.5Ca0.5MnO3 (AFM-I) and La0.5Sr0.5MnO3 (FM-M).

Bridging Charge-Orbital Ordering and Fermi Surface Instabilities in Half-Doped Single-Layered ManganiteLa0.5Sr1.5MnO4

The single-layered half-doped manganite La(0.5)Sr(1.5)MnO₄ (LSMO), was studied by means of the angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and resistivity measurements. STM revealed a smooth reconstruction-free surface; the density of states, extracted from photoemission and tunneling spectroscopy, is in agreement with transport measurements. The derived from ARPES Fermi surface (FS) nesting properties correspond to the known pattern of the charge-orbital ordering (COO), which implies that FS instability is related to the propensity to form a COO state in LSMO.

Multiferroicity and magnetoelectric coupling in half-doped manganite La0.5Ca0.5MnO3

We present experimental evidences to verify the peculiar multiferroics predicted in the half-doped manganite La0.5Ca0.5MnO3. The dielectric constant exhibits a frequency-independent peak just at the antiferromagnetic charge ordering transition temperature TCO, indicating a close correlation between the ferroelectriclike transition and charge ordering. In addition to a large magnetodielectric effect, the dynamic magnetoelectric coupling measurements provide solid evidences to an enhanced magnetoelectric coupling below TCO. Moreover, the magnetic field dependence of the coupling coefficient shows a close correlation with the magnetization hysteresis loop, which reveals that a large magnetoelectric coupling only persists in the charge ordering state.

Heisenberg-like ferromagnetism and percolative conductivity in the half-doped manganite Nd 0.5Ca 0.25Sr 0.25MnO 3

The magnetic behavior and electronic transport in the half-doped manganite Nd 0.5Ca 0.25Sr 0.25MnO 3 have been investigated. The critical exponents are studied by using isothermal magnetization methods. The results show that the paramagnetic–ferromagnetic transition is second order and the magnetic interaction is satisfied with the prediction of three-dimensional Heisenberg model. The electronic transport belongs to the percolation mechanism. These findings demonstrate that the critical behavior of the magnetic transition and conductivity for manganites are related to Mn-site ordering degree.

Magnetically Induced Electronic Ferroelectricity in Half-Doped Manganites

Using a joint approach of density functional theory and model calculations, we demonstrate that a prototypical charge ordered half-doped manganite La1/2Ca1/2MnO3 is multiferroic. The combination of a peculiar charge-orbital ordering and a tendency to form spin dimers breaks the inversion symmetry and leads to a ferroelectric ground state with a polarization up to several microC/cm2. The presence of improper ferroelectricity does not depend on the hotly debated structural details of this material: in the Zener-polaron structure we find a similar ferroelectric response with a large polarization of purely magnetic origin.

Excess specific heat and evidence of zero-point entropy in magnetic glassy state of half-doped manganites

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 conventional glass.

Size of orbital-ordering domain controlled by the itinerancy of the3delectrons in a manganite thin film

An electronic effect on a macroscopic domain structure is found in a strongly correlated half-doped manganite film Nd$_{0.5}$Sr$_{0.5}$MnO3 grown on a (011) surface of SrTiO3. The sample has a high-temperature (HT) phase free from distortion above 180K and two low-temperature (LT) phases with a large shear-mode strain and a concomitant twin structure. One LT phase has a large itinerancy (A-type), and the other has a small itinerancy (CE-type), while the lattice distortions they cause are almost equal. Our x ray diffraction measurement shows that the domain size of the LT phase made by the HT-CE transition is much smaller than that by the HT-A transition, indicating that the difference in domain size is caused by the electronic states of the LT phases.

Size and Nonstoichiometry Effects on Magnetic Properties of ${\hbox{La}}_{0.5}\hbox{Ca}_{0.5}\hbox{MnO}_{3}$ Manganite

AC and dc magnetic measurements in temperature interval 5-350 K and at external magnetic field H les 5 T were employed to probe the finite-size and nonstoichiometry effects on magnetic ordering of La0.5Ca0.5MnO3 half-doped manganite compound. It is shown that the complex mixed magnetic ordering in bulk samples appears to be unstable regarding the both effects. The strongest one is the size effect, suppressing the mixed order and inducing the ferromagnetic-like one, which is characteristic also for other half-doped manganites. The small variations in cation composition (nonstoichiometry), which don't influence the crystal structure, lead to strong perturbations in the mixed magnetic ordering of the bulk. The results obtained are discussed in the terms of pronounced competition of spin, charge and lattice degrees of freedom, characteristic for doped manganites.