Unraveling orbital ordering in La0.5Sr1.5MnO4

Orbital ordering (OO) in the layered perovskite La0.5Sr1.5MnO4 has been investigated using the enhanced sensitivity of soft x-ray resonant diffraction at the Mn L edges. The energy dependence of an OO diffraction peak over the L(2,3) edges is compared to ligand-field calculations allowing a distinction between the influences of Jahn-Teller distortions and spin correlations. The energy dependence of the diffraction peak at the Mn L1 edge is remarkably different from that observed at the Mn K edge.     

Spectroscopy of La0.5Sr1.5MnO4 orbital ordering: a cluster many-body calculation

Orbital ordering (OO) in La_0.5Sr_1.5MnO₄ has been studied using soft X-ray resonant diffraction (SXRD) at the Mn L_2,3 edges in combination with many-body cluster calculations. The SXRD intensity is modelled in second quantization using a small planar cluster consisting of a central active Mn site with first-neighbour shells comprising O and Mn sites. The effective Hamiltonian includes Slater-Koster parameters and charge transfer and electron correlation energies obtained from previous measurements on manganites. The energy dependence of the SXRD OO peak is calculated using the Jahn-Teller distortions of the oxygen octahedra and in-plane spin correlations as adjustable parameters. These contributions are clearly distinguished above the Néel temperature with a good spectroscopic agreement. The results also suggest a significant charge separation between the Mn sites.     

Direct observation of orbital ordering in La0.5Sr1.5MnO4 using soft x-ray diffraction

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.     

Independent freezing of charge and spin dynamics in La1.5Sr0.5CoO4

We present elastic and quasielastic neutron scattering measurements characterizing peculiar short-range charge-orbital and spin order in the layered perovskite material La1.5Sr0.5CoO4. We find that below T(c) approximately 750 K holes introduced by Sr doping lose mobility and enter a statically ordered charge glass phase with loosely correlated checkerboard arrangement of empty and occupied d(3z(2)-r(2)) orbitals ( Co3+ and Co2+). The dynamics of the resultant mixed spin system is governed by the anisotropic nature of the crystal-field Hamiltonian and the peculiar exchange pattern produced by the orbital order. It undergoes a spin freezing transition at a much lower temperature, T(s) less, similar30 K.     

Orbital ordering in La0.5Sr1.5MnO4 studied by soft X-ray linear dichroism

We found that the conventional model of orbital-ordering of 3x(2)-r(2)/3y(2)-r(2) type in the e(g) states of La0.5Sr1.5MnO4 is incompatible with measurements of linear dichroism in the Mn 2p-edge x-ray absorption, whereas these e(g) states exhibit predominantly cross-type orbital ordering of x(2)-z(2)/y(2)-z(2). LDA+U band-structure calculations reveal that such a cross-type orbital-ordering results from a combined effect of antiferromagnetic structure, Jahn-Teller distortion, and on-site Coulomb interactions.     

Microscopic stability mechanisms of a charge-ordered phase of La1.5Sr0.5NiO4

Using a pairwise potential approximation and a shell model, computer simulation is performed of a charge-ordered crystal phase of La_1.5Sr_0.5NiO₄, in which Ni²⁺ and Ni³⁺ ions are arranged in staggered rows in perovskite layers. This phase is found to be stable, and, in the process of its formation, the contribution to the lowering of the total crystal energy from the charge rearrangement is smaller than that from the relaxation of the crystal structure (the structure of NiO₂ layers, first of all) caused by this rearrangement. The decrease in the total energy is due to the long-range Coulomb interaction, predominantly the attraction between Ni³⁺ and oxygen ions in NiO₂ layers.     

Low temperature charge and orbital textures in La0.875Sr0.125MnO3

By using nuclear magnetic resonance techniques we show that for T<30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.     

Spin, charge, and orbital orderings in iron-based superconductors

In this article, we briefly review spin, charge, and orbital orderings in iron-based superconductors, as well as the multi-orbital models. The interplay of spin, charge, and orbital orderings is a key to understand the high temperature superconductivity. As an illustration, we use the two-orbital model to show the spin and charge orderings in iron-based superconductors based on the mean-field approximation in real space. The typical spin and charge orderings are shown by choosing appropriate parameters, which are in good agreement with experiments. We also show the effect of Fe vacancies, which can introduce the nematic phase and interesting magnetic ground states. The orbital ordering is also discussed in iron-based superconductors. It is found that disorder may play a role to produce the superconductivity.     

Physical properties of the doped manganites Ln0.5M0.5Mno3 (Ln = Pr,La; M = Sr,Ca, Ba)

We present the structural, magnetic and electrical properties of some manganese oxides such as Ln_0.5M_0.5MnO₃ (Ln = La, Pr and M = Ca, Sr, Ba) in which the average ionic radius <r_A> of the A site cation is varied while keeping the Mn³⁺/Mn⁴⁺ ratio fixed to 1. All the studied samples have a perovskite structure with rhombohedral or orthorhombic distortion. This distortion was related to the A cation size. The lattice volume increases lineraly with ( r _A) in all the range 1.179–1.3264 Å. The magnetic investigation shows that the magnetic transition temperature T _t, and magnetisation saturation M _s have similar behaviour by increasing and decreasing with (r _A). The electrical measurements show conduction by small polarons attributed to a large canting angle 9 in all the range 4–300 K in the two ferromagnetic compounds La_0.5Sr_0.5MnO₃ and La_0.5Sr_0.5MnO₃.     

Spin and charge ordering in three-leg ladders in oxyborates

We study the spin ordering within the three-leg ladders present in the oxyborate Fe3O2BO3 consisting of localized classical spins interacting with conduction electrons (one electron per rung). We also consider the competition with antiferromagnetic superexchange interactions to determine the magnetic phase diagram. Besides a ferromagnetic phase we find (i) a phase with ferromagnetic rungs ordered antiferromagnetically and (ii) a zigzag canted spin ordering along the legs. We also determine the induced charge ordering within the different phases and the interplay with lattice instability. Our model is discussed in connection with the lattice dimerization transition observed in this system, emphasizing the role of the magnetic structure.     

Zero-field cooled exchange bias and magnetization reversal in La1.5Sr0.5Co0.4Fe0.6MnO6

The polycrystalline sample La_1.5Sr_0.5Co_0.4Fe_0.6MnO₆ (LSCFMO) was prepared by sol-gel method and its magnetic properties were studied. The interesting magnetization reversal phenomenon and the zero-field cooled exchange bias (ZEB) effect were simultaneously observed in LSCFMO. ZEB effect can exist in a wider temperature range (0–200 K) compared with La_1.5Sr_0.5CoMnO₆ (0–10 K), which is very important in the potential applications. A schematic diagram based on the coupling between the Fe³⁺ spins, Mn³⁺ spins and Co²⁺ or Co³⁺ spins is used to understand the ZEB and the reversal behaviors. Due to the doping of 60% Fe ion, the magnetic microstructure of LSCMFO is more complex than that of LSCMO, resulting in the meta-stable spin structure and more interesting magnetic phenomenon.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

Giant radiofrequency magnetoabsorption in La0.5Sr0.5CoO3 cobaltite

Giant radiofrequency magnetoabsorption in La_0.5Sr_0.5CoO₃ polycrystalline lanthanum cobaltite is studied in the temperature interval 77–300 K and 0.5–12.5 MHz frequency range. The specimen’s Curie temperature T _C is about 250 K. The magnetoabsorption is found to increase with decreasing frequency, reaching 58% at f = 0.5 MHz near the Curie temperature. The results are satisfactorily explained based on the known physical ideas of high-frequency wave absorption in magnetic materials. The effects of giant magnetoabsorption and rapid variation of the absorption near the Curie temperature can be used in radiofrequency sensors of the magnetic field and temperature.     

Photoinduced melting of antiferromagnetic order in La(0.5)Sr(1.5)MnO4 measured using ultrafast resonant soft x-ray diffraction

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.     

Tuning charge and orbital ordering in DyNiO3 by biaxial strain

The first-principles calculations were used to explore the tunable electronic structure in DyNiO₃ (DNO) under the effects of the biaxial compressive and tensile strains. We explored how the biaxial strain tunes the orbital hybridization and influences the charge and orbital ordering states. We found that breathing mode and Jahn-Teller distortion play a primary role in charge ordering state and orbital ordering state, respectively. Additionally, the calculated results revealed that the biaxial strain has the ability to manipulate the phase competition between the two states. A phase transition point has been found under tensile train. If the biaxial train is larger than the point, the system favors orbital ordering state. If the strain is smaller than the point, the system is in charge ordering state favorably.     

Structural properties and charge ordered states in RMnO3 (R=La, Pr, Nd, Ca, Sr) and (La, Sr)2NiO4

Structural distortions arising from the condensations of two essential kinds of phonon modes: the triply degenerate rotational modes (phix, phiy, phiz) of MnO(6) and the doubly degenerate Jahn-Teller active modes (Q1, Q2) have been systematically investigated in the perovskite manganites. Microstructural features associated with certain types of distortions have been observed by transmission electron microscopy (TEM). In RMnO(3) and La(Sr)(2)NiO(4), we characterize the local structure, charge ordered states and orbital ordering by means of low-temperature TEM. We present direct evidence that the stripe modulation in La(Sr)(2)NiO(4) is indeed one-dimensional within each NiO(2) plane. Several typical kinds of defect structures, including antiphase boundaries and the 90 degrees -twin domains, appear commonly in the charge-ordered states.     

Stabilization of charge ordering in La1/3Sr2/3FeO3-delta by magnetic exchange

The magnetic exchange energies in charge ordered La1/3Sr2/3FeO3-delta (LSFO) and its parent compound LaFeO3 (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+-Fe5+ pairs (JF) and antiferromagnetic exchange between Fe3+-Fe3+ pairs (JAF) fulfills the criterion for charge ordering driven by magnetic interactions (|JF/JAF|>1). The 30% reduction of JAF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.