Showing papers on "Charge ordering published in 2011"
TL;DR: Mid-infrared femtosecond pulses are used to enable coherent transport between the copper oxide planes of a cuprate superconductor, and an upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which places stringent new constraints on the understanding of stripe order and its relation to superconductivity.
Abstract: One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional "striped" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La(1.675)Eu(0.2)Sr(0.125)CuO(4), into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.
983 citations
TL;DR: Using a combination of HAADF-STEM imaging and atomically resolved EELS in an aberration-corrected TEM it is demonstrated the possibility of 2D atom by atom valence mapping in the mixed valence compound.
Abstract: Using a combination of HAADF-STEM imaging and atomically resolved EELS in an aberration-corrected TEM we demonstrate the possibility of 2D atom by atom valence mapping in the mixed valence compound . The ELNES from and cation sites are similar to those of and references. Comparison with simulations shows that even though a local interpretation is valid here, intermixing of the inelastic signal plays a significant role. This type of experiments should be applicable to challenging topics in materials science, like the investigation of charge ordering or single atom column oxidation states in e.g. dislocations.
123 citations
TL;DR: In this paper, the magnetic, dielectric, and magnetodielectric properties of La2CoMnO6 ceramics were evaluated together with the polarization-electric field hysteresis loops.
Abstract: The magnetic, dielectric, and magnetodielectric properties of La2CoMnO6 ceramics were evaluated together with the polarization-electric field hysteresis loops. The phase segregation consisting of ordered and disordered regions was determined in the present ceramics. The ordering of Co2+ and Mn4+ gave rise to ferromagnetic transition temperature as high as 210 K, while the disordering of Co3+ and Mn3+ resulted in low ferromagnetic transition temperature of 80 and 150 K. The relaxor-like behavior combined with a giant dielectric constant (∼105) was determined in La2CoMnO6 ceramics, which was attributed to the charge ordering of Co2+ and Mn4+. Owing to the mutual origin of magnetism and dielectric relaxation, La2CoMnO6 ceramics showed considerable magnetodielectric effects (∼0.8% at 10 kOe) at room temperature.
118 citations
TL;DR: In this article, a phase diagram for stripelike charge ordering is obtained together with information on the structural transition into the low-temperature tetragonal phase, where a clear dome for the charge ordering around $x=\frac{1}{8}$ is detected well below the structural phase.
Abstract: Resonant soft x-ray scattering experiments with photon energies near the O $K$ and the Cu ${L}_{3}$ edge on the system La${}_{1.8\ensuremath{-}x}$Eu${}_{0.2}$Sr${}_{x}$CuO${}_{4}$ for $0.1\ensuremath{\leqslant}x\ensuremath{\leqslant}0.15$ are presented. A phase diagram for stripelike charge ordering is obtained together with information on the structural transition into the low-temperature tetragonal phase. A clear dome for the charge ordering around $x=\frac{1}{8}$ is detected well below the structural transition. This result differs substantially from other systems in which static stripes are detected. There the charge order is determined by the structural transition appearing at the same temperature. Furthermore, we present results for the coherence length and the incommensurability of the stripe order as a function of Sr concentration.
99 citations
TL;DR: In this article, the basic structural chemistry of superconducting pnictides and oxides is reviewed with respect to phase transitions upon charge-density formation, the ordering of vacancies, or the ordered displacements of oxygen atoms.
Abstract: The basic structural chemistry of superconducting pnictides and pnictide oxides is reviewed. Crystal chemical details of selected compounds and group subgroup schemes are discussed with respect to phase transitions upon charge-density formation, the ordering of vacancies, or the ordered displacements of oxygen atoms. Furthermore, the influences of doping and solid solutions on the valence electron concentration are discussed in order to highlight the structural and electronic flexibility of these materials.
92 citations
TL;DR: While the Landau theory has general applicability, it is shown that for the most conducting materials, R=Pr, Nd, both types of order can be understood in terms of a nearly nested spin-density wave.
Abstract: Guided by experiment and band structure, we introduce and study a phenomenological Landau theory for the unusual charge and spin ordering associated with the Mott transition in the perovskite nickelates, with chemical formula $R{\mathrm{NiO}}_{3}$, where $R=\mathrm{Pr} $, Nd,Sm, Eu, Ho, Y, and Lu. While the Landau theory has general applicability, we show that for the most conducting materials, $R=\mathrm{Pr} $, Nd, both types of order can be understood in terms of a nearly nested spin-density wave. Furthermore, we argue that in this regime, the charge ordering is reliant upon the orthorhombic symmetry of the sample, and therefore proportional to the magnitude of the orthorhombic distortion. The first order nature of the phase transitions is also explained. We briefly show by example how the theory is readily adapted to modified geometries such as nickelate films.
92 citations
TL;DR: A thorough investigation of single crystalline magnetite using broadband dielectric spectroscopy and other methods provides evidence for relaxor-like polar order in Fe3O4 as discussed by the authors.
Abstract: A thorough investigation of single crystalline magnetite using broadband dielectric spectroscopy and other methods provides evidence for relaxor-like polar order in Fe3O4. We find long-range ferroelectric order to be im-peded by the continuous freezing of polar degrees of freedom and the formation of a tunneling-dominated glasslike state at low temperatures. This also explains the lack of clear evidence for a non-centrosymmetric crystal structure below the Verwey transition. Within the framework of recent models assuming an intimate relation of charge and polar order, the charge order, too, can be speculated to be of short-range type only and to be dominated by tunneling at low temperatures.
47 citations
TL;DR: In this paper, an extended Hubbard model with the nearest-neighbor Coulomb interaction on the pyrochlore lattice at half filling was studied and an interaction-driven topological phase with nontrivial Z 2 invariants emerged at the Hartree-Fock mean-field level in the phase diagram.
Abstract: We study an extended Hubbard model with the nearest-neighbor Coulomb interaction on the pyrochlore lattice at half filling. An interaction-driven insulating phase with nontrivial Z 2 invariants emerges at the Hartree–Fock mean-field level in the phase diagram. This topological insulator phase competes with other ordered states and survives in a parameter region surrounded by a semimetal, antiferromagnetic and charge ordered insulators. The symmetries of these phases are group-theoretically analyzed. We also show that the ferromagnetic interaction enhances the stability of the topological phase.
43 citations
TL;DR: In this paper, a time-resolved RSXD experiment from the prototypical correlated electron material magnetite using soft x-ray pulses from the free-electron laser FLASH in Hamburg is presented.
Abstract: Resonant soft x-ray diffraction (RSXD) with femtosecond (fs) time resolution is a powerful tool for disentangling the interplay between different degrees of freedom in strongly correlated electron materials. It allows addressing the coupling of particular degrees of freedom upon an external selective perturbation, e.g., by an optical or infrared laser pulse. Here, we report a time-resolved RSXD experiment from the prototypical correlated electron material magnetite using soft x-ray pulses from the free-electron laser FLASH in Hamburg. We observe ultrafast melting of the charge-orbital order leading to the formation of a transient phase, which has not been observed in equilibrium.
37 citations
TL;DR: Magnetic reversal of an artificial square ice pattern subject to a sequence of magnetic fields applied slightly off the diagonal axis is investigated via magnetic force microscopy of the remanent states that result as discussed by the authors.
Abstract: Magnetic reversal of an artificial square ice pattern subject to a sequence of magnetic fields applied slightly off the diagonal axis is investigated via magnetic force microscopy of the remanent states that result. Sublattice independent reversal is observed via correlated incrementally pinned flip cascades along parallel dipolar chains, as evident from analysis of vertex populations and dipolar correlation functions. Weak dipolar interactions between adjacent chains favour antialignment and give rise to weak charge ordering of 'monopole' vertices during the reversal process.
TL;DR: In this paper, the authors investigated the charge ordering (CO) behavior of La 0.5−xBixCa0.5MnO3 (x=0, 1/16, 1 16, 1 8, 1 4, 3/8 and 1/2) compounds and found that the CO temperature increases with substitution of Bi3+ ion for La 3+ ion.
Abstract: Electronic transport properties of La0.5−xBixCa0.5MnO3 (x=0, 1/16, 1/8, 1/4, 3/8 and 1/2) compounds have been studied systematically to investigate their charge ordering (CO) behaviors. The results show that the CO temperature increases with the substitution of Bi3+ ion for La3+ ion, suggesting that the charge ordering is enhanced. This is attributed to the special role of the 6s2 lone pair of Bi3+. It is found that for all the samples the adiabatic small polaronic conduction mechanism is responsible for the transport behavior above CO transition, whereas Mott's variable range hopping mechanism dominates below the CO transition. In addition, the electronic transport behavior of La0.5−xBixCa0.5MnO3 compounds is high sensitive to an external magnetic field, which could raise fresh opportunities for application in magnetic sensors.
TL;DR: In this article, the effect of applied dc bias electric field on dielectric permittivity in bulk Y2NiMnO6 was investigated and it was found that a small bias field of 40 V/cm can greatly reduce the dielectrics around the room temperature, compared to the much larger electric field that is required for conventional ferroelectric materials.
Abstract: The effect of applied dc bias electric field on dielectric permittivity in bulk Y2NiMnO6 is investigated in this paper. It is found that a small bias field of 40 V/cm can greatly reduce the dielectric permittivity around the room temperature, compared to the much larger electric field that is required for conventional ferroelectric materials. The observed giant dielectric tunability is retained over a broad range of around room temperature and is most likely related to the charge ordering of Ni2+ and Mn4+ ions. This may further confirm the existence of electronic ferroelectricity in Y2NiMnO6.
TL;DR: For the first time, a switching from negative to positive TMR was observed across the Verwey transition, with a strong enhancement of TMR at low temperatures.
Abstract: Magnetic tunnel junctions sandwiching a superlattice thin film of iron oxide nanocrystals (NCs) have been investigated. The transport was found to be controlled by Coulomb blockade and single-electron tunneling, already at room temperature. A good correlation was identified to hold between the tunnel magnetoresistance (TMR), the expected magnetic properties of the NC arrays, the charging energies evaluated from current-voltage curves, and the temperature dependence of the junction resistance. Notably, for the first time, a switching from negative to positive TMR was observed across the Verwey transition, with a strong enhancement of TMR at low temperatures.
TL;DR: In this article, the origin of complex phase transitions in an electron-doped manganite was investigated using high-resolution photoemission spectroscopy and transmission electron microscopy.
Abstract: We study the origin of complex phase transitions in an electron-doped manganite, La${}_{0.2}$Sr${}_{0.8}$MnO${}_{3}$, using high-resolution photoemission spectroscopy and transmission electron microscopy. The results reveal evidence of phase coexistence and charge ordering in an intermediate temperature range unexpected in this high bandwidth system. The charge ordering nucleates above the phase transition temperature of 265 K, indicating a precursor effect. The high-resolution photoemission spectra exhibit unusual chemical potential shift and persistence of quasiparticles within the insulating phase below 265 K, where the charge-ordered phase dominates and hysteresis is observed in the resistivity. These results provide insight in the study of correlated electron systems exhibiting complex behaviors.
TL;DR: Well-defined magnetite nanocrystals with controllable size, shape and terminated surface are first employed to elucidate the first-order metal-insulator transition in magnetite, and new discoveries such as a highly suppressed phase transition temperature are identified by monitoring the variable-temperature electric resistance and infrared spectroscopy.
Abstract: The first-order metal–insulator transition (MIT) in magnetite has been known for a long time but is still controversial in its nature. In this study, well-defined magnetite nanocrystals (NCs) with controllable size, shape and terminated surface are first employed to elucidate this important issue, and new discoveries such as a highly suppressed phase transition temperature are identified by monitoring the variable-temperature electric resistance and infrared spectroscopy. Significantly, by carefully comparing the infrared vibrational bands of the as-prepared magnetite NCs with octahedral and cubic shapes, respectively, we found that these two forms of magnetite NCs exhibited different transmittance changes and frequency shifts of the infrared characteristics, presumably due to the differences in the lattice distortions on the corresponding {001} and {111} terminal surfaces. This result produced evidence in support of the charge ordering of Fe atoms along the low dimensionality at octahedral B sites undergoing the MIT. Taken together, infrared identification was proposed to be an available characterization strategy for MIT, which can reflect more information on the elusive lattice distortion of crystallographic structure or exposed surfaces.
TL;DR: In this paper, the crystal and magnetic structure of polycrystalline La 0.5−nd x Nd x Ca 0.5 MnO 3 (0.0 ≤ x ≤ 0. 5 ) samples have been investigated using magnetization, resistivity, transmission electron microscope, and neutron diffraction techniques.
TL;DR: In this paper, X-ray diffraction profiles were indexed with a tetragonal and orthorhombic structure for Ca 2 MnO 4 and Pr-doped compounds, respectively.
TL;DR: The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm and a second-order phase transition is revealed, which leads to charge ordering with transversal and vertical displacements and complex interchain correlations.
Abstract: The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm. By scanning tunneling microscopy and electron diffraction we reveal a second-order phase transition at 585 K. It leads to charge ordering with transversal and vertical displacements and complex interchain correlations. However, the structural phase transition is not accompanied by the electronic signatures of a charge density wave, thus precluding a Peierls instability as origin. Instead, this symmetry-breaking transition exhibits three-dimensional critical behavior. This reflects a dichotomy between the decoupled 1D electron system and the structural elements that interact via the substrate. Such substrate-mediated coupling between the wires thus appears to have been underestimated also in related chain systems.
TL;DR: In this article, a comparative study of the nanowires and nanoparticles of Ca 0.82La 0.18MnO3 was conducted, finding that the nanoparticles have similar magnetic properties to the bulk, whereas the nan wires behave evidently different.
Abstract: A comparative investigation has been conducted on the nanowires and nanoparticles (both in the size range of 20–35 nm) of Ca0.82La0.18MnO3, finding that the nanowires have similar magnetic properties to the bulk, whereas the nanoparticles behave evidently different. In the nanoparticles, charge ordering and antiferromagnetic phase disappear; instead, a ferromagnetic transition is observed. Analysis of the crystal structure indicates that, for nanosized manganite systems, whether charge ordering is suppressed and ferromagnetism is developed as the size scales down depends on the level of intrinsic structural distortions; nanodimensional effect or surface effect is not determinant.
TL;DR: In this article, structural and magnetic properties of pure and Co-doped Fe3O4(100) films were investigated, and it was found that the Fe 3O4 surface is blocked by Co doping due to the decrease of Fe2+ in the B site.
Abstract: Structural and magnetic properties, and the valence band structure of pure and Co-doped (up to 33%) Fe3O4(100) films were investigated. Reconstruction of the Fe3O4(100) surface is found to be blocked by Co doping. Doped Co ions in Fe3O4 are in a charge state of 2 + and substitute the Fe2+ in the B site of Fe3O4. All the films exhibit room temperature ferromagnetism. Co doping changes the coercivity and reduces saturation magnetization. The density of states near the Fermi level is reduced by Co doping due to the decrease of Fe2+ in the B site, which might responsible for the decrease in conductivity and magnetoresistance of Co-doped Fe3O4. The Verwey transition in the range of 100–120 K is observed for the pure Fe3O4 film, while no transition could be detected for Co-doped Fe3O4 films.
TL;DR: In this article, it was shown that the Fermi temperature scale of the homogeneous metal vanishes at the quantum phase transition to the ordered metallic phase driven by the Coulomb repulsion.
Abstract: by geometrical frustration. A comparison with the spinless model indicates that renormalization eects in the homogeneous metallic phase close to charge ordering are mainly due to charge rather than spin correlations. Spin degeneracy is, however, essential to describe the dependence of the system on geometrical frustration. Based on nite temperature Lanczos diagonalization we nd that the eective Fermi temperature scale, T , of the homogeneous metal vanishes at the quantum phase transition to the ordered metallic phase driven by the Coulomb repulsion. Above this temperature scale ’bad’ metallic behavior is found which is robust against geometrical frustration in general. Quantum critical phenomena are not found whenever nesting of the Fermi surface is strong, possibly indicating a rst order transition instead. ’Reentrant’ behavior in the phase diagram is encountered whenever the 2kF -CDW instability competes with the Coulomb driven three-fold charge order transition. The relevance of our results to the family of quarter-lled materials: -(BEDT-TTF)2X is discussed.
TL;DR: In this article, a transition between cos2θ dependent magnetoresistance (AMR) in an insulating state at low fields and sin 2 θ dependent AMR in a metal state at high fields is observed at intermediate fields, depending on the temperature and/or strength of an applied magnetic field.
Abstract: Angular dependent magnetoresistance (AMR) phenomena in Pr0.7Ca0.3MnO3 film have been investigated. A transition between cos2θ dependent AMR in an insulating state at low fields and sin2θ dependent AMR in a metal state at high fields is observed at intermediate fields, depending on the temperature and/or strength of an applied magnetic field. Although the AMR sign evolution process from cos2θ dependence to sin2θ dependence at low temperature is different from that at high temperature due to existence of ferromagnetic insulator besides charge ordering antiferromagnetic insulator, we believe that such AMR sign evolutions are closely related with magnetic-field-induced ferromagnetic metal percolation behavior.
TL;DR: An explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit is presented, which relies on the key effect of charge ordering.
Abstract: We present an explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit, which relies on the key effect of charge ordering Blocking a significant fraction of the lattice sites deeply modifies the electronic structure in a way that is shown to be quantitatively consistent with photoemission experiments It also makes the system highly sensitive to interactions (especially to intersite ones), hence accounting for the strong correlations effects observed in this regime, such as the high effective mass and quasiparticle scattering rate These conclusions are supported by a theoretical study of an extended Hubbard model with a realistic band structure on an effective kagome lattice
TL;DR: In this paper, Swedenborgite-type cobaltites and ferrites and derivatives, represent a very important class of oxides, LnBaM4O7 and CaBaM 4O7 (M = Co, Fe), where the transition element exhibits exclusively tetrahedral coordination.
Abstract: Swedenborgite-type cobaltites and ferrites and derivatives, represent a very important class of oxides, LnBaM4O7 and CaBaM4O7 (M = Co, Fe). They are one of the rare families of mixed valent transition metal oxides, where the transition element exhibits exclusively the tetrahedral coordination. Their complex crystal chemistry, involving closely related structures with various symmetries (orthorhombic, hexagonal cubic) and possibility of oxygen “hyperstoichiometry”, leading to LnBaM4O7+δ oxides is described herein, as well as their ability to exhibit structural transitions vs. temperature. It is also shown that the triangular geometry of these new strongly electron correlated systems, plays an important role in their physics, particularly in the competition that appears between the 1D magnetic ordering and the 2D magnetic frustration. The generation of new magnetodielectric properties for CaBaCo4O7 is also emphasized.
TL;DR: In this paper, the anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS2 and TbTe3 is investigated by means of multiple-pump pulse time-resolved femtosecond optical spectroscopy.
Abstract: The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS2 and TbTe3 is investigated by means of multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe remarkable anharmonic effects hitherto undetected in systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent in the broken symmetry state of the CDW systems.
TL;DR: Charge, orbital, and spin ordering of multiferroic BiMn(2)O(5) are investigated by the full-potential linearized augmented plane-wave (FPLAPW) method as implemented in the WIEN2K package.
Abstract: Charge, orbital, and spin ordering of multiferroic BiMn2O5 are investigated by the full-potential linearized augmented plane-wave (FPLAPW) method as implemented in the WIEN2K package. Both the generalized gradient approximation (GGA) as well as GGA plus the one-site Coulomb interaction (GGA+U) methods are considered for the exchange–correlation energy functional. The obtained results show that BiMn2O5 is found stable in ferrimagnetic state with band gap about 1.23 eV. The results suggest that BiMn2O5 contains two kinds of manganese: the ionicity of Mn1 (Mn4+) is +3.6 with magnetic moment of 2.40 μB and the ionicity of Mn2 (Mn3+) is +3.4 with magnetic moment of 3.22 μB. While charge disproportion between Mn1 and Mn2 is small, the difference between eg minority occupancies of Mn3+ and Mn4+ cations is large. Both these two properties give direct evidence of charge ordering. The analysis of the Born effective charge reveals that the partial ferroelectric polarization (Pele) originates from the charge ordering, which is in agreement with a recent work by Brink and Khomskii [J. Phys.: Condens. Matter, 2008, 20, 434217].
TL;DR: In this article, low-temperature infrared measurements of magnetite (Fe3O4) thin films on Si (111) substrate across the Verwey transition temperature (TV) were studied as a function of temperature.
Abstract: We present low-temperature infrared measurements of magnetite (Fe3O4) thin films on Si (111) substrate across the Verwey transition temperature (TV). The line parameters of the most intense t1u mode observed in the Fourier transform infra red spectrum of the film is studied as a function of temperature. We observe that mode frequency increases abruptly at 130 K, and full width at half maxima of the mode increases abruptly at 113 K. The observations point out that structural transition may start earlier at 130 K than the actual Verwey transition (121 K) and that complete at 113 K. These results are consistent with the higher transition temperature as observed in resistivity and magnetization measurements.
TL;DR: A dip-shaped anomaly appearing in the infrared spectrum of charge-transfer organic complexes has been investigated in this article, where the anomaly appears at approximately the same frequency (approximately 2700 cm), irrespective of light polarization as well as a composition of the complex, when the compounds undergo charge ordering.
Abstract: A dip-shaped anomaly appearing in the infrared spectrum of charge-transfer organic complexes has been investigated. The anomaly appears at approximately the same frequency (\ensuremath{\sim}2700 cm${}^{\ensuremath{-}1}$), irrespective of light polarization as well as a composition of the complex, when the compounds undergo charge ordering. Isotope-shift measurements for \ensuremath{\theta}-(BEDT-TTF)${}_{2}$RbZn(SCN)${}_{4}$ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] indicates a relationship between the overtone of a C=C stretching mode of the BEDT-TTF molecule and this anomalous signal. Calculations of electron-molecular vibration coupling based on a diatomic molecular dimer model reveals that the overtone is activated by an anharmonicity developed in the adiabatic potential in a charge-separated system. It is presented that numerical calculation based on the simple cluster model reproduces essential features of the experimentally obtained conductivity spectrum.
TL;DR: In this article, the magnetic properties of La1−x� Ca x MnO3 samples with an equivalent average particle size ~50nm prepared by a sol-gel method were investigated.
Abstract: Magnetic properties of La1−x
Ca
x
MnO3 (0.5 ≤ x ≤ 0.8) samples with an equivalent average particle size ~50 nm prepared by a sol–gel method were investigated. The charge ordering (CO) transition that is observed in the bulks disappears and the ferromagnetic (FM) transition occurs in all the prepared samples. For all the samples, the spontaneous magnetization (M
S) value is much lower than the corresponding theoretic value, which shows that the majority of the sample is antiferromagnetic (AFM). However, the M
S value is much larger than the corresponding value reported by some other groups. The invisible of CO transition and the large M
S value can be attributed to the good connection among the adjacent particles. Moreover, the exchange bias (EB) phenomenon is observed except the x = 0.5 sample. With x increasing, the M
S value decreases and the EB field increases, which can be understood by considering the coexistence of FM phase with Mn3+–Mn4+ spin clusters in the shell and the AFM phase in the core of the nanoparticles.