Site-specific magnetization reversal studies of magnetite

5 pages, 3 figures, 1 table.-- et al.The mechanism of magnetization reversal in magnetite Fe3O4 single crystals was studied using sitespecific magnetic sensitive diffraction anomalous near-edge structure. By exploiting the angular dependence of the cross section, we are able to show that the mechanism of reversal involves a mixture of coherent rotation and domain formation. The results reveal additional details to that provided by XMCD measurements, which average over nonequivalent sites.We thank J. W. Freeland, E. Kravtsov, and D. R. Lee for useful discussions of the results. Work at the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Sciences, under Contract No. W-31-109-ENG-38.Peer reviewe

Stability of Jahn-Teller distortion ordering in LaMn1-x ScxO3

We have investigated the role of Mn3+ Jahn-Teller distortion in driving ferromagnetism in the LaMn1-xScxO3 series. The replacement of Mn by Sc in LaMnO3 decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the LaScO3 sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in LaMnO3 into a nearly regular one in LaScO3. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc K edges reveal the stability of both Jahn-Teller distorted MnO6 octahedron and nearly regular ScO6 octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted MnO6 octahedra spatially ordered as in LaMnO3 and nearly regular ScO6 octahedra. This result contrasts with the pseudocubic phase and the appearance of regular MnO6 octahedra in LaMn1-xGaxO3 for x>0.5. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the Mn3+ atom is allocated. Besides, a ferromagnetic ground state is observed for x>0.5 in both series independently of the presence (or not) of Jahn-Teller distortions around Mn3+, which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in LaMn1-xBxO3 (B=Sc or Ga).The authors thank ESRF, ALBA, and Elettra Synchrotrons for granting beam time. Financial support from the Spanish MINECO (Projects No. MAT2012-38213-C02-01 and No. MAT2011-23791) and the Diputacion General de Aragón (CAMRADS) are acknowledged.Peer Reviewe

On the correlation between the X-ray absorption chemical shift and the formal valence state in mixed-valence manganites

7 páginas, 7 figuras.Here the correlation between the chemical shift in X-ray absorption spectroscopy, the geometrical structure and the formal valence state of the Mn atom in mixed-valence manganites are discussed. It is shown that this empirical correlation can be reliably used to determine the formal valence of Mn, using either X-ray absorption spectroscopy or resonant X-ray scattering techniques. The difficulties in obtaining a reliable comparison between experimental XANES spectra and theoretical simulations on an absolute energy scale are revealed. It is concluded that the contributions from the electronic occupation and the local structure to the XANES spectra cannot be separated either experimentally or theoretically. In this way the geometrical and electronic structure of the Mn atom in mixed-valence manganites cannot be described as a bimodal distribution of the formal integer Mn3+ and Mn4+ valence states corresponding to the undoped references.This work was supported by the Spanish MICINN FIS2008- 03951 project and Diputación General de Aragón (DGACAMRADS). VC thanks the FPU research grant from MICINN.Peer reviewe

Resonant x-ray scattering study of charge superstructures in layered L a2-x C axCo O4±d (0.4=x=0.7) and L a1.5 S r0.5Co O4 compounds

We propose a model for the Co charge disproportionation in La2-xCaxCoO4±d (0.4=x=0.7) and La1.5Sr0.5CoO4 layered cobaltites based on the photon energy, azimuthal angle, and polarization dependence of the scattered intensity of selected reflections in a resonant x-ray scattering experiment. Tetragonal superlattice (h/2, h/2, l)-type reflections were detected at room temperature for all the samples independently from the Ca (or Sr) doping rate in agreement with a checkerboard ordering of two different Co sites. The corresponding average charge disproportion is accounted for by a semiempirical model from which the imaginary part of the resonant atomic scattering factors of each of the two Co atoms is obtained resulting in about 0.5±0.1 electron. Moreover, no forbidden (h/4, h/4, l)-type reflections were observed at room temperature indicating the lack of any local anisotropy ordering in contrast with the behavior found in the related layered manganites. Finally, we found the pattern of small distortions (~0.05Å) around the two Co sites compatible with the resonant x-ray scattering results. The symmetry of this displacement pattern is consistent with the A2mm (or Ammm) orthorhombic structure of the ordered phase and the structural transition is accounted for by the condensation of two soft modes - X1+(B2u) and X1+(A1) - acting on the oxygen atoms

High-resolution Mn K-edge x-ray emission and absorption spectroscopy study of the electronic and local structure of the three different phases in Nd0.5Sr0.5MnO3

Nd0.5Sr0.5MnO3 is particularly representative of mixed-valent manganites since their three characteristic macroscopic phases (charge-ordered insulator, ferromagnetic-metallic, and paramagnetic insulator) appear at different temperatures.We here report a complete x-ray emission and absorption spectroscopy (XES-XAS) study of Nd0.5Sr0.5MnO3 as a function of temperature to investigate the electronic and local structure changes of the Mn atom in these three phases. Compared with the differences in the XES-XAS spectra between Nd0.5Sr0.5MnO3 and the single-valent reference compounds NdMnO3 (Mn3+) and Sr/CaMnO3 (Mn4+), only modest changes have been obtained across the magnetoelectrical transitions. The XES spectra, including both the Mn Kα and Kβ emission lines, have mainly shown a subtle decrease in the local spin density accompanying the passage to the ferromagnetic-metallic phase. For the same phase, the small intensity variations in the pre-edge region of the high-resolution XAS spectra reflect an increase of the p-d mixing. The analysis of these XAS spectra imply a charge segregation between the two different Mn sites far from one electron, being in intermediate valences Mn+3.5±δ/2(δ < 0.2 e−) for all the phases. Our results indicate that the spin, charge, and geometrical structure of the Mn atom hardly change among the three macroscopic phases demonstrating the strong competition between the ferromagnetic conductor and the charge-0rdered insulator behaviors in the manganites

High-pressure transformation in the cobalt spinel ferrites

We report high pressure angle-dispersive x-ray diffraction measurements on CoxFe3−xO4 (x=1, 1.5, 1.75) spinels at room temperature up to 34 GPa. The three samples show a similar structural phase transformation from the cubic spinel structure to an analogous post-spinel phase at around 20 GPa. Spinel and post-spinel phases coexist in a wide pressure range (~20–25 GPa) and the transformation is irreversible. The equation of state of the three cubic spinel ferrites was determined and our results agree with the data obtained in related oxide spinels showing the role of the pressure-transmitting medium for the accurate determination of the equation of state. Measurements releasing pressure revealed that the post-spinel phase is stable down to 4 GPa when it decomposes yielding a new phase with poor crystallinity. Later compression does not recover either the spinel or the post-spinel phases. This phase transformation induced by pressure explains the irreversible lost of the ferrimagnetic behavior reported in these spinels.Financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) (Projects no. MAT2012-38213-C02-01) and Diputación General de Aragón (DGA-CAMRADS) is acknowledged.Peer Reviewe

Resonant x-ray scattering in La1-xSr1+xMn0 4 (x 0.5): Incommensurate-lattice modulation vs. Charge-stripe models

Using resonant x-ray scattering at the Mn K-edge, we have investigated the nature of the charge and lattice modulation in the La1-xSr1+xMnO4 (x=0.5 and 0.6) manganites. Resonant reflections (h±e, h±e, 0) and (h±2e, h±2e, 0) of the tetragonal I4/mmm structure with a modulation vector of 2e=l-x were found in the insulating phases of both manganites but the intensity of these reflections is much weaker for La0.4Sr1.6MnO4. Resonant x-ray scattering data for the two samples are well explained by the presence of two types of sinusoidal modulations of the oxygen displacements, transverse and longitudinal to the tetragonal [110] direction. The amplitude of the oxygen displacements for any of the modulations decreases with the hole doping, in agreement with the change from a commensurate (x=0.5) to an incommensurate (x=0.6) ordered phase. The different polarization and azimuthal behaviour of the two sets of resonant reflections rule out any kind of stripe model composed by Mn3+-like and Mn4+-like charge-ordering. The maximum charge disproportionation among the different Mn atoms in the unit cell is about 0.15 e- and 0.04 e- for the x=0.5 and x=0.6 samples, respectively. These results thus confirm the existence of a charge-density-wave ordering in both the commensurate-phase of the half-doped La0.5Sr1.5MnO4 and the incommensurate-phase of the over-doped La0.4Sr1.6MnO4 manganites

Effects of substituting Mn with Ga in the multiferroic properties of TbMnO3

We here report the characterization of the crystal structure, magnetic properties and specific heat of TbMn1-xGaxO3 samples (x≤0.4). Metamagnetic transition was observed at low temperature for all compounds. Heat capacity was used to test the effect of Ga on the different phase transitions. The substitution of Ga for Mn is detrimental for the antiferromagnetic ordering of Mn3+ showing a continuous decrease of TN with increasing the Ga content. This substitution also decreases the entropy content of the transition due to the ordering of Tb3+moments. The anomaly ascribed to the ferroelectric transition is absent at zero field in Ga-based samples but it reappears at high magnetic fields in compounds with low Ga content. This result suggests that an external magnetic field induces ferroelectricity in the doped compounds.The authors acknowledge financial support from CICyT project FIS08-03951 and DGA (Camrads). V.C. thanks the FPU research grant from MICINN.Peer Reviewe

Magnetic order and magnetic properties of the oxygen deficient SmBaMn2O5 layered perovskite

Magnetism in SmBaMn2O5 was investigated on a single crystal by magnetic and neutron diffraction measurements. This is an oxygen deficient perovskite with a layered ordering of Sm and Ba cations. Mn atoms are coordinated with five oxygens forming a square pyramid and they are ordered in a checkerboard pattern of expanded-compressed pyramids in the ab-plane. The neutron diffraction study revealed a ferrimagnetic ordering of Mn moments below TN=134 K. Macroscopic measurements reveal a very anisotropic behavior. Measurements with the external magnetic field parallel (M||c) and perpendicular (M¿c) to the c-axis confirm that this is the easy axis above 10 K. Below this temperature, the Sm sublattice begins to polarize and the magnetization M||c decreases while M¿c experiences a huge increase. This indicates that Sm moments begin to order around 10 K in the ab-plane with a minor component on the c-axis that opposes the overall magnetization from Mn sublattices