1 Universität Paderborn, Department Physik, 33095 Paderborn, GERMANY

1. Magnetic and elastic properties of SrFeO3-x S. Janson1, Ch. Urban1, K. Rupprecht1, U. Ponkratz1,2, G. Wortmann1, T. Berthier3, W. Paulus3 1 Universität Paderborn, Department Physik, 33095 Paderborn, GERMANY 2 ESRF, 6 rue Jules Horowitz, 38043 Grenoble, FRANCE 3 Universitè Rennes, LCSIM, UMR 6511, F-35042 Rennes, FRANCE 1. Introduction / Aim 57Fe-Mössbauer spectroscopy is an extremely sensitive tool in analysing the chemical and physical properties of the different phases of the SrFeO3-x series, e.g. phase purity, Fe valence states (including a complex charge disproportionation of a Fe3.5+ state), as well as the rich variety of magnetic properties, depending sensitively on the valence state. This has been already demonstrated in 1964 [1] and in a lot of following publications [e.g. 2, 3]. Here we concentrated, beside the magnetic properties, on the elastic properties by analysing the local binding strength of the various Fe species in SrFeO3-x from the second-order Doppler shift (SOD), yielding local Debye temperatures. These are compared with the Debye temperatures obtained from nuclear inelastic scattering (NIS) of synchrotron radiation. The elastic properties of the end member of the SrFeO3-x series, SrFeO2.5, analysed in the same way, are described in another poster at this conference [4]. 2. Structural properties of SrFeO3-x depend on the oxygen content Sr2+ Fe3+,4+ O2- Fig.1c: Orthorhombic unit cell of SrFeO2.75:  2 different Fe sites: Fe4+ in octahedral and Fe3+ in pyramidal oxygen surrounding. Fig.1a: Cubic perowskit Structure of SrFeO3  Fe4+ cations in octahedral oxygen surrounding. Fig.1b: Tetragonal unit cell of SrFeO2.875: 3 different Fe sites  Fe4+ in plane with only octahedra, Fe3.5+ in plane with octahedral and pyramidal oxygen surroundings (electron hopping). 3. Mössbauer spectroscopy: Magnetic properties of SrFeO3-x 57Fe-Mössbauer spectroscopy was carried out with a 57Co(Rh) source, the absorbers were enriched with 30% 57Fe. The measurements were done in a coolfinger cryostat were the source remains at room temperature. SrFeO2.75 SrFeO3 SrFeO2.875 SrFeO3 SrFeO2.875 Fig.3: Magnetic hyperfine field Bhf of SrFeO3, SrFeO2.875 and SrFeO2.75. The red lines represent a Brilliouin fit to the data. SrFeO2.75 • Real stoichiometry of SrFeO3 is determined to SrFeO2.93 by analysis of the Fe4+ / Fe3+ subspectra ratio at 4 K. • For SrFeO2.875 a charge ordering of the Fe3.5+ component into Fe4+ and Fe3+ between 70 K and 75 K was observed, as described in Ref. [1]. • SrFeO2.75 shows no magnetic splitting of the Fe4+ sites  the magnetic structure at low temperatures is antiferromagnetic for Fe3+ with frustrated Spins for the Fe4+ component (see Fig. 4). Fig.2:57Fe Mössbauer spectra of SrFeO3, SrFeO2.875 and SrFeO2.75 at various temperatures. The red lines represent the Fe4+ component, the green lines the Fe3+. In the case of SrFeO2.875 the Fe3+ component is marked by green and blue lines and the light blue line represents the mixed valent Fe3.5+ site. Fig.4 (right): Modell of frustrated magnetism in SrFeO2.75 [2]. Antiferromagnetic exchange between Fe3+-ions, the Fe4+-ions remain in a non-ordered state. 4.Nuclear inelastic scattering (NIS) of synchrotron radiation on SrFeO2.93 • NIS experiments were carried out at beamline ID18 at the ESRF, Grenoble with an energy resolution of 3 meV. • From the NIS spectra the partial phonon density-of-states (DOS) at the Fe sites were calculated. • Energy resolution was 3 meV. • The spectral features of the DOS at 300 K as well as the derived parameters agree well with Ref. [5]. SrFeO2.93 Fig.8: Temperature dependence of the isomer shift S in SrFeO2.93 for the different Fe sites, yielding Debye temperatures of the Fe4+ and the Fe3+ sites. D(Fe3+) = 353 K D(Fe4+) = 551 K SrFeO2.93 References Fig.7:D of SrFeO2.93 from NIS. g(E) [1] P.K. Gallagher et al., J. Chem. Phys. 41, 2429 (1964). [2] J.P. Hodges et al., J. Solid State Chem. 151, 190 (2000). [3] A. Lebon et al., PRL 92, 032702 (2004). [4] Ch. Urban et al., poster to this conference. [5] W. Sturhahn et al., Phys. Rev. Lett. 74, 3832 (1995); W. Sturhahn and A.I. Chumakov, Hyp. Int.123/124, 809 (1999). [6] A.I. Rykov et al., Phys. Rev. B 68, 224401 (2003). [7] S. Janson, Diploma thesis (Paderborn 2005); S. Janson et al., in preparation. SrFeO2.75 SrFeO2.875 D(Fe3.5+) = 424 K D(Fe4+) = 540 K D(Fe3+) = 362 K D(Fe4+) = 660 K Fig.5:57Fe-NIS spectra of SrFeO2.93. Fig.9:Temperature dependence of S in SrFeO2.875 and SrFeO2.75, yielding D for the different Fe sites. Fig.6:Partial phonon DOS of SrFeO2.93 at various temperatures. 5. Conclusions SrFeO2.93: The NIS data reveal a Debye temperature D,HT = 500 K, in good agreement with the analysis of the SOD, giving a site-selective information with D(Fe4+) = 551 K and D(Fe3+) = 353 K. The temperature dependence of Fe phonon DOS show no anomaly in the position of the dominant mode at 20 meV at the magnetic ordering temperature TN, in contrast to the findings of a NIS study of Sr2FeCoO6-x [6]. There is only a slight anomaly in the variation of D,LT (see Fig. 7) and the corresponding sound velocuty [7]. SrFeO2.875: The analysis of the SOD delivers, with some restrictions for the Fe3.5+ sites, Debye temperatures of D(Fe4+) = 540 K and D(Fe3.5+) = 424 K, where the latter value is about in the middle between typical Fe4+ and Fe3+ values. The analysis of additional spectra in the complex transition region of the Fe3.5+ sites [3] with onset of magnetic order and charge disproportion into Fe3+ and Fe4+ sites is in progress [7]. SrFeO2.75: The ME spectra nicely display the unusual non-magnetic state of the frustrated Fe4+ site, even down to 2.1 K. Analysis of the SOD reveal of D(Fe4+) = 660 K and D(Fe3+) = 362 K; while the first very high value for Fe4+ sites eventually results from the difficult analysis of the unresolved quadrupole interaction, the latter one agrees excellently with the one obtained for octahedral Fe3+ in SrFeO2.5 [4].