1. Electronic phase separation in cobaltate perovskites Z. Németh, Z. Klencsár, Z. Homonnay, E. Kuzmann, A. Vértes Institute of Chemistry, Eötvös Loránd University, Budapest 1518 P.O.Box 32, Hungary Research Group of Chemical Research Center Hungarian Academy of Sciences at ELTE, Budapest 1518 P.O.Box 32, Hungary E-mail: hentes@chem.elte.hu

2. The ongoing interest in doped transition metal oxides is due to their complex electronic and magnetic structures which result in a series of intriguing phenomena such as high-temperature superconductivity and colossal magnetoresistance. One family of the promising magnetoresistive materials are LaCoO 3 based perovskites doped with divalent ions (e.g. Ca, Sr) at the rare-earth site, the latter resulting electron holes in the transition metal network (i.e. converting some Co 3+ ions into Co 4+ ) as well as oxygen vacancies. Consequently, the conductivity of Sr doped LaCoO 3 increases with strontium concentration. The classification of the magnetic states of La 1-x Sr x CoO 3 perovskites is even more problematic, because magnetic phase separation occurs from very low Sr concentrations onwards. Following the observation of the coexistence of ferromagnetic-like and spin glass properties in these perovskites, it was suggested that the magnetic behavior of La 1-x Sr x CoO 3 perovskites can be described as “glassy ferromagnetism”. That is, at low Sr doping levels only short range magnetic correlation is formed below the magnetic transition temperature, with a characteristic coherence length of a few nanometers. At a lower temperature the magnetization freezes out to some locally preferred direction, and the material enters a glassy phase. The proportion of magnetic clusters as well as the magnetic coherence length increases with increasing Sr doping level, and at about x = 0.18 the clusters coalesce, giving rise to a metallic and unconventional ferromagnetic state, which can be described as the coexistence of coalesced, long range ordered (with several hundred nm long magnetic coherence) magnetic clusters and isolated magnetically disordered regions. The ferromagnetic and conductive nature of the clusters is related to the double exchange process between intermediate-spin trivalent and low-spin tetravalent cobalt ions.

3. SG: spin glass, PS: paramagnetic semiconductor, FM: ferromagnetic metal, PM: paramagnetic metal Magnetic phase diagram of La 1-x Sr x CoO 3 perovskites Figure taken from J. Wu and C. Leighton PRB 67 174408 (2003)

4. The magnetoresistance (MR) of La 1-x Sr x CoO 3 depends strongly on the Sr doping level. While for x ≥ 0.2 a dominant MR peak was observed around the Curie temperature, for x ≤ 0.18 the MR was found to increase with decreasing temperature. For intermediate doping rates (0.18 ≤ x ≤ 0.2) the combination of the two types of magnetoresistance was found. Doping La 1-x Sr x CoO 3 perovskites at the transition metal site with iron ions influences magnetoresistance, as well. The alteration of the MR properties due to the insertion of iron can be interpreted either by the modification of the magnetic field induced low-spin to high-spin transition of Co 3+ ions, or by the changes in size distribution of the ferromagnetic clusters. Apart from the x and y concentrations of doping ions, the magnetic and electronic properties of La 1-x Sr x Fe y Co 1-y O 3-  are also expected to depend sensitively on oxygen deficiency. As the procedure of preparation as well as the type(s) and concentration(s) of applied dopant ions may well have an influence on the value of , the elucidation of the effect of oxygen deficiency on the electronic and magnetic structure of La 1-x Sr x CoO 3-  perovskites is also highly desirable. Figure taken from J. Wu and C. Leighton PRB 67 174408 (2003)

5. Effect of Sr on Mössbauer spectra of La 1-x Sr x 57 Fe 0.025 Co 0.975 O 3-  at 80 K left spectra (more Sr): FM + PM ↔ right spectra: FM with relaxation (SPM) + PM

6. Effect of oxygen vacancy and Fe doping on Mössbauer spectra of La 0.8 Sr 0.2 Fe y Co 1-y O 3-  at 80 K FM component decreases and relaxation increases with increasing  and y

7. Effect of T on Mössbauer spectra of La 0.8 Sr 0.2 57 Fe 0.05 Co 0.95 O 3-  at 80 K below T b (see next slide): FM with strong relaxation (SPM) + PM

8. Magnetic phase diagram of La 0.8 Sr 0.2 57 Fe y Co 1-y O 3-  SCG: spin cluster glass, T b : onset temp. of FM clusters,T f : freezing temp.

9. Isomer shifts vs. T of La 0.8 Sr 0.2 57 Fe y Co 1-y O 3- . Electronic phase separation!

10. Summary Mössbauer spectra were found to be very sensitive to the Sr and Fe doping driven magnetic transitions. With the help of Mössbauer spectroscopy the magnetic phase diagram for iron doping could be drawn. Besides the well-known magnetic phase separation, isomer shifts of Mössbauer spectra proved electronic phase separation, as well, a feature only predicted so far. Németh Z, Klencsár Z, Kuzmann E, Homonnay Z, Vértes A, Greneche JM, Lackner B, Kellner K, Gritzner G, Hakl J, Vad K, Mészáros S, Kerekes L: The effect of iron doping in La 0.8 Sr 0.2 Fe 0.05 Co 0.95 O 3−δ perovskite, European Physical Journal B 43: 297-303 (2005) Németh Z, Homonnay Z, Árva F, Klencsár Z, Kuzmann E, Hakl J, Vad K, Mészáros S, Kellner K, Gritzner G, Vértes A: Mössbauer and magnetic studies of La 0.8 Sr 0.2 CoO 3-  CMR perovskite, Journal of Radioanalytical and Nuclear Chemistry 271(1): 11–17 (2007) Németh Z, Homonnay Z, Árva F, Klencsár Z, Kuzmann E, Vértes A, Hakl J, Mészáros S, Vad K, de Châtel PF, Gritzner G, Aoki Y, Konno H, Greneche JM: Response of La 0.8 Sr 0.2 CoO 3-  to perturbations on the CoO 3 sublattice, European Physical Journal B 57: 257–263 (2007) Klencsár Z, Németh Z, Kuzmann E, Homonnay Z, Vértes A, Hakl J, Vad K, Mészáros S, Simopoulos A, Devlin E, Kallias G, Greneche JM, Cziráki Á, De SK: The role of iron in the formation of the magnetic structure and related properties of La 0.8 Sr 0.2 Fe x Co 1-x O 3-  (x = 0.15, 0.2, 0.3), Journal of Magnetism and Magnetic Materials 320: 651-661 (2008)