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Study of the orbital ordering in La1-xAxMn1-zBzO3 manganites 

    The discovery of colossal magnetoresistance (CMR) in the hole-doped Mn perovskites Ln1-xAxMnO3 (Ln- rare earths; A=Ca, Sr, Ba, Pb) has recently attracted much attention, because of their intriguing physical properties, that make these systems promising for applications as magnetic sensors. The most interesting property of these systems is the colossal magnetoresistance, which occurs near the Curie temperature TC, when the system undergoes a transition from the paramagnetic to the ferromagnetic state [1]. 

    The orbital degree of freedom is considered to play an important role in manganites. Due to the strong Hund coupling and the crystal field, two eg orbitals of the Mn ion are degenerated and one of them is occupied by an electron in the Mn3+ ion. In the undoped LaMnO3 the d3x2-r2/d3y2-r2 orbitals are alternatively aligned. Whereas a uniform alignment of the d--orbitals is experimentally confirmed in lightly and heavily doped manganites, experimental evidence of orbital ordering in optimally doped manganites (about 30% Mn4+) has not been reported.

    In this project we will use infrared and Raman spectroscopy together with magnetic and transport measurements to study orbital ordering in the optimally doped La1-x AxMn1-zBzO3 (A=Sr, Ba; B=Cu, Zn, Ga, Sc, Co, Cr) oxides. The purpose of this project is to clarify the orbital state of manganites in the ferromagnetic phase, what is crucial for understanding the colossal magnetoresitstance effect. Our preliminary results [2,3,4] show that orbital ordering takes place also in the ferromagnetic metallic phase. We have determined the critical temperature of the orbital ordering for doping concentration of 30% of Mn4+ ions. Continuation of investigation is necessary to confirm this finding. Because of that we propose additional investigation of LaMn1-xCrxO3+d system [5], where concentration of Mn4+ ions is fixed at about 28%. Besides that we, will analyze the influence of magnetic and valence nature of substitucional metals (Cu2+, Zn2+, Sc3+, Co3+, Cr3+, Ga3+, etc) on magnetotransport properties of La1-x AxMn1-zBzO3 manganites. 

    Experimental work will include many techniques as vibrational (Raman and infrared) spectroscopy, magnetization and magnetoresitivity measurements at liquid helium temperatures and high magnetic fields. 

References

  1. Physics of Manganites, T.A. Kaplan and S. D. Mahanti (eds), Kluwer, New York, 1998.
     

  2. Z. El-Fadli, M. R. Metni, F. Sapiña, E. Martinez, J. V. Folgado, D. Beltrán, and A. Beltrán:
    Structural effects of Co and Cr substitution in LaMnO3+d
    J. Mater. Chem., 10, 437 (2000). 
     

  3. De Marzi, Z.V. Popovic, A. Cantarero, Z. Dohcevic-Mitrovic, N. Paunovic, and F. Sapina: 
    Effect of A-site and B-site substitution on the infrared reflectivity spectra of La1-yAyMn1-xBxO3 (A=Ba, Sr; B=Cu, Zn, Sc; 0<y<0.3, 0<x<0.1) manganites
    Phys. Rev. B 68, 064302 (2003). 
     

  4. Z. . Popovic, A. Cantarero, W.H.A. Thijssen, N. Paunovic, Z. Dohcevic-Mitrovic and F. Sapina:
    Short range charge/orbital ordering in La1-xSrxMn1-zBzO3 (B = Cu, Zn) manganites
    J. Phys.: Condens. Matter 17 (2005) 351–360. 
     

  5. Z.V. Popovic, A. Cantarero, W.H.A. Thijssen, N. Paunovic, Z. Dohcevic-Mitrovic, F. Sapina:
    Novel phase transitions in B-site doped manganites
    Physica B 359–361 (2005) 1276–1278. 

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