[seminar] IF

[Ticijana Ban]

Poštovani,

Obavještavam Vas o održavanju seminara na Institutu za fiziku, Bijenička cesta 46, predavaonica u zgradi Mladena Paića u ponedjeljak 29. listopada 2007. u 15:00 sati.

Seminar pod naslovom Colossal magnetoelastic effects in Pr1-xCaxMnO3 compounds održat će dr. G. Reményi sa Institut Néel, C.N.R.S. Grenoble, Francuska.
Sažetak predavanja možete pronaći u nastavku poruke.

Srdačan pozdrav,
Ticijana Ban
_______________________________________________________________________________________________________________________

Colossal magnetoelastic effects in 

Pr1-xCaxMnO3 compounds





G. Reményi1,3, M. Saint-Paul1, M. Dörr2, M. Loewenhaupt2, S. Sahling2,  P. Lejay1



    1Institut Néel, associé a l'Université Joseph Fourier, C. N. R. S. , BP 166, 38042    Grenoble, France

    2Institut für Festkörperphysik TU Dresden D 01069 Dresden Germany 

   3Laboratoire des Champs Magnétiques Intenses, C. N. R. S. , BP 166, 38042   Grenoble, France


 

Manganese oxides with distorted perovskite structure have attached much attention during the last decade due to their colossal magneto resistance, and the strong correlations among the various degrees of freedom involved. In particular, Pr1-xCaxMnO3 compounds present in a wide Ca-doping range a charge ordering phenomenon, consisting of a real space ordering of Mn3+ and Mn4+ in alternate lattice sites below a certain temperature Tc0. This charge ordered phase is antiferromagnetic in nature, and simultaneous to orbital ordering. Tomioka et al. [1] have observed that an applied magnetic field can melt this charge ordered state and induce a transition from an insulating to metallic state. In order to study the effects of this charge order melting, (this ordering brings about a lattice distortion and a large hardening of the sound velocity [2] below Tc0) ultrasonic longitudinal sound velocity measurements were performed on polycrystalline Pr1-xCaxMnO3  (x= 0.35 ;  0.5) as a function of magnetic field and temperature. 

The d electrons of of Mn3+ present a configuration t2g3eg1 due to the crystal field. When doping with lower valence Ca cations, a proportion X of Mn3+  adopt a +4 valence, wich allows the eg electron in Mn3+ to migrate to unoccupied eg levels in Mn4+  depending on the Mn-O-Mn angle, and the relative orientation of T2g localised spins in the involved Mn sites. In the mechanism of double exchange (DE) the transfer of eg electrons is favoured when neighbouring T2g spins are aligned, thus introducing an effective ferromagnetic interaction between the Mn associated to the mobility of the charge. 

This charge, orbital, and magnetic ordering is suppressed by applying magnetic field. A drastic change of the electronic properties from the charge ordered insulating to a metallic state occurs in high magnetic fields which is caused by the melting of the charge order due to the ferromagnetic correlations. The strong correlation effects between the lattice and the electronic system indicates the importance of the magnetoelastic coupling in these colossal magnetoresistance compounds. The forced magnetostriction is characterized by a lattice contraction of about 10-3 which corresponds to an applied pressure of 0.14 GPa at the transition into the metallic state.

The transition to the high field metallic state is accompanied by a important softening. At low temperature the hysteresis effects are large, indicating the first order nature of the transition. The magnitude of the hardening when cooling at zero field indicates that there must be an important lattice deformation in the charge ordered phase, possibly due to cooperative Jahn-Teller effect on the localised Mn3+. The sound velocity decreases sharply with magnetic field below the zero-field Tc0, therefore the lattice strain associated to the charge order must relax when electrons delocalize into the metallic state due the ferromagnetic alignment of T2g localised spins.

Magnetostriction measurements on Pr0.65Ca0.35MnO3 polycrystalline samples are shown in magnetic fields up to 15 T. The charge ordering [1] occurs below TCO=230K, the application of a magnetic field causes the transition from the charge ordered insulator state to metallic sate [2]. It is found that this insulator-metal transition is accompanied by large negative jumps in the longitudinal and transverse magnetostrictions with a temperature dependent hysteresis. The application of a high magnetic field (13 T) causes the insulator-metal transition with suppression of the lattice distortions .

To analyze the data the anharmonic lattice has been evaluated. The thermal expansion coefficient a related to the thermal Grüneisen parmeter g is given by the relation a = gC(T)/B where C(T) is the Debye specific heat , B is the bulk modulus. Taking B=135 Gpa measured on the same sample the Debye temperature Q = 500 K, and g = 3, the calculated anharmonic contribution is shown in Fig2 and it allows to evaluate the additional contribution in the measured thermal expansion. Such an observation is in agreement with our present thermal expansion results measured at 13 T whose temperature dependence follows the anharmonic temperature variation in the whole temperature range. The 13 T thermal expansion curve coincides with the lattice contribution as no localized charge is present. Within the double-exchange frame work, the magnetic field favors the charge delocalization [1].

 

References and authors:

[1] Y. Tomioka, et al Phys Rev B 53 (1996) R1689

[2] S. Seiro, et al J. Phys. C 14 (2002) 3973
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