The Mn3+/Mn4+ mixed valence manganite systems have very rich phase
diagrams [1] exhibiting charge, orbital, magnetic and dielectric competing
orders depending on doping, magnetic and electric applied fields, temperature
and pressure. The recent interest in multiferroic materials, displaying
coexistence of ferroelectricity and magnetic order has triggered further research
in manganites, where phenomena such as the switching of the electric
polarization by a magnetic field in TbMnO3 [2] were reported. Recently, a new
mechanism that combines magnetic order and ferroelectricity was proposed by
Efremov et al [3]. This mechanism is based on the charge order state that
occurs in manganites, such as Pr1-xCaxMnO3, and conciliates the site versus
bond-centred charge order models, which brought up much debate during the
last years [4, 5, 6].
We have studied the Electrical-Field Gradients (EFG), via 111mCd/111Cd
Perturbed Angular Correlations (PAC), in the Pr1-xCaxMnO3 system to achieve a
better understanding on the charge ordered state (CO) and, in particular, to
find out about the existence of ferroelectric order in the Pr1-xCaxMnO3 system
using local probe information. The existence of a local paraelectric-to-
ferroelectric discontinuous phase transition, decoupled by a few degrees from
the charge order transition, is reported. Moreover, our data suggest that the
charge order transition is driven by the softening of vibration modes.
Finally, the preliminary PAC experimental results on RMnO3 (R, rare-earth)
multiferroic systems will be presented.
References
[1] B. Raveau et al. Phys. Rev. B, 60, 12191 (1999)
[2] T. Kimura, Nature 426,55 (2003)
[3] D.Efremov et al, Nature Materials, 3,853 (2004)
[4] P. G. Radaelli et all. Phys. Rev.B 55, 3015 (1997)
[5] Daoud-Aladine et al, Phys Rev. Lett 89, 97205 (2002)
[6] M. Coey, Nature 430,155 (2004)