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NUCLEAR AND MAGNETIC PHASES DIAGRAM OF THE Pr0.5Sr0.5-xCaxMnO3 SYSTEM
F. Damay1, C. Martin1, Z. Jirak2, A. Maignan1, M. Hervieu1, B. Raveau1, G. André3 and F. Bourée3
1Laboratoire CRISMAT, UMR 6508,
6, Bvd du Maréchal Juin, 14050 CAEN Cedex, France
2Institute of Physics of ASCR,
Cukrovarnicka 10, 16200 PRAGUE, Czech Republic
3Laboratoire Léon Brillouin,
CEA Saclay, 91191 Gif sur Yvette, France
Keywords : Manganese oxides, magnetoresistance, neutron
powder diffraction
The nuclear and magnetic structure of eight compounds belonging to the Pr0.5Sr0.5-xCaxMnO3 system have been studied by means of neutron and electron diffraction, in addition to electric and magnetic measurements, so as to understand the role of the average size of the A-site cations, <rA>, and of the disorder, s2, upon the physical and structural properties of this system. In spite of the complexity of the structural behaviors that can be observed depending on the <rA> value (Fig. 1), three types of compounds can be crudely distinguished.
From <rA> = 1.245 A (x = 0) to 1.239 A (x = 0.04), compounds exhibit a single structural transition around 150 K (biphasic domain), from an ap21/2 x ap21/2 x 2ap I4/mcm cell to a 2ap x 2ap x 2ap Fmmm one with decreasing temperature. The I4/mcm structure is paramagnetic from 300 K to about 250 K and becomes ferromagnetic below this temperature. The Fmmm structure is characterized by an A-type antiferromagnetic ordering of the magnetic moments. In these compounds, neutron diffraction data does not show any features of a possible ordering of Mn3+ and Mn4+ species. GMR properties are poor because of the weakly insulating low temperature state1.
From <rA> = 1.229 A (x = 0.12) to 1.219 A (x = 0.2), the high temperature paramagnetic or ferromagnetic phase is of Pnma space group. Several features distinguish, from that observed in the previous case, the antiferromagnetic transition occurring around 190 K : the antiferromagnetic ordering is of CE-type and moreover, both neutron and electron diffraction show that there is an ordering of Mn3+ and Mn4+ in the structure, that can be refined in a 2ap21/2 x 2ap x ap21/2 Pmmn cell (subcell Imma). This charge ordering (CO) is responsible for an electron localisation that leads to a highly insulating state at low temperature. The Pnma-Pmmn transition takes place abruptly without any biphasic region : in these compounds, the metastable state between ferromagnetism and CO antiferromagnetism can lead to large magnetoresistance effects. This is particularly perceptible in the case of Pr0.5Sr0.41Ca0.09MnO3 (<rA> = 1.233 A), that is a border compound between the x = 0.04 and x = 0.12 behaviors, and that exhibits the highest magnetoresistance ratios of the system. An interesting result is also the linear dependence of the paramagnetic to ferromagnetic transition temperature Tc with <rA>. The structural parameters deduced from high resolution neutron diffraction data show indeed that TC is strongly correlated to the <Mn-O-Mn> angle, which depends of the <rA> value. The decrease of TC with <Mn-O-Mn> has been predicted theoretically as due to a weakening of the electron exchange interaction between manganese species through the oxygen ion2,3, but experimental evidence of the <Mn-O-Mn> dependence of TC are quite rare so far.
From <rA> = 1.201 A (x = 0.3) to 1.180 A (x = 0.5), compounds are no more
ferromagnetic and exhibit only a paramagnetic to
antiferromagnetic transition. In both compounds, charge ordering
features4 are easily evidenced at low temperature. The
subcell keep the Pnma space group in both paramagnetic and
CE-type antiferromagnetic states but a significant distortion of
the cell, with a strong compression of the b parameter, is
observed as charge ordering developps. Pr0.5Ca0.5MnO3 (x = 0.5) is the only compound for which
the temperature of appearance of the charge ordered state and of
antiferromagnetism do not coincide ; moreover, in this case, the
charge ordered structure is quite complex, and shows an
incommensurate modulation with a typical wave vector q = (1/2-e, 0, 0) ( 0.02). Besides, Pr0.5Ca0.5MnO3 does not exhibit any magnetoresistance
properties, at least until 7 T.
Figure
1 : Nuclear and magnetic phases diagram in the Temperature -
<rA> plane of the
Pr0.5Sr0.5-xCaxMnO3
system (from neutron diffraction results)