High Pressure Measurements on TI 2 Mn 2 O 7
- PDF / 301,267 Bytes
- 6 Pages / 418.68 x 637.2 pts Page_size
- 96 Downloads / 254 Views
§ #
38042 Grenoble, France Laboratoire de Cristallographie, C.N.R.S., BP166 cedex 09, 38042 Grenoble, France European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France ICMAB-CSIC,Campus Universitat Autonoma de Barcelona, Bellaterra 08193, Spain
ABSTRACT Transport and structural properties of the colossal magnetoresistance pyrochlore TI 2Mn 20 7 are studied as a function of applied pressure up to -20GPa. This allows us to probe the effect of structural changes on the ferromagnetic transition and the transport properties. We observe a non-monotonous pressure dependence of the ferromagnetic transition temperature. We correlate this unusual variation with the structural parameters that, according to electronic band calculations, are key in controlling the properties of these materials. INTRODUCTION Colossal magnetoresistance (CMR) materials are largely studied due to its potential technological applications. The fundamental interaction at the origin of CMR for the perovskite manganites is well established : double exchange (DE) [1] where electron hopping between 3
mixed valence Mn "/Mn4 ions mediates ferromagnetic interactions leading to a ferromagnetic ordering with metallic conduction below Tc. Near Tr the applied magnetic field tends to align the local spins and the electron transfer increases. As a result, a sharp resistivity decrease is observed. The high resistivity above Tc and the CMR are explained by the combination of both Mn 3÷/Mn4+ DE and transport via Jahn-Teller polarons.[2]. The pyrochlore manganites[3] are interesting because the above scenario cannot be easily applied in T12Mn 2O7 compounds in which the high temperature state is metallic (dp/dT > 0) and there are no traces of significant Mn3÷/Mn'4 mixed valence since Mn• is the only Mn ion present[4]. Therefore, neither the DE mechanism nor any significant Jahn-Teller distortions seem to be operative in Mn pyrochlores. Strong carrier scaL:"ring by spin fluctuations (SF), associated with the ferromagnetic ordering of the Mn-O lattice, has ',een invoked to explain CMR in Mn pyrochlores. The mechanism would be similar to that of fe.-omagnetic transition metals[5], itinerant charge carriers ( TI 6s electrons) scattering against localized moments (Mn 3d electrons) with the small carrier density of the pyrochlores enhancing the otherwise well-known effect[6]. Recently, supplementary evidence for this picture has been put forward [7] through the compound Tl2Mn2-,RuxO7 . Ru 4d electrons seem to have an itinerant character for x•1, and both 41 Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
the ferromagnetic transition temperature T, and the magnetoresistance decrease with x. In other words, Ru substitution increases the TI bands carrier concentration without disrupting T1-O coordination. Another way of modifying charge concentration is by application of strong pressure, e.g. the increase of hole concentration of the superconducting Cu-O layers in cuprates under pressure [8]. We have thus performed electrical resistivity
Data Loading...
