Untitled - Materials Science Institute of Madrid - Consejo Superior de ...
Untitled - Materials Science Institute of Madrid - Consejo Superior de ...
Untitled - Materials Science Institute of Madrid - Consejo Superior de ...
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3. Magnetorresistencia colosal en perovskitas<br />
<strong>de</strong>rivadas <strong>de</strong> CaCu 3 Mn 4 O 12<br />
Hemos preparado la perovskita CaCu 3<br />
Mn 4<br />
O 12<br />
y diferentes<br />
<strong>de</strong>rivados, dopados en las subre<strong>de</strong>s <strong>de</strong> Ca y Cu, a<br />
presiones mo<strong>de</strong>radas <strong>de</strong> 20 Kbar con KClO 4<br />
como agente<br />
oxidante. El material sin dopar, con T c<br />
=345 K, muestra<br />
una magnetorresistencia (MR) sustancial con una<br />
estabilidad térmica comparable a la <strong>de</strong> los mejores óxidos<br />
magnetorresistivos. Hemos mejorado la MR a temperatura<br />
ambiente en la serie <strong>de</strong> <strong>de</strong>rivados RCu 3<br />
Mn 4<br />
O 12<br />
(R= La, Nd, Tb, Ce y Th)[1], en la que a<strong>de</strong>más se observa<br />
un incremento sustancial <strong>de</strong> T C<br />
. Por otro lado, los<br />
espectros <strong>de</strong> absorción <strong>de</strong> rayos X evi<strong>de</strong>ncian la presencia<br />
<strong>de</strong> dos diferentes bor<strong>de</strong>s K para Mn, que correspon<strong>de</strong>n<br />
a los dos distintos entornos <strong>de</strong> este catión en<br />
estos compuesto: Mn 3+ está distribuido al azar en la<br />
subred <strong>de</strong> Cu 2+ , mientras que Mn 3+ /Mn 4+ ocupa las<br />
posiciones octaédricas B <strong>de</strong> la perovskita.<br />
3. Colossal magnetoresistance in<br />
CaCu 3 Mn 4 O 12 perovskites <strong>de</strong>rivatives<br />
Polycrystalline CaCu 3<br />
Mn 4<br />
O 12<br />
perovskite and different<br />
<strong>de</strong>rivatives doped at Ca and Cu sublattices have been<br />
prepared un<strong>de</strong>r mo<strong>de</strong>rate pressure conditions <strong>of</strong> 20<br />
kbar, in the presence <strong>of</strong> KClO 4<br />
as oxidizing agent. The<br />
parent material, with a Curie temperature <strong>of</strong> 345 K,<br />
shows a significant magnetoresintance (MR) with a thermal<br />
stability much superior than that <strong>of</strong> many classical<br />
systems. We have improved both the T C and the roomtemperature<br />
MR in a series <strong>of</strong> stoichiometry RCu 3<br />
Mn 4<br />
O 12<br />
(R= La, Nd, Tb, Ce, Th) [1]. On the other hand, the x-ray<br />
absorption near-edge spectra evi<strong>de</strong>nce the presence <strong>of</strong><br />
two different Mn K-edges corresponding to two notably<br />
different crystallographic environments for Mn [2]:<br />
some Mn 3+ cations are randomly distributed over the<br />
Cu 2+ positions, whereas mixed valence Mn 3+ /Mn 4+ occupies<br />
the octahedral B positions <strong>of</strong> the perovskite structure.<br />
1. Sánchez-Benítez J., Alonso J.A., Martínez-Lope M.J., Casais M.T., Martínez J.L., De Andrés A., Fernán<strong>de</strong>z-Díaz M.T., Chem. Mat. 15<br />
(2003) 2193<br />
2. Alonso J.A., Sánchez-Benítez J., De Andrés A., Martínez-Lope M.J., Casais M.T., Appl. Phys. Lett. 83 (2003) 2623- 2625<br />
Proyectos: DGYCIT, Programa Sectorial <strong>de</strong> Promoción General <strong>de</strong>l Conocimiento, MAT2001-0539.<br />
4. Magnetorresistencia colosal en <strong>de</strong>rivados<br />
<strong>de</strong>l pirocloro Tl 2 Mn 2 O 7<br />
Hemos preparado a presiones mo<strong>de</strong>radas (20 kbar)<br />
nuevos <strong>de</strong>rivados <strong>de</strong> tipo pirocloro, basados en<br />
Tl 2<br />
Mn 2<br />
O 7,<br />
introduciendo cationes a<strong>de</strong>cuados en las<br />
subre<strong>de</strong>s <strong>de</strong> Tl y <strong>de</strong> Mn. Mediante medidas <strong>de</strong> espectroscopía<br />
<strong>de</strong> fotoemisión y absorción <strong>de</strong> rayos X en<br />
diferentes materiales dopados con Bi, Cd y Sb se ha<br />
<strong>de</strong>terminado la <strong>de</strong>nsidad <strong>de</strong> portadores, que se correlaciona<br />
con los valores <strong>de</strong> magnetorresistencia observados<br />
[1]. El análisis <strong>de</strong> los datos <strong>de</strong> magnetotransporte<br />
en el pirocloro sin dopar revelan que sólo los electrones<br />
<strong>de</strong> la banda down-spin contribuyen al transporte <strong>de</strong><br />
carga, confirmado su carácter semimetálico [2]. Por<br />
otro lado, el estudio por espectroscopía Raman e infrarroja<br />
<strong>de</strong> A 2 Mn 2 O 7 (A= Tl, In) ha permitido <strong>de</strong>terminar las<br />
constantes <strong>de</strong> fuerza <strong>de</strong> los enlaces A-O y Mn-O, asignar<br />
todos los modos <strong>de</strong> vibración y proponer una distribución<br />
<strong>de</strong> la energía potencial en estos compuestos<br />
[3].<br />
4. Colossal magnetoresistance in<br />
Tl 2 Mn 2 O 7 pyrochlore <strong>de</strong>rivatives<br />
At mo<strong>de</strong>rate hydrostatic pressures (20 kbar) we have<br />
prepared novel Tl 2 Mn 2 O 7 <strong>de</strong>rivatives, by replacing Tl or<br />
Mn for suitable cations. By photoemission and x-ray<br />
absorption near edge spectroscopy measurements in<br />
different materials doped with Bi, Cd and Sb we <strong>de</strong>termined<br />
the <strong>de</strong>nsity <strong>of</strong> carriers, which is correlated to the<br />
magnetoresistance values [1]. The analysis <strong>of</strong> magnetotransport<br />
data on Tl 2<br />
Mn 2<br />
O 7<br />
revealed that only the minority<br />
band conduction electrons contribute to the charge<br />
transport, confirming the half-metallic character <strong>of</strong> this<br />
material. On the other hand, the Raman and IR spectra<br />
and force field have been investigated for A 2 Mn 2 O 7 (A=<br />
Tl, In) by means <strong>of</strong> a short range force constant mo<strong>de</strong>l<br />
which inclu<strong>de</strong>s four stretching and four bending force<br />
constants. The assignment <strong>of</strong> all mo<strong>de</strong>s has been proposed,<br />
and potential energy distribution is also reported<br />
[3].<br />
1. J. Sánchez-Benítez, A. De Andrés, C. Prieto, J. Avila, L. Martín-Carrón, J.L. Martínez, J.A. Alonso, Martínez-Lope M.J., M.T. Casais, Appl.<br />
Phys. Lett. 84, 4209 (2004).<br />
2. P. Velasco, J.A. Alonso, M.T. Casais, M.J. Martìnez-Lope, J.L. Martínez, J. Phys.: Cond. Matter 16, 1 (2004).<br />
3. S. Brown, H.C. Gupta, J.A. Alonso, M.J. Martìnez-Lope, Phys. Rev. B 69 54434 (2004)<br />
Proyectos: DGYCIT, Programa Sectorial <strong>de</strong> Promoción General <strong>de</strong>l Conocimiento, MAT2001-0539.<br />
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