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align=center><A name=OLE_LINK2><STRONG><SPAN lang=ES-AR
style="COLOR: black; FONT-FAMILY: Arial; mso-ansi-language: ES-AR"><FONT
size=3>Seminarios del Dpto. de Química Inorgánica, Analítica y Química Física /
INQUIMAE</FONT></SPAN></STRONG></A><SPAN style="mso-bookmark: OLE_LINK2"><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><?xml:namespace prefix = o ns =
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lang=ES-AR
style="COLOR: black; FONT-FAMILY: Arial; mso-ansi-language: ES-AR">FCEyN -
Universidad de Buenos Aires</SPAN></STRONG></SPAN><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN
style="FONT-FAMILY: Arial"><o:p></o:p></SPAN></SPAN></FONT></P>
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align=center><SPAN style="mso-bookmark: OLE_LINK2"></SPAN><A
href="http://www.qi.fcen.uba.ar/es"><FONT size=3><SPAN
style="mso-bookmark: OLE_LINK2"><B><SPAN lang=ES-AR
style="FONT-FAMILY: Arial; mso-ansi-language: ES-AR">http://www.qi.fcen.uba.ar/es</SPAN></B></SPAN><SPAN
style="mso-bookmark: OLE_LINK2"></SPAN></FONT></A><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN
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align=center><SPAN style="mso-bookmark: OLE_LINK2"><SPAN
style="FONT-FAMILY: Arial"><FONT
size=3> <o:p></o:p></FONT></SPAN></SPAN></P>
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align=center><FONT size=3><SPAN style="mso-bookmark: OLE_LINK2"><STRONG><SPAN
lang=PT-BR
style="COLOR: black; FONT-FAMILY: Arial; mso-ansi-language: PT-BR">2do.
Cuatrimestre, 2006</SPAN></STRONG></SPAN><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=PT-BR
style="FONT-FAMILY: Arial; mso-ansi-language: PT-BR"><o:p></o:p></SPAN></SPAN></FONT></P>
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style="FONT-FAMILY: Arial; mso-ansi-language: PT-BR"><o:p><FONT
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align=center><SPAN style="mso-bookmark: OLE_LINK2"><SPAN lang=PT-BR
style="FONT-FAMILY: Arial; mso-ansi-language: PT-BR"><FONT
size=3> <o:p></o:p></FONT></SPAN></SPAN></P>
<P class=MsoNormal
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align=center><SPAN style="mso-bookmark: OLE_LINK2"><STRONG><SPAN lang=PT-BR
style="FONT-SIZE: 14pt; COLOR: black; FONT-FAMILY: Arial; mso-ansi-language: PT-BR">Miercoles
1 de Noviembre, 13.00h</SPAN></STRONG></SPAN><SPAN
style="mso-bookmark: OLE_LINK2"><B><SPAN lang=PT-BR
style="COLOR: black; FONT-FAMILY: Arial; mso-ansi-language: PT-BR"><BR><STRONG><SPAN
style="FONT-FAMILY: Arial"><FONT size=3>Aula Seminarios, DQIAyQF-
INQUIMAE<o:p></o:p></FONT></SPAN></STRONG></SPAN></B></SPAN></P>
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align=center><FONT size=3><SPAN style="mso-bookmark: OLE_LINK2"><STRONG><SPAN
lang=ES-AR
style="COLOR: black; FONT-FAMILY: Arial; mso-ansi-language: ES-AR">Ciudad
Universitaria, Pab. II, Piso 3</SPAN></STRONG></SPAN><SPAN
style="mso-bookmark: OLE_LINK2"><B><U><SPAN
style="mso-bidi-font-size: 10.0pt"><o:p></o:p></SPAN></U></B></SPAN></FONT></P>
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style="mso-bookmark: OLE_LINK2"><B><U><SPAN style="FONT-SIZE: 18pt"><o:p><SPAN
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face="Times New Roman"> </FONT></SPAN></o:p></SPAN></U></B></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center"
align=center><SPAN style="mso-bookmark: OLE_LINK2"><B><SPAN lang=FR
style="FONT-SIZE: 18pt; FONT-FAMILY: Arial; mso-ansi-language: FR; mso-bidi-font-style: italic">Octavio
Peña<o:p></o:p></SPAN></B></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center"
align=center><SPAN style="mso-bookmark: OLE_LINK2"><B><SPAN lang=FR
style="FONT-FAMILY: Arial; mso-ansi-language: FR; mso-bidi-font-style: italic"><FONT
size=3>Equipe Chimie du Solide et Matériaux. Unité de Sciences Chimiques de
Rennes<o:p></o:p></FONT></SPAN></B></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center"
align=center><FONT size=3><SPAN style="mso-bookmark: OLE_LINK2"><B><SPAN lang=FR
style="FONT-FAMILY: Arial; mso-ansi-language: FR; mso-bidi-font-style: italic">UMR
– CNRS 6226. Université de Rennes 1. </SPAN></B></SPAN><?xml:namespace prefix =
st1 ns = "urn:schemas-microsoft-com:office:smarttags"
/><st1:country-region><st1:place><SPAN style="mso-bookmark: OLE_LINK2"><B><SPAN
lang=EN-GB
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style="mso-bookmark: OLE_LINK2"><B style="mso-bidi-font-weight: normal"><SPAN
lang=EN-US
style="FONT-FAMILY: Arial; mso-ansi-language: EN-US"><o:p></o:p></SPAN></B></SPAN></FONT></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><B style="mso-bidi-font-weight: normal"><SPAN
lang=EN-US
style="FONT-SIZE: 16pt; FONT-FAMILY: Arial; mso-ansi-language: EN-US"><o:p> </o:p></SPAN></B></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: center"
align=center><SPAN style="mso-bookmark: OLE_LINK2"><B
style="mso-bidi-font-weight: normal"><SPAN lang=EN-US
style="FONT-SIZE: 16pt; FONT-FAMILY: Arial; mso-ansi-language: EN-US">Magnetic
interactions in manganese mixed-valence materials: from oxide perovskites to
sulfide spinels<o:p></o:p></SPAN></B></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-GB
style="FONT-FAMILY: Arial; mso-ansi-language: EN-GB; mso-bidi-font-weight: bold"><o:p><FONT
size=3> </FONT></o:p></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 1cm; LINE-HEIGHT: 150%; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-US
style="FONT-SIZE: 11pt; LINE-HEIGHT: 150%; FONT-FAMILY: Arial; mso-ansi-language: EN-US"><o:p> </o:p></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 1cm; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-US
style="FONT-FAMILY: Arial; mso-ansi-language: EN-US"><FONT size=3>Existence of
two magnetic sublattices in a well-defined crystalline structure may lead to
interesting cooperative phenomena. Basic mechanisms are double-exchange and
super-exchange magnetic interactions, in which the role of the ligands (e.g.,
oxygen anions) is fundamental. These cooperative phenomena may be coupled to
electrical conduction, leading to magneto-resistance effects, double phase
transitions or coexisting phenomena (e.g., ferromagnetism and
superconductivity).<o:p></o:p></FONT></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 1cm; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-US
style="FONT-FAMILY: Arial; mso-ansi-language: EN-US"><o:p><FONT
size=3> </FONT></o:p></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 1cm; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-US
style="FONT-FAMILY: Arial; mso-ansi-language: EN-US"><FONT size=3>In recent
years we have been involved with the study of transition-metal oxides of
perovskite and spinel structures, our main goal being the modification of the
magnetic interactions through subtle changes in the mixed-valence state of the
constituents. We will present several examples of our recent work: rare-earth
manganites REMnO<SUB>3</SUB> in which the oxidation state of manganese can be
modulated either by substitutions at the rare-earth site or by modifying the
environment of the transition-metal network ; double and triple oxides derived
from the spinel compound NiMn<SUB>2</SUB>O<SUB>4</SUB> and their solid solutions
(NiMn<SUB>2-x</SUB><I>Me</I><SUB>x</SUB>O<SUB>4</SUB>) or
(Co,Ni,Mn)<SUB>3</SUB>O<SUB>4</SUB> ; thiospinels obtained by modifications at
the anionic site (oxygen by sulfur).<o:p></o:p></FONT></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 1cm; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-US
style="FONT-FAMILY: Arial; mso-ansi-language: EN-US"><o:p><FONT
size=3> </FONT></o:p></SPAN></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 1cm; TEXT-ALIGN: justify"><SPAN
style="mso-bookmark: OLE_LINK2"><SPAN lang=EN-US
style="FONT-FAMILY: Arial; mso-ansi-language: EN-US"><FONT size=3>A
phenomenological model of two interacting sublattices with antiferromagnetic
exchange will be used to explain the spin reversal found in gadolinium-based
perovskites. Spin-flop mechanisms will be discussed in the case of erbium
perovskites. Finally, the ferrimagnetic properties of the oxide and sulfide
spinels will be shown and compared in
detail.<o:p></o:p></FONT></SPAN></SPAN></P>
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style="FONT-FAMILY: Arial; mso-fareast-font-family: Batang; mso-ansi-language: EN-US; mso-fareast-language: KO"><o:p><FONT
size=3> </FONT></o:p></SPAN></SPAN></P>
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style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 14.2pt; TEXT-ALIGN: justify; mso-layout-grid-align: none"><SPAN
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style="FONT-FAMILY: Arial; mso-fareast-font-family: Batang; mso-ansi-language: EN-GB; mso-fareast-language: KO"><o:p><FONT
size=3> </FONT></o:p></SPAN></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm -4.05pt 0pt 0cm; TEXT-ALIGN: center"
align=center><SPAN style="mso-bookmark: OLE_LINK2"><SPAN
style="FONT-FAMILY: 'ITC Zapf Dingbats'; mso-bidi-font-family: Arial"><FONT
size=3>iiiii</FONT></SPAN></SPAN><SPAN
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