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Séminaire MCBT

mardi 13 octobre 2015 à 11h00,
Salle Louis Weil, E424

Orateur : Justin Jeanneau (MagSup)
"Properties of new chromium compounds"

Abstract :

Chromium is a base for a large number of materials with very high c superconductivity develops from antiferromagnetic parents. Most interesting are the two-dimensional oxides of the Æn+1CrnO3n+1 Ruddlesden-Popper series which are poorly studied due to their extreme difficult synthesis conditions. Using the large volume presses available in the Néel Institute, I have synthesized the n=1 and 2 of the Sr series and studied their previously unknown physical properties in details.

In Sr3Cr2O7 I have found the appearance of AF order below TN=210K. The magnetization measurements were confirmed by neutron scattering and the AF structure was calculated from them. Specific heat measurements show a very large anomaly at TN, with an entropy much larger than expected from magnetic ordering alone. Electronic band structure calculations and theoretical models built on our data, suggest a total magnetic and orbital ordering coupled to a specific lattice distortion as observed in our neutron diffraction measurements.

Sr2CrO4 develops AF at a higher temperature TN=305K, both from magnetization and neutron diffraction measurements. The latter also show new magnetic peaks appearing below 150K, typical of a magnetic moment rearrangement. It is coupled to an unusual "Anti-Jahn-Teller" effect, i.e. the deformed CrO6 octahedron becoming un-deformed at low temperatures. The interpretation of this unexpected result is under study.

Recently, I have also synthesized the n=1 and 2 member of the Ca-based family that present roughly similar properties. Finally, I will briefly describe the pressure phase diagram of another Cr-based compound, CrSb2, that I have determined, following the report of superconductivity at Tc = 2 K under 0.8 GPa of pressure in CrAs.

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