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The peculiar magnetism of cobaltates


The peculiar magnetism of cobaltates

 

Guillaume Lang, Julien Bobroff, Henri Alloul, Gaston Collin, Nicole Blanchard

 

Sodium cobaltates show very peculiar properties: they are superconductors when you insert water into them, and they transform heat into electricity in an unexpectedly efficient way for metals. The electrons also display an odd way of moving in the cobalt layers. They are strongly "correlated", meaning that they avoid each others, as in high temperature superconductors. But here, they surprisingly avoid each other when they are only few of them. And when they are in a large number, on the contrary, they look like moving as in a standard metal again.

 

We have used nuclear magnetic resonance, a cousin of the MRI in hospitals, to get a better understanding of how electrons behave actually. Our study demonstrates that actually, correlations are always present. For a small number of electrons, the correlations were already known to exist and being ferromagnetic like. As this number increases, we have discovered that they survive, but in an antiferromagnetic pattern. These trends seem to be fundamental to understand the peculiar properties of these compounds. In addition, we have evidenced a new temperature scale, called T*, for which the answer of the system to a magnetic field, its susceptibility, is maximum. Finally, we have found that when there is more than 1 sodium for 2 cobalt, some new "inert" Co3+ sites appear, which refuse to participate in the metallic properties of the system. Here again, their actual number versus sodium content is totally unexpected.
 

 

Fig. 1 : The NaxCoO2 cobaltates consist of a superposition of cobalt (green), oxygen (red), and sodium (blue) layers.

 

Fig. 2 : New phase diagram of cobaltates, as a function of the sodium concentration, x. The upper panel shows the two correlated regions (AF=antiferromagnetic; FM=ferromagnetic), together with the new temperature scale T*. Lower panel shows how the inert Co3+ sites appear above x=0.5 because of some complex sodium ordering. SC corresponds to the superconducting phase.

 

Reference :
"Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates" G. Lang, J. Bobroff, H. Alloul, G. Collin, N. Blanchard - Phys. Rev. B 78, 155116 (2008) (Editor’s suggestion et voir aussi les "synopsis" de Phys. Rev

 

Contact :
Julien Bobroff
Laboratoire de Physique des Solides,
Université Paris-Sud 11, CNRS UMR8502,
91405 Orsay cedex