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by Staff Writers Berlin, Germany (SPX) Jul 09, 2015
A team at HZB has carried out the first detailed study of how magnetic and geometric ordering mutually influence one another in crystalline samples of spinel. To achieve this, the group synthesized a series of mixed crystals with the chemical formula Ni1-xCuxCr2O4 in which the element nickel was successively replaced by copper. They discovered through neutron scattering experiments at BER II not only how the crystal structure changes, but also uncovered new magnetic phases. The results were published in Physical Review B. Spinels consist of densely packed, highly symmetrical planes of oxygen atoms (somewhat like a densely packed box of marbles) where different metallic elements are lodged in the spaces between them. A great many different types of compounds arise as a result that are employed in extractive industries and as heat-resistant and magnetic materials. The embedded metal ions in the Ni1-xCuxCr2O4 spinel system cause a distortion of the crystal structure. In addition, they also display magnetic moments due to the geometrical structure that cannot be oriented as they otherwise would be. As a result, spectacular new temperature-dependent ordering arises. The HZB team has now comprehensively analysed the chromium-spinel system and have explained the complex phase diagram at a fundamental level for the first time.
Preparing a series of samples This causes the samples to be increasingly dark. With the copper proportion at 100 %, in the end the powder is black. The powders consist of small crystal grains whose diameters are between 30 and 50 microns. The exciting thing about this series of mixed crystals is that nickel or copper atoms sit at what are referred to as tetragonal sites of the crystal structure. Due to their different configurations of electrons, these tetrahedra become elongated along the crystallographic c-axis for nickel, while for copper they are compressed (Jahn-Teller effect). The distortion of the crystal structure can thus be controlled, which in turn has an effect on the magnetic ordering.
Phase diagramm between 2 and 900 Kelvin This shows that the crystal structure is cubic (three right angles, three equal edges) at high temperatures, since the kinetic energy of the atoms still suppresses the Jahn-Teller effect and magnetic ordering cannot become established. As the temperature declines, the Jahn-Teller effect comes to the fore and causes a reduction of the crystal symmetry initially to tetragonal (three right angles, two equal edges), and finally orthorhombic (three right angles, three unequal edges).
New magnetic phases
Peninsula of orthorhombic state Contrary to what has been assumed until now, the cause of this is the distortion of the nickel and copper tetrahedrons at 90 to each other rather than in the same direction. This results not in mutual cancellation of the distortions at this mixing ratio, but instead in a maximal distortion of the structure. "Atoms are not just spheres. They do crazy things, especially when they are in a geometrical system like a crystal, rather than in isolation", says Michael Tovar. To the publication: Phys. Rev. B 91, 024407. "Competing Jahn-Teller distortions and ferrimagnetic ordering in the geometrically frustrated system Ni1?xCuxCr2O4" M. Reehuis, M. Tovar, D. M. Tobbens, P. Pattison, A. Hoser, and B. Lake
Related Links Helmholtz-Zentrum Berlin fur Materialien und Energie Computer Chip Architecture, Technology and Manufacture Nano Technology News From SpaceMart.com
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