ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
Physics
Notes:
Contrary to the Invar effect, which is a volume enhancement in a magnetically ordered state and which is characterized by a smaller than normal thermal expansion coefficient (α), the anti-Invar effect is a volume enhancement in a paramagnetic state and is characterized by a larger than normal α. Both are caused by large anharmonicities due to the presence of moment fluctuations. The Invar effect occurs only in alloys, whereas the anti-Invar effect is observed in elements like fcc-Fe and fcc-Mn as well as in their alloys with other 3D elements. This property poses the anti-Invar effect as being more fundamental than the Invar effect. In order to understand the properties of moment fluctuations in anti-Invar, we have carried out temperature dependent paramagnetic neutron scattering experiments with 3D polarization analysis on Fe100−xNix alloys (x=10, 15, 20, 25 at. %) in their high temperature fcc states (500〈T〈1100 K) where they exhibit anti-Invar properties. At lower temperatures, these alloys undergo martensitic transformations into the bcc phase. The results show strong magnetic scattering in the forward direction in all alloys, even at temperatures as high as 1100 K, indicating ferromagnetic correlations, although no long range ferromagnetic ordering occurs in any of these alloys. The magnetic cross section decreases with increasing temperature for x=20 and 25 at. % (500〈T〈1100 K), and shows the unusual behavior of remaining constant with temperature for x=15 at. % and increases with increasing temperature for x=10 at. % (700〈T 〈1100 K). These observations are discussed by taking into account the competing effects of increasing lattice vibrational amplitude with increasing temperature, favoring the weakening of the correlations, and the anti-Invar property of the increase of the local magnetic moment with increasing temperature, favoring the strengthening of the magnetic correlations. © 1997 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.364900
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