Abstract:
The effect of non-collinear magnetic moments disorder on the properties of transition metal alloys will be discussed. In particular, for the case of the fcc Fe-Ni Invar, we have shown recently that the transition from the high-volume high-spin state to the low-volume low-spin state proceeds as a continuous transition to a disordered non-collinear configuration. The non-collinearity gives rise to an anomalous volume dependence of the binding energy curve, and explains many well-known peculiarities of Invar systems. I will also discuss recent experiments that provide a support for this theory. In particular, I will present the experiment in which the pressure - volume (P-V) relations for cubic iron-nickel alloys have been measured for different compositions. It was observed in the experiment, in agreement with theoretical predictions, that for a certain pressure range the bulk modulus did not change or even can decrease to some minimum value, after which it began to increase under still higher pressure. Also, a new effect was observed, namely that the Fe-Ni alloys with high Ni concentrations, which show positive thermal expansion at ambient pressure, become Invar system upon compression over a certain pressure range. Finally, I will show that the origin of magnetic frustrations in concentrated Fe-Ni alloys and in pure fcc Fe are different. While in pure Fe the frustrations are due to long-range and oscillating character of exchange interactions, in concentrated random alloys they are mainly a consequence of huge dispersion of the nearest neighbor exchange parameters caused by the local environment effects.