A NOVEL THEORETICAL DESCRIPTION OF VOLUME INSTABILITIES IN ITINERANT MAGNETIC SYSTEMS

We present a Ginzburg-Landau description of itinerant systems with possible magnetovolume instabilities near magnetic phase transitions. We emphasize that: (i) the expansion of the free energy is valid for any metallic system regardless of the magnitude of the magnetic moment per atom; (ii) the Landau coefficients are determined unambiguously by analyzing total energy surfaces in the (M, Ω) space (obtained from fixed-spin-moment (FSM) energy-band calculations); (iii) finite temperature analysis shows that in the space spanned by the Landau coefficients first order phase transitions are possible. Binding surfaces of systems near this region exhibit two local minima which lie lose in energy; (iv)the invar effect is connected with the possibility of the system to undergo a gradual high-spin-high-volume-low-spin-low-volume transition with increase in temperature. © 1989.