Curie Temperature of the Transitionmetal Ferromagnets

On the basis of the spin fluctuation model, the Curie temperature of a large number of transition-metal ferromagnets will be investigated systematically. The basis of this investigation are band structure calculations of the magnetic ground state using the full-potential linear-muffin-tin-orbital (FP-LMTO) method. The effects of exchange and correlation are treated within the local spin density approximation (LSDA). From these calculations one determines the magnetic moment and the susceptibility. Employing the augmented- spherical-wave method (ASW) we calculate spin spiral states which are mapped to a Heisenberg model in order to determine the respective exchange integrals. Within the mean field solution for the Curie temperature this treatment allows to determine Tc on an ab-initio basis. Applying this method to a large number of systems should check the applicability and the limits of these models, which so fax have only be applied for Fe, Co, and Ni.

The theoretical determination of the Curie temperature of magnetic alloys is still one of the great challenges of present solid state theory. While the description of the electronic structure and the resulting magnetic groundstate based on the local spin density approximation for exchange and correlation is usually in excellent agreement with experiment, the theoretical treatment of the finite temperature properties of itinerant magnetic systems is still under heavy debate. In the center of the discussion remains the question whether a magnetic system should be described in the itinerant electron or in the localized moment picture and how these two extreme views could be merged to a more unified picture. In general the relation between the band width and the width of the exchange splitting can be used to distinguish between localized and itinerant electron magnets. If the magnetic band splitting is larger than the bandwidth one speaks about localized moments, in the reverse case about itinerant moments. It becomes clear that the transition between these two models is gradual. In particular the pure itinerant behavior is hardly ever realized. However, in a general way one could say that the itinerant picture describes the ground state (non integer magnetic moments, weakly or strongly ferromagnetic behavior, etc.), when it comes to excitations a localized description of the magnetic behavior is required (local moments above Tc etc.). The project should thus try to resolve a discussion which is present in the literature for the last 30 years. With the application of the models worked out during this project it should become unambiguously clear how to treat the final temperature properties of transition metal ferromagnets. It must be pointed out, that the outcome of this project is not only of fundamental value since the determination of the Curie temperature on the basis of a microscopic model has a strong impact on alloy design. As an example we want to mention the high performance permanent magnet material Nd2 Fe14B which is characterized by the largest energy product of all known permanent magnet materials (up to 360 kJ/mole) (for a rectangular hysteresis, the energy product BH max measures the area of the hysteresis defined as the product of the remanent induction Br times the coercivity Hc. Unfortunately due to its Curie temperature of only about 600K, Nd2 Fe14B is restricted in its application to temperatures around room temperature. The reason for this limitation is the strong temperature dependence of the demagnetization which already at 400K makes Nd2 Fe14B inferior to the older SmCo 5 magnets. On the basis of the results of the present project it should be possible to make suggestions how to modify such systems in order to increase Tc.