Magnetic properties and stability of magnetically soft nanomaterials for high-temperature applications

Two-phase iron-based nanocrystalline alloys are the newest generation of magnetically soft materials. FINEMET and NANOPERM are the well known nanocrystalline materials obtained by partial crystallization of metallic glasses, with magnetic properties better than those found for the amorphous counterparts. The two-phase amorphous-nanocrystalline alloys exhibit very soft magnetic behavior but only up to temperatures close to the Curie point of the amorphous matrix. Thus, their application is limited only up to about 200degreesC. In order to increase the Curie point of the amorphous matrix and to elevate the application temperature of these alloys, iron is partially replaced by cobalt. Nanomaterials for high-temperature applications must fulfill two basic requirements: (i) very soft magnetic behavior at elevated temperatures, and (ii) stable performance at elevated temperatures for long time (application time). The second requirement is related to thermal stability of nanocrystalline structure and magnetic properties of these materials. This paper summarizes the current status of research in the field of magnetically soft nanocrystalline materials for high-temperature applications, especially highlighting the influence of alloy composition on structure and magnetic properties as well as their stability at elevated temperatures during very long time annealing. The original studies have been oriented on tailoring appropriate alloys for different application temperatures and studies of their stability during annealing for thousands of hours at temperatures up to 500degreesC. FINEMET and NANOPERM alloys have been modified by alloying elements like Co, Si, Cu, Hf, Zr, Nb and B. The structure was studied using Differential Scanning Calorimetry, X-ray Diffraction and Transmission Electron Microscopy. Hysteresis loop and VSM measurements were used for magnetic properties characterization at room and elevated temperatures.