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Evaluation of ferromagnetic shape‐memory alloys by the extended Hückel method

Ferromagnetic shape‐memory alloy (SMA) are powerful candidates as actuators, pressure sensors, magnetic sensors, etc. Magnetic‐field‐induced strain has been observed in many ferromagnetic SMA. The magnetic‐field‐induced strain is a reversible transformation in the martensite phase with the magnetic field. We have investigated the property of the ferromagnetic shape‐memory materials by the extended Hückel method, and estimated the ferromagnetic shape‐memory of Fe–Pt and Fe–Pd alloys at high temperatures. We used two physical quantities, i.e. cohesive energy and energy fluctuation, to measure the stability of the materials. On the basis of the cohesive energy and energy fluctuation, we discuss the characteristics of ferromagnetic SMA, in which the energy fluctuation is a measure of thermal stability of the metals and/or alloys. The martensite structure is unstable, which means that the energy fluctuation has to be controlled to a small value to keep the martensite phase. Furthermore, it is estimated that the energy fluctuation is associated with the Curie temperature. The Curie temperature is an essential parameter for ferromagnetic materials. From the discussion presented above, we can propose the following: (i) Alloys possessing a low cohesive energy are associated with a high mobility of atoms and are suitable for ferromagnetic shape‐memory materials; (ii) Alloys showing a low energy fluctuation show ferromagnetic shape‐memory and are favored for use as memory devices. We found that I (iodine) is the best dopant for Fe–Pt ferromagnetic SMA, and Tc (technetium) is the best dopant for Fe–Pd ferromagnetic SMA. Copyright © 2007 Institute of Electrical Engineers of Japan© 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.