S-d Exchange Interaction and the Invar Anomaly of Thermal Expansion of Weak Band Magnets (on the Example of Fe1–x Co x Si Solid Solutions)

In the present paper, the temperature dependence of the linear thermal expansion coefficient (LTEC) of weak band magnets is studied both theoretically and experimentally on the example of Fe_1–x Co _x Si solid solutions. The contributions of electron and magnetic components are also analyzed. It is demonstrated that the experimental data obtained cannot be explained within the framework of the well-known single-particle models. A theoretical approach describing contributions of electrons and fluctuations to the LTEC of weak band magnets is developed based on the spd-model which, in addition to the interatomic exchange interaction of d-electrons, considers the spd-interaction resulting in a noticeable shift of the chemical electron potential along the energy axis. It is demonstrated that the exchange interaction of magnetic moments of s,p- and d-electrons plays the important role in the formation of the Invar LTEC anomaly in Fe_1–x Co _x Si solid solutions.