Linear Thermal Expansion of FeSe Ferrimagnets

We have investigated the thermal expansion of the ferrimagnet FeSe of the hexagonal system by synchrotron radiation powder X-ray diffraction from room temperature down to 90 K. The lattice constants show anisotropic change against temperature. Especially, the c-axis expands down to 170 K, exhibiting negative linear thermal expansion. Almost the same behavior is found in the temperature dependence of the magnetization. This implies that the anomalous change of the c-axis has a strong correlation with magnetism. Magnetic fields up to 0.4 T do not affect the lattice constants at room temperature. Electrical resistivity indicates metallic behavior, i.e., there is a finite density of the state at the Fermi energy, consisting of the Fe3d and the Se4p states.