Thermopower and low-dc-field magnetization study of the layered ${\mathrm{Fe}}_{\mathrm{x}}$${\mathrm{ZrSe}}_2$ compounds: Anderson-type localization and anisotropic spin-glass behavior

The two-dimensional antiferromagnetic iron intercalates of ${\mathrm{ZrSe}}_2$, in which Fe is located in octahedral as well as in tetrahedral sites, have been investigated by thermopower and low-dc-field (5 mT) magnetization measurements. The samples studied were single crystals. In the direction parallel to the layers and at temperatures below 80 K, the thermopower value varies as the square root of the temperature, which suggests a variable-range hopping. As a result, the electrons transferred from Fe to ${\mathrm{ZrSe}}_2$ may be considered as localized in a band tail due to the random positions of iron within the van der Waals gap. In contrast, for T>80 K, the thermopower varies as the reciprocal temperature in agreement with the assumption that the carriers are thermally excited out of the polaron wells to extended states above the mobility edge. From the magnetic measurements performed using a high-sensitivity superconducting quantum interference device (SQUID) technique, a marked anisotropic spin-glass behavior appears below 10 K. The origin and the importance of the anisotropy is discussed on the basis of the ground-state degeneracy of Fe(II).