Role of Ce substitution in the magneto-crystalline anisotropy of tetragonal ZrFe10Si2

We report the study of Zr1-xCexFe10Si2 (x = 0.0, 0.3 and 0.6) compounds, with tetragonal ThMn12 structure, by means of neutron diffraction and Mössbauer spectroscopy to understand the role of Ce in the increasing magneto-crystalline anisotropy. Fitting of the 57Fe Mössbauer spectra to the three Fe atomic positions, namely 8i, 8j and 8f, indicate that Ce displaces Fe from the 8i to the 8j positions and in parallel increases the quadrupole splitting (QS) in position 8f, correlating with the increase of the magneto-crystalline anisotropy. However, the hyperfine field Bhf remains constant for all Ce substitutions. The full occupancy of the 8j positions by Fe at Ce = 0.6 can explain the instability of the ThMn12 structure for similar and higher Ce concentrations. Neutron diffraction experiments show the evolution of the magnetic moment and crystal structure as a function of temperature, showing Ce to cause an increase of the lattice parameters and tetragonal distortion. A large anomaly of the expansion coefficient, due to the spontaneous magnetostriction, is also disclosed. These results support a pure geometric influence of Ce on the magneto-crystalline anisotropy through a selective distortion of the lattice.