THE FERROMAGNETIC TO ANTIFERROMAGNETIC TRANSITION IN LA1-XYXMN2SI2(FE-57) COMPOUNDS

A set of La1-xYxMn2Si2(Fe-57) samples with x = 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5 and 1.0 have been prepared with the tetragonal ThCr2Si2 structure. Fe-57 Mossbauer spectra have been collected at 4.2 K for all samples. The spectra can be well fitted by two sextets, the predominant one (similar to 90% of the spectral area) being associated with Fe-57 atoms in the Mn 4d site, with the remainder located in the Si 4e site. The analysis shows that there are distinct changes in the hyperfine interaction parameters for samples above and below the critical concentration x(c) similar to 0.15, at which a transition from ferromagnetism to antiferromagnetism occurs. This transition is a result of a change in the exchange interactions linked with changes in the interatomic distances, due to the smaller atomic volume of Y compared to that of La.