A MOSSBAUER STUDY OF THE FERROMAGNETIC TO ANTIFERROMAGNETIC TRANSITION IN LA1-XYXMN2SI2-FE-57

The transition from ferromagnetism (F) in LaMn2Si2 to antiferromagnetism (AF) in YMn2Si2 has been investigated in detail by a Mossbauer effect study of a set of eight La1-xYxMn2Si2 samples (x = 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5 and 1) doped with(57)Fe. The room-temperature spectra are well fitted by two sextets, in which one dominant sextet(greater than or equal to 90% relative area) represents the occupation of the Mn 4d site by the Fe-57 atoms. We find a distinct change in the values of the magnetic hyperfine field and the quadrupole splitting between the spectra of x less than or equal to 0.20 and those of x greater than or equal to 0.30, which shows clear evidence for the F to AF transition. The transition is centred around a critical yttrium concentration x(c) similar to 0.25, although the spread in concentration over which the changes in the hyperfine parameters occur reflects the competition between ferromagnetic and antiferromagnetic interactions, leading to a complex spin-glass-type magnetic behaviour around x(c). The transition occurs as a result of changes in the exchange interactions related to the changes in the interatomic distances and the unit cell volume.