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

The transition from ferromagnetism (F) in LaMn2Si2 to antiferromagneti sm (AF) in YMn2Si2 has been investigated in detail by a Mossbauer effe ct 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 spectr a 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 tha n or equal to 0.30, which shows clear evidence for the F to AF transit ion. 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 competitio n between ferromagnetic and antiferromagnetic interactions, leading to a complex spin-glass-type magnetic behaviour around x(c). The transit ion occurs as a result of changes in the exchange interactions related to the changes in the interatomic distances and the unit cell volume.