Possible sign change of Mn-Mn exchange interaction between ThMnAl and ThMnIn

ThMnAl and ThMnIn have the C-15, cubic Laves phase crystal structure. prono unced thermomagnetic irreversibility is seen in the magnetisation of both c ompounds. A cusp in the AC susceptibility of ThMnAl near 43 K which shows f requency dependence establishes the spin glass freezing of the Mn moments i n this compound. This is further corroborated by the heat capacity data whi ch do not exhibit any anomaly near the cusp temperature. On the other hand, the zero-field-cooled magnetisation of ThMnIn shows a sharp ferromagnetic- like increase around 270 K, a broad peak around 180 K and then decreases mo notonically with temperature. Various scenarios based on the sign change of Mn-Mn exchange interaction between ThMnAl and ThMnIn due to the increased Mn-Mn bond length in the latter are discussed. Electrical resistivity is la rge in both compounds and shows a negative temperature coefficient indicati ng the disordered nature of both alloys. (C) 2000 Elsevier Science B.V. All rights reserved.