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

Abstract

ThMnAl and ThMnIn have the C-15, cubic Laves phase crystal structure. Pronounced thermomagnetic irreversibility is seen in the magnetisation of both compounds. A cusp in the AC susceptibility of ThMnAl near 43 K which shows frequency dependence establishes the spin glass freezing of the Mn moments in this compound. This is further corroborated by the heat capacity data which 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 monotonically 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 large in both compounds and shows a negative temperature coefficient indicating the disordered nature of both alloys.