* Corresponding author. Tel.: #91-22-215-2971; fax: #91- 22-215-2110. E-mail address: sudesh@tifr.res.in (S.K. Dhar). Journal of Magnetism and Magnetic Materials 213 (2000) 404}412 Possible sign change of Mn}Mn exchange interaction between ThMnAl and ThMnIn P. Manfrinetti , A. Palenzona, S.K. Dhar*, C. Mitra, P.L. Paulose, D. Pal, S. Ramakrishnan INFM and Dipartimento di Chimica e Chimica Industriale, Universita di Genova, Sezione di Chimica Fisica, Via Dodecaneso, 31-16146, Genova, Italy Condensed Matter Department, T.I.F.R., Homi Bhabha Road, Mumbai 400005, India Received 20 May 1999; received in revised form 9 February 2000 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-"eld-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 coe$cient indicating the disordered nature of both alloys.  2000 Elsevier Science B.V. All rights reserved. Keywords: Spin glass freezing; Disorder; Frustration ThMn  , which crystallises in the C-14 hexagonal Laves phase, orders antiferromagnetically at 115 K [1]. The substitution of Al for Mn is known to lead to single-phase alloys, ThMn  Al  , which have the C-15 cubic Laves phase structure for 0.4( x(1.4 [2]. The hexagonal and the cubic Laves phase structures are closely interrelated. The Mn atoms lie at the corners of regular tetrahedron in both the structures. However, while there is only one Mn site in the unit cell of the cubic Laves phase, there are two symmetry inequivalent Mn sites, 2a and 6h, in the hexagonal Laves phase. The Mn atoms at the 6h site order magnetically, but those at the 2a site remain non-magnetic even be- low the Ne H el temperature in ThMn  [1]. The non- magnetic behaviour of Mn atoms at the 2a site, along with large and anisotropic thermal expansion are believed to minimise the frustration of the mag- netic interactions arising from the topology of the magnetic atomic arrangement in ThMn  [1]. It was of interest to know the magnetic behaviour of Mn atoms in the cubic Laves phase alloys ThMn  Al  and with that in mind we have studied ThMnAl in this report. An extension of our 0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 0 4 2 - 1