On some magnetic phase transitions in CePtSn

We present measurements of the magnetization and specific heat of a CePtSn single crystal in the temperature range 1.7-15 K and in magnetic fields up to 14 T applied along the orthorhombic b axis. Possible microscopic mechani sms responsible for the observed magnetic phase transitions are discussed i n the context of previously reported magnetoresistance and neutron-scatteri ng data. Special emphasis is put on an irreversible field-induced transitio n around 4 T observed for temperatures T less than or equal to2 K. At this transition, found when sweeping the field up from the zero-field-cooled (ZF C) state, a tiny change in the magnetization is accompanied by a large nega tive GMR effect. Surprisingly, no transition is observed when sweeping the field back to zero, i.e. the 'field-annealed' phase (after the application of a sufficiently high field) persists even in zero field and a simple AF s tructure with a propagation vector q=(0,1/2,0) is proposed to characterize this phase. Clear evidence for this transition is observed also in temperat ure dependencies of the magnetization and specific heat in various magnetic fields. Based on these results, the completed magnetic-phase diagram of Ce PtSn in fields parallel to the b axis is presented. (C) 2001 Elsevier Scien ce B.V. All rights reserved.