Magnetisation inverted hysteresis loops in the molecular magnets [M(Cp)2][Ni(alpha-tpdt)2] (M = Fe, Mn).

The low temperature magnetic properties of the compounds belonging to the family [M(Cp)(2)][Ni(alpha-tpdt)(2)], where M = Fe (), Mn () and Cr () (alpha-tpdt = 2,3-thiophenedithiolate) are studied, with emphasis on the magnetisation curves. These compounds share a common structure, composed of alternating layers, each consisting of mixed parallel D(+)A(-)D(+)A(-)D(+)A(-) chains; the chains of adjacent layers being perpendicular. At low temperatures, a rather unusual inverted hysteresis loop with negative remanence is observed in 1, and a similar reversed hysteresis is also detected in the magnetic field induced transition of 2. This effect is associated with the layered structure of these compounds, together with the considerable magnetic anisotropy of the Fe(Cp)(2) and Mn(Cp)(2) donors, being to the best of our knowledge, the first time such inverted hysteresis loops are observed in a bulk molecule based material.