We have performed magnetic susceptibility and low and high field magnetization measurements on single crystals of the Ni tetramer compound [Ni4(μ-CO3)2(aetpy)8] (ClO 4)4 (aetpy = 2-aminoethyl-pyridine) with a butterfly (diamond) structure. We observed a two-step-like magnetization curve and a further increase of magnetization around 63 T in magnetic fields up to 68 T perpendicular to the Ni tetramer plane (c-axis). From the magnetic fields at steep jumps of the magnetization, we determine broadly the short diagonal (J1) and the side (J2) antiferromagnetic exchange constants between the Ni ions (J1/kB = 41.9 K and J 2/k2 = 9.2 K) in this compound. The long diagonal exchange interaction (J3) is ferromagnetic and the exchange constant is evaluated as J3/kB = -0.66 ± 0.04 K from the comparison between the magnetization measured in steady fields at 2 K and the calculated one. The origin of hysteresis of the magnetization in pulsed magnetic fields is discussed based on the calculated energy diagram.
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