Magnetic properties of a Ni tetramer with a butterfly structure in high magnetic fields

We have performed magnetic susceptibility and low and high field magnetization measurements on single crystals of the Ni tetramer compound [Ni-4(mu-CO3)(2)(aetpy)(8)] (ClO4)(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 (J(1)) and the side (J(2)) antiferromagnetic exchange constants between the Ni ions (J(1)/k(B) = 41.9 K and J(2)/k(B) = 9.2 K) in this compound. The long diagonal exchange interaction (J(3)) is ferromagnetic and the exchange constant is evaluated as J(3)/k(B) = -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.