Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Recent magnetic measurements on molecular magnets [Ni(hmp)(ROH)Cl]₄, where RCH₃, CH₂CH₃, or (CH₂)₂C(CH₃)₃, and hmp is the monoanion of 2-hydroxymethylpyridine, revealed a strong exchange bias prior to the external magnetic-field reversal as well as anomalies in electron paramagnetic-resonance peaks at low temperatures. To understand the exchange bias and observed anomalies, we calculate the electronic structure and magnetic properties for the Ni₄ molecules with the three different ligands, employing density-functional theory. Considering the optimized structure with possible collinear spin configurations, we find that the lowest-energy state has a total spin of S=0. The magnetic anisotropy barrier to reverse the magnetic moments of all four Ni ions simultaneously is calculated to be 4–6 K.