MnIII (pyrol) 3 (tren) (Hpyrol) 3tren = tris (1- (2-azolyl) -2-azabuten-4-yl) amine) is a mononuclear spin-transition compound switching between high spin (HS, S = 2) and low spin (LS, effective S = 1) around 47 K, preserving I4¯3d symmetry. Its magnetic anisotropy is studied by calculating the atomic susceptibility tensor from the refinement of polarized neutron powder diffraction. The analysis reveals that the weakly prolate-type atomic magnetic anisotropy in the HS state abruptly switches to uniaxial needle-shaped/Ising-type anisotropy in the LS state. However, the overall magnetic anisotropy of the unit cell remains isotropic due to the cubic nature of the crystal symmetry. Irreversible coexistence of mixed spin states HS/LS is observed in the vicinity of the cooperative spin crossover, where the average magnetic moment of Mn3+ shows a hysteretic temperature variation. This hysteretic mixing of HS and LS at intermediate temperatures suggests complex growth and nucleation of HS and LS domains. The study demonstrates that polarized powder neutron diffraction is a unique and powerful tool for describing complex magnetic anisotropies and magneto-structural correlations in molecular-based magnetic materials.
Pal et al. (Tue,) studied this question.