Magnetic exchange switching in a series of niobium(IV)-based molecular magnets: chemical design vs. mechanical stimulus

A series of isostructural magnetic coordination polymers {[MII(pyrazole)4]2[NbIV(CN)8].4H2O}n (M = Mn, Fe, Co, Ni for 1, 2, 3 and 4, respectively) show long-range magnetic ordering below 24, 8, 6 and 13 K due to the strong magnetic coupling between M and Nb. The exchange coupling changes from -6.8 for 1, through -3.1 for 2, +3.5 for 3 to +8.1 cm-1 for 4. This affects the nature of the long-range magnetic ordering in the 1-4 series: ferrimagnetic in 1 and 2 with antiparallael alignment of the Mn-Nb and Fe-Nb magnetic moments to ferromagnetic in 3 and 4 with parallel alignment. The proposed XAS/XMCD experiments performed at the Nb L2,3 edges with the application of sum rules are the only reliable way to unequivocally determine the magnetic structure of the four compounds. Moreover, compound 3 demonstrates switching of the type of long-range magnetic ordering under pressure. This will also be investigated by collecting XAS/XMCD spectra at the Nb L2,3 edge under high pressure.

一系列同构磁性配位聚合物{[MII(吡唑)4]2[NbIV(CN)8].4H2O}n（其中M对应化合物1至4分别为Mn、Fe、Co、Ni），因M与Nb间的强磁耦合作用，在24 K、8 K、6 K及13 K以下表现出长程磁有序。交换耦合常数从化合物1的-6.8 cm⁻¹，经化合物2的-3.1 cm⁻¹、化合物3的+3.5 cm⁻¹，变化至化合物4的+8.1 cm⁻¹。这一变化影响了1-4系列化合物的长程磁有序性质：化合物1与2中Mn-Nb及Fe-Nb磁矩呈反平行排列，为亚铁磁性；化合物3与4中磁矩呈平行排列，为铁磁性。在Nb的L2,3边开展XAS/XMCD实验并应用求和规则，是明确测定这四种化合物磁结构的唯一可靠方法。此外，化合物3在压力下存在长程磁有序类型的转变，这也将通过高压下采集Nb L2,3边的XAS/XMCD谱图进行研究。