Existence of Two Concomitant Magnetic Structures Below T_Néel for the Natrochalcite, NaFe^II₂(H₃O₂)(MoO₄)₂

A comparative study of the magnetic properties and magnetic structures of the natrochalcite, NaFe2(D3O2)(MoO4)2 (FeD) to those of the isostructural NaCo2(D3O2)(MoO4)2 (CoD) and NaNi2(D3O2)(MoO4)2 (NiD) is presented. The structural change is a shrinking of the unit cell in the order of the ionic radii of the transition metal, FeD > CoD > NiD. While NiD and CoD are canted-antiferromagnets with TN = 28 and 21 K, respectively, FeD is an anisotropic 2D-Ising antiferromagnet (TN = 17 K) with a spin-flop field of 14 kOe at 2 K and the presence of a hysteresis loop reaching only 1/4 of the saturation magnetization in 70 kOe. The critical field decreases almost linearly on warming to TN. The neutron diffraction patterns of FeD below TN display numerous magnetic Bragg peaks which cannot be assigned to any one magnetic structure but fits well to two superposed sets, one with a temperature independent line width and has a propagation vector k1 = (0, 0, 0) while for the other there is a clear dependence and k2 = (0, 0, 1/2). In the k1 = (0, 0, 0) magnetic structure the moments are parallel to each other within one chain and lie along the a-axis but are antiparallel to those in neighboring chains. In contrast CoD and NiD, for which k = (0, 0, 0), have their moments aligned along the b-axis and ac-plane, respectively. The second magnetic structure, k2 = (0, 0, 1/2), is characterized by four sublattices, two per layer, where the moments are in the ab-plane and canted with a resultant along the a-axis which is compensated by those of the adjacent layers. For the k2 = (0, 0, 1/2) structure, the scattering coherent length decreases, and the moments tend progressively toward the a-axis upon increasing temperature. The coexistence of two concomitant magnetic structures is unprecedented for compounds containing transition metal moment carriers.本研究针对具有钠明矾石（natrochalcite）结构的NaFe₂(D₃O₂)(MoO₄)₂（下称FeD），与同构化合物NaCo₂(D₃O₂)(MoO₄)₂（下称CoD）、NaNi₂(D₃O₂)(MoO₄)₂（下称NiD）的磁学性质及磁结构开展对比研究。结果表明，其晶胞收缩程度随过渡金属离子半径变化呈现FeD > CoD > NiD的顺序。其中NiD与CoD分别在28 K、21 K时发生倾斜反铁磁相变，属于倾斜反铁磁体；而FeD为各向异性二维伊辛（2D-Ising）反铁磁体，奈尔温度（T_N）为17 K，在2 K下的自旋翻转场为14千奥斯特（kOe），且在70千奥斯特场下仅呈现达到饱和磁化强度1/4的磁滞回线。临界场随温度升高至T_N时近乎线性降低。FeD在T_N以下的中子衍射图谱呈现大量磁布拉格峰，无法归属于单一磁结构，但可通过两组叠加的磁结构很好地拟合：一组的线宽与温度无关，传播矢量k₁=(0,0,0)；另一组则表现出显著的温度依赖性，传播矢量k₂=(0,0,1/2)。在k₁=(0,0,0)的磁结构中，同一链内的磁矩相互平行且沿a轴方向排布，而与相邻链内的磁矩反平行。与之相反，CoD与NiD的磁传播矢量均为k=(0,0,0)，其磁矩分别沿b轴与ac平面方向排布。第二组磁结构k₂=(0,0,1/2)包含四个亚晶格，每层含两个亚晶格，磁矩位于ab平面内且发生倾斜，最终合磁矩沿a轴方向，且该合磁矩被相邻层的合磁矩抵消。对于k₂=(0,0,1/2)结构，随着温度升高，相干散射长度逐渐减小，磁矩逐步向a轴方向靠拢。对于含过渡金属磁矩载体的化合物而言，两种磁结构同时共存的现象此前尚无报道。