Thompson Corey M, Greedan John E, Garlea V Ovidiu, Flacau Roxana, Tan Malinda, Nguyen Phuong-Hieu T, Wrobel Friederike, Derakhshan Shahab
Department of Chemistry and Brockhouse Institute of Materials Research, McMaster University , 1280 Main Street W, Hamilton, Ontario L8S 4M1, Canada.
Inorg Chem. 2014 Jan 21;53(2):1122-7. doi: 10.1021/ic4026798. Epub 2014 Jan 9.
The novel iron-based compound, BaYFeO4, crystallizes in the Pnma space group with two distinct Fe(3+) sites, that are alternately corner-shared FeO5 square pyramids and FeO6 octahedra, forming into Fe4O18 rings, which propagate as columns along the b-axis. A recent report shows two discernible antiferromagnetic (AFM) transitions at 36 and 48 K in the susceptibility, yet heat capacity measurements reveal no magnetic phase transitions at these temperatures. An upturn in the magnetic susceptibility measurements up to 400 K suggests the presence of short-range magnetic behavior at higher temperatures. In this Article, variable-temperature neutron powder diffraction and high-temperature magnetic susceptibility measurements were performed to clarify the magnetic behavior. Neutron powder diffraction confirmed that the two magnetic transitions observed at 36 and 48 K are due to long-range magnetic order. Below 48 K, the magnetic structure was determined as a spin-density wave (SDW) with a propagation vector, k = (0, 0, (1)/3), and the moments along the b-axis, whereas the structure becomes an incommensurate cycloid [k = (0, 0, ∼0.35)] below 36 K with the moments within the bc-plane. However, for both cases the ordered moments on Fe(3+) are only of the order ∼3.0 μB, smaller than the expected values near 4.5 μB, indicating that significant components of the Fe moments remain paramagnetic to the lowest temperature studied, 6 K. Moreover, new high-temperature magnetic susceptibility measurements revealed a peak maximum at ∼550 K indicative of short-range spin correlations. It is postulated that most of the magnetic entropy is thus removed at high temperatures which could explain the absence of heat capacity anomalies at the long-range ordering temperatures. Published spin dimer calculations, which appear to suggest a k = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure.
新型铁基化合物BaYFeO₄结晶于Pnma空间群，有两个不同的Fe(3+)位点，它们交替以角共享的FeO₅方锥和FeO₆八面体形式存在，形成Fe₄O₁₈环，并沿b轴呈柱状延伸。最近的一份报告显示，在磁化率中于36 K和48 K出现了两个可分辨的反铁磁（AFM）转变，但热容测量表明在这些温度下没有磁相变。高达400 K的磁化率测量结果出现上升，表明在较高温度下存在短程磁行为。在本文中，进行了变温中子粉末衍射和高温磁化率测量以阐明磁行为。中子粉末衍射证实，在36 K和48 K观察到的两个磁转变是由于长程磁有序。在48 K以下，磁结构被确定为具有传播矢量k = (0, 0, (1)/3)且磁矩沿b轴的自旋密度波（SDW），而在36 K以下，结构变为具有bc平面内磁矩的非公度摆线[k = (0, 0, ∼0.35)]。然而，对于这两种情况，Fe(3+)上的有序磁矩仅约为3.0 μB量级，小于预期的接近4.5 μB的值，这表明Fe磁矩的显著部分在研究的最低温度6 K时仍保持顺磁性。此外，新的高温磁化率测量揭示了在约550 K处有一个峰值最大值，表明存在短程自旋关联。据推测，大部分磁熵因此在高温下被消除，这可以解释在长程有序温度下热容异常的缺失。已发表的自旋二聚体计算似乎表明k = (0, 0, 0)磁结构，且既不考虑低维性也不考虑几何失配，不足以解释观察到的复杂磁结构。
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