YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ 层状钙钛矿：探索顺磁-共线-螺旋三重临界点之外的磁有序
YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ layered perovskites: exploring the magnetic order beyond the paramagnetic-collinear-spiral triple point
AI总结：
本研究通过 Sr 替代 Ba 增强 Cu/Fe 无序，发现 YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ 中螺旋态在临界无序度后转变为非阻挫反铁磁态，且两种有序温度同步升高，证实无序驱动的共同起源。
摘要：
通式 AA'CuFeO$_5$ 的层状钙钛矿是少数几种螺旋（cycloidal）自旋磁体之一，其中螺旋有序温度 $T_{spiral}$ 可通过在晶体结构中引入适量的 Cu/Fe 化学无序来调控至远高于室温。这一稀有特性使这些材料成为在室温下实现多铁性和磁电耦合的重要候选材料。此外，已有理论提出，该结构家族中可达到的最高 $T_{spiral}$ 值（约 400 K）对应于一个顺磁-共线-螺旋三重临界点，具有展现奇异物理的潜力。由于在实验中难以产生高程度的 Cu/Fe 无序，三重临界点之外的相图区域几乎未被探索。为填补这一空白，我们研究了 YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ 固溶体（$0 \leq x \leq 1$），其中用 Sr 替代 Ba，旨在增强实验上可获得的 Cu/Fe 无序的影响。结合体磁化强度、同步辐射 X 射线和中子粉末衍射测量，我们表明当 Cu/Fe 无序程度超过临界值时，螺旋态失稳，取而代之的是一种非阻挫、完全反铁磁的有序态，其传播矢量为 k$_{c2}$ = $(\frac{1}{2}, \frac{1}{2}, 0)$，有序温度 $T_{coll2}$ $\geq$ $T_{spiral}$，且该态在三重临界点之外逐渐稳定。有趣的是，$T_{spiral}$ 和 $T_{coll2}$ 随 $x$ 以相同速率增加，这表明它们具有共同的、由无序驱动的起源，与理论预测一致。
英文摘要：
Layered perovskites of general formula AA'CuFeO$_5$ are one of the few examples of cycloidal spiral magnets where the ordering temperatures $T_{spiral}$ can be tuned far beyond room temperature by introducing modest amounts of Cu/Fe chemical disorder in the crystal structure. This rare property makes these materials prominent candidates to host multiferroicity and magnetoelectric coupling at room temperature. Moreover, it has been proposed that the highest $T_{spiral}$ value that can be reached in this structural family ($\sim$ 400 K) corresponds to a paramagnetic-collinear-spiral triple point with potential to show exotic physics. Since generating high amounts of Cu/Fe disorder is experimentally difficult, the phase diagram region beyond the triple point has been barely explored. To fill this gap we investigate here the YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ solid solutions ($0 \leq x \leq 1$), where we replace Ba with Sr with the aim of enhancing the impact of the experimentally available Cu/Fe disorder. Using a combination of bulk magnetization, synchrotron X-ray and neutron powder diffraction we show that the spiral state is destabilized beyond a critical degree of Cu/Fe disorder, being replaced by a non-frustrated, fully antiferromagnetic state with propagation vector k$_{c2}$ = $(\frac{1}{2}, \frac{1}{2}, 0)$ and ordering temperature $T_{coll2}$ $\geq$ $T_{spiral}$, which is progressively stabilized beyond the triple point. Interestingly, $T_{spiral}$ and $T_{coll2}$ increase with $x$ at the same rate. This suggests a common, disorder-driven origin, consistent with theoretical predictions.
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