我們針對金屬三角晶格反鐵磁體EuCo2Al9 單晶的磁性、熱力學與輸運性質，進行了全面的研究。該化合物結晶於六方空間群P6/mmm，其中Eu2+ 離子形成堆疊的三角晶格。高溫磁化率遵循居禮—外斯定律（Curie–Weiss law），得出的有效磁矩與Eu2+ 自由離子的理論值（S =7/2）一致。此外，負的居禮—外斯溫度（θCW ≈−12.5 K）指出反鐵磁交互作用佔主導地位。長程磁有序在TN ≈3.6K以下形成。於1.8 K測量的等溫磁化強度具有強烈的各向異性。當H⊥c時，磁化強度平滑上升至飽和值≈7µB/f.u.，且未出現任何磁化平台。相反地，當H ∥c時，在1–2 T 附近出現了明顯的1/3磁化平台。該平台是幾何阻挫易軸反鐵磁體中，共線「上-上-下」（up-up-down, UUD）態的標誌性特徵。透過結合磁化強度、磁阻與熱容量測量，我們建構了兩種磁場方向的磁場—溫度相圖。此外，完整磁熵Rln8的恢復證實了整個S =7/2的多重態（multiplet）皆參與了磁有序過程。我們的H ∥c 結果獨立驗證了近期關於該化合物金屬自旋超固體（spin-supersolid）相圖的報導。此外，本研究進一步將探討延伸至過去未被探索的H⊥c方向。綜合而言，這些全面的結果確立了EuCo2Al9 作為金屬系統中阻挫磁性研究的模型平台。

We report a comprehensive study of the magnetic, thermodynamic, and transport properties of single crystals of the metallic triangular-lattice antiferromagnet EuCo2Al9. This compound crystallizes in the hexagonal space group P6/mmm, where the Eu2+ ions form a stacked triangular lattice. The high-temperature susceptibility follows the Curie–Weiss law, yielding an effective moment consistent with the free-ion value of Eu2+ (S = 7/2). Furthermore, a negative Curie–Weiss temperature (θCW ≈ −12.5 K) indicates dominant antiferromagnetic interactions. Long-range magnetic order devel ops below TN ≈ 3.6 K. The isothermal magnetization at 1.8 K is strongly anisotropic. For H ⊥ c, it rises smoothly to a saturation of ≈ 7 µB/f.u. without any plateau. In contrast, for H ∥ c, a clear 1/3 magnetization plateau appears near 1–2 T. This plateau is the hallmark of the collinear up-up-down (UUD) state in a geometrically frustrated easy-axis antiferromagnet. By combining magnetization, magnetoresistance, and heat capacity measurements, we construct the field–temperature phase diagrams for both field orientations. Additionally, the recovery of the full magnetic entropy Rln8 confirms that the entire S = 7/2 multiplet participates in the ordering. Our H ∥ c results provide an independent verification of the metallic spin-supersolid phase diagram recently reported for this compound. Moreover, our work extends the study to the previously unexplored H ⊥ c orientation. Together, these comprehensive results establish EuCo2Al9 as a model platform for frustrated magnetism in metallic systems.

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