The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr₂Si₂, where in contrast to its non-magnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.

自旋电子学的核心是材料的发展，这些材料在整体上是非磁性的，但在表面表现出二维（2D）磁性。在这里，我们介绍了价态波动材料EuIr ₂ Si ₂，与非磁性体相反，其Si端接表面显示出可控的2D铁磁性。靠近表面的Eu离子更喜欢具有磁性的二价构型，并且其较大的4 f矩在48 K以下的阶次。新兴的交换相互作用改变了最初由强Rashba效应诱导的2D表面电子的自旋极化。本体的取决于温度的混合化合价允许调整二维电子被限制在其中的投影带隙的能量和动量大小。这为处理表面的自旋极化电子提供了额外的自由度。我们的发现揭示了基于价态波动的稀土基材料，这是开发具有可控2D磁性能的系统的非常有前途的基础，这对于基础科学和应用都非常重要。