F-model phase transition in spin ice thin films

Spin ice is an exotic magnet known for its magnetic monopole excitations and disordered magnetic ground state. This is an experiment on half micron thick epitaxial films of spin ice, that behave like bulk spin ice under in-plane compression. Specific heat measurements suggest that this leads to an unusual phase transition from the disordered spin ice state (the magnetic monopole vacuum) to an ordered state. Such a phase transition is known in the "F-model" of statistical mechanics, but never hitherto observed in a magnetic material. The aim of this experiment is to measure magnetic Bragg peaks that should evolve at temperature below the transition and to map pout the phase diagram by measuring magnetic Bragg peaks as a function of applied magnetic field. In this way we will determine the magnetic order and phase diagram of this interesting system.

自旋冰（spin ice）是一类以磁单极子激发与无序磁基态为标志性特征的奇异磁体。本实验针对半微米厚度的自旋冰外延薄膜展开，该薄膜在面内压缩条件下可呈现体相自旋冰的行为。比热测量结果显示，这一体系会出现从无序自旋冰态（即磁单极子真空）到有序态的反常相变。这类相变在统计力学的“F模型”中已有理论预言，但此前从未在磁性材料中被观测到。本实验的目标为：测量相变温度以下应当出现的磁布拉格峰，并通过测量外加磁场下的磁布拉格峰绘制该体系的相图。借此我们将确定这一有趣体系的磁有序结构与相图。