AB堆叠双层蜂窝晶格中的阻挫铁磁相变

Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer

根据Mermin-Wagner定理,磁各向异性是二维体系磁有序的必要条件.而当出现相互竞争的各向异性时,磁相变会变得非同寻常.本文利用了扫描超导量子干涉仪在零场下对磁通的高灵敏度,对从单层到ABC堆叠的块材的CrBr3样品进行了磁化率和磁化强度的变温成像.研究发现厚层CrBr3在临界温区展现出层间与层内的各向异性竞争所导致的磁化率平台与尖峰.随着样品层数减少,这两个特征逐渐变宽并在双层达到与其临界温度类似大小的温度涨落,而在单层中则是更常规的铁磁相变行为.这表明二维极限下的量子涨落会增强层内与层间的竞争效应,产生类似阻挫的铁磁临界行为.这一结果揭示了一种在AB堆叠的双层蜂窝结构中的阻挫磁相变,并为进一步研究二维材料中的自旋阻挫打下基础.

In two-dimensional(2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked Cr Br3under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature(TC) in thick samples.It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of TC, which is distinct from both the monolayer and the bulk. Our observation demonstrates unconventional 2D critical behavior on a honeycomb lattice.