In 1929, Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontaneous transition to a fully magnetized state at very low densities, because the exchange energy gained by aligning the spins exceeds the enhancement in the kinetic energy¹. However, experimental realizations of this effect have been hard to implement. Here, we report the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism. Our platform utilizes the two-dimensional Fermi sea of composite fermions near half-filling of the lowest Landau level. We measure the Fermi wavevector—which directly provides the spin polarization—and observe a sudden transition from a partially spin-polarized to a fully spin-polarized ground state as we lower the density of the composite fermions. Our theoretical calculations that take Landau level mixing into account provide a semi-quantitative account of this phenomenon.

1929年，费利克斯·布洛赫（Felix Bloch）提出，顺磁性电子费米海应在非常低的密度下自发转变为完全磁化状态，因为通过排列自旋而获得的交换能超过了动能的增强¹。但是，很难实现这种效果的实验性实现。在这里，我们报告观察到相互作用驱动的突然转变为完全磁化，这非常让人联想到Bloch铁磁性。我们的平台利用复合费米子的二维费米海，将最低的朗道水位填满一半。我们测量直接提供自旋极化的费米波矢，并观察到当我们降低复合费米子密度时突然从部分自旋极化转变为完全自旋极化的基态。我们的理论计算将Landau水平混合考虑在内，提供了对该现象的半定量说明。