Discovery of intrinsic ferromagnetism in 2D van der Waals crystals

TL;DR

This paper reports the first experimental discovery of intrinsic long-range ferromagnetism in pristine 2D Cr2Ge2Te6 van der Waals crystals, demonstrating tunable transition temperatures and potential for spintronic applications.

Contribution

It provides the first evidence of intrinsic ferromagnetism in 2D vdW crystals and reveals how small magnetic fields can significantly enhance transition temperatures.

Findings

Intrinsic ferromagnetism observed in 2D Cr2Ge2Te6

Transition temperature can be increased by 35-57% with small magnetic fields

Field dependence of transition temperature is a hallmark of 2D soft ferromagnets

Abstract

It has been long hoped that the realization of long-range ferromagnetic order in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich electronic and optical properties, would open up new possibilities for magnetic, magnetoelectric and magneto-optic applications. However, in 2D systems, the long-range magnetic order is strongly hampered by thermal fluctuations which may be counteracted by magnetic anisotropy, according to the Mermin-Wagner theorem. Prior efforts via defect and composition engineering, and proximity effect only locally or extrinsically introduce magnetic responses. Here we report the first experimental discovery of intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers by scanning magneto-optic Kerr microscopy. In such a 2D vdW soft ferromagnet, for the first time, an unprecedented control of transition temperature of ~ 35% - 57% enhancement is realized via surprisingly small fields (<= 0.3 Tesla in this work), in stark contrast to the stiffness of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field enables an effective anisotropy far surpassing the tiny magnetocrystalline anisotropy, opening up a sizable spin wave excitation gap. Confirmed by renormalized spin wave theory, we explain the phenomenon and conclude that the unusual field dependence of transition temperature constitutes a hallmark of 2D soft ferromagnetic vdW crystals. Our discovery of 2D soft ferromagnetic Cr2Ge2Te6 presents a close-to-ideal 2D Heisenberg ferromagnet for studying fundamental spin behaviors, and opens the door for exploring new applications such as ultra-compact spintronics.