Real-space evidence for 2D-XY magnetism in atomically thin Fe5GeTe2

The discovery of two-dimensional (2D) magnetism in CrI3 [1] and CrGeTe3 [2] has overturned
the notion that a continuous symmetry cannot be spontaneously broken in the 2D limit [3]. For
CrI3 and CrGeTe3, the Hamiltonians can be approximated by the 2D Ising and Heisenberg models,
respectively. For the former material, magnetism is stabilized against the gapless magnon dispersion
by a robust magnetic anisotropy [1]. However, suitable candidate materials for 2D-XY magnetism
remain elusive. A novel magnetic ordering, the vortex-antivortex pair, is predicted to occur in 2D
XY magnets, despite a deceptively simple Hamiltonian H = −J 2 cos(ϕi −ϕj) [4, 5]. Real-space
probes which are directly sensitive to magnetization would be immensely valuable in demonstrating
the existence of such ordering.
In this seminar, I will share XMCD-PEEM images of real-space magnetization in Fe5GeTe2, a
vdW ferromagnet with bulk room-temperature TC [6, 7]. Most notably, a renormalization of magne
tization by vortex-antivortex pairs, rather than by conventional magnetic domain walls, is observed
for 4ML (≈ 3.2nm) and 9nmFe5GeTe2. Temperature-dependent XMCD-PEEM measurements also
reveal signs of a Berezinskii-Kosterlitz-Thouless transition in 9nm Fe5GeTe2, which likely originates
from the relative lack of anisotropy in this 2DvdW magnet. These results will serve as a basis for
the synthesis and characterization of other candidate vdW materials for XY magnetism, and for
studying dynamic properties of magnons in 2D vdW magnets.

References
[1] B. Huang et al. Layer-dependent ferromagnetism in a van der Waals crystal down to the mono
layer limit. Nature, 546(7657), 2017.
[2] C. Gong et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals.
Nature, 546(7657), 2017.
[3] N. D. Mermin et al. Absence of Ferromagnetism or Antiferromagnetism in One- or Two
Dimensional Isotropic Heisenberg Models. Phys. Rev. Lett., 17(1307), 1966.
[4] V. L. Berezinskiˇ i. Destruction of Long-range Order in One-dimensional and Two-dimensional
Systems having a Continuous Symmetry Group I. Classical Systems. JETP, 32, 1971.
[5] J. M. Kosterlitz et al. Ordering, metastability and phase transitions in two-dimensional systems.
J. Phys. C: Solid State Phys., 6(7), 1973.
[6] A. F. May et al. Ferromagnetism Near Room Temperature in the Cleavable van der Waals
Crystal Fe5GeTe2. ACS Nano, 13(4), 2019.
[7] A. F. May et al. Physical properties and thermal stability of Fe5-xGeTe2 single crystals. Phys.
Rev. Mater., 3(10), 2019.