Emergent magnetism in van der Waals materials offers
exciting opportunities
in fabricating atomically thin spintronic devices. One pertinent obstacle
has been the low transition temperatures (T
c) inherent to these materials, precluding room temperature applications.
Here, we show that large structural gradients found in highly strained
nanoscale wrinkles in Cr2Ge2Te6 (CGT)
lead to significant increases of T
c. Magnetic
force microscopy was utilized in characterizing multiple strained
CGT nanostructures leading to experimental evidence of elevated T
c, depending on the strain percentage estimated
from finite element analysis. Our findings are further supported by ab initio and DFT studies of the strained material, which
indicates that strain directly augments the ferromagnetic coupling
between Cr atoms in CGT, influenced by superexchange interaction;
this provides strong insight into the mechanism of the enhanced magnetism
and T
c.
Intrinsic two-dimensional (2D) multiferroics that couple ferromagnetism and ferroelectricity are rare. Here, we present an approach to achieve 2D multiferroics using powerful intercalation technology. In this approach, metal atoms such as Cu or Ag atoms are intercalated in bilayer CrI 3 to form Cu(CrI 3 ) 4 or Ag(CrI 3 ) 4 . The intercalant leads to the inversion symmetry breaking and produces a large out-of-plane electric polarization with a low transition barrier and a small reversal electric field, exhibiting excellent 2D ferroelectric properties. In addition, due to charge transfer between the intercalated atoms and bilayer CrI 3 , the interlayer coupling transits from antiferromagnetic to ferromagnetic, and the intralayer ferromagnetic coupling is also enhanced. Furthermore, the built-in electric polarization causes a distinct surface magnetization difference, generating a strong magnetoelectric coupling with a coefficient larger than that of Fe, Co, and Ni thin films. Our work paves a practical path for 2D multiferroics, which may have crucial applications in spintronics.
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