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Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

Journal Article · · Nature (London)
DOI:https://doi.org/10.1038/nature22060· OSTI ID:1489264
 [1];  [2];  [3];  [4];  [2];  [1];  [3];  [1];  [1];  [5];  [6];  [4];  [3];  [2];  [5]
  1. Univ. of California, Berkeley, CA (United States). Nano-scale Science and Engineering Center (NSEC)
  2. Univ. of California, Irvine, CA (United States). Dept. of Physics and Astronomy
  3. Univ. of California, Berkeley, CA (United States). Dept. of Physics; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Sciences Division
  4. Princeton Univ., NJ (United States). Dept. of Chemistry
  5. Univ. of California, Berkeley, CA (United States). Nano-scale Science and Engineering Center (NSEC); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Sciences Division
  6. Univ. of California, Berkeley, CA (United States). Dept. of Physics
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The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications1,2,3,4. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin–Wagner theorem5; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering6,7,8,9,10, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.
Research Organization:
Lawrence Berkeley National Laboratory-National Energy Research Scientific Computing Center
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
DOE Contract Number:
AC02-05CH11231
OSTI ID:
1489264
Journal Information:
Nature (London), Journal Name: Nature (London) Journal Issue: 7657 Vol. 546; ISSN 0028-0836
Country of Publication:
United States
Language:
English

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