To cite igrf in publications use:

  Hufkens K. (2022). igrf: The 13th Generation International
  Geomagnetic Reference Field (IGRF) implemented as an R package
  https://doi.org/10.5281/zenodo.6527638

A BibTeX entry for LaTeX users is

  @Misc{,
    title = {igrf: The 13th Generation International Geomagnetic Reference Field (IGRF) implemented as an R package},
    author = {Koen Hufkens},
    year = {2022},
    doi = {10.5281/zenodo.6527638},
    url = {https://bluegreen-labs.github.io/igrf/},
  }