Molecular beam epitaxy is widely used for engineering low-dimensional materials. Here, we present a novel extension of the capabilities of this method by assisting epitaxial growth with the presence of an external magnetic field (MF). MF-assisted epitaxial growth was implemented under ultra-high vacuum conditions thanks to specialized sample holders for generating in-plane or out-of-plane MF and dedicated manipulator stations with heating and cooling options. The significant impact of MF on the magnetic properties was shown for ultra-thin epitaxial magnetite films grown on MgO(111). Using in situ and ex situ characterization methods, scanning tunneling microscopy, conversion electron Mössbauer spectroscopy, and the magneto-optic Kerr effect, we showed that the in-plane MF applied during the reactive deposition of 10 nm Fe3O4(111)/MgO(111) heterostructures influenced the growth morphology of the magnetite films, which affects both in-plane and out-of-plane characteristics of the magnetization process. The observed changes are explained in terms of modification of the effective magnetic anisotropy.

Conversion electron Moessbauer spectra (CEMS) were collected for ultrathin magnetite films.  Data are the source spectra presented in Figs 3 and 4:

Fig 3a ba_Fe3O4_1_graze.txt : Source data to Fig. 3a. CEMS spectrum of a 100 Å magnetite film measured in situ under ultrahigh vacuum.
Fig 3b BA_Fe3O4_1_512.txt: Source data to Fig. 3b. CEMS spectrum of a 100 Å magnetite film measured ex situ after one months after exposure to the atmosphere
Fig 3c BA_Fe3O4_1_XII_2022.txt: Source data to Fig. 3c. CEMS spectrum of a 100 Å magnetite film measured ex situ, after six months after exposure to the atmosphere. 
Fig 4b ba_Fe3O4_5_bez_pola_512.txt: Source data to Fig. 4b. CEMS spectrum for the 10 nm Fe3O4(111) film on MgO(111) deposited with no-field.
Fig 4c ba_fe3o4__6_512.txt: Source data to Fig. 4c. CEMS spectrum for the 10 nm Fe3O4(111) film on MgO(111) deposited in-field.