Evolution of the ferromagnetic phase of ultrathin Fe films grown on GaAs(100)-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>4</mml:mn><mml:mo>×</mml:mo><mml:mn>6</mml:mn></mml:math>

Epitaxial bcc Fe has been grown on GaAs(100)-$(4\ifmmode\times\else\texttimes\fi{}6)$ at room temperature and studied with in situ magneto-optical Kerr effect (MOKE), low-energy electron diffraction, and alternating gradient field magnetometry (AGFM). The magnetic properties at room temperature were found to proceed via three phases; a nonmagnetic phase for the first three and a half monolayers, a short-range-ordered superparamagnetic phase, and a ferromagnetic phase above about five monolayers. The thickness dependencies of the coercivity and MOKE intensity further suggested that the ferromagnetic phase is subdivided into three distinct regimes with different magnetic properties. A combination of the in situ MOKE and ex situ AGFM measurements shows that the entire Fe film is ferromagnetic with a bulklike moment after the onset of the ferromagnetism, in contrast with previous studies, in which magnetic dead layers or half-magnetization phases due to the intermixing of Fe and As were proposed. The results show that it is the growth morphology of the ultrathin films, rather than the diffusion of As, that plays the dominant role in determining the magnetic properties in this system.