Coverage effects on the magnetism ofFe∕MgO(001)ultrathin films

Abstract

Different aspects of the structure-magnetism and morphology-magnetism correlation in the ultrathin limit are studied in epitaxial Fe films grown on MgO(001). In the initial stages of growth the presence of substrate steps, intrinsically higher than an Fe atomic layer, prevent the connection between Fe islands and hence the formation of large volume magnetic regions. This is proposed as an explanation to the superparamagnetic nature of ultrathin Fe films grown on MgO in addition to the usually considered islanded, or Vollmer-Weber, growth. Using this model, we explain the observed transition from superparamagnetism to ferromagnetism for Fe coverages above 3 monolayers (ML). However, even though ferromagnetism and magnetocrystalline anisotropy are observed for 4 ML, complete coverage of the MgO substrate by the Fe ultrathin films only occurs around 6 ML as determined by polar Kerr spectra and simulations that consider different coverage situations. In annealed 3.5 ML Fe films, shape or configurational anisotropy dominates the intrinsic magnetocrystalline anisotropy, due to an annealing induced continuous to islanded morphological transition. A small interface anisotropy in thicker films is observed, probably due to dislocations observed at the Fe/ MgO͑001͒ interface.

Interfacial geometry dependence of the iron magnetic moment: The case of MgO/Fe/MgO

“…Details have been previously described. 26 Briefly, experiments were conducted on sputtered films deposited at RT in order to avoid threedimensional island growth and minimize interdiffusion. The thickness of the Fe layer was accurately determined by measuring the x-ray reflectivity.…”

“…34 We have considered Fe coverages down to 0.5 ML, since in real samples, the interface cannot be perfectly flat (the MgO substrate steps being intrinsically higher than an Fe ML). 26 Two different slab structures were designed: a periodic thin film configuration separated by 15Å of vacuum, and superlattice arrangements. By changing the barrier width, we aimed to quantify the relative importance of the coordination determining the mm: superlattice structures with an even number of MgO monolayers consist of stacked layers alternating between those that contain only Fe atoms placed atop O atoms (Fe-O), consistent with the experimental data, 35,36 and those in which the Fe is forced to bind to Mg (Fe-Mg).…”

Correlations between magnetic properties and bond formation in Rh–MgO(0 0 1)

“…Experimentally, MgO(001) is used frequently as a substrate for metallic thin film growth (see references [40][41][42][43][44][45][46][47][48], for example). As for the 3d metals, Li and Freeman [40] have studied magnetism theoretically in the Fe/MgO(001) system.…”

Enhanced and oscillatory magnetoresistance of thin Fe(001) films

“…9 The Fe growth was confirmed to be layer by layer which allowed us to prepare ferromagnetic Fe films exhibiting cubic magnetocrystalline anisotropy and bulk magnetic moment values for coverages down to three atomic layers. Samples were either coated with MgO or Pt ͑ϳ25 Å͒ capping layers.…”