Coverage effects on the magnetism of $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}\left(001\right)$ ultrathin films

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 $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}\left(001\right)$ interface.

Figure 1

X-ray reflectivity simulations for a set of ultrathin Fe films of different thickness grown on and capped with different materials. Air-cap, cap-Fe and Fe-substrate interface roughness are $5\mathrm{\AA }$.

Figure 2

(a) X-ray reflectivity data and best fit for a series of $\mathrm{Mg}\mathrm{O}∕\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}$ samples with variable Fe thickness (the best fit is shown below the corresponding experimental curve). The indicated Fe thicknesses are those extracted from fits. The reflectivity for a sample with MgO buffer and capping layers deposited under equivalent conditions but with no Fe deposition (labeled as $0\mathrm{\AA }$ Fe) is also shown as a reference. (b) The electron density profile obtained from the fit of a representative sample. MgO electron density is $6.67\times {10}^{23}\text{electrons}∕{\mathrm{cm}}^3$. (c) Fe thickness extracted from fits to XRR data versus the nominal Fe thickness.

Figure 3

In-situ transverse Kerr hysteresis loops with the magnetic field applied along different directions (easy [100] Fe axis, hard [110] Fe axis) for 2 ML, 4 ML, 6 ML and 210 ML samples.

Figure 4

The calculated magnetic moment as a function of the film thickness. Inset: in-plane hysteresis loops for the 2 ML, 4 ML and 6 ML samples from SQUID measurements at RT, once the background from the MgO substrate and capping layers has been subtracted. The dashed line for the 2 ML curve is the fit to the Langevin equation.

Figure 5

Polar Kerr hysteresis loops for the 2 ML, 4 ML, 6 ML, and 8 ML samples.

Figure 6

Polar Kerr spectra for the same samples as Fig. 5; dots: experiment, lines: theoretical simulations assuming a Fe continuous layer 2 ML, 4 ML, 6 ML, and 8 ML thick.

Figure 7

Fe thickness dependence of the transverse magneto-optical response for $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}\left(001\right)$ films. The continuous line is a simulation assuming a perfectly continuous film.

Figure 8

(a)–(d) In-situ transverse Kerr hysteresis loops for different Fe thin films. (a) 3.5 ML sample, as prepared. The field is applied along the [100] direction; (b) 14 ML sample, as prepared. The field is applied along the [100] direction; (c) 3.5 ML sample after annealing at $400°\mathrm{C}$. The two curves have been obtained with the field applied along the [100] and [110] directions; (d) 14 ML sample after annealing at $400°\mathrm{C}$. The field is applied along the [100] direction. (e)–(f): simulated hysteresis loops for an ensemble of seventy-six iron particles. (e) The particles are interconnected forming three clusters; (f) the particles are interconnected forming a single cluster. (g)–(h) Particle distribution over a $124\times 124{\mathrm{nm}}^2$ area for the simulations shown in (e) (with the three clusters shown in black, dark gray and light gray) and (f), respectively.

Figure 9

(a) Calculated effective anisotropy constant $K_{\mathrm{eff}}$ for different Fe thicknesses. The broken line is the fourfold bulk anisotropy constant $K_1$ for bulk Fe $(4.6\times {10}^5\text{erg}∕{\mathrm{cm}}^3)$, whereas the solid line is the fit to a $1∕t$ dependence expected for an interface-type anisotropy. (b) HREM image (left) of a $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}\left(001\right)$ interface observed along the [100] zone axis and corresponding Inverse Fast Fourier Transform image (right), filtered by the ${020}_{\mathrm{Mg}\mathrm{O}}$ and ${110}_{\mathrm{Fe}}$ spots, evidencing the appearance of misfit dislocations.