Order and phase nucleation in nonequilibrium nanocomposite Fe-Pt thin films with perpendicular magnetic anisotropy

We report on the time evolution of mass transport upon annealing nonequilibrium Fe-Pt nanocomposite films, leading to nucleation of $L{1}_0$ chemically ordered phase. The nonequilibrium nanocomposite films were fabricated by applying ${\text{Fe}}^{+}$ ion implantation to epitaxial Pt films grown on (001) MgO substrates, yielding Fe nanoclusters embedded in a Pt matrix at a tailored penetration depth. Time-resolved x-ray diffraction studies were carried out using synchrotron radiation, allowing determination of the activation energy for nucleation of the FePt $L{1}_0$ phase within the segregated nanoclusters during annealing. The growth of the segregated $L{1}_0$ ordered phase was modeled using ideal grain-size law and found to be dominated by strain-driven surface nucleation. The activation energies were found to correlate with the nanocluster size. Magnetic characterization of selected annealed samples indicates perpendicular magnetic anisotropy with high coercive field coincident with high value of the chemical order parameter of the ordered phase within the magnetic nanoclusters.

Figure 1

(Color online) Time-dependent grain-size evolution upon two annealing temperatures (490 and $510°\text{C}$). Two grain sizes are found at both temperatures; the upper two curves correspond to the larger grain size $\left(d>20\text{nm}\right)$, whereas the lower two curves correspond to the smaller grain size $\left(d<10\text{nm}\right)$. A larger grain growth exponent [Eq. (1)] $n\approx 15$ is found for the larger grain size, whereas the smaller grain size exhibits $n\approx 6$.

Figure 2

(Color online) XRD symmetric scans for (a) Pt film as deposited, (b) Pt film after Fe ion implantation $\left(\text{dose}:5\times {10}^{16}\text{ions}/{\text{cm}}^2\right)$, and (c) Fe-implanted Pt film after 1 h annealing at $450°\text{C}$. The onset of $L{1}_0$ FePt ordered phase after annealing is evidenced by the fundamental FePt(002) and the superlattice FePt(001) reflections. A Gaussian fitting of the reflections is shown, with the Pt film reflections being represented by blue lines, the ordered FePt phase with green lines, and the cumulative fitting with red lines.

Figure 3

Polar and transverse Kerr loops along with MFM images for the Fe-implanted $\left(\text{dose}:5\times {10}^{16}\text{ions}/{\text{cm}}^2\right)$ Pt film (sample A) (a) before and (b) after annealing at $450°\text{C}$ for 1 h. The perpendicular loop measured after annealing shows the inhomogeneity of the sample, where areas with opposite magneto-optical constants are illuminated. MFM images before and after annealing (please notice their different scale) show the anisotropy change and the onset of domains with perpendicular magnetization for the latter in the region labeled as P, while the region without perpendicular magnetization is labeled O.

Figure 4

Polar Kerr loops for the Fe-implanted $\left(\text{dose}:2.5\times {10}^{16}\text{ions}/{\text{cm}}^2\right)$ Pt film (sample B) (a) after implantation and (b) after 5 min annealing at $450°\text{C}$. At this stage the sample exhibits magnetically decoupled nanoregions with strong perpendicular magnetic anisotropy. (c) After additional 8 min (13 min total) annealing at $450°\text{C}$. The inset to (c) depicts a MFM image of such sample after the two annealings, showing a homogeneous mazelike domain pattern on the surface indicating magnetic coupling.