First-principles prediction of the morphology of $L{1}_0$ FePt nanoparticles supported on Mg(Ti)O for heat-assisted magnetic recording applications

We perform first-principles calculations to predict the morphology of $L{1}_0$ ordered FePt nanoparticles grown on Mg(Ti)O substrates with relevance to application in heat-assisted magnetic recording (HAMR) media. We show how incorporation of Ti into MgO substrates reduces the FePt adhesion energy from $-1.29$ (pure MgO) to $-2.35\mathrm{J}/{\mathrm{m}}^2$ (pure TiO). This effect is due to the formation of strong Fe-Ti bonds at the interface. Consistent with experimental observations, the predicted equilibrium morphology of supported FePt nanoparticles is significantly changed, corresponding to increased wetting. This behavior is undesirable for HAMR media since it promotes grain growth which limits the storage density. We show how passivation of surface Ti atoms (e.g., with MgO) is sufficient to restore the wetting observed for pure MgO substrates offering a viable strategy for optimization of next generation recording media.

Figure 1

(a) An illustration of Wulff construction in two dimensions. Vectors corresponding to all low index miller indices are drawn with length proportional to their respective surface formation energy (e.g., as shown above for 01 and 11 surfaces). Lines (planes in 3D) are then drawn representing the surfaces intersecting the tips of these vectors (black dashed lines). The shape enclosed by these surfaces is the one that minimizes the total energy. (b) The Wulff-Kaichew constructions considers that one of the surfaces is in contact with a substrate and so the length of the vector corresponding to this direction is replaced by $\gamma +{\gamma }_{\mathrm{ad}}$.

Figure 2

(a) and (b) Two different views of the FePt morphology predicted to be most stable. (c) and (d) Corresponding illustrative examples of the atomic structure of a FePt nanoparticle. The brass and sliver spheres represent the iron and platinum atoms, respectively.

Figure 3

Different optimized configurations of Ti incorporated into MgO used to assess preferential incorporation sites. (a) 2 monolayers of Ti substituted into the center of a slab (left) and at the surface (right). The latter is found to be more stable $\left(\mathrm{\Delta }E=-0.252\mathrm{eV}\right)$. (b) An additional 2 monolayers of Ti substituted into the slab is again found to be more stable segregated at the surface $\left(\mathrm{\Delta }E=-0.184\mathrm{eV}\right)$. (c) A further 2 monolayers of Ti substituted into the slab is again found to be more stable segregated at the surface $\left(\mathrm{\Delta }E=-0.186\mathrm{eV}\right)$. The red, orange, and blue spheres represent oxygen, magnesium, and titanium atoms, respectively.

Figure 4

Optimized atomic structures of FePt/Mg(Ti)O systems [shown in (110) projection] with corresponding vertical distances between the interfacial Fe atom and adjacent cation (Mg or Ti) labeled. (a) FePt/MgO, (b) FePt/MgTi(2)O, and (c) FePt/MgTi(4)O. (d) Optimized atomic structure of FePt supported on a pure TiO slab passivated with 3 monolayer coating of pure MgO. In this case the vertical distance and adhesion energy is similar to that for a pure MgO substrate. The brass, sliver, orange, red, and blue spheres represent the iron, platinum, magnesium, oxygen, and titanium atoms, respectively.

Figure 5

Variation of adhesion energy with the number of Ti monolayers incorporated into the MgO substrate (red line). The inset shows the positive correlation between the vertical gap (see Fig. 4) and the adhesion energy.

Figure 6

(Left) A schematic showing an interface supercell with the interfacial Fe and Pt atoms as well as the Mg(Ti)O substrate atoms indicated. (Right) Variation of the charge transfer $(\mathrm{\Delta }q$, left scale) with number of TiO layers for the interfacial Fe and Pt atoms as well as the Mg(Ti)O substrate. Also shown is the magnetic moment on the interfacial Fe atom (right scale).

Figure 7

Predicted morphologies and illustrative atomic structures for FePt nanoparticles supported on (a) pure MgO and (b) MgO with more than 4 monolayers of TiO incorporated at the surface. The brass and sliver spheres represent the iron and platinum atoms, respectively.