Mechanism of $c$-axis orientation of $L{1}_0$ FePt in nanostructured $\mathrm{Fe}\mathrm{Pt}∕{\mathrm{B}}_2{\mathrm{O}}_3$ thin films

In oder to obtain a high-density magnetic recording medium using an $L{1}_0$ ferromagnetic ordered alloy, it is imperative to make its $c$-axis perpendicular to the thin film. In this work, we have investigated experimentally the $c$-axis orientation process of the $L{1}_0$ FePt in nanostructured $\mathrm{Fe}\mathrm{Pt}∕{\mathrm{B}}_2{\mathrm{O}}_3$ thin films, and its orientation mechanism is proposed on the basis of the micromechanics concept. From our present experimental results, we have found that there are three characteristic features when such $c$-axis orientation occurs in the thin films: the marked crystallographic orientation occurs during the cooling process, the axial ratio $c∕a$ experimentally observed tends to be considerably lower than the equilibrium value, and the degree of $c$-axis orientation is lowered for a relatively thick film, that is, the plane-stress state plays a key role to make the $c$-axis perpendicular to the film surface. In-plane (biaxial) tensile stresses are considered to be yielded due to the thermal shrinking difference between the two materials, and ordered FePt particles with the $c$-axis perpendicular to the film surface is considerably stabilized under such in-plane tensile stresses in terms of the mechanical interaction energy. The validity of this mechanism is also confirmed for $\mathrm{Co}\mathrm{Pt}∕{\mathrm{B}}_2{\mathrm{O}}_3$ thin films.

Figure 1

(Color online) (a) XRD profiles of ${(3\mathrm{nm}{\mathrm{B}}_2{\mathrm{O}}_3∕3\mathrm{nm}\mathrm{Fe}\mathrm{Pt})}_{10}$ thin films obtained for various annealing temperatures, and (b) FESEM micrographs. The $c$-axis orientation of ${(3\mathrm{nm}{\mathrm{B}}_2{\mathrm{O}}_3∕3\mathrm{nm}\mathrm{Fe}\mathrm{Pt})}_{10}$ is lowered when the annealing temperature is relatively high $(700°\mathrm{C})$. In contrast, the crystallographic orientation hardly occurs for the thin film annealed at $450°\mathrm{C}$. The annealing temperature of $550°\mathrm{C}$ is the most appropriate for the $c$-axis orientation and the enhancement of ordering.

Figure 2

(Color online) (a) XRD profiles of ${(3\mathrm{nm}{\mathrm{B}}_2{\mathrm{O}}_3∕3\mathrm{nm}\mathrm{Fe}\mathrm{Pt})}_{10}$ thin films obtained for various annealing times, and (b) the axial ratio $c∕a$ obtained from (002) and (111) diffractions. The experimental $c∕a$ values are considerably smaller than the equilibrium value 0.964 (PDF No. 43-1359).

Figure 3

(Color online) XRD profiles obtained for the multilayered thin films, ${(3\mathrm{nm}{\mathrm{B}}_2{\mathrm{O}}_3∕3\mathrm{nm}\mathrm{Fe}\mathrm{Pt})}_{10}$, that were cooled down at various cooling rates after annealed at $550°\mathrm{C}$ for $1\mathrm{h}$. The heating rate to the objective temperature was set at $60°\mathrm{C}∕\min$ in all cases. Despite the same annealing time, degree of the $c$-axis orientation is very different. The top XRD profile was obtained for the thin film that was again heated and cooled at $60°\mathrm{C}∕\min$ after cooled at $500°\mathrm{C}∕\min$ (after annealed at $550°\mathrm{C}$ for $1\mathrm{h}$). Remarkable $c$-axis orientation is found to occur in the subsequent slow cooling $(60°\mathrm{C}∕\min )$ process.

Figure 4

(Color online) XRD profile obtained for the triple-layered relatively thick film, $30\mathrm{nm}$ ${\mathrm{B}}_2{\mathrm{O}}_3∕30\mathrm{nm}$ $\mathrm{Fe}\mathrm{Pt}∕30\mathrm{nm}$ ${\mathrm{B}}_2{\mathrm{O}}_3$, annealed at $550°\mathrm{C}$ for $1\mathrm{h}$. The (111) diffraction peak was only detected in such a thick film.

Figure 5

(Color online) (a) XRD profiles obtained for the multilayered thin films, ${(3\mathrm{nm}{\mathrm{B}}_2{\mathrm{O}}_3∕3\mathrm{nm}\mathrm{Fe}\mathrm{Pt})}_{10}$, annealed at 450 and $550°\mathrm{C}$ for $1\mathrm{h}$ with or without a magnetic field of $100\mathrm{kOe}$. (b) FESEM micrograph of the thin film annealed at $550°\mathrm{C}$ for $1\mathrm{h}$. (c) $M\text{−}H$ loop measured by a SQUID for the thin films annealed at $550°\mathrm{C}$ (left) and $450°\mathrm{C}$ (right).

Figure 6

(Color online) (a) XRD profiles obtained for the triple-layered thin films, $3\mathrm{nm}$ ${\mathrm{B}}_2{\mathrm{O}}_3∕5\mathrm{nm}$ $\mathrm{Fe}\mathrm{Pt}∕3\mathrm{nm}$ ${\mathrm{B}}_2{\mathrm{O}}_3$, annealed at $550°\mathrm{C}$ for $1\mathrm{h}$ with or without a magnetic field of $100\mathrm{kOe}$ and (b) FESEM micrographs for the respective samples.

Figure 7

(Color online) Schematic illustration showing the mechanism of the $c$-axis orientation upon cooling process. (a) Accommodation of molten ${\mathrm{B}}_2{\mathrm{O}}_3$ to FePt nanoparticles. (b) Occurrence of tensile stress due to the thermal shrinking difference between FePt and ${\mathrm{B}}_2{\mathrm{O}}_3$. (c) Macroscopic shape change of FePt particles due to the tetragonal distortion and rotation of particles caused by ordering under the tensile stress. (d) Preferable rotation of the ordered FePt nanoparticle.

Figure 8

(Color online) XRD profiles obtained for CoPt multilayered thin film, ${(3\mathrm{nm}{\mathrm{B}}_2{\mathrm{O}}_3∕3\mathrm{nm}\mathrm{Co}\mathrm{Pt})}_{10}$. In the as-deposited film, the crystallographic [111] direction is oriented in the normal direction of the surface. The $c$-axis orientation occurs when the thin film is annealed at $550°\mathrm{C}$, but in contrast, it does not occur when cooled down from $900°\mathrm{C}$.