Domain wall properties of FePt: From Bloch to linear walls

An investigation of the orientation and temperature dependence of domain wall properties in FePt is presented. We use a microscopic, atomic model for the magnetic interactions within an effective, classical spin Hamiltonian constructed on the basis of spin density functional theory. We find a significant dependence of the domain wall width and the domain wall energy on the orientation of the wall with respect to the crystal lattice. Investigating the temperature dependence, we demonstrate the existence of elliptical as well as linear domain walls in FePt. The calculation and further analysis of the domain wall free energy results in the evaluation of a thermodynamic exchange stiffness and anisotropy constant.

Figure 1

(Color online) Sketches of the different types of domain wall: (a) circular, (b) elliptical, and (c) linear ones. The $z$ component and the transverse component of the magnetization are shown.

Figure 2

(Color online) Easy axis and transverse component of the magnetization versus space coordinate for parallel DWs at $311\mathrm{K}$. The solid lines are fitted following Eqs. (5, 6).

Figure 3

(Color online) $z$ component (below) and transverse component (above) of the magnetization versus space coordinate for perpendicular DWs at different temperatures. Results are from atomistic simulations for FePt. Solid lines represent fitted cosh and tanh profiles, respectively, to Eqs. (6, 5).

Figure 4

(Color online) $z$ component (below) and transverse component (above) of the magnetization versus space coordinate. Results are from MF calculations for the larger value of anisotropy. Solid lines represent fitted cosh and tanh profiles, respectively, to Eqs. (6, 5).

Figure 5

(Color online) The transverse and easy-axis amplitudes versus temperature. In the upper figure, results are from atomistic simulations for FePt; in the lower figure, results are from MF approximation for two different values of the anisotropy.

Figure 6

(Color online) Domain wall width $\delta$ versus temperature. The upper figure shows results for the atomistic FePt model for two different domain wall orientations. The lower figure shows results for the MF approximation for two different values of the anisotropy.

Figure 7

(Color online) Domain wall energy and free energy versus temperature for the two different orientations of the domain wall (above). The figure below shows results from MF calculations. The curves for $D=0.01442J$ are comparable with $\Delta F$ and $\Delta E$ from the FePt simulation.

Figure 8

(Color online) Temperature dependence of the uniaxial anisotropy constant and the exchange stiffness for FePt.