Composition dependence of exchange stiffness in ${\text{Fe}}_x{\text{Pt}}_{1-x}$ alloys

The exchange stiffness constants of chemically disordered ${\text{Fe}}_x{\text{Pt}}_{1-x}$ films with thickness around 50 nm were determined by means of ferromagnetic resonance. It was found to increase with increasing Fe content from $6\pm 4\text{pJ}/\text{m}$ for $x=0.27$ to $15\pm 4\text{pJ}/\text{m}$ for $x=0.67$. Theoretical results from fully relativistic and scalar-relativistic band-structure calculations using the Korringa-Kohn-Rostoker method confirm the experimentally obtained values. In addition, determination of the magnetocrystalline anisotropy by angular-dependent measurements of the ferromagnetic resonance gave the possibility to estimate the exchange length that was found to be 40–50 nm for all compositions investigated in this work.

Figure 1

Example of a spin wave propagating along the $y$ direction while the effective field is parallel to the $z$ direction.

Figure 2

(a) FMR spectra of epitaxial ${\text{Fe}}_{0.46}{\text{Pt}}_{0.54}$ at room temperature and ${\nu }_{rf}\approx 10\text{GHz}$ for two different angles between the external magnetic field and the sample normal. The first derivative of absorbed microwave power is plotted against the external magnetic field. (b) Contour plot of the first derivative of absorbed microwave power as a function of external magnetic field value and polar angle.

Figure 3

Exchange coupling for Fe-Fe, Fe-Pt, and Pt-Pt pairs as a function of radial distance $r$ in units of the lattice constant $a$.

Figure 4

Exchange stiffness in fcc-${\text{Fe}}_x{\text{Pt}}_{1-x}$ films as a function of Fe content obtained by KKR calculations (open symbols) and FMR experiments at room temperature (filled symbols). The solid line is a guide to the eye. Experimental value for bcc Fe is taken from Ref. 26.

Figure 5

Upper graphics: contour plot of the first derivative of absorbed microwave power as a function of external magnetic field value and azimuth angle of (a) ${\text{Fe}}_{0.27}{\text{Pt}}_{0.73}$, (b) ${\text{Fe}}_{0.46}{\text{Pt}}_{0.54}$, (c) ${\text{Fe}}_{0.58}{\text{Pt}}_{0.42}$, and (d) ${\text{Fe}}_{0.67}{\text{Pt}}_{0.33}$ at room temperature and ${\nu }_{rf}\approx 10\text{GHz}$. Lower graphics: extracted azimuthal dependence of the FMR resonance field. Symbols refer to experimental data and lines refer to simulations.