Probing the dynamical properties of the metastable bcc ${\text{Fe}}_x{\text{Co}}_{1-x}$ phase

The stabilization of the metastable bcc-${\text{Fe}}_x{\text{Co}}_{1-x}\left(0\le x\le 0.25\right)$ phase is studied via the dynamical properties of the atoms. By means of nuclear resonant inelastic scattering of synchrotron radiation the phonon density of states (PDOS) spectra of Fe in bcc and fcc ${\text{Fe}}_x{\text{Co}}_{1-x}$ are obtained. We analyze the measured PDOS spectra using first-principles calculations of phonon vibrations. Theoretical models of the thermodynamic unstable bcc phase, the stable fcc phase, as well as the bcc-${\text{Fe}}_4/{\text{Co}}_4$ superlattice are constructed. Via calculations of the force constants and the phonon-dispersion relations we show that the bcc-${\text{Fe}}_x{\text{Co}}_{1-x}$ phase confined between two adjacent bcc-Fe layers can be stabilized by the Fe atoms in the ${\text{Fe}}_x{\text{Co}}_{1-x}$ layer.

Figure 1

(Color online) RHEED patterns observed during the growth in the Fe/bcc-Co multilayer on MgO(100) for (a) the first Fe layer on the Au buffer layer, (b) the fourth Co layer, and (c) the thirteenth Co layer.

Figure 2

High-angle x-ray diffractometry scans $\left(\theta \text{−}2\theta \right)$ of (a) the bcc-Co sample ${\left[{}^{56}\text{F}\text{e}\left(2.4\text{nm}\right)/{{}^{56}\text{F}\text{e}}_{0.1}{\text{Co}}_{0.9}\left(1.7\text{nm}\right)\right]}_{20}$ and (b) the reference fcc-Co sample ${}^{57}\text{F}{\text{e}}_{0.1}{\text{Co}}_{0.9}\left(50\text{nm}\right)$.

Figure 3

(Color online) The supercell of the (a) bulk fcc ${\text{Fe}}_{0.0625}{\text{Co}}_{0.9375}$, (b) bulk bcc ${\text{Fe}}_{0.0625}{\text{Co}}_{0.9375}$, and (c) bcc-${\text{Fe}}_4/{\text{Co}}_4$ superlattice.

Figure 4

(Color online) (left) The phonon-dispersion relations and (right) the PDOS calculated for (a) fcc-Co crystal, (b) bcc-Co crystal, and (c) bcc-${\text{Fe}}_4/{\text{Co}}_4$ superlattice.

Figure 5

(Color online) (left) The phonon-dispersion relations and (right) the total and partial (calculated for Fe probe atoms) PDOS calculated for (a) fcc-${\text{Fe}}_{0.0625}{\text{Co}}_{0.9375}$ crystal, (b) bcc-${\text{Fe}}_{0.0625}{\text{Co}}_{0.9375}$ crystal, and (c) bcc-${\text{Fe}}_4/{\text{Co}}_4$ superlattice. All structures are illustrated in Fig. 3. The partial PDOS spectra for Fe probe atoms are multiplied by a factor of 5.

Figure 6

(Color online) The measured and calculated PDOS of ${}^{57}\text{F}\text{e}$ atoms in three different structures are compared: (a) the bcc-Fe crystal, (b) the bcc-Co structure doped by Fe atoms, and (c) the fcc-Co crystal doped by Fe atoms. In the calculations the ideal bulk systems (as described in the text) are used. The calculated spectra are convoluted with a Lorentzian function with a half width of 2 or 4 meV for the Fe crystal and ${\text{Fe}}_x{\text{Co}}_{1-x}$ crystals, respectively.