Magnetism of FePt Surface Alloys

The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, x-ray magnetic circular dichroism, and ab initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex noncollinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk $L{1}_0$ magnets is not essential to achieve high-anisotropy values.

Figure 1

STM images of ${\mathrm{Fe}}_x{\mathrm{Pt}}_{1-x}$ SAs formed during deposition of Fe atoms onto Pt(997) at 525 K. (a) ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$: Mixture of Fe and Pt atoms in the surface layer consistent with the expected stoichiometry. Fe $2\times 2$ and $2\times 1$ superstructures on a local scale are clearly visible (right side). (b) ${\mathrm{Fe}}_{25}{\mathrm{Pt}}_{75}$: Partial ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ SA with preferential $2\times 1$ superstructure confined close to the step edges.

Figure 2

(a) Geometry: XMCD was measured along different Pt(997) directions parallel and perpendicular to the substrate steps (within the planes in blue and red, respectively). $\theta$ is the polar angle with respect to the surface normal. (b) XAS of the 0.5 ML FePt SA at the Pt $N_{7,6}$ (left) and Fe $L_{3,2}$ edges (right). The dashed two-step function accounts for the contribution of the $2p\mathrm{\text{−}}4s$ transitions to the Fe XAS. The difference between the signals for opposite magnetization (blue and black traces) yields the XMCD spectra (red traces). The photon energies at which $M(B)$ for Fe and Pt is measured [Fig. 3a] are indicated by vertical lines.

Figure 3

(a) Upper panels: Hysteresis of the ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ SA taken at the Fe $L_3$ edge for directions $\theta =0°$, 70°. The data are normalized to the nondichroic XAS signal. The measured directions are color coded with orientation labels explained in Fig. 2a and in the text. Lower panels: Corresponding measurements at the Pt $N_7$ edge. (b) Hysteresis of ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ for $\theta =0°$, 35° and 70° together with a Stoner rotation fit using $K_u=0.35\mathrm{meV}/\mathrm{f}.\mathrm{u}.$ and $m_{\mathrm{f}.\mathrm{u}.}=3.53{\mu }_B/\mathrm{f}.\mathrm{u}.$

Figure 4

$M(B)$ perpendicular to the surface at the Fe $L_3$ edge of (a) the FePt SA prepared with 0.5 ML Fe, (b) a SA prepared with 30% less Fe, and (c) a nonalloyed Fe-stripe with 0.5 ML Fe. All three samples can be shown to exhibit a perpendicular easy magnetization axis.