High magnetic moments and anisotropies for ${\text{Fe}}_x{\text{Co}}_{1-x}$ monolayers on Pt(111)

The magnetism of 1-ML-thick films of ${\text{Fe}}_x{\text{Co}}_{1-x}$ on Pt(111) was investigated both experimentally, by x-ray magnetic circular dichroism and magneto-optical Kerr effect measurements, and theoretically, by first-principles electronic structure calculations, as a function of the film chemical composition. The calculated Fe and Co spin moments are only weakly dependent on the composition and close to $3{\mu }_B/\text{atom}$ and $2{\mu }_B/\text{atom}$, respectively. This trend is also seen in the experimental data, except for pure Fe, where an effective spin moment of only $S_{\text{eff}}=\left(1.2\pm 0.2\right){\mu }_B/\text{atom}$ was measured. On the other hand, both the orbital moment and the magnetic anisotropy energy show a strong composition dependence with maxima close to the ${\text{Fe}}_{0.5}{\text{Co}}_{0.5}$ stoichiometry. The experiment, in agreement with theory, gives a maximum magnetic anisotropy energy of 0.5 meV/atom, which is more than 2 orders of magnitude larger than the value observed in bulk bcc FeCo and close to that observed for the $L{1}_0$ phase of FePt. The calculations clearly demonstrate that this composition dependence is the result of a fine tuning in the occupation number of the $d_{x^2-y^2}$ and $d_{xy}$ orbitals due to the Fe-Co electronic hybridization.

Figure 1

(a) STM image of $0.60\pm 0.05\text{ML}$ Co/Pt(111) deposited and imaged at $T=35\text{K}$. The surface exhibits monolayer-thick grains containing on average 50 atoms each. (b) STM image of $1.00\pm 0.05\text{ML}$ Co/Pt(111). (c) STM image of $1.00\pm 0.05\text{ML}$ ${\text{Fe}}_{0.40}{\text{Co}}_{0.60}/\text{Pt}\left(111\right)$. Growth conditions for (b) and (c): deposition at $T=35\text{K}$ followed by annealing for 5 min at $T=300\text{K}$.

Figure 2

(Color online) XAS and XMCD spectra recorded at the $\text{Co}{L}_{2,3}$ edges for a $0.83\pm 0.05\text{ML}$ film of ${\text{Fe}}_{0.35}{\text{Co}}_{0.65}$ ($T=10\text{K}$, ${\mu }_{0}H=5\text{T}$, field and beam normal to the sample corresponding to $\theta =0°$). (a) Absorption spectra normalized to the incident photon flux $\left(I_0\right)$ for parallel $\left({\mu }_{+}=I_{+}/I_0\right)$ and antiparallel $\left({\mu }_{-}=I_{-}/I_0\right)$ alignment of photon helicity with respect to the field. The shown background signal corresponds to the XAS acquired on the clean Pt(111) surface. (b) XMCD spectrum with the corresponding integral calculated from spectra shown in (a). (c) Total XAS spectra with the corresponding integral calculated from spectra shown in (a) after subtraction of the background signal and of the two-step function accounting for nonresonant absorption (full gray line).

Figure 3

(Color online) Magnetization curves $M_{\theta }\left(H\right)$ of granular films measured at the Fe edge at $T=10\text{K}$, $\theta =0°$ (solid circles), and $\theta =70°$ (open circles) normalized to $M_S$ (see text). (a) $\Theta =0.79\pm 0.05\text{ML}$ pure Fe; (b) $\Theta =0.73\pm 0.05\text{ML}$ ${\text{Fe}}_{0.52}{\text{Co}}_{0.48}$, crosses represent the Co edge data; (c) $\Theta =1.17\pm 0.05\text{ML}$ ${\text{Fe}}_{0.55}{\text{Co}}_{0.45}$. Dotted lines are linear extrapolations of the experimental data when saturation is not reached. (d) MAE vs stoichiometry calculated from Eqs. (3, 4). The value for pure Co is taken from Ref. 8 and corresponds to one-monolayer-thick Co islands with an average size of 40 atoms. The dotted line is a guide for the eyes.

Figure 4

(Color online) (a) Polar MOKE magnetization curve recorded at $T=240\text{K}$ for $1.00\pm 0.05\text{ML}$ Co (red), Fe (black), and ${\text{Fe}}_{0.40}{\text{Co}}_{0.60}$ alloy (blue). (b) Coercive field vs stoichiometry for 1 ML of ${\text{Fe}}_x{\text{Co}}_{1-x}$ measured along the easy axis $\left(T=240\text{K}\right)$. The dotted line is a guide for the eyes.

Figure 5

(Color online) (a) Polar MOKE magnetization curves recorded at $T=260$ and 270 K for $1.00\pm 0.05\text{ML}$ Fe deposited on Pt(111) at 35 K and annealed to 300 K. (b) Arrot-Kouvel plot for the same sample at $T=260$ and 270 K. The Curie temperature $T_C$ is determined when the linearly extrapolated high-field data intercept the origin (solid line).

Figure 6

(Color online) Geometry of the system studied by means of SKKR calculations. 1 ML of FeCo with varying composition has been placed on top of an ideal Pt(111) surface.

Figure 7

(Color online) Spin and orbital magnetic moments as functions of Fe concentration $x$. Individual and averaged values are plotted. $m_L$ values are the ones calculated along the easy axis (see Sec. 4C). Note the linear dependence of the average spin moments on $x$ in contrast to what is observed in the bulk bcc FeCo alloy. The lines are guides for the eyes.

Figure 8

(Color online) (Bottom) Induced spin and orbital moments in Pt atoms of the topmost substrate layer as a function of Fe concentration $x$. Lines are guides for the eyes. The layer-resolved induced spin (top) and orbital (middle) moments (in units of ${\mu }_B$) in Pt atoms up to the fourth substrate layer for the ${\text{Fe}}_{0.5}{\text{Co}}_{0.5}$ monolayer are shown.

Figure 9

MAE as function of Fe concentration $x$. The separated contribution of the Pt substrate, per Pt surface atom, (filled circles) and of the ${\text{Fe}}_x{\text{Co}}_{1-x}$ overlayer (empty circles) to the total MAE (gray circles: dashed line) are also shown. Lines are guides for the eyes.

Figure 10

Total MAE as function of the interlayer distance between the magnetic film and the topmost Pt layer. The interlayer distance is expressed in percentage of the substrate interlayer distance. Lines are guides for the eyes.

Figure 11

(Color online) Calculated Fe DOS for ${\text{Fe}}_{0.50}{\text{Co}}_{0.50}$ and for pure Fe monolayer films. The inset shows the corresponding integrated DOS $\left(1\text{Ryd}=13.6\text{eV}\right)$.

Figure 12

(Color online) Orbital-resolved Co DOS calculated for ${\text{Fe}}_{0.5}{\text{Co}}_{0.5}$ and for the pure Co monolayer films.

Figure 13

(Color online) Orbital-resolved DOS for Fe and Co as a function of the alloy stoichiometry. Reducing the Co content produces for both elements the broadening of in-plane $\left(d_{xy}\right)$ and out-of-plane $\left(d_{yz}\right)$ orbitals. A similar trend is also observed for the other $d$ orbitals.

Figure 14

(Color online) Anisotropy of orbital moments (top) compared to the anisotropy of the occupation of spin-down orbitals of Fe (middle) and Co (bottom). Lines are guides for the eyes.