Interface-driven manipulation of the magnetic anisotropy of ultrathin Co films on Pt(111): Substrate deposition of hydrogen and model calculations

The magnetic anisotropy energy (MAE) and the resulting spin-reorientation transitions of Co/Pt(111) ultrathin films are investigated by manipulating the Co/Pt interface with controlled adsorption of hydrogen prior to Co deposition. In situ low-energy electron-diffraction and surface magneto-optical Kerr-effect measurements are performed on Co films grown at low temperatures. The results show that interface H deposition leads to a remarkable change in the magnetization direction from perpendicular to in-plane, even for the thinnest Co films, which is accompanied by an important increase in the coercive force. Layer-resolved self-consistent electronic calculations of the MAE are performed in order to identify the interface contributions responsible for perpendicular magnetic anisotropy (PMA) and to quantify how the MAE depends on various possible adsorbate-induced modifications in the local magnetic moments. The results show that the PMA is quite insensitive to changes in the local magnetic moments at the Co film surface even if they are relatively large. However, the PMA depends crucially on the Co-interface moments, on the Pt-interface moments induced by the proximity to Co, and on the resulting spin-orbit interactions at the Pt atoms. The observed suppression of PMA by interface H adsorption is interpreted as the consequence of the reduction in the interface-Pt moments which, originates either a reduction in the Co-interface moments or at a decoupling of the Pt substrate from the magnetic film. Experiment and theory thus prove the dominant role of the Co-Pt interface MAE on the development of the relative stability of perpendicular and in-plane magnetization directions. The magnetic properties of ultrathin films may thus be tailored to a large extent by adsorbates trapped at $3d\text{−}4d$ or $3d\text{−}5d$ film-substrate interfaces.

Figure 1

LEED patterns of unexposed $24.6\text{Å}$ thick Co films grown on Pt(111) at [(a) and (b)] 300 K and [(c) and (d)] 200 K. The primary electron beam energies are [(a) and (c)] 122.5 eV and [(b) and (d)] 252 eV.

Figure 2

Thickness dependence of the hysteresis loops of Co films deposited on a clean Pt(111) surface. The films were grown and measured at 200 K. The magnetic field direction is perpendicular to the film.

Figure 3

LEED patterns of a $29\text{Å}$ Co film grown at 200 K after H preadsorption. The primary electron beam energy is (a) 122.5 eV and (b) 252 eV.

Figure 4

Thickness dependence of the magnetic hysteresis loops of Co films grown and measured at 200 K after exposing the Pt(111) substrate to 20 Langmuirs of ${\text{H}}_2$. The applied magnetic field is (a) within the surface plane and (b) perpendicular to the surface.

Figure 5

MAE of ${\text{Co}}_4/\text{Pt}\left(111\right)$ films as a function of the exchange integral $J_{\text{Co}\left(1\right)}$ of the Co surface atoms. $J_{\text{Co}}=0.76\text{eV}$ corresponds to Co bulk. (a) Total anisotropy energy $\Delta E_{xz}=E_x-E_z=\Delta E_{xz}^{\text{MC}}+\Delta E_{xz}^{\text{DD}}$ (full circles) and magnetocrystalline anisotropy energy $\Delta E_{xz}^{\text{MC}}$ (open circles). (b) CoPt interface MAE $\Delta E_{\text{CoPt}}^I=\Delta E_{\text{Co}}^{\text{MC}}\left(4\right)+\Delta E_{\text{Pt}}^{\text{MC}}\left(5\right)$ (full circles), surface MAE $\Delta E^S=\Delta E_{\text{Co}}^{\text{MC}}\left(1\right)+\Delta E_{\text{Co}}^{\text{MC}}\left(2\right)$ (open circles), and uppermost-layer contribution $\Delta E_{\text{Co}}^{\text{MC}}\left(1\right)$ (crosses).

Figure 6

Local spin plus orbital magnetic moments $\mu \left(m\right)$ at the different layers $m$ of a ${\text{Co}}_4/\text{Pt}\left(111\right)$ film as a function of the exchange integral $J_{\text{Co}\left(1\right)}$ of the Co surface atoms. $J_{\text{Co}}=0.76\text{eV}$ corresponds to Co bulk. $\text{Co}\left(m\right)$ refers to the $m\text{th}$ Co layer, $m=1$ to the surface layer, and Pt(5) to the uppermost substrate layer.

Figure 7

MAE of ${\text{Co}}_4/\text{Pt}\left(111\right)$ films as a function of the exchange integral $J_{\text{Co}\left(4\right)}$ of the Co atoms at the CoPt interface. (a) Total anisotropy energy $\Delta E_{xz}$ (full circles) and magnetocrystalline anisotropy energy $\Delta E_{xz}^{\text{MC}}$ (open circles). (b) CoPt interface MAE $\Delta E_{\text{CoPt}}^I$ (full circles), surface MAE $\Delta E^S$ (open circles), and uppermost-layer contribution $\Delta E_{\text{Co}}^{\text{MC}}\left(1\right)$ (crosses).

Figure 8

Local orbital plus spin magnetic moments $\mu \left(m\right)$ at the different layers $m$ of a ${\text{Co}}_4/\text{Pt}\left(111\right)$ film as a function of the exchange integral $J_{\text{Co}\left(4\right)}$ of the Co atoms at the CoPt interface. See the caption of Fig. 6.

Figure 9

Magnetic anisotropy energy (MAE) of ${\text{Co}}_4/\text{Pt}\left(111\right)$ films as a function of the magnetic moment ${\mu }_{\text{Co}}\left(4\right)$ of the Co layer $m=4$ at the CoPt interface. The results are obtained by varying the exchange integral $J_{\text{Co}\left(4\right)}$ of the Co atoms at the CoPt interface, as in Fig. 7. ${\mu }_0=1.79{\mu }_B$ refers to the Co moment ${\mu }_{\text{Co}}\left(4\right)$ obtained for $J_{\text{Co}\left(4\right)}$ equal to the bulk value $J_{\text{Co}}$. (a) Total anisotropy energy $\Delta E_{xz}=\Delta E_{xz}^{\text{MC}}+\Delta E_{xz}^{\text{DD}}$ (full circles) and magnetocrystalline anisotropy energy $\Delta E_{xz}^{\text{MC}}$ (open circles). (b) CoPt interface MAE $\Delta E_{\text{CoPt}}^I$ (full circles) and surface MAE $\Delta E^S$ (open circles).

Figure 10

Local orbital plus spin magnetic moment ${\mu }_{\text{Pt}}\left(5\right)$ induced at the Pt atoms of the CoPt interface in a ${\text{Co}}_4/\text{Pt}\left(111\right)$ film as a function of the Co moment ${\mu }_{\text{Co}}\left(4\right)$ at the interface Co layer. The results are obtained by varying the exchange integral $J_{\text{Co}\left(4\right)}$ of the interface Co atoms.

Figure 11

MAE of ${\text{Co}}_4/\text{Pt}\left(111\right)$ films as a function of exchange integral $J_{\text{Pt}}\left(5\right)$ of the Pt atoms at the CoPt interface. $J_{\text{Pt}}=0.60\text{eV}$ corresponds to Pt bulk. (a) Total anisotropy energy $\Delta E_{xz}=\Delta E_{xz}^{\text{MC}}+\Delta E_{xz}^{\text{dip}}$ (full circles) and magnetocrystalline anisotropy energy $\Delta E_{xz}^{\text{MC}}$ (open circles). (b) CoPt interface MAE $\Delta E_{\text{CoPt}}^I$ (full circles), surface MAE $\Delta E^S$ (open circles), and uppermost-layer contribution $\Delta E_{\text{Co}}^{\text{MC}}\left(1\right)$ (crosses).

Figure 12

Local orbital plus spin magnetic moments $\mu \left(m\right)$ at the different layers $m$ of a ${\text{Co}}_4/\text{Pt}\left(111\right)$ film as a function of the exchange integral $J_{\text{Pt}\left(5\right)}$ of the Pt atoms at the CoPt interface.