Exchange-split interface state at h-BN/Ni(111)

The insulator/ferromagnet interface system of hexagonal boron nitride on Ni(111) was investigated with spin- and angle-resolved inverse photoemission and spin-polarized secondary electron emission. We identified the theoretically predicted boron nitride (BN) interface state at 1.7 eV above the Fermi level. The interface state is found to be influenced by the ferromagnetic substrate, which is reflected in a magnetic exchange splitting of $130\pm 50\text{meV}$. Parallel to the surface, it shows a free-electron-like $E\left({\mathbf{k}}_{\parallel }\right)$ dispersion with an effective mass of $1.1\pm 0.2m_e$. Furthermore, we found a spectral feature at 2.2 eV above the Fermi level, which we attribute to indirect transitions into a region of high density of BN-induced states.

Figure 1

(Color online) Inverse-photoemission spectra for normal electron incidence $\left(\theta =0°\right)$ on uncovered Ni(111) detected with ${\text{GM}}_{70}$ (black dots) and on h-BN/Ni(111) detected with ${\text{GM}}_{35}$ and ${\text{GM}}_{70}$ (blue dots). Peak positions are marked by dashed lines. The spectral features just above $E_F$ consist of contributions from a surface state (SS) and transitions into bulk $d$ states $\left(B_d\right)$. $A$ denotes the spectral intensity originating from the boron-nitride-induced interface state. $\Delta$ indicates the energy shift of the image-potential surface state IS between the clean and BN-covered surface. Inset: Inverse-photoemission spectra with improved energy resolution for h-BN/Ni(111) obtained with ${\text{GM}}_{70}$. The thin lines demonstrate the decomposition of the spectrum into two spectral features $A_1$ and $A_2$ plus background intensity.

Figure 2

Inverse-photoemission spectra of structure $A$ as a function of the angle of electron incidence $\theta$. The filled and open symbols represent our data obtained with ${\text{GM}}_{35}$ and ${\text{GM}}_{70}$, respectively. The spectra are vertically offset for reasons of clarity. Vertical dashed lines mark a final-state energy of 2.2 eV.

Figure 3

Inverse-photoemission spectra of h-BN/Ni(111) for $\theta =6°$ obtained with ${\text{GM}}_{35}$ (filled dots) and ${\text{GM}}_{70}$ (open dots). The solid black (for ${\text{GM}}_{70}$) and gray lines (for ${\text{GM}}_{35}$) represent decompositions of the spectra into two Gaussian peaks (left-hand panel) and one Gaussian peak (right-hand panel) on top of a quadratic background, respectively. The quality of the fits can be evaluated from the lines through the data points and from the difference between fit and data points as given in the bottom part of the figure.

Figure 4

(Color online) $E\left({\mathbf{k}}_{\parallel }\right)$ dispersion for the BN-induced states $A_1$ and $A_2$ and the image-potential state IS as derived from the spectra of Fig. 2.

Figure 5

(Color online) Spin-polarized secondary electron emission measurements as a function of the temperature for clean Ni(111) (black dots) and h-BN/Ni(111) (blue dots). The solid lines represent the saturation magnetization $M\left(T\right)$ of bulk Ni, described by Weiss’ theory, fitted to the data points for temperatures above $T/T_C=0.6$. Room temperature $T_R$ is marked by a dashed vertical line.

Figure 6

(Color online) Spin-resolved inverse-photoemission spectrum for normal electron incidence on h-BN/Ni(111), detected with ${\text{GM}}_{70}$. The solid (green) and open (red) triangles represent the IPE data for majority and minority spins, respectively. The data were obtained at room temperature and the spin asymmetry was rescaled according to Fig. 5. The solid lines were obtained from Gaussian line fits to the data. The spin-dependent peak positions for $A$ are marked by dashed lines.