Two-Dimensional Intrinsic Ferromagnetism at Nitride-Boride Interfaces

We predict theoretically novel two-dimensional interface ferromagnetism at $\mathrm{AlN}/{\mathrm{MgB}}_2(0001)$ using first-principles calculations, where the interface is employed as an ordered structure of spin sites instead of point defects. Although N dangling bonds are apparently saturated, interfacial states exhibit spin polarization. Hund’s coupling of the two N $p_{\parallel }$ orbitals as well as low density of states at the Fermi energy contribute to strong band ferromagnetism. Furthermore, first-principles electron transport calculations demonstrate that this interfacial spin polarization is responsible for quantum spin transport. The magnetization can be controlled by applied gate bias voltages.

Figure 1

(a) Two-dimensional energy bands of the $\mathrm{AlN}/{\mathrm{MgB}}_2$ interface structure. (b) Wave function for one of the spin-polarized states with the up-spin at the $\Gamma$ point. The contour value is 20% of the maximum, and the yellow isosurface is for the positive phase and the turquoise blue one is for the negative phase. (c) The LDOS of the interface N atom projected on the $p_{\parallel }$ states for the ferromagnetic configuration (solid lines, color) as well as the antiferromagnetic configuration (dashed lines, black). The LDOS of single atom in (d) bcc Fe as well as in (e) fcc Ni for comparison. Lines are defined as in (c).

Figure 2

(a) Atomic structure of the Pt-($\mathrm{AlN}/{\mathrm{MgB}}_2$)-Pt system used in our first-principles transport calculations. (b) Transmission functions calculated for the Pt-($\mathrm{AlN}/{\mathrm{MgB}}_2$)-Pt system. The dotted curve is the difference in transmission between the down-spin (dashed) and the up-spin (solid). (c) Same as (a), except the bulk-Pt electrodes are replaced by hypothetical 2D Pt sheets. (d) Transmission functions for the system depicted in (c).

Figure 3

The magnetic moment of the $\mathrm{AlN}/{\mathrm{MgB}}_2$ interface as a function of the gate-induced charge at the interface N atom.

Figure 4

(a) Electron density distribution (per spin) of the $\mathrm{GaN}/{\mathrm{ZrB}}_2$ interface at ${\epsilon }_F$. (b) Spin-resolved electron density distribution of the $\mathrm{AlN}/{\mathrm{MgB}}_2$ interface at ${\epsilon }_F$. Both yellow isosurfaces are for the value of 0.0008 a.u.