Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes

We present a systematic study of the electronic and magnetic properties of transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs). We considered the adsorption on the external and internal wall of SWNT and examined the effect of the TM coverage and geometry on the binding energy and the spin polarization at the Fermi level. All those adsorbed chains studied have ferromagnetic ground state, but only their specific types and geometries demonstrated high spin polarization near the Fermi level. Their magnetic moment and binding energy in the ground state display interesting variation with the number of $d$ electrons of the TM atom. We also show that specific chains of transition-metal atoms adsorbed on a SWNT can lead to semiconducting properties for the minority spin bands, but semimetallic for the majority spin bands. Spin polarization is maintained even when the underlying SWNT is subjected to high radial strain. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with nonmagnetic metal electrodes, these magnetic needles or nanomagnets can perform as spin-dependent resonant tunneling devices. The electronic and magnetic properties of these nanomagnets can be engineered depending on the type and decoration of adsorbed TM atom as well as the size and symmetry of the tube. Our study is performed by using first-principles pseudopotential plane wave method within spin-polarized density functional method.

Figure 1

(Color online) The configuration of adsorbed TM atoms (Co, Cr, Fe, Mn, and V) forming chain structures on the (8,0) SWNT are illustrated for various coverage geometries, such as $\theta =1∕2$, 1, 2, and 4.

Figure 2

(Color online) The spin-dependent band structure and TDOS of V, Co, and Fe chains adsorbed on the zigzag (8,0) SWNT for $\theta =1∕2$, 1, and 2 geometries. Solid and dotted lines are for majority and minority spin states, respectively. The zero of energy is set to the Fermi level $E_F$.

Figure 3

(Color online) The spin-dependent band structure and TDOS of Mn and Cr chains adsorbed on the zigzag (8,0) SWNT for $\theta =1∕2$, 1, and 2 geometries. Solid and dotted lines are for majority and minority spin states, respectively. The zero of energy is set to the Fermi level $E_F$.

Figure 4

(Color online) The variation of (a) magnetic moment $\mu$, and (b) the binding energy $E_b$ as a function of number of $d$ states for $\theta =1∕2$, 1, and 2 for external adsorption.

Figure 5

(Color online) Variation of the average magnetic moment $\mu$ and binding energy $E_b$ of an Fe chain adsorbed on the external walls of zigzag $(n,0)$ SWNT ($n=6$, 8, and 9) with the coverage geometry $\theta$ and tube index $n$. (a) $\mu$ versus $\theta$; (b) $E_b$ versus $\theta$; (c) $E_b$ versus $n$.

Figure 6

(Color online) Left panels: The number of states and corresponding structures of Fe atoms adsorbed on a finite (8,0) SWNT with open but fixed ends for (a) $\theta =1∕2$ (i.e, 64 carbon atoms + one Fe), (b) $\theta =1$ (i.e, 64 carbon atoms $+2$ Fe), and (c) $\theta =2$ (i.e, 128 carbon atoms $+8$ Fe). The zero of energy is set to the Fermi level $E_F$. The total net magnetic moment for each finite tube, $\mu$, is shown. Total density of states of majority and minority spin states of infinite and periodic systems are also shown by continuous lines for $\theta =1∕2$, 1, and 2. Right panels: Atomic configurations of Fe atoms adsorbed on the finite size (8,0) SWNT with closed ends. (d) One Fe atom is adsorbed on a tube consisting of 96 carbon atoms. (e) Two Fe atom are adsorbed on a tube consisting of 96 carbon atoms. (f) Eight Fe atoms are adsorbed on a tube consisting of 160 carbon atoms. The calculated total magnetic moments ${\mu }_T$ and TDOS of majority and minority spin states are illustrated for each configuration.