Nanoheterostructures with magnetic tunnel junction (MTJ) are promising composite materials for hard disk scanning heads, magnetoresistive memory elements, and other spintronic devices. In many cases an insulating layer determines the main physical properties of the nanocomposites. We suppose that boron nitride and carbon nanotubes (BNNTs and CNTs) can be used for this purpose. The present study is to characterize the interactions of CNTs or BNNTs with ferromagnetic transition metals, namely, Co and Ni by using local density approximation (LDA) within DFT formalism.
In this work the following possible configurations of NT/TM composite slabs were considered: top:fcc and top:hcp for CNTs and top(N):fcc(B), top(N):hcp(B), hereafter top:fcc, top:hcp, for BNNTs.
All Co-based nanocomposites were found to be substantially more energetically favorable than Ni ones. The CNT(9,0)/TM interfaces demonstrate slightly stronger bonding than CNT(10,0)/TM ones, which can be attributed to the difference in their conducting properties.
To analyze the electronic structure of the composites only the most energetically favorable interface configurations of the composites were considered. Interfaces of different CNTs with the same type of substrate display very similar state distribution near the Fermi level. In particular, atoms in the direct contact with the metal surfaces demonstrate significant differences between spin-up and spin-down density of states. However, atoms far from the interface are not spin-polarized. In the case of CNT(10,0)(top:fcc)/Co composite, a visible negative spin polarization of the top atoms and weak positive spin polarization of atoms far from the interface is detected with no spin polarization observed at fcc carbon atoms. In contrast, for CNT(10,0)(top:hcp)/Ni a positive spin polarization of hcp atoms as well as the top atoms negative spin polarization was detected.
In contrast with CNTs, the electronic structure of the BNNT’s atoms distant from the interface is quite similar to that of an isolated nanotube. However, contact-induced polarization leads to the filling of the unoccuppied states and vanishing of the band gap which causes an appearance of local conductivity in BNNT/TM composites. According to the spin density spatial distribution, the nitrogen atoms of BNNT/Ni are positively spin polarized, while only weak negative polarization is observed at boron atoms. In contrast, significantly stronger negative polarization on B is observed in the case of BNNT/Co due to the much shorter distance between boron and cobalt atoms in top:hcp configuration than between B and Ni in top:fcc.