Half-metallic properties of atomic chains of carbon–transition-metal compounds

Abstract

We found that magnetic ground state of one-dimensional atomic chains of carbon-transition metal compounds exhibit half-metallic properties. They are semiconductors for one spin-direction, but show metallic properties for the opposite direction. The spins are fully polarized at the Fermi level and net magnetic moment per unit cell is an integer multiple of Bohr magneton. The spindependent electronic structure can be engineered by changing the number of carbon and type of transition metal atoms. These chains, which are stable even at high temperature and some of which keep their spin-dependent electronic properties even under moderate axial strain, hold the promise of potential applications in nanospintronics. PACS numbers: 61.46.+w, 75.50.Cc, 71.20.Be Spin-dependent electronic transport has promised revolutionary applications using giant-magneto-resistance in magnetic recording and nonvolatile memories [1,2,3]. Half-metals (HM) [4,5] are a class of materials, which exhibit spin-dependent electronic properties relevant to spintronics. In HMs, due to broken spin-degeneracy, energy bands E n (k, ↑) and E n (k, ↓) split and each band accommodates one electron per k-point. Furthermore, they are semiconductor for one spin direction, but show metallic properties for the opposite spin direction. Accordingly, the difference between the number of electrons of different spin-orientations in the unit cell, N = N ↑ − N ↓ , must be an integer and hence the spin-polarization at the Fermi level,. This situation is in contrast with the ferromagnetic metals, where both spindirections contribute to the density of states at E F and spin-polarization P becomes less than 100%. Even though three-dimensional (3D) ferromagnetic Heusler alloys and transition-metal oxides exhibit HM properties [6], they are not yet appropriate for spintronics because of difficulties in controlling stoichiometry and the defect levels destroying the coherent spin-transport. Zinc-blende (ZB) HMs with high magnetic moment µ and high Curie temperature T c > 400K (such as CrAs, and CrSb in ZB structure) have been grown only in thin-film forms [7]. More recently, it has been predicted that four new ZB crystals can be HM at or near their respective equilibrium lattice constants [8].In this letter, we report that very simple and stable one-dimensional (1D) structures, such as linear atomic chains of carbon-transition metal compounds, i.e. C n (TM), show half-metallic properties. The prediction of half-metallic behavior in 1D atomic chains is new and of fundamental interest, in particular in the field of fermionic excitations with spin degree of freedom. Besides, the present finding may lead to potential applications in the rapidly developing field of nanospintronics, such as tunnelling magnetoresistance, spin valve, and nonvolatile magnetic devices.In earlier transport studies, the spin-direction of conduction electrons was generally disregarded, in spite of the fact that spin orientation of electrons decays much slower than their momentum [3]. Magnetic ground s...

Magnetic and Magnetotransport Properties in Nanogranular Co/C<SUB>60</SUB>-Co Film with High Magnetoresistance

“…The spin-dependent transport in the HB region might be associated with the highly spin-polarized molecular-like states generated in the C 60 -Co compound as theoretically predicted for various organic molecule-TM systems. [1][2][3][4] As for the existence of such shallow mid-gap states, the C 60 -Co compounds exhibit a small conductivity explained by the activated transport from the midgap states at less than one À10's meV from the Fermi level, 10,12) which is nearly equivalent to the interparticle voltage drop above a few meV (at V ¼ V T ) in the HB region. Analytical and theoretical approaches to the electronic structure of the C 60 -Co compounds itself and at the interface with Co crystal are indispensable to make obvious the electronic origin of the observed TMR effect, and are in progress.…”

“…The hybrid systems between organic molecules involving carbon nanotubes (CNTs) and transition metals (TMs) has recently attracted attention as a new class of materials systems for spintronics following the theoretical predictions for high spin-polarization. [1][2][3][4] In particular, magnetotransport properties in ferromagnetically contacted CNTs have been investigated. [5][6][7] The reported magnetoresistance (MR) in such CNT-TM systems, however, is as small as less than 10% different from the theoretical predications.…”

Spintronic properties of carbon-based one-dimensional molecular structures

“…40 Spin-dependent total density of states of ͑CrC n ͒ ϱ and ͑CoC n ͒ ϱ are presented for n = 3 and 4 in Fig. 11.…”

“…39 Half-metallic properties have been also predicted in simple 1D atomic chains composed of carbon and TM compounds. 40 Carbon chains in this respect are promising, since carbon has a strong tendency to form linear atomic chains, whereas other elements tend to make zigzag chains and they are more vulnerable to clustering. 30 In any real device the system length is certainly finite, so that the considerations regarding the properties of finite carbontransition-metal compound atomic chains, such as whether spin-polarized currents can be achieved in these structures, and the effects of metallic electrodes on their transport properties are interesting points to address.…”

Self-assembly mechanisms of short atomic chains on single-layer graphene and boron nitride

“…A short segment of carbon atomic chain containing n C atoms, indicated as CAC(n) hereafter, is a truly one-dimensional carbon allotrope, and has recently drawn attention due to its linear geometry, high strength, size-dependent quantum ballistic conductance, and interesting electronic properties. These properties of CACs were both theoretically [11][12][13][14][15][16][17][18][19][20] and experimentally [21][22][23] investigated. Theoretical studies revealed the Peierls distortion in CACs, 11,13,19 half-metallic and spintronic properties, [16][17][18] size-dependent quantum conductance, 12,14 and other geometrical structures.…”